The Laoheba Phosphate Mine Area in the Sichuan Basin stands as one of China’s primary locations for phosphate extraction, boasting a diverse array of rock types and complex rock layers. In recent years, frequent geological disasters, notably landslides, have occurred in the mining area. The safe extraction of phosphate rock faces significant challenges, necessitating an in-depth exploration of the physical and mechanical properties of the rocks within the mining area. This study employs nuclear magnetic resonance (NMR) and X-ray diffractometer (XRD) testing on six typical rock specimens, contrasting and analyzing their physical traits, thus unveiling the impact of rock composition and microstructure on their mechanical properties. The MTS815 Flex Test GT rock mechanics testing system was employed to perform uniaxial compression, triaxial compression, Brazilian disk splitting, and triaxial penetration tests. The study systematically examined the mechanical characteristics of typical rocks in the mining area. The correctness of the experiments was mutually validated by four types of tests. Finally, an analysis of rock failure modes and patterns was conducted. Research suggests that phosphate ore exhibits the highest porosity and permeability. Phosphate ore exhibits significant development of original joints and cracks internally, along with numerous defects, leading to its minimal compressive and tensile strength. Phosphate ore is typically situated in regions of weakened rock mass strength. Real-time monitoring of confining pressure is essential during mining operations to prevent the collapse of surrounding rock formations. The findings of this study offer theoretical backing for secure mining operations in the Laoheba Mining Area of the Sichuan Basin while also furnishing fundamental physical and mechanical parameters for regional geomechanical analysis.

The Sichuan Basin is situated in southwestern China. The mining area lies on the southwestern edge of the Sichuan Basin. The phosphate ore layer is found in the lower strata of the Cambrian System. The mining area boasts a diverse range of rock types. The rocks exhibit unique compositions. Their physical and mechanical features differ from those of ordinary rocks. We conducted research on six typical rocks selected from the mining area. Geological exploration and investigation into the physical and mechanical properties of rocks are essential prerequisites for the safe exploitation of phosphate mines.

With the exploration and development of mineral resources, and the increasing scale of various underground rock engineering projects, higher demands have been placed on the design and construction of geotechnical engineering. In recent years, there has been a surge in accidents in rock engineering, drawing attention to the safety of underground rock engineering. The physical and mechanical properties of rocks are closely associated with the stability of rock engineering. Therefore, investigating these properties is paramount. Unveiling the physical and mechanical properties of rocks can offer fundamental theoretical support for the construction and operation of rock engineering [1-4].

Around the physical and mechanical properties of rocks, scholars have conducted a substantial amount of fundamental research, laying the groundwork for the study of rock failure mechanisms [5-7]. Xue et al. investigated the law of rock porosity and grain size on rock fracture failure [8]. Ghazvinian et al. studied the mechanical behavior of rocks under different stresses and discussed the different crack failure thresholds on the anisotropic behavior of rocks influencing factors [9]. Based on experimental and theoretical analyses, Cai et al. summarized the failure mechanism of nodular rock masses and described the fracture surfaces in the principal stress space based on the stress criterion [10]. Kim et al. investigated the crack extension and failure criterion of granite-containing fractures using acoustic emission technique and analyzed the stress–strain curve of granite [11]. Damjanac et al. performed triaxial compression tests on cylindrical samples under three different stress paths [12]. Zhang et al. discussed the effect of cyclic loading on the strength and deformation of rocks [13]. Wang et al. explored the physical, mechanical, and fatigue properties of red sandstone [14]. Li et al. conducted impact compression tests on granite under high-temperature conditions to explore the laws of temperature and external loading on the mechanical properties of rocks [15]. To investigate the feasibility of the resistance strain gauge method, an experimental system was built to measure the fractured rock deformation due to cycling of confining pressure [16]. The above-mentioned scholars mainly used rock cylindrical samples to develop the physical and mechanical properties of rocks.

The test method and the test apparatus have been refined and improved by scholars. Brace et al. investigated the mechanical properties of rocks under tensile and shear stresses by means of triaxial tensile tests on rocks [17]. Rodriguez and Bobich studied the failure mechanisms of Carrara marble and Berea sandstone using a modified method, respectively [18, 19]. In addition, Aimone-Martin et al. performed tension shear tests on rocks using a self-developed symmetrical four-bar linkage [20]. Cai et al. performed a series of CPL tests on sandstone samples using a self-developed vibratory point loading device to study the CPL fatigue behavior of rock materials [21].

The properties of synthetic geotechnical materials have been investigated by several authors, for example, Consoli et al. obtained synthetic cemented silt by mixing cement, water, and soil and subjected it to uniaxial compression tests [22]. Fedrizzi et al. investigated the physical properties of carbonate rocks by using cement to bind calcite and quartz [23]. Arora et al. synthesized mudstone by mixing clay, cement, and water and compared its physical and mechanical properties with natural rock samples [24]. Mondol et al. synthesized a mudstone-like material by mixing water, kaolinite, and montmorillonite and used mechanical compression to investigate its physical properties [25].

Regarding the crack extension mechanism during rock blasting, Yue et al. set up three sets of PMMA samples and conducted a controlled blasting experiment with directional fracture; using a high-speed camera, the crack development was recorded [26]. Yang et al. used LS-DYNA to simulate the crack formation process during charge blasting and revealed the blast stress wave propagation path [27]. Wan et al. used the matter point method to explore the expansion pattern of rock cracks during blasting [28].

