Pore Water Pressure Build-up Research Articles

Numerical simulation of the effect of moving loads on saturated subgrade soil

The substantial increase in excess pore pressure and plastic strain in subgrade soil due to faster and heavier trains is a major cause of track instability. Stresses that occur in track substructure during the passage of trains have not often been captured accurately due to insufficient rigour in dynamic analysis. This paper presents a numerical investigation into the plastic/dynamic characteristics of a saturated porous medium (capping and subgrade) subjected to moving axle loads. In this study, a detailed numerical analysis is described, whereby a coupled fluid-dynamic framework is developed for the saturated porous medium in conjunction with a generalized plasticity model, in order to examine the cyclic loading response of a soft subgrade soil. The constitutive model is calibrated by undrained cyclic triaxial testing carried out on a soft foundation soil retrieved from a coastal railroad site having a historical record of mud pumping. Kelvin elements are attached to the transmitting boundary to absorb wave energy and to prevent any back-propagation of waves into the saturated subgrade domain. This approach is then validated against a semi-analytical solution and wayside measurements. The results show that the stress path corresponding to the applied moving load exhibits a series of heart-shaped envelopes along which the deviator stress and mean effective stress gradually decrease with the successive loading cycles. The build-up of pore water pressure and the associated soil deformation (e.g. accumulated settlement) are exacerbated when the train speed approaches a critical velocity of the saturated subgrade. A relationship between the train speed, track settlement and drainage capacity of a sub-ballast (capping) layer is also established. As a practical guide, the influence of permeability of the capping layer is highlighted in relation to controlling track settlement for a given operational train speed.

On the effects of pore water pressure buildup and dissipation on the seismic performance of a propped r.c. diaphragm wall in sand

The study concerns the analysis of a retaining structure composed by a couple of r.c. diaphragm walls propped at the crest in loose and medium-dense, variably saturated sand under seismic conditions. Fully coupled dynamic equilibrium conditions and pore water flow in the porous soil have been taken into account, in order to assess the effects that the development and subsequent dissipation of excess pore water pressures can have on the performance of such structures under seismic conditions. To this end, a series of simulations in which the saturated soil permeability is varied of about two orders of magnitude has been carried out, in order to consider different evolution rates for the dynamic consolidation process. The von Wolffersdorff hypoplastic model and the van Genuchten water retention equation have been used to describe the mechanical and hydraulic behavior of the sand. The results obtained in a large series of finite element simulations show a significant dependence of the seismic performance of the structure evaluated in terms of permanent rotations and structural loads, in view of the modern performance-based design criteria on the excess pore pressures buildup during the seismic shaking and on its dissipation with time. For the particular seismic input considered, neither fully drained nor fully undrained conditions can be considered applicable in most of the cases considered. In such conditions, the quantitative assessment of wall and soil displacements, pore water pressures and effective stress distributions within the soil requires necessarily the solution of a fully coupled, nonlinear dynamic consolidation problem.

The 2015 Shenzhen catastrophic landslide in a construction waste dump: analyses of undrained strength and slope stability

The 2015 catastrophic landslide in a 110-m-high waste dump in Shenzhen is recognized as one of the largest landfill failure worldwide. An earlier comprehensive field investigation revealed that the dominant component of the fill was completely decomposed granite (CDG), and the dumping operation was accompanied by a rise of the groundwater level. In this paper, the complex stress paths for the initially unsaturated fill materials being subjected to both rapid filling and wetting were investigated. A simplified method was proposed for estimating the gain of undrained shear strength under the complex stress paths. Soil samples were taken from the site to a laboratory to measure the undrained shear strength and validate the estimation method. Total stress-based stability analyses were carried out to calculate the factor of safety of the dump at failure. The triggering mechanism of the landslide is clarified as follows: The gain of shear strength with the surcharge loading for the wet layer in the lower part of the waste dump was limited by a build-up of excess pore-water pressure. The gain of shear strength for the relatively dry fill material was attenuated with the rise of groundwater level. When the shear strength was not enough to resist the increasing slip force with the surcharge loading, a deep-seated translational failure took place in the lower wet layer of the waste dump. The proposed method for analyses of undrained strength and slope stability are proven to be applicable to the waste-dumping operation with rapid filling and rising groundwater level.

