project martha apxr ground settlement assessment

R APPENDIX R

Ground Settlement Assessment (Engineering Geology Limited)

25 May 2018

Prepared for:

Oceana Gold (New Zealand) Ltd

P O Box 190

WAIHI 3641

OCEANA GOLD (NEW ZEALAND) LTD

PROJECT MARTHA

ASSESSMENT OF GROUND SETTLEMENT

8332

EXECUTIVE SUMMARY

1. Oceana Gold New Zealand Ltd (OGNZL) proposes to develop a new mining project

named Project Martha for extending the mine life by developing open pit and

underground mining comprising:

The Martha underground mine including the Rex lode

The Martha Phase 4 pit (MP4)

2. The groundwater level in Project Martha will have to be lowered to RL500m1 to allow

for the development of the Martha underground mine. This is approximately 200m below

the existing consented dewatering level to RL700m of the Correnso underground mine.

Dewatering will be achieved by the installation of stage pumping chambers within the

mine that pump into the Correnso dewatering system at RL780m.

3. This report addresses the potential effects on ground settlement of the dewatering to

RL500m, together with rebound that can be expected following closure when pumping

ceases and groundwater levels rise. Settlement caused by dewatering arises as a result of

lowering the groundwater and depressurisation of the water within the defects and pores

within the ground. This results in increased stresses within the ground and compression

resulting in settlement.

4. Monitoring of groundwater levels and settlement has been undertaken over a period of 28

years (1989-2017). The results indicate a very strong correlation between groundwater

level and settlement.

5. This report builds on previous assessments proposed for dewatering required for the

Martha, Favona, Trio and Correnso Mines. Settlement predictions have generally been

shown to be accurate. Where there has been a difference between predictions and

measured settlement, this has been explained in this report.

6. Over the life of the current mining operations, settlements due to dewatering at Waihi

have been small. No damage to property or infrastructure has been attributed to

settlement induced by mine dewatering.

7. The basement rock of the area surrounding Waihi is andesite and it extends to

considerable depth. An almost continuous cap of low permeability, highly weathered and

altered andesite is present on the surface of the andesite. The andesite rocks are overlain

by a series of younger volcanic deposits which are highly variable in thickness and

composition. A thin layer, up to about 8m thick, of geologically young ash soils,

alluvium and completely weathered rhyolitic tephra blankets much of the younger

volcanic deposits.

8. Groundwater drawdown associated with dewatering of Project Martha area is expected to

be confined to the andesite bedrock. Groundwater levels in the overlying younger

volcanic deposits are generally not expected to be affected. Minimal changes to the

shallow groundwater system have resulted to date from 28 years of dewatering and none

are expected. Full groundwater drawdown occurs in the mineralised veins. Partial

drawdown occurs in the andesite rock mass, with drawdown reducing with distance from

the veins.

1 Levels are expressed as mine datum, which is approximately 1,000m below mean sea level (i.e. RL1,000m mine

datum is approximately sea level).

9. Settlements due to the proposed dewatering have been assessed using two approaches.

Firstly, an observational approach based on extrapolation of historic settlements

assuming a simple linear relationship between settlement and time. Secondly, using

numerical modelling. The numerical modelling covers dewatering since 1989 and allows

assessment of tilt arising from the differential drawdown in groundwater about the veins.

The numerical modelling demonstrates that settlements predicted using the observational

approach are conservative and that ground surface tilts due to dewatering to RL500 will

be acceptable.

10. Broad settlements due to the proposed dewatering to the target level of RL500m have

been inferred from the observed settlements measured around Waihi arising from

dewatering from the Martha Pit from RL975m to RL770m, which occurred over the

period December 1999 to November 2017. Estimated settlements are summarised in

Table 1 for both dewatering to RL700m to allow for development of the Trio and

Correnso underground mine, which has already been consented, and from RL700m to

RL500m to allow development of Project Martha. Additional settlement arising from

dewatering now sought to be consented ranges from 24mm in Zone 1 to 140mm in Zone

7. Zones 1 to 7 represent different areas around Waihi in which settlements would be

expected to be similar. The Land Use Consent granted in 1997 refers to these zones. The

original zone boundaries are shown in attached Figure 15. We have reviewed and updated

the zonation taking into consideration the monitoring data since 1997. The recommended

new zone boundaries are shown in Figure 16.

11. Estimated settlements arising due to the proposed dewatering from RL700m to RL500m

are considered acceptable and expected to occur within the andesite rock mass below the

ground surface. Differential settlements, which are normally the concern for buildings

and shallow buried services, if they occur, are expected to be very small (less than 1 in

1,000). This is because settlements are associated with the andesite rock mass and not the

overlying younger volcanic deposit or surficial soils, and so damage potential is

considered negligible.

12. We expect there will be a lag between dewatering and settlement occurring. This is

because the andesite rock mass generally has low permeability and it will take time for

the pore pressures within the rock mass to decrease.

13. When dewatering is no longer required, groundwater levels are expected to return to

approximately their original levels. This will result in rebound of the ground. We

estimate that rebound will be approximately 50 percent of the settlement that occurs

during dewatering.

14. The Waihi area has already been subjected to ground dewatering to RL540m and for a

longer duration during historical underground mining undertaken between 1893 and 1952

than will be required for the existing projects including the period of Project Martha. The

historical dewatering will have resulted in pre-consolidation of the ground so that

settlements associated with the current phase of mining would generally be expected to

be of the same order or less than those that the area experienced between 1893 and 1952.

15. We recommend that the trigger levels in the Dewatering and Settlement Monitoring Plan

(and within the resource consent) be reset considering the settlements expected because

of the additional dewatering. The recommended revised trigger levels are provided in

Table 2.

CONTENTS

Page No.

1.0 INTRODUCTION ............................................................................................................. 1

2.0 BASIS OF ASSESSMENT OF SETTLEMENT ............................................................ 2

3.0 GROUND SETTLEMENT AT MARTHA MINE ......................................................... 2

4.0 SITE GEOLOGY .............................................................................................................. 3

5.0 PIT DEWATERING AND GROUNDWATER LEVELS ............................................. 4

5.1. Historical Dewatering ..................................................................................................... 4

5.2. Dewatering 1989-2017 .................................................................................................... 4

5.3. Dewatering associated with Project Martha ................................................................ 6

6.0 GROUND SETTLEMENT MEASUREMENTS ........................................................... 7

7.0 METHODS FOR ASSESSING SETTLEMENT ........................................................... 9

8.0 SETTLEMENT ESTIMATES ....................................................................................... 10

8.1. Observational Method .................................................................................................. 10

8.2. Numerical Modelling .................................................................................................... 11

8.3. Comments on Settlement Estimates ............................................................................ 13

9.0 EXCEPTIONS TO PREDICTIONS ............................................................................. 14

9.1. Gladstone Road ............................................................................................................. 14

9.2. Slevin Street................................................................................................................... 14

9.3. Favona Underground Mine ......................................................................................... 14

10.0 REBOUND ....................................................................................................................... 15

11.0 MITIGATION AND RECOMMENDATIONS ............................................................ 15

12.0 CONCLUSIONS.............................................................................................................. 16

REFERENCES

TABLES 1 TO 3

FIGURES 1 TO 24

APPENDIX A TWO DIMENSIONAL DECOUPLED SEEPAGE AND DEFORMATION

NUMERICAL MODELLING

Our Ref: 8332 25 May 2018 Page 1

8332 Ground Settlement Assess Report_Project Martha 25 May 2018.docx

25 May 2018

1.0 INTRODUCTION

Oceana Gold New Zealand Ltd (OGNZL) proposes to develop a new mining project

named Project Martha for extending the mine life by developing open pit and

underground mining comprising,

The Martha underground mine including the Rex lode

The Martha Phase 4 pit (MP4)

The project components are shown in Figure 1. Project Martha provides OGNZL with

the opportunity to continue mining operations in Waihi for further 11 years beyond the

existing life of mine (Correnso underground mine).

At its greatest depth Project Martha extends to RL500m2 (up to 620 metres below the

ground surface). This is 200m below the consented level for Correnso. Dewatering will

be achieved by the installation of stage pumping chambers within the mine that pump

into the Correnso dewatering system at RL780m. Water will continue to be pumped to

the Water Treatment Plant for treatment and discharge in full accordance with existing

conditions of consent.

This report addresses the potential effects on ground settlement of the additional

dewatering to RL500m, together with rebound that can be expected following closure

when pumping ceases and groundwater levels rise.

This report builds on the previous reports prepared for dewatering required for the

Extended Martha, Favona, Trio and Correnso Mines, the predictions for which have

generally been shown to be accurate. Where there has been a difference between

predictions and measured settlement, this has been explained within this report.

In this report, the term “settlement” applies to the wide spread and relatively minor

vertical displacement of the ground surface caused by dewatering. The term “tilt” refers

2 Levels are expressed as mine datum, which is approximately 1,000m below mean sea level (i.e. RL1,000m

mine datum is approximately sea level).

OCEANA GOLD (NEW ZEALAND) LTD

PROJECT MARTHA

ASSESSMENT OF GROUND SETTLEMENT

8332



to where there are differential settlements (i.e. differences in settlement over short

distances).

2.0 BASIS OF ASSESSMENT OF SETTLEMENT

Estimates of settlement have been based on the following data:

a) Settlement data over a period of 28 years (1989-2017) from surveys undertaken

in the area around the Martha Pit as required by condition 11 of the now-expired

Mining Licence 32 2388, condition 3 of the now-expired Water Permit 971286

and conditions 5 to 7 of resource consent 124860;

b) Groundwater level measurements over a period of 28 years (1989-2017) carried

out in boreholes around the Martha Pit and dewatering water levels in the

Martha Pit and Correnso underground;

c) Assessment of the likely groundwater conditions reported by GWS Ltd (Ref.1);

and

d) Geology of the immediate mine area from earlier reports and the current

geological model as recently updated by OGNZL.

3.0 GROUND SETTLEMENT AT MARTHA MINE

Settlement caused by dewatering arises as a result of lowering of the groundwater and

depressurisation of the water within the defects and pores within the ground. This

results in increased stresses within the ground. Various other physical processes can

also give rise to settlement. They include shrink and swell as a result of changes in

moisture content.

The amount of settlement due to dewatering/depressurisation of the groundwater

depends on the following:

a) The increase in the compressive stresses within the ground, which depends on

the difference between the initial and final groundwater levels;

b) The compressibility of the ground which is a natural property, and which varies

depending on the type of soil and rock; and

c) The thicknesses of the different types of soil and rock that underlie a site which

are depressurised.

