Dissipation Of Excess Pore Pressure Research Articles

Time scales in the primary and secondary compression of soils

AbstractThe last several decades have seen a surge of papers dealing with analytical and semi‐analytical solutions to the problem of one‐dimensional consolidation of soils. But rarely has any of these contributions focused on the time scales arising from combined primary and secondary compression. Primary compression has always been attributed to the dissipation of excess pore pressure as fluid is expelled from the soil skeleton to the drainage boundaries. However, there have been several schools of thought when it comes to the process governing the secondary compression. In this paper, we attribute the secondary compression to any of the following processes occurring either individually or in combination: (a) rate‐dependent (viscoplastic) constitutive response of the soil skeleton; (b) existence of a secondary pore scale system that expels fluid from the smaller‐scale pores to the larger‐scale pores; and (c) delayed compression due to creep following Bjerrum's concept of secondary consolidation. Contributions of the present work include closed‐form analytical solutions to the problem of combined primary and secondary compression of soils in one dimension, as well as a quantitative analysis of the time scales involved in such coupled hydromechanical processes.

Analysis of Shaft Resistance Setup of Driven Piles in Soft Sensitive Clays Considering Soil Consolidation and Creep

Abstract Soft sensitive clays around a pile shaft experience destructuration during pile installation, excess pore pressure (u) dissipation, and creep after pile installation, but their influence o...

Influence of stress history on undrained cyclic shear strength evolution

Offshore infrastructure often interacts cyclically with the seabed over the operational life of a project. Previous research on the evolution of soil’s undrained strength under long-term, large-amplitude cyclic loading has focused on contractile clays and demonstrated that this cyclic interaction can lead to the initial generation and later dissipation of positive excess pore pressure in the soil. This process generally leads to an initial strength reduction, with subsequent densification and soil strength gains that can have consequences on the performance of seabed infrastructure during its design life. In this paper, new experimental data from T-bar penetrometer testing in kaolin and reconstituted Gulf of Mexico clays is presented. The data illustrate how the stress history, quantified via the overconsolidation ratio, affects soil strength changes during large-amplitude cyclic loading. The experiments explore both long-term continuous loading cycles and episodic loading with packets of undrained cycles followed by quiescent consolidation periods. A critical state-based framework is used to interpret the experimental data and provide predictions of the long-term steady-state strength of both soils as a function of the initial in situ state of the soil.

Seismic analysis of a model tension leg supported wind turbine under seabed liquefaction

The paper presents numerical analyses of a pile supported Tension Leg Platform (TLP) wind turbine, during seismic loading and seabed liquefaction, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when subsoil liquefaction is extensive, leading to degradation of the pseudo-static safety factor against pile pullout well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking, but fully recovers during the following dissipation of earthquake-induced excess pore pressures and even exceeds the initial (pre-shaking) resistance. Pile head displacements develop steadily both during shaking and dissipation, but only while the static pullout safety factor remains less than unity. Due to the high tensional stiffness of the tendons relative to the buoyancy stiffness of the platform considered in this study, the pile head pullout is mostly transmitted to the platform with a relatively small part corresponding to reduction of tendon elongation. The resulting loss of buoy stability and tendon pretension may prove detrimental for the short-term operation of the platform. However, except possibly from the rare scenario of concurrent extreme environmental loads, both effects are recoverable, as they are unlikely to cause irreparable damage to the foundation and the superstructure.

Changes in sand mesostructure under repeated seismic liquefaction events during centrifuge tests

Previous studies have revealed that past shaking can substantially affect sand reliquefaction resistance. Researchers have attributed these effects to the formation of stable or vulnerable sand mesostructures during previous shaking events. However, no experiment has been performed to directly visualize how sand particles evolve into stable or vulnerable mesostructures. In this study, a sand deposit was shaken by four repeated seismic motions in centrifuge model tests, and simultaneously, mesoscopic images of sand particles were recorded by a newly developed microimage acquisition system. The mesoscopic images showed that pore fluid seepage caused particles previously in contact to be separated and form large voids during sand reconsolidation stages. The sand with large voids quickly contracted during the subsequent shaking event, exhibiting a decreasing sand reliquefaction resistance. The sand reliquefaction resistance gradually increased as the seepage damage effects attenuated, and the number of sand particle contacts increased with sand densification under multiple shaking events. The testing results suggested that influences of the excess pore pressure dissipation on the sand mesostructure should be considered in assessing sand reliquefaction resistance.

