Behaviour of piles in liquefiable deposits during strong earthquakes

Technical 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 saturated in their natural state and contain significant amounts of water. During strong shaking, the pressure in the water will increate and this will lead to ‘softening’ of the soil. In other words, the soil will loose some of its strength and capacity to support the structures resting on it. In the extreme case, the soil may liquefy and completely loose its strength. Soil liquefaction has been a major cause for damage to many engineering structures during strong earthquakes. In particular, pile foundations have suffered serious damage in liquefied soils. This report presents a research study examining appropriate methods for analysis of piles in liquefiable soils.

In general, two types of methods are of interest to the profession: simple methods and advanced methods for analysis. Simple methods are very attractive for preliminary assessment and design of piles, because they can be easily implemented in the engineering practice. Advanced methods on the other hand provide very detailed simulation of the performance during earthquakes and hence are needed for assessment of important structures. The latter methods require special expertise, high quality input data and are therefore relatively difficult to implement on a regular basis. The two methods are complementary in nature and address different needs, and are therefore equally valuable to the profession.

This study presents investigation on simplified and advanced methods for analysis of piles in liquefiable soils. Recently developed state-of-the-art methods were applied to the analysis of Fitzgerald Bridge crossing the Avon River in Christchurch. The analysis provided detailed information on the response of the pile foundation of the bridge in an extreme seismic event for Christchurch. The level of damage to the pile obtained from these analyses could be used into the engineering solution. The study addresses some critical issues in the application of the analyses such as definition of the model and selection of its parameters. It also provides guidelines for the application of these methods to New Zealand specific conditions.