Excess Pore Pressure Distribution Research Articles

Effect of tunnel-building separation distance on the dynamic soil-structure interaction response

Understanding how buried structures respond to cyclic loading is crucial for designing earthquake-resistant underground infrastructure. The complexity of the dynamic response of these underground structures is accentuated when accounting for their interaction with adjacent buildings. This study focuses on the impact of the lateral distance between a shallow rectangular tunnel and an adjacent residential structure on dynamic soil-structure interaction, employing two dynamic centrifuge tests. Particle Image Velocimetry technique was adopted to observe the developed mechanisms of liquefied soil deformation, tunnel uplift, and foundation settlement. Results show the pronounced effects on soil deformation above the tunnel crown with a reduction in the tunnel-building separation distance. The distribution of excess pore pressure along the tunnel lining will be presented. Tunnel manifested additional lateral movement and reduced uplift when the adjacent building was in close proximity. A decrease in the tunnel-building distance contributed to a reduction of building rotation, primarily due to the significant mitigation of non-uniform settlement under the footing. Overall, the soil deformation mechanism around the tunnel and the building will be shown to change depending on the separation distance between them.

Three-dimensional tunnel face stability using a new heterogeneous dynamic filter cake

In slurry-shield tunnelling, a heterogeneous dynamic filter cake occurs due to the interaction between slurry infiltrating and tool cutting, whose influence on tunnel face stability is not fully plumbed. Thus, a numerically based-limit analysis framework is proposed to assess the tunnel face stability considering a heterogeneous dynamic filter cake. The numerical model with a heterogeneous dynamic filter cake, which has spatio-temporally variable permeability coefficients according to cutter layouts, is built for solving a transient seepage flow. Numerically obtained seepage flow results are further incorporated into the 3D rotational failure mechanism, so as to give an upper-bound estimation on the tunnel face safety factor and the slurry pressure transfer efficiency. The proposed method is validated by comparing with previous studies regarding the slurry pressure drop, slurry pressure transfer efficiency, and tunnel face safety factors. Parametric analyses are performed to examine the excess pore pressure distributions at the filter cake-soil interface, and discuss the influences of cutterhead rotation and excess slurry pressures on the tunnel face stability. The results show that the defined three-dimensional pressure transfer efficiency is between the one-dimensional and the two-dimensional solutions published previously; the tunnel face stability reduces as cutterhead rotating and finally reaches a dynamic equilibrium state.

Coupled pore pressure analysis of cone penetration test in two-layered clay

PurposeThe paper aims to clarify the distribution of excess pore pressure during cone penetration in two-layered clay and its influence on penetrometer resistance.Design/methodology/approachAn arbitrary Lagrangian–Eulerian scheme is adopted to preserve the quality of mesh throughout the numerical simulation. Simplified methods of layered penetration and coupled pore pressure analysis of cone penetration have been proposed and verified by previous studies. The investigation is then extended by the present work to study the cone penetration test in a two-layered clay profile assumed to be homogeneous with the modified Cam clay model.FindingsThe reduction of the range of pore pressure with decreasing PF will cause a decrease of the sensing distance. The PF of the underlying soil is one of the factors that determine the development distance. The interface can be obtained by taking the position of the maximum curvature of the penetrometer resistance curve in the case of stiff clay overlying soft clay. In the case of soft clay overlying stiff clay, the interface locates at the maximum curvature of the penetrometer resistance curve above about 1.6D.Research limitations/implicationsThe cone penetration analyses in this paper are conducted assuming smooth soil-cone contact.Originality/valueA simplified method based on ALE in Abaqus/Explicit is proposed for layered penetration, which solves the problem of mesh distortion at the interface between two materials. The stiffness equivalent method is also proposed to couple pore pressure during cone penetration, which achieves efficient coupling of pore water pressure in large deformations.

A limit analysis approach to uplift bearing capacity of shallow plate anchors in marine environments

AbstractOffshore oil and gas exploration activities involve the installation of various subsea infrastructures on the ocean floor, often supported by shallow foundations such as anchor plates or mudmats. These foundations must be designed to withstand applied service loads along the structure lifecycle, including decommissioning loads required for subsequent removal. In this context, this work is dedicated to the evaluation of uplift bearing capacity of shallow anchors within the theoretical framework of limit analysis and its related kinematic approach. Based on the implementation of 3D failure mechanisms, rigorous upper bound estimates for the uplift force viewed as ultimate load for the material system are derived from either total or effective stress analyses. In that respect, emphasis is given to the formulation of limit analysis problem in the context of effective stresses, showing how the effect of seepage flow may be accounted for by means of driving body forces derived from the gradient of excess pore pressure distribution and how suction forces arise from pore pressure difference between upper and lower anchor surfaces. Evaluation of suction forces developed during anchor lifting is addressed by resorting to a simplified modeling applied to circular and rectangular anchors. The accuracy of the upper bound predictions derived for uplift force is assessed by comparison with available experimental data. Several numerical results are also provided to investigate the effects of relevant problem parameters on the uplift bearing capacity.

