wellington water ltd · 2018. 2. 16. · the following previous condition assessment reports have...

Wellington Water Ltd

Western WWTP Outfall Pipeline

South Karori Sewer Outfall: Remaining Life Interpretive Report

January 2018

Ch1290 Cross-section, Opus International Consultants Ltd

GHD | Report for Wellington Water Ltd - Western WWTP Outfall Pipeline, 5137412 | i

Executive Summary

Wellington Water Ltd engaged GHD Limited to provide materials consultancy services in

relation to the condition and likely remaining service life of the South Karori Sewer Outfall pipe

for the purposes of scheduling and budgeting replacement works. In this assessment, only the

reinforced concrete pipes have been considered, local welded steel pipe bridges at stream

crossings are outside the scope of the work reported herein.

Currently Wellington Water have a legislated requirement to replace the pipeline by 2023, but

want to examine options for retaining the asset in service for as long as is feasible.

Sample spools and patch samples were exhumed and recovered from eight locations on the

pipeline by a Contractor appointed by Wellington Water Ltd. Locations were reviewed by GHD

prior to sampling. Initially four spool samples and four “patch” samples cut from the pipe crown

were received, two of the “patch” samples were subsequently replaced by spool samples. The

two redundant patch samples were not assessed. The samples were individually identified by

location, with an arrow indicating flow direction marked at the 12 o’clock (crown/top of pipe)

position.

Based on examination of these samples, the following conclusions have been reached:

1. The primary cause of continuing deterioration of the inner surface is soft water leaching of

the cement paste below the operating water level. This is expected to continue

advancing at a nominal rate of 0.3 mm/yr for the internal surface below the flow level,

assuming no change in the corrosiveness of the sewage. Deterioration of the external

surface is expected to continue at approximately half of that rate.

2. The absence of evidence of biogenic acid attack on the interior surfaces of samples from

2008 and 2017 means that the change to carriage of treated sewage from 1997 is unlikely

to have reduced deterioration rates. Therefore, the previously reported assumption that

carriage of treated sewage from 1997 has made, and will continue to increasingly make,

historical average deterioration rates conservative is erroneous.

3. The primary causes of deterioration of the external surface are soft water leaching and acid

attack. The occurrence and rate of these mechanisms are location-specific, and unlikely to

apply where the pipe is concrete encased unless the encasement is severely damaged.

4. The average effective residual cross-section of intact alkaline cement paste was 34% of the

original cross-section, and a characteristic lower bound of 29%. The structural capacity of

the concrete is therefore expected to be significantly impaired. However, a structural

assessment is required to determine the probability of structural failure for any particular

section of the pipeline.

5. Biogenic acid attack of the internal surface above the operating water level is not expected

to be a future cause of deterioration.

6. Carbonation-induced corrosion of reinforcement is not expected to be a significant future

deterioration risk.

7. Chloride-induced corrosion of reinforcement is not expected to be a significant future

deterioration risk for the majority of the pipeline. A specific, as yet unquantified, risk exists

at the beach crossing and outfall where the pipe is directly exposed to marine conditions,

particularly Ch0 to approximately Ch150.

GHD | Report for Wellington Water Ltd - Western WWTP Outfall Pipeline, 5137412 | ii

8. Propagation of corrosion of the invert reinforcement will become more likely as the concrete

cover is reduced through erosion, although such corrosion s unlikely to be expansive and

may not be readily detectable.

9. ASR is present in the pipes, but at this time is not deleterious, and given the age of the

pipes is not expected to continue and present a significant future deterioration risk.

10. Non-deleterious secondary ettringite recrystallisation is present, but there is no evidence of

delayed ettringite formation (DEF). Given the age of the pipes and the presence of ASR,

DEF is not considered to be a future deterioration risk.

11. Contrary to Opus’s report, there is a risk of local structural overload from vehicles at an

identified beach crossing, and potentially at other locations, which will increase

progressively as the pipes continue to deteriorate over time.

12. Structural overload at other locations is possible, e.g. due to seismic events, unplanned

internal surcharge, or erosion of bedding.

13. There is evidence that at least two sections of the pipeline have been replaced:

– Around Ch4500: Extent and date of, and reason for, replacement not known.

– Around Ch6135: Extent of, and reason for, replacement not known. Possibly replaced

in conjunction with construction of the WWWTP.

14. As the pipes will become progressively more fragile with age, failure frequency and severity

will increase, and repairs will become increasingly difficult to perform. Based on the

measured loss of cement alkalinity data, it is considered that some sections of the pipeline

are likely to require major remedial or replacement works within 10 years to mitigate the risk

of failure.

15. Overall, the majority of the pipeline is considered likely to remain sufficiently serviceable as

a conduit for treated sewage until 2035, with periodic inspection and on-condition

maintenance as required, if local load cases do not exceed the reduced structural capacity.

As a result of this assessment we recommend that Wellington Water undertake the following

actions:

1. Continue to implement its ”Management and Monitoring Plan: Western Wastewater

Treatment Plant Effluent Pipeline”.

2. Review maintenance records to identify any trends in pipe failure type, frequency, severity

and location. Review this trend analysis on an annual basis, more frequently if the data

warrants.

3. Within two years, conduct a preliminary review of the technical, ecological and logistical

feasibility of relining and replacement options, or treating the effluent alkalinity to reduce

the rate of internal degradation, including estimates of the order of probable costs for

comparison with predicted repair costs.

4. As a precautionary measure, implement control measures at the southern beach crossing

and adjacent area to prevent vehicles driving over the pipeline without adequate protection.

5. Consider completing the following testing and assessment within the next 5 years to

increase certainty around the predicated remaining service life figures provided in this

report, and for input to structural capacity assessments:

– Residual load bearing capacity testing of exhumed pipe sections.

– Structural assessment to determine pipe minimum allowable sound cross section for a

range of structural load scenarios to inform assessment of structural failure risk.

GHD | Report for Wellington Water Ltd - Western WWTP Outfall Pipeline, 5137412 | iii

– Conduct additional detailed examination of the external surfaces of the current

samples to clarify the mechanisms of deterioration and their effect on residual

structural capacity.

– Conducting corrosion rate measurements on depassivated steel reinforcement of

exhumed and in-situ pipe sections.

– Obtaining samples from the Cook Strait end of the pipeline to assess chloride-induced

reinforcement corrosion risk.

This report is subject to, and must be read in conjunction with, the limitations set out in Section

1.4 and the assumptions and qualifications contained throughout the Report.

