Research On Geological Property And Deformation Mechanism Of Sandy Cobble Stratum In Chengdu Metro

Sandy cobble stratum distributes widely in Chengdu. Compared with a normal stratum,the nature of sandy cobble stratum is unique, and is more prone to large stratum deformation under the shield construction, which results in ground settlement and causes large-scale collapse if it’s severe enough.Currently, the research on the deformation of sandy cobble stratum under the shield construction is still at the foundation stage which can not completely solve practical problems.Based on the existing achievements of Chengdu Metro and the study of Chengdu Metro Line 4 West Extension Line,the engineering property of sandy cobble stratum has been summarized,and the deformation mechanism of sandy cobble stratum under the shield construction has been analyzed.The main contents and results of this paper are as follows:1.Causes of sand cobble stratum in Chengdu vary with time:the alluvial-diluvial deposits during the upper Pleistocene,then the moraine and outwash, alluvial origin during the middle Pleistocene,gradually converted to the cause of outwash during the late Pleistocene,and now are the causes of the modern river terraces and the floodplain.Sandy cobble stratum mainly distributes in the plains section, most of its thickness is 13～27 meters and the maximum thickness is over 30 meters, its buried depth is 1～3 meters and the maximum depth can be over 7 meters.Horizontally, from north west to south east in Chengdu, the buried depth of sandy cobble stratum is deeper and deeper.Longitudinally the Sandy cobble stratum is from new to old with depth, its degree of weathering is gradually weakened; the compactness and bearing capacity of gravel layer increase with depth.2.The study on the characteristics of sandy cobble stratum in the study area finds that its grading isn’t continuous,The mass ratio of the pebbles which are larger than 20mm is between 55%～90%, which have a absolute proportion in the stratum.The morphological characteristics of pebbles in the study area is insignificant, mainly spherical or ellipsoidal, and the friction coefficient is small.The arrangement of sand and gravel is stacked, and the grain size of pebbles is larger and larger from east to west.The roundness and sphericity are getting better from west to east.The overall distribution of boulders is as follows:the content of boulders gradually decreases from west to east, the grain size of boulders gradually increasing with the depth.The content of boulders is more than 0.75% in the sections, mainly concentrate in the depth of 10m～30m.The average compressive strength of boulders is 157Mpa which means a good anti-G ability.Sand lenses have a continuous grading and a uniform distribution in sections.The major cause of the loss of its liquefied is the change of inter-particle moisture content.3.The geological and engineering characteristics of sandy cobble stratum is poor as a whole.The stability of sandy cobble stratum is very poor under the shield construction,which interbeds seriously (the big difference of particle size,Stratigraphic interaction), so that the engineering properties of sandy cobble stratum at different palace have a big gap.Different regions of the excavation face are quite different,which results in the stability reduction of the excavation face; there will be block-off, collapse and instability of stratum structure in the case of external disturbance.4.The analysis of shield construction principle and the response of the sandy cobble stratum under shield construction figures out that:the deformation of sandy cobble stratum is mainly due to the ground loss.Then the composition of ground loss and the deformation law of sandy cobble stratum have been analyzed.5.Considering the discrete nature of sandy cobble stratum and the higher anti-G ability of pebbles than overall stratum,the particle discrete method (PFC2D) has been used to simulate the deformation process of sandy cobble stratum under shield construction.Deformation of the stratum and the change process of stress have been concluded.And also how factors such as the structure of stratum,boulders, sand lens influence the deformation has been studied.6.After the analysis and simulation.the range to control ground deformation has been summarized:The key control area of formation deformation is an arch region, which is located in the upper part of the tunnel, the thickness is 0.5D (D is the diameter of the tunnel), its transverse range is determined by the settlement trough width (W) and the depth of tunnel center (H).