Water Pressure Changes Research Articles

Effective stress and pore water dynamics in unsaturated soils: Influence of soil compaction history and soil properties

Effective stress is an important strength indicator for understanding and fundamentally predicting soil behaviour due to mechanical disturbance. This study investigated the influence of `soil properties on effective stress in relation to matric potential, and soil sampling direction (vertical/horizontal). Undisturbed soil samples representing 3 different locations were collected at 4 depths and in 2 sampling directions, to determine which soil properties mostly affect the effective stress in response to the magnitude of the stress applied. The soil properties included texture, organic carbon content, dry bulk density, void ratio, pre-compression stress and prevailing matric potential. The results showed that both the effective stress and the concomitant changes in pore water pressure largely depend on the prevailing matric potential in the soil. In addition, changes in soil structure caused by subsurface soil compaction as a result of tillage, organic carbon and clay content influenced the magnitude of the effective stress and pore water pressure in soils during loading. The porewater pressure measured under static loading is sampling direction dependent due to differences in hydraulic conductivity and pore continuity. Porewater pressure increases (become less negative) with increases in the fraction of fine pores in the soil while it may either remain constant or decrease, when the medium and coarse pore fractions in the soil increases. This implies that the smaller the pore connectivity and continuity, the less negative the changes in pore water pressure become. Also, the higher the pre-compression stress (sometimes due to higher organic carbon content), the more consistent the applied stress induced an increase in the calculated effective stress.

Stability of a 3D unsaturated vertical cut slope subjected to variable rainfall infiltration

Rainfall infiltration is one of the main triggers of slope failure. Based on the upper bound theorem of limit analysis, this work presents an analytical stability analysis of a 3D vertical cut slope subjected to rainfall infiltration. An analytical model that can directly provide pore water pressure change at any depth and time is adopted to calculate the time-dependent matric suction and shear strength of unsaturated soil during the rainfall infiltration process. Thereafter, based on a 3D rotational failure mechanism of a slope, the energy balance equation is built, and the factor of safety (FoS) of the slope is consequently determined in conjunction with the strength reduction technique. The influences of the 3D characteristics of the slope and rainfall patterns on both FoS and failure pattern of the slope are investigated. It is found that the 3D characteristic of slope is a key factor which determines the FoS of slope. Different rainfall infiltration patterns lead to diverse variation rules of both the FoS solutions and the critical failure patterns of slope. However, these rainfall patterns with a same accumulated rainfall result in the same FoS solution of slope at the end of rainfall infiltration. The present work may provide guidance on the stability analysis and preliminary design of slopes subjected to rainfall infiltration.

Design and application of hydro-mechanical coupling test system for simulating rock masses in high dam reservoir operations

A meter-level direct shear test system and a true triaxial test system were designed by placing the traditional test apparatus into sealed cabins subjected to high water pressure. The influences of three-dimensional seepage water pressure on the shear and compression deformation of rock mass in Xiluodu Hydropower Station were studied. The test results showed that the changes in water pressure caused obvious shear deformation of the interlayer dislocation zone and tensile deformation and reduction in the triaxial compression strength of the fractured rock mass. The effect of water pressure on shear displacement and tensile displacement had a hysteresis effect. This was consistent with deformation data collected through field monitoring. The deformation mechanism in the reservoir valley was the coupling of the stress and seepage fields caused by reservoir impoundment. The effective stress was reduced, the mechanical parameters were weakened, and the change of the initial stress field led to the slightly overall shear slip and tensile deformation of the bank slope.

Numerical Simulation of the Construction Process of Long Spiral CFG Piles

A numerical model for the construction process of long spiral CFG piles is established based on the coupled Eulerian–Lagrangian method. The construction process is divided into drilling process and concrete pouring process for modeling. The influence of long spiral CFG piles construction on saturated sand foundation is studied, and dynamic responses, changes of pore water pressure, and void ratio of saturated sand foundation are obtained. The rationality and accuracy of the simulation results are proved by comparing with the field test data and calculation results of the theory of cylindrical cavity expansion. The presented numerical results prove that the vibration load generated during the construction acts on saturated soil in the form of irregular reciprocating shear forces, which leads to a large excess pore water pressure in the soil and an increase in soil void ratio. Both the excess pore water pressure field generated during the construction and the soil pore ratio after the construction show a parabolic distribution in the vertical direction. The research results can provide reference and theoretical basis for future research and engineering practice.