To summarize, to the best of the author’s knowledge, there has been no reported geological and mechanical research on the Laoheba phosphate mine area. This study aims to fill this gap. Previous studies predominantly employed single experimental methods or focused on single rock types, with few utilizing multiple experimental methods to study various rock types. Given the unique composition of rocks in the mining area, distinct from typical rocks, and the diverse array of rock types within the region, there is a pressing need to explore the physical and mechanical properties of rocks in the area.

This study selected six representative rock types from the region for analysis to understand the physical and mechanical properties of rocks in the area. Utilizing nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and the MTS815 Flex Test GT rock mechanics testing system, various experiments including NMR, XRD, uniaxial compression, triaxial compression, Brazilian disk splitting, and triaxial permeability tests were conducted on six typical rocks from the Laoheba phosphate mine area in the Sichuan Basin, China. Through comprehensive analysis, this study systematically investigated the physical and mechanical properties of rocks in the phosphate mining area. Comparative analysis of the mechanical properties of rocks with different attributes provided insight into the stress–strain relationships of rocks with varying lithology. By integrating the physical and mechanical properties of rock materials, the study analyzed the failure modes and patterns of rocks. Starting from the perspective of rock properties, the study analyzed the mechanisms of rock failure and crack propagation patterns. The abundant experimental data presented in this paper can support regional geological mechanic’s research. The summarized rock failure modes in the study can provide guidance for the study of surrounding rock stability. The research findings offer original physical and mechanical parameters for mining engineering design and provide theoretical underpinnings for the safe construction and operation of mining projects.

2.1. Geographic Location, Geological Characteristics, and Meteorological Conditions of Rock Samples

The Laoheba Phosphate Mine Area is situated in Mabian County within the Sichuan Basin of China, located in southwestern China. According to statistics from the United States Geological Survey, China holds the world’s second-largest reserves of phosphate rock, accounting for 4.53% of the global total. Mabian County stands as one of the primary phosphate mining areas in China. As depicted in Figure 1, the extreme geographical coordinates of the mining area are between 103°21′50.095″ to 103°22′26.095″ east longitude and 28°39′48.313″ to 28°41′04.313″ north latitude. The mining area extends 2.345 km from north to south and 0.972 km from east to west, covering an area of 1.324 sq. km, with a mining elevation ranging from +1035 to +1675 m. Situated near the Dafengding National Nature Reserve and the local river water resources protection zone, the mining area faces stringent ecological environment protection requirements. Consequently, the demands for phosphate mining technology are exceedingly high, and thorough geological exploration prior to mining is of paramount importance.

Situated on the southwest edge of the Sichuan Basin, the mining area hosts the phosphate rock layer within the bottom strata of the Cambrian system. Nestled amidst high mountains, steep slopes, and intricate terrain, it resides in a mountainous region. The highest elevation of the exposed ore seam reaches 1675 m, while the lowest depth of the deep ore seam descends to 1035 m, resulting in a relative height difference of 640 m. The terrain exhibits a northward elevation, sloping southward, with higher elevations in the middle, and lower ones in the east and west. Predominantly covered by virgin forest, the Zhongshan area, where the mining site is located, features partially exposed rocks prone to geological hazards. These hazards encompass landslides, collapses, debris flows, among others. Moreover, the region experiences significant development of surface water bodies, with frequent heavy rainfall and a heightened propensity for debris flow occurrences. Given the complex geological conditions and the recurrent geological disasters, safe mining operations encounter substantial challenges [29].

2.2. Preparation of Samples

Due to the constraints imposed by the geological environment, the drill core sampling method is employed within the mining area to procure original rock materials. The area encompasses diverse rock types characterized by unique compositions. To facilitate a comprehensive assessment of the physical and mechanical properties of the rocks, five representative rock specimens alongside phosphate rocks from the mining site were meticulously chosen for examination, as detailed in Table 1. The raw rock material collected underwent processing to derive test samples. Adhering to the guidelines set forth by the International Society of Rock Mechanics (ISRM), the dimensions of the rock samples for uniaxial compression, triaxial compression, and triaxial penetration testing were maintained within a height-to-diameter ratio range of 2.0–3.0. This standardization aims to minimize the influence of end friction on test outcomes [30, 31]. Consequently, the original rock material underwent machining to produce cylindrical samples measuring 50 mm in diameter and 100 mm in height, with end parallelism of less than 0.05 mm, as illustrated in Figure 2.

For Brazilian disk-splitting tests, it is crucial to maintain the ratio of height to diameter of the rock samples within the range of 0.5–1.0. Employing water jet cutting technology, the initial rock material undergoes processing to yield disk specimens measuring 50 mm in diameter and 25 mm in height, as depicted in Figure 3. Furthermore, all specimens are situated in a controlled indoor environment at a constant temperature of 25°C. Completion of the tests is required within a 30 days timeframe to ensure the precision and reliability of the experimental outcomes.

