Development of Horizontal Soil Mixed Beams as a Shallow Ground Improvement Method Beneath Existing Houses

R. Hunter, S. van Ballegooy, J. R. Leeves and T. Donnelly

Following the 2010-2011 Canterbury Earthquake Sequence (CES), the vulnerability of residential houses in some areas of Christchurch to liquefaction-induced damage was realised. As a result of the ground surface subsidence caused by the CES, the liquefaction vulnerability has also increased in some parts of Christchurch (Russell et al., 2015). The liquefaction-induced damage resulted in a large number of residential houses in Christchurch that were uneconomic to repair. They are being demolished and rebuilt on stiffer and stronger foundation systems and in some areas which are particularly vulnerable to liquefaction, the stiffer and stronger foundation systems are being used in conjunction with shallow ground improvements. There are also a large number of houses that have liquefaction-induced damage, but are economic to repair. Until recently there was no practical ground improvement solution that could be economically constructed beneath existing repairable residential houses to decrease their liquefaction vulnerability. However, during a shallow ground improvement trial research project, commissioned by the New Zealand Earthquake Commission (EQC) in 2013, a method was developed to improve ground beneath residential houses, known as Horizontal Soil Mixing (HSM). HSM involves the mechanical mixing of injected grout into in situ soils using a modified directional drill and a specifically designed soil mixing tool to construct a series of HSM beams to improve the thickness and stiffness of the non-liquefying crust and decrease the vulnerability of the existing house to future liquefaction-induced damage. This paper describes the development of the HSM construction methodology, including constraints and issues that were encountered and overcome.