sustainability Article Study on Characteristics of Energy Storage and Acoustic Emission of Rock under Different Moisture Content Chuanming Li 1,2, Nan Liu 1,2,* , Wanrong Liu 3 and Ruimin Feng 4 1 Key Laboratory of Safety and High-Efficiency Coal Mining, Ministry of Education, Anhui University of Science and Technology, Huainan 232001, China; [email protected] 2 School of Mining and Safety Engineering, Anhui University of Science and Technology, Huainan 232001, China 3 School of Architecture & Civil engineering, Liaocheng University, Liaocheng 252059, China; [email protected] 4 Department of Civil Engineering, University of Arkansas, Fayetteville, AR 72701, USA; [email protected] * Correspondence: [email protected] Abstract: In order to study the energy storage and sound emission characteristics of rocks under different water content, uniaxial compression test, cyclic loading, unloading test and sound emission test were carried out using RMT-150B rock mechanics test system and DS5 acoustic emission system. The results show that the total strain energy of saturated rock samples and the area of hysteresis loop are the largest in the same period number, which indicates that the presence of water can reduce the elastic limit of rock samples, making the rock very easy to deform and even damage. Acoustic emission tests show that the damage energy of water-bearing rocks is small. The higher the water content, the smaller the peak damage energy. The bending energy index WET of the rock sample under saturated and natural state is smaller than that under dry state, which further indicates that the presence of water can reduce the elastic limit of the rock and soften it. The results can provide a basis for the prediction of underground engineering construction and rock failure instability. Citation: Li, C.; Liu, N.; Liu, W.; Feng, R. Study on Characteristics of Keywords: moisture content; sandstone; acoustic emission; energy storage Energy Storage and Acoustic Emission of Rock under Different Moisture Content. Sustainability 2021, 13, 1041. https://doi.org/10.3390/ 1. Introduction su13031041 In recent years, with the increase of intensity and depth of coal mining, dynamic dis- Academic Editor: Guang-Liang Feng asters such as rock burst seriously threaten production safety [1,2]. As water is common in Received: 9 December 2020 the coal mining environment and many deformations of engineering rock mass are related Accepted: 18 January 2021 to the water in the rock, and the elastic energy index is one of the critical measurement Published: 20 January 2021 indicators for the evaluation of impact tendency, accurate calculation of the elastic energy index is crucial to estimate the strength of coal rock impact tendency [3–5]. Therefore, the Publisher’s Note: MDPI stays neutral study of characteristics of mechanical properties and energy storage of rock under different with regard to jurisdictional claims in water-bearing conditions has very important engineering significance. published maps and institutional affil- The water inside the rock mass is the leading cause of rock softening and strength iations. reduction. Many scholars [6–9] have studied the influence of water content on rock de- formation, strength characteristics, and energy storage characteristics. Z. Pan et al. [10] analyzed the influence of water content on compressive strength and elastic modulus of sandstone through loading and unloading tests. Eberhardt et al. [11] studied the fracture Copyright: © 2021 by the authors. damage mechanical behavior of brittle rock samples through uniaxial loading and unload- Licensee MDPI, Basel, Switzerland. ing tests and acoustic emission monitoring. Su [12] carried out uniaxial cyclic loading This article is an open access article and unloading tests on saturated diorite, and found that the energy storage capacity and distributed under the terms and brittleness of sandstone were greatly weakened and plasticity was significantly enhanced conditions of the Creative Commons after saturating water. Xia [13] found the energy dissipated and the number of cycles was Attribution (CC BY) license (https:// approximately linear. Zhang [14] studied the stress–strain curve and energy evolution creativecommons.org/licenses/by/ law of sandstone under different water content and found that the curve was sparse in 4.0/). Sustainability 2021, 13, 1041. https://doi.org/10.3390/su13031041 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 1041 2 of 15 the saturated state, and the volume energy was small; and in the drying state, the curve was denser and the volume energy is the largest. Li [15] discusses the influence of water content and anisotropy on the strength and deformability of two meta-sedimentary rocks by triaxial compressive tests, and the experimental studies show that the anisotropy associ- ated with bedding in rock specimens plays a weakening effect on the triaxial compressive strength for both tested rocks. Roshan [16] discussed the X-ray computed tomography which was conducted on the representative samples from each type of sandstone