2.3. Determination and Analysis of Rock Physical Parameters

The density, water content, and porosity of rock materials directly influence their mechanical properties. Determining these physical parameters is essential for comprehensively studying the properties of rocks. In their natural state, rocks inherently contain moisture. Initially, the natural density of the sample is assessed, followed by the determination of its dry density. The procedure for testing the dry density involves placing the sample in an electric hot air-drying oven, setting the oven temperature to 105°C, and leaving it for 24 hours. Subsequently, the sample is removed from the oven, allowed to cool to room temperature (20°C ± 2°C), and then the dry density is measured. The water content of the sample is then calculated based on the disparity between the natural density and dry density. According to the standards set by the International Society for Rock Mechanics (ISRM), for the same type of rock, all experimental tests in this paper are conducted on three samples. This is done to obtain more reliable and representative results, considering the possible heterogeneity of samples and errors during the testing process. Tests for each sample should be conducted under similar conditions, including applied stress or strain ranges, temperature, humidity, and other environmental conditions. This ensures the comparability of results and reduces errors caused by variations in testing conditions. The average of the measured results is taken, and the test results are presented in Table 1.

The porosity of samples under saturated conditions is determined utilizing the principle of NMR, which exploits the resonance property of hydrogen protons in a magnetic field to ascertain the internal pore structure of rocks. The experimental apparatus employed is a NMR imaging device. The experimental procedure entails several steps: initially, the rock sample is placed within a vacuum saturation chamber, where clean water is injected to raise the water surface 20mm above the sample’s upper surface. To ensure a proper vacuum level inside the chamber, Vaseline is applied to the cylinder’s cover seam. Subsequently, the vacuum pump is connected to the saturation chamber and activated to attain a vacuum pressure of 100 kPa, maintaining it until no bubbles appear in the water. Following the evacuation, the sample is returned to its original container and left to equilibrate under atmospheric pressure for 4 hours before undergoing NMR testing. The saturated sample is then inserted into a NMR instrument for T2 spectrum analysis. The maximum cumulative porosity value in the spectrum serves as the representative porosity of the rock sample, with the experimental findings tabulated in Table 1. The test standard is “Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass.”

In literature 34, the physical parameters of mudstone are shown in Table 1. This study compares and analyzes the experimental results of this research with those of the literature [32].

From Table 1, it can be seen that the natural density and dry density of the six types of rock materials have relatively small differences. However, there are significant differences in moisture content and porosity; sample 1 has the lowest moisture content, while sample 4 has the highest moisture content; and sample 3 has the lowest porosity. Sample P has the highest porosity, which also suggests that sample P has the lowest strength. In theory, the higher the porosity, the higher the moisture content. However, the six types of rock materials studied in this paper do not conform to this rule, due to differences in rock composition, internal joints, rock strata, and depth of burial. Therefore, as a natural material, rocks have complex physical properties. The differences in the physical parameters of the six types of rock materials also indicate that there will be differences in their mechanical characteristics and failure modes.

Compared with the physical parameters of mudstone in literature 34, it is evident that the rock dry density in the Laoheba mining area is greater, with lower moisture content and porosity. Rocks in the Laoheba mining area are denser with fewer internal voids, hence resulting in greater density.

XRD was employed to conduct XRD on the rock samples in order to obtain their composition. The test standard is “Refractories and refractory products—X-ray diffraction (XRD)—Determination of the phase composition of the products.” The samples were ground into fine powders, and 50 mg of powder was used for testing. Copper (Cu) was used as the target material for XRD, with a wide-angle diffraction scanning angle range of 5–85° and a scanning rate of 5°/min. By subjecting the rocks to XRD, their diffraction patterns were obtained, allowing for the analysis of their composition and proportions. The experimental results and literature 7 are shown in Table 2 [7], and the results are presented in a pie chart as shown in Figure 4.

It is evident from Table 2 that the composition and proportions of the seven rock types vary significantly. Examination of Figure 4(a) reveals that sample 1 comprises Quartz, Muscovite, Moganite, and Earlshannonnite, with respective proportions of 56.2%, 21.4%, 13.9%, and 8.5%. In Figure 4(b), sample 2 is constituted by Quartz, Ramsbeckite, Polylithionite, and Boron nitride, with proportions of 52.2%, 21.8%, 14.7%, and 11.3%, respectively. Figure 4(c) illustrates that sample 3 is composed of Quartz, Muscovite, Clinochlore, and Whitmoreite, with proportions of 41.2%, 31.4%, 16.9%, and 10.5%, respectively. Turning to Figure 4(d), sample 4 consists of Dolomite, Ankerite, and Minrecordite, with proportions of 39.8%, 32.5%, and 27.7%, respectively. Figure 4(e) depicts that sample 5 comprises Dolomite, Minrecordite, and Calcium, with proportions of 40.2%, 34.2%, and 25.6%. Finally, Figure 4(f) demonstrates that sample 6 is made up of Fluorapatite, Hydroxylapatite, and Dolomite, with proportions of 43.2%, 37.9%, and 18.9%, respectively. According to Table 2, it can be inferred that the shale described in literature 7 is composed of Quartz, Calcite, Dolomite, Muscovite, Illite, and Pyrite, with proportions of 42%, 23.5%, 12.5%, 12.4%, 7.5%, and 2.1%, respectively.

