Class A and C predictions for Ballina trial embankment with vertical drains using standard test data from industry and large diameter test specimens

Consolidation using prefabricated vertical drains has been one of the most popular ground improvement methods in the past few decades. In this paper, a benchmarking exercise is performed to compare the accuracy of the available methods through both Class A and Class C predictions for a trial embankment stabilised by prefabricated vertical drains (PVD) at the site of the National Field Testing Facility (NFTF). In this paper, the main difference between Class A and Class C is that additional soil properties specially required to represent the visco-plastic behaviour or creep have been included, which were absent in the authors’ original Class A predictions made before the availability of field data. Naturally, the inclusion of visco-plastic behaviour improves the settlement and excess pore water pressure predictions significantly, especially after about 1–1.5 years. The site is located in Ballina, NSW and is owned by the Roads and Maritime Services (RMS) of NSW. A trial embankment was constructed over this soft Holocene clay under the auspices of the ARC Centre of Excellence in Geotechnical Science and Engineering (ARC-CGSE). Prefabricated vertical drains were installed with an array of field instrumentation including inclinometers, piezometers, settlement plates and total pressure cells. Large scale consolidometer testing was also carried out on undisturbed soil specimens (350 mm in diameter) retrieved from the site to obtain additional consolidation parameters needed for radial consolidation analysis. Class A and Class C predictions were performed via numerical and analytical approaches. The results show, that for this case history, the suggested approaches can match the observed field performance well, with the exception of long term excess pore water pressures (say beyond 1.5 years) and the lateral displacement approaching the ground surface