Numerical analysis of the post-installation consolidated response of skirted foundations

The effect of the installation process on the subsequent foundation consolidation response requires assessment when the preload is initially applied. In the framework of effective stress, the influence of large soil deformations during penetration is analysed in this study using a comparison of results from large deformation finite element (LDFE) analysis based on periodic mesh regeneration (Ritss technique) and a 'wished-in-place' small strain finite element (SSFE) model. The LDFE results demonstrate that considerations of the skirt penetration and soil plug bulge lead to different soil excess pore pressure and void ratio change distributions from that of the SSFE analysis, thereby affecting the foundation settlement and consolidated bearing capacity. The normalised fully consolidated undrained bearing capacity grows linearly with the preload ratio for preloads higher than the penetration resistance, and the capacity obtained from the LDFE is higher. The settlement ratio and the proportion of partially consolidated to fully consolidated capacity gain can be represented as a hyperbolic function of a revised dimensionless time factor. The methods integrated in this study can be used to predict the increase in the vertical undrained capacity of skirted foundations after consolidation, accounting for installation effects in engineering practice.

Semi-analytical solution for two-dimensional electro-osmotic consolidation of double-layered soil

This study proposed a semi-analytical solution for 2D electro-osmotic consolidation of double-layered soil. The method of Laplace transform is employed to derive the solution in the frequency domain. Then the numerical approach of inverse Laplace transform is used to obtain the explicit solution in the time domain. A finite element simulation is conducted to validate the proposed solution's accuracy. The verification example shows excellent agreement between the two methods. Based on the parametric study, it is found that the value of soil parameters in the second layer has significant influences on the behaviour of electro-osmotic consolidation. The larger the electro-osmosis coefficient in the second layer, the smaller the excess pore pressure at a given time. Additionally, a higher coefficient of volume compressibility in the second layer results in higher excess pore pressure. Compared with the 1D case, the distribution of excess pore pressure at the steady state is non-linear with depth in the 2D electro-osmotic consolidation, rather than a linear distribution. Furthermore, the dissipation rate and steady-state excess pore pressure could be underestimated if the electro-osmotic consolidation is modelled as a 1D case.

Numerical analysis of energy piles in a hypoplastic soft clay under cyclic thermal loading

AbstractThis work is a numerical investigation of the effect of thermally induced volumetric collapse of normally consolidated clays on the performance of energy piles. A series of coupled thermo‐hydro‐mechanical Finite Element simulations were carried out using the commercial software ABAQUS. These examined a single free‐head energy pile embedded in a normally consolidated clay layer subjected to a constant mechanical load and to a number of heating/cooling cycles, to reproduce operating conditions. The soil behaviour was described with two advanced hypoplastic constitutive models for clays, one of which incorporates the thermally induced volumetric collapse using an ad‐hoc algorithm developed by the authors. Both models predict a cyclic accumulation of settlement and excess pore water pressure, especially when the thermal collapse effect is considered. While the excess pore pressure distribution stabilises within a few cycles, the rate of settlement of the pile head does not show any tendency to decrease from one cycle to another. These results are in agreement with data from small scale tests on an isolated energy pile in normally consolidated clay, indicating that the numerical model developed in this study can be used to investigate the complex soil/pile/raft interaction processes occurring in real piled foundations incorporating energy piles.

Analysis of the most efficient horizontal drainage position in the large strain consolidation considering the self-weight effect

To study the large-deformation settlement of soft soil foundations, an analysis of the large-strain consolidation of soil in a Lagrangian coordinate system based on continuum mechanics is performed, and the control equations with the displacement gradient are obtained in this paper. Based on the nonlinear permeability and compressibility of soft soil foundations, the equation of large-strain nonlinear consolidation is established. Then, the finite difference method is used to solve the governing equations. Under the condition of two-sided drainage while considering the soil self-weight stress, it is found that the distribution of excess pore pressure is not symmetric along the depth of the foundation. The position of the maximum excess pore pressure appears below the midpoint of the soil foundation. The deviation degree increases with increasing foundation thickness and semilogarithmic compression index Cc value, and it decreases with increasing semilogarithmic permeability index Ck value. The calculations show that setting the drainage layer position close to the position of maximum excess pore pressure can effectively improve the drainage ability of the upper and bottom soil layers, which is beneficial to consolidation, and that the deviation distance is related to the Cc and Ck values.

