Muddy soft rock is the most prevalent sediment rock (Li et al. 2022a, b). It has been widely seen in underground excavations, including coal mines and water conveyance tunnels, etc. Its presence leads to complex geological structures and high water content in the rock mass surrounding the underground excavations. As a result of underground mining activities, in-situ stress would be re-distributed around the excavation surface and the stress balance would be disturbed leading to rapid release of strain energy in surrounding rock. This can create serious geotechnical issues. In this situation, the ground support and reinforcement become essential but face massive technical challenges. Those challenges and difficulties pose numerous hazards that should be paid significant attention during underground operations (He 2021; Bai et al. 2022; Ali et al. 2022; Wang et al. 2021a, b).

In micro scale, soft rock can be seen as a polymer of massive mineral particles cemented through cohesion (Shen 2002). The muddy soft mineral rock has three types of cohesion between various particles, including the cohesion between debris, cohesion between mineral particles, and cohesion between debris and mineral particles. As such, the clay type and content significantly affects the mechanical characteristics of the muddy soft rock. When rock comes into contact with water, water adheres to the mineral particles and ions of the dissolvable rock, leading to an inherent change in the rock structures and a reduction in the cohesion between mineral particles (Huang and Liu 2013; Vishal et al. 2015; Zhang et al. 2021). Particularly, the muddy soft rock normally possesses much clay mineral content like montmorillonite, illite and kaolinite, etc., that absorb water molecules easily. It also has a large amount of stable rock mineral particles like quartz and calcite, etc. In addition, it has some unstable rock mineral particles, like feldspar (Li et al. 2022a, b; Liu and Lu 2000). When the muddy soft rock comes into contact with water, the water penetrates the rock’s crystalline lattice and dilation occurs along the direction that is perpendicular to the lattice. As a result, the clay minerals become paste followed by dissipating and diffusing radially under the influence of water flow. The cohesion between rock mineral particles consequently vanishes (Li et al. 2015a, b). In the meantime, unstable rock particles, like feldspar, undergo an exchange reaction between its ions resulting in production of many by-products, including kaolinite and illite. This process further enhances and accelerates the interaction between water and rocks (Li et al. 2015a, b; Zhang et al. 2019; Zhao et al. 2017). Therefore, the muddy soft rock has a very low load bearing capacity and vollspdrd easily after contact with water due to the degradation (see Fig. 1). Water is inevitable during underground coal mining operations due to water usage for mining, ground water in the rock mass, as well as the moist environment underground (Fan and Ma 2018). If underground roadways pass through muddy soft rock, the rock support and reinforcement systems would, no doubt, be affected by the water. Particularly, the presence of water initiates cracks in the rock at the excavation surface and, hence, affect the rock integrity. This process, consequently, reduces the stability and the self-load bearing capacity of the surrounding rock (Bian et al. 2019; Sisodiya et al. 2021; Zhao et al. 2015).

Muddy soft rock collapse after contacting with water

As seen in Fig. 2, the rock reinforcement consists of surrounding country rock, grout, reinforcement element, as well as fixing device (such as support plate, nut) at the borehole collar (Li et al. 2021; Li et al. 2022a, b; Windsor 1997). When ground water flows through the rock mass, it not only corrodes the country rock, but also corrodes the reinforcement element, such as cables and rock bolts, through electrochemical reaction between water and steel wires. As a result, the load bearing capacity of the rock reinforcement is significantly jeopardized (Kim et al. 2018; Wang et al. 2021a, b). In addition, the water can also absorb massive heat generated during the cementitious grout curing, leading to an incomplete grout curing, and making the resin not reaching maximum strength and adhesion. When resin grout is used, the presence of the water might create some small voids in the resin annulus due to resin’s non-hydrophilic characteristics. As a result, water consequently jeopardizes the grouting quality. If no precautions are made to tackle the water-related hazards, the reinforcement system might fail easily and catastrophically (He 2021; Ma et al. 2021) (see Fig. 3).

