Formulae for overall stability of composite soil nailing walls

The prestressed anchors by pull-out resistance, cut-off curtains and mini-piles by shear strength together improve the overall stability of composite soil nailing walls. At present, it is the most mature and effective method for calculating the overall stability, which accumulates and reduces the resistance of these composite components. The key to the formulae is the reduction coefficient of breakdown resistance coefficient produced by the soil resistance, soil nails and composite components (the resistance coefficient is the ratio of the resisting torque produced by the components to the downturn torque produced by soil). They are empirical data and can be obtained from actual engineering data by inverse calculation arrangement according to certain assumptions. The studies on some critical excavations and the verified results show that there are reasonable ranges of combination coefficients when these components work individually and jointly, that is, they are 0.5~0.7 for prestressed anchors, 0.3~0.5 for cut off curtains and 0.1~0.3 for mini-piles. In addition, the sum of the breakdown resistance coefficients of soil and soil nails has the reasonable lower limit and it is about 0.86~0.97 (regardless of the other components function). When this condition is satisfied and the overall stability safety coefficient requirement is met, the movement of the excavations is generally small.