For checking of uplifting of underground structures due to liquefaction during the design of such structures in Japan, a formula based on the balance between the buoyancy acting on the structure from the liquefied soil and the dead load of the structure is used. The authors consider that (1) the liquefied soil applies considerably large dynamic loads on underground structures during an earthquake and the vertical component induces the uplifting of structures, and that (2) a vertical force larger than the statically acting buoyancy is highly likely to act on structures resulting in structures being uplifted only by the vertical force. In this study, to understand the action of the soil pressure on the pipe in liquefied soil during earthquake, shaking table tests were conducted. As result of the test, the following has been understood. The greater the shear strain of soil around the structure became, the larger vertical force induced by incremental earth pressure acted on the pipe. It was thus suggested that if the structure is subjected to shearing deformation together with the surrounding soil, upward vertical force acts on the structure. If it was a pipe of 1.25t/m3 in the appearance density, it was shown to surface by this action. In order to prove these hypotheses, shaking table tests are performed in this study.
With the present design practice, whether underground structures are uplifted or not during soil liquefaction is determined according to whether the buoyancy acting on the structure from the liquefied soil exceeds the sum of dead load of the structure and overlying soil mass. If the structure is found to be uplifted, liquefaction control measures shall be taken immediately.
From a practical viewpoint, however, moderate uplifting is often allowable. The present design practice is therefore economically detrimental.