Numerical investigation of dynamic soil response around a submerged rubble mound breakwater: II. Loose sandy seabed

Low-crested structures such as submerged rubble mound breakwaters (RMB) are commonly used as successful examples of coastal protection measures. This study addresses foundation issues related to the performance of a submerged RMB constructed on a liquefiable site where the foundation soils have been loosely deposited; these issues were investigated using an integrated numerical model proposed by Zhao et al. (2018). Unlike previous work in Zhao et al. (2018), the dynamic features of loose sand deposits (i.e. build-up of pore water pressures, development of plastic strains, degradation of soil stiffness) due to the fluid–seabed-structure interactions, accompanied by the onset of liquefaction, were simulated. The reliability of this model to predict the liquefaction susceptibility of loose sand deposits was validated against the experimental results available in literature. This study shows that the submerged RMB constructed on a liquefiable seabed experienced progressive and asymmetric downward settlement under successive loading cycles. The opposing currents in the wave field tended to enhance this asymmetric settlement whereas the following currents did the opposite. Regardless of changes to the magnitude and direction of current velocities, ignoring the currents in the wave field may lead to an overestimation of the increased rate of residual pore water pressures in the region underneath the breakwater, particularly in shallow soil layers where the maximum relative differences can reach up to 120% of (p̄current−p̄no-current)∕P0 at t∕T=25.

Controlling the Permeability of Sandy Soil Using Different Local Waste Materials

Advanced developed technologies associated with people demands have caused production and expansion of different local wastes where the process of managing such waste becomes a real need for controlling the pollution risk. One of the procedures for recycling can be made through using local wastes in permeability control for sandy soil since the soil permeability plays a crucial function on the water drainage, pore water pressure buildup and dissipation, and ground movement for saturated sand during and after earthquake occurrences. In addition, any economical mixture should maintain hydraulic constraints for practice use. In this study, a laboratory experiment was prepared to perform tests for estimating the water movements and permeability in a pure sand column and treated sandy soil with different locally waste materials including plastic, glass, rubber, and aluminum. In addition, a numerical investigation including finite element method has been adopted to verify the experimental procedure. It was shown that the permeability could be controlled to different rates using these local wastes.

A two-surface thermomechanical plasticity model considering thermal cyclic behavior

In thermal-related engineering such as thermal energy structures and nuclear waste disposal, it is essential to well understand volume change and excess pore water pressure buildup of soils under thermal cycles. However, most existing thermo-mechanical models can merely simulate one heating–cooling cycle and fail in capturing accumulation phenomenon due to multiple thermal cycles. In this study, a two-surface elasto-plastic model considering thermal cyclic behavior is proposed. This model is based on the bounding surface plasticity and progressive plasticity by introducing two yield surfaces and two loading yield limits. A dependency law is proposed by linking two loading yield limits with a thermal accumulation parameter nc, allowing the thermal cyclic behavior to be taken into account. Parameter nc controls the evolution rate of the inner loading yield limit approaching the loading yield limit following a thermal loading path. By extending the thermo-hydro-mechanical equations into the elastic–plastic state, the excess pore water pressure buildup of soil due to thermal cycles is also accounted. Then, thermal cycle tests on four fine-grained soils (natural Boom clay, Geneva clay, Bonny silt, and reconstituted Pontida clay) under different OCRs and stresses are simulated and compared. The results show that the proposed model can well describe both strain accumulation phenomenon and excess pore water pressure buildup of fine-grained soils under the effect of thermal cycles.