The rate of settlement depends on the permeability of the ground (i.e. on how quickly

soils can drain). Permeability is a measure of how easily water flows through a

material. The presence of holes, tunnels and other discontinuities in the ground aid the

draining of water from the ground and hence increase the rate of settlement.



Monitoring indicates a strong correlation between settlement and the depth of

dewatering. Dewatering is necessary so that mining can be undertaken in dry

conditions. Dewatering commenced at the Martha Pit in 1989. The water level in the pit

was reduced from RL1100m to about RL790m in April 2015 when the pit was

temporarily closed for operation due to pit wall instability blocking access to the base

of the pit. Since then the mine has been dewatered from the Correnso underground

mine. A graph of the depth of dewatering is shown in Figure 2.

Dewatering has also been undertaken since 2005 for the Favona underground mine,

located south east of the Martha Pit. Monitoring indicates ground settlement in the

vicinity of the Favona Mine is also strongly correlated to dewatering.

Over the life of the current mining operations, settlements due to dewatering at Waihi

have been small, but have been observed over a large area. The largest settlements have

occurred close to the Martha Pit because of dewatering of the more compressible

surface soils. These settlements occurred early in the life of the project. Settlements

have largely been in line with what was predicted, except for Favona. The reasons for

the differences are explained later in this report.

Other than in connection with an incident in 2012 described below, no damage to

property or infrastructure has been attributed to settlement induced by mine dewatering.

This is not surprising because damage from settlement usually only occurs where there

are high differential settlements (i.e. large differences in settlement over a short

distance, also known as tilt). Settlement due to dewatering at Waihi is now primarily a

result of settlement of deeper materials, not surface soils, so differential settlements

(tilt) tend to be very small.

An incident occurred in 2012 when damage to some houses in Gladstone Road to the

east of the Martha Pit was reported. Differential settlements were the cause of the

damage. Investigations confirmed the settlement was a result of dewatering of younger

volcanic deposits, and this was due to a deep geotechnical drillhole (CGD008) not

being properly grouted rather than mine dewatering. Following re-grouting of the

drillhole (CGD008) the original ground water levels were restored.

4.0 SITE GEOLOGY

The geology of the area is quite complex, but a better understanding has been possible

as knowledge has increased in conjunction with mining and exploration drilling.

Figures 3 to 6 show geological maps and sections of the area around Martha Pit,

including the various mineralised vein systems. Figure 3 includes the geological map of

the area around the Martha Pit and the location of the cross-sections radiating out from

the pit. The cross-sections are given in Figures 4 to 6 and show that the mineralised

andesite extends down to at least RL500m below the Martha Pit.

Figure 7 is a plan that shows the vein system, workings and drillhole locations between

the Martha and Favona system. Figure 8 is an indicative geological cross section from

Martha Pit through the Correnso, Trio and Favona systems along the same orientation

as Sections A-A' and B-B' in Figure 7. The section shows that andesite, the basement



rock, outcrops on the former Martha Hill, to the north of the Martha Pit and on Union

Hill. The andesite basement rock extends to depths greater than 600m. The andesite

rocks are extensively modified in places by weathering and hydrothermal alteration. An

almost continuous cap of low permeability, highly weathered and altered andesite

(upper andesite) is present on the surface of the mineralised (quartz) andesite. The

andesites are overlain by a series of younger volcanic deposits which are highly

variable in thickness and composition. The younger volcanic deposits consist of

rhyolitic tephras and ignimbrites in the form of flows, breccias and tuffs. Occasionally,

paleosols (buried soils) and sedimentary deposits such as alluvium and boulder

alluvium are present at the top of successive eruption sequences. The younger volcanic

deposits underlie much of Waihi Township and outcrop to the east and south of the

Martha Pit. A thin layer, up to about 8m thick, of geologically young ash soils,

alluvium and completely weathered rhyolitic tephras blankets much of the younger

volcanic deposits.

The younger volcanic deposits are more compressible than the underlying andesite

which means they have the potential to settle more when subjected to the same increase

in stress. Much of the dewatering induced settlement immediately adjacent to Martha

Mine is associated with these deposits.

5.0 PIT DEWATERING AND GROUNDWATER LEVELS

5.1. Historical Dewatering

Extensive underground mining was undertaken at the Martha Mine from 1893

until 1952. The workings were generally in the area of the Martha Pit but

extended to a much greater depth. Dewatering was required down to the full

depth of workings, approximately RL540m. Dewatering for Project Martha is

another 40m deeper (RL500m). The historic dewatering occurred over many

years, much longer than the recent phase of mining (1989-2018). By the time the

recent phase of mining commenced in 1989 the groundwater levels had returned

to pre-mining levels.

5.2. Dewatering 1989-2017

Dewatering occurred from Martha Pit from 1989 until 4 May 2015. Since then it

has been undertaken from the Correnso underground mine.

Dewatering of the Martha Pit occurred progressively. It was achieved by

installing pumps into the main shafts associated with historical underground

mining. A record of the mine dewatering level over the life of the mine is shown

in Figure 2.

The dewatering has been carried out at different rates during the period and

sometimes the water level has been held constant. The average rate of drawdown

in the pit from 1989 to 2000 was about 16m per year. Between 2001 and 2003

the groundwater level at the pit was maintained between RL940 - RL960. It was

then lowered progressively to about RL890 from about May 2003 to April 2006

and was held close to this until June 2009. The pit was dewatered further to



RL875 between June 2009 and December 2010. The average rate of drawdown in

this period was about 10m per year. The rate of dewatering was then increased to

about 60m per year from January 2010 and the water level was reduced from

RL875m to RL807 in November 2011. It was lowered progressively to about

RL790 from November 2011 to May 2015. After this the dewatering has occurred

from the Correnso underground mine. The dewatering level has lowered to

RL770 as at November 2017.

Monitoring of dewatering levels was undertaken from the bottom of Martha Pit

until April 2015. Dewatering levels since then are based on pump levels in

Correnso.

Monitoring wells, referred to as piezometers, have been installed at various

locations surrounding the pit to monitor the effect of dewatering on groundwater

levels as shown in Figure 9. The piezometers have been installed at different

depths to enable the monitoring of groundwater levels or water pressures in the

different geological units (alluvium, younger volcanic deposits and andesite

rock). There were no piezometers installed in the Golden Link Project Area prior

to 2011. As part of the investigations for the Golden Link Project, seven

boreholes, P90 to P95 and CGD008 were drilled and their locations are shown in

Figure 9. Piezometers were installed in different geological units to measure

groundwater levels.

GWS (Ref.1) provides a detailed review of the site hydrogeology and the effect of

pit dewatering on groundwater levels. Generally dewatering has resulted

comparatively in much greater changes in groundwater levels in the deeper

andesite rock than in the overlying weathered andesite, younger volcanic deposits

and alluvium as shown in Figures 10 and 11. In Figure 10, piezometer P2-1 and

P2-2 are in andesite and in Figure 11, piezometer P8-1 is located in andesite.

A shallow groundwater system associated with volcanic ash, alluvium and

completely weathered rhyolitic tephra is present at shallow depth. Monitoring

indicates that it is unaffected by dewatering of the pit except immediately

adjacent to the Martha Pit. Shallow groundwater levels are controlled, principally,

by rainfall infiltration, low surface permeability and natural and assisted drainage

to surface water systems. The response of piezometer P2-4 in Figure 10 is typical

of the response of the shallow groundwater systems.

Younger volcanic deposits are located within the shallow groundwater system.

Monitoring of groundwater levels indicates drainage primarily towards the open

pit where the younger volcanic deposits are close to the Pit. The response of

piezometer P2-3 in Figure 10 and P8-3 in Figure 11 is typical. Drawdown is

evident in the early stages of pit dewatering before a steady-state situation was

reached in about 1996. This contrasts with the continuing drop in groundwater

levels in the deeper andesite rock. Groundwater level changes and the associated

consolidation of the varying thickness of the younger volcanic deposits, which are

weaker and more compressible than the andesite, is responsible for much of the

settlement that occurred in the early stages of dewatering close to the Martha Pit.

The settlement was in accordance with predictions.



Groundwater levels in the mineralised andesite are directly affected by

dewatering in Martha Pit. Groundwater levels in the vein system respond rapidly

to mine dewatering with a slower response in the rock body (i.e. within the rock

mass away from the veins) due to its low permeability. The veins have good

connection due to both the high permeability nature and the historical mining.

Monitoring indicates that the amount of groundwater drawdown in the andesite

reduces with distance from the Pit. It also indicates that groundwater levels in the

previously mined and mineralised andesite are very similar to the level of

dewatering in the Pit, and that groundwater levels change quickly in response to

changes in the Pit dewatering level. However, at increasing distance from the Pit,

beyond the limits of historical underground mining and mineral vein system,

groundwater drawdown occurs much more slowly, before it reaches an

equilibrium between drainage and recharge. This is because of the lower

permeability of the unmined and non-mineralised rock, which means it takes

longer to drain.

Dewatering effects associated with the Martha Pit are abruptly attenuated to the

north and west of the mine because of faulting. Dewatering extends to the south-

west and east of Martha Pit. There is a connection between Martha, Correnso and

Trio groundwater systems. The Favona groundwater system was originally

hydrologically separate from Martha-Correnso-Trio groundwater system. Now

the construction of Favona to Trio inclines provides a connection of the two

systems above RL823m (Ref.1).

Monitoring in the andesite is limited by the depth to which the piezometers have

been installed. The deepest that a piezometer has been installed is at RL965m

(P92 in East Waihi) while the Project Martha underground mines will be

developed down to RL500m.

5.3. Dewatering associated with Project Martha

The Martha Underground mine extends to RL500m. Therefore, the water level

will need to be lowered at least 200m below the currently consented level

(RL700m). Dewatering will be achieved by the installation of stage pumping

chambers within the mine that pump into the existing Correnso dewatering

system located at RL780m. Water will continue to be piped to the Water

Treatment Plant for treatment and discharge in full accordance with existing

conditions of consent.

The observed response of veins within the Martha groundwater system (which

does not include the separate Favona groundwater system) to dewatering at

Martha Mine means that the RL500m water level will also be developed in veins

beneath Waihi, including Waihi East (Ref.1). The existing multi-level

piezometers in Waihi East shown in Figures 12 and 13 are considered a good

indication of the potential response to the additional dewatering at the location

and over much of the developed Waihi area. The groundwater levels in boreholes

P90 to P95 and P100 to P102 located in the Waihi East area are shown in Figures

12 and 13. These groundwater levels were recorded in November 2017. The

groundwater level of the Martha vein system at that time is estimated to be



approximately RL750m. The deepest piezometers in the boreholes are in the

upper andesite and show significant under drainage.

