Mechanical properties and permeability evolution of sandstone subjected to the coupling effects of chemical-seepage-stress

Abstract

In this study, a series of triaxial tests are conducted on sandstone specimens to investigate the evolution of their mechanics and permeability characteristics under the combined action of immersion corrosion and seepage of different chemical solutions. It is observed that with the increase of confining pressure, the peak stress, dilatancy stress, dilatancy stress ratio, peak strain, and elastic modulus of the sandstone increase while the Poisson ratio decreases and less secondary cracks are produced when the samples are broken. The pore pressure and confining pressure have opposite influences on the mechanical properties. With the increase of the applied axial stress, three stages are clearly identified in the permeability evolution curves: initial compaction stage, linear elasticity stage and plastic deformation stage. The permeability reaches the maximum value when the highest volumetric dilatancy is obtained. In addition, the hydrochemical action of salt solution with pH=7 and 4 has an obvious deteriorating effect on the mechanical properties and induces the increase of permeability. The obtained results will be useful in engineering to understand the mechanical and seepage properties of sandstone under the coupled chemical-seepage-stress multiple fields.

1. Introduction

As a consequence of anthropogenic activities, a significant influx of chemical pollutants progressively infiltrates the subsurface, leading to the contamination of groundwater [1, 2]. Chemical seepage not only redistributes chemical elements in the rock, but also alters the microstructure [3]. A large number of cracks and pores are then formed in rock after the corrosion induced by chemical seepage. These cracks and pores provide new flow paths for groundwater runoff and then change the seepage path of porous rock. This can change the mechanical and permeability properties of rock. In view of this, the present study focuses on the evolution of strength and deformation properties and permeability of sandstone under the coupling of chemical-seepage-stress fields.

Extensive experimental studies have been performed to study the mechanical properties and permeability of rocks under multi-field coupling. Prior studies [4−6] have shown that the chemical environments can initiate crack propagation in materials, e.g., quartz and ceramics. Later, it is also found that the mechanical properties of rocks are significantly affected by the chemical corrosion. For example, RUTTER et al [7] carried out a series of stress relaxation tests on fractured and intact sandstones to investigate the influence of pore water on the rock strength. Seismic velocity, attenuation, and acoustic emission of saturated granite in two chemical solutions were measured by ISHIDO et al [8]. They studied the effect of the pH and concentration of solutions on the propagation of microcracks and the fracture strength of the studied rock. LAJTAI et al [9] conducted a series of short-term and long-term tests to study the effect of water on the deformation and strength of granite. Triaxial compression tests on prefabricated cracked sandstone were carried out by FEUCHT et al [10] to study the effect of the hydrochemical solutions on the ultimate friction strength. They found that the ionic concentration, ionic charge, and pH value affected the friction factor and significantly reduced the friction strength of the cracked surface. FENG et al [11−15] conducted triaxial compression tests on nodular limestone under the action of percolation of different chemical solutions. They carried out real-time observation and digital recording of the microfracture process and analyzed the effects of chemical corrosion on the crack propagation rate, the crack morphology, the initial angle of the new cracks, and the mechanism of the microcrack propagation.