Parametric Study of Large Side Opening in Deep Shaft at Different K_o Ground Conditions Using 3D Nonlinear Finite-Element Analysis

The study investigates the complicated states of stresses around a large opening in an underground shaft and surrounding soil for a wide range of K_o (coefficient of earth pressure at rest) ground conditions by performing nonlinear 3D finite-element analysis. For this study, the magnitude of K_o ranged from 0.50 to 1.50 with an increment of 0.25, that is, a total of five cases were considered (K_o = 0.50, 0.75, 1.00, 1.25, and 1.50). The chosen geometry of the shaft is based on a construction site of North East Boundary Tunnel (NEBT) Project by DC Water. The original layered soil is simplified, and a uniform homogeneous silty-sand and clayey-sand (SM-SC) underlain by a bedrock layer was considered for the analysis. In the present study, the performance of the shaft and adjacent soil was analyzed when using 5.2-m (17-ft) diameter breakout into a 7.6-m (25-ft) diameter underground shaft at an approximate depth of 19.8-m (65-ft) from ground level. Ground improvement by soil-freezing was implemented to stabilize the adjacent soil mass at the breakout location prior to excavating the opening on the shaft wall. Sequential construction stage analysis (seepage and stress analysis) was performed in 3D nonlinear finite-element platform (using Midas GTS NX software) to simulate the construction sequences followed in the field. The most critical construction stage in terms of stability of shaft and soil is observed when the soil is excavated inside shaft and breakout opening on shaft wall is executed. Maximum hoop stress in the shaft is very sensitive at crown and invert locations of the opening to the variation of K_o magnitudes. It was observed that increasing the K_o from 0.5 to 1.5 (300% increase) caused the stress at the invert to increase by about 170%. In longitudinal direction, the influence of variation of K_o on stresses is observed to be not very significant. Formation of plastic zone in improved soil located around the breakout opening zone occurred and the extent of the plastic zone expanded to larger distances with the incremental increase of K_o magnitudes. The soil located in the bottom plug of shaft experiences cap failure due to unloading of overburden pressure for excavation.