Observation of groundwater levels and modelling by GWS (Ref.1) indicate

dewatering the Martha vein system from the current elevation (approximately

RL770m) to RL500m is not expected to significantly affect groundwater

pressures within the younger volcanics. This is consistent with the long-term

groundwater monitoring data which shows shallow groundwater pressures have

remained relatively unchanged while dewatering has been on going. The

groundwater pressures in the veins and other open rock defects connected to the

veins are expected to drop immediately in response to lowering of the

groundwater level required for the Martha Underground. The groundwater

pressures in the andesite rock mass are indicated to respond to the deeper

dewatering.

The weathering profile depth within Andesite rock does vary locally, resulting in

a variation in thickness of the low permeability material that perches the shallow

groundwater system. However, monitoring and modelling indicates that it is

always thick enough to maintain a perched water level in the overlying younger

volcanics (Ref.1).

6.0 GROUND SETTLEMENT MEASUREMENTS

Ground settlement measurements have been made around the Martha Pit and

throughout Waihi since 1989. Six monthly surveys have been undertaken as required

by condition 11 of the Mining Licence, condition 9 of Water Permit 971286 and

conditions 5 to 7 of resource consent 124860. The locations of the settlement markers

where settlement measurements are made are shown in Figure 14.

Land Use Consent (No.97/98-015, Condition 3.30g) granted in 1999 refers to Figure 8

of the evidence presented by Dr R Semple to the Environment Court in 1998. Seven

zones (Zones 1 to 7) are identified in Figure 8 and a copy of this figure is attached

(refer to Figure 15). The seven zones are intended to represent different areas in which

settlements would be expected to be similar. The zone boundaries were based on

settlement monitoring up to 1997, site geology and distance from the pit. Generally,

settlements within the different zones have been relatively consistent. However, there

are some settlement markers that don’t fit with the zones they are currently associated

with. Consequently, we have reviewed and updated the zonation taking into

consideration monitoring data since 1997. The recommended new zonation map is

shown in Figure 16. The same number of zones has been retained, and in many

locations the zone boundaries have not been changed. The new zones are primarily

based on examination of settlement data, but local geology has also been considered.

The monitoring data show that some of the settlement markers indicate greater

settlements than their original zone category. This is more significant for settlements to

the South and East of Waihi. This area is likely to be affected more by the Favona

underground mine rather than the Martha, Trio and Correnso system. We have reshaped

the boundaries of Zones 1, 2, 3, 4 and 5 in this area to provide consistency against the

monitoring results to November 2017. We have also modified the boundaries of Zones



5 and 6 slightly around the Martha Pit. The new zoning around the pit provides a better

transition and avoids localised, isolated zones. No changes have been made to the

boundary of Zone 7.

The results of settlement monitoring for Zones 1 to 7 are presented in Figures 17 to 23

respectively. The settlement data indicate that there have been varying amounts of

settlement but always over a large area and not resulting in unacceptable differential

settlement (tilt). Settlement markers along the southern and south eastern perimeter of

the pit have shown the greatest settlement. Generally, settlements decrease with

distance from Martha Pit. Settlements near the perimeter of the pit are within the range

of 30-290mm. Greater settlements have been recorded on the southern and eastern sides

of the Pit compared to the northern and western sides. This difference is due to the

presence of a greater thickness of the more compressible younger volcanic deposits on

the southern and eastern sides.

Comparison of settlements with groundwater levels indicates a correlation. The amount

of settlement is generally proportional to the drop in groundwater level. This is

apparent in Figure 24 where settlement at 4A, which is relatively close to Martha Pit,

has a similar trend to the groundwater level measured in the base of the pit. This same

trend is evident in Figures 21 and 22 which show settlements in Zones 5 and 6 that are

close to the pit.

The rate of settlement closer to the Martha Pit was generally higher during the initial

stages of Pit dewatering and has reduced with time. Settlement markers in Zone 5

(Figure 21) and Zone 6 (Figure 22) show this. The curves are generally steeper in the

early stages of mining, indicating a higher rate of settlement, than later. This is

attributed to the higher compressibility of the younger volcanic deposits and the

groundwater drawdown in these materials that occurred in the early stage of the mine

life. In the early stages of the mine life Martha Pit dewatering resulted in changes in

groundwater levels in both the younger volcanic deposits and underlying andesite rock.

This can be seen in Figure 10. The changes in groundwater level in the ignimbrite (P2-

3) were smaller than in the andesite, and equilibrium was reached in a shorter period.

However, ignimbrite is more compressible than andesite and this is the reason why

there were higher rates of settlements observed initially.

Settlements show a slower response to pit dewatering at increasing distance from the

Martha Pit and underground mines, as shown by settlement measurements in Zones 2, 3

and 4 (refer to Figures 18, 19 and 20 respectively). At these locations settlements are

much smaller and the rate of settlement is more uniform with time. Observations are in

line with predictions. Smaller settlements are expected because groundwater drawdown

away from the Martha Pit and underground mines is less and the rate of settlement is

more uniform with time due to the slow rate of depressurisation of the andesite rock

mass.

Settlements have continued to occur, albeit at very slow rates (typically 0.1-1mm year)

during periods when mine water levels have not changed significantly (July 1998 to

January 2000 and November 2006 to November 2010). We consider the most likely

explanation is ongoing drainage of the mineralised andesite rock mass, with slow

depressurisation occurring due to the low permeability of the rock mass away from the

quartz vein system. The proposed further dewatering to RL500m will have minimal



effect on any ongoing settlement in the upper andesite or younger volcanics. However,

it will result in additional depressurisation of the mineralised andesite rock mass. There

was a significant change in pit water level from November 2010 to November 2011. In

general, settlement markers (refer Figures 17 to 23) either showed no noticeable or only

a very small increase in the rate of settlement. Where there was a change in rate of

settlement it occurred either immediately or within one month following the change in

pit water level. However, the rates of settlement reduced to their original levels in

approximately six months.

The settlement measurements show cyclic patterns in some cases (refer to Figures 17 to

23). The cyclic variation is typically in the range of 5 to 15mm. The cyclical pattern is

believed to be either due to seasonal effects resulting in differences between base and

intermediate benchmark levels or due to errors in determining intermediate benchmark

levels. Most settlements are measured from intermediate benchmark levels that are in

turn checked against base benchmarks located away from the influence of mining

activity. It is possible that seasonal effects (e.g. changes in groundwater levels resulting

in differential settlements between the base and intermediate benchmarks or

shrink/swell of the surficial soils on which the benchmarks are located) could result in

seasonal differences between the base and intermediate benchmarks. The differences

could also be due to errors in survey measurement. The cyclical variations are relatively

small and are close to the level of accuracy expected from survey measurements.

Condition 7 of resource consent 124860 (and previously condition 10 of the Water

Permit 971286) requires that tilt be assessed based on the six-monthly settlement

survey data and reported in the Annual Dewatering and Settlement Monitoring report

(Ref.2). Tilt gradients are generally very low, less than 1:1,000, which is well below the

gradient that would give rise to concern for structures. Locations where tilt gradients

are greater than 1:1000 are discussed in Section 9.

7.0 METHODS FOR ASSESSING SETTLEMENT

Open-pit mining at the Martha Mine commenced in 1987. Prior to commencement of

mining, and subsequently again in 1997, theoretical predictions of the future ground

settlements because of mine dewatering were made. Since then, throughout the life of

the mine, ground settlements have been measured at 6 monthly intervals at 399

locations in Waihi. The large number of settlement observations, spanning some 28

years, provides a very reliable database for estimating likely future settlements

associated with further dewatering. In fact, the monitoring undertaken at Waihi makes

it one of the best monitored sites in New Zealand and provides an excellent database for

predicting future settlements.

The proposed dewatering will result in an approximately 250m drop in the current

groundwater levels in the andesite basement rock. Minimal effects on the shallow

groundwater system and younger volcanic materials are predicted by GWS (Ref.1).

This conclusion is based on review of monitoring data during mining of Martha Pit and

the different underground mines and theoretical seepage analyses. Consequently,

settlements caused by dewatering within the area of Project Martha will primarily be

associated with the changes in groundwater levels in the andesite basement rock.

Our Ref: 8332 25 May 2018 Page 10


Estimates of settlement in the andesite have been made using two approaches. The first

is an observational approach in which the settlement observations to date are used as a

basis for predicting what might occur with the proposed dewatering. The second is a

theoretical approach involving numerical modelling of seepage and deformation.

8.0 SETTLEMENT ESTIMATES

8.1. Observational Method

Estimates of settlements arising from further dewatering can be inferred from

settlements recorded during the dewatering of the Martha Pit and the different

underground mines. This is possible because:

(i) There is strong correlation between groundwater levels and observed settlement

and

(ii) Studies by GWS Ltd (Ref.1) indicate that the dewatering to RL500m will not

have any significant effect on the groundwater levels in the overlying younger

volcanic deposits and much of the upper andesite immediately underlying the

younger volcanic deposits, these being the layers most sensitive to dewatering

and responsible for much of the settlement that has occurred to date.

The initial dewatering of the Martha Pit from January 1989 through to 1996-1999

caused depressurisation of some of the younger volcanic deposits and the andesite units

as shown in Figure 10. After 1996, most of the piezometers in the younger volcanic

deposits have shown no reaction to further dewatering of the mine. Piezometric levels

have remained steady despite further dewatering of the pit suggesting that most of the

observed settlement after 1996 was due to depressurisation of the andesite rock mass.

From December 1999 to November 2017, the water level in the Martha Pit was lowered

by 205m from approximately RL975m to RL770m. The settlement measured during

this time can be assumed to be due to settlement within the andesite rock mass. This

period was selected because the water level was steady in the pit at about RL975m from

June 1998 to December 1999 and at about RL770m from December 2011 to November

2017. It can be assumed that steady conditions were established at the start and end of

the 205m drawdown.

As previously explained settlements measured around Waihi are grouped into seven

zones (Zones 1 to 7). Observed settlements in each zone are presented in Figures 17 to

23. The average settlement for each zone over the period December 1999 to November

2017 is shown in their respective figure and is also summarised in Table 1. The average

settlements due to 205m drawdown are estimated from the slopes of the settlement

curves ignoring any obvious outliers.

The greatest average settlement occurred in Zone 7. It was approximately 143mm. This

Zone is located near the Martha Pit. The higher rate of settlement in this area is likely

due to greater depressurisation of the andesite rock mass and possibly also due to

subsidence associated with the underlying historical Royal Lode workings.