Rock samples 1, 2, and 3, as well as the shale described in literature 7, all contain Quartz, with relatively high proportions. Quartz is typically white or gray. It has high compressive strength but lower tensile and shear strength, and it is brittle. As shown in Figure 2, rock samples 1, 2, and 3 are gray-white in color. Rock samples 4, 5, 6, and the shale in literature 7 contain Dolomite, which is white and heat resistant. Muscovite is usually present in rocks as thin flakes and has a high elastic modulus. Its presence increases the overall elastic modulus and stiffness of the rock. The presence of Muscovite affects the formation of rock fractures and crack propagation. Due to its flaky structure, areas with Muscovite in the rock may be more prone to forming fractures parallel to the cleavage. In some cases, the presence of Muscovite can increase the rock’s toughness, making it more resistant to fracture. The presence of Minrecordite may affect the strength and hardness of the rock. It may increase the rock’s hardness, making it more difficult to break or peel. However, if Minrecordite’s structure is more fragile than the surrounding rock, it may introduce local weaknesses, affecting the overall strength of the rock. The presence of Minrecordite may introduce cracks or fractures into the rock, especially when there are significant differences in physical properties between Minrecordite and the rock. This may affect the rock’s fracture pattern and fracture toughness. As seen in Figure 4(f), phosphate ore contains a large amount of Fluorapatite, confirming the validity of the experiment. Comparing with the results from literature 7, it is found that the rocks in the Laoheba mining area contain special components such as Earlshannonnite, Ramsbeckite, Clinochlore, and Hydroxylapatite. These components affect the density and strength of the rock, which are rarely seen in previous rocks. It indicates the unique physical and mechanical properties of rocks in the Laoheba mining area, making their study essential.

Rocks with higher porosity contain more pore water and have lower density, as evidenced by the results in Table 2. Rocks with higher porosity have higher permeability because more pore space provides more channels and storage space, allowing fluids to penetrate and flow through the rocks more easily. Rocks with higher porosity typically have lower strength because pores cause stress concentration in the rock, reducing its load-bearing capacity. This is because pores decrease the effective connection points within the rock, thus reducing its overall strength. Rocks with higher porosity exhibit lower stiffness and greater deformation capacity because pores can lead to microstructural damage within the rock, increasing its deformation capacity.

Following the determination of the rock’s physical parameters, this section will proceed to ascertain its mechanical properties. This step aims to furnish raw data essential for the subsequent analysis of the rock’s mechanical characteristics and the application of the broken ring model.

3.1. Test Equipment

The experiment was conducted using the MTS815 Flex Test GT rock mechanics test system from Sichuan University. Within this test system, uniaxial compression, Brazilian disk splitting, triaxial compression, and triaxial permeability tests were performed. As depicted in Figure 5, the test system consists of three main parts: the compression loading frame, the axial dynamic loading system, and the data acquisition system. The maximum axial compressive load is 4600 kN, the maximum axial tensile load is 2300 kN, and the maximum confining pressure is 140 MPa. Axial and circumferential deformations of the rock specimens were measured using axial and circumferential extensometers, respectively. The axial dynamic loading system is driven by hydraulic pressure with a flow rate of 40 Lpm and an output pressure of 21 MPa. The data acquisition system comprises signal conditioning and acquisition units, capable of collecting data on all channels at a sampling rate of up to 6 kHz and with 16-bit resolution. This system allows for the selection of various sensors for load control (typically load, strain, and displacement), with high control precision. All experiments were conducted under room temperature conditions (20°C ± 2°C) [33, 34].

3.2 Uniaxial Compression Test

To conduct uniaxial compression tests on rock samples to obtain uniaxial compressive strength, elastic modulus, and Poisson’s ratio. First, measure the dimensions of the sample and record them for later calculations. As shown in Figure 6, apply a thin layer of lubricant (Vaseline) to the top and bottom surfaces of the sample to reduce the friction between the loading plates and the end faces of the sample, ensuring a uniform stress state in the middle of the sample. Place the sample in the center of the loading plate, aligning it properly with the top and bottom loading plates without eccentricity. Start the equipment and apply 1 kN axial force to fix the rock sample for subsequent operations. Install axial and circumferential displacement gauges on the sample, adjust the positions of the axial and circumferential displacement gauges to obtain initial readings, and use them to monitor the axial and circumferential deformations of the sample. During loading, adopt a displacement-controlled loading method with an axial loading rate of 0.002 mm/s, uniformly apply axial pressure until the sample fractures, and end the loading. The computer control system will automatically record the loading time, axial pressure, axial deformation, and circumferential deformation. Internal defects may exist in the rock, resulting in a very low compressive strength, which is possible to occur during the experiment. The test standard is “Rock—Determination of the uniaxial compressive strength of rock specimens.”

3.3 Brazilian cleavage test

To conduct the Brazilian splitting test on rock samples to obtain tensile strength. Measure and record the dimensions of rock samples. Place the samples at the center of the pressure plate, as shown in Figure 7. Apply a thin layer of lubricant (Vaseline) to the top and bottom ends of the samples in contact with the pressure plate. Adjust the position of the pressure plate to ensure uniform loading of the samples. Apply loading using the displacement-controlled loading method, with an axial loading rate of 0.002 mm/s, uniformly applying axial pressure until the rock samples fracture. Observe the failure process of the samples under load and record the failure morphology of the specimens [35]. The test standard is “Standard Test Method for Splitting Tensile Strength of Intact Rock Core Specimens.”

3.4. Triaxial Compression Test

The triaxial compression test section mainly determines the internal friction angle and cohesive force of rock samples. It can also analyze the influence of confining pressure on the mechanical properties of rocks. To ensure representative test results, three confining pressure levels are set at 5, 10, and 15 MPa, respectively. The test standard is “Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures.” The test procedure is as follows:

  1. Sample preparation: Place a pressure platform at the upper and lower ends of the sample, each fitted with a rubber ring. Then, wrap the sample with two layers of heat-shrink film. This serves two purposes: preventing hydraulic oil from entering the rock sample, reducing its strength, and inhibiting lateral deformation of the rock sample.

