Pore Water Pressure Gradient Research Articles

Mud pumping in high-speed railway: in-situ soil core test and full-scale model testing

Mud pumping induced by moving train loads on rainwater-intruded roadbed causes intensive track vibrations and threatens safety of high-speed trains. In this paper, a vehicle–track–subgrade finite element model was established to analyze the dynamic responses of a ballastless track, and results showed that the concrete base and roadbed were detached because of the whipping effect arising from the rainwater intrusion channel. An in-situ soil core test showed that the intruded rainwater accumulated in roadbed to form standing water and saturated the roadbed. The flapping action of the concrete base caused by the whipping effect led to mud formation mixed with fine particles and rainwater, which migrated upward under the pore-water pressure (PWP) gradient. Mud pumping resulted from continuous particle migration in the saturated roadbed under moving train loads: under normal roadbed condition, coarse and fine particles were uniformly distributed in the roadbed; in early period of mud pumping, fine particles migrated downward to bottom of the roadbed because of the rainwater infiltration flow; in middle stage of mud pumping, fine particles migrated upward and gathered at the roadbed surface under PWP gradient; in later period of mud pumping, fine particles were entrained and removed with the dissipation of excess PWP. Moreover, a full-scale physical model was established to reproduce mud pumping, and polyurethane injection remediation against mud pumping was validated on this physical model. The remediation method was applied to an in-situ mud pumping. The deviation of the vertical track profile reduced remarkably and remained at a low level within half a year, showing a good long-term service performance of the polyurethane remediated roadbed.

A Copula-Based Simulation of Wave-Induced Pore Water Pressure Gradient and Local Acceleration Within Surf Zone for Natural and Laboratory Barred Beach Profiles

Wave-induced pressure gradients and local accelerations are important interconnected physical mechanisms involving several hydrodynamic and morphodynamic coastal phenomena. Therefore, to provide a reliable and realistic hydrodynamic and morphodynamic simulation, the dependencies among different parameters, such as water level, pressure gradient, local acceleration, and sediment concentration, should be considered. Herein, a copula-based simulation is presented for modeling multivariate parameters and maintaining their statistical characteristics within the surf zone. Archimedean and elliptical copula families are applied to investigate the dependency construction between the parameters in two case studies: one from a field site on the east coast of Japan, and another from a large-scale laboratory barred beach profile. The dependency between variables is evaluated using Kendall’s τ correlation coefficient. The water level, pressure gradient, and local acceleration are shown to be significantly correlated. The correlation coefficients between the variables for the natural beach are lower than the laboratory data. The marginal probabilistic distribution functions and their joint probabilities are estimated to simulate the variables using a copula approach. The performance of the simulations is evaluated via the goodness-of-fit test. The analysis shows that the laboratory data are comparable to the field measurements, implying that the laboratory simulation results can be applied universally to model multivariable joint distributions with similar hydrodynamic conditions.

Experimental investigation on hydraulic fracturing in cement mortar with tensile stress

A novel device is proposed to test hydraulic fracturing in specimen with tensile stress. The device is composed of splitting apparatus and sealing apparatus. The tensile stress in specimen is realized via splitting load. It is confirmed that the sealing apparatus has a less effect on peak splitting load and the Young’s modulus of specimen. Some hydraulic fracturing experiments of cement mortar specimens with tensile stress are conducted. Critical water pressure of hydraulic fracturing will decrease with increase of the tensile stress. Hydraulic fracturing mechanism of cement mortar with tensile stress is revealed based on the Biot’s theory and fracture process zone. Applied high water pressure on initial crack surface, water pressure in water flow zone, and drag force along initial crack surface and water flow zone have a promotional effect on fracture propagation. The T stress in the test and pore water pressure caused by tensile strain near crack tip will restrain fracture propagation. A critical water pressure model (CWP) is proposed based on general maximum tangential stress criterion, and it is verified that the CWP model is rational and accurate. The T stress and pore water pressure gradient along polar angle direction have an effect on direction of crack initiation.