Centrifuge modelling of wave-induced seabed response in clay

This paper describes a programme of centrifuge tests investigating the behaviour of clayey seabed under wave loading using an in-flight wave loading system. Three model seabeds of kaolin clay capturing typical unconsolidated, normally consolidated and overconsolidated soil responses were considered, with each seabed experiencing several episodes of wave loading and resting. Data acquisition measures included pore pressure transducers, accelerometer, bender elements and T-bar penetrometers. The depth-wise distribution of excess pore pressure, soil strength and modulus, as well as the motion of the liquefied layer of the seabed, was monitored throughout to enable a thorough investigation into the liquefaction and reconsolidation features of the soil. For the unconsolidated and normally consolidated soils, remarkable development of residual pore pressure was observed, and there was evidence that the strength/modulus recovery cannot be achieved by the surficial soil within a prototype time of 15 days. Within a certain depth below this surficial layer, there was a drastic increase in undrained strength, and this phenomenon was carefully examined by a modified moving-boundary model. For the overconsolidated soil, the build-up of residual pore pressure was rather limited, but discernible amplification of oscillatory pore pressure amplitude was observed. Implications for practice in offshore engineering are discussed based on the experimental findings.

Excess Pore Water Pressure and Ground Consolidation Settlement Caused by Grouting of Shield Tunnelling

This study investigates the effect of shield tunnel grouting on excess pore pressure distributions around tunnel and ground consolidation settlement. First, it presents half-plane excess pore pressure around an unlined tunnel caused by grouting pressure based on the complex variable theory and the conformal mapping method. Second, the analytical solution for the soil consolidation is derived using the conformal transformation method and the consolidation theory of Terzaghi–Rendulic. The drainage conditions of the tunnel lining are idealized into three cases: fully drained, partially drained, and impermeable along the tunnel circumference. The impacts of the burial depth of the tunnel, the permeability coefficient of the tunnel lining and the surrounding soil, the Poisson’s ratio of the soil, the lateral Earth pressure coefficient of the soil, and the shear modulus are investigated.

Investigation of saturation effects on vibrations of nearly saturated ground due to moving train loads using 2.5D FEM

An improved two-and-a-half dimensional finite element method (2.5D FEM) model is developed to predict ground-borne vibrations in coastal areas from passing high-speed trains (HSTs). The model treats the ground as nearly saturated layered half-space, and describes the pore water/air mixed fluid by the equivalent fluid approach. Reliability of the proposed model is verified by comparing with the field measurements for X-2000 train and published analytical solutions. On this basis, the ground vibrations and excess pore water pressure under different train speeds and on various soil cases are analyzed in detail. In particular, three kinds of water saturation are discussed to show the sensitivity of vibration responses to the minor change of saturation. Results show that the saturation would cause pronounced influence on vibration displacements and excess pore pressure, and a minor decrease in water saturation would lead to a much higher vibration displacement level and a big difference of excess pore pressure distribution along depth. The influence of saturation is dependent both on soil configurations and on train speeds. It is highlighted that the vibration level on coastal ground would be underestimated by using fully saturated model instead of the nearly saturated model.

Analytical Solution and Consolidation Analysis of One-dimensional Large Strain Consolidation Differential Equation

Abstract:: According to the universal one-dimensional consolidation equation introduced by Gibson, the governing equation with the excess pore water pressure as the control variable is derived, and the Fourier series solution under the boundary condition of single-sided drainage is deduced in detail by the standard mathematical physical method. It verifies the correspondence between the analytical solution and the numerical solution from a theoretical point of view. Using this analytical solution, the nonlinear distribution of the excess pore pressure along the depth direction is obtained, and the traditional small strain consolidation is compared in terms of the average consolidation degree and the final settlement. Soft soil foundation, large deformation foundation Derive a consolidation equation for soft soils with large deformations using the super-static pore pressure as the control variable Formula derivation, Example analysis Based on Gibson's general equation of consolidation and its theory, the detailed derivation process of differential equations with excess pore water pressure as the control variable is given. According to the example, the image shows the distribution of excess pore pressure with depth, and comparative analysis of large and small strains, If all other conditions are the same, When mv1=1 MPa-1, it can be calculated according to the large and small strains, but when mv1≥3MPa-1, the two errors are large, The calculation must be considered separately.

