Impact Of Foundation Softening On Inverted Arch Of Tunnel In Mudstone

As the "The Belt And Road"policy brings new chance to the development of the transportation network in the Northwest,the construction of transportation tunnels has entered the fast way.When the tunnel passes through the mudstone stratum,the physical and mechanical characteristics of the foundation mudstone change significantly due to the seepage of the foundation,which affects the mechanical deformation characteristics of the invert arch.Through the combination of theoretical analysis,laboratory test and numerical simulation.The damage characteristics of the tunnel arch under the softened environment of the tunnel foundation are studied,and the damage mechanism of the tunnel arch is summarized.Based on the existing literature,the mechanical calculation method of the invert arch of the mudstone tunnel is summarized,and the failure mode of the inverted arch is promoted.The existing cases and the disposal measures of the inverted arch of the tunnel are summarized and analyzed.Regarding the laboratory tests of mudstone,its permeability and swelling characteristics were mainly studied.In the permeation test of the original sample and the remodeled sample,by setting the dry density,pressure head,different load levels,initial moisture content and other factors of the sample,it is concluded that the permeability coefficient of the mudstone sample decreases significantly under the action of the overburden load.For remodeled soil samples with different dry densities,the permeability coefficient tends to decrease with the increase of dry density due to the fewer pores and seepage channels in the mudstone.In the permeation test of remolded soil samples with different head heights,as the head height increases,the permeation pressure increases,and the permeability coefficient gradually increases,and the larger the head height,the larger the permeability coefficient of the mudstone sample,and the more the rate of change Obvious;for different remolded soil samples with initial water content,as the initial water content increases,the permeability coefficient decreases first within a certain range,and after reaching a certain level,the permeability coefficient gradually increases,with the water content gradually increasing.In the expansiveness test of mudstone,it can be seen from the calculation of the expansion volume and expansion force that the expansiveness of the mudstone is significantly lower at a lower water content,and the expansibility is significantly weakened with the increase of the water content,and the expansion is stable.The duration has also increased significantly.And there is a good logarithmic function relationship between the increment of the expansion force and the increment of the initial water content.In order to study the influence of softening foundation mudstone on the mechanical effect of the inverted arch,Abaqus software was used to study the influence of the attenuation of the mechanical parameters of the foundation mudstone on the invert arch in combination with the change of the internal field variables.The paper is divided into uniform softening of the foundation and uneven softening of the foundation.The softening stage and softening depth are set.It is found that the change of the large principal stress and strain of the inverted arch structure has a great relationship with the softening depth,especially when the softening depth is less than plastic influence radius caused by tunnel excavation.The effect is most pronounced when the resulting plastic radius is used.When even softening occurs,the failure type of the inverted arch is the closest to the collapsed fracture type,and the principal stress failure location is near the arch foot.However,when uneven softening occurs,due to the existence of deflection stress,Except for the relatively concentrated stress on the arch foot positions on both sides,the stress level of the inverted arch on the softened side is in a safe state of compression,while the unsoftened side is affected at a distance of 4.5m from the centerline of the overhang arch,and the principal stress level is in a damaged state.Which belongs to uneven pull-type failure.In this paper,through the study of the softening stress mode of the mudstone tunnel foundation,combined with the seepage expansion law of the weakly expansive mudstone in the room,the stress and deformation law of the mudstone tunnel inverted arch is summarized,which provides a good reference for further research and foundation treatment.