Performance based design of Tension Leg Platforms under seismic loading and seabed liquefaction: A feasibility study

The response of pile supported Tension Leg Platforms during seismic loading and seabed liquefaction is analyzed numerically, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when liquefaction in the subsoil is extensive, leading to degradation of the pseudo-static factors of safety against pullout failure of the pile well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking due to subsoil liquefaction, but fully recovers during the following excess pore pressure dissipation phase and may even exceed the initial (pre-shaking) resistance value. Pile head displacements develop steadily during shaking and the following dissipation phase, but only during the finite time period when the static pullout factor of safety of the pile remains less than unity. Thus, final pile head displacements may increase considerably when this time period is elongated. Due to the generally high tensional stiffness of the tendons, relative to the buoyancy stiffness of the platform, the pile head pullout is mostly transmitted to the platform, with only a small percentage corresponding to reduction of tendon elongation. For the cases examined herein, the potential loss of platform buoyancy and tendon pretension may prove detrimental but is unlikely to threaten the safety and functionality of the platform. On the other hand, initially minor tilting of the platform (e.g. due to long period lateral loading and differential tendon pretension) may be magnified during seismic loading above allowable limits.

Determination of the ultimate consolidation settlement of jack-up spudcan footings embedded in clays

When jack-up platforms are deployed for long-term services such as those functioning as production units, the consolidation settlement of spudcan footings is an issue of particular concern. Unpredicted and thus unprepared settlement may do harm to the serviceability and safety of jack-up rigs. Through centrifuge experiment and large deformation finite element calculation, this paper first illustrates the inherent correlation between the excess pore pressure dissipation inside soil and spudcan settlement development. With the measured and calculated settlement time histories, subsequent efforts are directed towards exploring how to predict the ultimate consolidation settlement with accuracy. Particular attentions are placed on investigating the applicability and advantages of two observational methods: Asaoka's method and Hyperbolic approach. Their prediction performances for spudcan with various soil, loading, embedment and geometrical conditions are symmetrically evaluated through numerical parametric investigation. The upper and lower limits of their prediction errors are established. Explicit recommendations are given on the selection of appropriate prediction approaches for different consolidation conditions. The practical significance of these research outcomes lies in 1) providing guidance on the selection of settlement prediction approaches; 2) establishing the limits of prediction errors involved. These are expected to be particularly useful for the field monitoring of spudcan settlement.

Numerical-probabilistic modeling of the liquefaction-induced free fields settlement

Liquefaction is a phenomenon through which saturated sandy soil loses its shear strength and turns into a liquefied state. One of the most detrimental consequences of liquefaction is the reconsolidation volumetric settlements after the earthquakes, which is due to the dissipation of excess pore pressure caused by earthquakes. Severe floods can follow these settlements in free fields such as grounds close to the sea or rivers. Several researchers studied this phenomenon using data obtained from experiments in the lab or observations in the fields. Previous works were mainly based on a limited number of experimental observations and considered loadings and boundary conditions that were different from reality in the field. In actual field cases, loading does not occur in fully undrained conditions, and earthquake excitations do not comply with load application limitations on the laboratory samples. Accurate simulation of liquefaction-induced settlement phenomenon requires using fully coupled hydro-mechanical numerical analysis. In the current study, a Numerical-Probabilistic framework is proposed to estimate the liquefaction-induced settlement of the fully saturated sandy layers in the free field subjected to earthquake excitations. A fully coupled (u-P) formulation is employed to analyze soil deformations and pore water pressure variations in order to assess the soil volumetric strains and settlement. The main advantage of the current work is considering the probabilistic nature of the phenomenon and providing insight into the prediction uncertainties. Additionally, it uses a comprehensive database obtained from extensive numerical analyses. Based on the obtained settlements from the numerical analyses, a new method is proposed that predicts the post-earthquake settlement using the Bayesian linear regression method. The proposed method predicts the free field liquefaction-induced settlement in a confidence interval with high accuracy, which is validated by comparing the current work results with the observed liquefaction-induced settlements in the field case studies.

A large deformation finite element investigation of pile group installations with consideration of intervening consolidation

Piles are usually installed in both onshore and offshore sites to transit the structural loads through soft or loose soil layers to competent soil strata. Although the installation of a single pile is usually considered to be undrained, consolidation may take place during the installations of multiple piles, especially in the pauses between neighboring pile installations i.e. intervening consolidation. To date, the role that the intervening consolidation can play in affecting soil stress and strength states as well as set-up of pile capacities remains largely unclear. This article reports effective stress large deformation analyses of pile group installations with consideration of intervening consolidation. The results indicate pore pressure generation and accumulation, soil strength improvement, set-up effects on pile shaft and bearing resistances are clearly affected by intervening consolidation condition. It may be non-conservative to assume soil remains at in-situ strength state for the estimation of short-term pile capacity in design practice, if no intervening consolidation takes place. Timely dissipation of excess pore pressure amongst neighboring pile installations is beneficial notably for development of pile base resistances. The present findings are likely helpful in deepening the understanding of intervening consolidation and facilitating the incorporation of set-up effects into pile design practices.

