Study On The Characteristics Of Surface Deformation Caused By Loss Of Sandy Pebble Stratum

The Chengdu Plain is an alluvial fan plain formed under the hydrodynamic force of the Minjiang River and its tributaries.Sandy pebble stratum formed by alluvial and ice water subsidence is widely distributed in the area.In recent years,the construction of Chengdu Metro has entered a stage of vigorous development,with a large number of lines crossing the sandy pebble stratum in the area.Sandy pebble stratum is a kind of mechanically unstable stratum with special properties.In metro construction,due to excavation disturbance and its own characteristics,it will inevitably cause formation loss.This unstable factor will cause largescale subsidence or collapse of the surface,which affects the safety of surface structures,endangers personal and property safety of residents,and has aroused widespread concern in society.Therefore,it is of great engineering significance to study the characteristics of the surface deformation caused by the loss of sandy pebble stratum and analyze the influence of common factors on surface deformation.Based on the second phase project of Chengdu Metro Line 19,combined with the relevant engineering data,this paper summarizes and analyzes the characteristics of sandy pebble stratum,and studies the surface deformation characteristics caused by the formation loss in the sandy pebble stratum after the metro construction by using the methods of model test and numerical simulation.The main research results are as follows:(1)The analysis of the particle size,physical and mechanical characteristics and engineering characteristics of the sandy pebble stratum in the study area shows that the proportion of pebbles in the sandy pebble stratum with a particle size greater than 20 mm exceeds 65%.Pebble is the main part of the stratum,which plays a skeleton role in the stratum and determines the physical and mechanical properties of the stratum.The deformation modulus and bearing capacity of the sandy pebble stratum are shown as increasing from the top to the bottom as the density increases.(2)Based on the geometric similarity,the indoor model test was carried out,and the comparative test groups of different tunnel burial depth,formation cavity size and rainfall seepage flow were set up to measure the surface settlement and compressive stress in the formation,and the law of surface deformation and pressure stress change caused by the cavity under different simulation conditions was obtained.The change of surface subsidence in time can be roughly divided into four stages: the initial slight change stage,the early rapid increase stage,the mid-term slow increase stage and the later stable stage.The surface settlement curve has the characteristic of Peck curve.In the test scheme,the maximum settlement value of the surface under different burial depth is between 6.53 mm and 29.64mm;the maximum settlement value of the surface under different formation cavity size is between 3.62 mm and22.65mm;the maximum settlement value of the surface under different water infiltration amount is between 8.61 mm and 22.65 mm.(3)Using FLAC3D numerical calculation software,the law of formation displacement and vertical stress change caused by formation loss after excavation is obtained,and the characteristics of formation deformation under different burial depths and different formation cavity sizes are compared.The numerical simulation results show that the influence range of the surface deformation caused by the cavity is mainly concentrated in the horizontal 1.5D range directly above it,and the burial depth of 1.5D is a distinguishing point that has a significant influence on the surface deformation.