HARD ROCK GEOTECHNICAL PTY LTD Consulting Geotechnical Engineers

ABN: 24 066 600 002

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Introduction

A site investigation was undertaken on the 20th of November, 2019. The purpose of the investigation was to

provide foundation recommendations for the proposed five level mixed use development, constructed on

ground (no basement).

Scope of the Investigation

The preliminary investigation comprised the drilling of three hand augered boreholes. The subsurface profile

was logged and bulk sampled for visual identification. Borehole logs and locations are shown on pages 7 and

8 of this report.

Further detailed investigation is proposed following demolition and site clearing, and will include the drilling

of additional mechanically augered boreholes and exposure of adjacent boundary footings, as necessary.

Site Description

The site is occupied by a double storey solid brick shop fronted terrace building, with vehicle access via the

rear lane. Adjacent buildings are constructed to either side boundaries. The existing development extended to

all site boundaries. The investigation was limited to the rear ground level garage at 320 Smith Street. The site

is relatively flat and has poor surface drainage.

Subsurface Conditions

Regional geology

The site is identified on the ‘Geological Survey of Victoria’ Melbourne Sheet (1:63,360) as being within the

province of Silurian “Dargile Formation” and associated soils.

Subsurface profile

See borehole logs page 7. The boreholes encountered:

Concrete paving (80mm to 110mm nom. thickness) and FILLING of mixed composition to a

maximum depth of 0.60m, underlain by;

Natural stiff silty CLAY of medium plasticity/extremely weathered siltstone ROCK to depths of

between 0.55m and 0.90m, grading to;

Distinctly weathered (DW) siltstone ROCK.

All boreholes were extended to depths of between 0.55m and 0.90m, within the weathered siltstone ROCK.

The existing fill material can be considered the equivalent of rolled non sand fill in accordance with AS2870-

2011 clause 6.4.2.


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Soil moisture & groundwater

No groundwater was encountered in the boreholes; filling and natural soils were in a moist condition.

Site Classification

The site is classified as CLASS P in accordance with AS2870-2011.

The subsurface profile is moderately reactive (CLASS M).

The underlying siltstone ROCK is non-reactive (CLASS A).

Earthquake - Site Sub-Soil Class

A site sub-soil class of CLASS Be – Rock can be assumed with reference to AS1170.4-2007, Section 2.4.

Geotechnical Design Parameters

Geotechnical design parameters are provided in Appendix A of this report.

Foundation Recommendations

The siltstone ROCK is expected to exhibit a deep weathering profile; the upper rock is of relatively modest

strength. Higher bearing pressures may be available at depth and in consultation with this office and with

further detailed investigation.

Continuous strip & isolated pad footings

Conventional strip and pad footings should:

Penetrate through any FILLING and natural soils; and

Be founded within (DW) siltstone ROCK (bedrock).

Strip and pad footings can be proportioned for an allowable bearing pressure of 700kPa. This pressure can be

reviewed once the design pressures have been calculated.

Mass excavated pads/bored piers

Mass excavated pads/bored piers should penetrate and surface filling and natural soils, and be founded at

within (DW) siltstone ROCK (bedrock).

An allowable bearing pressure of 700kPa may be adopted beneath bored piers founding within (DW)

siltstone ROCK (bedrock). This pressure may be reviewed once design loads have been determined.

No skin friction is available for bored piers.

The base of the pier must to be clean and free of all loose material, and concrete poured without delay at the

completion of boring/excavation.

The contractor should be briefed and should supply means of insuring that the bored pier bases are clean and

free of disturbed and fallen material. An auger with a single cutting blade (i.e. no tines) may be an option to

cleaning the bases of the piers.

Rock socketed bored piles

Rock socketed bored piles should be founded a minimum of 2 pile diameters within competent siltstone

ROCK and can be proportioned for an allowable end bearing capacity of 1000kPa with an allowable skin

friction component of 100kPa within the competent SILTSTONE ROCK.

Excavation within the rock may be difficult and it is anticipated that specialist piling contactors may be

necessary.


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The piling contractor is to provide adequate means to ensure a clean base for the bored piles, free of any

debris.