Failure modes of the reinforcement muddy soft rock

Roadway bearing structure hydration instability

Over 20 incidents associated with stress corrosion by ground water have been reported in the underground coal mining industry in China annually. This makes the investigation into failure mechanisms of rock reinforcement due to the ground water corrosion essential. To date, research has only been carried out to investigate the influence of water on single reinforcement elements, for instance, the influence of water on surrounding country rock, grout or rock bolt, individually (Bai et al. 2016; Li et al. 2015a, b; Zhao et al. 2013). No research has been done to investigating the influence of water on the overall load bearing capacity of the whole rock reinforcement system, or the interactive influence between various elements involved in the rock reinforcement system. This research squarely addresses this research gap by conducting a series of laboratory tests on reinforced muddy rock to investigate the influence of water on the load bearing capacity of reinforced rock and its associated Acoustic Emission (AE) characteristics. The experimental results are intended to aid in the analysis of failure mechanisms of the reinforced muddy soft rock subject to the presence of water at various levels.

2.1 Materials for testing specimen preparation

Considering it is difficult to find similar rock-like materials to simulate the muddy soft rock in underground, it was decided to use the in-situ rock cored from the field for this research. Given that epoxy resin has similar ingredients and mechanical characteristics to the resin used in grouting rock bolts in the field, epoxy resin was used as grouting material in the laboratory tests in this study. Moreover, it has better flowability than the resin used in the field, making it more suitable for small scale tests in the laboratory (Chen et al. 2020; Luo et al. 2022; Qiu et al. 2021; Zhang et al. 2022). The setting time of the epoxy resin in this study was 0.5 h, and its Uniaxial Compressive Strength after curing 3 h was 47.68 MPa, confirming its strength was sufficient for the lab tests.

It was observed that the predominant type of failure of grouted bolts in underground coal mines occurs at the rock-to-grout interface and in the surrounding rock, especially where muddy soft rock and a great amount of water are present (Feng 2017; You 2005). As such, rupture failure mechanisms of bolt were not the interest of this study. The object of study in this paper is muddy soft rock, which exhibits low strength, poor structure, and subsequent need for water immersion treatment. A larger drilling size result in higher initial damage to the rock, thereby impeding the achievement of anchoring effects. Therefore, it is necessary to minimize the diameter of the bolt to avoid excessive damage caused by subsequent drilling. Simultaneously, the bolt diameter should not be too small to prevent significant deformation or even fracture during testing. Consequently, rebar with 5 mm diameter and 85 mm length was employed in this study along with a face plate with dimensions of 20 × 20 × 1.5 mm (Teng et al. 2018, 2017). The mechanical properties of the rebar can be found in Table 1.

In order to monitor the axial stress and deformation in the rock bolt, three strain gauges were installed at the bolt surface in each test according to the diagram in Fig. 4.

The locations of strain gauges installed on bolt surface

The dimensions of the strain gauge were 2.8 mm × 2 mm. A flat area with dimensions of 3 mm × 3 mm was sanded with a grinding wheel at the location where the strain gauges were adhered.

2.2 Rock casting

The in-situ rock was cored from the roof of the intake air roadway at the Level 1900 at the Zhenggao coal mine in Guizhou Province. In order to simulate the geological field conditions, test specimens with final dimensions of 100 × 100 × 100 mm were prepared from mudstone and sandstone grab samples. The moisture content in the sandstone was 1.03%, and the UCS of the sandstone when it was dry was 27.66 MPa. The Young’s Modulus was 3.39 GPa, and the Poisson’s Ratio was 0.19. The moisture content in the mudstone was 2.15%, and the UCS of the sandstone when it was dry was 11.68 MPa. The Young’s Modulus was 1.45 GPa, and the Poisson’s Ratio was 0.28. The mineral compositions of sandstone and mudstone can be found in Tables 2 and 3.

