A State-of-the-Art Review on the Study of the Diffusion Mechanism of Fissure Grouting

Abstract

China is renowned for its extensive underground engineering projects and the complex geological and hydrological conditions it faces. Grouting treatment technology is widely employed in deep-buried mines and tunnels, where grouting parameters such as materials, pressure, volume, and hole arrangement significantly impact the effectiveness of grouting. This review paper comprehensively examines current research on grouting materials, theories, experiments, and numerical simulations. It summarizes the various factors that must be considered during the grouting process of fissures and explores the diffusion mechanisms of grout under their influence. Furthermore, further research is needed on the mechanisms and treatment methods for poor grouting in rock masses, the distribution patterns of fissures, optimization methods for grouting parameters, and grout quality assessment techniques. Future research should focus on developing more efficient experimental methods with higher accuracy levels while advancing grouting technologies. Establishing comprehensive and accurate rock mass models along with improving monitoring capabilities are also crucial aspects to consider. Therefore, studying the diffusion mechanisms of grout in fissured rock masses is of significant importance for the practical operation of underground engineering projects.

1. Introduction

In recent years, with the improvement in the level of engineering and construction, research on the optimization of grouting methods and their effects has become increasingly urgent. Significant progress has been made within the theoretical and technical development of grouting in terms of the grouting mechanism, grouting methods and technology, grouting materials, experimental technology, and other aspects. It is widely acknowledged that cracks are formed in rocks due to geological processes. In engineering applications, fissure grouting stands out as the most effective approach for addressing crack damage in rock masses. Therefore, studying the diffusion behavior of slurry for repairing this type of ailment holds immense significance.

In terms of the diffusion mechanism of fissure grouting, current research primarily focuses on simulations and experimental investigations to explore the impact of parameters such as grouting material injection path, injection time, grouting pressure, and grouting volume on the diffusion mechanism. Furthermore, there is a growing emphasis on studying fundamental physical and chemical properties in domestic research, including aspects like grouting material viscosity [1,2,3,4], surface tension [5,6], ionic concentration [7], pH [8,9], etc. In engineering practice, the grouting process involves the complex distribution of rock fissure structures and various types of slurry flow, resulting in a highly intricate diffusion mode of slurry within rock fissures. To investigate the diffusion behavior of slurry in rock fissures, numerous scholars, both domestically and internationally, have conducted extensive studies on the diffusion mechanism of polymers. Zhang et al. [10] studied the flow of porous fissure media using an equivalent permeability tensor in the equivalent continuous medium model. This model accurately describes the slurry diffusion trend in rock fissures at a macroscopic level but not the diffusion process itself. Wei et al. [11] investigated the diffusion mechanism of fissure grouting using a step-by-step calculation method and partially discovered the diffusion mechanism of dynamic fissure grouting. Zhu et al. [12] proposed a grouting-process modeling technique that incorporates fluid-structure coupling, generates more realistic values for fissure apertures based on aperture-trace length relationships, and establishes numerical models to simulate fluid dynamics as well as structural deformation within fissured rock masses. These aforementioned methods are theoretical approaches put forth by scholars; additionally, some researchers have conducted experiments to study the interaction between fissure media and grouting materials, yielding significant findings. Liu et al. [13] developed a visual fissure constant-pressure grouting test system that includes pressure supply equipment, constant-pressure pumping equipment, fissure simulation equipment, and monitoring equipment. This system can create a specific fissure network, simulate the grouting flow process, and investigate the grout–water/gas displacement mechanism in fissured rock masses. Li et al. [14] presented a sequential diffusion coagulation (SDS) method that considers the spatiotemporal evolution of slurry viscosity. They examined the slurry diffusion process at different flow rates using the SDS technique and discussed the effects of flow rate on grouting pressure, counter-current diffusion distance, and diffusion behavior. The research demonstrated the usefulness and feasibility of investigating the slurry diffusion mechanisms of large fissures on an engineering scale. Wang et al. [15] conducted a true triaxial mechanical state freeze–thaw rock mass fissure grouting experiment. They performed computed tomography (CT) and scanning electron microscopy (SEM) tests on the grouted specimens to investigate the grout diffusion mechanisms of different grouting materials in freeze–thaw rock masses. The results indicated that low-viscosity silicate cement slurry exhibited better diffusion performance in unfrozen rock masses and rock masses subjected to 10 freeze–thaw cycles.

This paper presents a comprehensive overview of the current research status on grouting materials for fissured rock grouting, encompassing research methodologies, theories, simulation, and experimental applications, both domestically and internationally. Furthermore, it briefly outlines future research directions to provide valuable insights for further developments in this field.