to assess the porosity, rock composition, and texture, and the experimental studies show that volume and effective porosity is more accurately correlated to mechanical properties. Meng [17] studied sedimentary rock and the models and correlations between moisture content and bursting potential are established, and the rock after peak strength of stress–strain curve represents brittleness and shear failure and has obvious strain softening behavior in the condition of dried or with small moisture content. Acoustic emission (A.E.) technology is capable of dynamically monitoring the genera- tion and expansion of micro-cracks in rock materials and reflecting the damage evolution process. Therefore, it has been widely used in geotechnical engineering such as coal mining, slopes, tunnels and bridges [18–20]. Mansurov et al. [21] used acoustic emission to measure the information of rock failure process and predict the failure type of rock. Xiao [22] stud- ied the characteristic parameters of acoustic emission, and the plastic piecewise functions through the cyclic loading and unloading and acoustic emission experiments. Fu [23] conducted uniaxial compression tests and acoustic emission tests on the rock, and the results showed that the rock deformation rate has a good correspondence with the number of acoustic emission events. Liang et al. [24] studied the influence of the cyclic loading and unloading on the mechanical properties of the rock at the post-peak stage, and verified that the deformation behavior of the rock was in good agreement with the acoustic emission parameters. He et al. [25] studied the A.E. cumulative energy release phenomenon in the process of rock unloading, loading and failure. The aforementioned studies have greatly deepened the understanding of the law of energy changes in the process of rock instability and failure. However, the combined effect of periodic loads and water is common in actual underground engineering construction. Therefore, typical sandstone materials were selected to perform the uniaxial compression test, cyclic loading and unloading test, and acoustic emission test on rock samples with three different water content, and conduct research on the characteristics of mechanical properties and energy storage of the rock samples with water to provide a theoretical reference for rock fracture and instability. 2. Sample Preparation The white sandstone samples were produced from a sandstone mine in Renshou County, Sichuan Province, China. The main mineral components of white sandstone are quartz 60%, feldspar 20%, clay 15%, and a small amount of muscovite. All samples were manufactured in accordance with the standard of the International Society of Rock Mechanics and Rock Engineering. SC-200 core machine was firstly used to drill a cylindrical core with a diameter of 50 mm, and SCQ automatic stone cutter was then used to cut a sample with a length of 100 mm. Finally, the two ends of each specimen were polished to be smooth and parallel using SCQM automatic cutting and grinding machine. The specimens were divided into three groups: saturated state (a), natural state (b), and dry state (c). The average water content of the as-received sandstone is 0.202%. In order to obtain dry and saturated sandstone specimens, the white sandstone is dried and saturated with water, respectively. The sandstone specimens under two states are measured until the mass difference between the two consecutive times’ weighing is less than 0.05%. During tests, the rock samples under natural state are not processed; other rock samples are dried in an oven at 100 ◦C for 48 h, and then taken out, placed in a drying oven to cool to the room temperature, and finally weighed. When the change in weight is constant, the rock sample is considered to be completely dried, and an appropriate amount of rock sample Sustainability 2021, 13, 1041 3 of 15 will be selected as the drying group (c). The sample under the saturated state is prepared by using a proper amount of dried rock samples to absorb water by natural immersion method. Take out rock samples at regular intervals, wipe off the surface moisture of the rock sample with a towel, and weigh until the water content becomes stable. Figure1 shows the variation curve of the average water content of all saturated rock samples with the immersion time. It can be seen that white sandstone has strong water absorption, and the water content increases exponentially with the immersion time. In the initial stage of immersion (0–4 h), the water absorption rate is large, and the water content increases rapidly as time elapses; then (4–7 h) the water absorption rate gradually decreases, and the water content increases slowly; after more than 7 h, the average water content of the rock sample tends to stabilize, indicating that the sandstone is close to saturation with the average water content of 2.483%. The curve in Figure1 is the fitting equation with the correlation coefficient R2 of 0.9890. This equation is only applicable when white sandstone with φ50 mm and L100 mm is selected in this test.