Cyclic softening case study: Wendover Retirement Village, Christchurch

Wendover Retirement Village is located in Papanui, Christchurch, adjacent to a minor tributary of Dudley Creek on the edge of a drained peat swamp. The site suffered damage as a result of the recent major seismic activity in Canterbury, including ground settlement and lateral spreading.

To assess the subsurface profile, cone penetration tests (CPT) and hand auger boreholes were carried out and the results used to back-analyse the failures that occurred during the recent seismic activity. The site is typically underlain by 5–6m of peat and organic clay with a high water table. The organic material overlies 1–2m of sand, then gravel.

The initial assumptions of liquefaction causing the ground damage were dismissed when analysis predicted that liquefaction would be restricted to the sand that underlies the organic deposits at 5–6m depth. To determine the cause of the ground damage, a cyclic softening assessment was undertaken based on the method of Idriss and Boulanger (Idriss & Boulanger, 2008). By comparing the results with the earthquake records from seismographs located nearby, it was demonstrated that cyclic softening was expected in the 22 February 2011 earthquake event and unlikely in the 4 September 2010 event. This correlated well with the observations of ground damage. In addition, by using probabilistic predictions of the magnitude of ground accelerations (Gertsenberger, 2011), the likelihood of cyclic-softening-induced ground damage reoccurring was able to be estimated.