2. Current Status of Research on Grouting Materials

In recent years, for the fissure rock body, we have generally opted to enhance water sealing and reinforce grouting materials for better effectiveness based on engineering applications and examples. Currently, common grouting materials can be classified into four types, namely, inorganic materials, organic materials, composite materials, and biological materials, as illustrated in Figure 1. The organic-inorganic composite slurry possesses the combined advantages of both inorganic and organic materials, exhibiting excellent injection performance, rapid setting time, and being a more cost-effective alternative to pure organic grouting materials. However, the limited applicability of biological grouting materials stems from their environmental characteristics. Since the applicability of organic and inorganic composite grout to deep underground engineering is not clear at present, we will introduce organic grouting materials and inorganic grouting materials here.

2.1. Inorganic Grouting Materials

Inorganic grouting materials can be categorized into three types according to the slurry’s state: particulate materials, adhesive materials, and composite materials. Particle materials are more common, such as ordinary cement slurry [16,17], ultra-fine cement slurry [18], etc. Adhesive materials mainly include sodium silicate slurries [19] and silica sol [20]. Gelatinous materials mainly include sodium silicate and silica sol. Mixed materials mainly refer to a mixture of cement and sodium silicate slurry [21,22]. Inorganic grouting materials can meet requirements for grouting under a large part of engineering geological conditions. The most commonly used slurries for underground water-sealing engineering include the common cement type, the cement-based composite type, the cement–water glass type slurry, etc. However, different ratios are allocated according to the actual engineering situation, taking into account the properties of different cement-based slurries. For example, common cement slurries are advantageous because they are low-cost, easy to obtain, and have properties similar to those of cemented rock masses, making them the main choice of grouting materials. However, they also have characteristics of high viscosity and strong granularity, so they have disadvantages such as easy sedimentation of particles, poor stability, and low viscosity, making it more difficult to inject them into small fissures and thus more difficult to meet construction requirements [23]. At this time, ultrafine cement slurry is considered for grouting. Compared with traditional cement slurry, ultrafine cement slurry has the advantages of smaller particles, less sedimentation, a larger surface area, and a smaller particle size [24,25]. Nano-silica sol, created by uniformly dispersing nanoscale silica in water, undergoes gel formation upon mixing with cementitious materials [26]. This sol has the advantages of low initial viscosity, controllable gel time, and suitability for grouting engineering applications.

In the field of grouting material selection and optimization, new materials have emerged in recent years. Du et al. [27] investigated the dynamic properties of cement paste specimens with varying dosages of carbon nanotubes (CNTs) under impact loads. The findings showed that cement paste’s dynamic mechanical characteristics were somewhat improved with carbon nanotubes. Zhang et al. [28] developed anti-scouring grouts using a combination of bentonite, fly ash, potassium dihydrogen phosphate, sodium silicate, and light-burned magnesia. The grouts’ compressive strength, viscosity, and setting time were influenced by the molar ratio of MgO to KH₂PO₄ (M/P) and the contents of fly ash and sodium silicate. Zhang et al. [29] introduced a novel cement-based grouting material, SJP (S, Sichuan; JP, Jinping), which addressed diffusion and filling challenges in micro-fissured rock masses. Xu et al. [30] investigated the diffusion and sealing of cement–water glass slurry in rough fissures using an orthogonal experiment, which offered guidance for practical applications in grouting and sealing engineering. Pachta Vasiliki et al. [31] prepared and tested four lime-based grout compositions, studied the prismatic behavior and effectiveness of fiber-reinforced lime–volcanic grout, and made macroscopic observations and measurements of pulse velocity and compressive strength before and after grouting. The results showed that the application of fiber-reinforced lime-based grout in the restoration of historical masonry is feasible. Pan et al. [32] used a calcium sulfoaluminate cement (CSA)—silicate cement (PC)—gypsum ternary composite system to prepare a new type of grouting material. The factors affecting the performance of the grouting materials were analyzed by combining macro-experimentation and micro-analysis. Mujah et al. [33] enhanced the mechanical strength of a slurry by incorporating palm oil fuel ash into ordinary cement. Lin et al. [34] examined the impact of phosphogypsum and flue gas desulfurization gypsum on the performance and microstructure of thermally activated recycled concrete. Different gypsum types extended the initial setting times and improved the fluidity and mechanical properties of the grouts.

2.2. Organic Grouting Materials

Organic grouting materials (composite materials) can actually have better applicability. Organic materials typically have higher injectability, shorter and more controllable setting times, and higher early strength, but they are expensive. Polyurethane slurries [35,36] and acrylic slurries [37,38] have found extensive application in practical engineering.

