Model experiment and numerical simulation study on the instability of shield tunnel face in upper-clay and lower-sand composite strata

Abstract

Strata instability and environment disaster collapse caused by insufficient supporting pressure of shield tunnel excavation face is always a difficult engineering problem that cannot be ignored. Fixing the buried depth ratio as 1.5 and setting the upper-clay thickness ratio as 0, 0.2 and 0.5 respectively, the instability and failure law of shield tunnel excavation face in the upper-clay and lower-sand composite strata is studied by means of model experiments and numerical simulations. The quantitative analysis is carried out from the perspective of instability collapse pattern, ground subsidence mechanism, fluid-solid coupling mechanical response and spatial distribution change law. The study shows that the whole process of excavation face instability collapse can be divided into four stages: water loss, water and sand migration, loosening and instability, collapse and failure. Clay thickness ratio has little effect on pore pressure and effective stress in front of excavation face. The arch formed following excavation face instability and collapse extends to the vicinity of the interface of clay and sand, and with the increase of clay thickness, the arch moves away from the interface and the extension speed decreases. The thicker the clay is, the wider the subsidence range is and the smaller the subsidence value is. Distortion energy accumulates at top and bottom of the tunnel face and the top of the clay layer front-above the excavation face after its instability failure. In front of excavation face, the pore pressure isosurface shows a transitional change from bottom to top in the form of ‘basin-bowl-vase’. In the tunnel middle horizontal plane, the zone in the radius R semicircle in front of excavation face is defined as the pore pressure vacuum zone, the zone in the R ~ 4R semiring is defined as the hydraulic gradient zone, and the zone outside 4R is defined as the non-influence zone.