report on various lots prepared for project 78319 december

Report on Scoping Study and Stability Assessment

Various Lots Surfers Avenue, Tallwood Avenue

& Bannister Head Road Narrawallee

Prepared for Shoalhaven City Council

Project 78319 December 2011

Scoping Study and Stability Analysis Project 78319 Various Lots, Surfers Avenue, Narrawallee December 2011

Table of Contents

Page

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

2. Methodology ................................................................................................................. 1

3. Site Location, Geology and History ............................................................................... 1

4. Site Observations ......................................................................................................... 5

5. Stability Assessment ..................................................................................................... 6

5.1 Previous Stability Assessments ........................................................................... 6

5.2 Current Stability Modelling ................................................................................... 8

5.3 Future Instability .................................................................................................. 9

6. Comments & Recommendations .................................................................................. 9

7. Limitations .................................................................................................................. 10

Appendix A: About this Report

Appendix B: Drawings

Appendix C: Photographs

Appendix D: Results of Stability Analysis

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Report on Scoping Study and Stability Assessment Various Lots Surfers Avenue, Tallwood Avenue & Bannister Head Road, Narrawallee 1. Introduction

This report presents the results of a scoping study and stability assessment undertaken for the Surfers Avenue Precinct, Narrawallee, as identified in the Douglas Partners (DP) Coastal Hazards Study Report (DP Report 72051). The investigation was commissioned in an email dated 25 August 2011 by Mr Warwick Papworth of Shoalhaven City Council (SCC) and was undertaken in accordance with the DP proposal dated 30 September 2011. It is understood that the area is predominantly private land and that this study will assist in Council’s updating of the Draft Local Environmental Plan (LEP) with respect to land that is subject to natural hazards, in this case specifically land stability. Currently, only four lots are zoned as 2a(4), ie those subject to a restriction on development, and this review is intended to assess the 35 adjacent lots for potential re-classification by Council. 2. Methodology

The scoping study and stability assessment comprised:

• An initial review of the geotechnical reports submitted to the Council as part of development applications together with any relevant Council records of inspection;

• Detailed inspection of the site for assessment of slope conditions and estimate of comparable risk. The inspections were carried out from safe access points along publicly owned road reserves. Inspection of private land was carried out with Council obtained consent from landowners.

• Engineering assessment, including computer-based stability analysis of landslide risk areas.

• Recommendations for Lot reclassification based on apparent risk. 3. Site Location, Geology and History

The site comprises 39 residential lots and an area of beach front and public reserve, lying in the coastal zone between Narrawallee Beach and Bannister Head. The location and area under investigation is shown on Drawing 1 in Appendix A. Reference to the Ulladulla 1:250 000 Geological Series Sheet indicates that the site is underlain by sediments of Tertiary age underlain at depth by silty sandstones of the Conjola Formation of Permian age.

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The site has a long history of instability dating back to at least the late 1960’s and it is further recognised that some of the landform features seen are the results of ancient landslide activity probably dating back some 5000 years. The assessment comprised a review of the results of previous investigations carried out by DP and others, Council having provided DP with copies of relevant geotechnical reports and file notes relating to Development Approvals for the area briefly summarised in chronological order as follows:

• April 1969 - Report to the Shire Health Surveyor re. storm damage in the Milton – Ulladulla area on 15/4/1969 and destruction of dwelling during storm – dated 21/4/1969. This report details amongst others, storm conditions which resulted in a landslip that destroyed a timber framed dwelling on Lot 373 and states that ‘the storm included 9 – 10 inches of heavy rain’;

• July 1980 – A Soil Conservation Service Investigation report concluded that ‘in view of the degree of slope, indications are of impeded drainage and the evidence of slope failure, it is recommended that no further development proceed on these lots.’ i.e. Lots 370 to 376;

• May 1981 - Coffey & Partners (CP) report on geotechnical investigation of Lot 373. This report notes the presence of minor slumps relating to the previous works on the site as noted above. The report also notes the presence of major slope failures along the headland area to the east of the site with a major rotational slump block encroaching on the north-eastern portion of Lot 373. The report notes significant ground water seepages on the undeveloped slopes to the east of the site, flowing onto the road in front of the site. The report states that proposed ‘remedial measures are directed to stabilising the slopes over Lot 373 only’ and also that ‘the cause of major slope failures’ and ‘the extent of major slope instability and the principal failure mechanism could only be ascertained following a detailed study of the entire area of known instability’;

• July 1981 – CP report on proposed beach house foundations for Lot 365. This report records the presence of vertical cut faces which had failed over the week long period following excavation. The report further addresses the issue of slope stability and states that the Lot appears to be in a stable condition;

• January 1982 – CP revised report for Lot 373 following additional field investigation. There are significant changes to the content of the report from that submitted above with less emphasis on the general slope stability issues and more detail of the measures required to improve slope drainage and associated slope stability on this particular Lot;

• October 1982 – SCC file note detailing correspondence relating to the May 1991 CP report. Council states that ‘the Lot area cannot be taken in isolation’, that CP were in agreement with this view and that further investigation had been carried out since submission of the May 1981 CP report to satisfy Council’s concerns relating to the proposed development. Appended to this file note are several photographs showing the destruction of the property on Lot 373 and evidence of shallow (1 m approx.) slips on the hillside;