The rock specimen was composed of sandstone and mudstone in order to accurately simulate the field condition, where the sampling tunnel has a direct top layer of mudstone and a basic top layer of sandstone, with the bolt drilling through the mudstone layer into the underlying sandstone layer. In the field, layers with various thickness and cohesion were observed between different rock beddings. As such, in order to better simulate the field conditions, an adhesive infilling was placed between the mudstone and sandstone in the test specimen, a 1 mm thick plain cement mixture between the beddings. The water to cement ratio was 0.45, enabling the infilling layer to have a sufficiently lower strength and reasonable permeability (Fig. 5).

Testing specimen preparation

Drilling was carried out before immersing the test specimens in water to avoid premature fractures that are caused if drilling takes place after wetting the specimens. The borehole diameter for bolt installation was 8 mm according to guidelines that the borehole diameter for rock bolt installation should be 2–4 mm larger than the bolt diameter. The length of the borehole was 75 mm. Two boreholes were drilled in each testing specimen. After the drilling is completed, the test specimens were put in the oven to be dried. Considering the specimen has montmorillonite crystals, the oven temperature was set at 65 °C for 12 h according to Regulation for testing the physical and mechanical properties of rock—Part 2: Test for determining the water content of rock (Yao et al. 2015). As the muddy soft rock in the test specimen was very prone to collapse when completely immersed in water, a YH-40B constant temperature and humidity curing box was used for curing the specimens until the moisture content in the test specimen reached the required level. The curing environment was set at 25 °C and 95% humidity. Over the course of curing, the boreholes were sealed by water-proof membrane and the weight of the specimens were checked regularly to monitor the moisture content. The moisture content at varying curing times are illustrated in Fig. 6.

Moisture content in the testing specimen at varying curing time

Three stages could be categorized based on the increasing rate of the moisture content during specimen curing according to Fig. 6 including 1) rapid growth stage, 2) slow growth stage and 3) approaching saturation stage. As such, five moisture content levels including 0, 3%, 6%, 8%, 10.18% (Saturated moisture content) were selected for compression testing considering they are important milestones during the curing process. Once the specimens reached the set moisture level, rock bolts were installed in the pre-drilled boreholes by epoxy resin grout followed by attaching the face plate at the borehole collar. A spanner was used apply a pre-tension up to 1 kN. All rock bolts were fully encapsulated with resin grout. The testing facility MTS C64.106 was used to provide the axial load along the vertical direction during the tests (see in Fig. 7). A custom-made lateral loading facility made of hydraulic cylinder, self-servo hydraulic pump and mechanical frame was used to provide the lateral load along the horizontal direction.

Testing facility

In order to simulate loading conditions of the reinforcement rock unit in the field, dual-axial loads were applied to the test specimen using the MTS C64.106 and the custom-made lateral loading facility. In addition, the side with the borehole collar was a free surface, whereas the other side was fixed during the test (see Fig. 5). The lateral stresses σ₂ applied to the side of the testing specimen were set at 2, 4, 6 and 8 MPa. During the test, the lateral stress was increased up to the required levels first, followed by increasing the axial load on the specimen along the vertical direction until the specimen failed. The displacement rate along the vertical direction was 0.2 mm/min and the details of the test set up is found in Table 4. The DM-YB1820 dynamic and static strain monitoring system was used to capture the axial strain during the test as well as the real time AE signals. Four AE probes were installed to capture the signals, and the threshold value was set at 40 dB. The range of sampling frequency for AE during the test was set at 1 kHz–1 MHz.

3.1 Stress–strain relationship

Figure 8 illustrates stress–strain relationships of the reinforced rock test specimens at various lateral stresses σ₂ and moisture content levels w. The curves show that the stress–strain relationships are significantly different under different testing conditions, and they may be categorized into four types as seen in Table 5.

Axial stress–strain relationship curve

For type I, where moisture content level ranged from 0% to 6%, the deformation tended to increase with the axial stress σ₁. The first drop in stress occurred when the mudstone started cracking followed by a series of stress reductions. Owing to the rock bolt reinforcement and the cohesion at the rock interface, the mudstone still adhered to the intact sandstone and the whole assembly still exhibited a load carrying capacity to some extent. The load carrying capacity started reducing when the sandstone began to crack. It can be seen that there were a number of local peaks before the global peak occurred, and strain hardening was also observed during the testing especially when the specimen started failing.

