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 multi-crack interactions. It was found that the extent of the effect of corrosion depends on the type and concentration of chemical ions as well as the pH of the solution, the mineralogical composition of the rock, its geometry and the number of joints. Besides, there are many studies on the corrosion of rock mechanical properties by chemical environment [16-19], which are not detailed in the present work.

The propagation of cracks caused by chemical corrosion provides a conduit for the flow of fluids. KARFAKIS et al [20] carried out fracture mechanics tests on three rocks to study the fracture toughness, fracture specific energy, and strain energy index subjected to chemical corrosion. They found that the fracture toughness and strain energy were reduced, and the pore pressure accelerated the corrosion cracking of rocks. Hydrostatic pressure, triaxial compression, and creep tests on porous limestone after chemical corrosion were carried out by XIE et al [21]. They found that chemical corrosion led to a decrease in pore collapse limit stress, modulus of elasticity and material cohesion and enhanced deformation. RUTQVIST et al [22] concluded that temperature and chemical fields led to the closure of cracks and thus affected the permeability of fractured rocks based on the engineering test data. ZHANG et al [23] conducted uniaxial and triaxial compression tests to investigate the effects of brine on the mechanical properties and permeability of mudstone interlayers. LIN et al [24] conducted uniaxial compression tests on sandstone after chemical corrosion, accompanied porosity measurement. The results showed that the porosity and peak strain increased, while the strength and modulus of elasticity decreased. Double torsional load relaxation tests on silicified fault rocks in different chemical environments were conducted by CALLAHAN et al [25]. They found that the chemical environment induced the rock fracture and seepage channel formation. WANG et al [26, 27] conducted a series of chemical solution corrosion tests and triaxial compression tests on granite, sandstone, sand slate and sandstone with filled joints. They found that corrosion significantly changed the rock quality and reduced the mechanical properties. This change varied with the ionic composition of the solution and the pH value.

In summary, a large number of experimental studies on rocks subjected to chemical corrosion have been conducted. The changes in mechanical and permeability parameters have been obtained by qualitative and quantitative analytical methods. The studies reveal the corrosion process and corrosion mechanism of rocks. However, most of the studies did not consider the seepage effect of solutions, and only tests on rock samples soaked in solutions were conducted. The whole process of multi-field coupling could not be studied. Therefore, this paper carried out the experimental study of the full coupling of chemical-seepage-stress under the effect of seepage of chemical solutions on sandstones after soaking in different liquids.

2 Experimental programs

2.1 Preparation of sandstone specimens

The rock was a kind of fine-grained sandstone with good homogeneity, and its natural density was 2.3 g/cm³. The rock surface was in dark red color without obvious joints and fissures. The main constituents were quartz, feldspar, calcite, mica, and chlorite. The core collected from the engineering site was processed into a cylindrical specimen with a diameter of 50 mm and a length of 100 mm, as shown in Figure 1. NaCl, KCl and Na₂SO₄ were used as electrolytes to formulate two salt solutions with a concentration of 0.1 mol/L and pH of 4 and 7. The rock samples were immersed in these two salt solutions and distilled water, respectively.

Figure 1全屏查看

Images of the sandstone specimens soaked in the chemical solution for different periods of time

2.2 Testing apparatus and method

The mechanical tests were carried out on the rock automatic servo-controlled triaxial test system, as shown in Figure 2.

Figure 2全屏查看

Rock automatic servo-controlled triaxial test system

In this study, a steady-state method was used to measure the permeability of specimen. During the experiment, the liquid pressure at the inlet and outlet end of the pressure chamber were measured to deduce the permeability. The permeability k of rock specimens is derived as follows [28]:

(1)

where Q is the volume of fluid flowing through the rock per unit time; μ is the fluid viscosity and μ=1×10^-3 Pa·s in the study; L and A are the length and cross-sectional area of the specimen, respectively; P_u and P_d are the upstream and downstream pressures, respectively.

3 Analysis of mechanical characteristics

In this section, a series of triaxial compression tests were conducted on the sandstones immersed in chemical solutions for 30 d.