• August 1983 – Golder Associates (GA) report on stability assessment for Lot 372. This report concludes that the soil mass underlying the proposed development could move in the future and that ‘the greatest threat to the stability of this building area would be from the unstable steep slopes above’. The report suggests that the area has previously been quarried and the excavation involved have likely been the primary cause of landslips in this area;

• October 1984 – SCC memo relating to request for permission by owner of Lot 373 to construct a 2 m high retaining wall along road frontage of Surfers Avenue in order to retain the lower part of

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the slope. An accompanying letter from Harry Partridge and Associates, Consulting Structural and Civil Engineers (HPA) justified the wall design by suggesting that the increase in toe pressure and improvements in drainage would improve stability. The wall installation was approved by Council;

• October 1987 – CP report on Slope Failure, Lot 364 Narrawallee Beach – This report relates to slope failures which occurred during construction of a retaining wall at the western end of a cut face at the rear of the property. Several failures, 3 m to 4 m wide with back scarps up to 2 m in height, tension cracks and numerous seepage issues are displayed on an annotated site plan and accompanying sketch sections, which show the nature of the slope failures and their encroachment on neighbouring properties in detail. Of note is the identification on the cross sections of water bearing layers of sand which daylight from the slope above the retaining wall footing level. The report states that ‘the cause of the failures appears to be a result of undercutting of the slope and the presence of water bearing layers within the slope wash soils’;

• October 1987 – Photographs on file relating to Lots 320, 361, 362 and 363 Tallwood Avenue. These photographs show ground settlement and slumping of land along the crest of the slope immediately above Lot 364 and appear to relate to the failure of the retaining wall noted above;

• April 1988 – Peter J Burgess & Associates Geotechnical Investigation Report – Lot 376, suggested that there was a ‘potential slip failure over the whole slope’ and the report made a number of conclusions as follows: ‘there is no practical or economically feasible design for stabilising the allotment or providing foundations on the allotment which we would be prepared to endorse or guarantee’, ‘temporary stability may be achievable but permanent stability is not, in our opinion economically achievable’, ‘ the current investigation has only confirmed that the lot is unstable for the long term’ and ‘consideration must be given to stabilising neighbouring lots as any construction on the subject lot could trigger failures on neighbouring lots’.

• July 1989 - Development application for Lot 376 was refused following submission of a geotechnical investigation report by Grant Alexander and Associates ‘which appeared not to address factors dealt with in previous geotechnical reports’. Council records state the following: ‘Council formed the opinion that the development was at risk of the land being subject to slip’, that there was ‘no provision to ensure neighbouring lots made safe from failure resulting from development on the subject lot’ and that the ‘proposed development was contrary to provisions of Illawarra Regional Environmental Plan No 1 to allow development on hazard prone land.

• November 1990 – DP report (Project 14057) on geotechnical investigation, Lot 376. This report suggests that the block may be part of an existing ancient landslide probably triggered by marine erosion at the toe of the slope. The size and straightness of the large trees suggested that the instability took place in excess of 100 years before present. The report further recognises that minor instability has taken place in the lower 20 m of the slope and may have been related to the development of the lower part of the subdivision when road works and services were installed. The report concludes that the site has a medium risk of instability;

• May 1991 – Golder Associates (GA) report on a building application for Lot 376 comprising a critical review of previous reports, in particular the DP report (14057). The GA report concludes that the Lot should be classed as having a high risk of instability at that present time but also states that DP suggested remedial measures should result in the site having at best, a medium risk of instability;

• July 1991 – DP report (14057A) on Lot 376 is an update of the previous DP report noted above. A further three boreholes were drilled on the lot to supplement the initial investigation. The report states that the lot is presently stable but that increases in groundwater level would reduce the

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factor of safety. Recommendations relating to groundwater control and measures to isolate the block from potential movement on adjacent blocks comprising deep drainage trenches and sub-horizontal drainage holes were suggested;

• August 1991 – GA borehole drilled in rear garden of 30 Tallwood Avenue directly above Lot 376 – the bore was terminated at 11.65 m. SCC records indicate that the borehole was ‘squeezing’ at 5.8 m depth and was abandoned at 11.65 as driller was unable to recover a broken auger. No rock was encountered.

• September 1991 – DP report (Project 14057B) details various changes to design of drainage measures and the design of a catch fence for the proposed property on Lot 376;

• September 1993 – DP report (19404) on Lots 374 and 375 – This report suggests that the site forms the termination ends of two landslips which were caused by marine erosion at the toe of the slope, the instability occurring at least 100 to 120 years before the time of reporting due to the size and straightness of the large trees. Minor superficial instability is noted by the presence of smaller trees with curved trunks. The report concludes that “the site is currently stable but increases in groundwater level may reduce the factor of safety to an unacceptable level’. The report also recommends that appropriate groundwater and surface water runoff measures be incorporated into the design of the development and that gabion baskets be installed to add weight to and support the toe of the slope;

• June 2000 – PMA Consulting Geotechnical Assessment report - Lot 373 – This short report assigns a medium risk of instability to the lot but does not mention any of the previous, well documented history of instability in the area;

