Dissipation Of Excess Pore Pressure Research Articles

A closed-form criterion to identify high-mobility flowslides

This paper derives a closed-form criterion to assess the risk of flowslide runout in loose frictional soil. The derivations rely on a recently proposed framework to simulate pre- and post-failure motion in infinite slopes. An analytical solution of the coupled differential equations capturing flowslide hydromechanics is obtained by specifying them for a perfectly plastic constitutive law. This result enables a comprehensive examination of the factors that control whether the landslide motion, once triggered, autonomously comes to rest (self-regulating behaviour with low mobility) or continues to propagate (self-feeding behaviour with high mobility). It is found that the time history of motion is regulated by non-dimensional property groups reflecting the timescale of excess pore pressure dissipation and the inertial properties of the liquefied zone, which are in turn governed by material (e.g. hydraulic conductivity, dilation coefficient, elastic moduli) and slope properties (e.g. thickness, inclination). The solution is used to build charts identifying the critical ranges of soil properties and triggering factors that differentiate between high-mobility and low-mobility flowslides. Most importantly, it is shown that the fate of flowslide motions is predicted by a critical ratio expressed in terms of excess pore pressure and flow velocity, here defined as the factor of mobility, FM, with values above 1 indicating a self-feeding runout.

Large deformation analysis of spudcan settlement during operation

Jack-up rigs have been utilized to support booster stations in wind farms or to provide accommodation in oil and gas developments. In these scenarios, the dissipation of excess pore pressures and subsequent spudcan settlement during operation phase are of concern. However, predicting the spudcan settlement in naturally structured soil remains challenging. A large deformation finite element method, within the framework of effective stress analysis, has been developed to investigate the penetration and subsequent settlement of the spudcan footing. The inherent structure of cohesive soil and its degradation caused by spudcan penetration are described using the Structured Cam-Clay model. The large deformation approach is validated by comparisons with existing centrifuge tests. Parametric studies are then conducted with various operational loads and installation depths of spudcans. A backbone curve is provided, to estimate the history of spudcan settlement in kaolin with inherent structure. The distribution of excess pore pressures around the spudcan under different operation period is explored as well. A normalised dissipation curve at the largest cross-section of the spudcan during the operation phase is presented.

Centrifuge modeling of a large-scale surcharge on adjacent foundation

This study investigates the ground and structural response of adjacent raft foundations induced by large-scale surcharge by ore in soft soil areas through a centrifuge modeling test. The prototype of the test is a coastal iron ore yard with a natural foundation of deep soft soil. Therefore, it is necessary to adopt some measures to reduce the influence of the large-scale surcharge on the adjacent raft foundation, such as installing stone columns for foundation treatment. Under an acceleration of 130 g, the model conducts similar simulations of iron ore, stone columns, and raft foundation structures. The tested soil mass has dimensions of 900 mm × 700 mm × 300 mm (length × width × depth), which is remodeled from the soil extracted from the drilling holes. The test conditions are consistent with the actual engineering conditions and the effects of four-level loading conditions on the composite foundation of stone columns, unreinforced zone, and raft foundations are studied. An automatic layer-by-layer loading device was developed to simulate the loading process of actual engineering more realistically. The composite foundation of stone columns had a large settlement after the loading, forming an obvious settlement trough and causing the surface of the unreinforced zone to rise. The 12 m surcharge loading causes a horizontal displacement of 13.19 cm and a vertical settlement of 1.37 m in the raft foundation. The stone columns located on both sides of the unreinforced zone suffered significant shear damage at the sand-mud interface. Due to the reinforcement effect of stone columns, the sand layer below the top of the stone columns moves less. Meanwhile, the horizontal earth pressure in the raft foundation zone increases slowly. The stone columns will form new drainage channels and accelerate the dissipation of excess pore pressure.

