Excess Pore Pressure Distribution Research Articles

Numerical analysis of the set-up around the shaft of a closed-ended pile driven in clay

When a pile is driven into the ground, soil standing in the path of the pile is heavily distorted as it has to cede for the penetrating pile. At the end of installation, this soil is left in a distressed state, which progressively evolves to an equilibrium with time. As a result, for lightly overconsolidated clays, soil resistance generally decreases during driving but increases after installation; the latter phenomenon is referred to as set-up. Correct assessment of pile set-up is one of the most important considerations of displacement pile design, especially offshore. Indeed, underestimation of set-up leads to unnecessary expense and overestimation of set-up leads to a precarious overestimation of the pile capacity. This paper presents a numerical model, the aim of which is to predict set-up around the shaft of a driven pile in clay. The model was conceived after a careful literature review of the relevant field data, in order to support its assumptions. It is used to predict the stress and pore pressure distributions around the shaft of a driven pile during installation and subsequent equalisation. Numerical results are then compared to the literature results. Although the model underestimates the excess pore pressure created around the pile, it captures a key feature of displacement pile installation in clay: at the end of installation, the radial distribution of excess pore pressure presents a peak located a few radii away from the pile wall. This implies that, during equalisation, the radial effective stress at the pile wall decreases to a short-term minimum, leading to a short-term minimum in pile capacity, before eventually increasing.

Analytical solution to axisymmetric consolidation in unsaturated soils with linearly depth-dependent initial conditions

This paper introduces an analytical solution for the axisymmetric consolidation of unsaturated soils subjected to constant external loading. The analytical procedure employs variables separation and Laplace transformation techniques while capturing the uniform and linear initial excess pore pressure distributions with depth. Excess pore-air and pore-water pressures as functions of time, radial and vertical flows are determined using Laplace transforms, Fourier Bessel and sine series, respectively. In this study, the consolidation behavior, in terms of changes in excess pore-air and pore-water pressures and the average degree of consolidation, are investigated against the air to water permeability ratio. The effects of radial distance from the drain well on the dissipation rate are likewise highlighted in worked examples. Excess pore pressure isochrones and the matric suction varying with time are also presented.

Influence of initial stress distribution on liquefaction-induced settlement of shallow foundations

During earthquakes, saturated sandy soils may generate significant excess pore pressures and approach a state of liquefaction. Structures founded on shallow foundations above such soils may consequently undergo large settlements. Recent case history analysis has shown that the stress imposed by the foundation is a key factor in the estimation of such settlements. However, the case history data showed that although increasing bearing pressure caused an increase in settlements as expected, this was only true up to a point, and that very heavy structures appeared to settle less than some lighter structures. This work aims to investigate these counter-intuitive results by means of controlled experimental testing using a geotechnical centrifuge. Results of the centrifuge tests show that the trend derived from case histories is correct and that liquefaction-induced settlements peak for a given bearing stress (90 kPa for the models tested) and reduce for greater applied stresses. Further, by analysis of excess pore pressure distributions beneath the foundations it is shown that the main factor inhibiting pore pressure generation beneath the footings is not so much the confining pressure as the in-situ static shear stress around the edge of the foundation. This is supported by element test data from the literature. When this initial static shear stress is so high that the applied cyclic shear stress cannot exceed it (i.e. the direction of shear stress does not reverse) then pore pressure generation is greatly reduced, thus causing the observed reduction in expected settlements.

Pore Pressure Response to Groundwater Fluctuations in Saturated Double-Layered Soil

Analytical solutions are developed for one-dimensional consolidation of double-layered saturated soil subjected to groundwater fluctuations. The solutions are derived by an explicit mathematical procedure using Duhamel’s theorem in conjunction with a Fourier series, when groundwater fluctuation is described by a general time-dependent function and assumed to be the pore water pressure variations at the upper boundary. Taking as an example the harmonic groundwater fluctuation, the relevant response of the excess pore water pressure is discussed in detail, and the main influencing factors of the excess pore pressure distribution are analyzed. A dimensionless parameterθhas been introduced because it significantly affects the phase and the amplitude of excess pore pressures. The influences of the coefficients of permeability and compressibility of soil on the excess pore pressure distribution are different and cannot be incorporated into the coefficient of consolidation in double-layered soil. The relative permeability ratio of two clayey soils also plays an important role on the curves of the distributions of the excess pore pressures. The effects of the thickness of the soil layer on the excess pore pressure distribution should be considered together with the dimensionless parameterθand the permeability and compressibility of the double-layered soil system.