With the development of polymer grouting materials, their applications are becoming more and more extensive. The research team led by academician Wang Fuming from Zhengzhou University mainly investigates the development and application of new cementitious materials, polymer grouting materials, and geo-environmentally responsive grouting materials in the field of grouting materials. By selecting and blending different raw materials, they achieved the optimization design and preparation process of grouting materials and explored the performance characterization and application fields of grouting material properties. Aiming at the common diseases of road pavements, they independently developed a set of fast and effective grouting techniques and technologies for repairing diseases using the characteristics of polymer materials, which have been widely used in road pavements, pipelines, dams, tunnels, bridges, and other practical engineering fields [39,40,41,42]. This has played a great role in the South-to-North Water Diversion Project, Yi-Wan Railway Sudden Mud Control, Underground Engineering Sudden Water Surge Control, and other major restoration projects. Lefebvre and Keunings [43] developed a two-dimensional continuous flow model for high-polymer slurry, considering the geometric shape, chemical reactivity, and time-dependent flow of high-polymer systems. Mitani and Hamada [44] created a high-polymer slurry diffusion model based on Hele–Shaw flow to simulate the expansion and filling process of high-polymer materials in variously shaped molds, accounting for density changes. Zhang et al. [45] examined the toughening effect of in situ-prepared polyacrylamide on sulfoaluminate cement-based grouting material (SCGM), which showed significantly improved toughness compared with standard SCGM. Jia et al. [46] developed a chemical reaction–hydrodynamic model for polymer grout flow and diffusion in planar fissures, utilizing computational fluid dynamics theory and polymerization mechanisms. They achieved a coupled solution for the chemical reaction, temperature field, and flow field. Wang et al. [47] designed a water-soluble epoxy resin to modify cement-based grouting material, demonstrating superior strength, strain, flowability, stability, and bonding capacity compared with pure cement grouts. Li et al. [48] investigated the influence of polymer concentration, soil moisture, and soaking duration on the strength of silt solidified with permeable polyurethane grouting materials and found significant improvements in silt performance. Guo et al. [49,50] discussed the application of high-polymer injection technology in the treatment of hidden diseases, such as mud and the overturning of road surfaces, and the restoration of road-bearing capacity based on the performance and technical characteristics of high-polymer injection materials and comprehensive non-destructive testing techniques, such as ground-penetrating radar and hammer-type bending and sinking meters.

3. The Current Status of Research on the Theory of Seepage Diffusion of Fissure Grouting in the Rock Body

3.1. Existing Diffusion Theories

In the field of single-fissure grouting, researchers have conducted in-depth studies. Zhang et al. [51] studied the grouting process of rapidly solidified slurry in a single horizontal fissure. They proposed a step-by-step calculation method to address the uneven distribution of slurry viscosity. Xu et al. [52] developed a 3D rough single-fissure hydro-mechanical (HM) linked seepage flow model and investigated various fissure seepage properties and the factors influencing them. Zheng et al. [53] examined the mechanism of slurry diffusion in a single fissure under moving water conditions, established a mathematical model for slurry diffusion, and obtained theoretical solutions for diffusion parameters. Yang et al. [54] used the finite element method to simulate the diffusion process of cement slurry in a single rough fissure and analyzed the diffusion form of cement slurry and its influencing factors in a hydraulic environment. Zhang et al. [55] conducted cement–sodium silicate grouting experiments under dynamic water conditions in a single-plate fissure-grouting model and then established a transient equation to characterize the diffusion trajectory of slurry. It was found that under dynamic water conditions, slurry diffusion presents an asymmetric elliptical shape, and the ratio of slurry–water velocity has a significant correlation with the diffusion distance of slurry (against water and with water) and aperture. Zhang et al. [56] conducted chemical slurry grouting experiments under dynamic water conditions in a single-plate fissure-grouting model and found that the larger the velocity of dynamic water, the smaller the diffusion distance of slurry in the opposite water and vertical directions, and the smaller the area of slurry solidification.

In the field of network fissure grouting, Wang et al. [57] investigated the permeation behavior of slurry in fissures using a two-dimensional orthogonal fissure network diffusion model. They analyzed the influence of grouting parameters, including fissure aperture, slurry velocity, and viscosity, on the diffusion distance of the slurry. Zou et al. [58] proposed an extended two-phase flow model to study the behavior of yielding power-law fluids, especially cement slurry, in water-saturated fissure networks. The model was validated using experimental data, and numerical simulations were conducted to evaluate the effect of rheological parameters and time-varying rheological properties on the propagation process. In the context of pore-fissure grouting, Zhe et al. [59] conducted modification and reinforcement tests using nano-silica sol as an alternative to standard cement slurry. They investigated the injectability of slurry, gel impermeability, and macroscopic and microscopic pore closure properties in solidified mudstone. The study established fundamental principles for siliconsol grouting and mudstone anti-seepage reinforcement.

3.2. Current Status of Research on the Percolation Effect on Slurry Diffusion in Fissured Media

Due to the assumption of too many conditions in theoretical formulae and the large error in the real working conditions, with the gradual development of slurry diffusion theory, the influence of slurry gravity, the flow pattern of slurry, the time-varying viscosity of slurry, the percolation effect of slurry particles, and the zig-zag effect of slurry diffusion paths on the theory of infiltration grouting are constantly revised, thus making the theory of infiltration grouting more and more accurate.

