Experimental Study on Surrounding Rock Stability for Asymmetric Small Clear Distance Tunnels in Water-Rich Strata

Relying on the actual metro tunnels with grade V surrounding soil in the southeast coastal area of China, model test is conducted to investigate the deformation characteristics and instability mode of surrounding soil during the construction of asymmetric closely-spaced twin tunnels in water rich strata. Besides, the erosion characteristics and failure mode of surrounding soil is analyzed during the twin tunnels operation. The results show that: During the construction phase, considering the influence of twin tunnels geometry and the excavation sequences, the surface settlement curve presents an asymmetric form after the completion of twin tunnels excavation. Moreover, the surface settlement curve after tunnels excavation is in the shape of "single groove". Affected by the subsequent tunnel excavation, the position offset of the maximum settlement point on the surface is 4 times the twin tunnels' clearance. Due to the dual unloading effects of the middle soil pillar between asymmetric closely-spaced twin tunnels, the collapse range of the arch crown of the larger section tunnel gradually develops from a symmetrical form to an asymmetrical form with the twin tunnels' excavation steps. The lateral stress release of the middle soil pillar reaches 76% of its initial value after the twin tunnels construction. During the operation period, the surface subsidence value experiences four stages with the rapid rise of groundwater level and surface runoff, including slow increase, deformation rate increase, sharp increase and over-range damage. Under the effect of seepage erosion of surface groundwater, the overlying strata of asymmetric closely-spaced twin tunnels are subject to instability and failure, resulting in an asymmetric "collapse funnel" extending to the surface. The damage area of overlying strata on the side of larger section tunnel is larger.