  2. Sample placement: Position the rock sample at the center of the loading platform, as shown in Figure 8. Apply a 1 kN axial force to secure the rock sample for subsequent operations. Install axial and circumferential displacement extensometers on the sample, connecting them to the testing equipment to record axial and circumferential deformations.

  3. Loading: Lower the three-axis chamber, enclose the sample, and prepare for loading. Apply confining pressure at a rate of 0.02 MPa/s. Once the confining pressure reaches the set value, maintain its stability. Apply axial load at a rate of 0.002 mm/s until the sample fails, concluding the test.

3.5. Triaxial Permeation Test

The triaxial permeability test is conducted to determine the permeability coefficient of samples, which significantly influences the mechanical properties of rocks. The samples undergo testing in a water-saturated state, with preliminary estimates indicating a permeability pressure of 0.5 MPa and a confining pressure of 1 MPa during testing. The test standard is the “Standard Test Method for Permeability of Granular Soils (Constant Head).” The experimental procedure is as follows:

  1. Sample preparation: The rock sample is soaked in distilled water for 48 hours prior to the experiment to achieve saturation, ensuring single-phase seepage within the sample. A pressure-bearing platform with perforations at both upper and lower ends of the sample is employed to facilitate water flow. Rubber rings are installed on each upper and lower pressure-bearing platform, as depicted in Figure 9. Additionally, the sample is wrapped with two layers of heat shrink film to prevent hydraulic oil from infiltrating.

  2. Sample placement: The sample is positioned at the center of the loading platform, as illustrated in Figure 9, and secured with an axial force of 1 kN. The conduit on the test system is connected to the sample via the reserved holes on the pressure platform.

  3. Loading: The triaxial chamber is lowered to cover the sample, and loading preparations are made. Confining pressure is applied at a rate of 0.02 MPa/s, stabilizing at 1 MPa. A water pressure of 0.5 MPa is then applied to the upper end of the sample through a catheter, while no water pressure is applied to the lower end, resulting in a pressure differential of 0.5 MPa between the two ends. Due to the pressure difference, water inside the conduit will flow from the upper end of the sample to the lower end. The pressure difference between the upper and lower ends of the sample will gradually decrease. The experiment concludes when the pressure differential stabilizes between the upper and lower ends of the sample [36, 37].

4.1. Analysis of Uniaxial Compression Test Results

The data recorded by the MTS system were processed to obtain uniaxial compression stress–strain curves, as shown in Figure 10. As an example, the failure process of rock sample 2 was analyzed, as shown in Figure 10(a). The failure process of the rock can be roughly divided into five stages: (1) compaction stage (OA), (2) nonlinear deformation stage (AB), (3) linear elastic deformation stage (BC), (4) strengthening stage (CD), and (5) plastic failure stage (DE) [32, 38].

In the first stage (OA), when the axial pressure is first applied, the original fractures within the rock are squeezed and begin to close. As the axial pressure increases, the original fractures are completely closed, and the rock is in a compact and dense state.

In the second stage (AB), the rock stress–strain is in a nonlinear variation phase, with the stress amplitude increasing slowly and the strain amplitude increasing rapidly. The original fractures within the rock begin to expand or extend, and new microcracks begin to sprout.

In the third stage (BC), the stress–strain curve is linear, and the rock deformation is in a linear elastic phase. During this stage, the axial pressure continues to increase, the rock cracks continue to develop, and the cumulative failure continues to increase, while the rock remains intact.

In the fourth stage (CD), the stress–strain continues to increase and shows a nonlinear relationship; the stresses are intensified and eventually reach extreme values. Microcracks within the rock sample continue to expand and interconnect and penetrate. After crack expansion reaches a critical state of stable equilibrium, crack expansion accelerates.

In the fifth stage (DE), after the stress reaches the ultimate strength of the rock, the cracks expand sharply and the strain increases sharply, followed by rock failure. As the stress decreases the strain increases and the rock gradually loses its load-bearing capacity.

The uniaxial compression stress–strain curves for various rock samples are depicted in Figure 10(b). The shapes of these six curves exhibit a general similarity, traversing through the five stages outlined above before reaching failure. The gradient of each curve during the initial ascent varies, indicating discrepancies in the modulus of elasticity among the different rock samples; a steeper slope corresponds to a higher modulus of elasticity. Additionally, the ultimate stresses of the rock samples differ, with sample 5 exhibiting the highest ultimate stress and sample P displaying the lowest. As illustrated in Table 1, sample P possesses the highest porosity, indicative of more pronounced internal joints, cracks, and defects, directly contributing to its diminished compressive strength. Variances in texture and internal joints among samples further contribute to differences in compressive strength. The comparatively lower compressive strength of Sample P represents a notable weakness in rock engineering, warranting heightened attention in mining design and construction to avert rock collapse. Moreover, the ultimate strains of the rock samples vary, with samples 1 and 3 exhibiting smaller ultimate strains and greater brittleness, rendering them susceptible to brittle failure during mining. Thus, measures should be implemented to enhance the monitoring of rock displacement during mining operations.

