Use of a Rammed Aggregate Pier Ground Improvement trial to improve understanding of liquefaction potential in geologically aged soils

CMW Geosciences (CMW) with the support of CLL Service & Solutions Ltd (CLL) have carried out an in-house research and development (R&D) exercise involving a Rammed Aggregate Pier (RAP) ground improvement trial within Pleistocene-aged sand deposits that have been assessed using conventional methods to be susceptible to liquefaction. Although the results of the trial demonstrated that the Pleistocene-aged alluvial sands densified following RAP installation, shear wave velocity measurements following RAP installation were much lower than those undertaken prior to installation indicating that the high energy vibratory hammer used to install the piers had disturbed the soil microstructure. The results of our trial have highlighted an interesting effect of dynamic ground improvement techniques in geologically older soil deposits.

The updated MBIE guidelines in Earthquake Geotechnical Engineering Practice released in November 2021 have acknowledged that although the age of a deposit is an important factor to consider when undertaking a liquefaction assessment, it is often challenging to quantify the effects ageing has on the liquefaction resistance of soils with no widely accepted methods to do so. As a result, the guidelines have recognised the limitations of the methods and have outlined an approach for evaluating the ageing effects incorporating the use of shear wave velocity measurements and advanced lab testing. This provides the New Zealand geotechnical industry further guidance when assessing liquefaction potential on geologically aged soils.