Progressive Damage Pattern And Evolution Of Soil-rock Composite Strata Under The Influence Of Subway Tunnel Consteuction

With the vigorous development of subway tunnel construction,the geological conditions traversed by subway tunnels are more complicated,and accidents such as stratum instability and collapse caused by subway tunnel construction have been common in recent years.Compared with single strata,the deformation transmission process,mechanical evolution characteristics and destabilization damage mechanism of soil-rock composite strata under tunnel construction disturbance are different.In response to the inadequacy of existing research,this paper investigates the progressive damage pattern and evolution law of soil-rock composite strata under subway tunnel construction disturbance in Qingdao,relying on the National Natural Science Foundation of China(Grant No.51978356).Firstly,the deep and shallow burial limits of tunnels with soil-rock composite strata were studied using numerical calculation methods,and on this basis,similar model tests and discrete element simulations were used to reveal the deformation and progressive damage mechanisms of soil-rock composite strata under the disturbance of subway tunnel construction,and the main research work and results obtained in the thesis are as follows.(1)The existing methods of tunnel deep and shallow burial demarcation are summarized and analyzed,and the problem of tunnel deep and shallow burial demarcation for single strata and composite strata is studied based on the critical arch forming conditions.For single stratum,the critical arch burial depth increases with the weakening of the surrounding rock condition under the same excavation diameter,but the two are not linear,and the critical arch burial depth increases linearly with the increase of excavation diameter under the same surrounding rock level.For soil-rock composite strata,the critical arch depth of the strata is mainly related to the diameter of the tunnel,the location of the soil-rock interface,the stress-strain of the rock layer after excavation and the stiffness ratio of the rock layer to the soil layer.In general,the higher the height of the pressure arch,the greater the critical arch depth of the tunnel.The location of the soil-rock interface will determine whether the outer boundary of the pressure arch is formed in the rock or soil layer,which in turn affects the critical depth of the tunnel.(2)Based on similar model tests and discrete element software PFC2D,the deformation and progressive damage mechanism of soil-rock composite strata under underpass tunnel construction disturbance are studied,and the destabilization damage mode of the tunnel under different overburden rock thickness is analyzed.When the overburden thickness is small,the overburden soil layer is acting as a load,the rock layer above the tunnel is equivalent to an arch-shaped beam,the beam is damaged under the action of the upper soil layer load,so the collapse damage manifested starts from the top of the arch,after the rock layer above the arch completely collapses,at this time the soil layer will continue to expand upward with the collapse width of the rock layer at the soil-rock partition interface,in the case of a larger burial depth,the collapse will eventually stop and form in the soil layer Stable collapse arch.When the overburden thickness is larger,after tunnel excavation,the surrounding rock at the top of the arch position converges downward and begins to invade the contour surface boundary,the stratum generally occurs first from the arch waist shear damage,and then expand to the top of the arch,the surrounding rock loosening mainly in small pieces fall,and finally form a collapse arch.(3)When the actual burial depth of the tunnel is 4D,the tunnel collapses in an arch for both soil-rock composite strata and single strata.The collapsed arch range is the largest for soil strata,the smallest for rock strata,and the collapsed arch range for soil-rock composite strata is between soil and rock strata.When the overburden thickness of the arch is small(h_r=0,1/6D,1/3D),the collapse extends through the soil-rock partition interface to the soil layer,but does not extend to the ground,and the smaller the overburden thickness is,the larger the collapse range of the soil layer is.When the overburden thickness is larger(h_ris greater than or equal to 1/2D),the tunnel collapse arch is completely formed in the rock layer,and the height of the collapse arch gradually decreases with the increase of the overburden thickness,but the change is small,so in engineering,1/2D can be made as the minimum overburden thickness of the soil-rock composite stratum.When the overburden thickness exceeds D,the collapse process and the final collapse range of the tunnel are not much different from that of a single rock formation.The research results of this paper have some reference and significance for the analysis and evaluation of the design and construction of tunnels in soil-rock composite strata.