• January 2003 – Southern Geotechnics (SG) report on proposed extension to residence at Lot 371 – this report concludes that there is a medium risk of instability associated with the lot;

• April 2003 – Cottier & Associates (CA) Geotechnical Assessment – Lot 364. This report notes the previous history of the site and instability relating to the construction of the retaining wall and stabilising works ordered by Council as a result of the instability. Persistent ground water issues as previously noted by CP are mentioned together with recommendations on sub soil drainage and excavation support;

• January 2011 – DP report (Project 48920) on Geotechnical Assessment for proposed residence – Lot 373 – This report comprised a review of the various DP and CP reports relating to Lot 373 and adjacent lots and looks at the general history and instability in the Surfers Avenue area. The report considers the various types of slope stability hazard affecting the site at the time of reporting. The report makes a number of hazard reduction and precautionary works recommendations and also notes that these works ‘will not provide protection from deep seated instability, the main trigger of which are likely to be toe erosion which might be accelerated by climatic change and associated sea level rise’. A commentary on acceptable and tolerable risk based on AGS guidelines and the persons responsible for this ie Council. It indicates an acceptable risk level for slope instability within the lot following remedial works and a tolerable risk level for a deep seated landslide triggered by conditions outside of the control of the property owner.

Relevant borehole and test pit records have been extracted from the above geotechnical reports, the locations of which are displayed on Drawing 1. These records have been used to review the previous geological models and compile a current model for stability analysis.

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4. Site Observations

A geomorphological mapping exercise was carried out on 9 November 2011 by a Senior Engineering Geologist in order to record and interpret the various geological and environmental features on site. At the time of the site inspection, the weather was dry, however, there had been heavy rainfall during the preceding 24 hours. The site conditions and geomorphological features are shown in Drawing 2 and relevant photographs are included in Appendix C. A summary of the geomorphological features indicates that:

• There has been historic deep seated mass movement of the whole hillside predating the growth of mature trees ie. in excess of 100 – 120 years. This mass movement appears to extend from well into the public reserve at the south-eastern end of the site through to the public reserve at the north-western end of the site and is bounded by the foreshore and the property boundaries of the rear gardens of Tallwood Ave and Bannister Head Road.

• The deep seated mass movement may be split into four distinct slip zones, as identified on Drawing 2 and denoted as Slip Zones 1 to 4. The most recently active slip being Slip Zone 3 which is characterised by the lobe of material extending onto the foreshore comprising numerous silcrete boulders and beach sand. Two older zones, Zones 1 and 2 are recorded to the south of Zone 3 with Zone 4 lying to the north, straddling Surfers Avenue. It is suggested that due to the age of these features, the three outer lobes have long since been removed by marine erosion at the toe whereas Zone 3 is relatively recent and is still undergoing active toe erosion and denudation.

• At its highest point the back-scarp relating to this mass movement is in the order of some 20 m in height and lies at an angle of up to 51 degrees, the crest of which is at approximately RL 40 relative to Australian Height Datum (AHD).

• There is a distinctive flatter benched area at the toe of the main back scarp from approximately RL 10 to RL 20. A further benched area lies towards the toe of the slope and comprises the foreshore and reworked area including the turning circle at the end of Surfers Avenue.

• Ongoing creep appears to be occurring at a very slow rate on the foreshore and in the central area of Surfers Avenue, evidenced by cracking and distortion of road surfaces and driveways at various locations within the site.

• Smaller scale superficial landslips are recorded throughout the site, many of which relate to those described in the historical records. Some of these landslips relate to excavation works carried out during property development within the area and affect the over-steepened back-scarps which continue to show downslope movement and surface unravelling.

• Several groundwater issues are recorded on the slopes, some of which have been piped and fed into roadway drainage systems, or run directly into the sea on the foreshore.

• Varying ages of vegetation ranging from mature trees to young saplings, some showing signs of downslope movement with either leaning or bent trunks. Younger trees and saplings within areas of the public reserve at the north and south ends of the site, and a lack of older ground vegetation suggests that there has been recent superficial slumping, perhaps within the past 20 to 30 years.

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• There is evidence of downslope movement in some of the engineered properties and landscaped areas which has necessitated installation of stabilisation measures such as gabion walls, crib walls and timber post and board retaining structures. Some of these features display signs of slope movement such as cracking and rotation of retaining walls and opening up of driveway slab joints, notably that recorded in Lot 375, where up to 50 mm of extension is recorded.

• Damage to, and distortion of the road pavement and kerb lines adjacent to Lot 364 at the western end of Surfers Avenue, suggests that there is ongoing creep along a potential deep seated failure plane which may extend to the foreshore below Lots 318 and 319. Settlement and cracking of the patio on Lot 319 was noted.

