Experimental Study on Seepage Properties of Postpeak Fractured Rocks under Cyclic Loading-Unloading Confining Stress and Axial Stress

Xiaobo Zhang,Jianhua Yang,Chi Yao,Zuhao Xia,Mingdong Yang,Feng Xiong

doi:10.1155/2021/6629835

Abstract

Excavation in rock masses always encounters safety problems from rock fracture seepage in water-rich areas, which needs to be paid much attention, especially for fractured rocks under complicated stress state. For this reason, the permeability of fractured sandstone and granite is experimentally investigated under cyclic loading-unloading confining stress and axial stress. The variation of permeability coefficient and seepage flow with increasing and decreasing the confining stress and axial stress are comprehensively analyzed. Results show that the changing patterns of permeability with loading-unloading cycles of confining stress for both fractured sandstone and granite are similar. The permeability is most sensitive to the initial loading-unloading stages. After several loading-unloading cycles, the confining stress has little effect on permeability. The seepage flow decreases as the confining stress is unloaded to the same level in the loading process, indicating a hysteresis effect on the recovery of seepage capacity. The seepage properties under cyclic loading-unloading the axial stress are quite different from those under the confining stress. The permeability of fractured sandstone is most sensitive to the first cycle of loading-unloading of axial stress. The irrecoverable shear slide between fractures under the axial stress causes dilatancy or contraction, which makes the permeability coefficient to consecutively decrease at the subsequent cycles. The permeability of granite first decreases during the first loading of axial stress, while this trend is disordered at the subsequent stages no matter loading or unloading the axial stress. This is because of the accumulation of breakage fragments between fractures, which further disturbs the seepage flow. These findings may be useful for further understanding the seepage properties of fractured granite and sandstone under complex loading-unloading history.

Highlights

In the underground rock engineering, the continuous excavating inevitably breaks the original mechanical equilibrium state of rock masses, and the stress redistribution occurs in the surrounding rocks adjacent to the excavation [1, 2]
In contrast to the results of sandstone, the permeability coefficient of granite firstly decreases at the initial loading of axial stress at the first loading stage, while this trend changes to be disordered at the subsequent stages no matter loading or unloading the axial stress
Postpeak fractured sandstone and granite specimens were generated through triaxial tests, which were subjected to seepage tests under cyclic loading-unloading the confining stress and axial stress, respectively

Summary

Introduction

In the underground rock engineering, the continuous excavating inevitably breaks the original mechanical equilibrium state of rock masses, and the stress redistribution occurs in the surrounding rocks adjacent to the excavation [1, 2]. In order to maintain a self-equilibrium state, the stress field may be redistributed repeatedly in the rock masses under cyclic loading and unloading [5,6,7,8,9]. This usually results in the initiation, propagation, and coalescence of fractures within rocks subjected to postpeak failure. These fractures act as main seepage channels and significantly influence the permeability of the rock mass in water-rich regions [10, 11]. It is of practical significance to consider the effects of cyclic loading and unloading on the seepage properties of fractured rocks

Objectives

Methods

Conclusion