An experimental study of the relationship between the matric potential, unfrozen water, and segregated ice of saturated freezing soil

Frost heave damage is caused by the in situ freezing of pore water and segregated ice due to moisture migration. Previous studies have shown that in freezing soil, moisture migration is related to the pore water pressure gradient, and moisture migration is the dominant reason for the formation of an ice lens. However, the essential relationship between ice segregation and the matric potential is still controversial. Using a pF meter sensor, a 5TM volume water content sensor, and a digital image capture system, the relationship between the matric potential and the unfrozen water in saturated freezing soil was monitored in real time, and digital images of the formation of segregated ice were collected. Furthermore, the time space coupling relationships between the unfrozen water, the matric potential, the frost-heaving amount, the moisture migration, and the formation of an ice lens were systematically analyzed during soil freezing. The results demonstrate that there is an internal relationship between the moisture migration driven by the matric potential (from the micro-perspective) and the segregated ice layer effect (from the macroscopic perspective). In addition, the unfrozen water and the matric potential in the frozen area have a significant impact on the distribution of the segregated ice lenses.

Mathematical modeling of consolidation in unsaturated poroelastic soils under fluid flux boundary conditions

The spatiotemporal variability in precipitation could have a significant impact on ground subsidence. In this paper, we utilize a previously developed one-dimensional theory of consolidation for a two-fluid system, the generalization of Biot's theory, to investigate the impact of a time-dependent fluid flux across the surface of an unsaturated soil on the solid deformation and pore pressure responses. We consider three prescribed fluid flux patterns and the associated gradient of pore water pressure: constant, periodic, and exponential flux patterns. We employ a Fourier series representation and Laplace transformation in the space and time domains to derive a complete set of closed-form analytical solutions, including the complementary and particular solutions, describing both transient and steady-state excess pore water and pore air pressure responses as well as the total settlement induced by the fluid flux. Using two different types of soil, i.e. clay and sand, we demonstrate the dependence of the solution on the initial water saturation and hydraulic conductivity in the saturated and unsaturated regimes. Results of this study reveal that the fluid flux across the surface could have a significant impact on the dissipation of excess pore water pressure and the time evolution of the total settlement of the ground surface. The effect on the pore water pressure is more significant for sand than for clay irrespective of moisture content, mainly because of the much higher hydraulic conductivity of sand. Finally, we show that the hydraulic conductivity has a dominant effect on the consolidation behaviors of saturated and unsaturated soils subjected to surface fluid flux.

Pore Water Pressure Response during Tsunami Loading

AbstractTsunamis induce excess pore water pressure gradients in coastal sediment beds. The excess pore water pressure gradient induced during tsunami loading along with the shear stress caused by t...

Dimensional analysis of pore-water pressure response in a vegetated infinite slope

Pore-water pressure (PWP) induced by root water uptake has usually been investigated by individual physical quantities. Limited dimensional analysis has been available for investigating PWP response in a vegetated slope. In this study, dimensional analysis was conducted to explore dimensionless numbers controlling PWP distributions in a vegetated slope. Three dimensionless numbers governing unsaturated seepage were proposed, including capillary effect number (CN, describing the relative importance of water flow driven by PWP gradient over that driven by gravity), root water uptake number (RN, representing the effects of root water uptake), and water transfer–storage ratio (WR, ratio of water transfer to water storage rate). Dimensionless relationships were further proposed to estimate PWP and root influence zone in a vegetated slope. Then analytical parametric studies were conducted to study effects of RN, CN, and WR on PWP distributions. Thereafter, the proposed relationships were validated by published field and centrifuge tests. During the drying period, the effects of root water uptake on PWP and the root influence zone become more significant as CN decreases or RN increases. During the wetting period, the larger the WR, the deeper the wetting front moves and more reduction of negative PWP occurs. The proposed dimensionless relationships can determine PWP and the root influence zone in a vegetated soil reasonably well.