Mitigation of liquefaction-induced uplift of underground structures

Buoyancy is one of the most serious manifestations of underground structure’s instability during strong ground motions. Soil liquefaction along with significant reduction of its shear modulus beneath these structures are the main driving factors of this phenomenon. In this article, the seismic behavior of underground access structures embedded in sandy deposits was analyzed using the finite differences (FD) program (FLAC) with an emphasis on structural failures under real earthquakes. A new liquefaction model “energy-based approach” which simulates material cyclic behavior and estimates pore water pressure build-up, was incorporated into the numerical code as a constitutive model of the soil. Also, linear structural elements were used to model the underground structure. Several mitigation methods have been modelled against the structure flotation. As well as a new combined mitigation method and its impacts on the structural performance have been detailed herein. Numerical results showed that gravel drains would effectively dissipate the excess pore water pressure beneath the structure while increasing the burial depth of the structure and adding an impermeable layer under it would increase the vertical effective stress and therefore detracts the ability of excess pore water pressure to push the structure upward. It is found that compiling a gravel drains surround the structure with an impermeable layer beneath the structure would effectively reduce the structural uplift more than any other stand-alone method.

A Sequentially Coupled Catchment‐Scale Numerical Model for Snowmelt‐Induced Soil Slope Instabilities

AbstractThe frequency of snowmelt‐induced soil slope instabilities is increasing in some seasonally cold regions because of climate change. Reliable hazard assessment and risk mitigation of snowmelt‐induced landslides require physically‐based prediction models. However, existing models either apply only at the slope scale or assume precipitation as the sole landslide trigger. In doing so, they neglect the complexity and coupled nature of the thermo‐hydro‐mechanical processes leading to slope instability in seasonally cold regions (such as snow accumulation and melting, infiltration and surface runoff, soil saturation, pore water pressure buildup and dissipation). Here, we present a spatially distributed and sequentially coupled numerical model to simulate snowmelt‐induced slope instabilities at the catchment scale. The model accounts for temperature‐dependent changes in the soil hydraulic behavior related to changes in water state by means of a routine implemented in a geographic information system. We verified the performance of the model using a case study of spring snowmelt‐induced soil slope failures that occurred after the 2004 Mid‐Niigata earthquake in Japan. Considering limitations and simplifications, the model was able to predict the triggering condition, magnitude, and spatial distribution of the snowmelt‐induced landslides with a satisfactory degree of accuracy. We believe that the robustness and simplicity of our numerical approach make it suitable for implementation in early warning systems.

Undrained anisotropy and cyclic resistance of saturated silt subjected to various patterns of principal stress rotation

Although the undrained anisotropic behaviour of sand has been extensively studied, little is known about undrained anisotropy and liquefaction susceptibility of silt soil. This paper presents a systematic experimental study on the undrained response of a silt soil under different patterns of principal stress rotation, including the 90° jump of principal stress and the continuous rotation of principal stress. Particular attention has been focused on the influence of the cyclic stress path and the influence of cyclic stress ratio (CSR) on the pore pressure ratio ru and the deviatoric strain amplitude γqa. A remarkable finding of this study is that, for all loading patterns investigated, the pore pressure ratio ru is uniquely related to the deviatoric strain amplitude γqa, rather than to the CSR. Based on the experimental data, an explicit expression is proposed to relate γqa and ru, which can be used to predict the onset of failure as well as the build-up of pore water pressure at a specific strain level. The study also found that the relationship between the CSR and the number of cycles required to failure (Nf) is dependent on cyclic loading patterns and stress paths. However, by defining a unit cyclic stress ratio (USR) as a new index for cyclic resistance, a virtually unique relationship between USR and Nf can be established for all cyclic loading patterns and stress paths considered.

Pore water pressure accumulation and settlement of clays with a wide range of Atterberg’s limits subjected to multi-directional cyclic shear

In this study, normally consolidated specimens on four clays with a wide range of Atterberg’s limits were tested by applying several series of undrained multi-directional cyclic shear followed by drainage. The cyclic shear tests were carried out under the shear strain amplitudes (γ = 0.05%-2.00%), number of cycles n = 200 and the phase difference θ = 90o. Then the accumulation of cyclic shear-induced pore water pressure and the post-cyclic settlement in strain (εv, %) were observed and discussed. In conclusion, it is clarified that the pore water pressure ratio (Udyn/σ’vo) increases with g and the soils with higher Atterberg’s limits show lower Udyn/σ’vo, and under the multi-directional cyclic shear strain at γ > 0.4%, Hue clay and Kaolinite clay with relatively low plasticity suffer from cyclic failure. In addition, the post-cyclic settlement has a tendency of decreasing with the Atterberg’s limits in the range of plasticity index from Ip = 25.5 to 63.8, meanwhile when Ip < 25.5, different tendencies were observed e.g., Hue clay (with lower Ip) shows a smaller settlement compared with those on Kaolin (with higher Ip). Furthermore, the threshold number of cycles (ntp) and cumulative shear strain (G*tp) for pore water pressure buildup were then clarified.