GHD | Report for Wellington Water Ltd - Western WWTP Outfall Pipeline, 5137412 | iv

Table of Contents

1. Introduction..................................................................................................................................... 1

1.1 Background .......................................................................................................................... 1

1.2 Previous Condition Assessments ........................................................................................ 1

1.3 Purpose of This Report ........................................................................................................ 1

1.4 Limitations ............................................................................................................................ 2

2. Results ........................................................................................................................................... 4

2.1 Sample Recovery ................................................................................................................. 4

2.2 Opus Findings ...................................................................................................................... 4

2.3 Soil and Water Chemistry .................................................................................................... 5

2.4 Petrographic and SEM Examination .................................................................................... 6

2.5 Previous Assessment Reports ............................................................................................. 7

2.6 Vehicle Loading ................................................................................................................... 8

3. Discussion .................................................................................................................................... 10

3.1 Concrete Deterioration Mechanisms ................................................................................. 10

3.2 Reinforcement Deterioration Mechanisms......................................................................... 12

3.3 Effect of Deterioration on Structural Integrity ..................................................................... 14

3.4 Remaining Life Assessment .............................................................................................. 14

4. Conclusions and Recommendations ........................................................................................... 18

4.1 Conclusions ....................................................................................................................... 18

4.2 Recommendations ............................................................................................................. 19

Table Index

Table 1: 2017 Sample details reproduced Table 1 from Opus report. .................................................... 9

Table 2: Comparison of derived pipe deterioration rate constants. ...................................................... 15

Table 3: Predicted times to nil residual cross-section of sound alkaline matrix (linear –to-

time model). ....................................................................................................................... 16

Table 4: Predicted times to nil residual cross-section of sound alkaline matrix (pooled data,

optimised exponential model). ........................................................................................... 16

Appendices

Appendix A – Opus Report

Appendix B – Soil & Water Analysis Reports

Appendix C – Petrographic & SEM Reports

GHD | Report for Wellington Water Ltd - Western WWTP Outfall Pipeline, 5137412 | 1

1. Introduction

1.1 Background

Wellington Water Ltd engaged GHD Limited (GHD) to provide materials consultancy services in

relation to the condition and likely remaining service life of the South Karori Sewer Outfall pipe

for the purposes of scheduling and budgeting replacement works.

The South Karori Sewer Outfall pipe was originally constructed in the mid-1930’s, predominantly

using centrifugally cast reinforced concrete pipe, carrying wastewater from the locality of Karori

generally along the valley of the Karori Stream to the Cook Strait, terminating to the east of

Tongue Point. The pipe sections are rubber ring jointed, according to the New Zealand

Portland Cement Association this type of pipe was introduced to New Zealand around 1935.

One pipe sample recovered in 2008 carried a marking “1935” that might relate to the date of

manufacture. For the purposes of this assessment, the age of the pipes has been taken as

nominally 80 years.

Until 1997, the pipeline carried untreated primarily domestic sewage and stormwater from the

residential/semi-rural locality. Subsequent to commissioning of the Western Wastewater

Treatment Plant (WWWTP) in 1997, the pipeline has carried treated domestic sewage with

intermittent dilution by stormwater during high rainfall events. It is understood that the pipeline

has always operated as a gravity sewer main, typically running half full, that some untreated

sewage might also enter the pipeline during high rainfall events, and that there are no significant

sources of industrial or trade waste in the catchment.

A 2008 condition assessment of six spool samples found that five had reached their design

service life, with a future practical service life of 10-30 years. An update condition assessment

was recommended after 10 years, the current work represents that assessment.

It is understood that currently Wellington Water has a legislated requirement to replace and

upgrade the pipeline by 2023. Wellington Water seeks to confirm the remaining service life of

the pipeline, and to examine options for maximising the service life of this asset, preferably at

least beyond expiry of the current Resource Consent in 2035.

The current study comprises field investigations of the South Karori Sewer Outfall pipe by Opus

International Consultants Ltd (Opus) and interpretation of this and additional data by GHD.

1.2 Previous Condition Assessments

The following previous condition assessment reports have been provided to GHD, these reports

refer to six pipe spool samples recovered in December 2007:

Duffill Watts Consulting Group “Wellington City Council: South Karori Sewer Outfall

Pipeline Condition Assessment Report” issued June 2008 (Doc. Ref. R0031LGE).

Opus International Consultants Ltd report “South Karori Sewer: Condition of Pipes

Sampled in February 2008” issued 17 April 2008 (Doc. Ref.08-524A21.00).

1.3 Purpose of This Report

The purpose of this report is to provide interpretation of information contained in:

Opus International Consultants Ltd report “South Karori Sewer Outfall: Condition of Pipes

Sampled in September 2017” issued 22 December 2017 (Doc. Ref.3WW017.28/1OR).

Soil and water chemistry reports requested by GHD.


Petrographic and scanning electron microscopy (SEM) examination reports commissioned

by GHD.

The above information has been interpreted to provide:

The likely deterioration mechanisms affecting the pipe.

An estimate of the expected remaining service life of the outfall sewer.

1.4 Limitations

1.4.1 General

This report has been prepared by GHD for Wellington Water Ltd and may only be used and

relied on by Wellington Water Ltd for the purpose agreed between GHD and Wellington Water

Ltd as set out in Section 1.3 of this report.

GHD otherwise disclaims responsibility to any person other than Wellington Water Ltd arising in

connection with this report. GHD also excludes implied warranties and conditions, to the extent

legally permissible.

The services undertaken by GHD in connection with preparing this report were limited to those

specifically detailed in the report and are subject to the scope limitations set out in the report.

The opinions, conclusions and any recommendations in this report are based on conditions

encountered and information reviewed at the date of preparation of the report. GHD has no

responsibility or obligation to update this report to account for events or changes occurring

subsequent to the date that the report was prepared.

The opinions, conclusions and any recommendations in this report are based on assumptions

made by GHD described in Section 1.4.2 of this report. GHD disclaims liability arising from any

of the assumptions being incorrect.

GHD has prepared this report on the basis of information provided by Wellington Water Ltd and

others who provided information to GHD (including Government authorities), which GHD has

not independently verified or checked beyond the agreed scope of work. GHD does not accept

liability in connection with such unverified information, including errors and omissions in the

report which were caused by errors or omissions in that information.

The opinions, conclusions and any recommendations in this report are based on information

obtained from, and testing undertaken at or in connection with, specific sample points. Site

conditions at other parts of the site may be different from the site conditions found at the specific

sample points.

