Study On Stability Of Super-large Deep Foundation Pit In Swelling Stratum

Based on the engineering background of the super large-scale deep foundation pit of CHUANSHI depot and comprehensive base of Chengdu Metro Line 7,this paper analyzes the characteristics and applicable conditions of different retaining structures,optimizes and determines the types of retaining structures of different sections,and then analyzes the stability of the foundation pit and the law of the stress and deformation of the retaining structure when considering the expansion effect by FLAC^3D difference element software simulation.It is verified by the field monitoring results,and compared with the law without considering the expansion effect of expansive soil.The main research results are as follows:?1?When the swell effect is considered,the change law of the foundation pit and the supporting structure is basically the same as when the swell effect is not considered.?2?With the increase of excavation depth of foundation pit,the influence of swell effect on the vertical displacement of ground around the foundation pit,the deformation of soil and pile towards the foundation pit,the maximum positive and negative bending moment of pile,the axial force and the axial displacement of anchor cable increases gradually.The swell effect has a small effect on the soil and surrounding piles near the dormitory building,and the dormitory building is in a stable state.The swell effect has a greater effect on the axial force and axial displacement of the free section of the anchor cable than the anchor section.Under the action of swell force.?3?As the excavation depth of the foundation pit increases,the surface uplift at the edge of the foundation pit gradually decreases,and the surface settlement at other locations gradually increases;the deformation of the soil and piles into the foundation pit,the axial force and the displacement of the anchor cable gradually increase,but the increase gradually decreases;the position of the maximum positive and negative bending moment of the pile gradually moves downward.?4?The maximum surface settlement of the foundation pit varies from 6.0 m to15.0 m from the edge of the foundation pit.The largest surface settlement at the deepest position of the foundation pit,the maximum ground settlement is 4.7 mm,which is about 20%larger than when the swell effect is not considered,and the field monitoring value is 3.6 mm;the second slope grading second slope soil has the largest deformation into the foundation pit,with a maximum value of 41.8 mm,which is about 340%larger than when the swell effect is not considered,which are all within the control value range.Deformation of the foundation pit is stable.?5?The deformation of the pile into the foundation pit gradually decreases with the increase of the pile depth,and the deformation of the pile top into the foundation pit is the largest.The maximum bending moment outside the lower part of the pile is greater than the maximum bending moment inside the upper part of the pile.The pile at the deepest position of the foundation pit has the largest deformation and maximum bending moment in the foundation pit,the maximum deformation of the pile in the foundation pit is 21.6 mm,which is about 8%larger than when the swell effect is not considered,the field monitoring value is 29.1 mm;the maximum bending moment of the pile is 1270.0 k N·m,which is about 11%larger than when the swell effect is not considered;which are all within the control value range.Foundation pit retaining piles are in a stable state,but monitoring of the pile forces and deformation at the deepest position of the foundation pit should be strengthened.?6?With the increase of the distance from the locking end of the anchor cable,the axial force at the free section of the prestressed anchor cable first increases and then decreases,the axial force at the anchor end gradually decreases,and the axial displacement gradually decreases along the entire length of the anchor cable.The anchor cable played an obvious role above the middle depth of the foundation pit,and the Foundation pit bottom anchor cable did not play a corresponding role.The axial force of the anchoring section of the capsule anchor cable is relatively large,and the anchoring effect is significant.At the deepest position of the foundation pit,the axial force and axial displacement of the fifth row of anchor cables are the largest.The maximum axial force value of the anchor cable is 763.1 k N,which is about 29%larger than when the swell effect is not considered,Both are larger than the pre-stressed anchor cable lock value and smaller than the anchor cable tensile strength standard value;The maximum axial displacement of the anchor cable is 6.4 mm,which is about 30%larger than when the swell effect is not considered.The expansion effect has a large impact on the axial force and axial displacement of the anchor cable.The influence of the swell effect on the anchor cable should be considered during construction,and the monitoring of the stress and deformation of the anchor cable should be enhanced.?7?The construction of the cushion and the main structure and the construction of Zone B have no effect on the stability of the foundation pit and the stress and deformation of the supporting structure.