Deep Foundations - Mitigate Earthquake Effects.

Deep foundations are one of the most effective means of mitigating foundation movement during an earthquake. For example, the Niigata Earthquake resulted in dramatic damage due to liquefaction of the sand deposits in the low-lying areas of Niigata City. At the time of the Niigata Earthquake, there were approximately 1500 reinforced concrete buildings in Niigata City and about 310 of these buildings were damaged, of which approximately 200 settled or tilted rigidly without appreciable damage to the superstructure. The damaged concrete buildings were built on very shallow foundations or friction piles in loose soil. Similar concrete buildings founded on piles bearing on firm strata at a depth of 66 ft (20 m) did not suffer damage.

There are several important earthquake design considerations when using deep foundations, such as piles or piers, as follows:

1. Connection between pile and cap. It is important to have an adequate connection between the top of the pile and the pile cap. This can be accomplished by using steel reinforcement to connect the pile to the pile cap. Without this reinforced connection, the pile will be susceptible to separation at the pile cap during the earthquake.

2. Downdrag loads due to soil liquefaction. The pile-supported structure may remain relatively stationary, but the ground around the piles may settle as the pore water pressures dissipate in the liquefied soil. The settlement of the ground relative to the pile will induce downdrag loads onto the pile. The piles should have an adequate capacity to resist the downdrag loads.

The relative movement between the relatively stationary structure and the settling soil can also damage utilities. To mitigate damage to utilities, flexible connections can be provided at the location where the utilities enter the building.

3. Passive resistance for liquefiable soil. A common assumption is that the liquefied soil will be unable to provide any lateral resistance. If a level-ground site contains an upper layer of nonliquefiable soil that is of sufficient thickness to prevent ground fissuring and sand boils, then this layer may provide passive resistance for the piles, caps, and grade beams.

4. Liquefaction of sloping ground. For liquefaction of sloping ground, there will often be lateral spreading of the ground, which could shear-off the piles. One mitigation measure consists of the installation of compaction piles, in order to create a zone of nonliquefiable soil around and beneath the foundation.

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