Influence of geosynthetics reinforcement on liquefaction and post-liquefaction behaviors of calcareous sand

To provide new insights into the liquefaction and post-liquefaction behaviors of calcareous sand with and without geosynthetics reinforcement, a series of multi-stage cyclic triaxial tests were conducted. The geosynthetics employed in this study include geogrid, geotextile, and geotextile-geogrid composite. The multi-stage tests consist of an initial cyclic loading applied to cause liquefaction, followed by undrained monotonic loading without excess pore pressure dissipation. The effect of different arrangements of reinforcement layer on the behaviors of calcareous sand is examined and discussed in this study. The test results indicate that a unique relationship can be observed between the double amplitude axial strain and the pore pressure ratio of calcareous sand, irrespective of the influence of reinforcement layer arrangement, providing an effective means of predicting the strain at a given pore pressure level. The liquefaction resistance of calcareous sand increases with the increase in the number of reinforcement layer and decreases with the increase in the distance from the first layer of reinforcement to the sample’s top surface. Compared to geogrid and geotextile, the proposed geotextile-geogrid composite exhibits better efficiency in enhancing the liquefaction resistance of calcareous sand. The reinforcement also accelerates the recovery of strength for liquefied calcareous sand and increases the maximum shear strength of sand at large axial strain during the post-liquefaction stage.

Analytical solution for consolidation of saturated viscoelastic soils around tunnels with general Voigt model

This paper presents a novel analytical solution for the consolidation behavior of viscoelastic saturated soft soil subjected to large-scale ground loading. The rheological properties of clay are described using the general Voigt model. Based on the Terzaghi-Rendulic theory, the governing equations for the dissipation of excess pore water pressure in the surrounding soil mass of a tunnel are established under the first and second boundary conditions. The governing equations are solved using the complex variable method. The obtained solutions are verified by reducing them to the forms of three traditional rheological models, demonstrating the reliability of the proposed approach. Finally, based on the established solutions, the dissipation characteristics of excess pore water pressure around the tunnel are analyzed. The case results indicate that the soil permeability coefficient (k), the independent Newtonian viscosity coefficient (K0) and Hooke’s spring modulus (E0) in the general Voigt model have significant influences on the dissipation of excess pore water pressure and the degree of consolidation. A larger k, K0, and E0 lead to faster dissipation and consolidation development, while a greater tunnel buried depth (b) results in slower consolidation process. The influence of k on excess pore water pressure dissipation is more significant than that of K0 and E0. For the first boundary type, consolidation is instantaneous when k > 0.1 m/d. For the second type, when k < 0.0001 m/d, excess pore pressure remains unchanged within 100 d. The permeability condition of the tunnel has a considerable impact on the distribution of excess pore water pressure in the soil layer directly above the crown. When the tunnel is fully permeable, the effects of k, K0, and E0 on the dissipation of excess pore water pressure are more pronounced in the early stage, with almost complete dissipation of excess pore pressure above the tunnel before 100 d. However, when the tunnel is completely impermeable, their effects are more prominent in the later stage, and by 100 d, the maximum excess pore pressure within the depth range is 25 % of the initial.

Time-dependent processes influencing offshore foundations in clay: an experimental study on plate anchors

Motivation for this paper stems from experimental investigations that consider how the vertical capacity of a horizontal circular plate anchor in clay changes due to consolidation. These experiments produced interesting time-dependent measurements that prompted a follow-on study, designed to explore the underlying mechanisms. The new experimental data indicate that changes in anchor load during consolidation under a fixed anchor displacement are linked to three distinct mechanisms. The first is a ‘stress-relaxation’ reduction in anchor load that occurs quasi-instantaneously after the initial anchor movement stops, with a magnitude that is linked to the average strain rate associated with the initial anchor movement. The second is a local consolidation effect that causes a reduction in anchor load over durations that scale with the anchor diameter. The final mechanism occurs simultaneously with the second, but at a slower rate, such that the resulting increase in anchor load becomes apparent at larger values of time. This increase in anchor load is due to dissipation of excess pore pressures developed in the wake of the anchor during the initial anchor movement. These time-dependent changes are considered relevant for the post-installation capacity of offshore anchors and for the capacity of anchors following a large movement event.

Seismic Performance of Drained Piles in Layered Soils.