3.1 Stress-strain curves of sandstone

The obtained stress-strain curves of different triaxial compression tests are given in Figure 3. From left to right, the curves are respectively lateral strain, volumetric strain and axial strain. In the tests, the confining pressure σ₃ was 5 and 10 MPa while the pore pressure P_w was 0, 1 and 3 MPa, respectively. In the laboratory tests, three seepage fluids were used: distilled water, pH=7 solution and pH=4 solution.

Figure 3全屏查看

Stress-strain curves of the sandstones under different conditions: (a-c) Distilled water; (d-f) pH=7 solution; (g-i) pH=4 solution

The experimental data were processed to obtain the main mechanical parameters of sandstone, which are given in Table 1. DW is distilled water; E, μ, ε_c are elastic modulus, Poisson ratio, and peak strain, respectively; σ_c, σ_cd and σ_cd/σ_c are peak stress, dilatancy stress, and ratio of dilatancy stress, respectively.

Table 1全屏查看

Characteristic parameters of sandstone samples under different chemical, seepage, and stress conditions

Liquid	σ3/MPa	Pw/MPa	σc/MPa	εc/10-3	E/GPa	μ	σcd/MPa	(σcd/σc)/%
DW	5	0	81.65	11.21	9.85	0.20	55.05	67
		1	84.66	10.16	10.18	0.23	63.90	75
		3	70.17	11.63	6.64	0.26	51.80	74
	10	0	109.36	13.04	9.93	0.17	90.69	83
		1	106.71	11.27	10.32	0.18	85.21	80
		3	112.05	11.87	12.58	0.17	90.96	81
pH=7	5	0	81.92	10.21	9.94	0.24	67.44	82
		1	80.54	9.07	10.80	0.25	63.25	79
		3	75.28	9.32	8.79	0.17	61.14	81
	10	0	95.09	10.87	8.54	0.16	84.70	89
		1	102.38	12.28	10.55	0.17	82.62	81
		3	108.03	12.10	9.99	0.15	82.97	77
pH=4	5	0	79.01	10.69	8.85	0.17	57.91	73
		1	66.67	10.37	8.25	0.13	52.92	79
		3	74.28	13.31	7.95	0.21	55.49	75
	10	0	104.44	11.43	11.00	0.17	85.47	82
		1	83.81	12.04	8.60	0.13	79.85	95
		3	104.86	11.02	12.04	0.22	85.85	82

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Figure 3 shows that the shapes of the stress-strain curves were similar. The stress-strain process was divided into four successive stages: the compaction stage, elastic stage, plastic yielding stage, and destruction stage. Under low confining pressure, the sandstone was ductile. The sample had a distinct yielding phase and post-peak strain softening. Brittle damage occurred in sandstone under high confining pressure, because the texture became hard by the confinement of confining pressure. With the increase of confining pressure, the peak strength increased obviously, but the peak strain did not. Under high confining pressure, the curves were steeper than that under low confining pressure. Confining pressure had a compaction effect on rocks, which made mineral particles more closely connected and mineral structure more compact. In addition, the initial compaction stage was prolonged with the increase of pore pressure and the strength of rock decreased slightly. This is mainly due to the fact that the pore pressure weakened the confining pressure and thus slowed down the closure of the primary pores within the rock samples.

It can be observed that the chemical seepage fluid had a significant effect on the mechanical properties of the rock, as shown in Figure 4. One observes that the strength (σ_c) and deformation resistance properties (E, μ) of sandstone in distilled water are higher than the other two solutions. With distilled water, the internal structure of the sandstone is damaged by the softening, sludging, lubrication and other chemical reactions such as dissolution and hydration occurred. On the other hand, in the solutions with pH=7 and 4, the rock samples are degraded by ion exchange, oxidation reduction, and other chemical reactions, in addition to the above reactions [29, 30]. Therefore, the mechanical properties of the samples suffered greater deterioration. The existence of H⁺ also promoted the corrosion of chemical solutions on rocks.

