Sandy silt Sandy gravel
Figure 9.3 Failure of piles in a three-storeyed building in 1995 Kobe earthquake (Tokimatsu et al. 1997)
Pile foundations seem to suffer from earthquake loading for a variety of reasons. A comprehensive list of pile foundations of various structures that have performed poorly was compiled by Bhattacharya, Madabhushi and Bolton (2004). The load bearing soil strata into which the piles are transferring the load ay change their character under strong cyclic loading. In addition the piles have to bear the inertial and kinematic loading described in detail later in Section 9.4.1. In many cases the pile foundation failures appear to be associated with 'liquefaction' of the ground to some depth around the piles. Similarly presence of a non-liquefied layer such as stiff clay overlying a liquefiable layer appears to cause additional loading particularly if the ground is on a slope. Research has shown that slopes as gentle as 1o to 3o can result in lateral spreading of liquefied soil and that of any non-liquefied soil crust overlying the liquefied soil (Haigh et al, 2000).
The normal static design of pile foundations must be carried out under the provisions of EC7 Part 1 (1995). In addition, EC8 recommends that the liquefaction potential of all the soil layers at a given site be carefully determined based on the SPT tests conducted at the site. It is also suggested that careful consideration of any additional loading on the piles and pile caps that ay arise due to the lateral spreading of the soil, particularly in the presence of a non-liquefiable soil strata overlying a liquefiable layer. In addition, here liquefaction is anticipated it is suggested that the strength of the liquefied soil ust be ignored.
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