On strength property of gassy fine sand and model tests of pile foundation

Based on the phenomenon of blowout during the engineering geologic surveys in the Hangzhou Bay Bridge, Zhejiang province, China, the influence of shallow biogenetic gas on the engineering properties of sandy soil is measured and analyzed. It is found that the engineering behaviors of gassy sand is determined by its three-phase composition state and special stress path, the gas reservoir pressure of fine sand layer is 515 similar to 530kPa and its pressure coefficient is 1.03 similar to 1.06. The shear strength of gassy sand can be expressed as the formula tau (f)=c'+(sigma(n)-u(a))tan phi' + tau(us) and tau(us)=a(u(a)-u(w))(b) and increases with the decrease of the gas pressure level. Moreover, the model tests of steel pipe pile in the gassy sand show that the Q-s curve belongs to the type of steep fall, the ultimate bearing capacity of pile increases with the release of gas even though the increasing amplitude is different at the different stage of gas release, which can be explained by the internal mechanism that the increase of net stress makes predominance over the ultimate bearing capacity of pile compared with the decrease of suction under the condition of gas release slowly.