Seepage Pressure Research Articles

Experimental Study of Influence of Karst Aquifer on the Law of Water Inrush in Tunnels

Water inrush is a geological hazard often encountered in tunnel construction. In order to overcome problems encountered when using the existing water-inrush simulation model test, such as single function, low repetition utilization rate, and poor visibility, we developed a multi-type water-inrush model test system. Our test system can be a precursor to large-scale studies, handle multiple types, and perform serialization and visualization. We conducted cavity water-inrush simulation testing using this device, and studied the failure model of aquifuge rock. We reveal the evolution laws of water inrush from the cavity, including the initiation of fracture in the aquifuge rock, the formation of water-inrush channels, and the rupture of the aquifuge rock. By analyzing the seepage pressure changes at the monitoring points of the aquifuge rock, we divided the water-inrush process of the cave into a rack generation stage, a holing-through of fracture and forming of water-inrush channel stage, and a water-inrush stage. Test results show that the system is stable and reliable, only requiring a short test period, and can be used to guide large water-inrush testing and related projects.

Analysis on the Precursor Information of Water Inrush in Karst Tunnels: A True Triaxial Model Test Study

Water inrush in karst tunnels usually results in casualties, equipment damage, project delays and other serious consequences. An important measure to ensure the safety of tunnel construction is to provide accurate prediction of water inrush during tunnel excavation. To study the stability of the surrounding rock under effect of a water-bearing cave in front of the tunnel face, a true triaxial geomechanical model test was carried out based on the engineering background of the Xiema Tunnel in Chongqing Province. The response regularities of multivariate physical information of the surrounding rock during rock failure and water inrush were obtained by monitoring the displacement, stress and seepage pressure of different monitoring sections. The results show that some internal connections exist among the kinds of physical information. When the tunnel face was adjacent to the disaster source, the change of displacement and stress was in a decreasing trend, and the seepage pressure increased first and then decreased during the tunnel excavation. In addition, by fusion analysis, the sensitivity priority of precursor information for a rock failure and water inrush disaster in karst tunnels was summarized as stress, displacement and seepage pressure. Finally, the theoretical basis of the prediction of water inrush in karst tunnels had been established, which proved the feasibility of predicting disasters according to the precursor information.

Shear-Flow Coupled Behavior of Artificial Joints with Sawtooth Asperities

The coupling between hydraulic and mechanical processes in rock joints has significantly influenced the properties and applications of rock mass in many engineering fields. In this study, a series of regular shear tests and shear-flow coupled tests were conducted on artificial joints with sawtooth asperities. Shear deformation, strength, and seepage properties were comprehensively analyzed to reveal the influence of joint roughness, normal stress, and seepage pressure on shear-flow coupled behavior. The results indicate that the shear failure mode, which can be divided into sliding and cutting, is dominated by joint roughness and affected by the other two factors under certain conditions. The seepage process makes a negative impact on shear strength as a result of the mutual reinforcing of offsetting and softening effects. The evolution of hydraulic aperture during the shear-flow coupled tests embodies a consistent pattern of four stages: shear contraction, shear dilation, re-contraction, and stability. The permeability of joint sample is considerably enlarged with the increase of joint roughness, but decreases with the addition of normal stress.

Numerical Analysis of Surrounding Rock Stability in Super-Large Section Tunnel Based on Hydro-Mechanical Coupling Model

The groundwater seepage has an important effect on the stability of tunnel surrounding rock. Considering the effect of seepage, a hydro-mechanical coupling mathematical model is developed based on stress deformation equation of rock mass, flow equation of porous media and relation equation of hydro-mechanical coupling. In connection with the leakage situation of Jiangshuiquan Tunnel with super-large section, a geometry model of tunnel construction is built by COMSOL software platform. Furthermore, the hydro-mechanical coupling model is embedded in the PDE module of the software for numerical simulating the construction process of super-large section tunnel. Consequently, the surrounding rock displacement deformation under is discussed and verified by the data measured in field. Besides, the distribution characteristics of surrounding rock stress, seepage pressure and velocity are deeply analyzed. Results indicate that the displacement calculated by hydro-mechanical coupling model is well consistent with the site monitoring data and is greatly more than that of uncoupling model. After tunnel excavation, stress concentration phenomenon occurs around the cavern, especially at the arch foot part of the outer side wall, so the parts should be viewed as the key supporting part. The groundwater flows into the cavern and the surrounding pore water pressure is unloaded. A large hydraulic gradient occurs in the vault of the tunnel, prone to incur major engineering accidents; while the hydraulic gradient at vault is relatively small and usually has little harmful to engineering safety. The conclusion may provide guidelines for similar tunnel engineering construction.

