Polyurethane grouting serves as an important solution for both anti-seepage and reinforcement in underground construction projects. A comprehensive understanding of the propagation characteristics of polyurethane grout is essential for effective grouting design, particularly in the presence of flowing water and connected fracture systems. This study aims to investigate the propagation behaviors and sealing efficiency of polyurethane grout within connected fractures under flowing water conditions. A series of laboratory experiments were conducted employing a self-designed laboratory grouting device for connected fractures. The results demonstrate that the propagation patterns vary significantly with different fracture connectivity configurations. Dynamic water-grout interactions exhibit three distinct temporal stages that directly influence propagation dynamics: injection, expansion, and equilibrium phases. The propagation width is predominately established during injection, while the interaction between polyurethane grout and water is achieved through the expansion stage. The influence of initial water flow rate and fracture connecting pattern on the propagation and sealing efficiency of polyurethane grout was analyzed. The results show that higher initial water flow rates decrease the upstream propagation distance and width while altering the pressure distribution within fractures, decreasing pressure in the primary fracture but increasing it in the secondary fractures. This hydraulic response is quantitatively characterized by an exponential decay relationship between grout shear stress and initial water flow rate. The study reveals that fracture intersections play a critical role in grout accumulation and sealing enhancement. Notably, vertically connected fractures demonstrate superior and more stable sealing efficiency compared to horizontally-vertically connected fractures with significantly reduced sensitivity to flow rate variations. Additionally, viscosity and flow rate ratios between polyurethane grout and water are positively correlated with sealing efficiency, with viscosity ratios playing a more dominant role than injection rates. These findings provide valuable insights for optimizing polyurethane grouting performance in connected fracture systems under flowing water conditions, offering practical guidance for underground engineering applications.

Accurately estimating the evolutions of stress, aperture, and surface morphology of fractures during shear is crucial for analyzing the coupled processes in rock masses. Here, a new predictive model for analyzing the shear behavior of rock fractures that considers the shear energy-damage relation of contacted asperities is proposed. By incorporating the wear theory into the analysis of mesoscopic contact mechanics of asperities, this model enables comprehensive analysis of the deformation and damage at single asperities and their impacts on the overall shear behavior. Direct shear tests were conducted on two distinct types of tensile fractures, and the capability of this model in predicting the shear strength, shear-induced dilation, and surface damage was validated against experimental data. A comparison with the classical JRC-JCS model and previous semi-analytical predictive models was implemented to highlight the advantages and future prospects of this model. Sensitivity analysis of the shear behavior to key parameters involved in this model was put forward to comprehensively elaborate the mechanical interactions at asperity and fracture scales.

Cement grouts are typical non-Newtonian fluids, the flow behavior of which through fractured strata is governed by rheological properties of the grouts (e.g., yield stress and viscosity) and geometrical characteristics of rock fractures (e.g., surface roughness and variable aperture structures). Particularly, the complex void structure of rock fractures renders the flow field complex and heterogeneous, significantly increasing the difficulty of theoretically predicting the pressure gradient – flow rate relation that is essential for rock grouting in engineering practice. In this study, we aim to analyze the effect of aperture structure on flow behavior of Herschel-Bulkely (H–B) grout flow in rough-walled rock fractures. Here, H–B fluids were prepared using Sodium Polyacrylate solutions and flow experiments were conducted on 3D-printed rough-walled fracture and smooth parallel-plate models. Grout flow through these models were numerically simulated based on the regularized Herschel–Bulkely–Papanastasiou (H–B–P) rheological model. Well-matched results were obtained between experimental and numerical results that validated the numerical method. The fracture surface was then decomposed into three levels using a wavelet analysis subject to different normal stresses to create a series of aperture structures for numerical simulations of the flow process. A regressively fitted function based on numerical results was developed that can reflect the deviation of grout flow through a rough-walled model from a parallel-plate model. This modified theoretical model was validated by applying it to predict the pressure gradient-flow rate relation of another rough-walled fracture independently. The high-resolution experimental and numerical results revealed the response of H–B fluid flow to the applied normal stress and roughness; the modified theoretical model can readily be used to predict H–B fluids flow through rough-walled rock fractures, which is useful for rock grouting analysis in engineering practice.

