Excess Pore Pressure Generation Research Articles

Development of soil–pile interactions and failure mechanisms in a pile-reinforced landslide

The reinforcement effect of stabilizing piles is significant for ensuring the stability of major landslide mitigation projects. To investigate the work performance of piles under different boundary conditions, we introduce two physical modelings of a pile-reinforced landslide with the same pile parameters and soil conditions but different load conditions and analyze the consequent differences in the responses of the soil and piles and their interactions in these two physical modelings. Results show that piles in the same landslide/pile systems may have different reinforcement effects; this phenomenon has been explained by quantitatively describing the role of piles and analyzing the processes of soil–pile interactions in the physical modelings. Further, the dynamic responses between the excess pore pressure and the soil–pile interaction are investigated and the excess pore pressure-dependent mechanisms of soil–pile interactions are revealed. The generation of excess pore pressure and its accumulation around piles leads internally to a mechanism in which the arching effect is weakened and externally to a phenomenon in which the progressive failure of pile-reinforced landslides is accelerated. Eventually, we calculate the effective stress in the soil around piles, and the results reveal that a sharp reduction in effective stress in the soil around piles causes failure of the pile reinforcement. In addition, for a pile-reinforced landslide with a strong soil–pile interaction, an imbalance of soil arch footholds is the main failure macro-mechanism. Based on the analysis results, we present some feasible suggestions for the installation of rowed pile and their monitoring to ensure the long-term stability of a pile-reinforced landslide.

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.

Numerical investigation of liquefaction mitigation potential with vibroflotation

Vibroflotation is one type of ground improvement technique used for liquefaction mitigation in saturated sand. In order to study the feasibility of vibroflotation for liquefaction mitigation, two numerical models were created in the finite element framework. A nonlinear, coupled, hypoplastic u-p formulation was developed to simulate the behaviour of saturated sand. Numerical results were validated against on-field cone penetration test measurements after vibroflotation and centrifuge test simulating seismic site response of saturated sand. Seismic site response of saturated sand before and after vibroflotation was analysed. Saturated loose sand with fines and lower permeability underwent excess pore pressure generation, softening, and possibly liquefaction under seismic loading before vibroflotation. Sands with higher permeability were found to respond better to vibroflotation. The most effective degree of compaction was achieved in dry sands. Vibroflotation of saturated sand with fines ensured improved resistance against liquefaction whereas for coarse sands it led to complete eradication of the liquefaction.

Seismic performance of pervious concrete column improved ground in mitigating liquefaction

This paper investigates the performance of pervious concrete column improved ground subjected to seismic loading. The seismic performance of pervious concrete column improved ground is also compared with conventional stone column improved ground in responses of lateral displacement, excess pore pressure, ground surface acceleration, shear stress-strain behaviour and stress path. A fully saturated mildly sloping sand strata is considered as unimproved ground. OpenSeesPL software is used to analyse soil models representing unimproved ground and improved ground with stone column and pervious concrete column inclusions. It is found that the pervious concrete column improved ground has better seismic performance than stone column improved ground. The lateral displacement of ground is found to be significantly reduced while using pervious concrete column. Also the use of pervious concrete column has reduced excess pore pressure generation than stone column indicating that the improved ground with pervious concrete column inclusion is efficient in mitigating liquefaction than conventional stone column improved ground.

Coupled analysis of full flow penetration problems in soft sensitive clays

This study describes the development and implementation of a numerical procedure for the analysis of coupled geotechnical problems involving finite deformations, changing boundary conditions, multiphase porous media and softening behaviour. The numerical scheme is first validated through a benchmark problem concerning the laying of an on-bottom offshore pipeline, and then some further highlights of the effects of soil softening are presented. The scheme is then specifically employed to study the penetration process of a full flow cyclic T-bar test in soft sensitive clay. An advanced soil constitutive model has been implemented to capture the effects of soil structure and its progressive de-structuring as well as soil fabric anisotropy. Accordingly, the changing soil resistance due to the combined effects of soil softening, remoulding and reconsolidation is illustrated, by considering a T-bar undergoing large amplitude cyclic sequences interspersed with consolidation periods. The results from the numerical simulations reveal the effects of soil softening on the soil deformation pattern, the evolution of shear bands, the generation of excess pore pressures and the process of soil reconsolidation.

