This report prepared by Associate Professor Misko Cubrinovski of the University of Canterbury and Ian McCahon of Geotech Consulting Ltd discusses the nature of the soils in the Christchurch CBD, and their susceptibility to liquefaction.
It describes the effect of liquefaction in the Canterbury earthquakes on CBD buildings of different foundation types, identifies foundation types and land improvement measures suitable for buildings on deep alluvial soils, and discusses the importance of appropriate field investigations and the need for a holistic design approach that takes into account soil conditions, and foundation and structure systems. The report commences with a short Executive Summary.
The report has been broken down into the following sections, which can all be download from here.
- Title and authors; executive summary; introduction; chapter 2 general characteristics of seismic response of deep alluvial soils.
- Chapter 3 Christchurch CBD soils
- Chapter 4 Observed liquefaction and response spectra in CBD during the 2010 and 2011 Christchurch earthquakes.
- Chapter 5 Typical causes of failure in the CBD; chapter 6 comparison of extent of liquefaction between 2010-2011 earthquakes and a Mw = 8.0, alpine fault event.
- Chapter 7 typical methods of founding buildings which would avoid such failures; chapter 8 conclusions; acknowledgements; references.
CBD Building Foundations
The strong ground shaking triggered by the series of earthquakes in the period 4 September 2010 and 13 June 2011 caused widespread liquefaction throughout the suburbs of Christchurch and within the CBD. The 22 February 2011 earthquake was particularly damaging for the CBD buildings and their foundations. The principal zone of liquefaction stretching west-east along Avon River affected several high-rise buildings on shallow foundations and deep foundations in different ways.
- Buildings on shallow foundations, supported on loose to medium-dense sands and silty sands that liquefied, suffered differential settlements and residual tilts.The uneven settlements were often damaging to the structure. Several buildings underwent punching settlements and bearing capacity failures (sinking of the building in the soil).
- Pile supported structures in areas of severe liquefaction, particularly when the piles reached competent soils at depth, generally showed less differential and residual movements.
- Multi-storey and high-rise buildings supported on shallow foundations sitting on shallow gravels showed mixed performance. The variable thickness of the gravel layer and underlying soil layers resulted in some differential settlements, tilt and permanent lateral displacements. These adverse effects were especially pronounced in transition zones where ground conditions change substantially over short distances.
- There is evidence that hybrid building foundations (consisting of shallow and deep foundations or piles of different lengths) performed relatively poorly during the earthquakes. Structure-soil-structure interaction of adjacent (multistorey) buildings was another response feature that somewhat influenced the performance of the foundations.
- Within the CBD, zones of ground weakness (either localized over a relatively small area or sometimes continuous over several blocks) manifested pronounced ground distortion and liquefaction that adversely affected a number of buildings and their foundations. There was a marked difference in the performance of buildings only 20-30 metres apart, one that sat on the bad stretch of the liquefied soil and the other on a ground showing no signs of liquefaction or ground distress.
- The effects of lateral spreading within the CBD were localized but quite damaging to buildings causing sliding and stretching of the foundations and the structure.