Wang et al. [60] independently developed a visualized test system for micro-fissure grouting, and it was found that during the grouting process of micro-fissured rock, the percolation effect of the cement slurry has a significant influence on the grouting effect. Zhou et al. [61] used a central combination test (CCD) to investigate the effect of grouting pressure and soil mass fractal dimensions on the diffusion of cement slurry, considering the percolation effect. They discovered that as the grouting pressure increased, the viscosity as well as the effective diffusion radius of the leachate increased and then decreased. In addition, they found that the larger the soil’s fractal dimension, the smaller the effective diffusion range of the slurry. Li et al. [62] established the consolidation and diffusion mechanisms of compressible intelligent synchronous grouting material based on the percolation effect according to the composition characteristics of lightweight intelligent synchronous grouting material. The results show that the percolation effect has a significant effect on the consolidation and diffusion of grouting materials. Li et al. [63] established a differential control equation for slurry filtration diffusion based on the laws of mass conservation, solute transport, and mass transfer principles. They provided analytical solutions for constant pressure and constant flow rate grouting filtration diffusion and revealed the influence mechanisms underlying the filtration effect on the pore structure of the porous medium, the viscosity of the slurry, and the distribution of diffusion pressure. Li et al. [64] studied the influence of the percolation effect on the porosity, permeability, and slurry flow rate of an injected medium using the Kozeny–Carman model. Feng et al. [65] established a theoretical model for the three-dimensional front infiltration diffusion of cement slurry based on the law of mass conservation, the linear filtration law, and the permeability continuity equation. They found that the viscosity of the slurry had a significant impact on the filtration effect and the effective diffusion distance of the slurry. Specifically, their study revealed that the filtration effect became more pronounced with increasing slurry viscosity, leading to a shorter effective diffusion distance for the slurry. This suggests that controlling the viscosity of the slurry is an important factor in optimizing the grouting process and achieving the desired grouting effect. Fang et al. [66] constructed a cement slurry diffusion model in spherical pores based on the law of mass conservation and the linear filtration law. Their investigation revealed that the filtration effect induces a gradual increase in the permeability resistance of the grouting medium, leading to a progressive rise in grouting pressure over time. Qin et al. [67] utilized the flow–solid coupling mechanism and PFC2D particle flow numerical simulation technology to conduct a numerical simulation study on the transport of slurry particles in porous media and the coupling of slurry and a porous skeleton during the grouting process. Their study used the FISHTANK function library and FISH programming language embedded in PFC2D 6.0 software to carry out the numerical simulation calculations.

3.3. Current Status of Research on the Self-Expansion Effect of Slurry on the Diffusion of Slurry in Fissure Media

Most existing diffusion theories are based on chemical slurries such as cement slurry or sodium silicate slurry, often without considering the expansion of the slurry. However, polymer slurries have self-expansion characteristics, so the diffusion theory of polymers in the fissure medium needs to be studied separately.

Li et al. [68] used the finite volume method to discretize two-phase flow control equations, the Youngs algorithm to track the two-phase flow interface, and the Simple algorithm to solve the modified equations. They used the results to establish a diffusion model of high-polymer fissures and verified the correctness of the model through numerical examples. Li et al. [69] designed a plate fissure-grouting model test device to explore the influence of temperature on the diffusion law of self-expanding high-polymer in a fissure. The experimental results showed that there is a significant influence on the law of temperature on the diffusion of self-expanding high polymers, which helps to study the grouting mechanism and the development of grouting technology for high polymers in fissures. Liang et al. [70] established a diffusion theory formula for high polymers in the expansion stage by solving the force balance equation of the high-polymer fissure expansion diffusion stage microelement and verifying the correctness of the theoretical formula using numerical simulation.

3.4. Difficulties in Continuing Breakthroughs in the Future

The current research on fissure grouting seepage diffusion theory still has many difficulties and problems to be solved, including:

(1)

There are several factors that contribute to the complexity of the grouting process, and further exploration is needed. These factors include complex interactions between liquids and rocks, the morphology and distribution of fissures, the properties of grouting material, the control of grouting pressure and flow rate, heterogeneity in rock and soil layers, and environmental conditions such as temperature and humidity changes. The irregular and diverse morphology and distribution of fissures, such as horizontal, vertical, inclined, branching, and converging types, pose challenges to accurately describing the fluid flow and diffusion during grouting.

(2)

The existing research lacks practical guidance and often relies on ideal conditions, so it is unclear to what extent these findings are applicable to practical engineering. The theory of infiltration grouting mostly focuses on slurry diffusion in single network fissures while ignoring the diffusion of slurry in porous media or media with uneven pore sizes. Moreover, the current research primarily examines the influence of slurry fluid characteristics on diffusion, ignoring the role of pore structure characteristics in the diffusion mechanism.

