Great Western Highway Duplication

Our involvement

JK Geotechnics completed detailed slope stability analysis (using Slope/W) to assess temporary works soil nailing requirements (i.e. length and spacing) to support the cuts (up to 16.0m deep).  JKG also completed detailed analysis of the three permanent RE walls (external stability only) in accordance with the project specific RMS QA Specification R57.  The three walls were up to 160m long and a maximum of about 16m high.  Analysis was completed to assess global stability (using Slope/W) as well as overturning and sliding using conventional calculation methods and limit state factors.

Project challenges

The deep excavations were required immediately adjacent to the existing carriageway to allow for construction of new RE walls.  These cuts were up to 16m deep through highly weathered Bankswall Formation sandstone.  The analysis needed to consider all excavation stages prior to construction of the RE walls commencing.

During our analysis of the permanent RE walls, issues were discovered with the global stability for a number of the wall sections when more detailed survey information of the area downhill of the retaining walls was made available.  The survey showed the area downhill of the walls to be steeper than that adopted for initial design.  This steeper slope resulted in it not being possible to achieve the required factor of safety for the RE walls.

Our solutions

The results of our numerical analyses enable the selection of a temporary soil nail solution for the deep temporary excavations in consultation with our Client.  The designed soil nails ranged from 8m to 15m long on a regular pattern, with reinforced shotcrete adopted to provide weathering protection to the cut faces.

For the permanent RE walls, the design issues associated with the steeper sloping downhill section were resolved by reassessing the material properties for the Bankswall Sandstone unit, and adopting improved material properties for a deeper, better quality, portion of the strata.  Following our analysis using the revised material parameters, the wall designs were accepted by RMS and were constructed in accordance with the design documentation.