Mechanical and Failure Properties of Deep Grouted Fractured Rock Under Real-Time Coupling of Temperature and Dynamic Load

Abstract

Deep grouting rock engineering is faced with the dual influence of high temperature and dynamic load, which has become a hot issue in geotechnical engineering. This study analyzes the mechanical responses and failure properties of deep-grouted fractured rock under real-time coupling of temperature and dynamic loads through the high-temperature-split Hopkinson pressure bar (HT-SHPB), high-speed imaging, and scanning electron microscopy (SEM) tests. Key findings reveal that (1) the dynamic compressive strength of grouted fractured rock exhibits significant temperature dependency, where the strength increases with the increase of temperature, which has been verified by relevant references. From indoor temperature to 100 °C, the dynamic strength increases moderately, while a pronounced increase is observed between 100 °C and 300 °C. (2) In contrast, the dynamic peak strain demonstrates a two-stage evolution, which sharply rises from indoor temperature to 100 °C, followed by a slowly rise from 100 °C to 300 °C. (3) Macroscopically, impact fractures preferentially initiate as parallel lines at the extremities of pre-existing grouted fractures, consistent with stress concentration patterns under dynamic loading. Microscopic analysis reveals that grouting materials effectively suppress micro-crack generation and propagation at 300 °C, attributed to thermally enhanced cementation and pore-filling effects, explaining the variation of the macroscopic dynamic strength with temperature from the microscopic point of view.

1. Introduction

Fracture networks weaken rock mass by disrupting structural continuity, significantly reducing compressive strength and load-bearing capacity [1,2,3]. These discontinuities enable groundwater infiltration, which accelerates rock softening [4,5]. Anisotropic stress patterns develop along dominant fracture orientations, heightening collapse risks in tunnel walls [6]. Grouting techniques can enhance fractured rock stability by injecting cement-based or chemical grouts into fractures (Figure 1), where the hardened materials mechanically interlock with rock interfaces and fill fractures to restore structural continuity [7,8,9]. This process increases mechanical properties and deformation resistance through improved inter-particle bonding and reduced porosity [10,11].

The improvement of mechanical properties in grouted zones plays a pivotal role in ensuring the stability of surrounding rock masses. First, grout fills fractures and voids within the rock, effectively transforming discontinuous, fragmented structures into a cohesive composite medium [12]. This process significantly enhances the mechanical strength and deformation modulus of the rock, enabling it to withstand higher external loads without failure [13]. Second, the grouting reduces permeability, mitigating groundwater infiltration that could soften the rock or induce hydraulic fracturing—both critical factors in long-term stability. Moreover, by increasing the compressive strength and tensile resistance of weak zones, grouting minimizes stress-driven crack propagation, thereby delaying or preventing progressive collapse [14]. The improved stiffness of the grouted rock suppresses excessive deformation, a common trigger for instability in soft or jointed strata. In essence, grouting reinforcement transforms geologically defective, heterogeneous, and vulnerable rock masses into engineered materials with optimized load-bearing and failure-resistant characteristics, making it indispensable for modern geotechnical safety [15].

The mechanical properties of grouted rock have been studied by many scholars. These studies mainly relate to strength properties, deformation laws, and so on. For instance, Jin et al. conducted experimental research on the mechanical behaviors of grouted crushed coal rocks under uniaxial compression [16]; and Le et al. conducted a series of pull-off and direct shear tests to obtain the strength characteristic of the foliation in different metamorphic rocks [17]. However, these studies mostly focus on the mechanical response of grouted rock under static loading conditions. Actual underground rock engineering is more widely affected by dynamic loads [18,19]. The dynamic loads arise from seismic events, blasting operations, and vibrations induced by heavy machinery during construction [20]. These forces generate cumulative structural damage, propagate pre-existing fractures, and compromise the stability of grouted rock mass, significantly increasing collapse risks [21,22]. However, research on the mechanical properties of grouted rock under dynamic loads is insufficient.