Our Ref: 8332 25 May 2018 Page 11


Settlements in Zones 4, 5 and 6 over the period December 1999 to November 2017

averaged 38mm, 68mm and 96mm respectively. The distance from Martha Pit has a

significant effect on the magnitude of the settlement observed. Areas further away from

the Pit would have had less depressurisation of the underlying andesite rock mass and

therefore less settlement. Zone 4 is further from Martha Pit than Zones 5 and 6. Zone

5 has more points farther from the Pit than Zone 6.

Average settlements in Zones 1, 2 and 3 are between 24mm and 27mm. These areas are

least affected by drawdown.

As a check we have compared estimated settlements for the Trio Project (Ref.3) against

the measured settlements associated with dewatering from RL875 to RL770 (approx.

January 2011 to November 2017). The comparison showed that the estimated

settlements are consistent with the monitored settlements. This confirms the

observational method is a reliable predictor of future settlements.

Estimates of future settlement that will occur because of dewatering from the present

level of RL770 to RL700 for the Trio and Correnso Projects have been inferred from

the above observations. They have been made by scaling the observed settlements

arising from the 205m reduction in water level from December 1999 to November

2017. Estimates based on this approach are summarised in Table 1. Estimates of

settlements vary from 12mm in Zone 1 to 47mm in Zone 6. In Zone 7, settlement of

70mm is estimated. As noted above, the dewatering to RL700m for the Correnso

Project has already been consented. The dewatering has continued from November

2017 and therefore some of these settlements have already occurred.

Estimates of additional settlements due to dewatering from RL700m to RL500m to

allow for development of Project Martha are inferred from observations of historical

settlements from December 1999 to November 2017. The estimated settlements are

summarised in Table 1. Settlements range from 24mm in Zone 1 to 140mm in Zone 7.

Estimates of settlements in Zones 3, 4, 5 and 6 range from 26mm to 94mm. These

estimates are average estimates for the zones. At some locations settlements within

each zone will be less and at others more.

The rate of drawdown of the groundwater level in the Martha Pit that started after

November 2010 for the Trio Project is higher than the previous drawdown rates and

associated with observations used to estimate the expected settlement as shown in

Figures 17 to 23. It shows that the rate of drawdown has some effect on the rate of

settlement, but it is not that significant. The average rate of dewatering from RL700m

to RL500m will be at a faster rate than historical. However, the rate of drawdown is not

expected to significantly affect the magnitude of the settlement based on observations

to date and the numerical modelling. There could be a lag between dewatering and

settlement occurring. This is because the andesite rock mass is of generally low

permeability and it takes time for the pore pressures within the rock mass (i.e.

groundwater level) to decrease.

8.2. Numerical Modelling

Two-dimensional de-coupled numerical modelling of seepage and deformation has

been undertaken of an area to the east of Martha Pit. This area covers settlement Zones

Our Ref: 8332 25 May 2018 Page 12


4 and 5. The modelling provided estimates of settlements due to dewatering to RL770m

associated with mining since 1989 and with the proposed additional dewatering to

RL500m associated with Project Martha. The analyses have been undertaken using

GeoStudio 2D finite element analysis (FEA) programs, SEEP/W (Ref.7) and

SIGMA/W (Ref.8). The SEEP/W model of the ground and groundwater parameters was

provided by GWS. Some small adjustments were made to the model for deformation

modelling. The soil stiffness values were obtained from a geotechnical model

developed by PSM (Ref.4). The assumptions and results of the modelling are presented

in Appendix A.

The scope of modelling included:

i. Modelling of seepage and drawdown associated with dewatering of both single

and multiple veins to various levels using SEEP/W.

ii. Deformation analyses using SIGMA/W with changes in stress due to

dewatering to different levels predicted by the SEEP/W modelling.

iii. Comparison of the results of the numerical modelling with monitored

settlements

Modelling was undertaken for three stages of dewatering:

i. Stage 1: Drawdown to the midpoint of the weathered andesite (RL975m at

deepest).

ii. Stage 2: RL975m to RL770m (current level)

iii. Stage 3: RL770m to RL500m

The average modelled settlements for Stage 1 are 23mm for both single and multiple

veins. The average modelled settlements for Stage 2 are 89mm for a single vein and

152mm for multiple veins. For Stage 3 the average modelled settlements are 26mm for

a single vein and 66m for multiple veins.

Results of the modelling from the first two stages were compared with monitored

settlements. The average modelled settlements for Stage 1 are generally smaller than

monitored settlements (23mm compared to average monitored settlements of between

25mm and 45mm). The Stage 2 modelled settlements are greater. The average

modelled settlements for Stage 2 are 89mm for a single vein and 152mm for multiple

veins compared to average monitored settlements in Zones 4 and 5 of 38 and 68mm

respectively. The differences between modelled and monitored settlements could be

due to the assumption of a single stiffness value for the andesite rock mass. It is

possible that the rock mass has a lower modulus at higher elevations with increasing

modulus with depth.

The modelling indicates additional settlements due to Stage 3 dewatering (RL770m to

RL500m) are considerably less than for Stage 2 (average modelled settlements of

26mm for a single vein and 66mm for multiple veins compared to 89mm and 152mm

respectively for Stage 2). This is because groundwater drawdown and associated

incremental stress in the rock mass is localised around the veins. It results in arching

effects and lower settlements than if the rock mass was fully dewatered. Also, with

Stage 3 the depth of rock mass that is affected by drawdown is less than Stage 2.

Our Ref: 8332 25 May 2018 Page 13


The modelling allows estimation of ground surface tilts. The estimated maximum tilts

are associated with the multiple vein model and for Stages 1, 2 and 3 are 1 in 8,000, 1

in 4,600 and 1 in 3,800 respectively. The incremental change in tilt from Stage 2 to

Stage 3 is very small and consistent with the relatively small increment in settlement

predicted by numerical modelling. The measured tilts above the Correnso underground

mine (which is largely within Zones 4 and 5) are generally smaller than the maximum

tilt predicted by numerical modelling (1 in 4,600). This indicates the numerical model

is conservative. The estimated maximum tilt following dewatering to RL500m (1 in

3,800) is considerably less than 1 in 1,000 which is the current trigger for notification

of the Waikato Regional Council and for instigating additional monitoring.

8.3. Comments on Settlement Estimates

Estimates of the additional settlement due to dewatering from RL700m to RL500m (i.e.

Martha Project) by the observational method in Zones 4 and 5 are greater than those by

numerical modelling. The observational method gives values of 38-66mm. Estimates

from modelling for dewatering from RL770m to RL500m (70m more than associated

with Martha Project) are 26mm for a single vein and 66mm for multiple veins. For

dewatering from RL975 m to RL770m the monitored settlements are lower than

modelled, indicating that numerical modelling is conservative. Assuming the same

applies to dewatering to RL500m it is conservative to adopt settlements from the

observational approach for estimating the additional settlements associated with Project

Martha. We consider that estimates based on the observational method are reliable

because they are based on 28 years of monitoring. The numerical modelling estimates

rely on assessed values of rock deformation modulus and the extent of drainage within

the rock mass. There is considerable uncertainty associated with these assumptions.

However, the numerical modelling does provide valuable insight with respect to the

comparative difference between the different stages of dewatering and estimates of tilt

that arise due to greater drawdown at the veins.

Local variations in settlement can be expected, as has been evident with current

monitoring. The proposed dewatering could also result in an extension of the current

zone of influence of dewatering. Settlements within the new areas of influence are

likely to be very small based on monitoring of settlements within the current zone of

influence.

The proposed dewatering to RL500m is expected to result in groundwater drawdown in

the andesite rock mass, with no or minor impact on groundwater within the overlying

younger volcanic deposits and shallow groundwater system. Predicted settlements in

Zones 1 to 5 are small and are not expected to have any significant effect. Predicted

settlements in Zones 6 and 7 are greater but still no damage to existing structures or

infrastructure is expected.

Settlements at the ground surface are generally expected to follow those predicted in

the andesite rock mass. Differential settlements (tilt), which are normally the concern

for buildings and shallow buried services, are expected to be very small (less than 1 in

1,000) because settlements are associated with the andesite rock mass and not the

overlying younger volcanic deposits or surficial soils, and so damage potential is

considered negligible.

Our Ref: 8332 25 May 2018 Page 14


The total magnitude of the settlements is small and not considered significant because,

as noted above, differential settlements are normally the concern for buildings and

buried services. It is of interest to note that the Waihi area has already been subject to

groundwater dewatering to a similar depth and greater duration during historical

underground mining between 1893 and 1952 than required for the Project Martha. The

historical dewatering will have resulted in pre-consolidation of the ground so that

settlements associated with the current phase of mining would generally be expected to

be of the same order or less than those that the area experienced between 1893 and

1952.

In some areas the total magnitude of settlement arising because of dewatering to

RL500m will exceed the trigger levels derived for the Correnso Project. As

recommended in Section 11, new trigger levels have been set to account for the effects

of dewatering to greater depths for Project Martha.

9.0 EXCEPTIONS TO PREDICTIONS

Generally, settlements have been in-line with expectations. Where they have not they

have been investigated and, in all cases, there have been explanations. They are

discussed in the following sections.

9.1. Gladstone Road

An incident occurred in 2012 when damage to some houses in Gladstone Road to the

east of the Martha Pit was reported. Differential settlements were the cause of the

damage. Investigations confirmed the settlement was a result of dewatering of the

younger volcanic deposits in this area, and this was due to a deep geotechnical drillhole

(CGD008) not being properly grouted. Settlement was not directly related to mine

dewatering. Following re-grouting of the drillhole the ground water was re-stored to the

original levels and the land re-bounded close to original levels.

9.2. Slevin Street

Since May 2015 the tilt between marks 20C and 20D (Slevin Street) has been greater

than 1:1,000. OGNZL believes the settlement is not related to dewatering associated

with the current project, but to the weak nature of the ground and unfilled stopes that

underlie the area (Ref.2). The area is near a high hazard zone identified by GNS

Science (Ref.5). Slevin Park is a swampy area, historically infilled with weak material

and slumping/subsidence has occurred previously. No damage to any residences has

occurred. The closest residence to the affected area is 95m away.

9.3. Favona Underground Mine

Settlements at the Favona underground mine have been greater than originally

predicted and have exceeded 1 in 1,000 at three locations. The locations are directly

above the underground workings on farmland owned by OGNZL. No buildings have

been affected. The original settlement estimates only considered settlements in the

younger volcanic deposits (Ref.6). If allowance is made for settlement in the geologic

Our Ref: 8332 25 May 2018 Page 15


units that underlie the younger volcanic deposits (i.e. unweathered ignimbrite, dacite

units and both the upper and mineralised andesite rocks) then the observed settlements

are considered realistic.