Consolidation of soil induced by pile installation considering disturbance effect

In practice, the consolidation of soil around the pile has a great influence on the time-dependent bearing capacity of pile. However, most of the consolidation theory of soil around the pile neglects the disturbance effect of pile-driving on surrounding soil and regards the soil as homogeneous, which overestimates the consolidation efficiency of the soil, and obtains a higher pile bearing capacity. In view of this, a consolidation model of soil around a pipe pile considering soil disturbance effect is presented in this paper. Fourier transform and separation of variables are used to obtain the analytical solution, and then the solution is verified by degradation analysis and Finite Difference Method (FDM). Firstly, the radial and vertical distribution of excess pore pressure generated after pile-driving is analyzed. In the radial direction, the excess pore pressure decreases rapidly from the radius of pipe pile to the radius of the disturbed zone, then slowly decays to 0 from the radius of disturbed zone to the influencing radius of pipe pile. In the vertical direction, the excess pore pressure along the vertical direction grows linearly. Subsequently, the variation of the average excess pore pressure at the pile-soil interface with the permeability coefficient and radius of disturbance zone are analyzed. The permeability coefficient of disturbance zone has a remarkable negative effect on the excess pore pressure during the whole consolidation period. Increasing the radius of the disturbance zone will hinder the dissipation of the excess pore pressure in the intermediate and later stages. Finally, the validity of the proposed analytical solution is illustrated by comparing with the laboratory results.

Undrained cylindrical cavity expansion/contraction in stiff clays using a two‐surface plasticity model

AbstractThis paper presents a semi‐analytical solution of undrained cylindrical cavity expansion/contraction problem using a two‐surface plasticity model for natural stiff clays from the view point of poromechanics. The present solution reduces the classical boundary problem into a group of seven first‐order ordinary differential equations via an auxiliary independent variable ξ under undrained conditions. It strictly follows the premise of constant soil mass (CSM). Meanwhile, the solution for the constant soil volume (CSV) problem is used for comparison. It shows that CSM assumption is of paramount importance in developing solutions under high stress condition, while CSV induces non‐negligible errors in effective stress and pore pressure distributions. Based on the premise of CSM, extensive parametric studies on cavity expansion/contraction are undertaken to investigate the effects of the model key parameters on the stress components distribution, excess pore pressure distribution, and cavity expansion curves. The present solution could be useful while dealing with geotechnical and petroleum engineering problems involving stiff clays or soft claystones. The application in Praclay Gallery excavation in natural Boom clay well demonstrates the validity of the present solution.

Estimation of the critical buckling load of pile foundations during soil liquefaction

Buckling instability of slender piles may occur based on the effect of excessive axial loads acting on a pile with diminishing effective stress and shear strength owing to surrounding soil liquefaction. However, several guidelines for pile design often provide insufficient consideration of buckling failure, and it is significant to study the buckling failure of piles in liquefiable sites for the design and inspection of piles. In this paper, an efficient method of determining the critical buckling load of piles considering the material and geometric nonlinearities of piles based on the excess pore pressure distribution is proposed based on the beam-on nonlinear Winkler foundation (BNWF) model. Then, the method is verified and validated using results from centrifuge tests. Next, plots of the main effects and analysis of variance (ANOVA) were used for the sensitivity analysis of pile failure modes under axial loading. The initial imperfections and flexural rigidity of the pile have significant effects on the pile failure modes, and the pile is susceptible to buckling failure with low initial imperfections and flexural rigidity. Furthermore, the effects of the flexural rigidity of the pile, unsupported length ratio, initial imperfections, and degree of soil layer liquefaction on the critical buckling load of the pile in liquefiable soil are examined. The axial load-carrying capacity of the pile decreases with increasing initial imperfection amplitude, decreasing flexural rigidity of the pile, decreasing unsupported length ratio and severer degree of soil layer liquefaction. Finally, an estimating formula for predicting the critical buckling load of piles in liquefiable soil and an application example are presented.

Experimental Study on the Distribution of Wave-Induced Excess Pore Pressure in a Sandy Seabed around a Mat Foundation

Mat foundations are widely used in jack-up offshore platforms to support and transfer loads. Regarding mat foundations working on the seabed, the excess wave-induced pore pressure is critical to seabed stability, which may finally cause structural failure. Therefore, it is important to investigate the distribution of the excess pore pressure in the seabed around the mat foundation. In this study, experiments were performed to study the excess pore pressure distribution around a mat foundation in scale considering the true load state by recording wave profiles and pore pressures inside a sandy seabed. To guarantee the reliability of experiments, a numerical study was conducted and compared with the experimental results. Experimental results indicate that with the existence of the mat foundation, the excess pore pressure is higher at the region, the range of which is the width of the model mat (Wm) before the structure. The maximum pore pressure appears at 0.55 Wm in front of the center of the mat foundation. In addition, the current significantly increases the range of high pore pressure area and the amplitude of the excess pore pressure. As the mat orientation changes, the position of the maximum pore pressure changes from the front to the edge of the mat.