High temperature and dynamic load have great influence on geotechnical materials [23,24]. With the development of deep high-temperature engineering—such as geothermal development (Figure 2) (deep geothermal energy is renewable heat from deep within the Earth, resulting from the Earth’s molten magma and the decay of radioactive materials, including geothermal energy at an underground depth from 2000 m to 3500 m, with a temperature range usually above 150 °C; in addition, deep geothermal also includes the thermal energy of hot dry rocks at underground depths of more than 3500 m, where temperatures can reach much higher levels), underground coal gasification, deep mineral mining, and nuclear waste disposal—the deep rock mass will face the coupling effect of high temperature and dynamic disturbance [25,26]. The mechanical properties of rock under the coupling effect of temperature and dynamic load have been reported. For example, in the study [27], some traditional cracked-straight-through Brazilian disc (CSTBD) samples made of granite and sandstone were heated from 25 °C to 700 °C to obtain the dynamic fracture properties of high-temperature soft and hard rocks, and the modified traditional split Hopkinson pressure bar (SHPB) was applied for dynamic fracture experiments under impact loads. It can be found that the strength of the granite decreased gradually with high-temperature treatment, and the dynamic fracture strength of the green sandstone firstly increased and then gradually decreased. In another study [28], the SHPB with high-temperature function was adopted to systematically study the dynamic mechanical properties of sandstone. The research showed that with the increase of temperature, the dynamic compressive strength of sandstone first increased and then decreased; the dynamic peak strain increased gradually; and the dynamic elastic modulus decreased overall. Although these studies have made some achievements, research on the dynamic mechanics and failure behavior of deep grouted fractured rock under the coupling effect of temperature and dynamic load has not been seen.

In this paper, based on cement-based grouting material, the dynamic mechanical responses of deep grouted fractured rock under real-time temperature–dynamic load coupling were obtained by the high-temperature-split Hopkinson pressure bar (HT-SHPB) tests, and the macroscopic and microscopic failure properties were analyzed by high-speed camera and SEM tests. These studies are helpful to the development of deep high-temperature grouting engineering.

2. Experimental Materials and Methods

2.1. Preparation of Grout

The superfine cement grout (Figure 3) was formulated by blending water and superfine cement in a mass proportion of 24:40.

2.2. Preparation of Grouted Samples

Based on the reference [29], red cylindrical sandstone samples (Φ50 mm × H50 mm, Figure 4a) with a single artificial fracture were prepared. The 6 mm fracture aperture was based on the fracture classification standards and sandstone sample size. According to the classification of common fractures, fractures with a width ≥5 mm are defined as wide-tensile fracture. The fracture with 6 mm width in our research belongs to the category of wide-tensile fracture. Such fractures often occur in deep rock masses due to crustal movement and excavation unloading. Considering the sizes of the sandstone sample (Φ50 mm × H50 mm), it is difficult to set a larger fracture (more than 6 mm) in such small samples. However, if the fracture setting is too small, such as 1~4 mm, the grout filling effect is not obvious, which is not conducive to the study of the reinforcement effect of grout on the fractured rock. Based on the above analysis and after several sets of fracture size setting tests, the 6 mm width was selected.

Single-side sealing with butyl waterproof tape ensured that the grout did not leak from the fracture (Figure 4b), and the other side of the fracture was used as a grout inlet. Using a small syringe, the prepared superfine cement grout was injected into the sample fractures (Figure 5a). At last, these fractured samples were cured at indoor temperature for 7 days to form grouted fractured samples.

2.3. HT-SHPB Experimental System and Scheme

The HT-SHPB system (Figure 6a,b, developed by the Xuzhou University of Technology) includes the launching tube (Φ100 mm × 3000 mm), incident bar (Φ100 mm × 5500 mm), and transmission bar (Φ100 mm × 4000 mm). The strain gauges and ultra-dynamic strain indicator collect the signals in real time, while an i-SPEED 508 high-speed camera (iX Cameras Ltd., Rochford, UK) monitors real-time dynamic failure of the grouted sample.