10.0 REBOUND

Dewatering for Project Martha Area will eventually stop once underground mining in

the project area has ceased. The area will then be allowed to re-flood. The re-flooding

will occur naturally but will be accelerated by the addition of water pumped from the

Ohinemuri River. Re-flooding will result in the groundwater levels in the andesite and

the younger volcanic materials rising back close to their original levels. Rebound will

occur as rising groundwater re-saturates the ground, resulting in a reduction in effective

stress within the ground. As already noted, historical underground mining at Martha Pit

involved dewatering of the ground to about RL540m; approximately 40m above the

targeted RL500m for the Martha Underground. Following the cessation of mining in

1952 groundwater levels rose. Rebound will have occurred but was not monitored. We

are unaware of any damage arising from re-watering.

Rebound of the ground to its original level because of rising groundwater levels is not

expected. We estimate that rebound could be approximately 50% of the settlement that

has occurred during modern mining. We consider that no damage is expected because

of rebound.

11.0 MITIGATION AND RECOMMENDATIONS

Predicted settlements associated with additional dewatering to RL500m in Zones 1 to 5

are small and are not expected to have any significant effect. Predicted settlements in

Zones 6 and 7 are greater but still no damage to existing structures or infrastructure is

expected. This is because settlements are associated with settlement in the deeper

andesite rock mass which underlies the younger volcanic deposits that blanket much of

the area. Numerical modelling for Martha East (Zones 4 and 5) indicates incremental

settlements and tilts associated with Project Martha are small and well within

acceptable limits. Monitoring to date confirms this is the case, even in Zone 6.

Consequently no mitigation measures are considered necessary. We recommend

monitoring of settlement as required by the expired Mining Licence and resource

consent 124860 should continue. Figure 14 shows that settlement markers within the

area of the Martha Underground mine are closely spaced and therefore we believe it is

not necessary to install any additional settlement markers for Project Martha.

We recommend that the trigger levels be reset considering the settlements expected

because of the additional dewatering. The recommended revised trigger levels are

provided in Table 2. The recommended trigger levels are based on estimates of the

average settlement predicted for each zone. There is variation in observed settlements

within each zone as can be seen in the settlement observations shown for each zone in

Figures 17 to 23. There are a few reasons for this. They include the variation in

thickness and compressibility of the different geologic units, variations in groundwater

levels, the accuracy of the settlement surveys, variations in ground levels due to

shrink/swell effects, local disturbance or damage to settlement markers and changes to

Our Ref: 8332 25 May 2018 Page 16


groundwater levels due to new mining projects that have occurred following definition

of the zones. The zones should be interpreted as only providing broad definitions of

settlement response. Variations in settlements over distance are more likely to change

gradually and so in reality the zones are not hard boundaries.

We also recommend that similar conditions as for the Correnso Underground be

adopted to ensure that the groundwater regime affecting ground settlements is not

adversely impacted. These include:

i) No stoping above defined levels in the andesite. This is to preserve the integrity

of the top of the andesite that acts to maintain the perched groundwater level in

the younger volcanics.

ii) Grouting of all future surface-drilled holes to a depth below the top of the

andesite.

iii) Avoiding intercepting existing drillholes with mine workings.

iv) Grouting drillholes from underground where underground development

intercepts holes that are making water or geological defects with significant and

sustained water flows.

12.0 CONCLUSIONS

1. Dewatering of Project Martha will require a reduction in groundwater from the

consented level of RL700m to RL500m. Dewatering is proposed to be achieved

by the installation of stage pumping chambers within the Martha Underground

that pump into the Correnso dewatering system at RL780m. Water will continue

to be pumped to the Water Treatment Plant for treatment and discharge in full

accordance with existing conditions of consent. Now dewatering is being

undertaken down to RL700m for the Correnso Project. This dewatering is

already authorised by existing resource consents. The groundwater level will

then be lowered to RL500m for Project Martha. Drawdown is expected to be

confined to the andesite rock. Groundwater levels in the overlying younger

volcanic deposits are generally not expected to be affected. No changes to the

shallow groundwater system are expected.

2. Settlements due to the groundwater drawdown to the target level of RL500m

have been estimated by inference from observed settlement measurements and

numerical modelling. The settlements inferred from observed settlements are

greater than predicted by numerical modelling. They have been adopted as best

estimates. They are likely to be conservative based on comparison with the

results from numerical modelling. The numerical modelling provides useful

insight into the comparative settlements associated with different stages of

dewatering and provides estimates of ground surface tilt.

3. The inferred settlements have been based on the observed settlement measured

around Waihi arising from dewatering from the Martha Pit from levels RL975m

to RL770m which occurred over the period December 1999 to November 2017.

Settlements due to dewatering from RL770m to RL700m to allow for the

development of the Correnso underground mine are estimated to range from

12mm in Zone 1 to 47mm in Zone 6. Zone 7, which is around the pit, is

Our Ref: 8332 25 May 2018 Page 17


expected to have settlement of about 70mm. Additional settlement arising from

the dewatering now sought to be consented for Project Martha (to RL500m)

ranges from 24mm in Zone 1 to 94mm in Zone 6. Zone 7 is expected to have

additional settlement of about 140mm.

4. Estimated settlements are small and are expected to occur within the andesite

rock mass below the ground surface. Differential settlements (tilt), which are

normally the concern for buildings as well as buried services, are expected to be

very small and so damage potential is considered negligible.

5. We expect there will be a lag between dewatering and settlement occurring.

This is because the andesite rock mass generally has low permeability and it

will take time for the pore pressures within the rock mass to decrease.

6. When dewatering is no longer required, groundwater levels are expected to

return to their original levels. This will result in rebound of the ground. We

estimate that rebound will be approximately 50 percent of the settlement that

occurs during dewatering.

7. We recommend that the trigger levels be reset considering the settlements

expected because of the additional dewatering. The recommended revised

trigger levels are provided in Table 2.

Report prepared by

ENGINEERING GEOLOGY LTD

C Wu (Geotechnical Engineer) T Matuschka (CPEng)


REFERENCES

1. GWS (2018) ‘Project Martha Assessment of Groundwater Effects’.

2. OGNZL (2017) 'Dewatering and Settlement Monitoring Report 2017' Ref. WAI-200-

REP-007-003.

3. EGL (2012) 'Newmont Waihi Gold, Golden Link Project Area, Assessment of Ground

Settlement' Ref.6808.

4. PSM (2010) ‘Martha Mine East Layback Pit 66D, PSM125.R39’.

5. GNS Science (2008) ‘Waihi Subsidence Assessment’ GNS Science Consultancy Report

2006/235.

6. URS (2003) 'Favona Underground Mine - Assessment of Groundwater Issues', 14

March 2003.

7. GEO-SLOPE International Ltd (2007) ‘Seepage Modeling with SEEP/W: An

Engineering Methodology’, GEO-SLOPE International Ltd.

8. GEO-SLOPE International Ltd (2007) ‘Stress and Deformation Modeling with

SIGMA/W: An Engineering Methodology’, GEO-SLOPE International Ltd.


TABLE 1. SETTLEMENT ESTIMATES – OBSERVATIONAL METHOD

Zone

Typical Settlement

Response to

Dewatering from

RL975m to

RL770m (Dec 1999

to Nov 2017) (mm)

Estimated Settlement

Response to Dewatering

of Correnso Project from

RL770m to RL700m 1

(mm)

Estimated Settlement

Response to

Dewatering of Project

Martha from RL700m

to RL500m (mm)

1 24 12 24

2 24 12 24

3 27 13 26

4 38 19 38

5 68 33 66

6 96 47 94

7 143 70 140

1

The dewatering from RL770m to RL700m began in November 2017 and

therefore some of the estimated settlement has already occurred

TABLE 2. RECOMMENDED NEW TRIGGER LEVELS FOR SETTLEMENT (mm)

Zone Recommended New Trigger Levels for Settlement (mm)

1 55

2 65

3 95

4 160

5 260

6 340

7 540

LEGEND

As-Built Pit Crest(August 2018)

MP4 Pit Crest

MMZ Boundary

Expired MiningLicence Boundary

Extended Project LandUse Consent Boundary

Cambridge RoadRealignment

Martha Underground

Date: 14 April 2018

File:D:\...\Project Martha & Zone Boundaries-d.cdr

Figure 1

Project Martha and Regulatory Boundaries

NN

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Engineering Geology LtdUnit 7C, 331 Rosedale Rd, PO Box 301054, AlbanyPh (09) 4862546 Fax (09) 4862556

OCEANA GOLD (NEW ZEALAND) LTDPROJECT MARTHA

Mine Dewatering Level

Ref. No: 8332Date: Jan 2018Drawn: CW

Engineering Geology LtdUnit 7C, 331 Rosedale Rd, PO Box 301054, Albany, Auckland

Figure 2



Engineering Geology Ltd Unit 7C, 331 Rosedale Rd, PO Box 301054, Albany Ph (09) 4862546 Fax (09) 4862556

OCEANA GOLD (NEW ZEALAND) LTD PROJECT MARTHA

Geological Plan

Ref. No: 8332 Date: Jan 2018 Drawn: OGNZL

Engineering Geology Ltd Unit 7C, 331 Rosedale Rd, PO Box 301054, Albany, Auckland

Figure 3



AQF – Quartz Andesite A – Upper Andesite HBX – Hydrothermal Breccia CSH – Contact Shear (between BDAC and HBX or A) BDAC – Black Hill Dacite RT – Rhyolitic Tuff IS/ISC/ISS – Sandy Ignibrites IW – Welded Ignimbrite ASH - Ash REG – Regolith RHHY – Ruahorehore rhyolite



Geological Cross Sections A and B



Figure 4




Section A

Section B



Geological Cross Sections C and D



Figure 5




Section C

Section D



Geological Cross Sections E and F

Ref. No: 8332Date: Jan 2018Drawn: OGNZL


Figure 6



AQF – Quartz AndesiteA – Upper AndesiteHBX – Hydrothermal BrecciaCSH – Contact Shear (betweenBDAC and HBX or A)BDAC – Black Hill DaciteRT – Rhyolitic TuffIS/ISC/ISS – Sandy IgnibritesIW – Welded IgnimbriteASH - AshREG – RegolithRHHY – Ruahorehore rhyolite

Section E

Section F



Plan of Martha, Union and Favona Systems

Ref. No: 8332Date: Feb 2018Drawn: OGNZL


Figure 7





Indicative Geological Section from Martha to Favona

Ref. No: 8332Date: Feb 2018Drawn: OGNZL


Figure 8





Location of Groundwater Monitoring Piezometers and Water Table Contours

Ref. No: 8332Date: Jan 2018Drawn: GWS LTD


Figure 9



Notes from GWS Ltd Letter Reportdated 8 Jan 2018

Water table elevation were plotted at the monitoring well location. The elevation are shown as yellow numberswith wells with uncertain values shown as orange numbers.

