Yunnan Phosphate Mines, located in Yunnan Province, China, is a rich phosphate resource production area (Figure 1), mainly including phosphorite, phosphorite rock, and phosphorus iron ore. Its phosphate ore reserves rank first in China, with approximately 9 billion tons, accounting for over 70% of China’s total phosphate ore reserves. The development and utilization of phosphate resources are mainly focused on the agricultural production sector, playing an important supporting role in Chinese and global agricultural production. At the same time, Yunnan Phosphate Mines have also achieved a series of achievements in technological innovation and environmental protection, realizing the transformation and upgrading from traditional mines to modern mines. In the future, Yunnan Phosphate Mines have broad prospects for development. With increasing population and growing demand for global agricultural production, phosphate fertilizer, as an essential nutrient for farmland, will continue to have high demand. (Xiao et al, 2022; Zhang et al, 2022).

The rock sample in this paper is taken from the phosphorus block rock in the upper layer of phosphate mine roadway in Yunnan. The selected rock sample was cut and polished to make a standard cylindrical sample with diameter of 50 mm and height of 100 mm. Meanwhile, in order to avoid the influence of the end friction effect on the test, the flatness of the end face was controlled within 0.02 mm, the surface of the specimen was smooth without obvious joints and cracks, and the cylindrical specimen was manufactured in strict accordance with the standards of the International Society of Rock Mechanics, as shown in Figure 2.

1) Ultrasound Testing Device.

The longitudinal wave velocity was carried out by the UTA-2000A intelligent ultra-sonic monitor (as shown in Figure 3A). The sampling frequency is 10 MHz; sensor frequency is 35 kHz, ac-curacy is 0.1 µs, and Vaseline cream as coupling agent is used between the sample and sensors.

Measure the travel time t of the sound wave, and calculate the wave velocity v, of the rock ultrasonic wave according to the following formula: v p = L △ t (1) where, v p , is longitudinal wave velocity, m/s; L is sample length, m; and △ t is wave’s travel time, △ t = t − t 0 .

1) Grouping: The specimens are divided into three groups, labeled as Group A, Group B, and Group C, with 8 specimens in each group. Group A specimens undergo no treatment and are labeled as the natural group. Group B specimens are subjected to forced saturation in a vacuum saturation apparatus, and then sealed with plastic wrap, labeled as the saturated group. Group C specimens are first subjected to seismic wave simulation disturbance, and then subjected to forced saturation in a vacuum saturation apparatus, labeled as the post-seismic saturated group.

When the rock is saturation or loaded, cracks will occur. When the ultrasonic wave encounters cracks and defects in the propagation process, refraction, diffraction and other phenomena will occur. A decrease in waveguide performance and an increase in energy attenuation will lead to a decrease in P-wave velocity. Therefore, the mechanical deterioration characteristics of samples after saturation or loading can be indirectly reflected by ultrasonic testing of samples (Wei, 2020). Figure 5 shows the density and ultrasonic test results of the three groups of samples.

As can be seen from Figure 5, the average apparent density of 8 natural samples (Group A) is 2.77 g/cm³, and the average longitudinal wave velocity is 3093 m/s. After 48 h of saturation, the average density of 8 samples in Group B was 2.818 g/cm³, and the average P-wave velocity was 2056 m/s, which increased by 1.72% and −29.7% compared with Group A, respectively. After simulated seismic stress loading and water saturation twice, the average density and p-wave velocity of 8 samples in Group C are 2.825 g/cm³ and 1379 m/s, respectively, which are reduced by 2.00% and −52.9% compared with Group A.