One-Dimensional Consolidation of Double-Layered Foundation with Depth-Dependent Initial Excess Pore Pressure and Additional Stress

A model for one-dimensional consolidation of a double-layered foundation considering the depth-dependent initial excess pore pressure and additional stress and time-dependent loading under different drainage conditions was presented in this study and its general analytical solution was deduced. The consolidation solutions of several special cases of single-drained and double-drained conditions under an instantaneous loading and a single-level uniform loading were derived. Then, the average degree of consolidation of the double-layered foundation defined by settlement was gained and verified. Finally, the effects of the initial excess pore pressure distributions, depth-dependent additional stress, and loading modes on the consolidation of the soft foundation with an upper crust with different drainage conditions were revealed. The results show that the distributions of initial excess pore pressure and additional stress with depth and loading rates have a significant influence on the consolidation process of the soft foundation with an upper crust. This influence is larger with the single-drained condition than that with the double-drained condition. Comparing the consolidation rate with a uniform initial pore pressure and additional stress, their decreasing distribution with depth quickens the consolidation at the former and middle stages. Moreover, the larger the loading rate is, the quicker the consolidation of the soft foundation with an upper crust is.

Determination of the continuous stress-dependent permeability, compressibility and poroelasticity of shale

The continuous changes of the stress-dependent permeability, compressibility and poroelasticity of a tight oil and gas shale were characterized by using the Constant Rate of Strain (CRS) consolidation test. The CRS consolidation test compresses a thin disk test specimen at a given constant rate of strain under one-dimensional consolidation with one-sided drainage condition and measurement of excess pore pressure at the undrained end. Permeability is calculated from the one-dimensional consolidation equation assuming oedometric loading, incompressible solid grains, and an idealized excess pore pressure distribution in the sample. The CRS test method has been widely used for the determination of the stress-dependent permeability of soft sediments, but has not been utilized for very stiff and low permeability shale as well as to obtain their stress-dependent compressibility and poroelasticity parameters. To test its appropriateness to shale, CRS tests were performed on thin disk samples of Mancos shale using a high pressure and high temperature triaxial cell under high isotropic confining stresses. Two modifications were done to make the CRS test applicable to determination of the continuous stress-dependent hydro-mechanical properties of tight shale: (1) the test method was modified for isotropic loading, and (2) nonlinear poroelastic effects were accounted for in the solution of the pore pressure dissipation equation. The permeability values from the CRS tests with the modified analytical solution were found to be in agreement with those obtained from the Constant Pressure Gradient Permeability test, and Pressure-pulse Decay Permeability test using nitrogen as pore fluid and corrected for Klinkenberg and non-Darcy flow effects.

Upper bound limit analysis of roof collapse in shallow tunnels with arbitrary cross sections under condition of seepage force

The analytical solutions for predicting the exact shape of collapse mechanisms in shallow tunnels with arbitrary excavation profiles were obtained by virtue of the upper bound theorem of limit analysis and variation principle according to Hoek-Brown failure criterion. The seepage force was included in the upper bound limit analysis, and it was computed from the gradient of excess pore pressure distribution. The seepage was regarded as a work rate of external force. The numerical results of roof collapse in square and circular tunnels with different rock parameters were derived and discussed, which proves to be valid in comparison with the previous work. The influences of different parameters on the shape of collapsing blocks were also discussed.

