Study On Liquefication Uplift Mechanism And Anti-uplift Measures Of Rectangular Underground Structure Under Bidirectional Earthquake

The uplift response law and mechanism of underground structures in liquefiable foundation under earthquake action have always been a hot and difficult point in the field of geotechnical engineering seismic research.Based on the conclusion that vertical earthquakes do not promote sand liquefaction,few studies have considered the effects of horizontal and vertical coupling earthquakes on sand liquefaction and underground structure uplift.Therefore,in this paper,the finite element-finite difference coupling numerical method is used to analyze the influence of vertical earthquake on the underground structure in the liquefied foundation,and the uplift mechanism of the underground structure in the horizontal and inclined liquefied foundation under the action of bidirectional earthquake is revealed.Furthermore,an anti-uplift measure that can effectively suppress the uplift of underground structures under bidirectional earthquakes is proposed.The research can provide seismic fortification suggestions for the construction of underground structures in saturated sand foundation under horizontal and vertical coupling earthquakes.The main research contents and conclusions are as follows:(1)Two finite element numerical analysis models with and without underground structures are established by using the numerical program which can effectively simulate the seismic response of underground structures in saturated sand.The response laws of these two models under horizontal,vertical and bidirectional earthquakes are studied,and the differences of acceleration,excess pore water pressure and displacement response of soil in these two models are analyzed and compared.Furthermore,the acceleration,displacement and internal force response of the structure under different directions of earthquake action are compared and the floating mechanism of the structure under earthquake action is revealed.The results show that the vertical ground motion aggravates the soil-structure interaction by changing the shear wave propagation and increasing the vertical inertial force,which aggravates the degree of sand liquefaction and the seismic response of underground structures.(2)The effects of liquefaction layer thickness,sand density,groundwater level,buried depth of underground structure and ground tilt angle on the uplift and internal force of the structure under bidirectional earthquake are analyzed.The seismic response differences of underground structures in inclined and horizontal liquefied foundations are compared,and the uplift mechanism of structures in inclined liquefied foundations is further revealed.The analysis results show that the underground structure in the inclined site will have different vertical displacements,and as the ground inclination angle increases,the horizontal displacement of the structure and the soil gradually increases,and the vertical displacement difference and internal force difference on both sides of the structure gradually increase.In addition,the horizontal displacement of the structure mainly occurs during the earthquake,and the vertical displacement mainly occurs after the earthquake.(3)The limitation of the cut-off wall measure to suppress the seismic liquefaction of underground structures under bidirectional earthquakes(horizontal + vertical)is revealed.According to this limitation,the cut-off wall measures to suppress the uplift of the structure in seismic liquefaction are further improved,the optimal parameter design of the improvement measures is determined,the anti-uplift effect of the improved cut-off wall measures is analyzed,and its anti-uplift working mechanism under bidirectional earthquake is revealed.The results show that the improved cut-off wall can effectively restrain the uplift of underground structure during and after bidirectional earthquake.It prevents the structure from uplift by inhibiting the flow of soil on both sides of the structure,reducing the water pressure on the structure and inhibiting the uplift of the soil below the structure.