(3)

The existing research on high-polymer grouting diffusion theory primarily examines small-pore fissures and cleavage diffusion, ignoring the diffusion mechanism in large-pore gravel media and the influence of water flow. Experimental studies mainly investigate the relationship between grouting parameters, diffusion morphology, and solidification body radius without considering the spatial and temporal evolution of a medium’s dynamic response during the diffusion process.

(4)

In practical engineering, studying the diffusion mechanism of slurry in a single grouting hole is insufficient. In many cases, the area of the disease is relatively large, and multiple grouting holes need to be filled. This involves the selection of grouting hole types, the spacing between adjacent grouting holes, and the determination of the grouting sequence. Selecting appropriate grouting hole types and spacing can achieve twice the result with half the effort in terms of the effectiveness and cost of disease treatment. However, there is currently no mature theoretical basis for the layout of grouting holes in engineering disease treatment. The selection of existing grouting hole layout parameters often does not consider the filtration effect caused by slurry particles or only relies on on-site experience.

4. Research Status of the Numerical Simulation of Fissure Grouting in Rock Masses

4.1. Numerical Simulation Methods

The key to successful grouting is to select appropriate grouting parameters, including grouting pressure, grout volume, grouting rate, and grouting duration, to achieve the best grouting effect. With the development of computer technology, numerical simulation has become one of the most commonly used approaches for studying the grouting process in fissured rock masses. Numerical simulation technology provides a very intuitive and detailed display of the grouting process and is relatively easy and low-cost. This technology can be used not only to investigate the slurry diffusion law in fissures but also to determine the actual amount of grout utilized in practical grouting projects. In recent years, extensive study has been undertaken on numerical simulation methods and the application of grouting in fissured rock masses. The finite element method, the boundary element method, and the discrete element method are currently the most widely used numerical simulation methods.

4.2. Progress in Numerical Simulation Methods for the Study of Slurry Diffusion in Fissure Media

Numerical simulation is an important method for studying the diffusion law of grouting in fissured rock masses. Compared with theoretical research, numerical simulation does not rely heavily on assumptions and can continuously and dynamically obtain numerical solutions for the diffusion process of grout under complex conditions, with a wide range of applicability. For model experiments, numerical simulation can adjust various parameters in a model using programming or computer software, thus simulating grouting models under various working conditions without being limited by model size, flow field disturbances, personal safety, or measurement accuracy, which makes the research process more convenient and economical. These are of great significance for a correct understanding of the diffusion behavior of grout in fissures. At present, many scholars have conducted a large amount of work in this area.

Xue et al. [71] constructed height regression models for 33 water-conducting fissure zones using the PSO-SVR particle swarm-support vector machine, quantitatively evaluated the prediction accuracy of the water-conducting model, and then predicted water-conducting fissure zones for three mines. Ni et al. [72] used the finite element method to numerically simulate grouting in micro-fissured rock masses. They solved partial differential equations to simulate multi-physics field interactions. The calculation model included a single horizontal fissure within the surrounding rock masses, as shown in Figure 2. Their study focused on the diffusion process of grouting in micro-fissured rock masses, considering the fluid–solid coupling effect. In addition, they also investigated the impact of fissure aperture and the elastic modulus of rock masses on the grouting diffusion process. Liu et al. [73] used finite element software to simulate the two-phase fluid seepage of underground fluids in fissures within porous media. They established a relationship between flow velocity and permeability pressure differences. In numerical simulations using the finite element method, fissures can be simulated using line segments or surface elements to predict the deformation and changes in a fissure during the grouting process. In addition, the effects of seepage and consolidation during the grouting process on the mechanical behavior of rock masses can also be considered to predict and evaluate the stability of a rock mass after grouting. The advantages of the finite element method are that it can handle complex geometric and boundary conditions and that it considers the nonlinear and deformation characteristics of materials. However, the finite element method also has drawbacks, such as a high computational cost and a long computation time.

Shen et al. [74] used the lattice Boltzmann method to simulate the evolution process of velocity and the concentration field, considering the dissolution effect on the fissure surface, and established a rough rock fissure seepage–dissolution coupling model. They discussed the effects of different fractal dimensions of the surface of a fissure, two dimensionless variables: Pe (characterizing the relative size of convection and diffusion) and Da (characterizing the relative relationship between reaction rate and diffusion), and other factors on the mechanism of rough fissure seepage–dissolution coupling. Li et al. [75,76] introduced the basic assumptions of the Hele–Shaw model and transformed the three-dimensional problem of fluid flow in a plane fissure into a two-dimensional problem about the fissure aperture, thus establishing a quasi-three-dimensional method for high polymer grouting in plane fissures. They effectively simulated the diffusion process of high-polymer slurry in fissures, greatly reduced the model size, and saved computation time.

