Correlation between the rock mass properties and maximum horizontal stress: A case study of overcoring stress measurements

Abstract

Understanding the mechanical properties of the lithologies is crucial to accurately determine the horizontal stress magnitude. To investigate the correlation between the rock mass properties and maximum horizontal stress, the three-dimensional (3D) stress tensors at 89 measuring points determined using an improved overcoring technique in nine mines in China were adopted, a newly defined characteristic parameter CERP was proposed as an indicator for evaluating the structural properties of rock masses, and a fuzzy relation matrix was established using the information distribution method. The results indicate that both the vertical stress and horizontal stress exhibit a good linear growth relationship with depth. There is no remarkable correlation between the elastic modulus, Poisson’s ratio and depth, and the distribution of data points is scattered and messy. Moreover, there is no obvious relationship between the rock quality designation (RQD) and depth. The maximum horizontal stress σH is a function of rock properties, showing a certain linear relationship with the CERP at the same depth. In addition, the overall change trend of σH determined by the established fuzzy identification method is to increase with the increase of CERP. The fuzzy identification method also demonstrates a relatively detailed local relationship between σH and CERP, and the predicted curve rises in a fluctuating way, which is in accord well with the measured stress data.

1. Introduction

Various geological phenomena in the lithosphere and the associated physical and chemical processes are closely correlated with the action of in-situ stresses [1–6]. Understanding the state of crustal stress, especially the stress conditions at great depths, is a necessary foundation for solving the scientific issues associated with geodynamics, such as plate driving mechanism, earthquake mechanism, volcanism, and tectonism, and is also a prerequisite for designing and devising underground projects and guiding the development of subsurface energy resources [7–15]. The origin of in-situ stress is quite complicated and has not yet been fully understood so far. It is widely accepted that the generation of crustal stress is primarily related to various dynamic movements of the Earth, including both historical tectonic movements and present-day active geological processes [16]. As a consequence, the crustal stress state is often affected by multiple endogenous and exogenous factors.

As internal stress exists in the interior of rock mass, in-situ stress is inevitably affected by various physical and mechanical properties such as elastic constants of the rock mass. For example, several pointers have affirmed and confirmed the dependence of in-situ horizontal stress on elastic modulus [17]. Moreover, numerous smaller-scale studies, particularly in sedimentary basins, highlighted the role of elastic property contrasts in in-situ stress [18]. Three-dimensional (3D) stress tensors can be divided into vertical stress and horizontal stress. In most cases, the vertical stress can be well estimated using the overburden without causing too much error [19]. However, horizontal stress can be significantly influenced by plate tectonics, main geological features, and mountainous terrain, which may seriously confuse the relationship between horizontal stress and elastic constants [20]. Hence, to accurately determine horizontal stress magnitudes, it is crucial to understand the mechanical properties of the lithologies. In addition, stress measurements are generally carried out in relatively dense strata, but it is currently almost impossible to conduct reliable stress measurements in soft rocks such as coal measures and mudstone, thus leading to little knowledge of the horizontal stress in soft rocks [20].