The high-temperature box is used to heat the rock sample. The combined application of high-temperature box and SHPB realizes the real-time coupling of temperature and dynamic load. It is worth noting that the rock fragments generated by the impact will splash around, so the inner wall protection device is added to prevent the inner wall from being hit by these rock fragments (Figure 6c).

The high-temperature box is directly added to the SHPB equipment, overcoming the defect that the rock sample needs to be heated first and then put on the SHPB test equipment (temperature and dynamic load are not real-time coupling), and realizing the real-time coupling of temperature and dynamic load, which is more in line with the actual deep high-temperature engineering environment.

Cured superfine cement-grouted fractured samples undergo dynamic impact tests under the coupled condition of dynamic load and temperature (setting the impact air pressure at 0.1 MPa and using temperatures of indoor temperature, 100 °C, and 300 °C).

3. Dynamic Mechanical and Failure Properties of Grouted Fractured Rock Under Real-Time Temperature–Dynamic Load Coupling

3.1. Dynamic Mechanical Properties

SHPB data analysis software (the calculation and analysis software that comes with the HT-SHPB system) was used to obtain the data of the impact experiments. The software can identify and separate the incident wave and transmitted wave on the basis of the data collected by SHPB experiments and calculate the corresponding parameters such as strain rate, stress, strain energy, and high G value. In the above data, we selected dynamic stress-strain values, as shown in Figure 7. Based on the reference [30], we set the impact air pressure to 0.1 MPa, which corresponds to a relatively fixed strain rate.

Figure 7 shows the dynamic stress-strain curves of grouted fractured samples under impact air pressure of 0.1 MPa and varied temperature (indoor temperature, 100 °C, and 300 °C). Figure 8 presents the dynamic compressive strength of the grouted samples under different temperatures. In Figure 7, the stress-strain curves show repeated increases and decreases in stress, subsequently entering a phase of repeated fluctuations. The slopes of curves show long-term and reciprocating changes. This phenomenon indicates that the grouted samples suffer severe cracking (large deformations occur) and damage after impact. Further deformation and damage occur as the impact continues. From Figure 8, it can be seen that the dynamic compressive strength of grouted fractured rock increases with the increase of temperature. From indoor temperature to 100 °C, the dynamic compressive strength increases by 23%, while from 100 °C to 300 °C, the dynamic compressive strength increases by 192.01%. The degree of increase in strength from 100 to 300 °C is much greater than that from indoor temperature to 100 °C, which indicates that the dynamic compressive strength of the grouted samples is very sensitive to temperature. Although the samples are different, the trend of dynamic strength increasing with increase of the temperature is consistent with the trend found in the references [31,32], which proves the rationality of the conclusion of this study to a certain extent.

As can be seen from Figure 9, the dynamic peak strain also shows an increasing trend with the increase in temperature. From indoor temperature to 100 °C, the peak strain increases by 38.80 times, while from 100 to 300 °C, the peak strain increases by only 0.17 times, which is contrary to the degree of increase of dynamic strength at different stages of increasing temperature.

3.2. Macroscopic Dynamic Deformation and Failure Properties of Samples at Indoor Temperature

The i-SPEED 508 high-speed camera was used to capture the whole process of dynamic failure of the grouted samples, as shown in Figure 10. From Figure 10, the impact fractures first occur at the two ends of the pre-existing grouted fracture and are the pair of parallel lines (two dashed rectangles). According to elastic theory, the presence of a fracture in a sample induces stress concentration at the fracture tips. Under impact loading conditions, the stress concentration factor at the pre-existing grouted fracture tips undergoes significant amplification. This mechanical response drives localized stress magnitudes beyond material strength thresholds at the fracture tips, thereby preferentially initiating secondary crack propagation at these critical stress zones. In addition, the symmetrical configuration of pre-existing grouted fracture tips induces a corresponding stress distribution symmetry. This stress symmetry drives approximate parallel propagation of nascent fractures. Under sustained impact loading, the above-mentioned fractures propagate further, and the new fractures at the pre-existing grouted fracture’s periphery arise and develop (dashed circle). Meanwhile, the sample slides and deforms along the length direction of the pre-existing grouted fracture (opposite red arrows), generating a larger fracture that connects with and penetrates the fractures at the ends of the grouted fracture. Eventually, the sample is completely destroyed.