The elastic modulus, Poisson’s ratio, and compressive strength of six types of rock materials are shown in Table 3. Sample 5 has the largest modulus of elasticity, indicating its high stiffness. Sample 1 has the largest Poisson’s ratio, indicating that it has a large lateral deformation. Compared to the uniaxial compressive strength of shale in literature 7, the uniaxial compressive strength of rocks in the Laoheba mining area generally tends to be higher.

The compressive strength of phosphate rock sample P is relatively low, rendering it a vulnerable component in rock engineering. Its significance in mining design and construction is paramount to forestall potential roadway collapse. Additionally, distinct rock samples exhibit varying ultimate strains; notably, samples 1 and 3 display reduced ultimate strains, indicating heightened brittleness. This increased brittleness poses a risk of brittle failures during the construction process, necessitating enhanced monitoring of rock mass displacement.

4.2. Analysis of Brazilian Cleavage Test Results

Using the indirect method, the Brazilian disk tensile stress is calculated according to the following equation [39]:

σt=2pπDh
(1)

where, σ1 is tensile strength of rocks, p is failure load, D is sample diameter, and h is sample thickness.

The Brazilian splitting stress–strain curves for the six rocks are shown in Figure 11, from which the trend of the Brazilian splitting stress–strain curves for the six rocks is approximately the same. The tensile strength values of the rock are shown in Table 4.

Taking rock sample 2 as an illustrative case, the Brazilian cleavage failure process can be delineated into four distinct stages. OA Compaction Stage: In the initial loading phase, the original microfractures within the rock sample gradually close. Even a slight load application can induce significant deformation, resulting in an up-concave, nonlinear compaction deformation in the stress–strain curve OA section. AB Linear Deformation Stage: Subsequently, during the AB stage, the stress–strain relationship becomes linear, showcasing the rock’s elastic characteristics. BC Yielding Stage: When the stress approaches approximately 90% of the peak strength, the stress–strain curve deviates from linearity, marking the onset of the yielding stage. Here, the low-strength material within the sample yields initially at the same stress level, while the high-strength material undergoes gradual yielding. Consequently, failure within the sample progressively intensifies. CD Instantaneous Failure Stage: as soon as the stress attains its peak strength, the specimen undergoes sudden failure, manifesting the brittle characteristics inherent to the rock [40].

From Table 4, it can be observed that the tensile strength of sample P is similar in magnitude to the tensile strength of rocks in literature 40. Examining the graph reveals that sample P exhibits the lowest tensile strength, whereas sample 5 displays the highest. This observation aligns well with the results of the uniaxial compression test, affirming the accuracy of the conducted experiments. Following engineering design principles, it is recommended to consider the tensile strength of sample P as the standard value for designing tensile structures in mining applications.

4.3 Analysis of Triaxial Compression Test Results

The compressive strength of rocks under various confining pressures is presented in Table 5. Triaxial stress–strain curves for rocks under different circumferential pressures are illustrated in Figure 12, indicating similar shapes across various rock samples. Taking sample 2 as an example, as shown in Figure 12(b), the stress-axial strain curve can be divided into four stages:

  1. Compaction Stage (OA): This phase involves gradual reduction of microfractures or pores within the rock sample under axial pressure until closure.

  2. Linear Elastic Stage (AB): Characterized by a linear growth relationship in the stress-axial strain curve, demonstrating recoverable elastic deformation.

  3. Plastic Deformation Stage (BC): In this phase, rock microcracks progressively develop, causing local stress concentration due to original crack tips or internal defects, leading to crack extension and plastic failure.

  4. Post-Peak Failure Stage (CD): Following the stress peak, there is a significant decrease in stress while strain continues to rise, exhibiting brittle failure characteristics in the sample.

Due to the presence of a post-peak paratactic pressure retention system in MTS-815, the stress–strain curve experiences relief after a rapid decline [41, 42].

As depicted in Figure 12, the lateral constraint and peak stress of the same sample escalate alongside the rise in surrounding pressure. With increasing surrounding pressure, the plastic characteristics of the rock sample gradually manifest, a trend observed across all six rock samples. Illustrated in Figure 12(a), sample 1 exhibits a marginal increase in peak stress with rising confinement pressure, suggesting its insensitivity to confinement pressure. Conversely, the remaining samples demonstrate a substantial augmentation in peak stress amplitude with increasing confinement pressure, signifying a pronounced influence of confinement pressure on peak stress.

For different types of rock materials, the strain is also different. Sample P has the least strain and the most obvious brittleness, which is consistent with the results of uniaxial compression and Brazilian disk-splitting tests.

The internal friction angle and cohesion of the rock material are obtained by processing the test data, as shown in Table 6.

4.4. Analysis of Triaxial Permeation Test Results

The MTS815 Flex Test GT Rock Mechanics Test System measures the permeability and coefficient of permeability of rocks using the transient method. Rock permeability is calculated from the data automatically collected by the computer during the test [34]:

Ki=μβV[ln(ΔPiΔPf)2Δt(AsLS)]
(2)

where, Ki is permeability of the i-th sample, V is baseline volume, is ratio of initial pressure difference to final pressure difference, is duration of the test, Ls is sample height, As is cross-sectional area of the sample, μ is viscosity of pore fluids, β is compressibility of pore fluids.