Instability of flood embankments due to pore-water pressure build-up at the toe: lesson learned from the Adige River case study

A case study is presented of the Adige River embankment, a segment of which experienced instability of the landside slope due to the development of uplift pressures. Soil profile and hydromechanical properties of the embankment and foundation materials have been assessed via site investigation, laboratory testing, and field monitoring for two cross sections, within and outside the failure segment. The hydromechanical model developed thereof was first validated against its ability to reproduce the probability of failure for the two sections with a first-order, second-moment (FOSM)-based approach. Comparison of water flow regimes between the two sections was then used to highlight the importance of the hydraulic properties of the material on the landside for the development of uplift pressures at the toe of the embankment. The lesson learnt from this case study is that the hydraulic response of the ground on the landside may play a critical role on the stability of flood embankments and its characterization should therefore not be overlooked when planning site investigation.

Effect of principal stress rotation on dynamic characteristics of a sandy seabed under a partially reflected standing wave

Principal stress rotation (PSR) is one of the main stress conditions for a soil element subject to cyclic waves. It is one of the main causes behind generation of excess pore water pressure and cumulative plastic strain in soil deposits when drainage conditions are impeded, and thus compromises the stability of any supported marine structures. This paper aims to investigate the influence of PSR on the cyclic characteristics of soil under standing waves that are partially reflected, in which a generalized plasticity model considering the effect of PSR was adopted to model the soil behavior. Comparisons between the present model, previous hollow cylinder apparatus tests, and geotechnical centrifugal wave tests all show good agreement. Numerical results indicate that ignoring the PSR involved in the standing-wave–seabed interactions will significantly underestimate the build-up of pore water pressure, particularly at the standing wave nodes. Furthermore, the exclusion of the effect of PSR may lead to contradictory results in terms of susceptibility to liquefaction between the antinode and the node, when compared to the wave flume test and geotechnical centrifuge test. Numerical simulation results also demonstrate that while considering the impact of PSR, a sandy seabed exhibits higher liquefaction resistance when subjected to standing waves compared to the case when subjected to progressive waves with an equivalent wave height. Parametric studies show that the coefficient of wave reflection, wave characteristics (wave height, period, and water depth), and soil properties (permeability and saturation) significantly affect the liquefaction characteristics of soil under partially reflected standing waves.

Computational and experimental study of a hydro-dynamical landslide model based on laboratory flume tests

Hydrological factors such as volumetric water content (VWC) and pore-water content (PWC) have been cited widely as significant factors that trigger slope failures especially shallow landslides and debris flows. Over the years, researchers have studied these processes using a range of physically-based models which in many occasions are either too complex incorporating very many parameters or fail to mimic real field conditions. The principal objective of this study was to derive and incorporate a set of physically-based equations that describe the dependence of slope failures at laboratory scale on VWC into a factor of safety expression herein referred to as the hydro-dynamical landslide model. The model was validated by a series of physical tests on soil samples in the laboratory using the Chep-flume. Results showed a close agreement between computational and experimental data, confirming the hypothesis that cohesion, internal friction angle and pore-water pressure are modulated by VWC especially for slopes with sandy-loam soils. More so, rapid change of soil water content was observed to accelerate build-up of negative pore-water pressures (PWP) which triggers slope failure. Apart from giving a simplified expression for the factor of safety, the proposed model circumvents the difficulties associated with tedious procedures employed in the measurement of cohesive stress by limiting the tests to only three sets of parameters i.e. VWC, PWP and displacement.