A Study on the Dynamic Behavior of a Vertical Tunnel Shaft Embedded in Liquefiable Ground during Earthquakes

Since liquefaction was first observed in South Korea during the Pohang earthquake, public concerns regarding the seismic stability of major infrastructure have increased substantially. However, the seismic behavior of tunnel shafts, which are an important element of tunnel structures, has not been properly established, especially under liquefiable soil conditions. In this study, 3D numerical modeling with Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D) was performed to predict the dynamic behavior of a vertical tunnel shaft during liquefaction. This study demonstrates key aspects of the dynamic behavior of tunnel shafts by varying important parameters such as the thickness of the liquefiable soil layer and applied seismicity level. Moreover, important dynamic responses such as excess pore pressure generation, the seismic bending moment of the shaft, and lateral displacements are highlighted. Finally, meaningful discussion of the seismic risk analysis based on damage indices is conducted based on the analysis results.

Numerical modelling of the long-term effects of XCC piling in fine-grained soil

Although the development and utilization of X-section cast-in-place concrete (XCC) pile have been reported for some time, the long-term effects of XCC piling in fine-grained soil, in particular the set-up effects, have received little attention. This paper reports a coupled effective analysis of XCC piling using the dual-stage Eulerian-Lagrangian (DSEL) technique. The pile installation and subsequent soil consolidation was explicitly and consecutively modelled. The generation of excess pore pressure and alteration of stress states during the pile installation were explored. The distribution and magnitude of effective normal stress at pile/soil interface, in particular its evolution with time, were investigated. The influence of set-up effects on the XCC pile shaft resistance was assessed and quantified. It was found out, although the shaft resistances of both XCC and circular pile develops substantially with time, the consolidation in the wake of XCC pile installation can bring in more capacity enhancements and therefore practical benefits. The ultimate shaft resistance of XCC pile is 45% higher than that of the circular pile of the same cross-sectional area. Additionally, practical advice was given on how to optimize of the cross-sectional shape of XCC piles to take full advantage of set-up effects and achieve economical designs.

A three-dimensional investigation on performance of batter pile groups in laterally spreading ground

The performance of batter pile groups in laterally spreading ground has rarely been investigated. The present study evaluates the performance of 2 × 1 batter pile groups with four different configurations subjected to lateral spreading displacement through three-dimensional, fully coupled numerical investigations. Pressure dependent multi-yield model for the soil medium complemented with variation in permeability of liquefiable soil for excess pore pressure generation is considered. The nonlinear behaviour of the reinforced concrete pile is also incorporated. The behaviour of batter pile groups is evaluated for various earthquake time histories. The study indicates that the bending moment developed in the pile sections of both upstream and downstream sides of the ground displacement is observed to reduce under combined (kinematic + inertial) loading conditions for batter pile group configurations considered. However, shear and axial forces show increasing trends for batter pile groups, especially with higher batter angles for both kinematic and combined loading conditions. It is also observed that the presence of batter piles reduces both pile cap and superstructure displacements. However, significant pile cap rotations are recorded for pile groups with higher batter angles. The performances of the pile groups are confirmed for earthquake motions with a wide range of peak ground accelerations.

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.