Mechanisms of Crack Initiation and Propagation in Dense Linear Multihole Directional Hydraulic Fracturing

Artificially fracturing coal‐rock mass serves to form break lines therein, which is related to the distribution of cracked boreholes. For this reason, we use physical experiments and numerical simulations to study the crack initiation and propagation characteristics of dense linear multihole drilling of fractured coal‐rock mass. The results indicate that only in the area between the first and last boreholes can hydraulic fracturing be controlled by dense linear multihole expansion along the direction of the borehole line; in addition, no directional fracturing occurs outside the drilling section. Upon increasing parameters such as the included angle θ between the drilling arrangement line and the maximum principal stress σ1 direction, the drilling spacing D, the difference Δσ in principal stress, etc., the effect of directional fracture is gradually weakened, and the hydraulic fractures reveal three typical cracking modes: cracking along the borehole line, bidirectional cracking (along the borehole line and perpendicular to the minimum principal stress σ3), and cracking perpendicular to σ3. Five propagation modes also appear in sequence: propagating along borehole line, step‐like propagation, S‐shaped propagation, bidirectional propagation (along the borehole line and perpendicular to σ3), and propagation perpendicular to σ3. Based on these results, we report the typical characteristics of three‐dimensional crack propagation and discuss the influence of the gradient of pore water pressure. The results show clearly that crack initiation and propagation are affected by both the geostress field and the pore water pressure. The pore water pressure will exhibit a circular‐local contact‐to‐integral process during crack initiation and expansion. When multiple cracks approach, the superposition of pore water pressure at the tip of the two cracks increases the damage to the coal rock, which causes crack reorientation and intersection.

Finite element analysis of biological soft tissue surrounded by a deformable membrane that controls transmembrane flow

BackgroundMany biological soft tissues are hydrated porous hyperelastic materials, which consist of a complex solid skeleton with fine voids and fluid filling these voids. Mechanical interactions between the solid and the fluid in hydrated porous tissues have been analyzed by finite element methods (FEMs) in which the mixture theory was introduced in various ways. Although most of the tissues are surrounded by deformable membranes that control transmembrane flows, the boundaries of the tissues have been treated as rigid and/or freely permeable in these studies. The purpose of this study was to develop a method for the analysis of hydrated porous hyperelastic tissues surrounded by deformable membranes that control transmembrane flows.ResultsFor this, we developed a new nonlinear finite element formulation of the mixture theory, where the nodal unknowns were the pore water pressure and solid displacement. This method allows the control of the fluid flow rate across the membrane using Neumann boundary condition. Using the method, we conducted a compression test of the hydrated porous hyperelastic tissue, which was surrounded by a flaccid impermeable membrane, and a part of the top surface of this tissue was pushed by a platen. The simulation results showed a stress relaxation phenomenon, resulting from the interaction between the elastic deformation of the tissue, pore water pressure gradient, and the movement of fluid. The results also showed that the fluid trapped by the impermeable membrane led to the swelling of the tissue around the platen.ConclusionsThese facts suggest that our new method can be effectively used for the analysis of a large deformation of hydrated porous hyperelastic material surrounded by a deformable membrane that controls transmembrane flow, and further investigations may allow more realistic analyses of the biological soft tissues, such as brain edema, brain trauma, the flow of blood and lymph in capillaries and pitting edema.

Study on theory model of hydro-thermal–mechanical interaction process in saturated freezing silty soil

Abstract The freezing of frost susceptible soils is a dynamic hydro-thermal–mechanical (THM) interacting process. One-side freezing experiments of saturated soil in an open system with no-pressure water supplement are carried out. In these experiments, we analyzed the influence of temperature gradients, overburden pressures and cooling temperatures on moisture migration and frost heave. Based on these experiments, a mathematical model of frost heave is proposed with the variables of temperature, porosity and displacement, in which Clapeyron equation is employed as the phase equilibrium condition of water and ice in soil. Ice lens is one of the major aspects of the frost heave for frost susceptible soils. According to the mechanical and physical characteristics, a comprehensive criterion for the formation and end of new ice lens is presented. To solve the nonlinear equations, the finite element algorithm is applied to solve the general form of governing equations. Finally, numerical simulations are implemented with the assistance of COMSOL. Validation of the model is illustrated by comparisons between the simulation and experimental results. From this study, it is found that, (1) cooling temperature is the necessary condition for moisture migration and frost heave since pore water phases into ice under cooling temperature. Then negative pore water pressure occurs in soil. Pore water pressure gradient is the direct driving force for water migration in saturated soil. However, temperature gradient and overburden pressure have important influence on the pore water pressure gradient. The response of soil sample to the variation of water content lags behind the response to the change of temperature. (2) Discontinuously distributed ice lenses form near the cold front when the accumulated water phases to ice. Temperature gradient, overburden pressure and cooling temperature are key factors to determine the frost heave and moisture migration. (3) Freezing and migration of unfrozen water cause the change of porosity in soil. Ice lens will block the migration of unfrozen water. The discrete ice lenses result in the oscillation in the distributions of water content. (4) Unsaturated phenomenon occurs in the unfrozen zone under the frozen fringe which might be related to the suction effect of large negative pressure in this zone.