The permeability coefficient is calculated according to the following formula:

k=Kρg/μ
(3)

where, k is sample permeability coefficient, K is sample permeability, ρ is density of pore fluids, g is gravitational acceleration, μ is viscosity of pore fluids.

The seepage of water through rock significantly impacts the mechanical properties thereof. This seepage can induce alterations in the stress conditions within the rock, leading to various phenomena such as deformation, fracturing, softening, liquefaction, or dissolution, thereby posing risks to the stability of the rock mass. By substituting the experimental parameters into formulas (2) and (3), one can calculate the permeability coefficients, as presented in Table 7. Upon examination of Table 7, it becomes evident that the permeability coefficients of different rock types exhibit similar orders of magnitude, albeit with minor discrepancies. Notably, sample P demonstrates the highest permeability coefficient. Referencing Table 1, it is observed that sample P also boasts the highest porosity, indicative of an increased presence of pores within the rock, facilitating fluid infiltration into the rock mass. Under identical confining pressure conditions, an escalation in the porosity of the rock corresponds to a concurrent increase in its permeability coefficient. The ore-bearing rock exhibits a notably high permeability coefficient, which heightens the risk of rock mass collapse. Consequently, particular emphasis should be placed on ensuring the mechanical stability of ore-bearing rock formations throughout the mining process.

The rock samples after the test failure were compared and analyzed to find out the failure pattern and to provide theoretical support for the mine mining design.

5.1. Uniaxial Compression Failure Pattern Analysis

The samples experienced failure under uniaxial loading, as illustrated in Figure 13. The failure patterns varied considerably among different rock samples. This fragmentation occurred predominantly due to the absence of lateral deformation caused by surrounding pressure. Sample 1 exhibited a vertical primary crack with secondary cracks flanking it, accompanied by significant fragmentation at the upper end. Sample 2 displayed multiple vertical-splitting cracks and a transverse shear crack in the middle, resulting in severe fragmentation at both ends due to stress concentration effects. Analysis in Table 1 reveals that Sample 2, characterized by high feldspar content, exhibited increased brittleness and fragmentation, correlating with its higher compressive strength. Sample 3 primarily failed through vertical splitting, featuring two main cracks and localized short cracks near them, with severe fragmentation at the upper end. Sample 4 experienced severe failures, with an oblique penetration crack on the left side and a vertical crack on the right, resulting in the sample breaking into three pieces, with significant damage to both ends. Sample 5 demonstrated composite failure, with two vertical cracks and a transverse crack in the upper portion. Sample P had two vertical primary cracks, one of which changed direction in the lower portion and extended toward the right, reaching the bottom. The upper end of Sample P exhibited severe failure, attributed to its high phosphorus content and subsequent brittle behavior.

The physical and mechanical properties of rock materials vary due to differences in their properties, compositions, and internal joint structures among the six types of rocks. This leads to variations in failure modes and degrees of fragmentation during the failure process. Special attention should be given to sample 4 during tunneling, as it exhibits low compressive strength, evident brittleness, and high fragmentation. Additionally, its deformation during failure is relatively minimal, making it challenging to observe, and instantaneous failure is highly probable. Sample 4 belongs to an area characterized by weak surrounding rock, necessitating strengthened displacement monitoring during mining operations.

5.2. Analysis of Brazilian Splitting Failure Patterns

The fracture patterns of the six types of rocks in Brazilian disk splitting are illustrated in Figure 14. Owing to the diverse physical and mechanical properties of these rock types, the modes of disk-splitting failure vary. Specimens 1, 2, 4, and 5 exhibit the classic Brazilian disk-splitting failure mode. During loading, a primary crack emerges, traversing the specimen’s center and traversing its entirety. At the force-bearing end, localized fragmentation ensues due to stress concentration, manifesting evident brittleness. In the failure process of specimen 3, two symmetrical parallel cracks emerge about the disc’s central axis, dividing the specimen into three parts. The occurrence of these cracks in sample 3 may stem from pre-existing microcracks within the sample, which further propagate under loading to form the primary crack. Specimen P showcases a curved crack penetrating the specimen, though not intersecting its center. The crack in the upper section indicates tensile cracking, while the oblique crack in the lower section suggests shear cracking, resulting in a combined tensile-shear failure mode. Notably, significant fragmentation occurs at the specimen’s base.

5.3. Analysis of Triaxial Compression Failure Modes

The triaxial compression failure patterns of the six rock species at different envelope pressures are shown in Figure 15. The failure patterns of the rock samples are complex and morphologically diverse, and the failure patterns show significant surrounding pressure effects. At low surrounding pressure, the samples produce penetrating vertical cracks and multiple microcracks, forming a complex crack network. At higher pressures, the number of cracks decreases, and the complex disordered crack network gradually changes to a single shear crack. Increasing the envelope pressure reduces the anisotropy of the rock and the connectivity and effectiveness of the crack network should also be of concern when mining under high-ground stress.

To summarize the failure behavior and modes of the rocks in the Laoheba mine, two primary failure modes, namely vertical-splitting failure and conjugate shear failure, are observed under low envelope pressure. As the envelope pressure increases to high levels, three distinct failure modes emerge: partial shear failure, penetration shear failure, and tensile-shear composite failure. These five failure modes exhibit variability not only with changing surrounding pressure but also demonstrate a close correlation with the lithology. Table 8 presents the failure modes of rocks at the Laoheba mine under different envelope pressures and lithology. The analysis of these five failure modes is detailed below:

  1. Vertical-splitting failure (Ve-Sp) predominantly transpires in rock samples under low envelope pressures (5 MPa). At such pressures, the rock sample experiences minimal lateral constraint, leading to the development of vertical cracks. These cracks manifest as a principal crack traversing the entire sample, accompanied by multiple secondary cracks that branch out in proximity to the main crack, forming a network of fissures. Consequently, the sample becomes highly fragmented and exhibits brittleness.

