Experimental Investigation On The Earth Pressure And Settlement Of Adjacent Excavation Of Urban Shallow Tunnels

With the rapid development of China,The situation of tunnels,foundation engineerins and various types of pipe gallery engineerings intersect with each other and build at the same time has become more common.Existing theories are not sufficient for these complex boundary conditions,and can not get the accurate prediction of earth pressure of the retaining structure and stratum deformation,and lack of in-depth experimental and theoretical research.A two-dimensional retaining structure system and multi-trapdoor simulation test device was developed.Three working conditions has been considered,excavation of a tunnel near an existing underground retaining structure(stationary state),excavation of a tunnel near a newly excavated foundation pit(the active limit state is reached after the supporting structure is deformed),and the tunnel is excavated then excavation of the foundation pit.The earth pressures of retaining structure were measured with 18 cantilever load gauges.The particle image velocimetry technique was used to measure the surface settlement.For the first case,an excavation of an underground retaining structure tunnel is being built.The results shows that the earth pressure at the bottom of retaining wall decreases sharply and then increases at a turning point with certain height when shield passes through the adjacent underground retaining structure.The smaller the ratio of depth to the width of the tunnel,the greater the decrease in the earth pressure at the bottom,the higher the turning point,and the maximum ground settlement.The greater the ratio of the depth to the lateral width,the greater the range of reduced earth pressure and the maximum ground settlement.The larger the ratio of the depth to the distance of the tunnel,the greater the impact on the earth pressure of the retaining structure and the greater the maximum ground settlement.The correction coefficient of the maximum settlement of the ground settlement and the settlement coefficient of the settlement tank are introduced to obtain the revised Peck settlement prediction formula.The predictions show in good agreement with the measured values of model tests.For the second case,the tunnel is excavated near the newly excavated foundation pit.The results shows that with the formation loss caused by the tunnel excavation,the earth pressure increase of the retaining wall is positive and gradually increases and stabilizes.The smaller the ratio of depth to width,the greater the increase in earth pressure and the maximum displacement of the ground settlement curve.The ratio of the depth to distance of the tunnel increases,the increase in soil pressure does not change much,the maximum value of surface settlement increases,and the curvature of the soil surface settlement curve increases.As the ratio of the depth to the lateral width of the tunnel increases,the smaller the increase in earth pressure,the larger the maximum value of the curve,and the curve's curvature is opposite to that of the static retaining wall,which gradually decreases and becomes flat.Through analysis,a revised settlement prediction formula is also established.For the third case,the foundation pit is excavated after the tunnel is excavated.The results show that the ratio of depth to width of the tunnel,the ratio of the depth to distance,and the ratio of depth to the lateral width of the tunnel have little effect on the active earth pressure.However,when the ratio of depth to width of the tunnel are relatively large,the range of the shear strain distribution is wide,and multiple slip surfaces will appear.With the increase of the ratio of the depth to the distance of the tunnel,the two sliding surfaces formed by the fixed boundary at the bottom of the multi-trapdoor and the sides of the stuffing box are gradually moved from the sliding surface through the bottom of the retaining wall.With the increase of the ratio of the depth to the lateral limit width,the smaller the lateral limit width,the more obvious the frictional effects on both sides of the stuffing box,and the wider the range of the shear strain distribution.And the Inclination of slip surface has not changed much.