The provision of drains to geotechnical elements subjected to strong ground motion can reduce the magnitude of shaking-induced excess pore pressure and the corresponding loss of soil stiffness and strength. A series of shaking table tests were conducted within layered soil models to investigate the effectiveness of drained piles to reduce the liquefaction hazard in and near pile-improved ground. The effect of the number of drains per pile and the orientation of the drains relative to the direction of shaking were evaluated in consideration of the volume of porewater discharged, the magnitude of excess pore pressure generated, and the amount of de-amplification in the ground's motion. The following main conclusions can be drawn from this study. Single, isolated piles and a group of drained piles were tested in three series of shake table tests. Relative to conventional piles, the drained piles exhibited improved performance with regard to the generation and dissipation of excess pore pressure and stiffness of the surrounding soil, with increases in performance correlated with increases in the discharge capacity of the drained pile. The acceleration time histories observed within the pile-improved soil indicated a coupling of the rate and magnitude of porewater discharge, excess pore pressure generated, and de-amplification of strong ground motion. The amount of de-amplification reduced with increases in the number of drains per pile and corresponding reductions in excess pore pressure. The improved performance should prove helpful in the presence of sloping ground characterized with low-permeability soil layers that inhibit the dissipation of pore pressure and have demonstrated the significant potential for post-shaking slope deformation.

Analysis of nonlinear rheological consolidation with vertical drains: large or small strain

Given the importance of vertical drainage systems in facilitating the stability of soft ground, this paper presents a feasibility study of small-strain consolidation solutions with vertical drains and a quantitative comparison with large-strain consolidation solutions. This solution incorporates essential factors, such as non-Darcian vertical and radial flows, nonlinear void ratio and permeability relationship, initial self-weight stress, elastic viscoplastic constitutive equation, and time-dependent loading. In the design of vertical drains for large and small deformation consolidation equations are comprehensively addressed. The numerical results obtained by the finite difference method demonstrate that the conventional small-strain consolidation method for soft clay has the potential to overestimate the dissipation of excess pore pressure and underestimate the settlement at the later stage, yet correctly assess the effective stress reduction during the initial loading period. Based on the comparison of consolidation data, the relative difference in settlement between large-strain and small-strain consolidation theories will exceed 5% as the accumulated vertical strain reaches about 13%. Another threshold exists for the ratio of the soil thickness to the influence radius (H / r e) to affect the consolidation rate, whether or not the self-weight stress is captured.

A new analytical model to predict the effect of suction removal on vacuum preloading of fine copper tailings

This research deals with the vacuum preloading of fine-grained soft tailings related to the Sungun copper mine through laboratory tests and theoretical analysis. Vacuum preloading tests are conducted in two groups using a newly configured radial consolidation apparatus. During the first group of tests, the suction intensity is maintained constant similar to conventional vacuum consolidation tests, and in the second group, the suction is removed at certain steps after starting the preloading to investigate the vacuum removal effect on consolidation behavior. The samples in the slurry form are tested under three suctions of 20 kPa, 30 kPa, and 40 kPa individually considering influences of the initial water content of samples, vacuum pressure distribution, smear zone parameters, well resistance, and the dimensions of the vertical drain. Experimental results indicated that the initial water content and suction intensity severely governed the ultimate vertical strain of the sample. Also, depending on removal time, the sample could experience settlement, swelling, or no volume change after removing the suction. In the second part of the study, the vacuum removal effect is included in a new analytical model for vacuum preloading based on the Darcian law to predict the excess pore pressure dissipation and vertical strains. The proposed solutions successfully led to highly accurate predictions for vertical strains observed through comparative tests on the copper tailings. Besides, the proposed model effectively reproduced measurements of an in-situ vacuum consolidation of soda-ash tailings on a site near Dalian Bay in China, and the results of a numerical study on vacuum removal for a project in Ballina, Australia, with maximum errors of approximately 7%.

