Investigation On The Deformation Mechanism Of Sandy Ground Due To Tunnel Excavation Based On The Transparent Soil Testing Technology

Large deformation or potential instability caused by tunnel excavation in sandy ground is a common underground engineering problem.To prevent such failures,it is necessary to investigate the deformation pattern and mechanism of soil surrounding the tunnel.The ground deformation chatacteristics caused by the tunnel excavation can not only focus on the surface subsidence,but also the deformation inside the ground can not be ignored.Although the displacement information can be obtained through conventional experimental techniques by embedding the sensors in the model,during which process the soil properties has been changed,which in turn affects the authenticity and reliability of the displacement.Therefore,it is necessary to find a testing technique that can obtain the continuous displacement without significant influenceson the ground.The transparent soil model testing technology combines the synthetic transparent soil and digital image processing technique can directly obtain the displacement of inside ground.Based on this technique,this paper will investigate the deformation law caused by tunnel excavation in the sandy ground,and establish a mesoscopic model.It subsequently combines the particle discrete element method to explain the deformation and failure mechanism of the sandy ground.The main work and conclusions of this study are as follows:(1)The stress-strain chararacteristics of the glass sand with transparent soil material under loading and unloading paths were examineded using conventional triaxial test equipment.A conventional triaxial test model was established using the particle flow program to simulate the mechanical properties of the glass sand under the corresponding stress path in the triaxial test,thereby calibrating the corresponding mesoscopic parameters.The influence of mesoscopic parameters on macroscopic mechanical properties was studied by using grey relational model.(2)The transparent soil model test was designed to research the relationship between ground soil loss ratio and ground deformation during tunnel excavation in sandy ground.Based on the transparent soil test technology,it treats the the soil loss ratio as control variable,the evolution law of the ground deformation under different soil loss ratio and buried depth condition was studied.The relationship between the ground soil loss ratio,the tunnel depth and the settlement tank width parameter were established,and the calculation formulas for surface settlement and ground settlement were revised.(3)The particle discrete element method was used to capture the whole process of deformation and destraction of tunnel surrounding ground during tunnel excavation.The deformation process of shallow buried tunnel can be categoried into four stages as follows: the tunnel surrounding soil losens-the crown collapsed-the surface collapsed-the global destruction.The deformation process of deep buried tunnel can be categoried into four stages as follows: the tunnel surrounding soil losens-the crown collapsed-the tunnel convergence-the global destruction.(4)The evolution law of internal force chain of surrounding rock during tunnel excavation is studied.During the excavation,the contact force between the particles is rapidly adjusted,and the deformation of the soil around tunnel lags behind the change of contact force.Under the same time step,the change of the force chain structure is faster than that of the deformation,which indicates that there is a difference in time and space between the deformation and stress release of soil caused by unloading due to the tunnel excavation.From the perspective of the failure propagation,the stress release is larger than that of the horizontal direction.From the analysis of the force chain structure diagram,the soil arching effect of the shallow buried tunnel is less significant,compared with that for the deep buried tunnel.(5)The variation of pore ratio and coordination number at different locations in the ground was analyzed.It can be observed thathe change of the void ratio brought by the unloading of tunnel excavation leads to the expansion of the surrounding soil.The variation of the void ratio at different positions is different,results in the different magnitude of the volume change.Volume change is an important indicator representing the differential settlement.The change of coordination number can indirectly reflect the change of contact force.Based on the change of coordination number,the stress release of the tunnel crown after excavation is the greatest,followed by the springline,while the release from the lateral part is the least significant.The changes of the coordination number and the void ratio show opposite patterns.(6)A particle mesoscopic model was established to invetigate the deformation mechanism of the ground sandy soil surrounding the tunnel.A mesostructured model was proposed to establish the relationship between the friction coefficient between particls,the void ratio and the contact force between particles.Results shows that due to the stress deflection in the tunnel spandrel of the shallow buried tunnel,in some area,there is a phenomenon of volume shrinkage,in the other regions,there exists the phenomenon of volume expansion.The deformation at the springline is smaller than the deformation at the vault in the joint action of the volume shrinkage and volume expansion.After the tunnel excavation,the stress release in the horizontal and vertical directions of the side wall is smaller than that of the vaulted area,and the contact force variation at the side wall is less significant.Thus the deformation of the side wall is smaller than that of the vault.