Abstract
The interlayer shear weakness zone (ISWZ) is a special structural plane with different widths and spacing in stratified rock masses, it has higher permeability compared with surrounding rocks which is a risk factor for the safety of the hydropower station project. The high-pressure packer test (HPPT) by step injection is always applied to characterize the permeability of ISWZ. However, the non-Darcian flow is easy to appear under high pressure, which makes the Darcy law model no longer applicable. In this study, two non-Darcian flow analytical methods for confined aquifer were proposed to investigate the non-Darcian flow permeability parameters. The equivalent permeability coefficients of different non-Darcian models were derived as well. The in situ tests were conducted on the ISWZs at the Baihetan hydropower station to verify the proposed methods. The results indicate that the flow is non-Darcian flow in the test section from integrity to destruction during the whole HPPT process. Izbash’s law has a better fit than Forchheimer’s law in this complicated test situation. The equivalent permeability coefficients after destruction are one or two orders of magnitude larger than those before. Meanwhile, it is necessary to pay attention to the increased difference of two expressions of the equivalent permeability coefficients under higher gradient (i) or velocity (v). In general, these methods can be used to evaluate the characteristic of ISWZ to analyze the impact on engineering stability.
Highlights
The interlayer shear weakness zone (ISWZ) is a special structural plane widely distributed in the rock mass, especially in some large hydraulic projects in China
A conceptual model with a superposition of shear behaviours of interlayer soil and rock interface to predict the shear strength of ISWZ has been developed by Xu et al [1], he investigated the shear behaviour of a weak interlayered soil in the host rock and the soil-host rock interface by direct shear testing in the laboratory under different degrees of saturation [2], and used the equivalent continuum approach to evaluate the stability of the future Baihetan underground powerhouse that is affected by ISWZs [3]
The primary motivation of this paper is to evaluate the non-Darcian flow parameters of the ISWZ under high pressure caused by the high-pressure packer test (HPPT)
Summary
The interlayer shear weakness zone (ISWZ) is a special structural plane widely distributed in the rock mass, especially in some large hydraulic projects in China. With the construction of high-head hydraulic power stations, the influence of the ISWZ is important to the stability of the project. The structure of ISWZs is loose, with fractured rocks and high clay content which provides the main channels for groundwater flow. The groundwater flow may deviate from Darcy’s law as the velocity or hydraulic gradient increases due to high water head difference, it is completely different from the existing Darcian flow model. Considering the scaling variation in ISWZs, the hydraulic conductivity values of the interlayer staggered zones tend to converge as the scale increases, and the recommended values of hydraulic conductivity for ISWZs in the Baihetan hydropower project was proposed [5]
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