It should be noted that mapping of geomorphological features in developed areas relies to some degree on inference and experience. In this regard, previous landscaping, excavation works and filling may obscure natural features and alternatively, in rare cases, features may become more obvious. To this end the mapping exercise attempts to differentiate and exclude man made features which are not directly related to geomorphological processes. 5. Stability Assessment

5.1 Previous Stability Assessments

A number of geotechnical investigations have been carried out on various lots within the Surfers Avenue precinct. Very few of these investigations include detailed investigation and computational stability analysis. A summary of investigations and details of previous stability analysis are summarised in Table 1 (following page) with computed factors of safety where applicable. The various stability analyses recorded in Table 1 and summarised in the most recent report of January 2011, provides an in depth qualitative assessment in line with AGS guidelines on slope risk management. It is often easy for slope risk assessment to be based predominantly on quantitative assessments ie those derived from calculations and analysis based on limited knowledge of the ground model and little or no instrumentation and monitoring data. In the case of the Surfers Avenue precinct, a history of instability related investigation has been carried out to fulfil property development requirements. Much of this investigation has been of the shallow or near-surface type, however, as risks have become more apparent, often following slope failures, justifiable Council concerns and the development of more rigorous guidelines, property developers have been required to address the risks in a more detailed manner.

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Table 1: Summary of historic stability analysis

Date Author Lot No Factor of Safety Comments

May 1981 Coffey 373 N/A Five hand augers, maximum depth 1.90 m

July 1981 Coffey 365 N/A Two test pits, four dynamic cone penetration tests (DCP), maximum depth 1.50 m

January 1982 Coffey 373 N/A Two test pits, maximum depth 3.50 m August 1983 Golder 372 N/A Two test pits, maximum depth 3.40 m November

1984 HPA 373 N/A No logs provided in report

October 1987 Coffey 364 N/A Four logs of existing excavations on partially failed face.

November 1990 DP 376 1.4 – 1.2

Two bores, maximum depth 10.10 m, includes sensitivity analysis, worst case scenario driven by ground water changes.

May 1991 Golder 376 N/A Review of DP report – no additional bores

July 1991 DP 376 1.4 – 1.2

Three additional bores to prove rock – rock head proven, piezometers installed, max depth 12.0 m. Likely FOS 1.4, worst case scenario 1.2.

August 1991 Golder 376 & 350 N/A

Failed attempt to drill deep bore on crest of slope behind 30 Tallwood Avenue. Maximum depth 11.65 m. Not clear whether piezometer successfully installed.

September 1991 DP 376 N/A Catch fence design changes.

September 1993 DP 374 &

375 1.5 – 1.3 Four bores, maximum depth 23.90 m, sandstone proven at 16.0 m depth, artesian ground water recorded.

June 2000 PMA 373 N/A No intrusive investigation carried out. January 2003 SG 371 N/A One test pit, maximum depth 3 m.

April 2003 CA 364 N/A Four test pits, maximum depth 4 m.

January 2011 DP 373 Qualitative

Detailed qualitative risk assessment carried out; moderate risk relating to large scale deep seated failures, low risk relating to various shallow failures after proposed remedial works.

The most detailed ground model to date, relating to development of Lot 376, was initially developed by DP during the early 1990’s, underwent review by GA at the request of Council and was updated with ground information from additional deeper boreholes, which improved understanding of the ground model, rock head level and associated risks. Similarly, investigation and analysis of the two adjoining lots, Lots 374 and 375 included deeper boreholes which proved rock and provided information on groundwater levels, which in this case, were recorded at the time as being artesian in nature. Computational analysis derived a factor of safety ranging from 1.5 to 1.3 in the worst case scenario where the soil exhibited no cohesion and the water table was at ground surface.

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Previous assessments suggest that the slopes carry a moderate risk of instability as summarised in Table 2, based on information extracted from DP Project 48920 relating to Lot 373 and amended here to cover the area in general: Table 2: Slope Risk Assessment for property development – Lot 373

Hazard Likelihood Consequence to Property Risk

Extremely slow soil creep in steep and very

steep slopes

Likely but localised and controlled by site

development including soil cover and drainage

improvements

Insignificant for engineered structures designed to resist soil

creep

Low

Rapid surficial soil slumps or shallow rotational failures

Unlikely after site development including

drainage and stabilisation measures

Minor to Medium Low

Slow, intermediate depth failure Rare Major Low

Deep seated rotational failure Unlikely to Rare Major to Catastrophic Moderate

5.2 Current Stability Modelling

Computational stability analyses were run on a ground model derived from previous reports. The analyses were carried out using GeoStudio 2007 Slope/W program and investigated variations in water level, material parameters and slip geometries. Modern slope stability analysis programmes such as Slope W are able to carry out multiple iterations and model various slip geometries with reasonable ease in comparison to older programmes. The analyses show that marginal factors of safety are inherent within the slopes. The contemporary FOS are well below those previously computed and suggest that the original analyses and programmes which generated them were over optimistic. In order to generate analyses in which the slopes had a FOS of above 1, the critical soil strength parameters had to be increased beyond normally recognised values in order to prevent the slope from undergoing deep seated failure during modelling. Three dimensional relationships may explain why the slopes are currently standing even though the modelling suggests otherwise or the higher strength parameters may be real, possibly being due to a certain degree of cementation having developed over the millions of years that the Tertiary aged sediments have been in place Extensive modelling and experience of similar slopes, indicates that elevated ground water levels and removal of toe weight by marine erosion will significantly reduce the FOS for the slopes. Also, long term softening of the very stiff clays within the hillside will have a significant effect on overall stability Sample stability analysis results are included in Appendix D. It should be noted that the stability analyses carried out for this report relate to a ground model and profile near to, but not at the highest and steepest point along the extensive back scarp. The highest

