Woolgoolga To Ballina Pacific Highway Upgrade – Reliability Assessment Of Soft Ground Treatment Design
Eleven road sections with an approximately 20 km total length out of the 155 km Pacific Highway Upgrade project between Woolgoolga and Ballina (W2B), NSW traverse areas having significant depths of soft soils. At Maclean Interchange or Clarence River Interchange, soft soil thickness was up to 20 – 25 m under the road alignment. Soft ground treatment design for the identified soft soil areas was undertaken in 2014. The main objectives of the soft ground treatment were to provide certainty of delivery of the highway upgrade within a given time during the main contract with a satisfactory long-term pavement performance.
The highway section between Whytes Lane and Pimlico Road of approximately 3.85 km is one of the longest road sections underlain by up to 8 m thick soft soil that required ground treatment. Due to the significantly length of the soft ground treatment for this road section, one of the main objectives was to reduce or optimise the cost of soft ground treatment.
During the detailed design stage, soft ground treatments using preloading with or without Prefabricated Vertical Drains (PVD) were considered. Due to issues such as sample disturbance during soil sampling and transporting, limitations of the adopted soil testing methods and equipment, limitations of the available geotechnical investigation information, there was a possibility that the actual ground behaviour could be different from the predicted behaviour using the design soil parameters. Reliability analyses were carried out to assess the potential variability of material parameters on embankment settlement and ground treatment requirements.
The reliability assessment provided quantitative confident levels of the ground treatment designs and suitable contingency measures. The reliability assessment provided indication of the cost and risk balancing. The target confidence level was minimum 70% for the soft ground treatment design with the proposed observation method and contingency measures such as placement of additional surcharge or additional preloading time to respond to changes during the preloading period. The reliability assessment also effectively assisted the client’s decision on the preferred soft ground treatments.
The adopted reliability assessment method as described in Duncan (2000) and the assessment results for the soft ground treatment design were presented. The embankment settlement was monitored during the preloading stage and was back analysed. The reliability assessment results, which were analysed in the design stage, and the ground treatment design were reviewed against the actual embankment settlement performance.