Study on Unloading Relaxation Characteristics of Columnar Jointed Rock Masses Based on Displacement Back Analysis

We aim to understand the relaxation of columnar joint rock masses during the excavation process of the diversion tunnel of the Baihetan hydropower station. This paper inverts the deformation parameters of the relaxed columnar joint rock based on the displacement monitoring data, and introduces a relaxation factor to describe the deterioration degree of anisotropic parameters of the relaxed columnar jointed rock. The equivalent strain is proposed as the criterion of unloading relaxation and the threshold is also given. Based on the software Flac3d, a program for calculating anisotropic elastoplastic model is developed. The distribution of the relaxation zone of the diversion tunnel after excavation is simulated, and compared with the results of the acoustic detection to verify the correctness and rationality of the program, which can provide a necessary reference for the design and construction of hydropower projects.

An Analytical Solution for Non-Darcian Flow on a Constant Head Packer Test in the Interlayer Staggered Zone

Groundwater flow in an aquifer has frequently been found to be non-Darcian by performing in situ tests. A novel analytic model is proposed in this study for describing the unsteady non-Darcian flow in a confined aquifer by taking advantage of the observed flow rate and injection pressure during the constant head packer test. A linearization approximation of the Izbash equation is used to approximate the nonlinear term in the governing equation. This analytic model is applied to describe the non-Darcian flow in the interlayer staggered zone at the Baihetan hydropower station, China. The test results inversed by the genetic algorithm show that non-Darcian flow happened during the test under the injection pressure 0.3 MPa with the power index n is 1.278, non-Darcian hydraulic conductivity k 1 is 1.613 × 10 − 5 cm/s and the specific storage S s is 9.757 × 10 − 5 m-1, respectively. The sensitivity analysis indicated that when the power index n or the specific storage S s is larger, and the hydraulic head will increase more slowly and needs longer to stabilize, but the non-Darcian hydraulic conductivity k 1 shows the opposite trend. Moreover, the hydraulic head is more sensitive to the power index n compared to other parameters at late times. The findings of this study reveal the non-Darcian flow during the constant head packer test and provide a simple and fast way to estimate parameters for more accurate seepage field simulation.

Study on Non-Darcian Flow in Interlayer Shear Weakness Zones in the Basalt by High-Pressure Packer Tests

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.

Study on the Effect of Amygdales on Mechanical Characteristics of Basalt Using the Combined Finite-Discrete Element Method

Amygdaloidal basalt, as a heterogeneous rock, is widely exposed at Baihetan hydropower station, China. The geometric effect of amygdales needs further studies and quantifying the shape, orientation, and statistical distribution of amygdales plays an important role in the laboratory and numerical experiments. Therefore, digital image processing (DIP) was first utilized to build a heterogeneous model (HM) to calibrate against the laboratory test results. Then, the heterogeneous models (HMs) with prescribed geometric features were generated by the inverse Monte-Carlo (IMC) algorithm. The uniaxial compression experiments based on HMs were conducted to study the mechanism of the crack initiation and propagation in the amygdaloidal basalt. The tensile fractures were mainly occurred in the matrix, and the shear fractures were mainly occurred in the amygdales. With the increase in the elliptic coefficient of amygdales, the uniaxial compressive strength (UCS) showed a linear growth trend. With the increase in the orientation of amygdales, the UCS exhibited a “V-shaped” distribution characteristic. This paper provides a numerical method for studying the mechanical properties of rocks with flaws.