Steady state harmonic response of nonlinear soil-structure interaction problems through the Preisach formalism

Steady state harmonic response of nonlinear soil-structure interaction problems is addressed in this paper. Due to well-known stiffness degradation phenomenon, the steady-state study is referred to small-medium strain condition whereas no significant pore water pressure change is observed (e.g. clayey soils and sands in drained conditions). A novel cyclic hysteretic model based on the Preisach formalism is proposed to describe the cyclic behavior of soil-foundation interaction. Through a harmonic balance procedure, furthermore, the steady state response of nonlinear soil-structure interaction problems is determined. Equivalent amplitude-dependent stiffness and damping have been derived in closed form to reliably describe the nonlinear soil-structure interaction phenomenon. Those expressions are function of the elastic foundation soil stiffness, the maximum attainable horizontal force and the moment capacity of the foundation. Furthermore, handy expressions of the equivalent fundamental period and damping, also dependent from the level of the excitation, are also derived. The proposed hysteretic model, moreover, has the advantage to be easily calibrated to match the modulus reduction and damping curves determined either from numerical or experimental tests. Numerical applications and comparisons with more advanced models and experimental data available in literature are also presented.

Deformation early-warning index for heightened gravity dam during impoundment period

The mechanical parameters of materials in a dam body and dam foundation tend to change when dams are reinforced in aging processes. It is important to use an early-warning index to reflect the safety status of dams, particularly of heightened projects in the impoundment period. Herein, a new method for monitoring the safety status of heightened dams is proposed based on the deformation monitoring data of a dam structure, a statistical model, and finite-element numerical simulation. First, a fast optimization inversion method for estimation of dam mechanical parameters was developed, which used the water pressure component extracted from a statistical model, an improved inversion objective function, and a genetic optimization iterative algorithm. Then, a finite element model of a heightened concrete gravity dam was established, and the deformation behavior of the dam with rising water levels in the impoundment period was simulated. Subsequently, mechanical parameters of aged dam parts were calculated using the fast optimization inversion method with simulated deformation and the water pressure deformation component obtained by the statistical model under the same conditions of water pressure change. Finally, a new early-warning index of dam deformation was constructed by means of the forward-simulated deformation and other components of the statistical model. The early-warning index is useful for forecasting dam deformation under different water levels, especially high water levels.

Cyclic Secant Shear Modulus and Pore Water Pressure Change in Sands at Small Cyclic Strains

AbstractWhen fully saturated sandy soil is subjected to cyclic straining in undrained conditions, pore water pressure increases and effective stress decreases. Therefore, it has been assumed that i...

Experimental and Numerical Studies on Permeability Properties of Thermal Damaged Red Sandstone under Different Confining Pressures

The stability and safety of underground rock mass engineering are closely related to the permeability process of fluids and permeability properties of rocks. To reveal the flow behavior of fluid in thermal damaged rock, first, a rock seepage testing system was applied to study the permeability properties of red sandstone specimens after different high-temperature treatments from 200 to 800°C under different confining pressures of 10 to 30 MPa. Meanwhile, the microstructures of the red sandstone specimens were characterized by the mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Then, the permeability process of pore water pressure and the flow form of fluid also were investigated by the numerical modeling method. The results show that the permeability properties of red sandstone specimens after high-temperature exposure follow linear Darcy’s law, and the relation between confining pressures and equivalent permeability coefficient ( K 0 ) can be described by a power function. Besides, the phenomenon that microscopic structural deterioration is intensified with increasing temperature and the average pore size and porosity of the red sandstone specimens are both power functions is related to the equivalent permeability coefficient. Furthermore, the results of numerical modeling indicated that the flow field within the range affected by confining pressures gradually becomes stable and orderly from disorder, and flow lines of the fluid become smooth and straight, and perpendicular to the isosurface of pore water pressure as time goes by. Moreover, the nonlinear correlation between pore water pressure and seepage path length changes to a linear correlation, which is consistent with linear Darcy’s law.