Pavements

Sub-grade preparation

Preparation of pavements and floor slab subgrades should comprise the stripping of any organic material and

the surface fill to expose the natural silty CLAY (or weathered rock) subgrade.

The exposed surface should be proof rolled with the aim of achieving a dry density ratio of 98% as measured

by standard compaction (AS1289 5.1.1). Any soft, wet or loose material which does not respond to

compaction should be additionally excavated to expose a firm working base.

Depending on the moisture content of the subgrade at the time of construction, it may be necessary to add

water or allow the subgrade to dry back to achieve satisfactory compaction.

We recommend that the pavement construction be commenced without delay once the subgrade has been

exposed and proof rolled. A minimum compacted lift of 200mm imported filling should be placed

immediately once subgrade preparation is completed to protect the subgrade from moisture changes and

provide a working platform for vehicles.

Subgrade drainage

The subgrade should be provided with subsurface drainage to maintain any groundwater table to at least

300mm beneath the underside of the pavements, or a lower CBR value should be used in the design.

Pavements

Based on experience within the immediate area and the site investigation to date, it is recommended that

pavements be designed on the:

Natural silty CLAY soils using an estimated C.B.R value of 4.5% and a long term Young’s modulus Esl

of 18MPa and a correlation factor of 0.7 can be adopted as per the Cement and Concrete Association of

Australia ‘Industrial Floors and pavements’ section 3 Design for strength.

Natural weathered siltstone ROCK soils using an estimated C.B.R value of 15% and a long term

Young’s modulus Esl of 33MPa and a correlation factor of 0.9 can be adopted as per the Cement and

Concrete Association of Australia ‘Industrial Floors and pavements’ section 3 Design for strength.

Earthworks

The use of on-site filling and top soil material should be avoided and removed to spoil. The excavated

weathered siltstone ROCK should be suitable for structural filling.

Any imported filling used should comprise of clean, essentially of a granular nature, non-organic and have

a plasticity index of less than 12%. Suitable material may include crushed scoria, non-descript crushed rock,

mudstone or siltstone or equivalent. Fill material should have a nominal particle size of 75mm or less and as

a guide, appropriate material would meet the following criteria:

P.I x %pass 0.425 (AS sieve) 600

Any filling should be placed in lifts not exceeding 250mm loose thickness. Each layer should be compacted

to a dry density ratio of 98% measured by standard compaction (AS1289 5.1.1) using an appropriate medium

weight vibratory roller. The recommended moisture content is within 2% of optimum moisture content under

standard compaction.


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Construction & Maintenance

Excavation within fill material and natural sand soils may experience short-term instability (particularly if

undertaken during the wetter months) and shoring and/or over excavation should be anticipated.

Disturbance of the filling and any natural soils may have resulted from previous site works and should be

anticipated.

Blinding concrete should be poured in the base of the footing trenches where founding depths exceed footing

dimensions.

Articulation of pavements and floor slabs where they abut walls should be provide to allow for the

differential movement between the foundations and the pavements.

Articulation of masonry walls should be provided as per details contained in reference (2) below, where

relevant.

Where footings are located in close proximity or adjacent to a backfilled service trench or easements, the

footing must be deepened and founded at a depth at or below the level of plane of inclination of 45 above

horizontal extending outwards from the base of the trench or filling (as illustrated by figure C6.4.1 AS 2870

2011). This includes service trenches which may be present on adjacent sites or on site prior to the current

development (such as abandoned stormwater and sewer trenches).

All contractors should be well briefed as to the requirements and specifications in this report. The subgrade

preparation, compaction testing, and inspection of foundation excavations should be conducted to insure that

they are in accordance with this report. It is recommended that fill material type is verified and that the

compaction be tested during placement. Testing should be conducted in accordance with AS3798-1990,

‘Guidelines of earthworks for commercial and residential developments’.

This report is based on the assumptions that conditions revealed through selective sampling are indicative of

the actual conditions throughout the site, i.e. correlation between boreholes. Variations between boreholes

may exist due to previous land use or natural geologic processes. Additional deepening of the foundations,

deeper than the minimum specified founding depths in this report, may be required. The actual subsurface

conditions can be discerned only during earthworks when the subsurface profile can be directly observed.

