Field evaluation of large-scale, shallow ground improvements to mitigate liquefaction triggering

Much of the devastation wrought by the 2010-2011 Canterbury Earthquake Sequence (CES) in Christchurch, New Zealand, was caused by extreme levels of liquefaction-induced damage to structures with shallow foundations. In response to this disaster, the New Zealand Earthquake Commission (EQC) funded a large study known as the Ground Improvement Trials to evaluate and identify shallow ground improvement methods that are not only effective at increasing the soil’s resistance to soil liquefaction, but are also cost effective and practical to build for lightweight structures. Of the nine ground improvement methods included in the trials, three were selected for extensive analysis in this dissertation. These three ground improvement methods are the Rapid Impact Compaction (RIC), the Rammed Aggregate PiersTM (RAP), and the Low-Mobility Grout (LMG).

At three test sites along the Avon River in Christchurch neighborhoods that were among the worst hit by liquefaction-related damage, full-scale test panels of natural soil and ground-improved soil were constructed and evaluated using a variety of in situ test methods. The analysis in this dissertation primarily relies on data from excavation trenching, cone penetrometer testing (CPT), direct-push crosshole testing (DPCH), and shake testing with T-Rex. These tests capture changes in density and stiffness, and therefore liquefaction resistance, due to the ground improvement methods in comparison to the natural soil. Shake testing with T-Rex is further able to define the relationship between cyclic shear strain and the generation of excess pore pressure that ultimately determines whether or not a soil will liquefy under cyclic loading. Under this framework, the effectiveness of each of the three ground improvement methods is evaluated and discussed.