Dynamic, in situ, nonlinear-inelastic response and post-cyclic strength of a plastic silt deposit

This study presents the use of controlled blasting as a source of seismic energy to obtain the coupled, dynamic, linear-elastic to nonlinear-inelastic response of a plastic silt deposit. Characterization of blast-induced ground motions indicates that the shear strain and corresponding residual excess pore pressures (EPPs) are associated with low-frequency near- and far-field shear waves that are within the range of earthquake frequencies, whereas the effect of high-frequency P-waves is negligible. Three blasting programs were used to develop the initial and pre-strained relationships between shear strain, EPP, and nonlinear shear modulus degradation. The initial threshold shear strain to initiate soil nonlinearity and to trigger generation of residual EPP ranged from 0.002% to 0.003% and 0.008% to 0.012%, respectively, where the latter corresponded to ∼70% of the maximum shear modulus, Gmax. Following pre-straining and dissipation of EPPs within the silt deposit, the shear strain necessary to trigger residual excess pore pressure increased two-fold. Greater excess pore pressures were observed in situ compared to those of intact direct simple shear (DSS) test specimens at a given shear strain amplitude. The reduction of in situ undrained shear strength within the blast-induced EPP field measured using vane shear tests compared favorably with that of DSS test specimens.

Accelerating consolidation of soft ground by aerosol injection technique: a field study

We present an aerosol injection technique (AIT) to accelerate the consolidation of soft soils for ground improvement. We employ high-pressure aerosol injections at different depths to enhance the drainage in soft soils for faster consolidation. The technique is briefly described. A well-instrumented field test is carried out to demonstrate its performance. Compared to the traditional methods, our approach gives rise to faster dissipation of excess pore pressure and larger ground settlement. This method is particularly attractive for the improvement in soft ground in medium depths.

Large deformation coupled analysis of embedded pipeline – Soil lateral interaction

Subsea pipelines buried in the seabed may undergo large lateral displacement under environmental, operational, and accidental loads at different interaction rates and hence different drainage conditions. The undrained shear strength is commonly used in practice to assess the pipe-soil interaction assuming a sufficiently high displacement rate. This approach neglects consolidation effects and the rate-dependent response of the soil and may significantly underestimates the lateral resistance for a pipeline moving slowly relative to the ground. In this study, a coupled large deformation finite element (LDFE) framework is developed via a remeshing and interpolation technique with small strain (RITSS). A Modified Cam-Clay (MCC) model with efficient numerical integration is used. The proposed coupled LDFE framework is verified against selected physical model tests. Effects of the interaction rate and hence drainage condition on the p-y curve, excess pore pressure generation and dissipation, and failure mechanisms are discussed. An empirical relationship between the ultimate resistance and the normalized velocity of the pipe (denoting the drainage condition) is proposed, which may be applied for the integrity and safety analysis of buried pipes in landslide or fault-crossing regions.

Experimental and numerical modeling on the liquefaction potential and ground settlement of silt-interlayered stratified sands

Recent seismic events indicate that the simplified liquefaction-evaluation procedures are incapable of depicting general trends in liquefaction damage for stratified sands interlayered with silts. The conditions and mechanisms affecting the liquefaction potential of stratified sands exist in the field and ground settlement after liquefaction remain poorly understood. This work aims to investigate the seismic response of nonhomogeneous soil deposits by large-scale model tests and numerical simulations using an advanced constitutive model. A comprehensive experimental program was undertaken in which a total of three shake-table tests were performed on uniform sand and two stratified-sand deposits interlayered with different thicknesses of silt to investigate the ground settlement and distribution and dissipation of excess pore pressure during and after shaking. The shake-table test results and the numerical simulations of the silt-interlayered stratified sands, first indicate that the thickness of the silt seam has a significant influence on the liquefaction resistance of stratified-sand deposits beneath the silt layer. The second conclusion of this study reveals that the thickness and coefficient of consolidation of the silt and the liquefied sand below the silt layer significantly alter the degree of dissipation after the shake, and this causes different deformation/settlement at the ground surface. Therefore, there will be probably inaccuracies in applying simplified liquefaction evaluation procedures to the actual soil profile characterized by various patterns of layering in the field.