For type II, where moisture content level ranged from 6% to 8%, the strength of the mudstone and sandstone, the grout strength, and the cohesion between two different rocks were seriously jeopardized. The overall load carrying capacity was significantly lower than that of Type I category. After the peak stress, the reinforcement by the rock bolt gradually decreased as the rock damage became more severe. In addition, the higher moisture content reduced the cohesion and friction at the interface between the rocks. After the peak stress, the stress reduced in steps and exhibited no stress hardening.

For type III, where the testing specimen was fully saturated and the lateral stress level σ₂ was less than 4 MPa, the compaction stage was much longer as the moisture content degraded the rock assembly significantly. Also, the mudstone became more muddy and failed dramatically, leading to the loss of rock reinforcement and cohesion at the interface. As a result, the rock assembly was very loose and the stress–strain curve only exhibited one single peak stress, following which the stress reduced quickly and sharply. No stress hardening was observed in this case. Similarly, for type IV, where specimens were fully saturated but the lateral stress σ₂ was greater than 4 MPa, only one peak stress was observed and stress hardening was observed after a moderate decrease in stress that followed the peak stress. The increase in stress after the peak stress might be attributed to accumulated internal friction between rock fractures under high lateral stress. However, the reinforcement by the rock bolt could not restrain the dilation and expansion at the free surface of the rock assembly, and consequently, the reinforced rock lost its load carrying capacity.

The influence of the moisture content on the strength of the reinforced rock is seen in Fig. 9a. Consistent with Fig. 7, Fig. 9a also shows the peak strength of the rock specimen increased with the lateral stress σ₂, whereas it decreased as the moisture content increased. When σ₂ = 2 MPa and moisture content ranged from 3% to 10.18%, the strength of the rock decreased by 24.33%–69.05%. When σ₂ = 4 MPa, the strength of the rock decreased by 24.29%–70.41%. When σ₂ = 6 MPa, the strength of the rock decreased by 23.96%–65.04%. When σ₂ = 8 MPa, the strength of the rock decreased by 22.38%–67.66%. As seen in Fig. 9b, the strength of the rock was influenced by both lateral stress σ₂ and moisture content. When the moisture content was less than 3%, σ₂ had a higher influence on the rock strength compared with the specimens with moisture content over 3%. An increase in σ₂ significantly increased the rock strength. When the moisture content was over 3%, the influence of σ₂ on the rock strength tended to be minimized as the moisture content increased, suggesting that the increase in the moisture content jeopardizes the rock reinforcement quality and rock strength.

The strength of the reinforced rock at various moisture content levels

3.2 Failure modes of testing specimens

Four types of failure modes could be categorized consistent with the four types of stress–strain relationships (see Figs. 10, 11, 12, 13).

Failure mode I (moisture content 0 ≤ w ≤ 6%)

Failure model II (moisture content 6% < w ≤ 8%)

Failure mode III (Fully saturated, σ₂ < 4 MPa)

Failure model IV (Fully saturated, σ₂ ≥ 4 MPa)

As see in Fig. 10, the type I failure mode shows a significant dilation and expansion of the rock specimen towards the free surface. Slabbing cracks dominate the failure mode at the free surface after the test. Minor radial cracks also were observed originating from the borehole collar, whereas the middle area between the two bolts remained intact, confirming the reinforcement achieved by the bolts. A number of shear cracks and tensile cracks were developed inside the test specimen. A significant amount of dry rock powder was observed at the failure plane. The mudstone was flaky and spalling at the free surface. Furthermore, when the moisture content was less than 3%, rock bursts of fracturing occurred during the test.

Similar to failure model I, failure model II also exhibited a combination of tensile and shear failure (Fig. 11). Due to the increase in moisture content, the sandstone powder became wet and lumpy, whereas the mudstone particles are argillite and detached from the testing specimen. More rock fractures were observed at the free surface. Due to the degradation of the bolt grouting subject to high moisture content, the reinforced area between the two bolts decreased. Radial cracks propagated further, leading to a reduction in the rock specimen strength and more fractures, even at the lower lateral stress.