The Micro Damage Model of the Cracked Rock Considering Seepage Pressure

The damage constitutive model and the evolution characteristics of the cracked rock, considering seepage pressure, are critical to the stability of the cracked rock engineering. Based on the Rice Theory of Thermodynamics and the friction-bending crack model, we proposed the damage evolution model for the cracked rock in the compressive-shear state, considering the seepage pressure, the initial damage of the cracked rock, and the accumulated damage caused by crack evolution. According to the influences of the normal deformation and the branched crack (caused by the seepage pressure and the normal stress) on the opening of the crack, we established an evolution equation for the seepage tensors with the accumulation of the crack damage. In addition, based on the visual development platform, VB 6.5, we developed the software, WFRD2D, to comprehensively calculate the nonlinear damage deformation, the strength and the seepage indexes of the cracked rock, subjected to seepage pressure. Then, we obtained the pre-peak stress–strain curve of the cracked iherzolite rock using WFRD2D. The results show that the rock strength decreased by 48%, whereas the lateral strain increased by 240%, considering the seepage pressure of 3 MPa. These phenomena indicate that seepage pressure damages the cracked rock, then, further decreases the rock strength and increases the lateral strain. Thus, the seepage pressure significantly affects the mechanical properties of the cracked rock.

Experimental Study on the Creep Characteristics of Coal Measures Sandstone under Seepage Action

The seepage action of underground water accelerates the deformation of roadway surrounding rock in deep mines. Therefore, the study of creep characteristics of surrounding rock under seepage action is the basis for the stability control of roadway surrounding rock in deep water-rich areas. In this paper, a seepage-creep coupling test system for complete rock samples was established. Combined with a scanning electron microscopy (SEM) test system, the seepage-creep law of coal measures sandstone and the damage mechanism were revealed. The study results showed that the maximum creep deformation of sandstone under natural and saturation state decreased gradually with the increase of confining pressure, and the maximum creep deformation under saturation state was greater than the corresponding value under natural state when the confining pressure was same. When the confining pressure was constant, the creep deformation, the constant creep deformation rate and the accelerated creep deformation rate of sandstone increased rapidly with the increase of infiltration pressure. With the change of time, the change of permeability parameters went through three cycles; each cycle was divided into two stages, slow change stage and rapid change stage, and the rate of variation increased with the increase of the seepage pressure. Based on the macroscopic and microscopic characteristics of sandstone rupture, the connection between macroscopic and microscopic mechanism on sandstone rupture was established. The results in this paper can provide a theoretical basis for stability control of roadway surrounding rock in water-rich areas.

Influence Analysis of Mining’s Effect on Failure Characteristics of a Coal Seam Floor with Faults: A Numerical Simulation Case Study in the Zhaolou Coal Mine

A fluid–solid coupled numerical simulation was carried out for the Zhaolou coal mine using Flac3D software. The results showed that: (1) when the failure depth of the floor exceeded the extension depth of the floor rock mass and the hydrostatic seepage pressure of the fractured structure surface exceeded the stress state of the fractured element, floor water inrush will occur. (2) With other factors being equal, if the footwall is mined first, the pore pressure concentration near the fault is more obvious, and the floor-confined water is lifted 10–25 m higher than when the hanging-wall is mined first. Mining the floor adversely affects its plastic zone, and the failure depth is ≈ 10 m greater than when the hanging wall is mined first. (3) When advancing against the fault dip, the pore pressure concentration near the fault is more obvious, and the floor confined water is lifted 5–20 m higher than when the mine advances along the fault dip. Again, mining the floor first disrupts the floor plastic zone, and the failure depth is approximately 5 m greater than when the mine is advanced along the fault dip. (4) The risk of floor-water inrush is minimized when the hanging-wall is mined first and the mine advances along the fault dip; the risk is greatest when the footwall is mined first and is advanced against the fault dip. These results provide a theoretical basis for preventing mine water inrush.

Mudcake effects on wellbore stress and fracture initiation pressure and implications for wellbore strengthening

Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure (FIP) either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near-wellbore region, rather than by mudcake strength. Fluid loss pressure (FLP) should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake.

