Characterisation of undrained behaviour of Christchurch soils

Abstract

Strong earthquakes are recognised as one of the principal natural hazards for New Zealand. The intense ground shaking during such earthquakes may cause damage to wooden houses, buildings, bridges and industrial facilities, loss of function of lifelines (water and electricity supply), and will affect the society in a very profound way.

All these structures and lifelines rest on the ground or are buried into it, and therefore, it is critically important to know how the ground will behave during strong earthquakes. Typically, soils are satruated in their natural state and contain a significant amount of water. During strong shaking, the pressure in the water will increase and this will lead to “softening” of the soil. In other words, the soil will lose some its strengh and capacity to support the structures resting on it. In the extreme case, the soil may liquefy and completely lose its strength. The “quick-sand” illustrates well this state of the soil. The pore pressure build-up, eventual liquefaction and consequent deformation of soils are all embodied in the technical term “undrained behaviour” of soils.

The undrained behaviour of soils depends on their grain-size composition. Clays having very fine particles respond to earthquakes in a very different way from sands, which have particles between 0.06 mm and 2 mm and are recognised as the most susceptible soils to liquefaction. The effects of fines on the undrained behaviour of sands are quite complex and not well understood. This is a particularly relevant issue for Christchurch because this city has highly variable sandy deposits with a predomination of fines and relatively high seismic hazard.

This report presents the initial phase of an experimental study on the undrained behaviour of Christchurch soils carried out at the University of Canterbury under the support of EQC. Since soil testing is quite complex and based on rigorous procedures, the first phase of the study was used to verify the performance of a newly acquired apparatus and to establish testing procedures for the Christchurch soils. In the second phase, soil samples were collected from several sites in Christchurch and were tested in the laboratory. In these tests, the soils were loaded in a way that resembles the loads imposed on field deposits during actual earthquakes. A series of tests were conducted on two soils with different fines contents (FC = 0% and 10%) in order to investigate the effects of fines on undrained behaviour. This report summarises the results of the laboratory tests.

The ultimate goal of this long-term study is the development of a geotechnical model that will allow reliably predicting the behaviour of Christchurch soils during strong earthquakes. This in turn will result in an improved design and performance of engineering structures during extreme seismic events.