Monitoring ground improvement by Rammed Aggregate Piers using a combined CPTU and SDMT approach at a silty sand liquefaction-prone site in Emilia-Romagna

Following the 2012 Emilia-Romagna earthquake, widespread liquefaction of silty sands was observed, providing the opportunity to gain a better understanding of the influence of fines content on liquefaction hazard and mitigation works. This paper presents the results of a thorough geotechnical investigation performed as part of a full-scale liquefaction experiment involving controlled blast tests in Bondeno, a small village that suffered liquefaction in 2012. Piezocone (CPTU) and seismic dilatometer (SDMT) tests were performed in natural and improved soils after Rammed Aggregate Pier® (RAP) treatment to provide accurate soil characterization and to evaluate the effectiveness of liquefaction mitigation. CPTU and SDMT results revealed a good agreement in the geotechnical characterization of the site, detecting homogenous soil properties in both the natural and treated soils and estimating CPTU-DMT coupled parameters in sandy layers (e.g. overconsolidation ratio, at-rest earth pressure coefficient), that are usually not determinable by the use of a single type of in situ test. In particular, the combined use of CPTU-DMT data provided verification of the increase in the lateral stress produced by the RAP installation. Data analyses revealed that the RAPs were an effective ground improvement technique despite the high percent of fines (≈ 25-35%).