Water elevation data from shallow monitoring wells was averaged over the last 12 months (Jan-Dec 2017) giventhe seasonal fluctuations evident in therecords and plotted at the monitoringwell location.

Inferred contours of the water table at10m intervals were drawn having regardto the topography. These contoursare shaded yellow.

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P2-1 in Andesite - Tip Level RL974.26m


P2-3 in Ignimbrite - Tip Level RL1073m

P2-4 in Boulder Alluvium - Tip Level RL1101m



Piezometer Readings in Borehole P2



Figure 10



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-De

c-8

9

08

-De

c-9

0

08

-De

c-9

1

07

-De

c-9

2

07

-De

c-9

3

07

-De

c-9

4

07

-De

c-9

5

06

-De

c-9

6

06

-De

c-9

7

06

-De

c-9

8

06

-De

c-9

9

05

-De

c-0

0

05

-De

c-0

1

05

-De

c-0

2

05

-De

c-0

3

04

-De

c-0

4

04

-De

c-0

5

04

-De

c-0

6

04

-De

c-0

7

03

-De

c-0

8

03

-De

c-0

9

03

-De

c-1

0

03

-De

c-1

1

02

-De

c-1

2

02

-De

c-1

3

02

-De

c-1

4

02

-De

c-1

5

01

-De

c-1

6

01

-De

c-1

7

01

-De

c-1

8

Gro

un

dw

ater

Lev

el R

L (m

)



P8-3 in Ignimbrite - Tip Level RL1092.9m

P8-4 in Boulder Alluvium - Tip Level RL1111.8m



Piezometer Readings in Borehole P8



Figure 11



940

960

980

1000

1020

1040

1060

1080

1100

1120

0 20 40 60 80 100 120 140 160

Pie

zom

ete

r Ti

p R

L (m

)

Head of Water (m)

P90

P91

P92

P93

P94

P95



Piezometer Readings in Boreholes P90 - P95



Figure 12



940

960

980

1000

1020

1040

1060

1080

1100

1120

0 20 40 60 80 100 120 140 160

Pie

zom

ete

r Ti

p R

L (m

)

Head of Water (m)

P100

P101

P102



Piezometer Readings in Boreholes P100 - P102



Figure 13



P

9

0

P

9

1

P

9

2

P

9

3

P

9

4

P

9

5

-1000E

0E

1000E

2000E

3000E

4000E

5000E

-2000E

2000N

1000N

0N

-1000N

3000N

T

R

I

O

F

A

V

O

N

A

M

O

O

N-

L

I

G

H

T

C

O

R

R

E

N

S

O

DAYBREAK

E

M

P

I

R

E

BM19B

19BB

17CB

17BB

17AB

34GC

2.04B

18EE

34H

18IB

18C

34AD

2.10

34BE

34C

BM34

11AC

10BC

18AB

10AB

2.11

18B

1.28B

2.08

2.09B

BM16

BM17A

34I

2.06

-280

-274

-258

-220

-183

-178

-161

-154

-152

-152

-150

-149

-138

-136

-130

-129

-125

-119

-118

-115

-114

-113

-112

-112

-107

-106

-103

-101

-94

19CB

A10B

20C

A11D

16BC

BM25

25E

10DB

21O

21DC

25D

25A

10CB

2.03

18F

34EB

12CE

2.02

1.28A

18G

20D

25B

34D

13AC

2A

25F

BM12

21N

25G

BM18

13BC

25CB

25H

25I

12DC

12AC

24I

15A

AP22A

24DC

20AC

21C

24L

15BC

20E

22F

24J

24E

12BC

24K

11BB

24AC

24B

BM13

1.10A

24G

24F

24H

21EB

4DB

BM24

BM2

21M

2BC

24CD

4B

20F

20BB

BM21

21BC

21K

30C

AP3

7CB

BM9B

26EE

26F

-255

-143

-139

-137

-131

-128

-125

-124

-123

-122

-122

-121

-120

-118

-118

-117

-116

-116

-114

-113

-113

-111

-111

-110

-110

-110

-109

-108

-108

-108

-107

-104

-103

-103

-102

-102

-101

-100

-99

-98

-97

-96

-95

-95

-94

-94

-94

-93

-92

-92

-92

-92

-91

-91

-91

-90

-90

-90

-89

-89

-88

-86

-84

-83

-83

-82

-80

-80

-79

-76

-74

-70

-69

-69

-68

-47

-39

23AB

2.14A

23C

22C

23B

BANK1

2HB

2.25

BARRY3

23D

22G

2.24

23E

BM23

MATAURA1

STAFORD

2.16

BARRY2

2.13

22E

22BC

2.15

23F

22H

MORTON

22M

22I

GW

CUBA

22L

21P

22A

2.28

22D

22J

1.06

2GB

21FB

2.26

BM22

21L

1.09B

21AC

2.29

30BB

26BE

BM15

21Q

SM822

21I

22K

26CE

21GC

2.31B

21J

15DB

7BB

27E

27KB

2.30

21HC

3.04

26AE

4.07

27N

3.01

4.08

27H

27G

4.05

3.02

4.09

27J

BM30

27F

26G

26H

27I

26PB

3.11A26R

30AB

3.09

27DC

26I

26Q

3.10A

BM26

27M

3.13

27L

27AB

27O

3.6A

2.41

34FC

2.19B

2.18

2.20

2.23

2.17A

BARRY8

2.21

1.05

2.22

15C

2.27

2.34

2.36

2.40B

4.02

2.33

4.03B

BM31

31BC

4.04

4.01C

31AC

29DB

26JB

26MB

31CC

3.25

29AC

29CE

3.24

29B

-124

-104

-103

-103

-102

-102

-100

-100

-99

-99

-98

-97

-97

-96

-94

-92

-92

-90

-90

-89

-86

-86

-84

-84

-83

-82

-81

-81

-81

-81

-78

-78

-78

-77

-76

-75

-74

-73

-70

-69

-68

-67

-66

-66

-64

-63

-63

-63

-60

-60

-58

-56

-55

-55

-54

-54

-54

-53

-50

-50

-49

-47

-45

-43

-42

-41

-41

-41

-40

-39

-38

-37

-37

-36

-36

-32

-32

-31

-31

-31

-31

-30

-30

-30

-29

-27

-27

-26

-23

-22

-22

-19

-19

-13

-8

-118

-113

-109

-101

-99 -94

-92

-87

-87

-86

-86

-84

-79

-74

-70

-56

-52

-47

-46

-45

-44

-41

-38

-36

-33

-33

-31

-25

-25

-23

-23

-17

-17

-12

-9

1.11B

2CE

14DB

1.07

A33C

14CB

2FC

14EA

1.08

14BC

4A

2DA

4EC

2EB

14FB

1K

3.03

7AC

1.12

1.04

1JB

BM14

1I

1SC

BM7

BM4

4FB

1C

3.12

33A

14AC

3.14

33F

33E

6A

1B

1FB

3.07

1O

1HC

1EB

33DB

1NB

1LC

BM6

1GB

1MD

1A

5C

3.15

1PA

1.01

AP2

33GA

1RA

1.14

BM1

33B

1QC

BM293.22A

3.05

33C

5BC

5AC

BM5

1.16

1D

-104

-76

-69

-55

-53

-51

-50

-50

-47

-46

-45

-42

-42

-41

-41

-40

-35

-34

-33

-33

-32

-28

-28

-27

-27

-27

-26

-26

-26

-25

-25

-24

-24

-23

-21

-21

-21

-20

-20

-19

-18

-18

-17

-17

-16

-15

-14

-13

-12

-12

-12

-12

-11

-11

-11

-10

-10

-9

-6

-4

-4

-3

-3

-3

-2

-2

-2

-1

AP100

1.26

2.05

31MD

31ND

31LC

1.25

31KC

31FC

2.35

31JD

31HC

31IC

1.21

31GC

1.22

31DD31PC

31QC

31EC

1VA

1.20A

1.24

1.23

31OD

AP2A

1.13

1.27B

1UA

27CD

3.30

1TB

3.26B

3.29

3.28A

1.17B

1.15

AP1

AP6

-93

-79

-79

-69

-66

-61

-58

-54

-53

-51

-47

-46

-46

-44

-44

-41

-36-36

-33

-30

-23

-22

-20

-20

-18

-18

-18

-12

-9

-9

-8

-5

-3

-3

-3

-2

-1

0

0

BLOCK-N

F16B

F11C

F26

F27B

F10B

F09A

F12C

F15C

F28B

F14C

F13C

F30B

F31B

F08A

F29B

F33

F32B

ITXCIVB

F04

F06

F02

F03

F05

FP1

TRIG24

TRIG22

-147

-145

-144

-140

-135

-128

-128

-126

-124

-122

-120

-119

-114

-113

-112

-111

-110

-106

-97

-96

-95

-94

-94

-92

-71

-41

-33

F22

BLOCK-S

-172

-166

F17B

F21

-184

-183

3.23

3.27B

3.21

3.16

3

4

4

12

57

AP20No2

1

BM20 Dist'd

18HC

New mark

1.02B

2.44 Dist'd

-475

F18 Dist'd

F23 Dist'd

F24 Dist'd

F20 Dist'd

-238

-230

-210

-204

F25 Dist'd

-178

F07 Dist'd

-148

1.02C

New Mark

Lost

1.03

Lost

1.03B

New Mark

26OB

New Mark26NC

New Mark

BM20A

-271

-332

2.09C

New mark

28AD

New mark

28AC

Lost

BARRY1

-102

BA

RR

Y4B

-103

F35B

BA

RR

Y6

BARRY5

-98

BARRY7

F34C

-130

ENGINEERING GEOLOGY LTD

OCEANA GOLD NZ LIMITED - WAIHI

Project Martha

Proposed Settlement Zones

Settlement in millimetres

Zone 7

Settlement

Zone 6

Settlement

Zone 5

Settlement

Zone 4

Settlement

Zone 3

Settlement

Zone 2

Settlement

Zone 1

Settlement

Zone

Figure 16

(2019)

Martha

55

65

95

160

260

340

540

AutoCAD SHX Text

Unit 7C, 331 Rosedale Rd, PO Box 301054, Albany

AutoCAD SHX Text

Ph (09)486-2546, Fax (09)486-2556

AutoCAD SHX Text

Drawing No.

