Objectives When the drilling water pressure test is applied to the in-situ permeability test of the waterproof curtain, especially the underground continuous wall, the conventional semi-infinite space model prescribed in rock mass testing standards differs significantly from the actual rectangular seepage field of the curtain, resulting in potential errors. The lack of field drilling water pressure test cases and corresponding calculation methods for waterproof curtains has hindered the accurate evaluation of their permeability. Investigating the influence of parameters (such as the thickness of the waterproof curtain the length of the test section, and the position of the test section) on the development process of the P-Q curve and the permeability rate q can lay a foundation for the calculation of its theoretical permeability coefficient.

Method Based on the similarity theory, a laboratory-scale model test of drilling water pressure on waterproof curtains was carried out to investigate the variation of flow rate Q under different test pressures P. Existing calculation methods and test parameters for determining the permeability coefficient were critically evaluated, and an empirical formula was derived from the model test data

Result The results show that in the drilling water pressure test, the flow rate increases approximately linearly with the pressure, and the length of the test section is approximately proportional to the rate of flow increase but has little influence on the permeability rate. The position of the test section has little effect on the flow growth rate and Lu value. With the increase of the thickness of the waterproof curtain, the permeability rate decreases. Permeability coefficients calculated using conventional formulas are generally lower than those obtained from laboratory permeability tests. Based on the model test data, the empirical formula for calculating the permeability coefficient is derived, which is suitable for the estimation of the permeability coefficient of the complete curtain when the pressure is not large. The test section is lower than the water surface outside the wall, with a wall thickness of 0.5～1.5 m. When there are defects in the waterproof curtain, the flow is affected by the distance from the test section to the defects, the width of defect, and the permeability of the rock or soil outside the wall.

Conclusion Based on these findings, this study puts forward the recommended parameters of the test section with a length of 5 m and a pressure value of 0.2～0.3 MPa. These results make the use of the drilling water pressure test method more reliable and convenient for evaluating the permeability of waterproof curtains