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Liquefaction Induced Damage

Soil liquefaction or lateral spreading has resulted in major structural damage to hundreds of houses, many of which will have to be demolished.

Residential areas that were significantly affected by liquefaction were Avonside, Dallington, Avondale, Bexley, Kaiapoi and Pines Beach. In Avonside and Avondale, settlement and lateral spreading increased with proximity to the Avon River. Liquefaction induced damage has been observed in a few other areas, but limited to a smaller number of dwellings.

The amount and uniformity of ground settlement varied greatly from site to site. On sites where the settlement was mainly uniform the structural integrity of the house foundations remained essentially intact but some buildings tilted, causing minor structural damage. On sites where differential settlement or lateral spreading occurred, the buildings became distorted, resulting in cracks in concrete slabs and foundations, brick veneers, and internal linings. On sites where severe lateral spreading occurred, such as Courtnay Drive in Kaiapoi, buildings suffered from extensive structural damage (see Figure 1). Sinking into soft soils appeared to be more severe for buildings with heavy brick cladding and heavy tile roofs.

Concrete slabs and pile foundations were observed to perform very differently on liquefied sites. This was clearly illustrated at an Avondale residence consisting of an older (1950s) house, partly built on concrete pile foundations and newer (1997) extensions built on a concrete slab. The older part of house on piles remained relatively plumb compared to the newer part. The large settlements of the slab foundations caused extensive structural damage at the interface between the old and new structure (see Figure 2). Similar observations were made on sites where a garage or carport on a concrete slab was connected to a house on pile foundations, giving differential movement between the two foundation types resulting in damage to the wall and roof structure of the building. In general, for areas where there has been soil liquefaction, pile foundations performed better than concrete slabs. The better performance of houses on piled foundations may also be a consequence of the timber ground floor which can accommodate differential settlement of the foundation system more easily than a concrete slab.

A small number of buildings on poor soil sites have driven timber or concrete piles under the concrete slab, but it is not possible to identify these buildings from a visual inspection. There are several verbal accounts of buildings with driven piles performing much better than neighbouring buildings with simple concrete slabs.

Several concrete floor slabs fractured due to uneven liquefaction induced settlements. This occurred mostly in modern buildings, some of which had little or no observable slab reinforcement. Structural damage may have been reduced if more reinforcing had been used in the slabs, or if a thicker solid slab or thick ribbed slab (Ribraft) had been used.

On all sites with differential settlement, there was significant non-structural damage to the wall linings, windows, doors and fixtures due to cracks in the concrete slab and building envelope. Outside the houses, yards were often covered in ejected soil from liquefaction, and driveways and garages were distorted due to settlement. On most liquefied sites there was no structural damage to houses due to shaking, possibly because the soft soil isolated the structure from the worst ground motion (similar to a base isolation system).