4.3. Future Research Priorities

Numerical simulation of grouting in rock fissures is a fundamental technology for important engineering projects such as mines and tunnels. It can provide important references and guidance for grouting scheme design and actual construction. In addition, numerical simulation technology can effectively improve the efficiency and quality of grouting, reduce construction difficulties, and provide important guarantees for environmental protection and personnel safety. In the future, research on numerical simulation of grouting needs to continue to expand into new fields, increase research depth to meet the needs of practical applications, and promote the continuous upgrading and development of grouting technology and methods. In the authors’ opinion, in future research on numerical simulation of grouting in rock fissures, further exploration is needed in the following areas:

(1)

An optimization method for grouting based on numerical simulation technology should be developed while continuing to improve the accuracy and computational efficiency of simulation technology. Interdisciplinary collaboration should be strengthened by combining numerical simulation with traditional construction processes and physical experiments to optimize grouting schemes and improve engineering quality. In-depth research on grouting processes and technologies should continue to comprehensively improve construction efficiency and engineering benefits.

(2)

Polymers are complex diffusers with phase-transition characteristics. When establishing numerical models, the phase transition characteristics of high polymers from liquid to solid during diffusion should be considered. The internal driving force of the expansion of polymer slurry due to chemical reactions and the influence of water on the chemical reaction of the slurry should be considered to establish a more accurate numerical model of polymer diffusion. In future research on grouting in rock fissures with high polymers, the influence of various factors such as fissure dip angle, fissure networks, and fissure seepage should be comprehensively considered. Further analysis should be carried out to establish a prediction model with lower error in order to obtain results that are closer to the real grouting environment in fissured rock masses.

(3)

Due to the uneven distribution of fissures in rock masses, the grouting volume has high uncertainty. Therefore, accurately determining the properties, occurrence, roughness, density, and scale of grouting holes and nearby rock fissures in rock masses is the basis for predicting the diffusion distance of slurry correctly. Currently, research on strengthening the detection methods for rock fissure parameters is not sufficient. While further exploring the fissures themselves, more superior and intelligent numerical simulation methods need to be developed to better simulate real fissures.

5. Current Status of Experimental Research on Grouting Modeling of Fissured Rock Bodies

5.1. Research, Development, and Testing of Spatiotemporal Diffusion Modeling Systems for the Cement Slurry Grouting of Fissures

Generally, in theoretical analysis and numerical computation, it is necessary to establish an appropriate fissure model to systematically observe the grouting parameters, mainly to obtain important physical parameters that are difficult to observe intuitively in practical engineering. A crucial prerequisite for conducting these experimental studies is to ensure that the environment constructed at the experimental site is similar to the actual engineering project so that the experimental results have practical engineering significance.

Zhou et al. [77] conducted a study on the temporal and spatial diffusion patterns of cement particles in porous media under varying hydraulic conditions, as depicted in Figure 3. They used a self-developed diffusion test device to investigate changes in filtrate viscosity, receiving medium porosity, and slurry diffusion pressure at different testing points during a specific period. The study successfully determined the temporal and spatial diffusion patterns of slurry in porous media while considering the filtration effect.

Wuhan Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, has developed an experimental device that can achieve visualized injection grouting in fissures with dynamic water [78]. The principle of the experimental device is shown in Figure 4. The experimental device is equipped with a high-definition camera, which can visualize the grouting diffusion process by changing different experimental conditions. It then obtains a clear and intuitive understanding of the influence of dynamic water velocity, fluid characteristics of the slurry, and structural characteristics of the fissures on the diffusion law of the slurry.

Shandong University has developed a grouting experimental platform that can change the inclination angle of fissures, as shown in Figure 5 [79]. The experimental platform can adjust the distance between two glass plates to vary the width of the fissure. It can also simulate a fissure by filling it with porous media, simulating the diffusion morphology and distribution law under the conditions of receiving different media.

In addition, Yang et al. [80] first used pre-made templates to pour single fissures and fissure network rock masses and then conducted slurry diffusion experiments with given grouting pressures and water–cement ratios in rock fissures with different apertures. Their study found that under the conditions of large apertures and a slurry with a high water–cement ratio, the diffusion law of the slurry conforms to the fluid characteristics of a Newtonian fluid, while under the conditions of small apertures and a slurry with a low water–cement ratio, the seepage characteristics of the slurry are consistent with the characteristics of a non-Newtonian fluid. The aperture opening, the angle (bending degree) between the fissures, and the viscosity of the slurry have a significant impact on the distribution of slurry diffusion pressure in fissures.

