Shoalhaven City Council (Geotechnical Investigation)

Introduction:

At the request of Shoalhaven City Council (SCC), Terra Insight Pty Ltd (Terra) has carried out geotechnical inspections and subsequently a geotechnical investigation to determine the subsurface conditions along the western shoulder of Bunkers Hill Road where a landslip has occurred a result of an extreme weather event in early 2022. The assessment was undertaken in relation to the proposed stabilisation and reinstatement of the road. The investigation was undertaken with the objective of determining the ground conditions within the slip and determination of:

• Foundation conditions;

• Excavation conditions including the depth to rock, the ease with which it can be excavated and safe batter slopes that can be adopted; and

• Retaining wall design parameters.

The findings contained in this report are the result of discrete/specific methodologies used in accordance with normal practices and standards. Under no circumstances can it be considered that these findings represent the actual state at all points. The subsurface conditions may vary significantly on the other parts of the site, particularly where no nearby sampling and testing work has been carried out. The report is limited to the scope or work and objectives as outlined herein.

Scope of work:

The proposed scope of work for the geotechnical and environmental assessment included the following:

• Inspection of the site;

• Preliminary assessment of the potential causes of the landslides;

• Recommendations for short-term measures to reinstate the roads;

• Recommendations for potential remedial solutions if required;

• Site preparation including:

▪ Review of existing information;

▪ Dial before you dig submission (service clearance was provided by council);and

▪ Safety plan and management of subcontractors such as traffic control and service clearance.

• A Desk top study including review of aerial images and geological mapping;

• A site walkover by a principal geo-environmental engineer;

• Site investigation including

▪ Auguring of at least two (2) boreholes along above the slip to 4.0m depth or refusal in rock, whichever is shallowest and coring to obtain rock core samples, using a drilling rig;

▪ In situ testing (including DCPs) and sampling will be undertaken at regular intervals at each test site; and

▪ Backfilling of the test sites with concrete;

• Geotechnical laboratory investigation including Atterberg Limits and/or gradings.

• Preparation of a report summarising the results of fieldwork, presenting the subsurface conditions encountered, interpreting findings and providing engineering advice relevant to the objectives outlined above. The scope of work was carried out in general accordance with the above, except for the following exceptions:

• As the depth to rock was shallow on the site, three boreholes were augered to depths of 2.85m to 4.05m with borehole BH03 extended to 5.2m depth using coring techniques.