Investigation of cross-interactions of coupled thermal-hydraulic-mechanical model using stochastic simulations

The fundamental study of coupled thermal–hydraulic-mechanical (THM) systems is an important issue in multi-physical fields. This study developed a THM model in which the hydraulic and mechanical effects are fully coupled and the temperature change serves as the perturbation source of the system. A porothermoelasticity model was applied to investigate the cross-interactions among the temperature, change in pore water pressure, and displacement of a buffer material in the near-field and a host rock in the far-field with the conceptual model in a nuclear waste repository. The results show that the size scale of the domain of interest and the THM properties play important roles in the coupled THM system. The changes in pore water pressure depended on the combined effect of the deformation of the porous space and pore water controlled by the thermal expansion coefficients of the solids and fluids. Stochastic analyses show that the uncertainties of variables varied spatio-temporally due to the thermal influences and approached zero due to the presence of a stable condition. The cross-interactions between displacement and change in pore water pressure induced by the thermal effect were complex and can be determined from statistical moment analyses.

A Cyclic Macro-Element Framework for Consolidation-Dependent Three-Dimensional Capacity of Plate Anchors

This paper presents a new macro-element modelling framework for plate anchors which enables the effect of pore water pressure changes and the related evolution of soil strength during the process of cyclic loading and consolidation to be captured. The proposed modelling framework combines an advanced macro-element model for plate anchors, expanded to capture the cyclic loading behaviour, with a simple one-dimensional model of undrained shearing and consolidation for a soil element representative of the whole soil mass around the anchor. The representative soil element tracks the effects of changes in effective stress on the soil strength, which in turn governs the anchor capacity in the macro-element model. The two modelling components are linked through a mobilised capacity compatibility condition. It will be firstly shown that such modelling framework is able to capture the expected changes in an anchor’s capacity related to cyclic pore pressure generation and consolidation under one-dimensional cyclic loading of the anchor. Then, the model will be used to explore the plate anchor’s behaviour and failure mechanisms under loading conditions which mobilise its full three-dimensional cyclic loading capacity. The macro-element model will identify some conflicting mechanisms (i.e., the anchor’s kinematic/rotation and soil weakening/strengthening) governing the three-dimensional capacity of the anchor.

Soil Layer Disturbance Caused by Pipe Jacking: Measurement and Simulation of a Case Study

In this paper, areas of soil disturbed by pipe jack construction are categorized and analyzed in detail. Mechanisms of soil disturbance are discussed and patterns of soil deformation are studied using random medium theory. Lateral deformations in deep soil, pore water pressures, stratified settlement, and earth pressures are investigated using measurements from an electrical transmission pipeline project in China. The measurements show that soil layer movement can be predicted by monitoring changes in pore water pressure, and the amplitude of soil disturbance transverse to the pipeline is larger than that above the pipeline in this project. The distributions of stress components in the direction of, and vertical to, the pipeline axis are studied by numerical simulation. It shows that the closer to the pipeline axis, the greater the additional stress, the faster the attenuation speed, and the smaller the range of influence. Relationships of positive propulsion, friction, and stratum loss with longitudinal and transverse surface deformations are also discussed. The ground displacement is a coupling of the three factors to the action of the soil. The surface transverse subsidence with a main influence range of about ±4 D (D is outer diameter of pipe jacking). The findings from this study can be used as technical foundation and reference for the construction of similar projects.

Deformation and pore water pressure change during static and cyclic loading with subsequent shearing on soils with different textures and matric potentials

Soil deformation on arable lands due to heavy machine traffic always results in compaction and stress dependent shearing. The deteriorated effects of compaction on soil pore functions have been often reported, but less is known about the shearing-induced deformation. To better understand the combined effect of compaction and shearing on soil deformation and pore water pressure uw, measurements were performed on repacked soils with three textures (silt loam, sandy loam, and sand) and two matric potentials (-60 hPa and -300 hPa). Soils were sheared after two types of loading (static loading and cyclic loading) with three compaction levels (50, 100, and 200 kPa). Compaction was the main stress accounting for the loss of soil void ratio e and volume, on the contrary, shearing slightly increased void ratio and volume due to particle rotation in the shear zone. At -60 hPa, soil water ratio stepwisely decreased with compaction and subsequent shearing process. The soil volume change induced by compaction and shearing resulted in an obvious increase in uw. The higher the stress levels and initial matric potential, the more prominent changes in soil deformation and uw. Compared with static loading, further decrease of soil volume and increase of uw were observed under cyclic loading. Soils pre-compacted by cyclic loading always showed high and relatively constant uw values during shearing. The results demonstrated that compaction increased uw by decreasing e and increasing the degree of water saturation while shearing enhanced these effects by means of rearranging particles and blocking the water-filled pores. Therefore, after shearing, soil pore functions were further deteriorated compared with the initial state before compaction.

