Geotechnical design aspects for large gravity retaining structures under seismic loading

Large wharf gravity retaining walls are often constructed in areas of Canada where seismic activity of a level that has damaged marine structures in other countries can be anticipated. From a geotechnical viewpoint, much of this damage is related to the liquefaction of sands and silts in foundations or backfills. It is critical that zones of potential liquefaction be detected, and this may require more extensive field and laboratory testing than for static designs. Simplified procedures for evaluating the liquefaction potential of sands and silts are presented, and some guidelines for sensitive clays are discussed. Since hydraulic fill is often used, remedial measures and specifications to avoid liquefaction of backfills are described. These include densification of the soil and provision of materials outside the gradation range that is most susceptible to liquefaction. While tsunamis arrive after the strong shaking, they can be the source of significant damage in areas where they might occur. Conservative earth pressure parameters are given for areas exposed to tsunamis, and these may be employed for initial assessment of potential problems from offshore earthquakes. The various geotechnical aspects considered must be closely related to other design factors such as seismic exposure, earth pressures, and allowable short-term safety factors.