Investigation of trench effect on fatigue response of steel catenary risers using an effective stress analysis

The design of Steel Catenary Risers (SCR) heavily relies on the interaction between the riser and seabed in the touchdown zone (TDZ). To accurately estimate the SCR-seabed stiffness, the soil behaviour must be assessed in two main phases: (1) during cyclic SCR motions, which generate excess pore pressure, leading to soil softening and remoulding in the TDZ, and (2) during inactivity periods throughout the SCR’s lifespan, causing dissipation of excess pore pressure associated with the consolidation state. The latter was neglected by the existing non-linear hysteretic soil models, which are based on the total stress approach. In the current study, a global riser analysis was conducted and the consolidation effect was incorporated using an effective stress framework. Long-term soil stiffness was determined by capturing both the remoulding and consolidation effects, which are respectively associated with the damage accumulation during the SCR cyclic motions and soil strength recovery during the intervening pause period. The constructed model was then combined with a new methodology named Hybrid Trench Model to investigate how the consolidation effect contributes to the fatigue performance in different trench depths. Stochastic fatigue analysis showed that the consolidation effect might increase the damage by around 10%-40% over the long-term assessment.

Modelling biogeochemical clogging affecting piezometers in acid sulfate soil terrain

This study offers an analytical solution for radial consolidation that captures the biogeochemical clogging effect in acid sulfate soils. Field sites and personal communication with industry practitioners have provided evidence of piezometers exhibiting retarded pore pressure readings that do not follow conventional soil consolidation and seepage principles when installed in coastal acidic floodplains. This retarded response together with a variation in pH, ion concentrations, and piezometric heads provided evidence of clogging at and around the piezometers. This paper uses the proposed biogeochemical clogging model, which is an analytically derived system of equations to estimate the excess pore water pressure dissipation of piezometers installed in clogging-prone acid sulfate soils. The inclusion of the total porosity reduction attributed to biological and geochemical clogging improves the predictions of the retarded dissipation of excess pore pressure, especially after about 1 year. This method is validated for two previously identified acidic field sites in coastal Australia, where piezometers measured a very slow rate of dissipation. It is concluded that this model has potential to accurately monitor the performance of critical infrastructure, such as dams and embankment foundations built on acidic terrain.

Numerical study of piezoball dissipation test with penetration under partially drained conditions

The piezoball, a full-flow penetrometer equipped with a pore pressure transducer, is increasingly being used in in-situ investigations due to its advantages of characterizing soft soils relative to the piezocone. The operative coefficient of consolidation, ch, can be estimated by monitoring the dissipation of excess pore pressures after piezoball penetration. However, the determination of ch from the dissipation test becomes complicated if the piezoball is penetrated under partially drained conditions. The performances of piezoball penetration and subsequent dissipation are investigated using a coupled large deformation finite-element approach. The numerical results are verified by comparing with existing centrifuge tests, against both undrained and partially drained conditions. A comprehensive interpretation method is then presented for inferring ch from piezoball tests against various drainage conditions. It is found that the variation of ch with drainage conditions is essentially dependent on the distribution of pore pressures before dissipation. Two backbone curves determining the nominal initial excess pore pressure at the equator of the probe and the coefficient of consolidation are obtained to assess the effect of partial drainage, which capture the influences of the piezoball geometry, stress level and over-consolidation ratio.

One-dimensional self-weight consolidation of layered soil under variable load and semi-permeable boundary condition

In this study, an analytical solution is proposed to describe one-dimensional self-weight consolidation of two-layer soil under variable load and semi-permeable boundary condition. The analytical solution is comprehensively validated against existing field test, analytical solution and numerical simulation results. The influence of drainage capacity of boundary, self-weight, load rate, and soil layering features are then investigated. The difference of consolidation ratio between the complete pervious boundary condition and the semi-permeable boundary condition decreases with increasing semi-permeability coefficient R. Considering the self-weight will increase the excess pore pressure and make the maximum pressure appear below the middle depth, especially at the early time. The effect will be weaker and weaker and finally disappear. The dissipation of excess pore pressure is significantly impeded if the soil layer with lower consolidation coefficient is at the bottom. Finally, the design curve of the time for practical completion of consolidation (tst) is provided. An almost flat part can be observed in the curve if the relative thickness of the layer with lower consolidation coefficient ranges from 0.1 to 0.23, indicating that the influence of relative thickness on tst is negligible over this range, which provides a helpful reference for two-layer soil consolidation in engineering practices.