Calibrating water distribution network models and identifying interference areas between water sources on the network in the North of Ho Chi Minh City

The realistic simulation model of a water distribution network has been widely applied in water resource management, from optimization design to operation management. However, the calibration of the model simulation to adapt with reality is still a challenging task, because within the water distribution network, there are numerous unknown variables, such as: variant consumptions, pipeline characteristics, losses and uncontrollable or non-revenue water, especially flows at potential leak points. This study presents the simulation and calibration of the hydraulic model of the water distribution network in the Northern area of Ho Chi Minh City, including Go Vap District and a part of District 12. The network currently supplies water for more than 140.000 water meters and the water flow into and out of the network in real time is controlled in the study area. The results of the calibration of the water distribution network model are assessed to be consistent with the actual operation in terms of flow and water pressure changes during the day at the observation points by performance metrics. On that basis, the study continues to apply the calibrated model to analyze the water source tracing to determine the interference areas between two water sources in the study area from two water treatment plants.

The effect of rainfall on the slope stability with numerical simulation on Tawaeli-Toboli road, Central Sulawesi

Tawaeli-Toboli is an arterial road that often experiences landslides due to rainfall, which increases pore water pressure and thus, decreases the soil’s shear strength. This research analyzes the effect of rainfall on Tawaeli-Toboli road with numeric simulation to determine the characteristics such as changes in pore pressure and slope stability. Frequency analysis with HAVARA software was used to determine the maximum rainfall, while the dominant rain duration was analyzed using WindRose. Furthermore, the SoilVision software was used to fit grain size distribution, and the results are used to estimate the soil-water characteristic curve (SWCC). Analysis of pore water pressure changes was carried out using the SEEP/W program. The results were used as input data to determine the slope stability analysis using the SLOPE/W program. The frequency analysis result showed that the maximum amount of rainfall on Tawaeli-Toboli road is 205.698 mm after 11 days. During normal rainfall (20 mm/day), the Western slope remains stable with a safety factor value of 1.50. Simultaneously, in heavy rains, it decreases significantly to 1.350 after 11 days of rain. The eastern slope is quite stable because it has a safety factor value of 1.721 during heavy rain conditions.

The failure characteristics around shallow buried tunnels under rainfall conditions

The instability of weak surrounding rock around shallow buried tunnels induced by rainfall is a challenge for tunnel engineering and threatens construction safety. To reveal the influence of rainfall on the rock surrounding a tunnel, the results of experiments on the failure of surrounding rock under different rainfall intensities and durations using a self-developed test system are presented. First, the stress and seepage behaviors of the surrounding rock under rainfall were analysed, and the results showed that the stress increased while the pore water pressure decreased as the rainfall intensity decreased. It was more difficult to maintain vault collapse as rainfall intensity increased, and the particle erosion in the model intensified as the rainfall duration increased. The change in pore water pressure was greater than the change in stress, but the stress and pore water pressures influenced each other during rainfall. The influence of rainfall on the rock at the top of the tunnel was greater than that at the two sides of the tunnel because the seepage channels at the top of the tunnel were more developed and collapsed more easily due to smaller stresses. According to the phenomena of the surrounding rock observed during the test, the evolution of progressive failure under rainfall was roughly divided into eight stages: stability, water drop, continuous water flow, partial collapse, mud and water mixture, vault collapse, throughgoing collapse, and massive water and mud gushing. Furthermore, the final failure pattern of the surrounding rock consists of an excavation disturbance zone, vault damage zone and collapse failure zone. This research helps to reveal the progressive failure of surrounding rock around tunnels under rainfall conditions and can be used as a reference for designing disaster prevention strategies in practical engineering.