Modelling of the hydro-mechanical response of sedimentary rocks of southern Ontario to past glaciations

Glaciation is considered as one of the main natural processes that can have a significant impact on the long term performance of a deep geological nuclear waste repository (DGR) located in the Northern Hemisphere. The northern part of the American continent has been subjected to a series of strong glaciation and deglaciation events over the past million years. The last glacial cycle in the Northern Hemisphere started approximately 110,000 year ago. During that cycle, southern Ontario was buried under a continental ice sheet, with a maximum thickness of up to 3000 m at about 20,000 years ago. The ice cap retreated approximately 10,000 years ago. However, field data from deep boreholes in sedimentary rocks of southern Ontario show anomalous pore water pressure including underpressure and overpressure zones. In this paper, a large-scale coupled hydro-mechanical (HM) model is developed to investigate the hydro-mechanical (HM) response of the sedimentary rocks of southern Ontario to past glacial cycles. Particular emphasis has been placed on the evolution of pore water pressures and surface displacements. The HM model is verified using analytical solutions. The results of the large-scale HM modeling study shows that the past glaciation, particularly the second cycle (22,000 apb) had significant impact on the pore water pressure gradient and effective stress distribution in the sedimentary rocks of southern Ontario. Furthermore, good agreement between the large scale modeling results and anomalous pressures leads us to the conclusion that these anomalies could be glacially induced. The results of this research can provide information that will contribute to a better understanding of the impact of future glaciations on the long term performance of DRGs in sedimentary rocks.

Numerical study on the interaction among a nonlinear wave, composite breakwater and sandy seabed

Most previous investigations related to composite breakwaters have focused on the wave forces acting on the structure itself from a hydrodynamic aspect. The foundational aspects of a composite breakwater under wave-induced cyclic loading are also important in studying the stability of a composite breakwater. In this study, numerical simulations were performed to investigate the wave-induced pore water pressure and flow changes inside the rubble mound of the composite breakwater and seabed foundation. The validity and applicability of the numerical model were demonstrated by comparing numerical results with existing experimental data. Moreover, the present model clearly has shown that the instantaneous directions of pore water flow motion inside the seabed induced by surface waves are in good agreement with the general wave-induced pore water flow inside the seabed. The model is further used to discuss the stability of a composite breakwater, i.e., the interaction among nonlinear waves, composite breakwater and seabed. Numerical results suggest that the stability of a composite breakwater is affected by not only downward shear flow generating on the seaward slope face of the rubble mound but, also, a high and dense pore water pressure gradient inside the rubble mound and seabed foundation.

Pore Pressure and Viscous Shear Stress Distribution Due to Water Flow within Asphalt Pore Structure

Abstract: There has been a concentrated effort in modeling fluid flow in asphalt pavements, and most of the existing work has been limited to evaluation of directional permeabilities. The effects of other phenomena, such as development of flow‐induced pore pressure and viscous shear stress distributions at the pore–solid interfaces, on moisture damage in an asphalt pore structure have not been fully understood. A three‐dimensional (3D) numerical fluid flow model was developed using the lattice Boltzmann (LB) method to simulate water flow within the pore structures of various hot mix asphalt (HMA) pavements. In this article, an in‐depth analysis of variations of pore pressures and viscous shear stresses caused by water flow within the 3D pore structure of different HMA specimens is provided. The relationship between the pore geometry and the pore water pressure and viscous shear stresses were studied. Pore pressures and viscous shear stresses were computed after 3D fluid flow simulations conducted within the real pore structures of numerous HMA specimens. X‐ray computed tomography (CT) was used to acquire 3D pore structures of HMA specimens, and the obtained images were input into the LB model. The results indicated that the pore water pressure gradient is highly nonlinear within the pore structure of HMA pavements, as opposed to a linear gradient commonly observed in homogeneous pore structures (e.g., granular soils). The viscous shear stresses were observed to be the largest at the constrictions located at the mid‐depth of the specimens. Furthermore, a one‐to‐one relationship was observed between the reduction in the pore area and viscous shear stresses developed during the water flow.

