Study On The Stability Of Excavation Of Deep Foundation Pit Of Buried Karst Cave In Medium Weathered Limestone

This paper relies on the foundation pit project of the airport North tunnel of the second expressway of Guangzhou New Baiyun International Airport.In the process of foundation pit excavation,the influence of hidden karst cave on the stability of foundation pit supporting structure and surrounding soil cannot be ignored.By analyzing and studying the law of karst development and combining with the practice of site engineering,the main problems and difficulties of foundation pit construction in karst development area are clarified.By analyzing field data and based on previous studies,the ratio of similar materials in the indoor model test was determined according to Fuzzy comprehensive evaluation criteria of Fuzzy mathematics.Taking salt carving as a reference,the material for making karst caves was determined through several combination ratio tests,which solved the problem of the formation of karst caves inside the materials in model tests.Combined with the actual situation of the engineering section,the indoor model test of the excavation of the buried double karst cave in moderately weathered limestone was carried out to obtain the deformation characteristics of the supporting structure and surrounding soil during the construction,which was verified with the numerical simulation results.Through the Midas GTS NX finite element analysis software,the numerical calculation model of the excavation of the buried double karst cave in the middle weathered limestone was established,the model test was verified,and the influence of the relative position,relative size and relative connectivity of the concealed double karst cave on the excavation stability under different working conditions was numerically simulated.The specific research results are as follows:(1)by analyzing the typical foundation pit excavation period of physical and mechanical parameters of rock mass,combined with a large number of relevant weathering of limestone and the ratio of similar material test of silty clay,grouping design the experiment scheme and the determination of the basic physical and mechanical parameters of similar material,using Fuzzy Fuzzy comprehensive judgment as the basis,16 limestone group 6 selected materials,gravel layer group,the silty clay piece together 6 than as indoor rock similar material experiment,respectively.Through the analysis of the physical and mechanical properties of the cave itself and its mechanism of action,taking the salt carving as a reference,several combination ratio tests were designed,and the ratio of fine grain salt: starch: water =10:1:1 was selected to make the cave,thus solving the problem of the formation of the cave inside the material in the model test.(2)By carrying out the indoor model test of vertical excavation of the buried double karst cave foundation pit in moderately weathered limestone,it was found that the soil settlement near the diaphragm wall was relatively large,and the settlement value reached the maximum at 5cm.After being far from the diaphragm wall,the settlement gradually decreased to 0;The horizontal displacement of the diaphragm wall first increases and then decreases with the increase of depth,among which the maximum horizontal value is at the buried depth of 14 cm and increases with the increase of excavation depth.When the first layer of soil is excavated,the first support is pulled.After the second support is set,the axial force of the first support tends to shrink.The maximum axial force still appears in the middle lateral support,and the lateral supports on both sides are less affected.The stress of the soil outside the foundation pit gradually increases with the excavation depth,and the maximum value appears at the buried depth of-24 cm under the second excavation.The compressive stress of the soil inside the foundation pit decreases with the increase of the excavation depth,and tends to change to the tensile stress.(3)After the laboratory model test was verified by Midas GTS NX numerical analysis software,the influence of different working conditions on the stability of foundation pit excavation was analyzed in relation to the relative position,relative size and relative connectivity of the concealed double karst caves in the active zone.It is found that the surface settlement in each working condition presents a trend of first increasing and then decreasing.When the position relationship is vertical,the maximum settlement value appears in a position closer to the wall than in a parallel position.At the same time of connection,the maximum value of surface subsidence in the parallel distribution of karst caves is always greater than the vertical distribution.When the two caves are vertical,the surface settlement value is larger and closer to the wall when the size of the upper cave is larger.When two karst caves are parallel,the soil deformation on one side of the larger cave is larger.The maximum uplift value of foundation soil is in the middle of foundation pit and increases with the increase of excavation step.The first excavation curve basically satisfies the quadratic polynomial,and the curve changes as the excavation progresses.Whether the karst caves are connected or not has no significant influence on the soil at the bottom of the pit.The maximum lateral movement of the underground diaphragm wall appears in the third excavation under various working conditions,and the lateral movement of the diaphragm wall first increases and then decreases with the depth.Under different working conditions,the maximum displacement fluctuates in the range of 9-11 m.Through this study,the soil deformation and the overall stability of deep foundation pit excavation caused by the presence of karst caves in the middle weathered limestone stratum are discussed,and the main conclusions obtained are instructive to the project and have reference significance for similar projects.