Effects of Relative Density in Progressive Sliding of Tailings deposits: Insights from Flume Tests

We present a series of flume tests to observe the process of tailings slope failure when subjected to a concentrated water flow. Progressive flow slides were observed in a range of tailing deposits with varying relative densities, in which hydraulic conductivity reduced with increasing relative density. Slope densification by compaction of very loose or loose tailings deposits can lead to a lower permeability, generating excess pore pressure, and subsequently reducing slope stability. This is evident from the great number of acoustic emission signals before sudden failure, and from a sharp increase in the surface displacement and internal displacement observed after excess pore pressure generation. The variation of fluid-pressure distribution results in successive sliding, onset at the toe and progressing from the toe to the rear of the slope. Excess pore pressure generation is found to occur after, rather than prior to sliding, and it only occurs along the slip surface. An optimal relative density of 0.35 was identified, in which the slope failure changes from a sudden collapse to a progressive failure. However, the optimal relative density may vary with the height of the slope. This research extends existing knowledge about behavior of tailings materials, while the observed discrepancy between the failure mechanisms of slopes made of loose and dense deposits could provide insights for preventing failures of tailings dams.

Cyclic performance of a monopile in spatially variable clay using an advanced constitutive model

The performance of monopiles in cohesive soils is of great interest for future offshore wind farm developments, particularly under the cyclic loads experienced in the ocean environment. Clay behaviour during undrained cyclic loading is complex and involves the accumulation of plastic strains, generation of excess pore-water pressures and degradation of initial stiffness. In this paper, the cyclic performance of a laterally-loaded monopile in spatially variable clay is investigated for the first time. A kinematic hardening constitutive model is used in a 3D finite element analysis to capture the hysteretic stress-strain behaviour of the clay. The monopile is installed in overconsolidated London Clay, which is present at several offshore wind farms in the Thames Estuary. The finite element model is coupled with random field representations of initial stiffness and clay structure. The statistical characterisation of the random fields was undertaken considering parameter ranges observed in laboratory tests. Under one-way cyclic loading, the monopile showed ratcheting behaviour, where pile rotation accumulates with increasing numbers of load cycles. The cyclic secant stiffness also increased due to the generation of negative excess pore-pressures in the clay. This behaviour occurred in both homogeneous and spatially variable clay. The monopile was also subjected to an extreme dynamic event and the soil response around the monopile showed increasing variability in stress-strain response and generation of excess pore-water pressure over time as plastic strain accumulated. However, the overall behaviour of the foundation was governed by a spatial average of the mobilised clay. The range in monopile response demonstrates how the natural spatial variability of clay can have a strong influence on monopile performance.

Unified thixotropic fluid model for soil liquefaction

The geotechnical earthquake engineering profession has struggled with the inherent complexity of the multiphase soil response to cyclic loading owing to the progressive nature of the generation of excess pore pressure (EPP) and degradation of soil stiffness and strength. One approach to improve understanding of the cyclic response and correlate the transition from a two-phase saturated soil to a single, fluid-phase liquefied soil is to treat the soil as a non-Newtonian viscous liquid. However, the work to date suggests that the viscous fluid model approach can only be implemented following the onset of sustained soil liquefaction. This paper presents a unified thixotropic fluid model and framework that effectively links the pre-shear soil fabric and its progressive cyclic response to the onset and maintained state of soil liquefaction. The framework treats the soil fabric as a fluid net-type structure proposed for use with thixotropic fluids, and presents the constitutive state and rate equations describing the deconstruction of the liquefiable soil fabric in response to cyclic loading. The unified framework uses physically meaningful soil parameters that can be obtained from common cyclic laboratory tests to seamlessly link the state-dependent and shear-strain-rate-dependent nature of soils to the generation of EPP, the latter of which is shown to increase in significance as EPP accumulates. The proposed thixotropic-induced excess pore pressure model should prove advantageous for use in forward modelling of the stress–strain rate response of liquefiable soil and generation of EPP.

