Pile design for liquefaction effects

Peter W. Mitchell

Although liquefaction is a rare event in Australia, with only two documented instances, a pile design must still cater for liquefaction in a design earthquake. Liquefaction can lead to a loss of shaft resistance, additional horizontal displacements and bending moment under inertia loads, pile buckling and lateral spreading. This paper describes the four loading stages suffered by a pile before, during and after an earthquake when liquefaction may or may not occur. The method of Youd et al (2001) is used to predict the potential for liquefaction with depth. The important parameters for use in a ‘design earthquake’, namely peak ground acceleration and earthquake magnitude, depend on the seismic activity of the region and have been derived for the area of Adelaide, South Australia. If the soil profile has the potential to liquefy, the geotechnical capacity, the lateral behaviour and the buckling potential of the pile under the inertia loads must be determined for the loss of soil support. If lateral spreading can occur, a further lateral analysis is required. This method is used by the author in routine pile design and an example of the design process together with four case histories is given. The importance of continuous sampling and very careful logging of the soil profile during the geotechnical investigation is emphasised for an accurate liquefaction assessment.