Discussion on several problems in design of retaining structures of deep excavation

Based on the laboratory tests on excavation stress paths and field test data, some problems in the design of the supporting structures of foundation pits are analyzed. The idea is proposed that the consolidation state of the foundation soil should be recovered and the stress paths corresponding to the active and passive areas of foundation soils should also be considered. It is found that the capability of soil resistance to shear failure in active and shallow passive areas is greater than the calculated results using the conventional CU strength index, and it is beneficial to engineering safety. However, the capability of the soils in the deep passive zone to resist shear failure is less than the calculated results using the conventional CU strength index. For the projects requiring a large degree of resistance capability of the passive foundation soils, the embedment depth of the supporting structures needs to be checked to ensure that the passive resistance is less than the limit passive earth pressure. The 'strong impact zone’ of excavations in the passive zone is proposed, and the method for calculating its depth is developed. The value of secant modulus of soils under the corresponding stress paths is employed as the horizontal reaction coefficient of the foundation soils, and the formula for calculating the reaction force of distributed soils acting on the supporting structures and the horizontal displacement of the supporting structures is deduced. It can reflect the characteristics of load deformation of foundation soils under the action of the excavation stress path, and the calculated results are closer to those of engineering reality. This study has a good guiding significance for the design of foundation pits mainly dominated by deformation control.