Design, excavation and performance of rock cuttings on the Karratha to Tom Price Road, Stage 2

Sean Eyre and John McDermott

Construction of the Stage 2 Karratha Tom Price Road (KTP2) situated in the Pilbara region of WA, included excavation of about 130 cuttings along the route, 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 Pty Ltd.

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. Rock fall modelling, which was calibrated based on the review of the adjacent rail cuttings and confirmed by rock rolling trials just prior to opening the road to the public, indicated that less than 1% of rocks would reach the road. These rock fall “escape-rates” were included in the QRA, which indicated the risk to life associated with rock falls along the road would be acceptable.

The ground conditions made for very difficult cleanup conditions, and assessment of the “condition and quality” of the constructed cut faces. The rock instability mechanisms in such ground conditions typically do not comprise textbook wedge, planar sliding or toppling failures conducive to stability calculations. Rather, as the three significant rock falls that occurred during construction and other smaller failures attest, the rock instability that has occurred has resulted from a diverse range of mechanisms, such as ravelling of weak layers underlying massive rock, rolling of rounded rocks from the soil profile of a cutting and sliding failure on uncommon defect sets.

In the two annual stability reviews since the road was opened in August 2008, there have been about 2,000 fallen rocks. Only three rocks travelled far enough to reach the traffic lanes. Performance of the rock slopes since the road was opened to public traffic has been in reasonable accord with the design intent and it is judged that the agreed performance criterion is satisfied.