Soil liquefaction following strong earthquakes causes extensive damage to civil engineering structures. Foundations of buildings, bridges etc can suffer excessive rotation/settlement due to liquefaction. Many of the recent earthquakes bear testimony for such damage. In this article a hypothesis that “Superstructure stiffness can determine the type of liquefaction-induced failure mechanism suffered by the foundations” is proposed. As a rider to this hypothesis, it will be argued that liquefaction will cause failure of a foundation system in a mode of failure that offers least resistance. Evidence will be offered in terms of field observations during the 921 Ji-Ji earthquake in 1999 in Taiwan and Bhuj earthquake of 2001 in India. Dynamic centrifuge test data and finite element analyses results are presented to illustrate the traditional failure mechanisms.
TopIntroduction
Soil liquefaction plays a major role in the damage suffered by many a civil engineering structures. This is illustrated by many of the recent earthquakes including the Kobe earthquake of 1995 in Japan, the Kocaeli earthquake in Turkey and the 921 Ji-Ji earthquake in Taiwan during 1999 and Bhuj earthquake of 2001 in India. In these earthquakes many examples of failures have been observed, for example settlement and/or rotation of structures owing to foundation liquefaction, bowing out of quay walls owing to liquefaction of the backfill, excessive settlement and damage of pile foundations etc.
It is well known that soil liquefaction occurs in loose, saturated sand or silt layers. The cyclic shear stresses generated by the earthquake loading will cause the excess pore pressures rise over and above the hydrostatic pore pressures. The degree of liquefaction is often measured by the excess pore pressure ratio ru defined as;
(1)While this factor is easy to visualise in the case of free-field sites, its estimation is more involved when liquefaction below foundations of existing structures is being considered primarily as it involves the calculation of initial effective stress σ′vo. The range of ru varies from 0 to 1 i.e. no excess pore pressures generated to full liquefaction condition. When the value of ru reaches 1, the foundations of buildings, bridges and other civil engineering structures would suffer excessive settlements and/or rotations.
The main emphasis of this article will be on the failure mechanisms suffered by the foundations when full liquefaction is reached. It will be argued that the superstructure stiffness has a role to play in determining the actual failure mechanism by which the foundation will fail. This aspect is important in understanding the failure mechanisms suffered by foundations in past earthquakes and in attempts to carryout liquefaction resistant designing of future structures.
TopLiquefaction During Recent Earthquakes
Liquefaction has been observed in many of the recent major earthquakes. In this article, examples from the Kocaeli earthquake in Turkey and the 921 Ji-Ji earthquake in Taiwan in 1999 and the Bhuj earthquake in India in 2001 will be considered.
Liquefaction played a major role in damage suffered by buildings during the Kocaeli earthquake, Free et al. (2003). Liquefaction was observed at several locations during the 921 Ji-Ji earthquake, Madabhushi (2007). The Taichung harbour witnessed excessive settlements of backfill behind quay walls. Similarly liquefaction and liquefaction induced lateral spreading was observed at several bridge sites.
Widespread liquefaction was observed during the Bhuj earthquake of 2001, Madabhushi et al. (2005). Sand boils were observed at several locations in the Rann of Kachchh. Moderate to severe damage was recorded to quay walls at Navalakhi port, rotation of Harbour Master Tower at Kandla port, damage to piles supporting wharfs, settlement of railway lines and cracks and lateral spreading of earth dam slopes. In this article, one particular bridge site that was studied following this earthquake will be considered in detail.