As seen in Fig. 12, the whole specimen became muddy due to the moisture saturation. Cracks were observed even before the test. The cohesion between the bolts and mudstone was heavily degraded leading to only friction at the rock-to-grout interface to transfer load. For this case, the load bearing capacity of the testing specimen only came from the strength of the sandstone as well as the middle-to-bottom section of the grouted bolt. The dominant failure in the sandstone was shear failure.

Similar to failure mode III, the failure mode IV also demonstrated that the load bearing capacity was reduced significantly due to high moisture content (Fig. 13). Massive tensile failure cracks were seen at the surface, whereas the dominant failure in the sandstone was shear failure. As the lateral stress in this case was very high, it provided a high confinement to the fractured rocks in the specimen, leading to a distinct stress hardening behaviour. However, the capacity for rock reinforcement was significantly jeopardized as the grout couldn’t couple the muddy rock and bolt.

4.1 Acoustic emission characteristics

We introduced a damage factor, DF (Zhao et al. 2021), in order to better characterize the internal fractures subject to the influence of moisture content and lateral stress according to:

where D is the accumulated AE counts at a particular time point and D₀ is the total accumulated AE counts.

Taking the test under lateral stress σ₂ of 4 MPa as an example (see in Fig. 14), when the moisture content was low, say less than 6%, the AE counts were also very low at the beginning of the test leading to a low DF. During the yielding stage, the AE counts increased significantly resulting in a rapid increase in the DF magnitude. This indicated that a large number of internal cracks in the test specimen developed and propagated in a violent manner. In addition, when the axial stress experienced the first drop, a large number of AE counts were recorded indicating that the internal cracks in the specimen propagated throughout the full size and maximum strain energy was released at this point in time.

AE counts of testing specimen at various moisture content levels

When the moisture content was over 6%, numerous AE counts were recorded at the beginning of the test. It was observed that at the axial stress equivalent to 30% of the compressive strength of the specimen, DFs were 0.0341, 0.0776, 0.2375, 0.4245, 0.1431 at moisture contents 0, 3%, 6%, 8% and 10.18%, respectively. By contrast, when the specimen was fully saturated, the DF magnitude was smaller than that for moisture content at 6%–10.18%. This might be related to the pre-mature damage of the mudstone because of the high moisture content even before the test and, hence, less cracks were initiated for the fully saturated specimens at the beginning of the test. In addition, the total accumulated AE counts were recorded as 13,973, 10,818, 4372, 3533 and 1984 under moisture contents 0, 3%, 6%, 8% and 10.18%, respectively. The total accumulated AE counts decreased as the moisture content in the test specimen increased. This suggests that the water decreased the specimen’s hardness, initiating the internal cracks more easily with less energy.

4.2 Crack propagation of the reinforced specimen

At present, b-value analysis is one of the most important tools for studying the AE characteristics of rock materials. The concept of b-value originates from the study of seismology. It was first proposed by Gutenberg and Richter (1994) and used to describe the relationship between the frequency and magnitude of earthquakes. In the AE study of rock materials, the initiation and propagation of microcracks during rock fracture can be described by the change of b-value (Dang et al. 2023; Wang et al. 2021a, b; Zhang and Zhang 2017). The equation is as follows:

where M is the magnitude; N is the cumulative number of events in the range M + ΔM; a and b are the constants. When calculating the AE b-value, the AE amplitude is usually divided by 20 instead of M (Yu et al. 2022). Then Eq. (2) can be changed to the following form:

where A (dB) is the amplitude of the AE event. For microcrack propagation in rock materials, increasing b-values indicate that small-scale microcrack initiation predominates in the rock, decreasing b-values indicate that large-scale microcrack development predominates in the rock at that stage. And constant b-values represent a persistent distribution of microcracks at different scales. If the b-value fluctuates a lot, it means that the microcrack is expanding in a sudden destabilizing way. Otherwise, it means that the microcrack presents a gradual expansion.

