Western Australia Chapter

AGS Young Geotechnical Professionals Evening Seminar and the 9th Dr Baden Clegg Award

Various speakers

This annual event gives a platform for our younger (under 35) geo-professionals to present their work, gain feedback from experienced colleagues and become more involved in AGS activities. Please support them by coming along to what should be a varied, interesting and exciting evening. There will be four presentations, each 10 minutes long, with 5 minutes for questions after each talk. The Dr. Baden Clegg Award will be awarded to the best contribution.

John McDermott

Design, Excavation and Performance of Rock Cuttings on the Karratha to Tom Price Road, Stage 2

Construction of the Stage 2 Karratha Tom Price Road (KPT2) included excavation of about 130 cuttings along, which total about 18.5 km in length. Design and Construction was undertaken by the Millstream Link Alliance, which is a “pure alliance” that comprises Main Roads Western Australia, MacMahon Contractors Pty Ltd, Coffey International Pty Ltd and GHD Group Pty Ltd.

A total engineering geology approach was adapted to the design of cuttings, which focussed on understanding the geology rather than “drill first and think later”. Cut slopes, catch ditch widths and bench widths were based on a combination of field observations on existing rail cuttings, geological mapping, modelling of fallen rocks using the computer simulation programme “ROCFALL” and a Quantitative Risk Assessment (QRA). An ongoing design assessment was also applied where after initial excavation had begun, the batters were flattened to suit the exposed ground conditions.

Tim Nash

The Effectiveness of an Impact Roller for Foundation Treatment in Alluvial Sandy Clays

During ground-breaking geotechnical investigations for a heavy haul railway that crosses a section of the Robe River floodplain in the Pilbara region of Western Australia, the upper alluvial soils displayed unusual and concerning behaviour by what appeared to be a dramatic loss of strength when saturated. In general, the geological profile comprises up to 4.0m of sandy clay overlying variably cemented calcrete and gravels which are underlain by basement sandstone.

Preliminary laboratory testing results pointed to a classic ‘Pindan’ type soil with a relatively high void ratio, partial collapse potential and a significant order of magnitude reduction in strength when saturated. This, combined with the investigation observations and the cost of treating up to 14km of foundations, lead to a re-think of traditional deep boxout foundation treatment by assessing the effectiveness of a square dynamic impact roller.

Russel Pol

Piezocone Penetration Testing of Fallow and Operational Tailings Storage Facilities

Piezocone Penetration Tests (PCPTs) on both operational and fallow Tailings Storage Facilities (TSFs) are a useful tool to further increase our understanding of tailings behaviour both during and after deposition. PCPT investigations have been carried out on two TSFs, five years apart. One of the TSF was fallow for the five year period, while the other was operational. Where possible, the PCPTs were carried out adjacent to previous locations so that a comparison of the shear strength and pore pressure profiles could be carried out.

Comparing the results of the piezocone investigations conducted on the fallow TSF, it was identified that the majority of the tailings strength profile remained relatively consistent with depth. However, near the surface, an increase (up to 100%) in the shear strength was observed up to 5 m depth. The strength increase of the tailings near the surface is due to evaporative desiccation, leading to the over-consolidation of the tailings and a subsequent gain in strength. It was also observed that the in situ silty clay material below the tailings had increased in strength, with normal consolidation occurring due to the load applied by the tailings. A consistent hydrostatic pore pressure profile, used to estimate the phreatic surface within the TSF, was not readily identifiable within the fallow TSF for either the 2004 or 2009 investigation.

Gary Webley

A case history for the Southern Seawater Desalination Plant

WA’s Water Corporation announced in 2007 that the Southern Seawater Desalination Plant (SSDP) would be commissioned as the state’s second major source of desalinated water. On completion, the plant at Binningup (35km North of Bunbury) will produce 50 gigalitres of water per year (with the potential to expand to 100 GL). GFWA were contracted by the Southern SeaWater Alliance (SSWA) to construct the diaphragm walls that form the Intake Pump Station (IPS). Construction was carried out between September 2009 and April 2010.

Engineers Australia members participating in AGS technical sessions can record attendance on their personal CPD logs. Members should refer to Engineers Australia CPD policy for details on CPD types, requirements and auditing guidelines.