Damage from the Canterbury earthquakes varied greatly from region to region of Christchurch. One street may have been completely destroyed, while another in close proximity was untouched. The reason for this variance is multifaceted. Different soil types react differently to seismic activity, and each area has differing styles of buildings. This means that areas with older buildings may have had more earthquake damage.

Land near waterways vulnerable

Much of Christchurch city and its immediate neighbouring settlements are situated on river deposits, beach dune sand, estuaries, lagoons and swamplands that have been drained over time. Before the construction of stopbanks and river realignment in the mid 19th century, the Waimakariri River regularly flooded the area now settled as Christchurch and Kaiapoi. This means that the types of land houses are now built on in these areas are more susceptible to damage from seismic events.

At the Avon and Heathcote Rivers serve as the major drainage channels to the estuary and the sea. Similarly, so is the Kaiapoi River which passes through Kaiapoi. Many areas of liquefaction were near these waterways, and lateral spreading of the ground towards the rivers was also evident.

Limited checks before 1978

Most house foundations are shallow constructions, extending little more than 300–400 mm into the ground. Foundation investigation requirements were limited until 1978 when the first NZS 3604 introduced requirements for checking the bearing capacity of the soil beneath a foundation prior to construction. Several observational criteria must be satisfied and a subsurface investigation carried out to define bearing capacity down to a depth of generally not more than 1.2 m below the underside of the proposed footings.

Liquefaction caused most problems

The major problem in Christchurch has been the occurrence of liquefaction in soil layers beneath the upper 1.2 m or so where the soil investigation was carried out.

During the earthquake, the liquefied sand often made its way to the surface – usually through cracks that formed as the ground spread towards a waterway or through cracks that formed as the ground rippled.

Although ground settlements from liquefaction occur anyway, where ejection of sand occurred, it caused additional ground surface settlement, often by uneven amounts. Concentrated loads from house foundations with heavy wall and roof linings have sometimes caused greater settlement around the perimeter of a house than inside the perimeter.

Source: Graeme Beattie, BRANZ Principal Engineer