The scourge of Christchurch: Liquefaction

Residents clean up liquefaction in eastern Christchurch.
Residents clean up liquefaction in eastern Christchurch.

Liquefaction, sand volcanoes and vast quantities of grey sandy silt became the scourge of Christchurch on February 22, bubbling out of the ground during the biggest shakes and also during some of the magnitude 5.0 aftershocks.

Liquefaction occurred across the whole city but was a major issue in eastern parts and around the central business district. Whole suburbs in the east were covered in the clinging, stinking mud, especially those around the Avon River and the Avon-Heathcote Estuary.

In the west of the city, some streets in Merivale and Fendalton were also badly affected while adjacent roads and properties were untouched.

The worst-hit areas of the city were those where houses had been built on loosely compacted ancient river or beach sands and sandy soils close to bodies of water, generally around the Avon River and close to the estuary.

Properties on the western side of the city and across the Canterbury Plains sitting on stony, gravelly soils were mostly unaffected by liquefaction.

Geotech Consulting geologist Mark Yetton said liquefaction occurred within the top four to five metres of the ground where there were layers of loose sand which had been deposited naturally. The space between the sand grains was occupied by water.

In a significant quake, the sand grains became packed closer together, squeezing the water up and out of the ground. Where that occurred near a river, lateral spreading became an issue, allowing more rigid soils to slide down into the river on top of the weaker soils that had liquefied.

The September 4, 2010, quake was the first time major liquefaction was experienced in the region. Christchurch residents had been warned of the dangers from liquefaction for many years before that quake struck and have faced further clean-ups after February, most notably after the June 13 and December 23 quakes.