A unified monotonic model for sand based on modified hyperbolic equation and state-dependent dilatancy

Accurate prediction of the state-dependent shearing behaviors of sand in a rational way is a challenging task of the development of constitutive models. In the present study, a deformation type model is proposed based on a modified Duncan-Chang hyperbolic equation, which provides a simple and efficient way to characterize the stress–strain relationship. The hyperbolic equation is improved to tackle the limitations when predicting the strain-softening behavior of sand. By coupling with a flow rule, the volumetric response under a drained condition and excess pore pressure generation under an undrained condition can be modeled as well. An empirical state variable, known as the relative dilatancy index, is implemented in the modified hyperbolic equation and the flow rule to model the state-dependent shearing behaviors in a unified way. The flow rule evolves with the state of soil during shearing and provides realistic simulations of dilatancy-related response. The calibration process for dilatancy and hyperbolic equations is suggested. The model is validated by drained and undrained triaxial compression tests of two silica sand, showing a fairly good capability of unified modeling of the sand behaviors with a wide range of densities and stress levels.

Excess pore pressure generation during slurry deposition of gold tailings

ABSTRACT Excess pore water pressure (PWP) in gold tailings, deposited in the laboratory in a settling column, was observed for various rates of deposition. Rates of rise (vertical increase in height of settled tailings over time expressed in metres per year) of 10 m/yr and 20 m/yr were evaluated by depositing the gold tailings in layers. The maximum deposition rate achieved in the laboratory by depositing tailings to the full height of the settling column at once (full depth deposition) was also investigated. Maximum excess PWPs of 6 kPa generated within 3 to 9 minutes from the start of tailings slurry deposition were recorded. 90% of the maximum generated excess PWPs dissipated within the first 10 hours for coarse tailings and 30 hours for fine tailings. Dissipation of the remaining 10% occurred within 1.1 days and 10.4 days respectively. The excess PWPs were the highest for full depth (FD) deposition. Excess PWPs at an rate of rise (RoR) of 20 m/yr and 10 m/yr were 50% and 30% of the FD deposition. Dissipation of the excess PWPs occurred mostly during the sedimentation phase. It is shown that excess PWP build up on its own cannot be the mechanism that limits the RoR of gold tailings dams.

Dynamic properties of a sand–nanoclay composite

The paper describes the influence of 1–3% (by dry mass of sand) Laponite, a highly plastic synthetic nanoclay, on the dynamic properties of Ottawa sand, based on undrained resonant column tests. The effect of Laponite depends on the amount added, the confining stress and consolidation time. With 1% Laponite, there is an increase in the very small-strain shear stiffness at all confining stresses, which increases with extended consolidation time, reflecting the thixotropic nature of the nanoclay. Added Laponite also produces an increase in small-strain damping, an extension of the linear strain threshold; and it delays the generation of excess pore pressure, degradation of shear modulus and increase in damping with shear strain. These effects, which become more significant with increasing Laponite content, can be attributed to the impact of the clay on the fabric and grain-to-grain contacts. Evidence is provided that Laponite interferes with direct interaction between the sand grains, and the thickness of the clay layer present at the contacts appears to control the small-strain behaviour of the sand–Laponite mixtures. The formation of a gel-like pore fluid from the hydration of Laponite in the voids, and the degree to which it occupies the pore space, are responsible for the behaviour in the non-linear region.

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.

Mechanical behaviour of undisturbed diatomaceous soil

The mechanical behaviour of sedimentary marine soils containing diatoms does not follow classical trends usually found in Soil Mechanics, and evidence of geotechnical singularities of undisturbed samples are still scarce. This article presents new insights on the behaviour of natural diatomaceous soil in compression, shearing and cyclic loading, on undisturbed highly plastic diatomaceous silt from Mejillones Bay in northern Chile. Microscopic observations show that the soil contains significant amount of siliceous diatom. Isotropic compression results in high overconsolidation (OCR > 20) and high compressibility after yielding. Similarly, at low confining stress the undrained shear strength does not depend on pressure and is virtually equal to the UCS = 730 kPa. At higher stresses, a transition to frictional behaviour is observed, characterized by high variability in excess pore pressure generation during undrained triaxial shearing tests, presumably due to heterogeneous diatom layering in undisturbed soil samples. Microscopic observations indicate that high compressibility could be attributed to massive diatom breakage and disturbance of the soil microstructure. Compared to conventional data on fine soils, the cyclic behaviour of the diatomaceous soil is more brittle at low strains, with higher normalized shear modulus and lower damping at shear strains lower than 2 × 10−1%; however, it presents higher degradation at larger strains.