Pile driving and submarine slope stability: a hybrid engineering approach

During pile installation into a submerged, sandy slope, liquefaction mechanisms including flow and cyclic liquefaction warrant attention. Because of the interconnection of these mechanisms, evaluating slope stability during and as a result of vibration-inducing construction activity is not trivial. This paper presents a practical approach to such an evaluation. The primary focus of any slope stability analysis must lie with flow liquefaction as the form of failure with the most hazardous potential. Given the importance of excess pore water pressure in giving rise to (delayed) slope failures due to cyclic loading events, excess pore pressure (EPP) generation and dissipation is the mechanism of most interest in modelling cyclic liquefaction. Currently, no engineering method exists which is able to capture the interconnected processes. Therefore, a hybrid model, consisting of a numerical tool which computes EPP generation and dissipation in time, is combined with empirical relations to describe the decay of EPPs generated due to pile driving in space and time. The proposed numerical tool predicts the evolution of EPP in a one-dimensional soil column close to a vibratory-driven pile, taking into account sustained static shear stresses, interim drainage, and pre-shearing. Radial EPP dissipation is considered the dominant mode of drainage. This engineering tool fits within a holistic slope stability analysis procedure, which is demonstrated for a submerged slope in the IJmuiden harbour of the Netherlands, where mooring piles and sheet piles are installed through a relatively loose layer of sand.

Centrifuge modeling of geosynthetic-encased stone column-supported embankment over soft clay

Geosynthetic-encased stone column (GESC) has been proven as an effective alternative to reinforcing soft soils. In this paper, a series of centrifuge model tests were conducted to investigate the performance of GESC-supported embankment over soft clay by varying the stiffness of encasement material. The enhancement in the performance of stone columns encased with geosynthetic materials was quantified by comparing the test with ordinary stone columns (OSCs) under identical test conditions. The test results reveal that by encasing stone columns with geosynthetic material, a significant reduction in the ground settlement, relatively faster dissipation of excess pore pressure and enhanced stress concentration ratio was noticed. Moreover, with the increase in the encasement stiffness from 450 kN/m to 3300 kN/m, the stress concentration ratio increased from 4 to 6.5, which signifies the importance of encasement stiffness. In addition, a relatively lower value of soil arching ratio observed for GESCs compared to OSCs indicate the formation of a relatively strong soil arch in the GESC-supported embankment. Interestingly, under embankment loading, GESCs fail by bending while OSCs fail by bulging. The stress reduction method can be used to calculate the settlement of GESC-supported embankment with larger stress reduction factor than that in the OSC-supported embankment. Finally, the limitation of the construction of the embankment at 1 g was addressed.

A general analytical solution for one dimensional consolidation of unsaturated soil incorporating impeded drainage boundaries

This paper presents a novel analytical solution for one dimensional consolidation system of unsaturated soil incorporating impeded drainage boundary conditions. Through diagonalizing the coefficient matrix, the consolidation system is firstly transformed into a diagonal one with respect to a new variable vector. Then, using the methods of eigenfunction expansion and undetermined coefficients, the analytical solution is developed through deriving the eigenvalues, corresponding eigenfunctions and undetermined coefficients. Its orthogonality and convergence are proven, and several comparisons against existing solutions and two typical experimental data are performed to verify its validity. Finally, two typical examples are provided to illustrate the consolidation behavior of unsaturated soil with different drainage parameters. The results indicate that the present analytical solution converges rapidly, and it is consistent with existing analytical, semi-analytical and numerical solutions for various drainage boundary conditions. The proposed analytical solution achieves an overall better agreement with experimental data, and the impeded drainage boundary condition is more reasonable to describe the consolidation phenomenon of unsaturated soil. In general, larger drainage parameter accelerates excess pore-pressure dissipation and soil settlement.

Consolidation effects on monotonic and cyclic capacity of plate anchors in sand

This study investigated the change in monotonic and cyclic capacity of a plate anchor across different degrees of consolidation in dense sand. To quantify the effect of consolidation on anchor capacity, a framework is introduced and validated using centrifuge model anchor test data. The centrifuge tests considered a rectangular plate loaded at varying rates in dense sand, under both monotonic and irregular cyclic conditions, at a fixed embedment depth and with a horizontal load inclination (at the seabed). In order to vary from drained to undrained conditions, the sand was saturated using both water and a viscous pore fluid with viscosity approximately 700 times higher than water. The anchor's ultimate monotonic capacity in dense sand increased by up to 173% as the consolidation response evolved from drained to undrained with generation of dilation-induced suction. This increase in capacity across the consolidation regime can be adequately quantified using the proposed framework; however, uncertainty arises in achieving the theoretical undrained capacity. Both drained and undrained irregular cyclic loading resulted in anchor capacity increases of up to 33%, attributed to soil volume changes associated with cyclic densification under drained cyclic loading and excess pore pressure dissipation under undrained cyclic loading.