Effects of drainage condition on the stress-strain behavior and pore pressure buildup of sand-PET mixtures

Recycling and usage of Polyethylene Terephthalate (PET) in civil constructions will not only decrease the environmental challenges, but also it will result in considerable saving in the overall project costs. Introducing PET chips as a capable material for the improvement of sands, this paper presents the results of consolidated drained (CD) and undrained (CU) triaxial tests on different sand-PET mixtures. Applying four different PET contents and two relative densities, tests are conducted under a wide range of confining pressures from 50 kPa to 500 kPa and the mobilized shear stress and the pore water pressure buildup are studied. Changing in the drained and undrained dilative-contractive behavior of sand-PET mixtures in different PET contents, relative densities and confining pressures showed the multi-variable nature of the problem emphasizing on the need for the comprehensive stress-strain modeling of mixtures. Such a behavior is then assessed through proposing new predictive, evolutionary models for the shear stress and the pore pressure based on the PET content, relative density, confining pressure and axial strain. Parametric studies showed that (0.2–0.3)% PET content, also, (2–4)% axial strain are determinative ranges for the evaluation of undrained and drained shear stresses of mixtures, respectively. A three-dimensional, graphical comparison using the developed relationships introduced the (0.5–0.7)% range as the critical value for the PET content distinguishing the drained and undrained behavior of mixtures. Next, according to the sensitivity analysis, confining pressure and axial strain are, respectively, presented as the most influencing parameters on the drained and undrained, mobilized shear stresses.

A coupled mathematical model for accumulation of wave-induced pore water pressure and its application

Prediction of the wave-induced instability of seabed due to the build-up of pore water pressure is essential for coastal engineers involved in the foundation design of marine infrastructure. Most Previous studies have been limited to decoupled residual mechanism for the rise in excess pore water pressures. In this study, the existing decoupled approach has been improved to consider the coupling effect between the development of pore water pressures and evolution of seabed stresses. Comparisons with the existing wave flume tests and geotechnical centrifuge wave tests have demonstrated that the developed coupling approach is capable of reproducing the accumulation of pore water pressure under cyclic wave loadings, and shows more promising predictions compared to the existing decoupled model, especially for the case of standing waves. The validated framework is further extended to a field scale numerical model to investigate the development of pore water pressures and the corresponding liquefaction susceptibility of seabed to water waves. The numerical results have revealed the different mechanism for wave-induced pore pressure build-up in marine soils between the developed model and the existing decoupled model. The coupling effect of residual pore pressure and seabed stress could accelerate pore pressure accumulation, which implies that the existing decoupled model may underestimate the liquefaction potential of seabed, particularly under standing wave loadings. Furthermore, results of the developed model have shown that the effect of wave nonlinearity on advancing seabed liquefaction is more noticeable for progressive waves than that for standing waves.

Influence of initial conditions on the liquefaction strength of an earth structure

Liquefaction has been the major source of damage for structures and infrastructures in most recent earthquakes, as it induces loss of strength and stiffness in soils, resulting in settlement of buildings, landslides, and failure of pipelines and earth dams. Such a phenomenon is primarily associated with saturated cohesionless soils and strong-motion seismic events able to induce pore water pressure build-up. Even though the degree of soil saturation is strictly related to the oscillation of groundwater table and to the interaction flows (namely precipitation and evaporation) between the exposed surface and the atmosphere, the initial distribution of pore water pressure is commonly simplified in liquefaction potential analysis and the presence of unsaturated soil layers is almost always neglected. To try to fill this gap, this work investigates the effect of the initial distribution of pore water pressure on the liquefaction strength evaluation, considering the potential presence of unsaturated soil layers. The work uses as case-study a well-investigated inhabited levee damaged by the 2012 Emilia earthquake. The initial distribution of pore water pressure within the dyke is carried out by solving the Richards equation in steady-state and transient conditions, assuming as boundary conditions the evolution of potential flows recorded at the site in the time period including the earthquake event. The cyclic resistance of unsaturated soils is considered during the execution of dynamic analyses in effective stress conditions. Results of the analyses show that initial conditions influence the liquefaction strength of earth structure and that neglecting the cyclic resistance of unsaturated soils is not a conservative assumption.