Investigations undertaken in respect of this report are constrained by the particular site

conditions, such as the location of buildings, services and vegetation. As a result, not all

relevant site features and conditions may have been identified in this report.

Site conditions (including the presence of hazardous substances and/or site contamination) may

change after the date of this Report. GHD does not accept responsibility arising from, or in

connection with, any change to the site conditions. GHD is also not responsible for updating this

report if the site conditions change.

1.4.2 Assumptions

In preparation of this report, GHD has assumed that:

The recovered sample spools are representative of the condition of the pipeline.


That the locations and orientation of the recovered sample spools have been accurately

marked and recorded by the Contractor performing the sample recovery.

The Opus report listed at Section 1.3 is an accurate record of the condition of the sample

spools received.

The recovered soil and water samples are reasonably representative of the burial

conditions along the alignment.

That the mechanisms of deterioration deduced from the petrographic and SEM

examinations are representative of the pipeline.

Although the pipes might pre-date the earliest known precast reinforced concrete pipe

standards in the region (AS A.35-1937, adopted in New Zealand as NZSS 594), they are

expected likely to be generally consistent with those standards.

As a gravity sewer main, the pipeline is not subject to elevated internal pressurisation

during operation or potential adverse events.

GHD has no knowledge of the original design load cases, nor of potential loading

conditions such as seismic events, uplift/floating, wave/tidal effects, etc. on the pipeline.

1.4.3 Issues Outside the Scope of This Assessment

The following issues are outside the scope of this assessment:

Consideration of the condition of the rubber sealing rings between concrete pipe sections.

Consideration of the condition of local sections of welded steel pipe bridges at above-

ground stream crossings.

Structural assessment.


2. Results

2.1 Sample Recovery

2.1.1 Sample Description

The sample spools were exhumed and recovered from the pipeline by a Contractor appointed

by Wellington Water Ltd. Locations marked on a schematic plan were reviewed by GHD prior to

sampling.

Initially four spool samples and four “patch” samples cut from the pipe crown were received, two

of the “patch” samples were subsequently replaced by spool samples. The two redundant patch

samples were not assessed. The samples were individually identified by location, with an arrow

indicating flow direction marked at the 12 o’clock (crown/top of pipe) position.

Section 2 of the Opus report in Appendix A provides an extensive description of the samples as-

received for examination, Table 1 (with attendant Notes) from the Opus report is reproduced

below as Table 1.

2.1.2 Location Naming Convention

The samples were identified by the chainage along the pipeline at which they were recovered,

Ch0 being the downstream/outfall end of the pipeline at Cook Strait and Ch6135 being the

closest sample to the WWWTP at Karori. This convention is consistent with the 2008

assessment reports.

The actual chainage of the upstream end at the WWWTP has not been provided at the time of

writing. Based on drawings of the alignment received, the last marked chainage at Ch6268 is

believed to represent the plant boundary.

2.2 Opus Findings

A copy of the 2017 Opus International Consultants report is contained in Appendix A, the

following provides a summary of the conclusions:

The condition of the pipe sections is defined in terms of three Serviceability Limit States

(SLS’s) (paraphrased, refer to Section 4.2.4.1 of the Opus report in Appendix A for

complete definitions):

– Serviceability Limit State 1 (SLS1): Reinforcement no longer embedded in sound,

alkaline concrete, at which time corrosion of reinforcement can commence and risk of

physical failure increases. This SLS was used to estimate remaining life in the 2008

assessment reports, and likely defines the minimum remaining service life.

– Serviceability State 1.5 (SLS1.5): Reinforcement is in contact with concrete that has

lost its alkalinity, is visibly discoloured and slightly softened.

– Serviceability Limit State 2 (SLS2): Reinforcement is no longer embedded in

physically sound concrete and the bond between the cement matrix and steel

reinforcement has been compromised. Risk of physical failure significantly increases,

and continued pipe integrity can become dependent on a number of local parameters

that cannot be estimated.

Six of the eight pipe samples are 15 inch nominal diameter, with a wall thickness of 17/8 to 2

inches (48 to 51 mm), considered to represent the original pipeline dating from the 1930’s.

The sample from Ch4500 is of smaller diameter and thinner walled, visible features of the

concrete suggest more recent manufacture. This is considered to represent a section


installed more recently. The date of, and reason for, installation is not known at the time of

writing. As the age of this sample is unknown, it has been excluded from the remaining life

assessment.

The sample from Ch6135 appears to be a modern DN450 (approximately 18 inch) concrete

pipe, probably installed at construction of the WWWTP, possibly to replace deteriorated

pipe. This has not been verified at the time of writing. For the purposes of this

assessment, the service life of this pipe has been taken as nominally 20 years.

The pipe samples do not show evidence of significant deterioration of the internal surfaces

above the operational water level. Deterioration of sewer crowns by biogenic acid attack is

a common feature of gravity sewers carrying untreated sewage, due to metabolisation of

H2S by sulfur-oxidising bacteria living on the surface and excreting sulfuric acid onto the

concrete. The report poses ventilation, flow conditions, and sewage composition and

septicity as contributing factors. Pipe sample Ch5650 showed neutralisation of the cover

concrete at the crown to approximately 10 mm depth, however the morphology of

deterioration was not considered to be consistent with biogenic acid attack.

Minor scouring of the internal surface in the persistently wetted zone below the springline

was evident.

All of the spool samples with the exception of Ch6135 have reached or slightly exceeded

SLS1 described above at some part of the circumference. The pipe represented by Ch

6135 has potentially reached SLS1 at any areas where the cover is at the minimum cover

of 10 mm specified in the standards. None of the spool sections received have reached

SLS2.

The minimum measured covers were 7-19 mm for internal surfaces and 6-12 mm for

external surfaces. Whilst nominally centrally located within the pipe wall, the reinforcement

location varies due to minor displacements and distortions. In 2008 the typical original

cover depths to the circumferential wires were reported as approximately 15-25 mm (min

10-16 mm) to the inner surface and approximately 10-20 mm (min 5-9 mm) to the outer

surface (measured to the few undamaged surfaces).

Remaining life predictions by Opus are discussed further in Section 3.4. It should be noted that

the definitions of the SLS’s have been developed by Opus, and are not recognised standards.

These definitions do not represent physical failure of the pipe, they represent levels of

deterioration that are observable, and to some extent quantifiable (in particular SLS1).