Fluid flow in rock fractures is significantly influenced by cyclic shear. This phenomenon arises due to seismic activity or repeated stress changes resulting from excavation, blasting, and operational loads. In this study, experiments are carried out to investigate non-Darcian flow in single rough fractures after cyclic shearing. The evolution of inertial and viscous permeability is analyzed, and a predictive model for non-Darcian flow is established. Cyclic shearing experiments are first conducted to examine shear characteristics and geometric variations, using four groups comprising 24 rough rock fractures. Subsequently, 360 non-Darcian flow experiments are performed to study the evolution of inertial and viscous permeability under cyclic shearing. It is observed that both types of permeability tend to decrease with an increasing number of shearing cycles. The most significant reduction occurs during the first cycle, followed by a slower decline that eventually stabilizes. A predictive model for non-Darcian flow is then developed, considering the geometry before shearing, rock properties, and cyclic shear characteristics. This model is validated against experimental data. Based on the proposed predictive model, a method for determining the critical number of shear cycles is also proposed. These findings contribute to understanding the evolution of non-Darcian flow in fractures subjected to seismic activity or repeated stress changes.

Non-Darcian flow in rock fractures exhibits significant anisotropic characteristics, which can be affected by mechanical processes, such as cyclic shearing. Understanding the evolution of anisotropic nonDarcian flow is crucial for characterizing groundwater flow and mass/heat transport in fractured rock masses. In this study, we conducted experiments on non-Darcian flow in single rough fractures under cyclic shearing conditions, aiming to analyze the anisotropic evolution of inertial permeability and viscous permeability. We established quantitative characterization models for the two types of permeability. First, we conducted cyclic shearing experiments on four sets of 24 rough rock fractures, investigating their shear characteristics. Then, we performed 480 non-Darcian flow experiments to analyze the anisotropic evolution of viscous permeability and inertial permeability of these rock fractures. The results showed that viscous permeability exhibited significant differences only in the orthogonal direction, while inertial permeability exhibited differences in both orthogonal and opposite directions. With increase in the shear cycles, the differences in the orthogonal direction gradually increased, while those in opposite direction gradually decreased. Finally, we established characterization equations for the two permeabilities based on the proposed directional geometric parameters and validated the performance of these equations with experimental data. These findings are useful for the quantitative characterization of the evolution of non-Darcian flow in fractures under dynamic loading conditions.

Significant differences in non-Darcian flow between different directions (i.e., forward and reverse flow directions) exist in rock fractures, and understanding of these differences holds crucial implications for evaluating and characterizing flow within fractured rocks. This study proposes a directional aperture parameter to quantitatively characterize the differences in flow between different directions. Firstly, a directional aperture parameter capable of quantitatively distinguishing geometric information of fractures in different directions is proposed. Then, 900 sets of linear and nonlinear flow numerical experiments based on 90 rough fractures are conducted. The results reveal that the differences between forward and reverse flow are shown in the nonlinear flow regime, with equal viscous permeability but significant differences in inertial permeability between the two flow directions. The main reason for the differences lies in the variations of aperture along the two flow directions. A dual-parameter model characterizing the inertial permeability is established by using the directional aperture parameter based on the numerical experimental data from the 90 rough fractures. The critical condition where the significant differences between the forward and reverse flow starting to appear are identified. The quantitative characterization of differences in three-dimensional rough fractures between different directional flows is discussed. The findings from this study could be helpful in advancing our understanding of fluid flow behaviors in natural rock fractures.