3.3. Micro-Structure Characteristics of Samples at Temperature of 300 °C

Due to the presence of the high-temperature box, the high-speed camera is unable to monitor the process of sample failure at 300 °C (Figure 6c). The SEM tests were used to obtain the failure properties of grouted samples (Figure 11 and Figure 12). It can be seen from these figures that there are no obvious micro-cracks or pores on either the structural surface or the cementation surface, which indicates that the existence of the grouting material restrains the generation and development of micro-cracks to some extent under the condition of 300 °C. The high temperature promotes the hydration product transformation of the grout material, enhances the bond force of the grout and rock interface, and effectively inhibits the initiation and development of micro-cracks. The internal micro-structure of the grouting area is dense and has few defects, which shows that the empty spaces are fully filled by the grout during grouting, and the loose particles are consolidated into the whole through cementation. Meanwhile, the high bonding strength in the transition zone of the micro-interface effectively reduces the stress concentration and weak points, thus significantly improving the macroscopic mechanical properties, which is basically consistent with the law of dynamic mechanical strength variation with temperature revealed by the macroscopic dynamic stress-strain curves (Figure 7) and dynamic compressive strength (Figure 8).

4. Discussion

The higher temperatures may introduce thermal stresses, which negatively affect the strengths of the rock. For example, the reference [28] studied the dynamic mechanical properties of sandstone, the results of which showed that, with increasing temperature, the dynamic compressive strength of sandstone first increases and then decreases. The above studies indicate that when the temperature exceeds a certain threshold, the dynamic compressive strength of the rock sample will decrease. In our study, the maximum temperature is 300 °C, which may not reach the temperature threshold, so the mechanical strength of the rock sample only increases with the increase of temperature; that is to say, the current temperature (300 °C) has not yet had a negative impact on the rock.

5. Conclusions

The mechanical and failure properties of deep grouted fractured rock under real-time coupling of temperature and dynamic load are obtained by the HT-SHPB impact experiments, high-speed imaging, and SEM tests. The following conclusions are obtained:

• The dynamic compressive strength of grouted fractured rock increases with the increase of temperature. The dynamic strength increases by 23% from indoor temperature to 100 °C and by 192.01% from 100 to 300 °C. The trend of dynamic strength increasing with the temperature is verified to a certain extent by the existing references.
• The dynamic peak strain shows the increasing trend with the increase in temperature. The peak strain increases by 38.80 times from indoor temperature to 100 °C and by 0.17 times from 100 to 300 °C, which is in contrast to the degree of increase in dynamic strength during different stages of increasing temperature.
• The macroscopic failure properties show that the impact fractures first occur at the two ends of the pre-existing grouted fracture and are a pair of parallel lines. Micro-structure characteristics indicate that the existence of grouting material restrains the generation and development of micro-cracks at 300 °C, explaining the variation of the macroscopic dynamic strength with temperature from a microscopic perspective.

In this paper, an impact pressure of 0.1 MPa and three temperatures (indoor temperature, 100 °C, and 300 °C) were selected to study the dynamic mechanical responses of grouted fractured rock. In the next work, higher impact pressures (such as 0.3, 0.5, 1.0 MPa, etc.) and temperatures (500, 800 °C, etc.) will be selected to study the dynamic mechanics and failure characteristics of grouted fractured rock in deep high-temperature engineering. Moreover, the notch experiments’ important role in the fracture of the rock is played by the fracture toughness of the rock, so in addition to these mechanical properties of sandstone, this value will need to be studied in our next work.