A cohesion loss model for determining residual strength of deep bedded sandstone

Abstract

Rock residual strength, as an important input parameter, plays an indispensable role in proposing the reasonable and scientific scheme about stope design, underground tunnel excavation and stability evaluation of deep chambers. Therefore, previous residual strength models of rocks established were reviewed. And corresponding related problems were stated. Subsequently, starting from the effects of bedding and whole life-cycle evolution process, series of triaxial mechanical tests of deep bedded sandstone with five bedding angles were conducted under different confining pressures. Then, six residual strength models considering the effects of bedding and whole life-cycle evolution process were established and evaluated. Finally, a cohesion loss model for determining residual strength of deep bedded sandstone was verified. The results showed that the effects of bedding and whole life-cycle evolution process had both significant influences on the evolution characteristic of residual strength of deep bedded sandstone. Additionally, residual strength parameters: residual cohesion and residual internal friction angle of deep bedded sandstone were not constant, which both significantly changed with increasing bedding angle. Besides, the cohesion loss model was the most suitable for determining and estimating the residual strength of bedded rocks, which could provide more accurate theoretical guidance for the stability control of deep chambers.

1. Introduction

Rock residual strength, as an important input parameter, plays an indispensable role in proposing the reasonable and scientific scheme about stope design, underground tunnel excavation, stability evaluation of deep buried chambers and their support design [1−5]. Since the accurate determination of rock residual strength not only helps to facilitate cost-effective design of these engineering structures, but also improves the safety factor of the construction personnel [6−9].

As shown in Figure 1, the elastic zone (initial stress zone), plastic zone, yield zone (plastic softening zone), excavation zone and caved zone were from the deep chambers excavated, respectively. Correspondingly, the mechanical behavior characteristics were produced, including the elastic deformation, joints slip, crack growth and extension, and cohesion loss. Meanwhile, the phenomenon of dilatancy and bulk modulus softening were accompanied. Additionally, the corresponding strength thresholds were the initial high in-situ stress, damage threshold, yield strength, peak strength (failure) and residual strength in the corresponding buried depth.

Besides, as shown in Figure 1, the in-situ stress evolution characteristics of deep chambers during the whole life-cycle evolution process were responded to the stress path of the mechanics tests in this study. The specific in-situ stress evolution characteristics were as follows. Firstly, there was the in-situ stress states of deep chambers corresponding to a certain buried depth before excavation; Then, deep chambers corresponding to a certain buried depth were the state of the minimum principal stress unloaded in excavating; Subsequently, the vertical stress in deep chambers corresponding to a certain buried depth reached to the peak strength at failure; Finally, the vertical stress in deep chambers corresponding to a certain buried depth reached to the residual strength when deep chambers collapsed. Generally, the bearing capacity of the surrounding rocks of deep chambers was the residual strength.

Additionally, a series of residual strength models were established and applied for providing better theoretical guidance for rock engineering, including the cohesion loss model, Joseph-Barron (J-B) model, Hoek-Brown (H-B) model, Mohr-Coulomb (M-C) model, and geological strength index (GSI)-softening model.