Inertial permeability is a critical parameter that quantifies the pressure loss caused by inertia in fluid flow through rough-walled fractures, described by the Forchheimer equation. This study investigates the size effect on the inertial permeability of rough-walled fractures and establishes a characterization model for fractures of varying sizes. Numerical simulations are conducted on five large-scale fracture models (1 m x 1 m) by resolving the Navier-Stokes equations. Smaller models are extracted from these large-scale fracture models for detailed size-dependent analysis. The results show that the peak asperity height (xi), asperity height variation coefficient (eta), and the fitting coefficient of the aperture cumulative distribution curve

Grouting is widely used in tunnel construction as a measure to reduce water seepage through rock fractures. Fresh cement-based grout often comes into contact with flowing water after being injected into rock fractures, especially in post-excavation grouting scenarios in rock tunnels or pre-excavation grouting in deep tunnels and remedial grouting in dam foundations. The flowing water can cause erosion of the fresh grout and viscous fingering in the grout, which reduces the efficiency of the grouting. In the present study, experimental tests using a simulated fracture were carried out to investigate grout erosion and viscous fingering in the time period after the injection stops until the grout has gained sufficient strength. The aim of the tests was to evaluate the validity of the existing criteria used to determine grout erosion and viscous fingering. The test results showed significant grout erosion and viscous fingering caused by the flowing water despite these behaviors not being expected according to the existing criteria. The reduction in the grouted area was up to 50% after 10 min and up to 64% after 60 min. Based on these results, the mechanism of grout erosion and viscous fingering between water and grout is discussed with respect to grouting design strategy. The present study provides a deeper understanding of grout erosion and viscous fingering after the grouting is completed, indicating complex mechanisms of these behaviors and oversimplification in the existing criteria. The results are useful for the design of grouting in fractures with flowing water.

Co-exploitation of coal and geothermal energy provides a solution for the efficient utilization of hydrothermal resources. To analyze the feasibility of this approach, a hydraulic erosion and heat transfer coupling model is established and validated through laboratory experiments. Then, the evolution of hydraulic and thermal characteristics within the reservoir is analyzed. Finally, the sensitivity of geothermal extraction efficiency is investigated, and the effect of hydraulic erosion on geothermal energy extraction is discussed. The simulation results indicate significant increases in porosity, fluidized particle volume fraction, permeability, and flow velocity near the production well. The hydraulic pressure within the fractured rock zone gradually decreases, and the reservoir temperature decreases non-linearly over time. At the end of extraction, the porosity near the production wall approaches 0.9, and the temperature is reduced to 304 K. Besides, the increase in extraction pressures, production well radii, initial reservoir temperatures, and decrease of cementing strengths of fractured rocks could cause earlier thermal power peaks, while they may also result in rapid depletion of geothermal resources. In addition, the proposed model exhibits significantly higher thermal power compared to the model disregarding hydraulic erosion effects, with the peak thermal power being 38 times greater than the latter.

Thermal stress can trigger slip in rock fractures. Understanding this thermally induced slip behavior of fractures in crystalline rock is crucial for the design and operation of enhanced geothermal systems, and deep geological repository. The present study aims to investigate mechanism and mechanical response of thermal-induced slip in a single fracture. We develop a FEM-based model to simulate the thermally induced slip in fractures with different surface roughness, and under different boundary conditions. The model is validated against a unique set of thermally induced slip test data in laboratory. The results show that thermal stress can cause a decrease in normal stress on the fracture surface, accompanied by an increase in shear stress. The mechanical boundary conditions and fracture roughness can significantly influence the fracture aperture evolution during the slip process. Specifically, results show that increasing the vertical stress can induce a transition from fracture closure to dilation (3.8% closure at z= 1 MPa versus 23.7% aperture increase at z= 50 MPa), while a higher normal stiffness suppresses slip and limits aperture changes. Additionally, rougher fractures exhibit greater closure (the roughest fracture exhibits 3.8% closure vs 1.0% for the smoothest), and neglecting fracture surface plasticity leads to an underestimation of fracture closure in the slip process.