2.3 Soil and Water Chemistry

2.3.1 Test Reports

Copies of the soil and water analysis reports for samples recovered where the pipe was

excavated for sampling are contained in Appendix B, the following provides interpretation of the

data in relation to the effects of burial condition on the durability of the pipe.

2.3.2 Soil Samples

All of the soil samples were similar in chemistry, with near neutral pH (pH 6.2 to 7.0), sulfate ≤50

mg/kg as SO3 and chloride generally


concentrations in interstitial moisture, and therefore cannot be assessed in terms of the

durability of the pipeline.

Rain or surface water percolating through the soil surrounding the pipe will not be modified by

the soil chemistry, and will remain soft and unbuffered, resulting in a continuing level of soft

water leaching when the soil is damp. Buried concrete encased pipe sections are unlikely to be

significantly affected unless the encasement is very severely deteriorated or physically

breached.

2.3.3 Groundwater Sample, Ch1290

Only one groundwater sample was recovered, from Ch1290. The water was slightly alkaline

(pH 7.6) with minor concentrations of deleterious (chloride, sulfate, ammonium, magnesium)

ions and protective (calcium, alkalinity) ions.

The water chemistry has been assessed for corrosiveness to concrete using several standard

models:

Langelier Saturation Index (LSI): -1.8 (7 indicates calcium leaching, magnitude is indicative of

severity).

Leaching Corrosion Index (LCI): 1,661 (magnitude is indicative of severity, >1,000

classified as “very highly aggressive”).

2.3.4 Internal Wastewater

At the time of writing, no definitive information was available on the chemistry of the wastewater

inside the pipe.

Typical chemistry values for water supplied to the Karori area from the Te Marua WTP were

obtained from the Wellington Water website. The water chemistry has been assessed for

corrosiveness to concrete using several standard models:

Langelier Saturation Index (LSI): -6.4 (7 indicates calcium leaching, magnitude is indicative of

severity).

Leaching Corrosion Index (LCI): 4,724 (magnitude is indicative of severity, >1,000

classified as “very highly aggressive”).

Although the corrosivity of the water supply might be ameliorated somewhat in the process of

becoming wastewater entering the pipeline, the contents of the pipe is expected to be

aggressive to the concrete by a soft water leaching mechanism.

2.4 Petrographic and SEM Examination

Copies of the petrographic and SEM examination reports on two selected pipe sections are

contained in Appendix C. The following provides interpretation of the results in terms of the

deterioration mechanisms and their likely effect on remaining service life.

This testing was conducted on cross-section samples from the inverts of pipe samples Ch1290

(partially buried) and Ch3800 (concrete encased above ground). The inverts were selected

using the results from the Opus report where deterioration of the invert was reported, but no

deterioration of the crown reported. Sample Ch1290 was selected for the unusually high depth

of deterioration from the external face.


The petrographic report identified the following features:

The coarse aggregate in both samples was rounded to sub-angular and of mixed

lithologies, indicating a screened river gravel. Evidence of minor alkali-silica reaction (ASR)

was present, but had not progressed to a deleterious stage.

The internal surface at the invert of both samples showed deterioration consistent with

calcium leaching or “soft water leaching”.

The external surface at the invert of Ch1290 showed deterioration consistent with attack by

acidic groundwater. This finding is inconsistent with the chemistry of the groundwater

sample obtained from this site, the composition of which is consistent with soft water

leaching.

The external surface of the Ch3800 invert sample showed minimal (


The 2008 Duffill Watts report records that 1,900 m of the pipeline is partially or fully

exposed above ground, of which 40 m was classified as “very poor condition” and 310 m as

“poor condition”. However, no photographic references or classification criteria are

provided to enable interpretation of these classifications, and no chainages are provided to

allow comparison with recovered spool samples.

The 2008 Duffill Watts report contends that the use of averaged deterioration rates over

time should be conservative given that the change to carriage of treated sewage after 1997

will have reduced the deterioration rates.

The Opus report states that five of the six spool samples examined in 2008 had reached

SLS1.

2.6 Vehicle Loading

As noted by Opus, the pipeline is not crossed by official roads and therefore, in theory, is not

subject to load from vehicular traffic.

Examination of recent satellite imagery of the beach crossing and outfall area has revealed the

presence of vehicle tracks crossing the pipeline, apparently as a shortcut between an existing

track and the neighbouring beach. In some cases, the cover has been eroded and the tracks

indicate vehicles driving directly on the exposed pipeline.

Any sections of the pipeline intersected by access tracks within properties will be subject to

vehicle loads, sections crossing paddocks or other open space within properties might also be

subject to occasional vehicle loads.

As the pipeline continues to deteriorate over time, this presents a risk of local breakage by

structural overload.


Table 1: 2017 Sample details reproduced Table 1 from Opus report.

Site (chainage)

Opus sample

ref

Date received

Sample length

(as received), mm

Nominal diameter

Sample type

(note 1)

Installation type Comment

450 4-17/445 21/9/17 530 15” Ring Buried -

1290 (note 2)

4-17/425 14/9/17 490 15” Ring Partly buried (top is above ground)

From spigot end. Invert subsampled for petrographic analysis.

2200 4-17/426 14/9/17 460 15” Ring Buried From spigot end

3055 4-17/446 21/9/17 530 15” Ring Buried -

3800 4-17/447 21/9/17 530 15” Ring Exposed (concrete encased)

Invert subsampled for petrographic analysis.

3800 (note 3)

4-17/408 11/9/17 - 15” Patch Exposed (concrete encased)

From collar end

4500 4-17/409 11/9/17 375 375 mm? Patch Exposed From collar end

5650 4-17/424 14/9/17 390 15”? Patch Exposed (concrete encased)

From collar end

6135 (note 2)

4-17/483 5/10/17 530 450 mm Ring Buried From collar end

6135 (note 3)

4-17/410 11/9/17 - 450 mm- Patch Buried From spigot end

1. Patches were cut from the top of the pipe. Rings represent the circumference of the pipe barrel excluding spigot and collar.

2. Corresponds to a sampling/CCTV location proposed in 2008.

3. We were subsequently advised to examine the full ring samples from these pipes instead.


3. Discussion

3.1 Concrete Deterioration Mechanisms

3.1.1 Biogenic Acid Attack

Uncharacteristically for a partially filled 80-year-old gravity sewer that has carried untreated

sewage for most of its service life, typical biogenic acid attack of the headspace surfaces has

not been observed.