In this paper, a scanning algorithm is used to obtain the b-value, i.e., the b-value is calculated for a fixed number of AE events as the sampling window each time, and then the b-value is calculated by sliding and repeating in time order with a certain number of AE events in steps. Existing studies show that although the length of the sampling window and the step size affect the magnitude of the b-value at different times, they have no effect on the variation of the b-value over time (Dong et al. 2022; Zhang and Zhang 2017). Therefore, taking the σ₂ = 4 MPa test results as an example, the sampling window and step size were determined according to the total number of AE events and the amplitude distribution of the specimens at different moisture contents as follows: A sampling interval of 2000 AE events and a step length of 1000 AE events when the moisture content is 0%–3%; A sampling interval of 2000 AE events and a step length of 500 AE events when the moisture content is 6.00%–10.18%. To obtain the b-value, ΔA = 2 dB is used to perform statistics on the AE amplitude data.

Figure 15 shows the variation of the b-value (Normalization) of the specimens with time for different moisture contents (σ₂ = 4 MPa). As can be seen from the figure, when the moisture content is 0, the b-value increases slightly in the compaction and elasticity stage and then gradually decreases with a small fluctuation range.

b-value of the testing specimens at various moisture contents

At this stage, the progressive expansion of the large-scale microcrack is dominant. And in the yielding stage, the b-value fluctuates dramatically, the microcracks of different scales develop alternately and rapidly, and the brittle characteristics of the fracture initiation and propagation are significant. As the moisture content increases, the degree of b-value fluctuation gradually decreases, showing a smooth change and accompanied by a small number of abrupt changes, the specimen gradually changing from sudden and brittle failure to progressive and ductile failure. The figure shows that the b-value increases and then decreases in the initial stage of loading when the moisture content is 0%–3%, i.e., small-scale microcracks accumulate and then expand to large-scale microcracks. The b-value decreases at the initial stage when the moisture content is 6.00%–10.18%. This indicates that as the moisture content increases, the initial damage caused by the hydration of the specimen extends through the initial loading stage and develops into large-scale cracking.

To further determine the damage development characteristics of the specimens at different moisture contents, the relationship between the AE RA-value and the average frequency (AF) was analyzed. As in Fig. 16, RA-value is the ratio of rise time to amplitude, and AF is the ratio of AE count to duration. The AE signal with high AF and low RA-value represents the formation of a tensile crack and vice versa for the formation of a shear crack (Lotidis and Nomikos 2021; Wang et al. 2019).

Simplified waveform and basic parametersof acoustic emission signal

The AF-RA scatter distribution of the specimens at different moisture contents is shown in Fig. 17. From the figure, it can be seen that the microcracks under different moisture content are all tensile and shear compound cracks, and they are all dominated by tensile cracks with a small number of shear cracks. With the increase in moisture content, the total number of microcracks continues to decrease and the number of shear cracks is significantly lower. The distribution tends to be closer to the vertical axis, and the dispersion range becomes smaller and smaller. The maximum value of AF decreases from 243.50 kHz to 182.97 kHz. Combined with the failure model of the specimens, it is clear that the increased hydration damage causes the specimens to contain more initial damage, resulting in less de novo crack development.

AF-RA scatter distribution of the specimens at different moisture contents

The axial load (P) distribution in the rock bolt subject to four different failure modes were analysed in this section.

For failure mode I at testing condition w = 0 and σ₂ = 8 MPa, P distribution was recorded at four points in time (see Fig. 18b). These points include p₁, when the axial stress reached 30% of the magnitude of the first stress drop; p₂, when the axial stress first dropped; p₃, when the axial stress reached 80% of the peak stress; and p₄, when the axial stress reached it peak (see Fig. 18a). The collar location was labeled at zero distance in the horizontal axis. As can be seen from the figure, the bolt presents the characteristic of progressive failure from collar to bottom. It was found that before reaching p₂, P at all three locations increased with time, and the maximum rate of increase was at the collar.