Cumulative cyclic response of offshore monopile in sands

Satisfactory performance of offshore wind turbines is governed by the cyclic response of its foundation under wind and wave. Most methods of evaluating the lateral cumulative deformation of monopile due to cyclic loading are based on small-diameter pile tests installed in dry sand. This paper investigates the cumulative deformation of large-diameter monopiles installed in saturated sand. A numerical model was developed employing FLAC3D and was validated by comparing its predictions with the results of triaxial tests and centrifuge tests. The validated numerical model was used to evaluate and compare the cyclic responses of monopiles between saturated case and no pore pressure case. To evaluate the cumulative deformation for offshore monopile due to cyclic loading, the numerical model was refined to account for the effects of number and amplitude and frequency of cyclic loading, soil permeability coefficient, soil relative density and pile diameter. It was found that the larger subsidence and the less capacity of soil around the pile is caused by accumulation and dissipation of transient excess pore pressure, thus the monopile in saturated sand would experience the larger cumulative deformation. Correspondingly, based on this parametric study, an analytical model was developed to calculate the pile-top cumulative displacement of offshore large-diameter monopile under lateral cyclic loading considering the effects of pile diameter. This analytical model was validated against the results of centrifuge tests and model pile tests in saturated sands.

Variation in axial load distribution of piles in liquefiable slope by centrifuge test

During seismic loading, the axial pile response is significantly affected, due to the development and dissipation of excess pore pressure in the foundation soil. Positive skin friction is reduced by liquefaction during shaking, while negative skin friction is caused by reconsolidation after the end of the shaking. In particular, the negative skin friction causes a large drag load, resulting in pile compression and settlement. Thus, this study investigated the axial pile response under seismic loading by performing dynamic centrifuge tests on liquefied sand with the slopes of 15° and 27°. The centrifuge tests were conducted to simulate the end-bearing single and 2 × 2 group piles embedded in a liquefied slope. During the centrifuge spinning, the obtained drag load due to the negative skin friction agreed with the value calculated with the beta method for shaft resistance. The model was shaken with sinusoidal base motion with peak amplitudes of 0.1 and 0.2 g, and it was found that the liquefaction caused a loss of spin-induced drag load. The dissipation of excess pore pressure resulted in reconsolidation settlement, leading to large drag loads acting on the pile. In addition, the negative skin friction was found to be larger than the liquefied residual strength recommended in the current practice codes. As a result, it is seen that the consideration of the strength reduction is not necessary for the calculation of negative skin friction induced by reconsolidation, since the liquefied ground regains skin friction after the dissipation.

Evolution of breakout force with operation period for deeply embedded spudcan in clay

The extraction of spudcan foundation in clay can often be difficult and time-consuming considering the large pull-out force required to overcome the breakout resistance. This is particularly an issue for long operation periods, during which the dissipation of excess pore pressure in the soil will increase the base suction and undrained shear strength of soil. This paper investigates the spudcan breakout force in clay deposits through a coupled pore fluid-effective stress large deformation finite element approach. The complete process of spudcan installation, operation and removal is simulated. The numerical approach is verified against existing centrifuge test results. A good agreement can be obtained between the numerical and experimental results in terms of normalised breakout force and operation period. A series of parametric study is then carried out to explore the effects of installation depth, operational load and soil parameters. Based on the numerical results, a unified relationship can be established between the dimensionless forms of breakout force and operation period, for which a simple algebraic expression is proposed for the convenient assessment of breakout force recovery.