For further information regarding geotechnical site investigation reports, refer to reference (4) below.

Inspection of all foundation excavations, site works and compaction must be conducted by a suitably

qualified, experienced engineer, engineering geologist, building surveyor or similar to ensure that the

founding material and site works are in accordance with this report. Should there be any doubt, this office

should be immediately contacted.

Please do not hesitate to contact us, should there be any further queries.

Yours faithfully,

HardRock Geotechnical Pty Ltd

James Harrison B.E. (Geo.) M.E. (Env.)

(Geotechnical Engineer)


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References

1. AS2870-2011. “Residential slabs and footings.”

2. The Cement and Concrete Association of Australia. Technical Note: TN61.

3. Institution of Engineers, Australia. 1987. “Guidelines for the Provision of Geotechnical Information in

Construction Contracts”.

4. AS1726-2017. “Geotechnical Site Investigations”.

5. AS2159- 2009. “Piling – Design and Installation”.

6. AS 3798-1996 “Guidelines on Earthworks for commercial and residential developments”.

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Hard Rock Geotechnical Pty Ltd Consulting Geotechnical Engineers

Borehole Logs

File:

191469

Date:

20/11/2019

Supervisor:

JH / CW

Client: Orclar Nominees Pty Ltd

Project: 318-320 Smith Street, Collingwood

Borehole No. 1 Drilling Method: HA Location: see Figure 1

Depth (m) Structure Description Cohesion/

density

Soil moisture/

groundwater Testing:

0.06

- 0.11

0.55

Pavement

SP

concrete pavement

silty CLAY(CL), medium plasticity, orange/light

brown/grey - extremely weathered (XW) siltstone

ROCK

Refusal at 0.55m on weathered siltstone rock

MD

M


0.09

0.60

0.70

Pavement

Fill

SP

concrete pavement

clayey SILT/gravel



ROCK


L/VL

MD

M

M


0.10

0.25

0.90

Pavement

Fill

SP

concrete pavement

non-descript crushed rock/gravel/clayey SILT/silty

CLAY



ROCK


L/MD

MD

M

M

Legend: Density Cohesion Moisture HA - Hand Auger A - Flight Auger Drill Rig

VL - Very Loose S - Soft W - Wet Unified Soil Classification Symbols: CL, SM, SW

L - Loose F- Firm M - Moist SP - Soil Profile

MD - Medium Density ST - Stiff D - Dry Some< 15%

D - Dense VST- Very Stiff Trace< 5%

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LOCATION PLAN

File: 191469

Date: 20/11/2019

Figure 1

Project: 318-320 Smith Street, Collingwood

Scale:

Not to Scale (sketch for borehole locations)

Legend:

Borehole

Footing inspection

BH1

BH3

BH2


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Please note these parameters are difficult to measure and exhibit large natural variations. Sound engineering judgement is recommended in their use.

Appendix A - 318-320 Smith Street, Collingwood

Material

Design Parameters

Po

isso

n’s

Ra

tio

Dra

ined

Yo

un

g’s

Mo

du

lus

E´

(MP

a)

K0 Kp Ka

Drained Undrained Moist

bulk

weight

γ (kN/m3)

Saturated

bulk

weight

γsat

(kN/m3)

(1 -

sin

´)

(1 +

sin

´)

(1 -

sin

´)

(1 -

sin

´)

(1 +

sin

´)

c´ (kPa) ´ (°) Cu (kPa) (°)

FILL (loose) & top soils 0 28 0 28 17 20 0.1-0.2 1-5 0.53 2.76 0.36

silty CLAY (stiff)

including extremely weathered siltstone

ROCK

3 25 100 0 19 19 0.2- 0.4 8-15 0.58 2.46 0.41

Distinctly weathered siltstone ROCK

(high to moderately weathered)

3

(joint

strength)

25

(joint

strength)

0 37 24 24 0.2-0.3 150-500 0.58 4.02 0.41

preliminary geotechnical site investigation report

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