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point on the back scarp is located in the rear garden of Lot 349 (32 Tallwood Avenue) and lies within the area of affected by Slip Zone 2 rather than Slip Zone 3. 5.3 Future Instability

Evidence recorded in reports from the past 40 years indicates that accelerated toe erosion and changes in ground water level have triggered further movement of the deep seated slip in Zone 3. Gradual creep of this slip mass is evident and is expected to continue in the future. Ongoing erosion and water infiltration together with softening of the soil mass will exacerbate this problem. There is likely to be ongoing deep seated creep of the soil mass referred to as Slip Zone 2 and although this does not directly affect property located on the slope at present, removal of support to the steepest and highest section of the back scarp may result in the propagation of slips which could potentially extend into the backyards of properties along Tallwood Avenue. Excavation works in Slip Zone 4 have previously triggered failures which have encroached on the rear gardens and properties of Tallwood Avenue. These appear to have been stabilised in the short term but may suffer from ongoing creep or catastrophic deep seated failure in the longer term. The slope instability seen on this site is difficult to monitor and quantify by visual means, and is often only seen when significant movement has occurred. Limited instrumentation in the form of piezometers, insufficient long-term monitoring and a lack of slope movement instrumentation means that accurate modelling of the major slip blocks is difficult and ongoing movement, with potential to cause catastrophic failure is very difficult to predict. The effects of climate change, including sea level rise, elevated ground water levels, more frequent storm events and accelerated coastal erosion will all contribute to ongoing stability issues in the area and increase the potential for catastrophic failures to occur. Modifications to and increases in surface water runoff and domestic drainage systems will only add to the potential for groundwater changes within the area. Progressive failure beyond the existing back scarp, with potential to affect the properties of Tallwood Avenue, Bannister Head Road and Surfers Avenue cannot be ruled out and as such the majority of the properties are at risk to some degree. 6. Comments & Recommendations

A review of historical geotechnical records together with detailed geotechnical mapping of the area in question indicates that there has been significant instability in the distant past which has affected much of the land subject to this review, with continuing intermittent, though generally minor instability. There is potential for further instability to occur which may be caused by both natural processes and/or human intervention. Encroachment of slips towards the properties of Tallwood Avenue and Bannister Head Road cannot be ruled out and water infiltration into the ground surrounding these lots has potential to cause instability if not adequately controlled.

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Gradual creep of the deep seated slip masses, if it is occurring, may affect the properties located on these lots and lead to loss of support to the land above the main back scarp which would have the potential to affect the adjoining lots. It is recommended that all the lots within the precinct be zoned as 2a(4) with regards to restrictions on development resulting from potential land instability. Furthermore it is recommended that ground water and ground movement instrumentation, such as inclinometers, be installed in the high risk areas on public land and monitored over the long-term to improve Council’s understanding of the processes, locate possible shear planes and provide rates of movement on these planes. Similar monitoring regimes should also be a requirement of development applications on private land in order to provide warning to property owners of potential problems prior to development or redevelopment taking place. Areas of similar topography and geology (ie steep slopes developed in Tertiary aged sediments) outside of the Surfers Avenue Precinct may also be at risk from mass movement as described above. It would be beneficial from a risk management perspective to review such areas in light of the above findings. 7. Limitations

Douglas Partners Pty Ltd (DP) has prepared this report for this project at Surfers Avenue Precinct, Narrawallee in accordance with DP’s proposal dated 30 September 2011 and acceptance received from Warwick Papworth dated 24 October 2011. The work was carried out under DP’s Conditions of Engagement dated 30 September 2011. This report is provided for the exclusive use of Shoalhaven City Council for this project only and for the purposes as described in the report. It should not be used by or relied upon for other projects or purposes on the same or other site or by a third party. In preparing this report DP has necessarily relied upon information provided by the client and/or their agents. The results provided in the report are indicative of the sub-surface conditions on the site only at the specific mapping and previous sampling locations, and then only to the depths investigated and at the time the work was carried out. Sub-surface conditions can change abruptly due to variable geological processes and also as a result of human influences. Such changes may occur or have occurred after the field testing was completed. DP’s advice is based upon the conditions encountered during this investigation. The accuracy of the advice provided by DP in this report may be affected by undetected variations in ground conditions across the site between and beyond the mapping and previous sampling locations.

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This report must be read in conjunction with all of the attached and should be kept in its entirety without separation of individual pages or sections. DP cannot be held responsible for interpretations or conclusions made by others unless they are supported by an expressed statement, interpretation, outcome or conclusion stated in this report. This report, or sections from this report, should not be used as part of a specification for a project, without review and agreement by DP. This is because this report has been written as advice and opinion rather than instructions for construction.