Consolidation analysis around cavity expansion in presence of vertical drains

As a ground improvement technique, the new method composed by a cylindrical cavity expansion (CCE) combined with vertical drains (VD) greatly accelerates the consolidation of soft soil. In fact, the cylindrical cavity applies a radial displacement in the soil. Excess pore pressures will be consequently generated and the consolidation process will occur at the vicinity of the cavity wall. The dissipation of excess pore pressures can be enhanced in presence of VD installed at a radial distance rd from the center of the cylindrical cavity. Because of CCE installations in soft soil, surrounding soil properties are altered in such a manner that horizontal permeability of the soil in the smear zone reduces. The purpose of the present paper is to investigate the subsequent consolidation around CCE using the Finite Difference Method (FDM) by taking into account the CCE installation disturbance. Parametric analysis aimed to investigate the effect of the extent of the disturbed zone, the soil permeability reduction in the smear zone on the rate of consolidation around CCE.

Dynamic analysis of beams vibrating on nonlinear poroelastic multi‐layered continuum

AbstractThe paper presents a framework for analysis of beams interacting with nonlinear‐poroelastic, layered continuums (e.g., clayey soils) when subjected to time‐dependent loads. The poroelastic layered continuum is characterized by a nonlinear‐elastic constitutive relationship that relates the secant shear modulus to the induced shear strain. The Biot's theory of consolidation is combined with a dynamic beam‐continuum interaction model to develop the analysis. The vertical consolidation settlement of the beam and the excess pore pressure in the porous continuum are assumed to be products of separable functions, and the extended Hamilton's principle of least action is applied to obtain the differential equations governing the inertial consolidation motion of the beam‐continuum system and the dissipation of excess pore pressure. An iterative numerical algorithm is used to solve these coupled differential equations following one‐dimensional finite element analysis in which the implicit Wilson‐Θ time integration scheme is used to obtain the time history of beam and continuum responses. The novelty of the framework is that it rigorously takes into account the nonlinear poroelastic soil‐structure interaction within a dynamic time‐integration framework with minimal computational resources. The characteristics of this newly developed model are illustrated through examples.

Numerical Modeling of Liquefaction-Induced Downdrag: Validation against Centrifuge Model Tests

Earthquake-induced soil liquefaction can cause soil settlement around piles, resulting in drag load and pile settlement after shaking stops. Estimating the axial load distribution and pile settlement is important for designing and evaluating the performance of axially loaded piles in liquefiable soils. Commonly used neutral plane solution methods model the liquefiable layer as an equivalent consolidating clay layer without considering the sequencing and pattern of excess pore pressure dissipation and soil settlement. Moreover, changes in the pile shaft and the tip resistance due to excess pore pressures are ignored. A TzQzLiq numerical model was developed using the existing TzLiq material and the new QzLiq material for modeling liquefaction-induced downdrag on piles. The model accounts for the change in the pile’s shaft and tip capacity as free-field excess pore pressures develop or dissipate in soil. The developed numerical model was validated against data from a series of large centrifuge model tests, and the procedure for obtaining the necessary information and data from those is described. Additionally, a sensitivity study on TzLiq and QzLiq material properties was performed to study their effect on the developed drag load and pile settlement. Analysis results show that the proposed numerical model can reasonably predict the time histories of axial load distribution and settlement of axially loaded piles in liquefiable soils both during and postshaking.

Centrifuge modeling of seismic response of slope on partially replaced soft soil ground

The slope on coastal soft soil ground creates a significant risk of landslide and settlement during an earthquake, causing damage to overlying structures. Soil replacement is an effective method to improve the seismic stability of coastal soft soil ground. The seismic response and stability of soft soil ground and slopes before and after replacement are investigated by centrifuge shaking table tests. Results show that the acceleration response of the backfill slope increases after the soft soil is replaced. The instantaneous settlement increases while post-earthquake settlement significantly decreases. There is a significant hysteresis in the seismic pore pressure response of soft clay. The time required for the pore pressure to reach its peak value decreases after the soft soil is replaced, while the peak seismic pore pressure value decreases significantly. Replacing the soft soil foundation can improve the drainage boundary, accelerate the dissipation of seismic excess pore pressure, and greatly reduce post-earthquake settlement. When a seismic wave is applied, a slip occurs along the interface between gravel and clay; this slip range is also reduced after the soil is replaced. The results of this work indicate that soft soil ground replacement can enhance seismic stability, providing a workable reference for the treatment of coastal soft soil ground.