Study On The Earth Pressure And Mechanical Properties Of The Pile-supported Enclosure Structure Of Deep Foundation Pit In Soil And Rock Combination Stratum

Deep foundation pit engineering has always been a hot spot in construction engineering.The design and construction of deep foundation pit engineering is a complex and difficult problem in geotechnical engineering.Jinhua has strong regional geological conditions,and the geological conditions are soil-rock combination.The design and construction of the deep foundation pit retaining structure are difficult.In order to study the applicability of the pile-supported retaining structure used in the metro deep foundation pit in soil-rock combination.The article relies on the deep foundation pit project of Wan Da Plaza Station on Jin-Yi-Dong Line 1 in Jinhua City.The paper uses theoretical analysis,monitoring data analysis and numerical simulation analysis to study the earth pressure and other mechanical properties during the excavation of the deep foundation pit at Wanda Plaza Station.It can provide guidance for the use of pile-supported enclosure structures in Jinhua soil-rock combination area.The main research results are as follows:The distribution law of active earth pressure outside the pit during excavation:the active earth pressure is triangularly distributed when the excavation depth is above the rock surface;the active earth pressure has evolved from a triangular distribution to an"R"shaped distribution when the rock surface is below the rock surface.The maximum active earth pressure in the soil-rock combination area is about 0.45 times that of the soft soil area.Above the rock face,the measured value is about 0.81 times the theoretical value;below the rock face,the measured value is about0.48 times the theoretical value.The stress of the steel bar at the lower buried depth inside the foundation pit changes from compressive stress to tensile stress as the excavation depth increases;the stress of the steel bar outside the foundation pit mainly manifests as compressive stress,and the stress on both sides increases with the increase of the excavation depth,When the excavation to the end,the stress of each place reaches the maximum.The stress changes on both sides of the inside and outside are approximately symmetrically distributed.Near the support,there will be a maximum positive bending moment in the middle of the retaining pile.The bearing capacity of the retaining pile is relatively low,between 10%and 20%,and the safety margin of the retaining pile is relatively large.The depth of the rock surface and the prestress of the steel support have a great influence on the earth pressure outside the pit and the internal force of the enclosure structure.The results show:H₁/H=0.4 is a dividing line.When H₁/H<0.4（H₁represents the depth of the rock surface,H is the excavation depth of the foundation pit）,as the rock surface depth increases,the enclosure structure horizontal displacement changes little;The axial force of the first concrete support,the second and third steel supports basically remain unchanged;The earth pressure outside the envelope structure changes little and reaches a maximum value near the two supports.When H₁/H>0.4,as the rock surface depth increases,the maximum horizontal displacement value increases linearly;The bending moment of the pile body increases greatly,and the position of the maximum bending moment shows a downward trend;The outside of the enclosure structure earth pressure has increased greatly,the maximum earth pressure above the excavation surface shows a downward trend.The prestressing of the steel support is beneficial to control the value and position of the maximum horizontal displacement,thereby controlling the deformation of the foundation pit...