Douglas Partners Pty Ltd

Appendix A

About this Report

July 2010

Introduction These notes have been provided to amplify DP's report in regard to classification methods, field procedures and the comments section. Not all are necessarily relevant to all reports. DP's reports are based on information gained from limited subsurface excavations and sampling, supplemented by knowledge of local geology and experience. For this reason, they must be regarded as interpretive rather than factual documents, limited to some extent by the scope of information on which they rely. Copyright This report is the property of Douglas Partners Pty Ltd. The report may only be used for the purpose for which it was commissioned and in accordance with the Conditions of Engagement for the commission supplied at the time of proposal. Unauthorised use of this report in any form whatsoever is prohibited. Borehole and Test Pit Logs The borehole and test pit logs presented in this report are an engineering and/or geological interpretation of the subsurface conditions, and their reliability will depend to some extent on frequency of sampling and the method of drilling or excavation. Ideally, continuous undisturbed sampling or core drilling will provide the most reliable assessment, but this is not always practicable or possible to justify on economic grounds. In any case the boreholes and test pits represent only a very small sample of the total subsurface profile. Interpretation of the information and its application to design and construction should therefore take into account the spacing of boreholes or pits, the frequency of sampling, and the possibility of other than 'straight line' variations between the test locations. Groundwater Where groundwater levels are measured in boreholes there are several potential problems, namely: • In low permeability soils groundwater may

enter the hole very slowly or perhaps not at all during the time the hole is left open;

• A localised, perched water table may lead to an erroneous indication of the true water table;

• Water table levels will vary from time to time with seasons or recent weather changes. They may not be the same at the time of construction as are indicated in the report; and

• The use of water or mud as a drilling fluid will mask any groundwater inflow. Water has to be blown out of the hole and drilling mud must first be washed out of the hole if water measurements are to be made.

More reliable measurements can be made by installing standpipes which are read at intervals over several days, or perhaps weeks for low permeability soils. Piezometers, sealed in a particular stratum, may be advisable in low permeability soils or where there may be interference from a perched water table. Reports The report has been prepared by qualified personnel, is based on the information obtained from field and laboratory testing, and has been undertaken to current engineering standards of interpretation and analysis. Where the report has been prepared for a specific design proposal, the information and interpretation may not be relevant if the design proposal is changed. If this happens, DP will be pleased to review the report and the sufficiency of the investigation work. Every care is taken with the report as it relates to interpretation of subsurface conditions, discussion of geotechnical and environmental aspects, and recommendations or suggestions for design and construction. However, DP cannot always anticipate or assume responsibility for: • Unexpected variations in ground conditions.

The potential for this will depend partly on borehole or pit spacing and sampling frequency;

• Changes in policy or interpretations of policy by statutory authorities; or

• The actions of contractors responding to commercial pressures.

If these occur, DP will be pleased to assist with investigations or advice to resolve the matter.

July 2010

Site Anomalies In the event that conditions encountered on site during construction appear to vary from those which were expected from the information contained in the report, DP requests that it be immediately notified. Most problems are much more readily resolved when conditions are exposed rather than at some later stage, well after the event. Information for Contractual Purposes Where information obtained from this report is provided for tendering purposes, it is recommended that all information, including the written report and discussion, be made available. In circumstances where the discussion or comments section is not relevant to the contractual situation, it may be appropriate to prepare a specially edited document. DP would be pleased to assist in this regard and/or to make additional report copies available for contract purposes at a nominal charge. Site Inspection The company will always be pleased to provide engineering inspection services for geotechnical and environmental aspects of work to which this report is related. This could range from a site visit to confirm that conditions exposed are as expected, to full time engineering presence on site.

July 2010

Description and Classification Methods The methods of description and classification of soils and rocks used in this report are based on Australian Standard AS 1726, Geotechnical Site Investigations Code. In general, the descriptions include strength or density, colour, structure, soil or rock type and inclusions. Soil Types Soil types are described according to the predominant particle size, qualified by the grading of other particles present:

Type Particle size (mm) Boulder >200 Cobble 63 - 200 Gravel 2.36 - 63 Sand 0.075 - 2.36 Silt 0.002 - 0.075 Clay <0.002

The sand and gravel sizes can be further subdivided as follows:

Type Particle size (mm) Coarse gravel 20 - 63 Medium gravel 6 - 20 Fine gravel 2.36 - 6 Coarse sand 0.6 - 2.36 Medium sand 0.2 - 0.6 Fine sand 0.075 - 0.2

The proportions of secondary constituents of soils are described as:

Term Proportion Example And Specify Clay (60%) and

Sand (40%) Adjective 20 - 35% Sandy Clay Slightly 12 - 20% Slightly Sandy

Clay With some 5 - 12% Clay with some

sand With a trace of 0 - 5% Clay with a trace

of sand

Definitions of grading terms used are: • Well graded - a good representation of all

particle sizes • Poorly graded - an excess or deficiency of

particular sizes within the specified range • Uniformly graded - an excess of a particular

particle size • Gap graded - a deficiency of a particular

particle size with the range Cohesive Soils Cohesive soils, such as clays, are classified on the basis of undrained shear strength. The strength may be measured by laboratory testing, or estimated by field tests or engineering examination. The strength terms are defined as follows:

Description Abbreviation Undrained shear strength

(kPa) Very soft vs <12 Soft s 12 - 25 Firm f 25 - 50 Stiff st 50 - 100 Very stiff vst 100 - 200 Hard h >200

Cohesionless Soils Cohesionless soils, such as clean sands, are classified on the basis of relative density, generally from the results of standard penetration tests (SPT), cone penetration tests (CPT) or dynamic penetrometers (PSP). The relative density terms are given below:

Relative Density

Abbreviation SPT N value

CPT qc value (MPa)

Very loose vl <4 <2 Loose l 4 - 10 2 -5 Medium dense

md 10 - 30 5 - 15

Dense d 30 - 50 15 - 25 Very dense

vd >50 >25

July 2010

Soil Origin It is often difficult to accurately determine the origin of a soil. Soils can generally be classified as: • Residual soil - derived from in-situ weathering

of the underlying rock; • Transported soils - formed somewhere else

and transported by nature to the site; or • Filling - moved by man. Transported soils may be further subdivided into: • Alluvium - river deposits • Lacustrine - lake deposits • Aeolian - wind deposits • Littoral - beach deposits • Estuarine - tidal river deposits • Talus - scree or coarse colluvium • Slopewash or Colluvium - transported

downslope by gravity assisted by water. Often includes angular rock fragments and boulders.