Load vs time curve and P distribution curve at w = 0, σ₂ = 8 MPa

During the stage from p₂ to p₃, the mudstone gradually collapsed and debonded from the rock bolt, leading the axial load in the rock bolt to transfer to the sandstone. Accordingly, the figure shows that the maximum increase in P occurred at the middle monitoring position. During the stage from p₃ to p₄, the cracks tended to propagate toward the interior of the test specimen, leading to an increase in P at the bottom monitoring position. The part of the bolt inside the sandstone provides the main anchoring force at this time. Under this condition, the anchoring capacity of the bolt is intact, the cracked rock specimen was still reinforced by the rock bolt, and the specimen remained intact to some extent.

As seen in Fig. 19, for failure mode II with w = 8% and σ₂ = 2 MPa, P distribution was recorded at four points in time. These points include p₁, when the axial stress reached 30% of the peak stress; p₂, when the axial stress reached it peak; p₃, when the axial stress reached 80% of the peak stress after the peak; and p₄, when the axial stress dropped to 70% of the peak stress. The bond between the rock bolt and mudstone degraded under the influence of increasing moisture content, which reduced the anchoring performance of the collar section. During the stage from p₁ to p₂, P at the collar increased from 656.89 N to 1069.53 N, whereas that at the middle position increased from 1988.76 N to 2947.37 N. The increasing rate at the middle position was higher than that at the collar position. During the stage from p₂ to p₄, due to the friction in the interior of the specimen, P at the middle and borehole end positions tended to increase and the rock gradually failed. After p₄, P decreased at all locations. In addition, it was found that the moisture content has a greater influence on the bond between rock bolt and mudstone than that between rock bolt and sandstone.

Load vs time curve and P distribution curve at w = 8%, σ₂ = 2 MPa

As seen in Fig. 20, for failure mode III with σ₂ = 2 MPa and fully saturated condition, P distribution was recorded at four points in time. These points include p₁, when the axial stress reached 10% of the peak stress; p₂, when the axial stress reached 30% of the peak stress; p₃, when the axial stress reached 70% of the peak stress; and p₄, when the axial stress reached it peak. At p₁, P at collar and middle positions showed the greatest increase, whereas at the stage from p₁ to p₂, P at the borehole end increased more significantly than at other positions. This indicated that due to the hydration reaction, the bond between the mudstone and rock bolt was almost completely lost and, hence, the load bearing capacity contributions mainly came from the sandstone and rock bolt reinforcement. After p₄, as the sandstone started failing, the axial load carrying capacity at the borehole end was gone, the whole assembly failed, and the rock bolt slipped out.

Load vs time curve and P distribution curve at σ₂ = 2 MPa and fully saturated condition

For failure mode IV with σ₂ = 6 MPa and fully saturated condition, P distribution was recorded at four points in time (see in Fig. 21b). These points include p₁, when the axial stress reached 70% of the peak stress; p₂, when the axial stress reached it peak; p₃, when axial stress dropped after the peak; and p₄, when the axial stress reached the strain hardening point. As seen in Fig. 21, before p₁, P distribution in the rock bolt was similar to that of failure mode III. During the stage from p₁ to p₂, P at the borehole end increased slowly, whereas P at the middle position increased stably. This was related to the friction between rock fractures due to the high lateral stress leading to some load bearing capacity, even after cracks initiated in the rock. During the stage from p₂ to p₃, after the sandstone failure, P at the borehole end reduced. During the stage from p₃ to p₄, P bearing capacity increased, but the rock reinforcement completely failed.

Load vs time curve and P distribution curve σ₂ = 6 MPa and fully saturated conditions

Various failures were observed under lateral stress of σ₂ = 4 MPa, subject to different moisture content levels ranging from 0% to 10.18% (see in Fig. 22). It was found that the bolt-to-grout interface remained intact after the tests, regardless of the moisture content levels. At a moisture content of 0%–3%, some residual mudstone cracks were observed on the grout surface. At a moisture content of 6%, only residual mudstone powder could be seen on the grout surface. When the moisture content became 8%–10.18%, the rock completely debonded from the grout, with only muddy materials sitting on the grout. Furthermore, even before the tests, the borehole wall was very loose and muddy rock grains were seen detached from the borehole wall, leading to numerous difficulties in forming a rigid bond between rock and rock bolt.