It is considered that the minimal sulfate levels in the supply water, combined with minimal

contributions of sulfate-bearing industrial and trade wastes, has resulted in an operational

environment such that H2S generation was limited. Historically, sulfate and H2S concentrations

appear to have been too low to support the bacterial colonies necessary for this type of

deterioration to occur in the majority of the pipeline (noting that replacement of the section

nearest to the WWWTP might have been related to local biogenic acid attack).

The commissioning of the WWWTP resulting in carriage of treated sewage makes future

biogenic acid attack a minimal risk.

The lack of evidence of historical biogenic acid attack at sampled locations is an important

matter. It means that the change to carriage of treated sewage from 1997 is unlikely to have

resulted in any reduction of the deterioration rate from 1997 onwards (as suggested in the Duffill

Watts report), therefore the historically averaged rates are less conservative than previously

proposed.

3.1.2 Internal Leaching and Erosion

The primary mechanism of deterioration of the internal pipe surface is soft water leaching, due

primarily to the chemistry of the potable water supply to the locality, which makes up the

majority of the liquid (i.e. residential grey water) passing through the pipeline.

Assuming that the current incoming water quality represents the historical average, and remains

essentially unchanged into the future, then the currently estimated historical average

deterioration rates can be expected to continue. If there is a significant change in the chemistry

of the sewage, e.g. an increase in trade waste, future deterioration rates might increase.

Erosion rates of concrete weakened by soft water attack are related to flow volume, turbulence,

and grit loading. Restriction of flow to 190 L/sec prior to production of the 2008 reports (referred

to in the Duffill Watts report) might have reduced the flow-related effects, depending on

historical and current flow rates, while construction of the WWWTP is likely to have reduced

incoming grit loadings. Unless soil is drawn into the pipeline at locations of groundwater

infiltration, overall rates of erosion are considered likely to have reduced, making the historical

average slightly conservative. Future increases in flow rates, or increases in duration of peak

flows, might increase erosion rates.

3.1.3 External Leaching

The occurrence and severity of soft water leaching of the external surface will be location-

specific. The chemistry of the soil samples recovered suggests that the most likely mechanism

is soft water leaching by percolating or seasonal groundwater.

The presence and condition of any concrete encasement will have a very significant effect on

the occurrence and rate of deterioration of the pipe sections external surfaces. The measured

external depths of deterioration in samples from encased pipe at Ch3800 (4-10 mm) and


Ch5650 (2-6 mm) suggest that the encasement has limited protective ability in at least some

locations, noting that the mechanism has not been identified in these samples.

3.1.4 External Acid Attack: Ch1290

The evidence of acid attack in sample Ch1290 from the petrographic examination is

unequivocal, but is inconsistent with the soils and groundwater chemistry sampled at that

location. It cannot be determined whether the acid attack is historical and exposure conditions

have changed over time, or whether the chemistry at the time of sampling is a seasonal effect

due to recent rainfall at the site.

As the groundwater is expected to be highly aggressive by soft water leaching, for the purposes

of this report, it will be assumed that the measured mean historical rate will continue.

3.1.5 Soft Water/Acid Attack and Strength Reduction

It is important to note that, in this assessment, the extent of deterioration is primarily determined

by measurement of the depth to which the natural alkalinity of the cementitious matrix has been

reduced, as indicated by a phenolphthalein indicator test. Phenolphthalein indicates whether

the pH of the cementitious matrix is higher or lower than approximately 9.5, but is not able to

indicate the cause of that loss of alkalinity. Different deterioration mechanisms that cause loss

of alkalinity have different implications for the remaining service life of the concrete:

Carbonation: Carbonation is a result of absorption of carbon dioxide, most commonly from

the atmosphere. The reaction process involves conversion of calcium hydroxide

(portlandite) contained in the cementitious matrix to calcium carbonate (calcite) without

removal of calcium from the matrix. This conversion process can result in slight

densification and strength increase of the concrete. The durability concern is the reduction

of pH from approximately 12.5 (at which steel reinforcement is passive) to approximately

8.5, at which the reinforcement is no longer passive and can corrode once the carbonation

front reaches the reinforcement surface.

Soft water leaching and acid attack: These two mechanisms exhibit slightly different

symptoms in relation to rate and severity, and in the deposition of characteristic iron-based

reaction products by acid attack. Acid attack is usually more rapid and severe than soft

water leaching. Both mechanisms degrade concrete by dissolution of the calcium

hydroxide (portlandite) phase and removal of the dissolved calcium from the matrix,

however the pH is reduced to a level where the calcium silicate hydrates that provide the

physical strength of the cementitious matrix become unstable and break down. This

breakdown of the calcium silicates results in a progressive loss of strength, until ultimately

the concrete erodes or frets away completely. Once the pH is reduced at the

reinforcement, the reinforcement can corrode, and once the matrix surrounding the

reinforcement starts losing strength the bond to the reinforcement is reduced and

composite structural action is eventually lost. These mechanisms therefore result in a

progressive loss of structural capacity in the concrete.

Biogenic acid attack: Biogenic acid attack is a specific case of acid attack, where the acid

affecting the concrete is sulfuric acid excreted by sulfur oxidising bacteria as they

metabolise hydrogen sulfide gas. In this case, the cementitious matrix is not only

weakened, but the weakened matrix is physically disturbed by expansive deposition of

calcium sulfate dihydrate (gypsum).

The above mechanisms can be readily differentiated by petrographic examination, and by

examination under an electron microscope (SEM/EDX) as applied in this study.


Determination of the mechanism that is occurring in the concrete is therefore important in

interpreting how the test results are used in estimation of remaining service life:

In the case of carbonation, remaining life estimation revolves around the rate of progress of

the carbonation front, and the subsequent rate of corrosion of the reinforcement.

In the case of cementitious matrix weakening by soft water leaching or acid dissolution,

estimation of remaining life revolves around the rate of loss of structural capacity as the

residual cross-section of sound concrete diminishes, and eventually the reduced composite

effect of the reinforcement. Once the cementitious matrix starts to weaken, the extent of

strength loss cannot be quantified. It is therefore prudent to consider only sound, alkaline

concrete as contributing to the structural capacity of the element.

In the current assessment, the dominant deterioration mechanisms are soft water leaching of

the internal surface below the flow level, and soft water leaching or acid attack of the external

surface. For these reasons, the remaining life assessment for the pipeline should ignore that

part of the cross-section that is not sound and alkaline to phenolphthalein.