It can be seen from the above analysis that the average density of Group C samples under simulated seismic stress loading and saturated water treatment is the highest, and the average P-wave velocity is the lowest. This is because, compared with the natural group, due to the vacuum forced water saturation, water molecules enter the samples and fill the primary cracks and bedding, thus reducing the propagation velocity of longitudinal waves in the samples. The samples of Group C underwent simulated seismic stress loading first, and many new cracks were generated inside the samples. After being saturated with water, more water molecules entered into the samples, resulting in a further increase in the sample density compared with Group B, and a further decrease in the propagation velocity of P-wave. When ultrasonic wave meets water, cracks and defects in the propagation process, it has to undergo refraction and diffraction, and the weakening of waveguide property and the increase of energy attenuation will lead to the decrease of P-wave velocity. Therefore, density and ultrasonic testing of samples can not only eliminate abnormal samples and reduce the influence of rock anisotropy on mechanical test results, but also indirectly reverse simulate the seismic stress and mechanical deterioration characteristics of samples after saturation.

Figure 6 shows the stress-strain curves of the samples in the conventional triaxial compression test. As can be seen from Figure 6, the variation law of stress-strain curve under conventional triaxial compression of samples of each group roughly went through four stages: compaction, elasticity, yield and failure. 1) Compaction stage. The original pores and cracks in the sample are slowly compressed, and the stress-strain curve is concave. 2) Elastic stage. The stress-strain curve is basically linear and follows Hooke’s law. 3) yield stage. As the axial stress continues to load, the material with low strength inside the sample first enters the yield failure stage, and the increase rate of axial stress decreases gradually. 4) failure stage. When the ultimate strength of the specimen is reached, macroscopic slip occurs along a fracture surface inside the specimen, and the bearing capacity of the specimen decreases rapidly with the increase of deformation.

Among them, the stress-strain curves of Groups A and B showed good linear characteristics before the peak, with obvious peak points, but less jitter after the peak, showing no obvious plastic characteristics. After simulated seismic stress loading, the samples of Group C show obvious plastic characteristics, and the stress-strain curve jitter obviously after the peak, showing multiple steps, which is the result of crack dislocation and closure.

To further analyze the deformation characteristics of samples in each group, the circumferential strain and volume strain-stress curves of typical samples (σ ₃=4 MPa) in each group were plotted, as shown in Figure 7. As can be seen from Figure 7, during the loading process, the variation trend of the volume strain of the three groups of samples is quite different, and this difference is mainly determined by the circumferential deformation. At the initial stage of loading, the circumferential strain of Group A was almost zero, and the circumferential deformation did not begin to increase until the end of the elastic stage. After the specimen was broken, the increase rate of circumferential deformation began to increase, but it was still not much faster than the axial deformation at the same period. As a result, although the specimen showed signs of slight expansion within a period of time after the specimen was broken, it still showed a state of continuous compression on the whole.

Sample B is similar to sample A in the initial loading stage, with small circumferential deformation. However, after the fracture of the sample, the annular deformation of Group B samples showed an explosive increase, and the volume of the sample rapidly changed from compression to expansion. After specimen fracture, the axial strain continues to increase, but with the rapid drop of axial stress, under the action of confining pressure, the increase trend of the circumferential strain of the specimen is restrained. The volume variation of the sample tends to be stable.

The deformation of the samples in Group C is significantly different from that in Groups A and B. The mechanical properties of the samples in Group C have deteriorated due to the influence of fatigue stress before loading, resulting in relatively large transverse deformation of the samples in the initial loading stage, and the growth rate of the transverse deformation gradually increases with the extension of time. After the compaction stage, the growth rate of the circumferential strain exceeds the axial deformation. In the elastic stage, the sample has entered the whole expansion stage until the sample cracks.

The above analysis shows that water saturation and fatigue stress loading have great influence on the deformation characteristics of the sample. The samples of the natural group were in the overall state of compression during the experiment, the samples of the water-saturated group were first compressed and then expanded, and the samples of the seismic stress disturbance and then saturated were in the overall state of expansion.

