On 4 September 2010, a magnitude Mw 7.1 earthquake struck the Canterbury region on the South Island of New Zealand. The epicentre of the earthquake was located in the Darfield area about 40 km west of the city of Christchurch. Extensive damage was inflicted to lifelines and residential houses due to widespread liquefaction and lateral spreading in areas close to major streams, rivers and wetlands throughout Christchurch and Kaiapoi. Unreinforced masonry buildings also suffered extensive damage throughout the region. Despite the severe damage to infrastructure and residential houses, fortunately, no deaths occurred and only two injuries were reported in this earthquake. From an engineering viewpoint, one may argue that the most significant aspects of the 2010 Darfield Earthquake were geotechnical in nature, with liquefaction and lateral spreading being the principal culprits for the inflicted damage.
Following the earthquake, an intensive geotechnical reconnaissance was conducted to capture evidence and perishable data from this event. The surveys were performed on foot, by car and from a helicopter over a period of six days. A broad-brush field reconnaissance was conducted in the first two days, followed by pin-point investigations at specific locations including detailed site inspections and field testing using: Dynamic Cone Penetration Test (DCPT), Swedish Weight Sounding (SWS), and Spectral Analysis of Surface Waves (SASW).
This paper summarizes the observations and preliminary findings from this early reconnaissance work.
Many slope failures have been observed to occur during or immediately after rainfall. Although conditions leading to these failures have been described as caused by a rapid rise in pore-water pressure as a result of rainwater infiltration, the important factors that influence the initiation of slope failures have not been adequately clarified. To investigate these factors, a series of laboratory experiments was conducted on model sandy slopes to determine the initiation process of rainfall-induced slope failure. In the tests, failures were induced in small-scale model slopes either by water percolation from the side upslope or by artificial rain falling on top of the slope. Besides monitoring pore-water pressure, changes in soil moisture contents and ground deformation were measured. Test results showed that slope failure was always induced when the soil moisture content within a certain region near the toe of the slope reached nearly full saturation, even though other parts of the sliding mass were still in a partially saturated state. In addition, minute deformations along the slope were shown to precede failure. The findings presented here show that by monitoring the soil moisture content of slopes and performing displacement measurements, it is possible to predict the occurrence of rainfall-induced slope failure.
In the 4 September 2010 (M W 07.1) and 22 February 2011 (M W 06.2) earthquakes, widespread liquefaction and lateral spreading occurred throughout Christchurch and the town of Kaiapoi. The severe soil liquefaction and lateral spreading in particular caused extensive and heavy damage to residential buildings, Christchurch business district (CBD) buildings, bridges and water supply and wastewater systems of Christchurch. After the earthquake, comprehensive field investigations and inspections were conducted to document the liquefaction-induced land damage and lateral spreading displacements and their impact on buildings and infrastructure. The results of ground surveying measurements of lateral spreads at approximately 120 locations along the Avon River, Kaiapoi River and streams in the affected area reveal permanent lateral ground displacements at the banks of up to 2Á3 m that progressed inland as far as 200Á250 m from the waterway, causing significant damage to structures located within the spreading zone. Different features and magnitudes of spreading were identified, which were often affected by a complex interplay of ground conditions, topography, meandering river geometry and local depositional environment. The spreading was characterised by very large and highly non-uniform ground deformation causing stretching of building foundations and the buildings themselves. Road bridges suffered a characteristic spreading-induced damage mechanism including back-rotation of the abutments associated with deck pinning and damage at the top of the abutment piles. The wastewater system of Christchurch was hit particularly hard by the liquefaction and lateral spreading, and approximately 60% of the damaged pipes of the potable water system were located in areas of severe liquefaction and lateral spreading.
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