Study of surface subsidence above an underground opening using a trap door apparatus

Abstract Physical model simulations have been performed to determine the effects of underground opening configurations on surface subsidence under super-critical conditions. This paper indicates the importance of the main factors that control the extent of subsidence produced on the surface and determines the effects of geometry of underground openings on the angle of draw, the maximum subsidence and the volume of the subsidence trough. A trap door apparatus with the test area of 95 × 95 cm2 has been fabricated to perform the scaled-down simulations of surface subsidence. Gravel is used to represent the overburden in order to exhibit a cohesionless frictional behavior. In plan view the excavation dimensions are sufficient to induce maximum possible subsidence. The findings can be used to evaluate the subsidence profile for tunnels and caverns in soft ground. The results show that the angle of draw and the maximum subsidence are controlled by the width (W), length (L), height (H) and depth (Z) of the underground openings. The angle of draw and maximum subsidence increase with increasing L/W ratio and tends to approach a limit when L/W equals 3. For the same L/W ratio and H/W ratio, increasing the Z/W ratio reduces the angle of draw and maximum subsidence. The volume of the subsidence trough increases with increasing H/W ratio and L/W ratio. The width of the subsidence trough can be represented by sets of empirical relations. The relation between opening depth and subsidence trough developed by Rankin (for cohesionless soils) is in good agreement with most physical model results for deep openings (Z/W = 2–4), while for Z/W = 1, the predicted trough width is less than the physical model simulation. The volume of the subsidence trough is largest for Z/W = 2.5 and for H/W = 0.6, and is about 60% of volume of the underlying opening.