Study On Engineering Characteristics Of Sand Lens In Chengdu

Chengdu City is located in the Chengdu Plain,the geological situation is very complex,Chengdu Plain area is widely distributed by alluvial diluvial and glacial-water subsidence formation of sand and gravel.In recent years,Chengdu subway construction has entered a stage of vigorous development,and a large number of lines pass through the sand and gravel strata.Chengdu area sand pebble bed is extremely unstable,river sand alluvium of lenticular sand distribution in sandy pebble stratum in great quantities,shield between disturbance or sand once appeared water erosion,very easy to cause sliding,sand gravel stratum in the subway construction because of the influence of excavation disturbance and its own characteristics,inevitably cause formation damage.The main function of sand lens collapse is to increase the deformation and settlement of the stratum.After the collapse of sand lens,a large amount of voids will be left in the stratum and a cavity will be formed.If this part of the formation loss is not filled in time,it will lead to upper subsidence and deformation.Therefore,this paper takes Chengdu Metro Line 7 as the boundary of the study area,mainly relies on the collection and collation of a large number of engineering survey data of Chengdu Metro Line 1,2,3,4,5,7 and 8,as well as a large number of borehole data and hydrological survey data of downtown buildings,and chooses the geological modeling software GOCAD.The threedimensional spatial distribution model of sand and pebble strata in the main urban area of Chengdu is constructed.The threedimensional spatial distribution morphology of sand pebble layer and the spatial distribution of sand lens are studied.In the case of shield tunneling,the change law of the influence of sand lens on the surface subsidence characteristics caused by ground loss in sandy pebble stratum is analyzed,and the influence law of common factors such as overlying soil depth and formation cavity size on the surface subsidence is analyzed,which can better understand the basic law of movement of sandy pebble stratum.This paper further explores the perturbation analysis of the second tunnel excavation in the longitudinal sand pebble stratum containing lens body,analyzes the bearing capacity of the foundation of the sand pebble stratum,and ensures the normal use of the buildings around the surface,which provides certain reference significance and reference value for the construction of the subway and buildings in Chengdu in the future.The main research results are as follows:(1)the characteristics of sandy pebble stratum in the study area size,physical and mechanical characteristics and the characteristics of engineering analysis,the results show that the area of sandy pebble formation of particles larger than 20 mm of pebble accounts for more than 55%,the pebble for formation main part of in formation skeleton effect,decision effect to the formation of physical and mechanical properties,deformation modulus of sandy pebble stratum and the bearing capacity of topdown as density increases.(2)The three-dimensional model of sand and pebble layer in the study area of Line 7 is analyzed,and the distribution rule of sand and pebble layer in the study area is obtained.The overall thickness of sand and pebble layer in the study area is higher in the west and lower in the east.From the northeast to the southwest,there is more in the southwest.From the east-west direction,there are more lens bodies in the west and less in the east.There are more lens bodies in the middle section from Tianfu Square to Nijiaqiao,and the trend is in line with the trend of water flow.A small number of lenses exist in the pebble soil layer of the Lower and Middle Pleistocene,which has little influence on engineering construction due to the deep strata.(3)Is obtained by using FLAC3 D numerical simulation software,the results show that when the lens contained in the formation,its hysteresis settling time shorten,when lens above the tunnel hole is easy to cause the occurrence of large-area subsidence,impact on surface subsidence is one of the biggest,its embedment position 0.5 D and 1 D effects.Compared with previous experiments by scholars,it can be seen that its maximum subsidence is 13.6 times that of the lensless sand and pebble stratum,which accelerates the delayed subsidence of the stratum and accelerates the instability of the cavity.When the lens is on both sides of the tunnel,the variation of its settlement is greater than that of the sand pebble stratum without lens,but the variation of its settlement is smaller than that of the lens directly above the tunnel.Shield tunneling at bedrock has little influence on surface deformation,but the presence of lens still affects its settlement,so attention should be paid to sand lens in engineering.