Water-rock interaction is a vital factor to affect the stabilities of rock projects. This paper conducted a series of experiments on argillaceous limestones to investigate the influences of saturation and wetting-drying cycle on the physical and mechanical performances of rocks. The results show that the increasing saturation increases the dissolution of clay minerals and lubrication among mineral grains, resulting in an obvious reduction effect on the strength and deformation performances of argillaceous limestones. Wetting-drying cycle increases the porosity and changes the pore structure of argillaceous limestones, leading to the pore transformation from small pore (0.01∼0.1
The performance of rocks is affected by the geologically buried environment, e.g., temperature, corrosion, and water [
In some specific natural engineering environments such as water-level fluctuation in reservoir or dam and seasonal rainfall, the wetting-drying cycle phenomenon is very common to the rock, which has recently become the issue of interest among researchers [
The argillaceous limestone was collected from the Liupanshan tunnel, located in the Ningxia Autonomous Region in northwest China, as shown in Figure
Sampling location of argillaceous limestone in this study.
Chemical compositions of argillaceous limestones.
SiO2 | CaO | Al2O3 | MgO | Fe2O3 | K2O | Na2O | Others |
---|---|---|---|---|---|---|---|
45.85% | 18.28% | 13.70% | 10.44% | 5.80% | 2.30% | 1.53% | 2.09% |
After drilling, cutting, and polishing in the laboratory, the argillaceous limestone block was processed into the standard cylinder specimens with the diameter and height of 50 mm and 100 mm, respectively, as shown in Figure
(a) Argillaceous limestone specimens and (b) RTX-4000 GCTS rock mechanics test system.
To study the saturation effect, the specimens were firstly filled with the water under the pressure of 90 Pa for 72 hours, using a BH-1 vacuum pressure saturation device. Then, the specimens were taken out from the saturation device and were immediately utilized to conduct the conventional triaxial compression tests, with the confining pressures of 0, 5, 10, and 15 MPa, respectively, using a RTX-4000 GCTS rock mechanics test system (Figure
For the cycle effect of wetting-drying, the argillaceous limestone specimens were firstly saturated in the water for 3 days and then dried in the natural state for 3 days [
The variations in the triaxial compression strength
Effect of confining pressure on the (a) triaxial compression strength and (b) elasticity modulus of the argillaceous limestone specimens in the saturated and natural states, respectively.
Figure
Based on the linear Mohr–Coulomb (M-C) strength criterion (equation (
Cohesion and friction angle of argillaceous limestone in the natural and saturated states, respectively.
In the natural state, the argillaceous limestone has the constant cohesion and friction angle of 18.61 MPa and 43.08°, respectively. However, in the saturated state, both the cohesion and friction angle show decreasing trends, by the rates of 25.90% and 6.59%, respectively. The degradation effect of saturations can be attributed to two reasons. One is that some clay minerals, which widely exist in the argillaceous rock, are dissolved in the water, leading to the increasing porosity and structural damage of rocks. The other reason is that the lubrication performance of water in rocks decreases the friction among the mineral grains, resulting in the attenuation of bearing capacity of argillaceous limestones.
The performance of argillaceous limestones, which does not just depend on the saturated state, is also influenced by the cycle effect of wetting-drying. In order to visually investigate the variation in the mass of argillaceous limestone treated by the wetting-drying cycle, mass loss level was defined as
Figure
Variation in mass loss level with different wetting-drying cycle number.
Variations in density and ultrasonic velocity of argillaceous limestones with different
Variations in density and ultrasonic velocity with different wetting-drying cycle number.
The variations in compression strength
Variations in (a) compression strength, (b) peak strain, (c) elasticity modulus, and (d) secant modulus versus the wetting-drying cycle number.
Mechanical and pore parameters of argillaceous limestones after the wetting-drying cycle.
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Pe (%) | |||
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0∼0.01 |
0.01∼0.1 |
0.1∼1.0 |
>1.0 | ||||||
0 | 81.83 | 1.15 | 8.89 | 6.17 | 8.73 | 0.10 | 10.18 | 87.88 | 1.84 |
3 | 80.13 | 1.17 | 8.55 | 5.75 | 8.82 | 0.27 | 7.83 | 90.48 | 1.42 |
6 | 79.16 | 1.21 | 8.46 | 5.61 | 8.84 | 0.23 | 6.13 | 91.32 | 2.32 |
8 | 71.19 | 1.32 | 6.65 | 4.75 | 9.41 | 0.02 | 7.83 | 90.86 | 1.29 |
10 | 64.49 | 1.33 | 5.71 | 4.53 | 9.52 | 0.01 | 7.06 | 91.46 | 1.47 |
12 | 51.25 | 1.43 | 4.96 | 3.28 | 9.55 | 0.02 | 7.03 | 92.24 | 0.71 |
As shown in Figure
SEM images of argillaceous limestones after the cycle effect of wetting-drying: (a)
The AE count distribution of the argillaceous limestone specimens in the loading process of uniaxial compression is also dominated by the cycle effect of wetting-drying, as shown in Figure
AE count distributions of the argillaceous limestone specimens in the loading process of uniaxial compression.
The increase in
Relationships between physical and mechanical parameters. (a)
The porosity
Variation in porosity versus the wetting-drying cycle number.
The
T2 curves of argillaceous limestones after the cycle effect of wetting-drying.
As tabulated in Table
Variations in Pe (0.01∼0.1
In this study, argillaceous limestones from the Liupanshan tunnel (China) were saturated and treated by the wetting-drying cycle to investigate the variations in the physical and mechanical performances. The SEM and NMR tests were also conducted on the specimens to reveal the microscopic feature and structure. Some conclusions can be obtained as follows: The saturation effect promotes the dissolution of clay minerals in argillaceous limestone and reduces the friction among the mineral grains, which leads to attenuations in the strength and deformation performances of the specimens. The cohesion and friction in the saturated state decrease by 25.90% and 6.59%, respectively, compared with those in the natural state. With an increase in the wetting-drying cycle number, density, ultrasonic velocity, compression strength, elasticity modulus, and secant modulus decrease gradually, while mass loss level increases. The variation process of these physical and mechanical parameters follows the exponential function with the increase in the wetting-drying cycle number in general. Wetting-drying cycle effect gives rise to the development of microscopic fractures, as well as the increase in porosity, especially after the 6th cycle. The pore structure of argillaceous limestone under the wetting-drying cycle effect also redistributes. The percentage of the small pore (0.01∼0.1
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This paper was financially supported by the National Natural Science Foundation of China (nos. 51704279, 51504247, and 51734009) and the Natural Science Foundation of Jiangsu Province of China (no. BK20170270).