July 2010

Rock Strength Rock strength is defined by the Point Load Strength Index (Is(50)) and refers to the strength of the rock substance and not the strength of the overall rock mass, which may be considerably weaker due to defects. The test procedure is described by Australian Standard 4133.4.1 - 1993. The terms used to describe rock strength are as follows:

Term Abbreviation Point Load Index Is(50) MPa

Approx Unconfined Compressive Strength MPa*

Extremely low EL <0.03 <0.6

Very low VL 0.03 - 0.1 0.6 - 2

Low L 0.1 - 0.3 2 - 6

Medium M 0.3 - 1.0 6 - 20

High H 1 - 3 20 - 60

Very high VH 3 - 10 60 - 200

Extremely high EH >10 >200 * Assumes a ratio of 20:1 for UCS to Is(50)

Degree of Weathering The degree of weathering of rock is classified as follows:

Term Abbreviation Description Extremely weathered EW Rock substance has soil properties, i.e. it can be remoulded

and classified as a soil but the texture of the original rock is still evident.

Highly weathered HW Limonite staining or bleaching affects whole of rock substance and other signs of decomposition are evident. Porosity and strength may be altered as a result of iron leaching or deposition. Colour and strength of original fresh rock is not recognisable

Moderately weathered

MW Staining and discolouration of rock substance has taken place

Slightly weathered SW Rock substance is slightly discoloured but shows little or no change of strength from fresh rock

Fresh stained Fs Rock substance unaffected by weathering but staining visible along defects

Fresh Fr No signs of decomposition or staining Degree of Fracturing The following classification applies to the spacing of natural fractures in diamond drill cores. It includes bedding plane partings, joints and other defects, but excludes drilling breaks.

Term Description Fragmented Fragments of <20 mm Highly Fractured Core lengths of 20-40 mm with some fragments Fractured Core lengths of 40-200 mm with some shorter and longer sections Slightly Fractured Core lengths of 200-1000 mm with some shorter and loner sections Unbroken Core lengths mostly > 1000 mm

July 2010

Rock Quality Designation The quality of the cored rock can be measured using the Rock Quality Designation (RQD) index, defined as:

RQD % = cumulative length of 'sound' core sections ≥ 100 mm long total drilled length of section being assessed

where 'sound' rock is assessed to be rock of low strength or better. The RQD applies only to natural fractures. If the core is broken by drilling or handling (i.e. drilling breaks) then the broken pieces are fitted back together and are not included in the calculation of RQD. Stratification Spacing For sedimentary rocks the following terms may be used to describe the spacing of bedding partings:

Term Separation of Stratification Planes Thinly laminated < 6 mm Laminated 6 mm to 20 mm Very thinly bedded 20 mm to 60 mm Thinly bedded 60 mm to 0.2 m Medium bedded 0.2 m to 0.6 m Thickly bedded 0.6 m to 2 m Very thickly bedded > 2 m

Appendix B

Drawings

Appendix C

Photographs

Photo 1 – View of entrance to Lot 373 showing cracking to wall and water issues at toe of slope

Photo 2 – Lot 374 & 375 Gabion retaining wall at toe of slope and seepage issues

Project No 78319 Site Photographs – Plate 1

Date: December 2011

Surfers Avenue Stability Assessment Client: Shoalhaven City Council

Photo 1 – View of Lot 364 retaining wall and damage to road surface of Surfers Avenue

Photo 2 – View of Surfers Avenue adjacent to Lot 364 showing distortion to kerb line and road surface


Date: December 2011


Photo 1 – View down driveway of Lot 374 & 375 showing opening of driveway slab joint (~50 mm)

Photo 2 – View of active erosion on back scarp at rear of Lot 376 & public reserve


Date: December 2011


Photo 1 – View along foreshore showing active toe erosion

Photo 2 – View of north west of site showing leaning trees and silcrete boulder deposits against trees


Date: December 2011


Photo 1 – Ground water issues on foreshore

Photo 2 – Weathered sandstone exposure on foreshore showing signs of active erosion


Date: December 2011


Photo 1 – View of back scarp at rear of Lots 349 & 350, Tallwood Avenue - note bent and leaning trees

Photo 2 – View of back scarp above public reserve showing soil creep and leaning tree trunks


Date: December 2011


Appendix D

Results of Slope Stability Analysis

1

2

3

4

5

67

8

91011

121314

15

16

1718

19

20

21

22

23

24

2526

27

28 29

3031

32

3334

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0

Sandy Clay - Very Stif f (9)

