The effects of water on the strength of black sandstone in a brittle regime

Highlights

• The UCS and elastic modulus decrease with the soaking duration of black sandstone.

• Relationships between mechanical properties and the water content are established.

• Tensile failure transitions to shearing in partially to fully saturated specimens

• The water distribution in black sandstone affects the strength.

Abstract

The mechanical behavior of rock is significantly affected by water. To further study the influence of water on the mechanical behavior of rocks, uniaxial compressive loading experiments were performed on black sandstone specimens (collected at a site in Longchang, Sichuan Province, China) with different water contents. The water absorption was first tested to study the evolution of the water content in the specimens. Then, the mechanical behaviors of the specimens after different durations of soaking in water (from dry to saturation) were tested under uniaxial compression. The results showed that, in the stress-strain curves for the specimens, the initial slope and strength decrease with increasing soaking duration and that the curves are strongly nonlinear in the prepeak region. Furthermore, with increasing water content, the elastic modulus and uniaxial compressive strength decrease, while the Poisson's ratio increases. A comprehensive set of predictive empirical relationships is established between the mechanical properties and the water content/soaking duration for the black sandstone. As the soaking duration increases, the failure pattern gradually changes from tensile to shear-dominated failure. A novel observation is that, for two specimens with the same water content, the strength of a specimen with a homogeneous water distribution is lower than that of a specimen with a non-homogeneous water distribution. The results suggest that the water soaking duration, water distribution and loading-saturation sequence of a rock should all be considered in experiments to gain a deeper understanding of the mechanical responses of rocks with different water contents.

Introduction

Water is thought to be one of the most notable factors in reducing rock strength. Many studies have reported the effects of water on the uniaxial compressive strength (UCS) of sedimentary rocks, such as shale, chalk, sandstone, tuff and mudstone (Colback and Wiid 1965; Hawkins and McConnell 1992; Heap et al. 2015; Török and Vásárhelyi 2010; Talesnick and Shehadeh 2007; Vásárhelyi and Ván 2006; Van Eeckhout and Peng 1975; Wasantha and Ranjith 2014; Yilmaz 2010). Okubo et al. (1992) indicated that the UCS and Young's modulus values of granite, andesite, and tuff are lower under wet conditions than under dry conditions, whereas the stress-strain curves of the rocks are similar in shape in both wet and dry conditions. Price (1960) indicated that the compressive strength of coal measure sandstones under air-dried conditions varies from 51 to 80% that of samples are “completely” dry, while the compressive strength of saturated samples is only 45% that of “completely” dry samples. Van Eeckhout (1976) conducted numerous studies on coal mine shales and concluded that moisture lowers the amount of work needed to generate a fracture and increases internal crack length, hence lowering the shale strength. Rajabzadeh et al. (2012) found that a saturated limestone collected from Fars Province, southern Iran, exhibited a UCS that was 70% that of a dried limestone. Verstrynge et al. (2014) reported that the continuity of the binder (limonite) was a key factor driving the strength reduction due to water saturation in ferruginous sandstones. Wong et al. (2016) reviewed the terminologies commonly used to quantify the amount of water stored in rocks and the water-weakening effects on rock strengths and moduli. Generally, a statistically negative exponential relation was found between the water content and the UCS of sedimentary rock (Hawkins and McConnell 1992; Vásárhelyi and Ván 2006). In addition to UCS tests, many experimental studies have discussed the effects of water on the triaxial compressive strength (TCS) of different rock types. Baud et al. (2000) performed triaxial compression tests on Boise, Darley Dale, Berea and Gosford sandstones under saturated conditions at room temperature, and they explained the relation between the water weakening and failure modes using micromechanical models. Li and Reddish (2004) studied the effect of water on both intact and broken rocks. Chang and Haimson (2007) found that water also significantly affects the strength of nearly impermeable crystalline rock.

In the past, most studies focused on the relationship between the water content and mechanical behavior parameters, such as the UCS, TCS, elastic modulus and Poisson's ratio. However, such studies treated the water content as the only variable affecting the mechanical behaviors of the rock specimens, neglecting the effect of the duration that the rock was exposed to the water and the failure features related to the penetration of water into the rock. When a specimen begins to soak in water, the water is believed to initially remain only on the surface, while the core of the rock remains in a dry state; then, the water gradually fills the pores of the rock, and the water content increases until the specimen is saturated, as experimentally observed via nuclear magnetic resonance (NMR) measurements (Zhou et al. 2016). During the water saturation process, the exterior of the rock comes into contact with the water first, followed eventually by the core. Thus, degradation or water-rock interaction occurs near the outside first then gradually spreads into the rock. However, the results reported by Vásárhelyi and Ván (2006), Erguler and Ulusay (2009) and Masoumi et al. (2017) reveal the effects of the water content on the mechanical behaviors of rocks but neglect the effects of the water distribution and the water soaking duration of the rock on the mechanical behavior of the rock. This issue has been largely left to the discretion of researchers, even though the heterogeneous distribution of water in rock is a major factor that has an effect on the rock's strength and other mechanical properties. Therefore, further investigation is needed to understand the influences of water on the mechanical properties of rock by considering both the water content and the water soaking duration of rock as important factors.

To this end, in the current study, the effects of water content and water soaking duration on the mechanical behavior of black sandstone were experimentally tested. Different soaking durations were applied to obtain specimens with different water contents, including unsaturated, saturated and long-term saturated specimens. The unsaturated specimen has a water content that has not reached its maximum value, whereas the saturated specimen was removed from water when the water content of the specimen reached its maximum value. A long-term saturated specimen is obtained by continuing to soak a saturated specimen in water for many hours or days. Notably, few experiments have been conducted specifically on the mechanical behaviors of black sandstone at different water contents under uniaxial compressive loading. However, this rock type is widely distributed in Sichuan, Anhui and Guizhou Provinces in China. For example, the Wenchuan earthquake memorial wall in Sichuan Province is composed of many black sandstone blocks. The roadway in the Qidong mine in Anhui Province and the oil reservoir in Guizhou Province also contain black sandstone layers. It is believed that the findings regarding the mechanical properties of black sandstone in this study will be beneficial for the designing of many projects.