Deterioration of shear strength parameters of limestone joints under simulated acid rain condition

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Abstract

Deterioration of shear strength parameters of carbonate joints under the influence of acid rainwater is a serious concern in rock engineering. Considering this, limestone specimens with planar joints (i.e. joints having roughness coefficient <5) collected from the Lanjiberna limestone mine (in the state of Odisha, India) were investigated. Depreciation of shear strength parameters of the joints concerning short-term exposure to a simulated typical acid rain (i.e. pH 4 solution) condition was evaluated. A total of 24 direct shear tests were performed on natural limestone joints and their degraded equivalents. Both peak shear strength and peak friction angle of a degraded joint are found to be less than those of its natural counterpart. The drop in peak friction angle is found to be 8–10% and 14–19% due to submergence of the joints in pH 4 solution for 5 days and 10 days, respectively. It was ascertained that the deterioration of the asperity strength due to the short-term exposure of the planar joint surfaces to the pH 4 solution overrides the effect of insignificant macroscopic change of the surface roughness caused by solution induced micro-cavities and hence, controls the degradation of the shear strength parameters.

Introduction

Rock joints, usually, are excellent pathways for flow of rainwater which is one of the major factors for chemical weathering of a rock mass. Chemical weathering of most igneous, metamorphic and sedimentary rocks is a slow and intricate natural process that leads to the changes of mineralogy and microstructure through mineral alteration and associated physical disintegration. Carbonate rocks are, however, more vulnerable to chemical weathering than other rocks if acidic environments prevail. The formation of acidic environment due to anthropogenic activities (e.g. urbanization, industrialization, use of vehicles, mining activities etc.) is very common nowadays. During the period of precipitation, if the rain water comes in contact with acidic pollutants, it leads to acid rain. The interaction of acid rain with carbonate rock has a stoichiometry, consistent with the reaction: CaCO3 (solid) + H+ ⇌ Ca2+ + HCO3−.1 Because of this, although carbonate rocks may portray solution induced cavities/marks, the existing/remaining material is nothing but carbonate. In other words, unlike other rocks, carbonate rocks cannot be categorized in terms of weathering grades (a manifestation of different degrees of decomposition) following the common 6-fold weathering classification of rock materials.2, 3, 4

Rock mass behavior is heavily dependent on the shear behavior of joints.5, 6, 7, 8 There are some investigations existing in the literature that dealt with shear behavior of joints with reference to weathering grades of joint surface materials of granite and quartzite 9, 10. There is also a study by Mohtarami et al.11 which is about shear behavior of andesitic joints with reference to different acidic pH conditions. Nouailletas et al.12 investigated the influence of acid attack on the shear behavior of fractures in Campanian flysch (a carbonate rock with more than 60% calcite). However, it should be noted that the investigated fractures in Campanian flysch were not natural fractures and were induced by three-point bending test. Moreover, an acidic condition of very low pH of 0.2 was considered by Nouailletas et al.12 in relation to leakage of CO2 through a geological fault in the context of carbon sequestration. However, to the best of the authors’ knowledge, there is no study on the influence of acid rainwater on the shear behavior of carbonate joints. This is an important topic as far as quarries, tunnels, dam foundations, etc. In carbonate rocks are concerned. In the present study, with due need, shear behavior of natural limestone joints and their degraded equivalents (treated with pH 4 solution, a typical pH of acid rain as per Environmental Protection Agency, US13) is investigated. How the different durations of short-term exposure to such acidic environment change shear behavioral characteristics of the limestone joints is also explored. In order to carry out the research, block specimens of limestone with natural joints collected from the Lanjiberna limestone mine (in the state of Odisha, India) are utilized.

Section snippets

Specimens

Block specimens of limestone with joints were collected from the Lanjiberna limestone mine of Odisha Cement Limited (OCL) (22°15′ 45″ N, 84°31′ 1″E, Sundargarh district of the state of Odisha, India). As far as geology of the area is concerned, the limestone exposed in and around Lanjiberna area belongs to the Birmitrapur Formation of Gangpur Group14 (Fig. 1). An example of the outcrop from where oriented specimens of jointed limestone were collected is shown in Fig. 2. The dark colour of the

Laboratory investigation

Each jointed block of limestone was saw-cut to obtain a pair of blocks with equivalent natural joint surfaces for direct shear tests to be performed under natural and degraded (by acidic solution) conditions (Fig. 3, Fig. 4). Additionally, joint surface roughness measurement using a Barton comb was done in order to ascertain this surface equivalence (Fig. 5). It should be noted that all considered joint surfaces had a joint roughness coefficient (JRC) less than 5 which implies that the

Results and analysis

Following the direct shear tests, the shear stress was plotted against shear displacement for all specimens as shown in Fig. 6. The corresponding normal stress and peak shear strength are presented in Table 2. Although shear behavioral patterns for the natural joints and degraded joints look similar (Fig. 6), peak shear strength of a degraded joint is always found to be less than that of its natural counterpart (Fig. 6, Table 2). It should be noted that considering a particular phase of

Conclusions

Shear behavior of natural limestone joints and their degraded equivalents (i.e. treated with pH 4 solution), was investigated in this study. How the different durations of short-term exposure to such acidic condition change shear behavioral characteristics of the joints was also explored. In order to carry out the research, block specimens of limestone with planar natural joints collected from the Lanjiberna limestone mine (in the state of Odisha, India) were utilized. The concluding remarks of

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the anonymous reviewer and Prof. Zimmerman, the Editor-in-Chief (IJRMMS), for their constructive suggestions that helped enhance the clarity of the paper. The authors gratefully acknowledge the overall support by Indian Institute of Technology Kharagpur, for carrying out this investigation. BKR acknowledges research fellowship from the University Grants Commission (UGC), Government of India.

References (21)

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