How Is the Choice of Mitigation Options Made?
The choice of mitigation options depends very much on the particular characteristics of the site. If there is not a significant lateral movement hazard, mitigation for a new facility is largely a matter of finding the most cost-effective solution to providing vertical support and control settlement. For existing facilities, mitigation is generally more difficult and expensive because of the presence of the structure. Techniques that densify the soil may be precluded for an existing facility because they would cause settlement of the structure.
When a lateral spreading hazard is present, the mitigating measures, to be effective, may in some cases need to be employed beyond the boundary of the specific site. This may preclude effective mitigation by an individual property owner, requiring instead action by public entities or groups of property owners.
Does Mitigation Work?
Several different ground improvement techniques have been used for sites identified as having the potential for liquefaction. For example, several sites had been improved in Treasure Island, Santa Cruz, Richmond, Emeryville, Bay Farm Island, Union City, and South San Francisco, California, prior to the Loma Prieta earthquake in 1989. The sites where ground improvements were carried out had little or no damage to either the ground or facilities built upon the improved sites even though they experienced peak ground accelerations ranging from 0.11g to 0.45g. In contrast, untreated ground adjacent to these improved sites spread, oscillated, or settled, due primarily to liquefaction (Leighton and Associates, 1993).