Figure 4全屏查看

Stress-strain curves of the sandstone under different chemical solutions: (a-c) σ₃=5 MPa; (d-f) σ₃=10 MPa

3.2 Characteristic stresses of sandstone

It can be seen from Figure 5 that the peak stress and dilatancy stress increased with the increase of confining pressure. This observation is related to the fact that the rock dilatancy is limited by confining pressure. As a result, one observes that the dilatancy stress ratio increases. The peak stress and dilatancy stress ratio decreased with the increase of pore pressure because the pore pressure had a weakening effect on the confining pressure. It resulted in a decrease in the peak stress and dilatancy stress. However, the dilatancy stress decreased more, so the dilatancy stress ratio decreased. Under the same confining pressure and pore pressure, the dilatancy stress ratio under the action of distilled water was less. Chemical solutions corroded the rock and reduced its peak stress and dilatancy stress. The peak stress decreased more than the expansion stress, so the dilatancy stress ratio increased. The dilatancy stress ratio under the action of pH=4 salt solution was generally least because H⁺ promoted the corrosion on rocks.

Figure 5全屏查看

Peak stress and dilatancy stress ratio of sandstone under different conditions: (a, c) σ₃=5 MPa; (b, d) σ₃=10 MPa

3.3 Deformation characteristics of sandstone

As shown in Figure 6, the peak strain and elastic modulus increased significantly with the increase of confining pressure, and Poisson ratio decreased. The sandstone was compacted by the confining pressure and its lateral deformation was restricted. Therefore, the sandstone can withstand greater deformation and the strain occurring in the lateral direction increased less than that in the axial direction. The peak strain and elastic modulus decreased, and Poisson ratio increased with the increase of pore pressure because the pore pressure weakened the confining pressure. The change of pore pressure was not significant, so the variation of parameters with increasing pore pressure was not obvious. Under the same confining pressure and pore pressure, the elastic modulus of the samples under the action of distilled water were greater, because the corrosion of rocks in distilled water was weaker.

Figure 6全屏查看

Peak axial strain (a, b), elastic modulus (c, d), and Poisson ratio (e, f) of sandstone under different conditions: (a, c, e) σ₃=5 MPa; (b, d, f) σ₃=10 MPa

4 Permeability evolution of sandstone

In this section, the effects of different conditions on the permeability of the sandstone were investigated. The change of permeability during the whole process of triaxial compression test was analyzed to study the evolution law between deformation and permeability.

4.1 Effect of hydrochemical action on permeability

The permeability-strain curves are given in Figure 7, where k₀, k₁ and k_max denote the initial permeability, stable average permeability, and maximum permeability, respectively. As can be seen from Figure 7, the curves of sandstone under the action of pH=4 solution are higher than other samples. It indicates that the permeability was greater. The original pores and micro-fissures in the rock were subjected to stronger corrosion under the action of solution. Corrosion not only dissolved the mineral components, but also made the rock more susceptible to physical corrosion under the action of solution seepage. It corroded the mineral composition and destroyed the internal structure of the rock, which resulted in the propagation of the original seepage channels and the generation of new seepage channels. Hence, the permeability increased significantly [31-33].

Figure 7全屏查看

Permeability of sandstone under different chemical solutions: (a, b, c) σ₃=5 MPa; (d, e, f) σ₃=10 MPa

Some of the primary pores and micro-fissures were narrowed or closed under the extrusion of pore pressure in the process of seepage. It reduced the original seepage channels and resulted in a decrease in the permeability. Thus, the permeability decreased with the increase of pore pressure. Besides, the permeability decreased with the increase of confining pressure because the confining pressure compacted the pores and micro-fissures.

4.2 Relationship between permeability and strain

As shown in Figure 8, the stress-strain curves are mainly divided into the initial compaction stage, linear elasticity stage, and plastic deformation stage. The effective porosity and fissure openness inside the samples decreased first and then gradually increased with the increase of axial stress. It made the permeability to produce a similar pattern of change. The permeability curve decreased first, then remained stable, and finally rose sharply when the stress-strain curve reached its peak.