Unsaturated seepage and stability of double-layer slope under rainfall infiltration

Taking a homogeneous double-layer soil slope as an example, the SEEP/W module and SLOPE/W module in the finite element analysis software GeoStudio were used in this paper. Then, the changes of pore water pressure and stability under different rainfall patterns and soil parameters were studied. Finally, the variation curves of pore water pressure and slope safety factor with rainfall time were obtained. The results show that: Soil parameters a and m are directly proportional to the slope safety factor, while n is inversely proportional to the slope safety factor. Under the condition of continuous rainfall, the decreasing rate of slope safety factor is directly proportional to the rainfall intensity.Under different rainfall patterns, the continuous rainfall in the advanced and normal rainfall patterns will cause the slope stability to decline and then gradually recover, while delayed and averaged rainfall patterns rainfall will cause the slope stability to decline continuously.In addition, there is a lag period in the change of slope safety factor, and the whole lag period lasts about 6 hours. During the lag period, the pore water pressure inside the soil began to decrease, while the slope safety factor continued to decrease. The safety factor starts to recover after the lag period ends.

Experimental Study on the Negative Skin Friction of the Pile Group Induced by Rising and Lowering the Groundwater Level

Negative skin friction (NSF) has been one of the important factors in the design of pile foundation; especially, the influence of water level on the pile negative skin friction should be paid attention. In this paper, a series of model tests were carried out to analyze the bearing characteristic of the pile group influenced by groundwater level. The pile axial force and negative skin friction, settlement, and soil pore pressure were investigated. The results showed that both the water level rising and lowering cycle could increase the axial force of the pile along the upper part of the pile, yet reducing it along the lower part of the pile; both the axial force and the negative skin friction of the pile presented a feature of time effect; the value of negative skin friction was positively correlated with that of the pile head load, and the neutral plane ranged from 0.57 L to 0.64 L as the water level changed; the soil featured settling in layers, and the change of pore water pressure was accordant with the water level changing regulation.

Study on the Hydro‐Thermal‐Mechanical Characteristics of the Lime‐Soil Compaction Pile in the Forming Process

To study the effect of the lime‐soil compaction pile on the collapsible loess foundation and the influence of water‐heat‐force characteristics on the soil during compaction, the field test of lime‐soil compaction pile treatment of collapsible loess foundation is carried out. This paper monitors the process of soil temperature, water, and soil pressure change in the process of pile forming. According to the macroprocess and micromechanism of lime‐soil compaction pile treatment of collapsible loess foundation, the qualitative law of water‐heat‐force change in the process of pile forming is obtained. Meanwhile, the influence of soil damage is introduced in the process of compaction expansion. The model of pore expansion under linear damage conditions in the plastic zone is established. The formula of radial stress distribution under damage conditions is given. The influence of different damage factors on the expansion process is analyzed.

Analysis of Influence Factors of Pore Water Pressure Change in Frozen Soil

In this article, changes of pore water pressures (PWP) in silty clay subjected to freezing and thawing were measured under an open‐system condition. A total of five soil samples were tested, with water contents of 10.70%, 18.28%, 23.98%, 31.00%, and 37.65%, respectively. Each experienced a first‐step freezing stage, a thawing stage, and a second‐step freezing stage. The results showed that changes in PWP depended on the water content, soil type, salinity, ice content, air, pressure, temperature, and others. The PWP minimum with initial water content has a “w‐shaped double‐valley” characteristic, which described two PWP minima existing in two optimum water contents as initial water content increased. An influence‐factor analysis of PWP was proposed and gave a reasonable interpretation on the “w‐shaped double‐valley” characteristic of PWP. In addition, the tensiometer method to measure PWP in frozen soil was further discussed with regard to its advantages and disadvantages.

Thermal Conductivity of Water-in-Oil Emulsions

A correlation for calculating the thermal conductivity of binary dispersed systems depending on their composition is proposed. The results of measuring the thermal conductivity of reverse type water-in-oil emulsions at a pressure of up to 20 MPa in the temperature range of 293–353 K with a change in the volumetric water content in the emulsion up to 50% are presented. The obtained correlation is suitable for calculation of the thermal conductivity of water-in-oil emulsions in a wide range of changes of water content, temperature and pressure.