Water Retention and Drainage Technology at Weakly Permeable Soil Layers Interface in Foundation Pits

In the interlayer profiles of soil layers with significant differences in permeability, "interface water retention" is a major technical problem with high construction risks and is also very difficult to disposal during the engineering dewatering process. In order to ensure the quality of underground structure construction, improve soil strength, and reduce construction risks, an interlayer stagnant water drainage technology combining precipitation wells, collection wells, and gravel wells is proposed. Based on the on-site observation of a foundation pit dewatering project in the Loess Plateau area, the influence of gravel well size, well spacing, and setting depth on the stagnant water drainage effect of the sidewall of the foundation pit is calculated and analyzed, and the feasibility and rationality of this stagnant water drainage technology is preliminarily verified. Results are as follows. The setting of gravel wells can provide a smooth vertical seepage channel for stagnant water at the interface of low permeability soil layers, achieve proactive groundwater control in advance, and have beneficial effects on ensuring the quality of underground structure construction, soil layer drainage effect, and improving soil strength. The range of influence of gravel well drainage is mainly determined by the horizontal permeability characteristics of the low permeability soil layer. As the diameter of the gravel well increases, its stagnant water drainage range is 6 m～8 m, and it is recommended to arrange wells with a spacing of 10 m～15 m. The depth of the gravel well setting should penetrate the low permeability soil layer within the range of water level drop, and should not exceed the actual precipitation level to avoid the risk of water accumulation caused by the gravel well setting during the project implementation.