A nonstationary parameter model for the sandstone creep tests

The rock masses of hydro-fluctuation belt experience seepage pressure following impoundment in the Three Gorges Reservoir; its creep behaviors are significant for reservoir bank slopes. To study the creep behaviors under seepage pressure (0, 1.45, and 1.75 MPa), we performed creep tests using representative landslide sandstone in the Three Gorges Reservoir and investigated the sandstone creep behaviors under the coupling effects of seepage pressure and stress. Previous researches on rocks have usually regarded the creep constitutive parameter as a constant; however, in this study, a nonlinear, nonstationary, plastic-viscous (NNPV) creep model which can describe the curve of sandstone creep tests is proposed. The rock-creep parameters under three levels of seepage pressure were identified, and theoretical curves using the NNPV model agreed well with the experimental data, indicating that the new model cannot only describe the primary creep and secondary creep stages under varying seepage pressures but also, in particular, perfectly describes the tertiary creep stage. Finally, the sensitivity of the NNPV model parameters is analyzed, and the result shows that the nonstationary coefficient α and the nonlinear coefficient m are main parameters affecting the tertiary creep stage.

Long-term kinematics and mechanism of a deep-seated slow-moving debris slide near Wudongde hydropower station in Southwest China

Long-term kinematic research of slowmoving debris slide is rare despite of the widespread global distribution of this kind. This paper presents a study of the kinematics and mechanism of the Jinpingzi debris slide located on the Jinsha river bank in southwest China. This debris slide is known to have a volume of 27×106 m3 in active state for at least one century. Field survey and geotechnical investigation were carried out to define the structure of the landslide. The physical and mechanical properties of the landslide materials were obtained by in-situ and laboratory tests. Additionally, surface and subsurface displacements, as well as groundwater level fluctuations, were monitored since 2005. Movement features, especially the response of the landslide movement to rainfall, were analysed. Relationships between resisting forces and driving forces were analysed by using the limit equilibrium method assuming rigid-plastic frictional slip. The results confirmed a viscous component in the long-term continuous movement resulting in the quasioverconsolidated state of the slip zone with higher strength parameters than some other types of slowmoving landslides. Both surface and subsurface displacements showed an advancing pattern by the straight outwardly inclined (rather than gently or reversely inclined) slip zone, which resulted in low resistance to the entire sliding mass. The average surface displacement rate from 2005 to 2016 was estimated to be 0.19~0.87 mm/d. Basal sliding on the silty clay seam accounted for most of the deformation with different degrees of internal deformation in different parts. Rainfall was the predominant factor affecting the kinematics of Jinpingzi landslide while the role of groundwater level, though positive, was not significant. The response of the groundwater level to rainfall infiltration was not apparent. Unlike some shallow slow-moving earth flows or mudslides, whose behaviors are directly related to the phreatic groundwater level, the mechanism for Jinpingzi landslide kinematics is more likely related to the changing weight of the sliding mass and the downslope seepage pressure in the shallow soil mass resulting from rainfall events.

Safety assessment for seawall based on constrained maximum entropy projection pursuit model

The safety of seawalls is crucial. A scientific and effective safety evaluation method for seawalls is an important measure to protect seawall safety. Based on the monitoring information, seepage pressure, tide level, and rainfall were taken as the safety assessment indexes of seawalls. The projection pursuit model was improved by taking the primary and secondary relations of the assessment indexes known in the professional field as constraints. The water cycle algorithm was used to solve the projection vector, and the weights of the safety assessment indexes of the seawalls were calculated using the improved projection pursuit model. The safety assessment method of seawalls was established by Fuzzy Theory. A method where the maximum membership degree principle was replaced by the comprehensive safety grade value was put forward to determine the seawall safety grade. The analysis of examples showed that the seawall safety assessment results of the constrained maximum entropy projection pursuit model conformed to the objective reality, and were more scientific and reasonable than the maximum entropy projection pursuit model and the analytical hierarchy process model.

Experimental Study on the Permeability Characteristics of Sandstone in Different Chemical Solutions

Groundwater seepage is one of the important factors which affect the physical and mechanical properties of soft rock. In this study, the permeability evolution of sandstone was investigated using water solution seepage tests with different pH values. The results show that the chemical nature of the solutions has a great influence on the evolution of the permeability coefficients of sandstone specimens. Over time, it was determined that the permeability coefficient under the solution with PH = 2 was significantly greater than those under the PH = 7 and PH = 12 solutions used in this study. During the infiltrations, the ions of H+ and OH- which were involved in the water-rock interaction made the pH value of the solution change with different degrees. After the seepage, the ions in the solutions changed significantly. The ion concentrations seeping from sandstone specimen in the acidic solution, such as Ca2+, Mg2+, Fe3+ (Fe2+), Al3+ and K+, were found to be much greater than in the neutral and alkaline solutions. However, the ion concentrations seeping from sandstone specimen in the neutral solution displayed little differences from those in the alkaline solution. The concentration of the Si ion in the alkaline solution was much higher than those of the acid and neutral solutions, and the concentration of the acid solution was slightly higher than that of the neutral solution. The changes in the ion concentration in the osmotic solution reflected that there had been different forms of chemical reactions between the minerals of the sandstone and the solutions. The seepage pressures also had some effects on the permeability coefficients, pH values, and concentrations of ions in the solutions. After the seepage tests, the rock microstructures were observed by the means of a Scanning Electron Microscope (SEM). The results show that some new holes have been formed in the sandstone specimen due to the seepage, especially in the sample with the acid solution. The internal structural changes of the sandstone are found to be the main reasons for the changes in the permeability coefficients.

