Monitoring ground improvement using the seismic dilatometer in Christchurch, New Zealand

This article illustrates the use of the seismic dilatometer test (SDMT) in combination with the piezocone test (CPTu) to monitor the effects of ground improvement by different techniques at seven test sites in Christchurch, New Zealand. The results of SDMTs carried out in treated soils and in adjacent natural soils are compared in order to assess the effectiveness of different ground improvement techniques (Rammed Aggregate Piers, Low Mobility Grout, Stone Columns, Deep Soil Mixing) aimed at reducing the liquefaction susceptibility. Moreover, the article presents the variation of significant parameters obtained from parallel SDMT and CPTu soundings in treated versus natural soils. SDMT and CPTu results are also used in combination to estimate the overconsolidation ratio and the at-rest lateral earth pressure coefficient (K0) before and after the treatment. The effectiveness of the different ground improvement techniques is also evaluated in terms of the variation of the before and after integral liquefaction vulnerability indicators, such as the liquefaction potential index and the liquefaction severity number, which are calculated using different methods based on CPTu, the DMT horizontal stress index (KD), and the shear wave velocity (VS) for a design earthquake. The results confirm that the DMT, as well as the CPTu, is sensitive to changes in stress/density in sands and silty sands and is, therefore, well suited to detecting improvements in these soils, while VS provides less evident results.