Load-bearing characteristic of methane hydrate within coarse-grained sediments – Insights from isotropic consolidation

Gas hydrate, as a solid pore constituent, can potentially share external load with sediment matrix, and critically impact geological evolution of its hosting sediments, well integration and sediment settlement during gas production depending on its contribution to support external loading. We aim to address the question of whether hydrate shares the load with the soil skeleton through isotropic consolidation experiments. Our results highlight the formation of methane hydrate does not affect the integrity of sediment skeletons if the effective stress previously applied on the skeleton remains constant, which indicates that methane hydrate would not actively support the load. Subsequent external load applied to the sediments after hydrate formation can be supported by hydrate. Secondly, hydrate as well as hydrate-bearing sediments exhibit creep behavior. As hydrate creeps, the load transfers to the sediment skeleton with time and hydrate gradually retreats from the status of load-bearing. When hydrate saturation is low and the coexisting pore fluid can percolate through sediments, the local deformation of hydrate due to sediment consolidation induces excess pore pressure, which dissipates by pushing out the pore fluid. When the coexisting pore fluid is enclosed in hydrate mass in high hydrate saturation environments, the dissipation of excess pore pressure relies on the viscous flow of hydrate. This pressure dissipation could last for millions of years according to analytical analysis based on Terzaghi's consolidation theory, during which hydrate cannot completely escape the status of load-bearing. These observations conclude that load-bearing is a dynamic status rather than a stagnant pore habit, countering conventional thinking that load-bearing joins grain-coating, cementing and pore-filling as a pore habit of gas hydrate.

An effective stress analysis for predicting the evolution of SCR–seabed stiffness accounting for consolidation

Steel catenary risers (SCRs) are an efficient solution to transfer hydrocarbons from deep-water seabeds to floating facilities. SCR design requires an assessment of the fatigue life in the touchdown zone, where the riser interacts with the seabed, which relies on reliable estimates of the SCR–seabed stiffness over the design life. Current models for SCR–seabed stiffness consider only undrained conditions, neglecting the development and dissipation of excess pore pressures that occur over the life of the SCR. This consolidation process alters the seabed strength and consequently the SCR–seabed stiffness. This paper summarises experimental data that show that long-term cyclic vertical motion of an SCR at the touchdown zone leads to a reduction in seabed strength due to remoulding and water entrainment, but that this degradation is eclipsed by the regain in soil strength during consolidation. The main focus of this paper is on prediction of the temporal changes in seabed strength and stiffness due to long-term cyclic shearing and consolidation, to support calculations of SCR–seabed interaction. The predictions are obtained using a framework that considers the change in effective stress and hence soil strength using critical state concepts, and that considers the soil domain as a one-dimensional column of elements. The merit of the model is assessed by way of simulations of SCR centrifuge model tests with over 3000 cycles of repeated undrained vertical cycles in normally consolidated kaolin clay. Comparisons of the simulated and measured profiles of SCR penetration resistance reveal that the model can capture accurately the observed changes in SCR–seabed stiffness. Example simulations show the merit of the model as a tool to assess the timescale in field conditions over which this order of magnitude change in seabed stiffness occurs. It is concluded that current design practice may underestimate the seabed stiffness significantly, but the new approach allows rapid checking of this for particular combinations of SCR and soil conditions.

Inclined Perimeter Drains Performance as Liquefaction Countermeasure Techniques Below Existing Buildings

Reducing the risk of structural damage due to earthquake-induced liquefaction in new and existing buildings is a challenging problem in geotechnical engineering. Drainage countermeasure techniques against liquefaction have been studied over the last decades with an emphasis on the use of vertical drains. This technique aims to allow a rapid dissipation of excess pore pressures generated in the soil during the earthquake thereby limiting the peak excess pore pressures and consequently improve the structural response. Rapid drainage in the post-earthquake period in the presence of these drains helps quick recovery of the soil strength. Recent studies propose different variations in the vertical drains arrangement to improve the excess pore pressure redistribution in the soil around structures. However, conventional arrangements for existing buildings do not achieve an adequate proximity from the drains to the soil below the foundation. To address this, the performance of inclined and vertical perimeter drain arrangements are studied in this paper. Dynamic centrifuge tests were carried out for the different arrangements in order to evaluate the excess pore pressure generation due to ground shaking and the following dissipation together with the foundation settlement and dynamic response.