Discrete Element Analysis And Physical Model Test Of Deformation In Cobble Stratum Tunnel Excavation

The construction of transportation infrastructure is transferring from ground to underground,because of the traffic diversion and future urban space expansion.In Urumqi district of Sinkiang,alluvial and diluvial cobbles are widely distributed along the zonal terrain of Urumqi River.The urban underground far-flung cobble stratum brings about many technical problems to the underground engineering.The geotechnical properties of cobble and conventional engineering are essentially different.In the process of tunnel excavation,due to the uneven distribution and discreteness of cobble particles,it is easy to collapse and the construction difficulty increases.Based on the project of Urumqi underground subway line 1,the author considered the future demand for the construction of urban rail transit and municipal tunnel.Combining the city subway tunnel buried depth is shallow and the characteristic of the large excavation span,the author explored the cobble stratum deformation law of undermining method and put forward the corresponding stratum settlement control index.The research is as follows:(1)Starting with the physical and mechanical characteristics of cobble stratum,the author expounded the reason of using discrete element numerical calculation method.Based on the particle flow code(PFC)software,the author established the numerical triaxial test model.Through repeating adjustment of macroscopic parameters of model,the the influences of different types of cobble grain parameters was analyzed into the influences of the macroscopic mechanical properties.And combining with the characteristic curve of the measured formation mechanics of indoor test,the author demarcated the macroscopic particle parameters for Urumqi cobble stratum.(2)On the basis of discrete element numerical simulation,the deformation of surface under different span and buried depth is calculated and the corresponding regression formula of surface subsidence is summarized.The data of surface and vault deformation are analyzed,and then get the change between the two about depth.According to the principle of best square approximation,the solution function of Peck formula parameter was established.The corresponding formation loss rate and cobble geological condition influence coefficient were obtained.The prediction method of ground surface settlement which can be referenced in the corresponding stratum of the region is obtained.(3)With the similarity ratio is 1:20,the relationship between the physical parameters and the prototype is derived.And the author determined the physical and mechanical parameters of the similar model.With the similar materials to be selected,the method of orthogonal test is adopted.The influence of each factor on its mechanical properties was studied,and the proportion of similar materials was determined.(4)The model test case of cobble stratum was established.And the excavation model test was carried out in different span and depth.The author used the earth pressure box to measure the inner pressure of surrounding rock.And the data of soil deformation was collected by using the non-contact video measurement system.The deformation and unloading condition of cobble stratum after excavation are reflected by the experiment.Compared with the aforementioned discrete element simulation results,the deformation characteristics and laws of the cobble stratum are further verified.(5)The control value of ground surface settlement is analyzed under the influence of different external factors.On the basis of the calculation results of numerical simulation and model test,the influence coefficient of the corresponding cobble stratum is given.And the allowable value of surface subsidence is determined.Based on the study of ground deformation law of excavation process,the corresponding surface settlement control index is controlled by stages.