Groundwater environments generally exist in dynamic and static water conditions. Due to the relative simplicity of grouting diffusion in a static water environment, there are more studies on the diffusion of slurry in different fissures in dynamic water conditions. In order to intuitively explore the diffusion of slurry in multiple fissures or even fissure networks, many current researchers have mainly focused on flat fissure models or models similar to them. These models include single-flat fissure model experiments, approximate single-flat fissure model experiments, and multi-fissure, intersecting, branching fissure, and fissure network model experiments. Currently, research on grouting reinforcement technology for groundwater environment control is somewhat limited to simulating experiments on single fissures or uniformly distributed fissures.

5.2. Research, Development, and Testing of Spatiotemporal Diffusion Modeling Systems for the Polymer Grouting of Fissures

High-polymer slurry, in contrast to non-expansive slurry, utilizes its own foaming reaction to generate secondary pressure, enhancing flow within a fissure after injection with a high-pressure gun. As the foaming process of high-polymer slurry gradually stops, it solidifies due to gel, resulting in an increase in dynamic viscosity and the formation of rigid polyurethane foam. Liang et al. [81] categorized the grouting process of fissures using high-polymer slurry into the following three stages: static pressure grouting, expansion and diffusion, and gel solidification. Their research provides insights into the temporal and spatial evolution of foaming-type high-polymer slurry diffusion, as illustrated in Figure 6.

Figure 6 shows a schematic diagram of the grouting model test system. The data monitoring module in this test system mainly consists of a strain acquisition system and a high-definition camera. The scale is marked along the direction of slurry flow from the grouting hole on the fissure model so that the spatiotemporal diffusion law of the slurry can be observed using a high-definition camera.

In a study by Du et al. [82], a center combination design (CCD) experiment was conducted using a self-made visual diffusion device with pressure and water-blocking rate monitoring functions to study the spatiotemporal diffusion law (diffusion morphology, diffusion pressure, and water-blocking rate) of the slurry in receiving media. Their study revealed the influence of grouting parameters (dynamic water pump pressure, grouting volume, and mass fractal dimensions) on the spatiotemporal distribution of the diffusion morphology, diffusion dynamic response, and water-blocking rate of high-polymer slurry, as shown in Figure 7. The expansive nature of high-polymer slurry makes its diffusion mechanism more complex in the water-rich gravel layer. The main influencing factors, including dynamic water pump pressure, grouting volume, and mass fractal dimensions, were investigated to explore the diffusion law and grouting effect of high-polymer slurry in water-rich gravel layers.

Due to the control of stress and characteristics on the cavity structure inside real rough fissures, it is difficult to directly observe internal seepage. Currently, there is a lack of high-precision numerical analysis and experimental verification of the displacement law of the slurry–water two-phase flow in rough fissures. Zou et al. [83] conducted research on the displacement seepage law of slurry–water two-phase flow and developed a visualization displacement test system and method based on particle image velocimetry (PIV) technology. They obtained the flow field distribution and the relationship between flow rate and pressure drop inside a transparent 3D-printed rough fissure. As shown in Figure 8, the 3D-printed model is fixed on a test bench with the smooth side facing up and the rough side facing down. Before the experiment, the model is filled with water, and bubbles are removed to ensure that the fissure is in a saturated state. During the experiment, a simulated slurry is injected into the fissure model at a constant flow rate using a plunger pump. The non-Newtonian fluid (slurry) and invading water, as the displaced phase, produce a two-phase flow, while data are collected simultaneously.

5.3. Development and Testing of the Multi-Hole Grouting Diffusion Model Test System

The effective selection of grouting materials, grouting pressure, grouting volume, and the arrangement of grouting holes are crucial factors in achieving economic and practical goals during the grouting process. The next crucial step is to arrange grouting holes, including their shape and spacing, after choosing the appropriate type of grout. In addition to producing the desired strengthening effect, a sensible grouting hole and spacing design can also prevent needless financial expenditures. Therefore, some scholars have conducted research on this and achieved certain results.

Soga et al. [84] conducted experiments on porous grouting for soil reinforcement. They found that injecting a small amount of grout into multiple holes can achieve better reinforcement compared with a single hole. The reinforcement effectiveness decreased with the increase in grouting interval time in the consolidated soil layer, but in the over-consolidated soil layer, the reinforcement effect was independent of interval time. Their study also examined the effects of different hole layouts. Peng et al. [85] investigated the interaction problem of multi-hole grouting in seepage grouting. They used natural boundary element theory and the Fourier series method to derive an analytical solution for the seepage pressure of an external well, considering the wellbore wall as an impermeable formation. Ren et al. [86] introduced an optimization approach for multi-hole grouting in fissured rock masses. Their method involved strategically placing multiple holes along the longer sides of the rock masses in different directions and cycles to prevent potential issues of porous crossing. The effectiveness of this optimization scheme was confirmed by calculating the water yield in the grouting area and analyzing the composition of the core cement. In a related study, Zhou et al. [87] developed a diffusion model that considers both single-hole and porous grouting. This model was based on fractal geometry and the filtration effect. The researchers applied the grouting hole parameters derived from the model to the actual site and evaluated the reinforcement effect of grouting based on these specific hole parameters.