3.1.6 Alkali-silica Reaction (ASR)

Although ASR is present in both samples examined, and expected to be present in all pipe

sections assuming the same aggregate source was used in production, it has not progressed to

a deleterious stage. The micro-cracking caused by ASR if significant may reduce the strength of

the concrete and facilitate corrosion initiation.

Given the age and persistent wetness of the pipes throughout their service life to date, this

mechanism is considered unlikely to progress any further. ASR is not considered to be a

significant risk to the future service life of the pipeline.

3.1.7 Delayed Ettringite Formation (DEF)

No evidence of DEF was reported. Although DEF is often synergistic with ASR, given the

current age of the pipes, DEF is not considered to be a future deterioration risk.

3.2 Reinforcement Deterioration Mechanisms

The pipe testing has demonstrated that the pipe concrete has areas where the concrete

alkalinity is reduced through soft water attack or atmospheric carbonation, coincident with

reinforcement that has low cover. Under these conditions the reinforcement is expected to lose

its passive film that requires high alkalinity for stability, and which prevents the steel from

corroding.

In addition, chlorides that ingress or are present in the concrete might induce corrosion of

reinforcement, with the concentration required to initiate corrosion dependent on several factors

including the concrete paste’s alkalinity, and porosity/density/compaction which are influenced

by the concrete mix design and manufacture/construction method.

Corrosion initiation is more likely when the reinforcement is located in concrete that has both

low alkalinity and a quantum of chlorides.

Once corrosion can initiate, the reinforcement will corrode with potential loss of steel cross-

section and loss of bond to the cement matrix. The rate of subsequent corrosion propagation

depends on several complex factors including oxygen and moisture availability, concrete

resistivity and permeability, ratio of anodic/cathodic zones and presence of micro or macro voids

in the cement paste at the steel interface.


3.2.1 Carbonation-induced Reinforcement Corrosion

Carbonation of buried sections of the pipeline is not expected to occur due to persistent

dampness.

Atmospherically exposed sections are expected to undergo carbonation, but the sample from

Ch4500 showed no significant carbonation by phenolphthalein test.

The Ch1290 sample showed neutralisation of the exposed top surface to approximately 3 mm,

but given the presence of surface etching it is unclear as to whether this is due to carbonation or

rain water leaching. If it was due to carbonation, the rate taken over the service life to date is

minor.

Given the measured depths of cover to reinforcement, carbonation-induced reinforcement

corrosion is not considered to be a significant risk within the remaining service life of the pipe.

3.2.2 Soft Water Attack-Induced Reinforcement Corrosion

There is very limited published information on the risk or rate of corrosion of reinforced concrete

pipes subject to loss of alkalinity primarily from soft water or acidic groundwater attack. Most

pipe durability studies have assessed chloride induced risk, or concentrated on rate of loss of

the cement paste due to contact with a conveyed liquid or environment.

The internal pipe surface above maximum flow level would normally be exposed to air, and

depassivated reinforcement might corrode as for atmospherically carbonated concrete. Typically

the relative humidity within a long unventilated half-full pipe will be high and the rate of corrosion

is expected to be low.

The normally submerged concrete surface will be water saturated, and the rate of corrosion is

expected to be minimal provided that the reinforcement remains fully embedded in sound

concrete. A low-oxygen, low expansion corrosion mechanism is possible where the submerged

cover is very thin, say of the order 5 mm or less. Hence, as erosion of the invert progresses,

the risk of the reinforcement corrosion will increase. Such corrosion is unlikely to be expansive

and might not be readily detectable.

For external buried concrete surfaces, the corrosion rate is similarly expected to be very low due

to reduced oxygen availability.

There is scope to undertake corrosion rate studies on the exhumed pipe sections that might

help quantify the possible rates of corrosion of depassivated reinforcement.

3.2.3 Chloride-Induced Reinforcement Corrosion

Historically, centrifugally cast reinforced concrete pipe has been found to have good resistance

to marine exposure, in particular in relation to very long times for propagation of corrosion to

cause cracking1. AS/NZS 4058:2007 requires minimum 20 mm cover for barrel and socket type

pipe in normal marine exposure for 100-year life (excludes wetting and drying exposure).

Unless high levels of chloride-bearing trade waste are entering the wastewater stream, the

Karori town water supply and available groundwater chemistry results do not exhibit sufficient

chloride concentrations for this mechanism to be considered to be a significant risk within the

remaining service life of the pipe.

There is a specific local risk of chloride-induced reinforcement corrosion at the beach crossing

and outfall at the Cook Strait end of the pipeline (Ch0 to approximately Ch150). This has not

been evaluated in this assessment, as none of the available samples were obtained from

locations sufficiently close to Ch0 to obtain meaningful data.

1 Durability of Concrete Pipe in a Marine Environment, Concrete Pipe Association of Australasia, July 2000


It is recommended that a specific assessment of the current status of marine exposure risk be

made.

3.3 Effect of Deterioration on Structural Integrity

The leached cement matrix has considerably reduced strength compared to intact (non-

leached) matrix, therefore the pipes have already lost significant structural capacity where

extensive leaching has occurred. The remaining structural capacity of the pipe will be primarily

related to the remaining intact pipe cross-section area.

Depending on bedding conditions, some locations will be at higher risk of failure than others.

Where the bedding provides a high level of soil arching action, and the deterioration is primarily

below the springline, higher levels of deterioration might be acceptable. The presence and

condition of any concrete encasement will have a very significant effect on the acceptable

extent of deterioration of the pipe sections.

The pipe’s embedded steel reinforcement will lose its protective passive film once the

surrounding protective alkalinity is lost (i.e. once SLS1 is reached). If reinforcement corrosion

progresses, it would further reduce the structural capacity of the affected pipe over time.

However, the rate of reinforcement corrosion is expected to be very slow based on the pipe

exposure conditions.

As the pipeline continues to deteriorate, there is a progressively increasing risk of breakage due

to structural overload:

At any point where the support is compromised by erosion due to stream flow, as

unsupported pipe becomes less able to span gaps in the bedding.

At bell-and-spigot joints in the event of soil movement.

At the beach crossing by vehicles driving over or directly on the pipe to access the beach

from a nearby track.

During seismic events.

If the pipeline is subject to loads such as internal surcharge pressure or uplift/floating

forces.

There is a potential attendant liability and reputational risk in relation to injury to members of the

public, damage to vehicles and environmental effects should the pipe collapse or rupture.