Analysis of the lag effect of embankment dam seepage based on delayed mutual information

Environmental variables, such as the upstream level, significantly affect the embankment dam seepage lag time. The lag effect should, therefore, be given adequate consideration when determining the saturation line and establishing a mathematical model for seepage. At present, however, the lag time is mainly estimated qualitatively. A method for analyzing the seepage lag effect is presented herein based on the mutual information principle of information science theory, wherein the upstream water level is considered an information source and water level of the seepage pressure is considered an information function point. The method explores application of the directed information transfer index (DITI) of mutual information in order to establish an information transfer model between seepage levels of measurement points and upstream levels of the embankment dam. Furthermore, an extreme value of the DITI function is adopted as the judging criterion to determine the lag time of the influence of the upstream level on corresponding seepage levels. Results obtained through analysis of an engineering case study, demonstrate the effectiveness and feasibility of the proposed method.

An Experimental Study on Cracking Behavior of Precracked Sandstone Specimens under Seepage Pressure

This paper aims to investigate the strength and failure mechanism of fractured rock under seepage pressure. For this purpose, precracked sandstone specimens were prepared with different fissure angles, and a seepage pressure loading device was created. Together with the acoustic emission (AE) system, the loading device was adopted to perform uniaxial compression tests with or without seepage pressure. The main results are as follows. Combined with axial stress‐strain curves, photographic monitoring results and the output of AE counts and rock failure process can be generally divided into four stages: microcrack closure, elastic deformation, crack growth and propagation, and final failure. The seepage pressure had a significant effect on the mechanical properties of the specimens: the specimens under seepage pressure lagged far behind those without seepage pressure in peak strength but maintained a comfortable lead in peak strain. Under seepage pressure, the typical failure features of the specimens varied with the fissure angles: the specimens with small fissure angles (i.e., [0°,30°]) mainly underwent tensile failure; those with medium fissure angles (i.e., [30°,60°]) suffered from shear failure; and those with large fissure angles (i.e., [60°,75°]) were prone to tensile‐shear failure.

Structural Analysis of Offshore Cofferdam Subjected to Wave Load and Suction Pressure

This study presented a novel circular cofferdam for offshore bridges consisting of ring segments and investigated its structural behaviors using ANSYS Mechanical. Because the bottom segment of the cofferdam which has a double sleeve cross-section was installed by suction, contact behaviors of the cofferdam wall and the lid plate during installation are important for design and the behaviors were also analyzed. Prior to numerical investigation of the bottom segment and complete cofferdam after dewatering, a suction modeling for structural analysis was proposed and evaluate by seepage analysis. Hydrodynamic loads applied to the cofferdam were also evaluated using panel method based on the potential flow theory. Through numerical analyses, structural behaviors of the cofferdam during installation were then investigated. First, contact behaviors between the lid plate and the wall were analyzed using different contact conditions imposed on the interface. Sharp stress increases were shown while the stress jumps were limited to contact area. Next, structural behaviors were investigated by considering seepage pressure. Using an axisymmetric seepage model, the total water pressure considering seepage was estimated and applied to structural analysis. The analysis results showed that strong effects of seepage on the stress change in cofferdam occurred and the seepage effects are necessary to be considered in design of the cofferdam induced by suction.

Simulation Analysis on Water’s Micro Seepage Laws under Different Pressure Gradients Using Computed Tomography Method

The aim of this paper was to develop a model that can characterize the actual micropore structures in coal and gain an in‐depth insight into water’s seepage rules in coal pores under different pressure gradients from a microscopic perspective. To achieve this goal, long‐flame coals were first scanned by an X‐ray 3D microscope; then, through a representative elementary volume (REV) analysis, the optimal side length was determined to be 60 μm; subsequently, by using Avizo software, the coal’s micropore structures were acquired. Considering that the porosity varies in the same coal sample, this study selected four regions in the sample for an in‐depth analysis. Moreover, numerical simulations on water’s seepage behaviors in coal under 30 different pressure gradients were performed. The results show that (1) the variation of the simulated seepage velocity and pressure gradient accorded with Forchheimer’s high‐velocity nonlinear seepage rules; (2) the permeability did not necessarily increase with the increase of the effective porosity; (3) in the same model, under different pressure gradients, the average seepage pressure decreased gradually, while the average seepage velocity and average mass flow varied greatly with the increase of the seepage length; and (4) under the same pressure gradient, the increase of the average mass flow from the inlet to the outlet became more significant under a higher inlet pressure.

