Mitchell Freeway Extension

Background

CMW was engaged as sub-consultant to Aurecon for a package of work for Main Roads WA which included a 12km extension of the Mitchell Freeway north of Perth from Burns Beach Road to Romeo Road, a 3km extension of Neerabup Road, various underpasses, intersections and interchanges as part of the overall scheme.

Scope

CMW carried out a geotechnical investigation that incorporated the following:

Supervision of drilling of 43 boreholes up to 20m deep;
Supervision of 36 Cone Penetration Test (CPT) probes and 9 Dilatometer tests up to 15m deep;
Undertaking of 2 ripping trials up to 4m deep;
Preparation of geotechnical cross sections showing the distribution of the various sub-surface formations;
Provide details of soil/rock parameters recommended for design of shallow foundations and retaining walls, type of recommended foundation, settlement analysis including soil/rock parameters and relationships used;
Safe slope angles for all cut and fill slopes including a statement of the factors of safety of each slope and output from stability analyses.

Findings/Challenges

The investigation and reporting was required over a relatively short time frame and, as such, multiple drilling rigs were required to ensure timely completion. In addition, access and environmental constraints, related to protected flora, had to be carefully considered when planning and executing the investigation.

Potential geohazards that were identified for this development were material variability, loose sand zones, pinnacle limestone and karst features.

These hazards had to be carefully considered in foundation and cut slope design recommendations. Practical advice for earthworks, subgrade preparation, stormwater management and construction process were provided to help mitigate the risks associated with the geohazards identified.