  2. Conjugate shear failure (Co-Sh) also occurs primarily in rock samples subjected to low envelope pressures (5 MPa). Under these conditions, the sample initially fractures along one side due to the end face effect, resulting in the concentration of axial stress on the opposite side, leading to the formation of a shear crack. Ultimately, these cracks on opposing sides converge to form a V-shaped conjugate main crack, culminating in sample failure.

  3. Partial shear failure (Pa-Sh) predominantly arises in rock samples under medium to high surrounding pressures (10, 15 MPa). The failure mechanism is constrained by significant lateral confinement, resulting in the manifestation of oblique shear cracks that are short and only partially penetrate the sample.

  4. Penetrating shear failure (Pe-Sh) predominantly occurs in rock samples subjected to medium to high envelope pressures (10, 15 MPa). In this scenario, a single oblique macroscopic crack emerges and penetrates throughout the sample, which generally remains devoid of secondary cracks.

  5. Tensile-shear composite failure (Te-Sh) primarily manifests in rock samples under medium perimeter pressure (10 MPa). These samples exhibit both vertical tensile cracks and transverse shear cracks, which collectively contribute to sample failure.

In the deep part of the rock mass, under the action of high-ground stress, rocks experience significant initial stress. During the process of tunnel excavation, if the drill and blast method is used for construction, the high-ground stress will suppress the blasting action, resulting in slow crack propagation and fewer cracks. In deep blasting construction, it is necessary to increase the amount of explosives or decrease the spacing between blast holes to achieve effective blasting purposes.

The investigation utilized NMR, XRD, and the MTS815 Flex Test GT rock mechanics testing system to examine six representative rock types from the Laoheba phosphate mining area in the Sichuan Basin, China. Various tests, including NMR, XRD, uniaxial compression, Brazilian disk splitting, triaxial compression, and triaxial permeability, were performed. Triaxial compression tests were conducted with three different confining pressure levels to explore their impacts on the compressive strength and failure modes of the rocks. Comparative analysis was conducted on the physical properties of the six rock types, encompassing density, porosity, composition, Poisson’s ratio, elastic modulus, internal friction angle, cohesion, and permeability. The stress–strain relationship during loading was visually depicted. By amalgamating rock physical properties with mechanical characteristics, a comprehensive investigation of the physical and mechanical properties of the six rock types was undertaken. Additionally, the study analyzed the failure modes and patterns of the six rock types in correlation with their properties, offering insights for regional geological mechanics research. The research findings contribute by providing original physical and mechanical parameters for mining engineering design and offering theoretical underpinnings for the safe construction and operation of mining engineering projects. The principal conclusions of this study are summarized as follows:

  1. According to the XRD results, all three types of rocks contain Quartz, accounting for as high as 41.2%. Quartz is one of the most common minerals in the Earth’s crust and can enhance the strength and toughness of rocks. The XRD analysis revealed that the rocks within the Laoheba phosphate mining region of the Sichuan Basin harbor unique constituents including Earlshannonnite, Ramsbeckite, Clinochlore, and Hydroxylapatite, which were seldom encountered in prior geological formations.

  2. Sample P exhibits the highest porosity and permeability coefficient, with porosity and permeability coefficients of 1.87% and 2.45E-09, respectively. It is characterized by pronounced original joints and cracks; resulting in heightened internal defects and consequently, diminished compressive and tensile strength. Compared to sample 5, the compressive and tensile strength of sample P decreased by 34.4% and 23.7%, respectively. Given the vulnerability of phosphate ore deposits within the rock mass, prioritizing stability in mining design is imperative. Continuous real-time monitoring of surrounding pressure and deformation during mining operations is essential to mitigate the risk of rock collapse.

  3. With increasing burial depth of rocks, there is a corresponding rise in initial in situ, leading to heightened peak stress upon failure. The initial in situ increases from 5 MPa to 15 MPa, and the average compressive strength increases by 66.4%. Moreover, escalating surrounding pressure augments the elastic modulus of rocks and accentuates their plastic characteristics. In excavating tunnels within deep underground environments using drilling and blasting techniques, adjustments such as increasing explosive quantities or reducing blast hole spacing are warranted.

  4. The failure modes of the six rock types under varying surrounding pressures are intricate and diverse, with a total of five failure patterns, demonstrating significant pressure-induced effects. Furthermore, heightened surrounding pressure tends to diminish the anisotropic properties of rocks. Hence, in deep mining endeavors, attention must also be directed toward optimizing the connectivity and efficacy of the rock fracture network.

All data and material generated during this study are included in this published paper.

The authors declare no competing interests.

Conceptualization: MZ, ZMZ and MW. Formal analysis: MZ, WTG. Software: TP and MW. Writing-original draft: MZ, ZMZ. All authors discussed the results and commented on the manuscript.

This research was funded by the National Natural Science Foundation of China (12272247), the major research and development project of Metallurgical Corporation of China LTD. in the nonsteel field (2021-5).

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