Dynamic characteristics of a sandy seabed under storm wave loading considering the effect of principal stress rotation

This paper aims to investigate the cyclic characteristics of soil under storm waves with the typical irregular character, in which a generalized plasticity model considering the effect of principal stress rotation (PSR) was adopted to model soil behavior. In the present model, the plastic strain generated by principal stress rotation was considered to be an additional item in the constitutive relationship of soil, and the normalized loading and plastic flow directions were determined based on the stress tensor invariants. Comparisons between the present model, previous hollow cylinder apparatus (HCA) tests, and geotechnical centrifugal wave tests all show good agreement. Numerical results indicate that the moveable and random characteristics of the storm wave result in similar and irregular dynamic responses in sandy deposits along the extension direction of the seabed. Moreover, the severest wave loading in a real storm will significantly magnify both the speed and extent of liquefaction of the sandy seabed. Numerical results also demonstrate that ignoring the PSR involved in the storm–wave–seabed interaction will significantly underestimate the build-up of pore water pressure. Additionally, when considering the impact of PSR, the sandy seabed exhibits higher liquefaction resistance when subjected to the random wave compared to the representative regular wave. However, since the sharply weakening liquefaction resistance of soil under the severest wave condition cannot be captured with the regular wave, it is suggested that the random wave be adopted to further investigate the storm–wave–seabed interaction. Parametric studies show that soil properties (e.g., permeability and saturation) significantly affect the liquefaction characteristics of soil under storm waves.

Excess Pore-Water Pressure Generation in Cyclic Undrained Testing

AbstractA new strain-based model is presented in this paper for assessing the residual excess pore-water pressure (ue) buildup in fully saturated sands. In this model, the generation of residual ue...

Bench-Scale Experiments on Effects of Pipe Flow and Entrapped Air in Soil Layer on Hillslope Landslides

Soil pipes are commonly found in landslide scarps, and it has been suggested that build-up of pore water pressure due to clogged soil pipes influences landslide initiation. Several researchers have also suggested that entrapped air in the soil layer increases the pore water pressure. We carried out bench-scale model experiments to investigate the influence of soil pipes and entrapped air on the build-up of pore water pressure. We installed a water supply system consisting of an artificial rainfall simulator, and used a water supply tank to supply water to the model slope and artificial pipe. We used two types of artificial pipe: A straight pipe, and a confluence of three pipes. Furthermore, we placed a layer of silica sand on top of the model slope to investigate the effect of entrapped air in the soil layer on the build-up of pore water pressure. Silica sand is finer than the sand that we used for the bulk of the model slope. Our results indicate that, although artificial pipes decrease the pore water pressure when the amount of water supplied was smaller than the pipe drainage capacity, the pore water pressure increased when the water supply was too large for the artificial pipe to drain. In particular, the confluence of pipes increased the pore water pressure because the water supply exceeded the drainage capacity. The results also indicate that entrapped air increases the pore water pressure in the area with relatively low drainage capacity, too. Based on these results, we found that although soil pipes can drain a certain amount of water from a soil layer, they can also increase the pore water pressure, and destabilize slopes. Furthermore, entrapped air enhances the trend that the pore water pressure can increase in the area with relatively low drainage capacity, as pore water pressure increases when too much water is supplied, and the artificial pipe cannot drain all of it.

Cyclic undrained behavior and post liquefaction settlement of a nonplastic silt

This study presents an investigation into liquefaction and post-liquefaction behavior of a nonplastic silt under cyclic loading. A systematic approach was adopted for assessment of above-mentioned behavior by performing cyclic triaxial tests on saturated specimens in terms of pore water pressure build-up characteristics, effective stress, number of cycles and double amplitude axial strain. The specimens, consolidated under an effective confining pressure of 100 kPa, were prepared at relative densities ranging among 30–80%. The results revealed that the axial strains of specimens of lower relative densities were increased at higher CSR levels. For prediction of pre- and post-liquefaction behavior, a family of curves for establishing relationships among factor of safety to liquefaction, post-liquefaction volumetric strain and relative density were established. These curves, along with the calculation of post-liquefaction volumetric strain, may be useful in estimation of settlements of silt after being subjected to cyclic loading.