It is noted that even a fully intact and sound pipe section might fail depending on the actual

load, overburden and bedding conditions.

The need to undertake further tests to assess the risk of structural failure, or implement

protection or strengthening works to sections of the pipeline must be compared to the risk of

pipe failure by delaying such works prior to replacement of the whole asset, and then needing to

repair or replace sections of failed pipeline as maintenance actions. The cost effectiveness of

such actions will depend on the length of affected pipeline subject to the identified possible

failure mechanism.

3.4 Remaining Life Assessment

3.4.1 General

Remaining life assessment has to consider the ability of the pipe to withstand operational loads.

Cement matrix that has been leached of calcium phases by soft water leaching or acid attack is

considerably weaker than sound, alkaline matrix and composite action with the reinforcement


will be reduced. This loss in strength cannot be measured, therefore a conservative assumption

is to discount any contribution to structural capacity by the thickness of deteriorated concrete.

It should be noted that where the neutralisation is due solely to atmospheric carbonation of the

cement matrix, there will be no significant loss of strength.

3.4.2 Deterioration Rates

Table 2 summarises averaged deterioration rate constants obtained from measured data for the

full service life, along with characteristic upper bound (95% values) rate constants. The

characteristic values shown are the mean rate constant plus 1.65 standard deviations, noting

that the small data pools will result in the standard deviation being inflated.

As the internal deterioration at the invert is not known for the two crown patch samples at

Ch4500 and Ch5650, they cannot be included in a remaining life assessment, although it is

likely that these sections will be in similar condition to the rest of the pipeline. Similarly, Ch6135

also has to be excluded as being of different manufacture and lower age.

In broad terms the rate of internal concrete degradation has been approximately twice that of

external concrete degradation. Potentially the rate of future internal concrete leaching could be

reduced by increasing the treated water hardness and buffering capacity by lime dosing or

similar treatment, subject to the ability to comply with the applicable environmental discharge

requirements. Such an option would require a suitable technical and economic evaluation.

Alternatively the pipe could be lined internally to exclude the pipe from the soft water.

The internal surface of the pipe represented by the Ch6135 sample appears to be deteriorating

faster than the older pipe. It is suggested that this section should be included in any major

relining or replacement works.

The average rate constants in Table 2 have been derived from the phenolphthalein test results

reported by Opus. The internal rate constants were calculated individually using the highest

depth of deterioration below the springline for each sample, then averaged across samples of

similar age. The external rate constants have been determined in the same manner.

Table 2: Comparison of derived pipe deterioration rate constants.

Surface 2017 2017

Ch6135

2008 All Data

Internal Mean / mm/y

(2017 excl. Ch4500, Ch5650, Ch6135)

0.27 0.50 0.32 -

Internal Characteristic / mm/y

(2017 excl. Ch4500, Ch5650, Ch6135)

0.30 - 0.45 -

External Mean / mm/y

(2017 excl. CH4500, Ch6135)

0.12 0.15 0.16 0.14

External Characteristic / mm/y

(2017 excl. CH4500, Ch6135)

0.16 - 0.19 0.19

Given the loss of effective cross-section, structural integrity might already be marginal at certain

locations, and might become so at others in the near to medium term. Quantitative

consideration of the potential effect of reduced sound cross-section under various applicable

load cases is outside the scope of this assessment, and cannot be performed without empirical

data from load testing of ring samples.


As noted in the Opus report in Appendix A, the structural capacity required at a given location

will depend on local factors including loading, support from the bedding, condition of any

concrete encasement, and effectiveness of soil arch action in the bedding.

3.4.3 Comparison with 2008 Assessment

Data from the current assessment cannot be compared directly with the 2008 assessment on a

sample-by-sample basis, as the samples are not identified by chainage in the previous report.

A gross comparison of rate constants is provided in Table 2. At this time, the data for the

interior surface at the two ages is not considered sufficiently similar to combine without

additional knowledge of sample locations.

The data indicate that the relationship of deterioration to time is not directly linear, but might

have a time exponent of slightly less than 1. The use of a linear-to-time model is therefore not

considered to be unduly conservative.

3.4.4 Predicted Time to Nil Residual Sound Alkaline Matrix

The rate constants summarised in Table 2 have been used to predict the time until certain

proportions of the pipeline have no residual cross-section of sound alkaline concrete at some

part of the circumference. These predictions are presented in Table 3 using a linear-to-time

model, and in Table 4 from applying an optimised exponential model (exponent 0.95) to pooled

data from 2008 and 2017.

This assessment could be refined by detailed examination of additional samples to confirm the

mechanism of deterioration of the exterior surface.

Table 3: Predicted times to nil residual cross-section of sound alkaline

matrix (linear –to-time model).

Extent 2017 Data 2017

Ch6135

2008 Data

5% of pipeline (characteristic) 12 years - (-16 years)

50% of pipeline (mean) 25 years 42 years 10 years

Table 4: Predicted times to nil residual cross-section of sound alkaline

matrix (pooled data, optimised exponential model).

Extent Pooled 2008 and selected 2017 data

5% of pipeline (characteristic) (-4 years)

50% of pipeline (mean) 18 years

3.4.5 Remaining Service Life

The pipeline remaining life cannot be definitively quantified, as the current load cases at various

locations are not known, nor are the original design load cases. The average effective residual

cross-section of intact alkaline cement paste was 34% of the original cross-section, and a

characteristic lower bound of 29%. The structural capacity of the concrete is therefore expected

to be significantly impaired, and the concrete will become increasingly fragile as cement

leaching continues.


Section 3.3 above outlines the effects of the observed deterioration in relation to potential

structural overload cases. In essence, the pipeline will continue to convey sewage from the

WWWTP to the outfall for as long as structural overload does not compromise its integrity at

particular locations, noting that the risk of failure increases with time.

For the purpose of this assessment, the indicative future service life has been considered

probabilistically using SLS1 as described in the Opus report (loss of alkalinity as measured by

phenolphthalein test without appreciable softening) resulting from soft water leaching (the

dominant mechanism of deterioration) at some proportion of the circumference. When SLS1 is

reached throughout the cross-section of the pipe wall, the residual structural capacity is

considered minimal. At present, the pipeline remains generally serviceable as a conduit for

sewage, but the risk of failure due to physical loading is significantly increased, and

requirements for maintenance intervention are also likely to be increased. Future failure

locations will depend on local installation parameters and load cases.