19 kN/m³

15 kPa

25 °

16 kN/m³

1

Clay - Very Stiff (8)

20 kN/m³

5 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1

Sandstone Boulders (5)

21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1

Sandy Clay - Firm to Stiff (3)

19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Ground Model - Measured Water Level

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Ele

va

tion

-20

-10

0

10

20

30

40

50

0.859

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

15 kPa

25 °

16 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1


21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1


19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Slip Analysis - Measured Water Level

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Elevation

-20

-10

0

10

20

30

40

50

1

2

3

4

5

67

8

91011

121314

15

16

1718

19

20

21

22

23

24

2526

27

28 29

3031

32

3334

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

15 kPa

25 °

16 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1


21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1


19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Ground Model - Measured Water Level raised by 7m

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Ele

va

tion

-20

-10

0

10

20

30

40

50

0.625

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

15 kPa

25 °

16 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1


21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1


19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Slip Analysis - Measured Water Level raised by 7m

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Ele

va

tion

-20

-10

0

10

20

30

40

50

1

2

3

4

5

67

8

91011

121314

15

16

1718

19

20

21

22

23

24

2526

27

28 29

3031

32

3334

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0

Sandy Clay - Very Stiff (9)

19 kN/m³

15 kPa

25 °

16 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1


21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1


19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Ground Model - Measured Water Level Lowered by 6m

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Ele

va

tion

-20

-10

0

10

20

30

40

50

1.095

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

15 kPa

25 °

16 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1


21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1


19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Slip Analysis - Measured Water Level Lowered by 6m

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Ele

va

tion

-20

-10

0

10

20

30

40

50

1

2

3

4

5

67

8

91011

121314

15

16

1718

19

20

21

22

23

24

2526

27

28 29

3031

32

3334

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

0 kPa

25 °

16 kN/m³

1


20 kN/m³

0 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

0 kPa

20 °

16 kN/m³

1


19 kN/m³

0 kPa

27 °

16 kN/m³

1


21 kN/m³

0 kPa

27 °

20 kN/m³

1


20 kN/m³

0 kPa

20 °

17 kN/m³

1


19 kN/m³

0 kPa

15 °

16 kN/m³

1


20 kN/m³

0 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Ground Model - Long Term Scenario (c'=0 kPa).Forced slip through toe of slope.Measured Water Level.

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Ele

va

tion

-15

-10

-5

0

5

10

15

20

25

30

35

40

45

50

0.733

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

0 kPa

25 °

16 kN/m³

1


20 kN/m³

0 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

0 kPa

20 °

16 kN/m³

1


19 kN/m³

0 kPa

27 °

16 kN/m³

1


21 kN/m³

0 kPa

27 °

20 kN/m³

1


20 kN/m³

0 kPa

20 °

17 kN/m³

1


19 kN/m³

0 kPa

15 °

16 kN/m³

1


20 kN/m³

0 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Slip Analysis - Long Term Scenario (c'=0 kPa).Forced slip through toe of slope.Measured Water Level.

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Elevation

-15

-10

-5

0

5

10

15

20

25

30

35

40

45

50

0.925

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0

Sandy Clay - Very Sti ff (9)

19 kN/m³

15 kPa

25 °

16 kN/m³

1

Clay - Very Sti ff (8)

20 kN/m³

5 kPa

20 °

17 kN/m³

1

Col luvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1 Sandstone Boulders (5)

21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

11 kPa

20 °

17 kN/m³

1 Sandy Clay - Firm to Sti ff (3)

19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Measured Water Level (Layer 4, c' = 11 kpa)

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Ele

vati

on

-20

-10

0

10

20

30

40

50

1.004

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

15 kPa

25 °

16 kN/m³

1


20 kN/m³

5 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1


21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

21 kPa

20 °

17 kN/m³

1


19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Measured Water Level (Layer 4, c' = 21 kpa)

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Elevation

-20

-10

0

10

20

30

40

50

0.985

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

15 kPa

25 °

16 kN/m³

1


20 kN/m³

15 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1


21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

15 kPa

20 °

17 kN/m³

1


19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Measured Water Level (Layers 4 & 8, c' = 15kpa)

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Elevation

-20

-10

0

10

20

30

40

50

1.015

RL34

RL31

RL36

RL29

RL27

RL24

RL17RL16

RL7RL6

RL4

RL2RL0


19 kN/m³

15 kPa

25 °

16 kN/m³

1


20 kN/m³

20 kPa

20 °

17 kN/m³

1

Colluvium (7)

19 kN/m³

10 kPa

20 °

16 kN/m³

1


19 kN/m³

15 kPa

27 °

16 kN/m³

1


21 kN/m³

40 kPa

27 °

20 kN/m³

1


20 kN/m³

20 kPa

20 °

17 kN/m³

1


19 kN/m³

5 kPa

15 °

16 kN/m³

1


20 kN/m³

5 kPa

25 °

17 kN/m³

1

Sandstone (1)

21 kN/m³

50 kPa

30 °

20 kN/m³

1

Measured Water Level (Layers 4 & 8, c' = 20 kpa)

Distance

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Ele

va

tion

-20

-10

0

10

20

30

40

50

report on various lots prepared for project 78319 december

Documents