Observe the peak stress of samples in each group in Figure 6. For Group A, the peak strength of sample is 169.9 MPa when the confining pressure is 4 MPa. When the confining pressure is 6, 8, 10, and 12 MPa, the peak strength of each sample increases by 18.7%, 38.1%, 60.9%, and 87.1%, respectively. For the samples of Group B, the peak strength of the samples under the confining pressure of 4 is 91.5 MPa. Compared with that under the confining pressure of 4 MPa, the peak strength of the samples under the confining pressure of 6, 8, 10, and 12 MPa increases by 41.93%, 63.30%, 127.0%, and 154.7%, respectively. For the samples of Group C, the peak strength of the samples at the confining pressure of 4 is 84.17 MPa. Compared with that at the confining pressure of 4 MPa, the peak strength of the samples at the confining pressure of 6, 8, 10 MPa, and 12 MPa increases by 20.7%, 31.1%, 70.7%, and 113.0%, respectively. It can be seen that the peak strength of samples increases gradually with the increase of confining pressure, but the influence of confining pressure on the peak strength of samples varies greatly among all groups.

Meanwhile, the transverse comparison of samples of each group shows that the peak strength of samples of Group A is the largest, and the average peak strength of samples of Groups B and C under various confining pressure conditions is 32.2% and 48.3% lower than that of Group A, respectively, indicating that saturation water and simulated seismic stress loading have obvious deterioration effects on samples. The strength of the sample after seismic stress disturbance is lower, because the long-term fatigue loading promotes the expansion of the primary cracks and the initiation of new cracks in the sample, and the vacuum saturation after fatigue loading promotes the water molecules to enter these new cracks, causing secondary damage to the sample, leading to substantial deterioration of the mechanical properties of the sample.

The peak strength of samples under various confining pressures was plotted, as shown in the figure, from which it can be found that the approximate linear relationship between the two was in line with Coulomb strength criterion. According to Coulomb strength criterion, the maximum shear stress the sample can bear is determined by cohesion and internal friction Angle, which can be expressed as: τ = c + μ σ (3) where, c is cohesion; μ for internal friction coefficient, and μ = tan ⁡ φ , φ is the angle of internal friction, σ is the normal stress on the failure surface. If expressed as principal stress, then: σ 1 = k σ 3 + Q (4) where, σ ₁ is the peak strength; k and Q are material strength parameters, and the relationship between their value and the strength parameter (c, f) of the material is: φ = arcsin k − 1 k + 1 (5) c = Q 1 − sin ⁡ φ 2 cos ⁡ φ (6)

According to Eq. 4, the relationship between peak strength and confining pressure of samples was obtained by regression, as shown in Table 1; Figure 8.

It can be seen that the influence coefficients of confining pressure of the samples are roughly 9–18.4, and the correlation coefficients are all greater than 0.94, indicating that the peak strength of triaxial compression of the samples of each group has a good correlation with the confining pressure, in line with the Coulomb strength criterion. According to Formula 5, 6, the cohesion and internal friction Angle of each group of samples are obtained, as shown in Table 1.

In order to explore the deterioration of samples caused by saturation water and seismic stress disturbance, CT scanning was performed on typical samples from Groups B and C. Figure 9 shows the failure morphology of samples under triaxial compression (Bohloli et al, 2007; Wood, 2015; Wang et al, 2023). Since the failure patterns of Groups A and B are similar, only the fracture scanning maps of Group B samples are listed to save space.

As can be seen from Figure 9, the failure characteristics of water-saturated samples are similar to those of traditional natural samples, showing shear failure. The shear plane starts from the edge of the sample end face and presents typical diagonal shear failure, and some powdery failure occurs at the junction of the failure plane and the bottom end face.

The failure characteristics of the samples disturbed by seismic stress are obviously different from those of the traditional samples. There are multiple fracture zones extending inward from the end of the sample, and multiple fracture zones extending inward from the side surface of the sample. From the side view, it can be seen that the sample has obvious transverse expansion, which is consistent with the analysis results of the bulk strain. This is because after the sample of this group experienced seismic stress disturbance, the primary cracks continued to develop and expand, and even produced new cracks. These cracks were saturated by vacuum water, resulting in secondary failure. The internal damage of the sample was significantly increased compared with that of the sample of this group. When triaxial compression test is carried out, it can still produce certain resistance to deformation under confining pressure. When the specimen is broken and the confining pressure is gradually discharged, the specimen will produce heavy shear and penetrating failure due to the fracture penetration, and then appear large transverse deformation.