Failure modes after the test under different moisutre content

The weakening damage process of anchorage system after water erosion is extended from the mesoscale to the macroscale. Therefore, the hydration destabilization process of the rock-resin coupled structure is analysed by combining the results of SEM tests of specimens under the conditions of w = 0, 6% and 10.18%.

As seen in Fig. 23, the resin and rock can be effectively bonded under dry conditions, with no obvious separation interface, and the interface shows a coupling area of some width. The rock part is intact, without obvious holes or cracks. When the groundwater intrudes into the extraction space, it is seen that a large number of fissures and dissolution holes are formed inside the rock under the action of hydration. There is an obvious separation interface between the rock and the resin, and the rock near the interface is in a fractured state. The interfacial bonding effect is deteriorated. As the hydration damage intensifies, the degree of rock fragmentation increases. And as the main cement (clay minerals) between the rock particles swell and soften, the surrounding rock muddies and disintegrates, losing its self-supporting capacity. Due to the influence of free water and the muddying of the rock particles, the resin and the rock lose their ability to bond.

Diagram of hydration damage of rock-resin coupled structure

Figure 24 shows the failure progress of the anchorage system without the influence of moisture content. At the beginning of the test, the surrounding rock had good integrity and a full bond was developed at the rock-to-grout interface. As the external stress increased, the external part of the surrounding rock started failing, but owing to the reinforcement and friction between fractured rocks under external stress, the rock assembly still had some load bearing capacity. As the rock kept failing further, leading to more interior crack propagation, the reinforcement near the collar was lost. When the interior part of the surrounding rock completely failed and cracked, the bond between the rock and grout completely detached, leading to the complete loss of reinforcement. The anchorage system at this stage completely failed.

Failure progress of anchorage system without influence of moisture content

Figure 25 shows the anchorage system with different initial hydration damage. It was found that the anchorage system remained intact without any excessive water present. As the groundwater penetrated into the rock, a large number of voids formed, and the self-supporting capacity of the surrounding rock was greatly reduced. Due to the rock grains detaching from the rock surface as a result of water penetration, the bond between the grout and the muddy rock degraded, leading to a significant initial damage to the reinforced rock unit. At this stage, the bearing capacity of anchorage system is not sufficient to maintain stability. Under the influence of the hydration reaction and external stress, the anchorage system rapidly fails.

Anchorage system with different initial hydration damage

The failure mechanisms of the reinforced muddy soft rock under various moisture contents and lateral stresses were studied and results presented in this paper and, hence, the following is concluded:

1. (1)

   The load bearing capacity of the reinforced rock increased with the lateral stress σ₂ but decreased as the moisture content increased. An increase in the moisture content reduced the extent of the influence of lateral stress on the load bearing capacity of the reinforced rock.

2. (2)

   The failure modes were affected by the moisture content level and the lateral stress σ₂ so that as the moisture content increased, the reinforced zone decreased, more rock cracks were observed, and hydration reaction became more significant. The rock assembly would completely fail easily when fully saturated.

3. (3)

   At low moisture content level, very few AE signals were captured at the beginning of the test while a significant number of AE events were recorded when the rock specimen started failing. By contrast, with high moisture content, a significant number of AE signals were captured even at the beginning of the test, but the overall AE counts were significantly lower than that at a low moisture content level. In this case, the energy required to initiate and propagate the cracks was much less.

4. (4)

   With the increase in moisture content, the specimen gradually changed from sudden and brittle failure to progressive and ductile failure, and the initial damage within the specimen continued to increase.

5. (5)

   As the moisture content increased, the effective reinforced zone decreased and the bond between the grout and the rock degraded. Particularly when the rock was fully saturated, reinforcement to the rock by the bolt was minimal.