As recorded in Table 3 and Table 4, the results of the assessment depend on how the available

data is interpreted. The 5% (characteristic) ages represent the ages at which 5% of the pipeline

length is expected to reach SLS1 throughout the cross-section at some proportion of the

circumference, and a significant increase in maintenance and possible local replacement of

affected sections might be required depending on load conditions. The 50% (mean) ages

reflect the ages at which SLS1 will have been reached throughout the cross-section of half of

the alignment, with the attendant potential consequences. Statistically, some local sections of

the pipeline will already have reached this state.

Overall, the majority of the pipeline is considered likely to remain sufficiently serviceable as a

conduit for sewage until planned replacement in 2035 (refer Section 1.1, 18 years from 2017),

with periodic inspection and on-condition maintenance as required, provided that local load

cases do not exceed the reduced structural capacity.

The rate of pipeline degradation will not have decreased after the change from untreated to

treated sewage conveyance in 1997. The future rate of internal degradation might be

decreased, and future reliability increased, if the treated sewage calcium concentration and

buffering capacity are increased through dosing, or an internal liner is installed.

At this time, no additional assessment of the increased risks associated with the marine

environment at the beach and reef crossings has been made. This is a short and relatively

accessible section of pipe, and the environmental implications of a failure are limited.

Therefore, it is considered that this section can be managed through routine inspection and on-

condition maintenance.


4. Conclusions and Recommendations

4.1 Conclusions

The following conclusions are reached from the assessment presented in this report:

1. The primary cause of continuing deterioration of the inner surface is soft water leaching of

the cement paste below the operating water level. This is expected to continue advancing

at a nominal rate of 0.3 mm/yr for the internal surface below the flow level, assuming no

change in the corrosiveness of the sewage. Deterioration of the external surface is

expected to continue at approximately half of that rate.

2. The absence of evidence of biogenic acid attack on the interior surfaces of samples from

2008 and 2017 means that the change to carriage of treated sewage from 1997 is unlikely

to have reduced deterioration rates. Therefore, the previously reported assumption that

carriage of treated sewage from 1997 has made, and will continue to increasingly make,

historical average deterioration rates conservative is erroneous.

3. The primary causes of deterioration of the external surface are soft water leaching and acid

attack. The occurrence and rate of these mechanisms are location-specific, and unlikely to

apply where the pipe is concrete encased unless the encasement is severely damaged.

4. The average effective residual cross-section of intact alkaline cement paste was 34% of the

original cross-section, and a characteristic lower bound of 29%. The structural capacity of

the concrete is therefore expected to be significantly impaired. However, a structural

assessment is required to determine the probability of structural failure for any particular

section of the pipeline.

5. Biogenic acid attack of the internal surface above the operating water level is not expected

to be a future cause of deterioration.

6. Carbonation-induced corrosion of reinforcement is not expected to be a significant future

deterioration risk.

7. Chloride-induced corrosion of reinforcement is not expected to be a significant future

deterioration risk for the majority of the pipeline. A specific, as yet unquantified, risk exists

at the beach crossing and outfall where the pipe is directly exposed to marine conditions,

particularly Ch0 to approximately Ch150.

8. Propagation of corrosion of the invert reinforcement will become more likely as the concrete

cover is reduced through erosion, although such corrosion s unlikely to be expansive and

may not be readily detectable.

9. ASR is present in the pipes, but at this time is not deleterious, and given the age of the

pipes is not expected to continue and present a significant future deterioration risk.

10. Non-deleterious secondary ettringite recrystallisation is present, but there is no evidence of

delayed ettringite formation (DEF). Given the age of the pipes and the presence of ASR,

DEF is not considered to be a future deterioration risk.

11. Contrary to Opus’s report, there is a risk of local structural overload from vehicles at an

identified beach crossing, and potentially other unidentified locations, which will increase

progressively as the pipes continue to deteriorate over time.

12. Structural overload at other locations is possible, e.g. due to seismic events, unplanned

internal surcharge, or erosion of bedding.

13. There is evidence that at least two sections of the pipeline have been replaced:


– Around Ch4500: Extent and date of, and reason for, replacement not known.

– Around Ch6135: Extent of, and reason for, replacement not known. Possibly replaced

in conjunction with construction of the WWWTP.

14. As the pipes will become progressively more fragile with age, failure frequency and severity

will increase, and repairs will become increasingly difficult to perform. Based on the

measured loss of cement alkalinity data, it is considered that some sections of the pipeline

are likely to require major remedial or replacement works within 10 years to mitigate the risk

of failure.

15. Overall, the majority of the pipeline is considered likely to remain sufficiently serviceable as

a conduit for treated sewage until 2035, with periodic inspection and on-condition

maintenance as required, if local load cases do not exceed the reduced structural capacity.

4.2 Recommendations

Based on the above conclusions, GHD recommends that Wellington Water implement the

following actions:

1. Continue to implement its ”Management and Monitoring Plan: Western Wastewater

Treatment Plant Effluent Pipeline”.

2. Review maintenance records to identify any trends in pipe failure type, frequency, severity

and location. Review this trend analysis on an annual basis, more frequently if the data

warrants.

3. Within two years, conduct a preliminary review of the technical, ecological and logistical

feasibility of relining and replacement options, or treating the effluent alkalinity to reduce the

rate of internal degradation, including estimates of the order of probable costs for

comparison with predicted repair costs.

4. As a precautionary measure, implement control measures at the southern beach crossing

and adjacent area to prevent vehicles driving over the pipeline without adequate protection.

5. Consider completing the following testing and assessment within the next 5 years to

increase certainty around the predicated remaining service life figures provided in this

report, and for input to structural capacity assessments:

– Residual load bearing capacity testing of exhumed pipe sections.

– Structural assessment to determine pipe minimum allowable sound cross section for a

range of structural load scenarios to inform assessment of structural failure risk.

– Conduct additional detailed examination of the external surfaces of the current

samples to clarify the mechanisms of deterioration and their effect on residual

structural capacity.

– Conduct corrosion rate measurements on depassivated steel reinforcement of

exhumed and in-situ pipe sections.

– Obtain samples from the Cook Strait end of the pipeline to assess chloride-induced

reinforcement corrosion risk.

– Obtain data on the basic chemistry of the sewage entering the pipeline to evaluate its

aggressiveness.

GHD | Report for Wellington Water Ltd - Western WWTP Outfall Pipeline, 5137412

Appendices


Appendix A – Opus Report

wellington water ltd · 2018. 2. 16. · the following previous condition assessment reports have...

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