A simple method to account for non-uniform initial excess pore water pressures in settlement computations

The variation in percentage consolidation with time within a clay layer subjected to a non-uniform initial excess pore water pressure distribution can be difficult to evaluate, and as a result, often a uniform initial distribution is assumed in most analyses. However, by utilizing some of the key features of consolidation in terms of excess pore water pressure dissipation, it is possible to simply adjust the uniform case to account for any number of non-uniform initial excess pore pressure distributions. By observing the decay of excess pore water pressure with time resulting from various non-uniform initial distributions, it is clear that any initial asymmetry or skewness is quickly dispersed, and the distribution of excess pore pressure with depth becomes sinusoidal (or half-sinusoidal if singly drained) shortly after consolidation has commenced. In other words, once the pore pressure decay due to a non-uniform initial distribution has become sinusoidal, it will actually decay at the same rate as the uniform case – however, it will be “ahead” or “behind” the uniform case by some constant factor. Once this factor has been determined, it is possible to simply adjust the rate of consolidation resulting from a uniform initial pore pressure distribution (the values of which are widely available in literature) to account for any number of realistic non-uniform initial excess pore pressure distributions.

Limit analysis of rectangular cavity subjected to seepage forces based on Hoek-Brown failure criterion

On the basis of Hoek.Brown failure criterion, a numerical solution for the shape of collapsing block in the rectangular cavity subjected to seepage forces is obtained by upper bound theorem of limit analysis. The seepage forces obtained from the gradient of excess pore pressure distribution are taken as external loadings in the limit analysis, and the pore pressure is easily calculated with pore pressure coefficient. Thus the seepage force is incorporated into the upper bound analysis as a work rate of external force. The upper solution of the shape of collapsing block is derived by virtue of variational calculation. In order to verify the validity of the method proposed in the paper, the result when the pore pressure coefficient equals zero, and only hydrostatic pressure is taken into consideration, is compared with that of previous work. The results show good effectiveness in calculating the collapsing block shape subjected to seepage forces. The influence of parameters on the failure mechanisms is investigated.

Design and analyses of floating stone columns

Two-dimensional (2D) finite element analyses were performed on floating stone columns using the unit cell concept to investigate the settlement and the consolidation characteristics of an improved foundation system. Undrained analyses, followed by consolidation analyses, were conducted throughout the study. The computed values for settlement and excess pore pressure distribution over time are compared for different area replacement ratios. Based on these coupled consolidation analyses, a simple approximate method is developed to predict the degree of consolidation for floating stone columns. In addition, a simple method to calculate the settlement improvement factor for floating columns is proposed. The proposed method may provide more realistic answers than other design methods in view of the yielding characteristics and the influence of key parameters that are considered in the analyses. The key parameters relevant to the design of floating stone columns include the area replacement ratio, the friction angle of the column material, the loading intensity, and the post-installation earth pressure. Closer agreements are obtained with the proposed method than with the established Priebe׳s method or α–β method.

Effect of perforations on uplift capacity of skirted foundations on clay

The retrieval of deep water subsea installations resting on soft soil, such as “mudmat” shallow foundations, can be a difficult and costly operation if significant resistance to uplift is experienced. At the mudmat invert, suctions may develop, increasing the uplift resistance to greater than the weight of the mat. In this paper, a series of centrifuge model tests are performed to determine the uplift resistance of rectangular mudmats resting on lightly overconsolidated kaolin clay. The study investigates the influence of perforation, in combination with skirt length and eccentric uplift, on the uplift resistance and suction generation at the foundation invert. The outcomes demonstrate that the central and eccentric uplift of mudmats have different failure mechanisms, resulting in a different distribution of excess pore pressure at the foundation invert. In contrast, perforations do not change the failure mechanism and only alter the magnitude of suction generated. The two different configurations of perforation investigated significantly reduce the suction at the mat invert and the uplift resistance, and may potentially shorten the operating time for centred uplift. The combination of perforation and eccentric uplift has the most beneficial effect on the reduction of the uplift resistance.