Figure 8全屏查看

Relationships between volumetric strain and permeability of sandstone under different conditions: (a, c, e) σ₃=5 MPa; (b, d, f) σ₃=10 MPa

After the confining pressure loading was completed, the primary pores and fissures inside the rock samples were compacted partly. The permeability fluctuated up and down around a relatively stable value. At the beginning stage of axial stress loading, the permeability decreased. This stage was the initial compaction, and the main factor affecting the permeability was the axial stress. The axial stress gradually compacted the primary pores and fissures, which made the permeability decrease steadily. After entering the linear elasticity stage, the permeability was less affected by axial stress. The primary pores and cracks have been closed completely, and no new cracks can be generated. Therefore, the permeability remained stable and basically unchanged. The permeability increased rapidly with the increase of axial stress in the plastic deformation stage. It reached the maximum value when the slope of the curve was gradually close to zero. In this stage, plastic deformation characteristics appeared in the sandstone. From a microscopic point of view, a large number of pores and cracks were formed. The newly formed pores and fissures were continuously connected to each other and to the primary pores. They formed new seepage channels and made the permeability increased.

5 Failure modes of sandstone

The main damage form of sandstone under chemical-seepage-stress multi-field coupling was tensile-shear damage. Most of the samples produced a through crack with multiple secondary cracks. The pore pressure caused more deformation and macroscopic cracks when the rock was fractured [34]. This phenomenon became more obvious as the pore pressure increased, as shown in Figure 9(a). The sudden release of energy made the low stress zone around the main rupture surface affected when the rock reached the peak stress. It took the form of the extension of secondary cracks from the main cracks to the ends of the specimen. Pore pressure weakened the connection between mineral particles and destroyed the mineral structure, which made cracks develop more easily. The confining pressure compacted the samples and strengthened the mineral particles connection and structure, so the cracks were limited. With the increase of confining pressure, the number of cracks decreased obviously, as shown in Figure 9(b).

Figure 9全屏查看

Failure modes of sandstone under different conditions: (a) Distilled water and σ₃=5 MPa; (b) pH=4 salt solution and P_w=3 MPa; (c) pH=7 salt solution, P_w=3 MPa and σ₃=5 MPa

Under the action of chemical corrosion, the fracture surfaces became rough and difficult to slide. The friction coefficient of the surface increased, which inhibited the expansion of secondary cracks. The damage form of specimens corroded by pH=7 solution was dominated by a through main crack and only a small number of specimens produced secondary cracks, as shown in Figure 9(c). However, the rocks under the action of distilled water or pH=4 solution produced more secondary cracks. Distilled water had a weak corrosive effect on rocks, and the friction coefficient increased slightly. Therefore, the fracture surfaces were smoother and the number of cracks was fewer than the samples corroded by pH=7 solution. The corrosion of rock by pH=4 solution was strong and more pores were formed. Thus, the rock was more likely to crack and many fractures can be observed in rocks.

6 Conclusions

A series of triaxial compression tests were carried out on red sandstone under the action of different chemical solutions, confining pressures, and pore pressures to study its mechanical and permeability characteristics. The main conclusions are as follows.

1) The stress-strain curves have basically the same shape and are divided into the compaction stage, elasticity stage, yielding stage, and destruction stage. It is found that the confining pressure and pore pressure have opposite influences on the mechanical properties. The peak stress, dilatancy stress, the dilatancy stress ratio, the peak strain, and elastic modulus increase with the increase of confining pressure, while the Poisson ratio decreases. The failure mode of rock changes from plastic failure to brittle failure, and the rock produces less cracks when fractured. The chemical corrosion also has an important influence on the mechanical properties. The peak stress, dilatancy stress, the dilatancy stress ratio, and elastic modulus of the samples under the action of distilled water are greater. The pH=7 and 4 salt solutions have stronger corrosive effects on sandstone. The existence of H⁺ also promotes the corrosion.

2) The permeability decreases with the increase of pore pressure. The confining pressure and pore pressure have similar effects on the permeability. The permeability under the action of pH=4 solution is larger than other samples. The change process of permeability with axial stress is divided into three stages: initial compaction stage, linear elasticity stage, and plastic deformation stage. With the increase of axial stress, the permeability first decreases, then remains stable, and finally increases rapidly.

3) It is found that the change of chemical-seepage-stress field has a large impact on the strength, deformation, and permeability of sandstone. The achievements can be useful to deepen the understanding of underground rock engineering and prevent engineering accidents.