Performance of tied-together piled foundation in liquefiable soils: New Zealand case study

Piled foundations are widely used to transfer structural loads through a weak stratum to competent underlying ground. In earthquake-prone areas, these piles have to consider kinematic loading and potential loss of bearing where there is an added liquefaction potential. Earlier practice in parts of Christchurch, New Zealand, did not acknowledge the potential loss of bearing in liquefiable sand layers with numerous instances of driven piles through peat layers bearing on liquefiable sands with consequential damage following the earthquakes of the Canterbury earthquake sequence. Localised instances of screw-type piles installed into liquefiable sands are also evident where the design was driven by vertical structural load specification with torque readings used to confirm bearing capacity. This paper presents a case study of performance of a two-storey 3000 m² building complex on piled foundations through peat, where the original buildings were founded on driven piles generally into liquefiable layers and later extensions on screw piles of varying depths. Particular focus is given to an approximate 500 m² extension founded on screw piles of varying depths through peat and generally embedded in underlying liquefiable sand that performed surprisingly better than expected, presumably because of a tied-together foundation system.