AutoCAD SHX Text

Date:

AutoCAD SHX Text

Drawn:

AutoCAD SHX Text

Scale:

AutoCAD SHX Text

Filename:

AutoCAD SHX Text

1:20000 (@A3)

AutoCAD SHX Text

JL

AutoCAD SHX Text

2 Feb 2018

AutoCAD SHX Text

8332-Fig16

AutoCAD SHX Text

8332-Fig16.dwg

500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

May-1

988

May-1

989

May-1

990

May-1

991

May-1

992

May-1

993

May-1

994

May-1

995

May-1

996

May-1

997

May-1

998

May-1

999

May-2

000

May-2

001

May-2

002

May-2

003

May-2

004

May-2

005

May-2

006

May-2

007

May-2

008

May-2

009

May-2

010

May-2

011

May-2

012

May-2

013

May-2

014

May-2

015

May-2

016

May-2

017

Min

e D

ew

ate

rin

g R

L (

m)

Sett

lem

en

t (m

)

1TB AP1 AP2A 1UA 1VA 31ND

31OD 31PC 31QC 2.05 31FC 31JD

31DD 31EC 31GC 31HC 31IC 31KC

31LC 31MD 1.17B 1.20A 1.24 1.27B

1.25 1.22 1.23 1.13 2.35 28AD

3.16 3.23 3.26B 3.27B 3.28A 3.29

3.30 BM28/2 3.21 1.15 27CD ITXCIVB

Avg. Settlement Mine Dewatering Level



Ground Settlement - Zone 1

Ref. No: 8332Date: Feb 2018Drawn: CW


Figure 17



500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

May-1

988

May-1

989

May-1

990

May-1

991

May-1

992

May-1

993

May-1

994

May-1

995

May-1

996

May-1

997

May-1

998

May-1

999

May-2

000

May-2

001

May-2

002

May-2

003

May-2

004

May-2

005

May-2

006

May-2

007

May-2

008

May-2

009

May-2

010

May-2

011

May-2

012

May-2

013

May-2

014

May-2

015

May-2

016

May-2

017

Min

e D

ew

ate

rin

g R

L (

m)

Sett

lem

en

t (m

)

14BC 14CB 14DB 1A 1B 1C

1D 1GB 1HC 1I 1JB 1K

1NB 1O 1XB 33A 33B 33C

33DB 33E 33F 33GA 4EC 4FB

5AC 5BC 5C 5DA 6A AP2

BM1 BM14 BM29 BM4 BM5 BM6

BM7 7AC 1MC 1.01 1EB 1QC

3.07 1RA 1SC 3.22A 14EA 1LC

1.02C 1.03 1.04 1.12 14FB 1FB

3.03 3.05 3.07 3.12 3.14 3.15

1.14 1.16 Avg. Settlement Mine Dewatering Level





Engineering Geology LtdUnit 7C, 331 Rosedale Rd, PO Box 301054,

Figure 18



500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

May-1

988

May-1

989

May-1

990

May-1

991

May-1

992

May-1

993

May-1

994

May-1

995

May-1

996

May-1

997

May-1

998

May-1

999

May-2

000

May-2

001

May-2

002

May-2

003

May-2

004

May-2

005

May-2

006

May-2

007

May-2

008

May-2

009

May-2

010

May-2

011

May-2

012

May-2

013

May-2

014

May-2

015

May-2

016

May-2

017

Min

e D

ew

ate

rin

g R

L (

m)

Sett

lem

en

t (m

)

29AC 29CE BM31 2FC 2.33 29B

29DB 4.03B 2.34 2.40B 26OB 3.24

4.04 31CC 31BC 31AC 4.01C 2.36

1.08 1.21 1.22 1.25 1.07 2CE

2DA 2EB 26NC 3.25 4.02 4A

14BC 14CB 14DB 14EA 14FB 15DB

A33C Avg. Settlement Mine Dewatering Level






Figure 19



500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

May-1

988

May-1

989

May-1

990

May-1

991

May-1

992

May-1

993

May-1

994

May-1

995

May-1

996

May-1

997

May-1

998

May-1

999

May-2

000

May-2

001

May-2

002

May-2

003

May-2

004

May-2

005

May-2

006

May-2

007

May-2

008

May-2

009

May-2

010

May-2

011

May-2

012

May-2

013

May-2

014

May-2

015

May-2

016

May-2

017

Min

e D

ew

ate

rin

g R

L (

m)

Sett

lem

en

t (m

)

7BB 1.06 1.09B 2.13 2.14A 2.15 2.16

2.21 2.24 2.25 2.26 2.28 2.29 2.30

2.31B 21AC 21GC 21HC 22A 22BC 22C

22D 22E 23AB 23B 26AE 26BE 26CE

26G 26H 26I 27AB 27DC 27E 27F

27G 27H 27I 27J 27KB 27L 27M

27N 27O 2GB 2HB 3.01 3.02 3.04

3.09 3.10A 3.11A 3.13 3.6A 30AB 30BB

4.05 4.07 4.08 4.09 BM15 BM22 BM23

BM26 BM30 21FB 21J 21L 21I 21Q

21P 22G 22H 22I 22J 22K 22L

22M 23D 23C 23E 23F GW MATAURA1

Morton SM822 Stafford Bank1 Barry1 Barry2 Barry3

Cuba 1.05 1.11B 1.26 2.18 2.19B 2.20

2.22 2.23 2.27 2.41 15C 26F BM2

AP100 Barry4B Barry5 Barry6 Barry7 Barry8 Avg. Settlement







Figure 20



500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

May-1

988

May-1

989

May-1

990

May-1

991

May-1

992

May-1

993

May-1

994

May-1

995

May-1

996

May-1

997

May-1

998

May-1

999

May-2

000

May-2

001

May-2

002

May-2

003

May-2

004

May-2

005

May-2

006

May-2

007

May-2

008

May-2

009

May-2

010

May-2

011

May-2

012

May-2

013

May-2

014

May-2

015

May-2

016

May-2

017

Min

e D

ew

ate

rin

g R

L (

m)

Sett

lem

en

t (m

)

4B 4DB 7CB 10CB 10DB 11BB

12AC 12BC 12CE 12DC 13AC 13BC

15A 18F 18G 18HC 2.03 20AC

20BB 20C 21BC 21C 21DC 24B

24CD 24DC 25A 25B 25CB 2A

2BC 34D 34EB A10B A11D AP22A

AP3 BM9B BM12 BM13 BM18 BM21

BM25 24AC 1.10A 1.28A 16BC 30C

20D 20E 20F 21K 21O 21N

21M 24E 24F 24G 24H 24I

24J 24K 24L 25D 25E 25F

25G 25H 25I 22F 26EE 26PB

15BC 2.02 21EB 2.17A BM16 BM24

18B 26Q 26R Avg. Settlement Mine Dewatering Level






Figure 21



500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

Ma

y-1

98

8

Ma

y-1

98

9

Ma

y-1

99

0

Ma

y-1

99

1

Ma

y-1

99

2

Ma

y-1

99

3

Ma

y-1

99

4

Ma

y-1

99

5

Ma

y-1

99

6

Ma

y-1

99

7

Ma

y-1

99

8

Ma

y-1

99

9

Ma

y-2

00

0

Ma

y-2

00

1

Ma

y-2

00

2

Ma

y-2

00

3

Ma

y-2

00

4

Ma

y-2

00

5

Ma

y-2

00

6

Ma

y-2

00

7

Ma

y-2

00

8

Ma

y-2

00

9

Ma

y-2

01

0

Ma

y-2

01

1

Ma

y-2

01

2

Ma

y-2

01

3

Ma

y-2

01

4

Ma

y-2

01

5

Ma

y-2

01

6

Ma

y-2

01

7

Min

e D

ew

ate

rin

g R

L (

m)

Sett

lem

en

t (m

)

10AB 17AB 17BB 18AB 18C 18EE

2.04B 2.06 2.08 34BE 34C 34H

34I BM17A BM34 34AD 18IB 1.28B

10BC 11AC 2.09C 2.10 2.11 34GC

19CB 34FC BM20 BM20A Avg. Settlement Mine Dewatering Level






Figure 22



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ay-1

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ate

rin

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L (

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Sett

lem

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t (m

)

Settlement Zone 7

17CB 19BB BM19B Avg. Settlement Mine Dewatering Level






Figure 23



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4A




Ground Settlement of Marker 4A and Mine Dewatering Level



Figure 24



File: 8332 Appendix A Final.docx.doc

APPENDIX A

TWO DIMENSIONAL DECOUPLED SEEPAGE AND DEFORMATION

NUMERICAL MODELLING

Our Ref:8332 25 May 2018 Page 1

File: 8332 Appendix A Final.docx

APPENDIX A

TWO DIMENSIONAL DECOUPLED SEEPAGE AND DEFORMATION

NUMERICAL MODELLING

1.0 INTRODUCTION

This memorandum summarises the assumptions and results of two-dimensional (2D)

seepage and deformation numerical modelling of the area to the east of the Martha Pit,

referred to as Waihi East. The modelling was undertaken to enable estimates of settlements

due to dewatering to RL770m associated with mining since 1989 and with Project Martha

which will involve dewatering down to RL500m from the currently consented level of

RL700m.

The analyses have been undertaken using Geostudio 2D finite element analysis (FEA)

programs, SEEP/W and SIGMA/W (Refs. 1 and 2). The SEEP/W model of the ground and

groundwater parameters was provided by GWS Ltd (Ref.3). Some small adjustments were

made to the model for deformation modelling.

2.0 SCOPE OF WORK

The scope of work has included:

i. Modelling of seepage and drawdown associated with dewatering of both single and

multiple veins to various levels using SEEP/W.

ii. Deformation analyses using SIGMA/W with changes in stress due to dewatering to

different levels predicted by the SEEP/W modelling.

iii. Comparison of the results of the numerical modelling with monitored settlements

3.0 SOIL PROFILE AND GROUND MODEL

Figure A1 provides the model geometry and the associated boundary conditions. The

location of this section is also shown in Figure A1. The model was constructed as four

layers. From the surface these layers were: younger volcanic rocks, weathered andesite,

and undisturbed andesite comprising higher permeability (K) andesite and lower K

andesite. The higher K and lower K andesite are both referred to as andesite rock mass in

this document. The seepage modelling parameters within the GWS model have not been

altered. The depth of andesite extended to RL400 in the GWS model. We have extended

down to RL255. This is conservative for estimating deformations.

The parameters summarised in Table A1 have been adopted for SIGMA/W deformation

analyses. These parameters are generally consistent with the parameters adopted in PSM's

report (Ref.1). The only difference is we have adopted a Young's modulus of 2,000MPa for

the weathered andesite. This number is only important for modelling settlements from

January 1989 to December 1999. The settlements since December 1999 are believed to be

mostly associated with the undisturbed andesite rock mass.