Evaluation of coefficient of consolidation for dilatory dissipation in piezocone test in overconsolidated cohesive soils

The interpretation method for dilatory dissipation, showing an initial temporary increase in pore pressure, in piezocone tests in overconsolidated clayey soils was investigated in this study. Finite difference analyses on the dissipation after cone penetration were performed with various ground conditions. The spatial distribution of initial excess pore pressure induced by cone penetration was determined from experimental estimations through in-situ tests and laboratory physical model tests and theoretical predictions by the cavity expansion theory and critical state soil mechanics. A method to evaluate the coefficient of consolidation from dilatory dissipation results was developed through the assessment of numerical simulation results. Comparing the application results with the laboratory-determined values, it was verified that the proposed approach provided a reliable and consistent evaluation of the coefficient of consolidation.

Coupled DEM–LBM simulation of internal fluidisation induced by a leaking pipe

This paper presents a numerical study of soil fluidisation induced by local leakage from a buried pipe using the coupled discrete element and lattice Boltzmann approach (DEM–LBM). The behaviour of the system is studied using different flow rates and initial bed heights. The excess pore pressure distribution and the soil behaviour due to a localised leak are investigated and compared with previous experimental findings. The comparison indicates that DEM–LBM is capable of capturing the leakage–soil interaction at the particle scale. Moreover, as the flow distribution is found to be one-dimensional in the radial direction rather than one-dimensional in the vertical direction, an extension to the Ergun flow equation is derived and the numerically measured fluidising pressure shows good agreement with this analytical solution. In addition, as indicated by the numerical results and the derived analytical solution, the relationship between fluidising pressure and pipe buried depth as well as leak size is also established. The simulation results suggest that DEM–LBM is a promising tool to carry out further studies into the internal fluidisation phenomenon.

Numerical Analysis for Excess Pore Pressure Dissipation Process for Pressed Pile Installation

The finite element analysis is performed on the excess pore pressure dissipation for pressed pile installation using the ABAQUS finite element software. The modified Cam-Clay model is used to model the soil behavior. The finite slide contact model is used to model the pressed pile installation process. Based on the geology stratum of soils and drainage conditions, the excess pore pressure dissipation process is analyzed using the proposed method. The initial excess pore pressure distribution along the pile depth and the pile radius direction is obtained. The excess pore pressure dissipation after 98 days is analyzed.

Limit analysis of roof collapse in tunnels under seepage forces condition with three-dimensional failure mechanism

The state of roof collapse in tunnels is actually three-dimensional, so constructing a three-dimensional failure collapse mechanism is crucial so as to reflect the realistic collapsing scopes more reasonably. According to Hoek-Brown failure criterion and the upper bound theorem of limit analysis, the solution for describing the shape of roof collapse in circular or rectangular tunnels subjected to seepage forces is derived by virtue of variational calculation. The seepage forces calculated from the gradient of excess pore pressure distribution are taken as external loading in the limit analysis, and it is of great convenience to compute the pore pressure with pore pressure coefficient. Consequently, the effect of seepage forces is taken as a work rate of external force and incorporated into the upper bound limit analysis. The numerical results of collapse dimensions with different rock parameters show great validity and agreement by comparing with the results of that with two-dimensional failure mechanism.

Time Factor in Consolidation: Critical Review

The magnitude of consolidation settlement is often calculated using Terzaghi's expression for average degree of consolidation (U) with respect to time. Developed during a time of limited computing capabilities, Terzaghi's series solution to the one-dimensional consolidation equation was generalized using dimensionless time factor (T), where a single U-T curve is used to describe the consolidation behaviour of both singly and doubly drained strata. As a result, any comparisons between one- and two-way drainage are indirect and confined to discrete values of time. By introducing a modified time factor T* in terms of layer thickness (D) instead of the maximum drainage pat length (Hdr), it is now possible to observe the effect of drainage conditions over a continuous range of time for a variety of asymmetric initial excess pore pressure distributions. although two separate U-T plots are required (for singly and doubly drained cases), the time factor at specific times remains the same for both cases, enabling a direct visual comparison. The importance of a revised time factor is evident when observing the endpoint of consolidation, which occurs as U approaches 100%. This occurs at T*≅0.5 for two-way drainage and at T*≅2 for one-way drainage, an observation not possible using the traditional expression for time factor.