Combined wave-current induced excess pore-pressure in a sandy seabed: Flume observations and comparisons with theoretical models

Waves are coexisting with currents in coastal zones; nevertheless, previous experimental studies for excess pore-pressure responses in a porous seabed were predominantly limited to the wave-only condition. In this study, the combined wave-current induced excess pore-pressures in a sandy seabed were experimentally simulated with a specially-designed flume, which can concurrently generate periodic waves and a following/opposing co-directional current. The effect of a current on the wave profile is firstly examined. The wave steepness is decreased by a following current, but enhanced by an opposing current. Flume observations indicate that, under combined wave-current loading, the wave-induced pore-pressure is increased for the following-current case, but reduced for the opposing-current case. Such wave-current combination effect becomes more significant for shorter wave periods. The variation trend of the excess pore-pressure distribution in the present flume observations is consistent with that of the existing analytical solutions. Nevertheless, due to the existence of wave and/or current boundary layer and non-lineartiy of wave-current interactions as indicated by the flume observations, certain deviations exist between the flume results for excess pore-pressure and the analytical solutions, which can not be ignored especially for the opposing-current case. The effects of the boundary layer on the combined wave-current induced pore-pressures in the seabed are further highlighted by supplementary numerical simulations. A favorable prediction by the analytical solution would be expected for following-current cases and smaller pore-pressure amplitudes would be obtained for opposing-current cases.

A study of the excess pore water pressure during pile-sinking construction of nodular pile composite foundation

In combination with the pile-sinking construction of the experimental section of the nodular pile composite foundation at the Dongqian Lake depot of Line 4 of the Ningbo rail transit line, an excess pore water pressure test was conducted and the influence of nodular inter-joint voids on the vertical pore water pressure was considered. The theory of calculating the discretization of the vertical distribution of excess pore pressure caused by nodular pile sinking was deduced. The established theory was used to calculate the 2 m pile spacing test conditions, and the calculation results were compared with the test results. The calculated results were in good agreement with the measured results. Considering the vertical excess static pore pressure calculation theory influenced by the nodular joints, nodular can be better described. The excess pore water pressure caused by the construction of piles can provide design and construction parameters for the pile foundation.

Coupled CFD–DEM method for undrained biaxial shear test of methane hydrate bearing sediments

Methane hydrate (MH), a potential source of future energy, is extensively deposited in marine sediments. It is essential to understand the mechanical properties of methane hydrate bearing sediments (MHBS) for applications relevant to mining and geotechnical engineering. This study aims to investigate the undrained shear strength of MHBS through coupled computational fluid dynamics and discrete element method (CFD–DEM) numerical approach. The Tait’s fluid state equation is implemented into the Navier–Stokes equation-based CFD, while the DEM is used to model granular particle system of MHBS. The CFD–DEM tool is first verified by two typical geomechanics problems where analytical solutions are available. The simulations show that the stress–strain behavior of MHBS depends on temperature, back pressure and MH saturation, as observed in reported experimental results. The presence of MH alters the hardening response of clean sand into softening response due to the bonding effects of MH. The friction angles and cohesions described by total stress and effective stress both increase as the back pressure and MH saturation increase or the temperature drops. There is significant localization in MH bond breakage events but no localization effect is observed in fluid flow and excess pore pressure distribution. This is because fluid is mostly controlled by the boundary conditions instead of specific fluid–particle interactions locally in the simulated quasi-static loading.

Analysis of consolidation of a soil layer with depth-dependent parameters under time-dependent loadings

In this paper, a Laplace transform method is proposed to solve the one-dimensional consolidation problems considering depth-dependent parameters of the soil layer, which is subjected to complicated time-dependent loadings at ground surface. The coefficient of volume compressibility and the coefficient of permeability for the soil layer are continuous functions of vertical coordinate. The solution is obtained and described in detail. The characteristics of consolidation under suddenly imposed constant loading are analysed. The results show that the excess pore pressure distribution of a soil layer with depth-dependent parameters is dissimilar to the solution Terzaghi has proposed. The characteristics of consolidation under time-dependent loadings are also analysed and the influences of depth-dependent parameters are considered. The results also suggest that the effective stress develops with time under time-dependent loading follows the trend under suddenly-imposed constant loading but exhibit certain oscillations. The oscillation amplitudes of the effective stresses don’t decrease with time. Therefore, it is inaccurate to simplify the cyclic loading to constant loading.