TABLE A1 Material Deformation Parameters

Unit Unit Weight

(kN/m3)

Young's Modulus

(MPa)

Poisson's

Ratio

Younger Volcanics 25 8000 0.2

Weathered Andesite 20 2000 0.2

Andesite -Undisturbed (both

Higher K and Lower K)

27 8700 0.2

4.0 MODELLING AND RESULTS

Decoupled seepage and deformation analyses using SEEP/W and SIGMA/W have been

undertaken for three stages of groundwater drawdown. The SEEP/W model initially

calculates the change in pressure at each stage of groundwater drawdown. The SIGMA/W

model then calculates the ground deformation that is caused by the change in pressure.

4.1. Seepage Modelling

The SEEP/W model was used to undertake steady state seepage analyses for three stages of

groundwater dewatering. The stages of the modelling are shown in Figure A2 and detailed

described below.

Stage 1. Undertake groundwater dewatering to the Weathered Andesite (at approximately

RL975 at it deepest). This approximately represents dewatering from January

1989 to December 1999.

Stage 2. Undertake groundwater dewatering from RL975 to RL770. This approximately

represents dewatering from December 1999 to November 2017.

Stage 3. Undertake groundwater dewatering from RL770 to RL500. This approximately

represents dewatering from November 2017 to 2031 (estimate ending time for

Project Martha).

The average rate of groundwater drawdown for Stage 3 is greater than previous. However,

we assume that dewatering of the andesite rock mass will achieve steady state conditions

with the proposed drawdown rates that are shown in Figure A2.

Drawdown analyses for both single vein and multiple veins have been carried out. The

results are presents in Figure A3 to A8. Figures A3 to A5 show the results of modelling for

a single vein for Stages 1, 2 and 3 respectively. Figures A6 to A8 show the results of

modelling for multiple veins.

4.2. Settlement Modelling

The SIGMA/W modelling has been undertaken to calculate the deformation induced by the

pressure change associated with each stage of groundwater dewatering. The results are

shown in Figures A9 to A14. Figures A9 to A11 show the results of modelling for a single

vein for Stages 1, 2 and 3 respectively. Figures A12 to A14 show the results of modelling

for multiple veins.

Seven settlement zones have been identified in the Waihi area in which measured

settlements are relatively consistent. The modelling covers a section through Martha East



which is covered mostly by Zones 4 and 5. Comparisons of the predicted settlements

against measured settlements in Zones 4 and 5 are shown in Figures A15 and A16

respectively. Figures A17 and A18 show the ground surface settlements profiles for Stages

2 and Stage 3 of the model respectively. Results are shown for both single and multiple

veins. The estimated average settlements are summarised in Table 2. These are the average

of the settlements across the whole profile being modelled.

TABLE A2 Estimated Settlements

Stage

Single Vein Multiple Veins

Average

Settlement

(mm)

Cumulative

Settlement

(mm)

Average

Settlement

(mm)

Cumulative

Settlement

(mm)

1 23 23 23 23

2 89 112 152 175

3 26 138 66 241

The average modelled settlements for Stage 1 are 23mm. The average modelled

settlements for Stage 2 are 89mm for a single vein and 152mm for multiple veins. For

Stage 3 the average modelled settlements are 26mm for a single vein and 66mm for

multiple veins. The cumulative settlements are also provided in Table 2.

The modelling allows estimation of maximum ground surface tilts. They are summarised

in Table 3. They have been obtained by selecting the maximum tilt from the surface

profiles shown in Figures A17 and A18.

TABLE A3 Maximum Tilts

Stage Tilts

Single Vein Multiple Veins

1 1 in 8,000 1 in 8,000

2 1 in 10,000 1 in 9,000

3 1 in 30,000 1 in 18,000

1 and 2 1 in 5,500 1 in 4,600

1, 2 and 3 1 in 4,600 1 in 3,800

The estimated maximum tilts are associated with the multiple vein model and for Stages 1,

2 and 3 are 1 in 8,000, 1 in 4,600 and 1 in 3,800 respectively. Figure A17 show the

incremental change in tilt from Stage 2 to Stage 3 is small. The measured tilts above the

Correnso underground mine (which is largely within Zones 4 and 5) are generally smaller

than the maximum tilt predicted by numerical modelling of Stages 1 and 2 (1 in 4,600).

This indicates the numerical model is conservative.

5.0 DISCUSSION

The estimated settlements at the ground surface due to groundwater dewatering from

RL975 to RL770 (Stage 2) are summarised in Table 2. They are 89mm for a single vein

and 152mm for multiple veins. The monitored settlements in Zones 4 and 5 are lower, 38

to 68mm on average. The possible explanations for this difference are:



1. The adopted rock mass stiffness (Young's modulus) for the undisturbed andesite may

be too low especially for andesite at greater depth.

2. The andesite rock mass has not fully depressurised to steady state condition.

Irrespective of the above reasons the results indicate that application of the numerical

model to estimate deformation induced by dewatering from RL770 to RL500 (Stage 3) is

likely to be conservative.

The results shown in Figures A17 and A18 and Table 2 indicate that the estimated ground

surface surficial settlements in Stage 3 are significantly less than Stage 2. This is because

of two reasons,

1. Soil arching effect reflects less surficial settlement due to deeper and more localised

groundwater dewatering

2. The effective depth of andesite rock mass that are affected by drawdown reduces with

increase in drawdown depth. The effective thickness affected by groundwater

drawdown for Stages 2 and 3 are 720m (RL975 to RL255) and 515m (RL770 to

RL255) respectively.

The tilts arising from settlement due to dewatering summarised in Table 3 show that the

incremental tilt due to dewatering to RL500m are small. The maximum tilt for multiple

veins is 1 in 3,800 which is well below the level that would give rise to any concerns with

damage to structures or infrastructure.

6.0 SUMMARY

The numerical modelling provides useful insight into the comparative settlements

associated with different stages of dewatering and provides estimates of ground surface tilt.

The results show that settlements due to groundwater dewatering from RL770m to

RL500m (Stage 3) are lower compared to dewatering from RL975m to RL770m (Stage 2).

This is because groundwater drawdown and associated incremental stress in the rock mass

is localised around the veins. It results in arching effects and lower settlements than if the

rock mass was fully dewatered. Also, with dewatering from RL770m to RL500m the depth

of rock mass that is affected by drawdown is less than Stage 2.

The numerical modelling allows estimation of ground surface tilts. The estimated

maximum tilt following dewatering to RL500m is 1 in 3,800. This is well below the level

that would give rise to any concerns with damage to structures or infrastructure. It is also

considerably less than 1 in 1,000 which is the current trigger for notification of the

Waikato Regional Council and for instigating additional monitoring.

The modelled section is for Waihi East which is largely covered by settlement Zones 4 and

5. However, the comparative results between Stage 2 and 3 would apply to the other

settlement zones.

SEEP/W Model

SIGMA/W Model

SEEP/W and SIGMA/W Model Figure A1

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Gro

un

dw

ater

Lev

el R

L (m

)Monitored Dewatering Level

Inferred Dewatering Level of Project Martha

Stage 3




Ref. No: 8332Date: May 2018Drawn: CW


Figure A2



Stage 1 Stage 2

SEEP/W Model

SEEP/W Steady State Pressure Head Distribution, Unit: metre

Single Vein Drawdown Stage 1 SEEP/W Model and Results Figure A3

SEEP/W Model



SEEP/W Model

SEEP/W Steady State Pressure Head Distribution, Unit: Metre

Multi Veins Drawdown Stage 1 SEEP/W Model and Results Figure A6

SEEP/W Model



SIGMA/W Single Stage Vertical Settlements, Unit: metre

SEEP/W Steady State Pressure Head Distribution, Unit: metre (for reference)

Single Vein Drawdown Stage 1 SIGMA/W Settlements Figure A9



Multi Veins Drawdown Stage 1 SIGMA/W Settlements Figure A12

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7BB 1.06 1.09B 2.13 2.14A2.15 2.16 2.21 2.24 2.252.26 2.28 2.29 2.30 2.31B21AC 21GC 21HC 22A 22BC22C 22D 22E 23AB 23B26AE 26BE 26CE 26G 26H26I 27AB 27DC 27E 27F27G 27H 27I 27J 27KB27L 27M 27N 27O 2GB2HB 3.01 3.02 3.04 3.093.10A 3.11A 3.13 3.6A 30AB30BB 4.05 4.07 4.08 4.09BM15 BM22 BM23 BM26 BM3021FB 21J 21L 21I 21Q 21P 22G 22H 22I 22J 22K 22L 22M 23D 23C 23E 23F GW MATAURA1 MortonSM822 Stafford Bank1 Barry1 Barry2Barry3 Cuba 1.05 1.11B 1.262.18 2.19B 2.20 2.22 2.232.27 2.41 15C 26F BM2AP100 Barry4B Barry5 Barry6 Barry7Barry8 Avg. Settlement SIGMA/W Single Vein Avg SIGMA/W Multiple Veins Avg Mine Dewatering LevelInferred Dew Lvl for Martha



Comparison of SIGMA/W Settlement with Observed Settlement - Zone 4



Figure A15



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e D

ew

ate

rin

g R

L (

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Sett

lem

en

t (m

)

4B 4DB 7CB 10CB 10DB

11BB 12AC 12BC 12CE 12DC

13AC 13BC 15A 18F 18G

18HC 2.03 20AC 20BB 20C

21BC 21C 21DC 24B 24CD

24DC 25A 25B 25CB 2A

2BC 34D 34EB A10B A11D

AP22A AP3 BM9B BM12 BM13

BM18 BM21 BM25 24AC 1.10A

1.28A 16BC 30C 20D 20E

20F 21K 21O 21N 21M

24E 24F 24G 24H 24I

24J 24K 24L 25D 25E

25F 25G 25H 25I 22F

26EE 26PB 15BC 2.02 21EB

2.17A BM16 BM24 18B 26Q

26R Avg. Settlement SIGMA/W Single Vein Avg SIGMA/W Multiple Veins Avg Mine Dewatering Level

Inferred Dew Lvl for Martha



Comparison of SIGMA/W Settlement with Observed Settlement - Zone 5



Figure A16



SIGMA/W Vertical Surficial Settlements, Unit: metre

SIGMA/W Model

SIGMA/W Surficial Settlements (Cumulative) Figure A17

SIGMA/W Vertical Surficial Settlements, Unit: metre

SIGMA/W Model

SIGMA/W Surficial Settlements Stage 3 Figure A18

project martha apxr ground settlement assessment

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