Axisymmetric Consolidation of Unsaturated Soils by Differential Quadrature Method

Axisymmetric consolidation in a sand drain foundation is a common problem in foundation engineering. In unsaturated soils, the excess pore-water and pore-air pressures simultaneously change during the consolidation procedure; and the solutions are not easy to obtain. The present paper uses the differential quadrature method (DQM) for axisymmetric consolidation of unsaturated soils in a sand drain foundation. The radial seepage of sand drain foundation is considered based on the framework of Fredlund’s one-dimensional consolidation theory in unsaturated soils. With the use of Darcy’s law and Fick’s law, the polar governing equations of excess pore-air and pore-water pressures of axisymmetric consolidation are derived. By using DQM, the two governing equations are transformed into two sets of ordinary differential equations. Then the solutions of excess pore-water and pore-air pressures can be obtained by Rong-Kutta method. The DQM solution can be used to deal with the case of nonuniform initial pore-air and pore-water distributions. Finally, case studies are presented to investigate the behavior of axisymmetric consolidation of unsaturated soils. The convergence analysis and average degree of consolidation, the settlements in radial and vertical direction, and the effects of different initial excess pore pressure distributions are presented, and discussed in this paper.

Application of recycled aggregate porous concrete pile (RAPP) to improve soft ground

In this study, a series of laboratory chamber tests was carried out to evaluate the applicability of a porous concrete pile fabricated with recycled aggregates for soft ground improvement. The recycled aggregate porous concrete pile (RAPP) has been developed to replace natural aggregates and to overcome technical problems associated with the conventional compaction piling systems. The laboratory chamber tests for evaluating the performance of RAPP were carried out with a cylindrical mold of 280 mm in internal diameter and 580 mm in height. A replacement area ratio of 5 % and three different loading steps were applied in the chamber tests. The experimental results of the surface settlement, excess pore pressure and vertical stress distribution versus time were compared with those of the sand compaction pile (SCP) reinforced composite ground under the same experimental condition. In addition, the experimental results were compared with the numerical simulation using ABAQUS. The current study shows that the settlement reduction in the RAPP-reinforced system is significantly enhanced due to load transfer from the soil formation to the RAPP. Furthermore, the comparison of consolidation rates shows that the RAPP can also accelerate the consolidation of soft clay formation because the RAPP behaves as a vertical drain.

Calculating cv based on non-uniform initial excess pore pressure

The coefficient of consolidation (cv) is often determined by comparing the characteristics of the experimental and theoretical consolidation using empirical curve-fitting procedures which are based on the theoretical U−T curve generated by a layer subjected to a uniform initial excess pore water pressure distribution. However, in cases where settlement–time data are a result of a non-uniform initial pore pressure distribution, these curve-fitting procedures are no longer valid. In this technical note, a generalised procedure for Taylor and Casagrande's popular curve-fitting procedures is proposed, where the user is directed to select appropriate adjustment factors, depending on the type of non-uniform initial excess pore pressure distribution encountered. These factors were determined by approximating separate regions of the U−T curves using simple power and exponential functions. In non-uniform cases where the power approximation only captures a small portion of the U−T curve it may be difficult to use the corresponding modified curve-fitting procedure objectively.

Stability analysis of rock slopes subjected to seepage forces using the modified Hoek–Brown criterion

The paper deals with the stability analysis of a water-saturated rock slope by means of the kinematic approach of limit analysis theory. Particular emphasis is first given to the effects of pore water pressure on the global stability of geotechnical structures. The rock strength properties being formulated in terms of effective stresses, it is shown how the effect of seepage flow may be accounted for such an analysis through driving body forces derived from the gradient of excess pore pressure distribution is shown. The latter is obtained as the solution to a hydraulic boundary value problem, and then incorporated as external loading in the subsequent stability analysis. The rock strength properties are modeled by a modified Hoek–Brown failure criterion, for which closed-form expressions of the support functions have been derived in a previous paper. The approach is then applied to investigate the stability of rock slopes. Computational results are given, providing ample evidence of the destablizing effects induced by the seepage forces. The influence of relevant geometrical, strength and loading parameters is also discussed.