Post-liquefaction pore pressure dissipation in sand under cyclic stress triaxial testing

Soil liquefaction has conventionally been studied as a pure undrained condition subject to cyclic shearing in the laboratory. However, in the field, ground settlement and expulsion of pore water in the form of sand boils are frequently observed, which conventional triaxial and simple shear apparatus are unable to replicate. In this study, a novel insight of coupled pore pressure generation and dissipation in liquefiable clean sand is discussed with the use of a modified cyclic triaxial testing set-up. Stress-controlled cyclic triaxial tests with controlled pore water drainage far lower than the soil's permeability were carried out to investigate ‘near-perfect undrained’ conditions, which are more representative of field conditions. Comparison of results from three sets of stress-controlled cyclic triaxial testing were carried out: (a) conventional shearing under an undrained condition; (b) shearing under controlled drainage; and (c) undrained shearing until liquefaction, and thereafter continual shearing under controlled drainage. The critical cyclic shear stress ratio where excess pore pressure dissipation dominates excess pore pressure generation is identified from the coupled cyclic shear loading and pore water drainage tests. In addition, this study also noted a critical void ratio where the soil is no longer susceptible to liquefaction even under considerable shearing amplitudes. These findings may offer guidance on the tolerable cyclic shear stress and minimum degree of densification to avoid occurrence of soil liquefaction in the field.

A semi-empirical model to predict excess pore pressure generation in partially saturated sand

Past studies revealed that excess pore pressure generation due to cyclic loading is highly governed by induced strains, volumetric deformation potential of soil, number of cycles, and bulk stiffness of pore fluid. It is well established that partial saturation can significantly reduce bulk stiffness of pore fluid and consequently excess pore pressure generation during seismic loading. On the basis of that, a number of researchers have investigated induced partial saturation as an effective soil improvement technique to increase the liquefaction resistance of fully saturated soils. This paper focuses on development of a semi- empirical model to interpret the effects of partial saturation on the excess pore pressure generation in sands. In this regard, an existing strain based excess pore pressure ratio (ru) prediction model originally developed for fully saturated soils was modified to incorporate the effect of partial saturation on the excess pore pressure generation. The literature data as well as data from a series of strain-controlled direct simple shear test were used to evaluate the reliability of the proposed equation in predicting the excess pore pressure ratio in partial saturation condition.

Effect of relative density and biocementation on cyclic response of calcareous sand

Microbial-induced calcium carbonate precipitation (MICP) represents a promising approach to improve the geotechnical engineering properties of soils through the precipitation of calcium carbonate (CaCO3) at soil particle contacts and soil particle surfaces. An extensive experimental study was undertaken to investigate the influence of initial relative density on the efficiency of the biocementation process, the reduction of liquefaction susceptibility, and the cyclic response in biocemented calcareous soils. For this purpose, stress-controlled undrained cyclic triaxial shear (CTS) tests were carried out on untreated and MICP-treated calcareous sand specimens for different initial relative densities and magnitudes of biocementation. Improvement in the cyclic response was quantified and compared in terms of excess pore pressure generation, evolution of axial strains, and the number of cycles to liquefaction. The cyclic experiments show that MICP treatment can change the liquefaction failure mechanism from flow failure to cyclic mobility and can significantly change the excess pore pressure generation response of initially loose specimens. Scanning electron microscope (SEM) images indicate the CaCO3 crystals alter the characteristics of the sand particles and confirm the physical change in soil fabric that impacts the dynamic behavior and liquefaction resistance of MICP-treated specimens. Furthermore, the effect of biocementation was contrasted against the effect of relative density alone, and MICP treatment was shown to exhibit greater efficiency in improving the cyclic resistance than densification.