5.4. Future Directions for Research Development

Experimental research on grouting models in fissured rock masses is the basis for rock grouting engineering design and practical operation plans. There are various experimental methods, including studying the properties of fissures and grout, the effects of grouting parameters on grouting efficiency, and the deformation and stress characteristics of rock masses after grouting. This can help to better understand the effectiveness of rock grouting and develop more optimized grouting plans. In the future, research in this field should focus on developing more efficient and accurate experimental methods, developing advanced grouting technologies, establishing more comprehensive and accurate rock models, and improving the control and monitoring of grouting circuits to achieve more comprehensive results. We recommend conducting a more in-depth and detailed analysis of the mechanisms and treatment methods for poorly grouted rock masses, the distribution patterns of fissures, and the exploration of optimization methods for grouting parameters and grouting quality detection technology.

(1)

For experimental research on fissured rock masses, exploring the spatiotemporal evolution of the rheological properties of grout and porosity at any testing point and time (in space and time) can help to better reveal the micro-mechanisms of the evolution of grout diffusion pressure. However, current research on high-polymer grouting experiments mostly focuses on the diffusion of fissures and small pores, while there is relatively little research on large porous gravel media. The influence of water flow on high polymer diffusion is mostly ignored. The experimental research mainly focuses on the relationship between grouting parameters, the form of diffusion, and the radius of the diffusion solidification body without considering the spatiotemporal evolution of the medium’s dynamic response during grout diffusion.

(2)

There are still many factors that affect the diffusion of cement slurry or high polymers in fissured media, and relatively few factors have been examined in actual experiments. Therefore, more factors need to be considered, such as introducing smarter devices, developing new experimental systems, and examining the influence of different water flow rates on grout diffusion simultaneously. The influence of high-polymer foaming agent dosage, preheating temperature, gel time, and other factors on diffusion also needs to be examined.

(3)

There are many materials available for grouting technology currently, and in the future, new grouting materials and composite materials can be developed specifically for fissured rock mass grouting. These new materials can be used for the development of experimental systems to further improve grouting efficiency.

6. Conclusions and Prospects

We systematically reviewed the current development status of fissure grouting in terms of theory, numerical analysis, and experimental aspects. In future research, a combination of theoretical analysis, numerical simulation calculations, and on-site experimental monitoring could facilitate a better understanding of the slurry diffusion mechanism in fissure grouting. We drew the following conclusions based on their previous research:

(1)

The current research on fissure grouting flow and diffusion theory faces several challenges and unresolved issues. Firstly, the interaction between the grout and the rock mass during the grouting process is complex and influenced by various factors such as fissure morphology and distribution, grouting materials, grouting pressure, and flow rate. This complexity makes it difficult to accurately describe fluid flow and diffusion. Secondly, existing research lacks guidance for practical engineering applications and mostly focuses on idealized conditions without considering diffusion in actual engineering porous media and the heterogeneity of pore channels.

(2)

The diffusion theory of polymer grouting tends to focus on fissures and small pores, neglecting the influence of large-pore gravel media and water flow on diffusion. Moreover, in engineering practice, the diffusion mechanisms and layout parameters of multiple grout holes need to be considered, but there is currently a lack of a mature theoretical basis and comprehensive methods that account for the filtration effect caused by grout particle movement. Therefore, further research is needed to address these issues and improve the accuracy and practicality of the fissure grouting flow and diffusion theory.

(3)

The numerical simulation study of rock fracture grouting is of great significance for engineering projects such as mines and tunnels and can provide guidance for grouting design and construction. It improves efficiency, quality, environmental protection, and safety. Future research should focus on optimizing grouting methods, considering phase transition characteristics of high polymers, comprehensively analyzing factors like fissure characteristics, and developing advanced detection and simulation methods for accurate prediction and simulation of grouting in real fissured rock masses.

(4)

Experimental research on grouting in fractured rock masses forms the basis for the engineering design and practical implementation of rock mass grouting projects. This research includes studying the properties of fractures and grouting characteristics, the influence of grouting parameters on grouting effectiveness, deformation, and stress characteristics of rock masses after grouting, and exploring the spatiotemporal evolution of slurry characteristics and porosity in fractures. Future research will focus on developing more efficient and accurate experimental methods, advanced grouting technologies, comprehensive and precise rock mass models, and improving the control and monitoring capabilities of grouting processes. Additionally, in-depth analysis can be conducted on the mechanisms and treatment methods for poor grouting in rock masses, distribution patterns of fractures, exploration of optimization methods for grouting parameters, and grout quality testing techniques. The development and application of new materials also hold the potential to enhance grouting efficiency.