Highway Rock Cuttings

Brief

Our client awarded a contract to construct a dual four-lane highway totalling 41 kilometres.  The design was completed by the Employer with the detailed design of geotechnical works, including cutting slopes, falling under the responsibility of the appointed contractor.

After the commencement of the excavation, some elements of the rock cuttings became unstable, and collapse was noted at two locations.  A review of the as-encountered ground conditions and cutting design determined that other cuttings were likely to experience the same instability.  The contractor issued claims on a variety of issues including the requirement to excavate rock cuttings at a shallower angle, and the requirement for blasting exceeding that which could be determined from the initial site investigation provided at tender.

HKA was retained to provide geotechnical engineering services in arbitration, detailing opinion on the cause of instability and adequacy of the contractor’s site investigation and design methodology.

What we did

HKA conducted an assessment, including the appraisal of the contractor’s interpretation of ground conditions and stability calculations, with respect to established rock mechanics principles.  Additionally, an assessment of the pre-tender site investigation was undertaken to reassess the likely volume of rock that required blasting during excavation.

The investigation concluded that, in our opinion, the contractor had employed methods used in soil slope design and incorrectly applied this to the rock slopes.  Further, it was opined that the contractor had insufficiently researched the in-situ rock structure and accounted for this in the slope design.  Our expert’s opinion was that the actual blasting volume was predictable from the pre-tender information.

Outcomes

HKA’s expert analysis allowed the parties to understand the mechanism controlling the stability of the cuttings and how it was that this was not accounted for in the design, resulting in the consequential inevitability of the failures.