The effects of rock fragments on hydrologic and hydraulic responses along a slope

AbstractA model describing the three‐dimensional matrix flow along a slope with rock fragments or impermeable blocks was developed. The model was combined with modified Picard's iteration to ensure mass conservation in the unsaturated flow. We found that rock fragments obstruct water flow along the slope. The groundwater table must be raised to provide a sufficient pore water pressure gradient to facilitate water flow, but higher pore water pressure may induce slope failure. We also conducted a bench‐scale laboratory flume experiment to examine the effects of impermeable blocks on downstream seepage flow. In addition, a numerical experiment was conducted to examine how different arrangements of impermeable blocks affect downstream seepage flow and pore water pressure. This research demonstrated that the hydraulic phenomena were affected when impermeable blocks were present, and pore water pressure increased as the position of impermeable blocks was lowered. Copyright © 2006 John Wiley & Sons, Ltd.

Formation of Thrust Structures in Front of Coastal Landslides

Abstract Small rotational landslides developed on the Quaternary glacial till coast of West Runton, north Norfolk (eastern England), show small‐scale thrust structures that formed in front of the landslide toe. The thrust sheets (a few tens of centimeters in breadth) developed in unconsolidated beach sand in the upper intertidal zone and may be laterally discontinuous, overlapping, and vertically stacked. Formation of these toe‐thrust sheets, rarely described in the literature, likely occurs during low tide when a high pore water pressure gradient exists between the landslide toe and subbeach water table.

Shale Compaction

Abstract The compaction of shales or other fine-grained compressible rocks is described by a mathematical model, and specific solutions are presented graphically. The model treats the presented graphically. The model treats the upward and downward movements of the water the solid matrix in very extensive, uniform, flat-lying units. It may readily be adapted treat the compaction of sands. The principal elements of the model are: (1) continuity equations for the water and solid matrix; (2) Darcy's law; (3) an expression for the fluid potential; (4) an equation for the total vertical stress; (5) an empirical relationship between porosity and the difference between the total vertical stress and the fluid pressure; and (6) an empirical relationship between permeability and porosity. From these elements an expression is derived for the porosity within the unit in terms of the space and time coordinates and boundary conditions, for the approximation that the densities of the water and the solid matrix are constant. Numerical solutions for the fluid pressure, the total vertical stress, the pressure, the total vertical stress, the porosity, the permeability, and the velocities porosity, the permeability, and the velocities of water and solid matrix were obtained as profiles through the unit at close time intervals, profiles through the unit at close time intervals, and representative results are displayed. The events followed are: (1) shale sedimentation; (2) a time lapse following shale sedimentation; (3) sedimentation of a normally pressured unit over the shale unit; and (4) a final time lapse with no sedimentation. Two boundary conditions for the base of the shale unit are considered: (1) the underlying unit is impermeable, and (2) the underlying unit is a normally pressured sand. In the latter case, water flows both upward and downward out of the compacting unit. The solutions show that pore water pressures much greater than normal are obtained and may persist for tens or hundreds of millions of years. It is also found that a shale unit rapidly buried beneath a thick normally pressured sand develops a zone near the sand-shale boundary of reduced porosity and permeability in which the pore water pressure permeability in which the pore water pressure gradient is very large. Introduction The presence of low density overpressured shales or mudstones in a sedimentary sequence influences the operations of petroleum exploration, drilling and production. During the exploration phase such low density fine-grained rocks influence the interpretation of seismic and gravity surveys. During the drilling of prospects, the mud casing and log programs and prospects, the mud casing and log programs and safety are affected by high pressures. During production, the possible influx of shale water production, the possible influx of shale water requires investigation.