Study on surface adhesion of Plasma modified Polytetrafluoroethylene hollow fiber membrane

Polytetrafluoroethylene (PTFE) is popular membrane material because of its excellent thermal stability, chemical stability and mechanical stability. However, the low surface energy and non-sticky property of PTFE present challenges for modification. In the present study, plasma treatment was performed to improve the surface adhesion of PTFE hollow fiber membrane. The effect of discharge voltage, treatment time on the adhesion of PTFE hollow fiber membrane was symmetrically evaluated. Results showed that the plasma treatment method contributed to improve the surface activity and roughness of PTFE hollow fiber membrane, and the adhesion strength depend significantly on discharge voltage, which was beneficial to seepage pressure of PTFE hollow fiber membrane module. The adhesion strength of PTFE membrane by plasma treated at 220V for 3min reached as high as 86.2 N, far surpassing the adhesion strength 12.7 N of pristine membrane. Furthermore, improvement of content of free radical and composition analysis changes of the plasma modified PTFE membrane were investigated. The seepage pressure of PTFE membrane by plasma treated at 220V for 3min was 0.375 MPa, which means that the plasma treatment is an effective technique to improve the adhesion strength of membrane.

Reliability analysis of tunnel roof in layered Hoek-Brown rock masses

This paper develops a two-layer model and upper bound solutions for the shape of collapse block in rectangular tunnel subjected to seepage pressure, and then the reliability-based analysis is performed. According to the layered Hoek-Brown rock masses with different characteristics, a continuous failure mechanism of roof collapse in the deep cavities is derived by using upper bound solutions. The performance function of roof stability is proposed and is adopted for reliability analysis and design. Parameters involved in the performance function are random or deterministic variables. For comparison, improved response surface method (RSM) and Monte-Carlo simulation are utilized to calculate the Hasofer-Lind reliability index and the failure probability. It is found that the improved RSM is a reliable method in probabilistic calculation and the supporting pressure has a significant influence on the reliability index. The normal or lognormal distribution of variables has no effect on the failure probability, while the coefficient of variation (COV) of random variables would greatly influence the failure probability. A reliability-based design is performed to determine the probabilistic tunnel pressure for a target reliability index when different COVs of random variables are considered.

Numerical study on seepage flow in pervious concrete based on 3D CT imaging

Pervious concrete specimens are prepared for CT (Computed Tomography) imaging and numerical simulation to observe pore characteristics and study law of seepage flow. 3D reconstruction based on CT imaging is utilized to generate virtual 3D pervious concrete structures, extract the connected pore network models, and analyze the target porosity, actual porosity, 3D porosity etc. Pore characteristics analysis shows the 3D porosity, and average planar porosity can generally represent the actual porosity, and 3D connected porosity is close to the effective porosity. The connected pore network models are used to simulate the water seepage flow using CFD (Computed Fluid Dynamics) method. The seepage flow simulation shows the permeable capacity increases clearly with increasing porosity under the same inlet pressure. The relationship between seepage velocity and pressure gradient is obtained and fitted by the nonlinear Forchheimer formula. The acquired hydraulic conductivity can be used to assess the seepage flow capacity.

Numerical analysis and fluid-solid coupling model test of filling-type fracture water inrush and mud gush

The geological conditions surrounding the Jijiapo Tunnel of the Three Gorges Fanba Highway project in Hubei Province are very complex. In this paper, a 3-D physical model was carried out to study the evolution process of filling-type fracture water inrush and mud gush based on the conditions of the section located between 16.040 km and 16.042 km of the Jijiapo Tunnel. The 3-D physical model was conducted to clarify the effect of the self-weight of the groundwater level and tunnel excavation during water inrush and mud gush. The results of the displacement, stress and seepage pressure of fracture and surrounding rock in the physical model were analyzed. In the physical model the results of the model test show that the rock displacement suddenly jumped after sustainable growth, rock stress and rock seepage suddenly decreased after continuous growth before water inrushing. Once water inrush occured, internal displacement of filler increased successively from bottom up, stress and seepage pressure of filler droped successively from bottom up, which presented as water inrush and mud gush of filling-type fracture was a evolving process from bottom up. The numerical study was compared with the model test to demonstrate the effectiveness and accuracy of the results of the model test.