Development Of Excess Pore Pressure Research Articles

Effect of Loading Rate on Lateral Pile-Soil Interaction in Sand Considering Partially Drained Condition

Reliable assessment of the lateral pile-soil interaction is of pronounced importance for the design of mono-pile foundations of offshore wind turbines. As the offshore engineering moves to deeper waters, the diameter of mono-piles is getting larger, usually about 5 m and could be up to 8 m, which may lead to partially drained behaviors of sand in the vicinity of the pile and thus imply limitations of conventional design methods in which fully drained conditions were assumed. To shed light on this issue, a fully-coupled finite element model was established using an in-house developed finite element code DBLEAVES, incorporating a cyclic mobility constitutive model that is capable of describing the instantaneous contractive and dilative response of sands simultaneously. Triaxial and centrifuge model tests were conducted to calibrate the constitutive model and validate the pile-soil interaction model respectively. This is followed by a parametric study primarily focusing on the effects of loading rates. The initial stiffness of the p-y curve was found to increase with the loading rate whilst the bearing capacity showed the inverse, and the mechanism behind this phenomenon is examined in detail. Then an explicit model was developed to evaluate the development of excess pore pressure in the pile front upon lateral loading, and an upper boundary of normalized loading rate was identified to distinguish fully and partially drained conditions.

Centrifuge modelling for seismic response of single pile for wind turbine subjected to lateral load

The soil around the pile foundations of offshore wind turbines (OWTs) would suffer from earthquakes which are widely believed to undermine the bearing capacity and serviceability of foundations. The non-intermittent lateral environmental loads in the ocean (e.g., wave, wind, and current) continuously act on OWTs. However, the seismic response of pile foundations and the soil around is less clear when considering the OWTs are subjected to lateral environmental loads. This paper presents two sets of centrifuge modelling tests on single piles, with a prototype diameter (D) of 1.5 m and a first-order natural frequency highly comparable to that of a prototype wind turbine. Two model piles with the same dimension and stiffness, but one with a lateral load on the pile head, were both installed in a dry sand ground and a saturated sand ground. Owing to the fact that the lateral load increased the initial static shear stresses of the soil in front of the pile, the development of excess pore pressure was inhibited and the permanent ground lateral displacement was accumulated during shaking. The existence of the lateral load led to a pile head displacement as much as 34 cm and increased the bending moment along the pile up to 6 MN·m. Moreover, the seismic response of piles and soils were compared between the dry sand ground and saturated sand ground to clarify the influence of excess pore pressure. It was also found that the residual bending moment after unloading the pile was still very large up to 17 MN·m because of the rebalanced soil-pile interaction. These findings highlight the necessity to consider the lateral load effect in the seismic design of pile foundations for OWTs.

Deep-Seated Slope Stability Analysis and Development of Simplistic FOS Evaluation Models for Stone Column-Supported Embankments

Inclusion of stone columns in the underlain soft soil is one of the most prominent methods for improving the stability of embankments. The stone columns are encased with a geosynthetic material to further enhance the stability. The influence of this partial replacement of weak foundation soil with stone columns on the performance of embankments needs to be quantified. In this study, performance of ordinary stone column (OSC) and geosynthetic-encased stone column (GESC)–supported embankments is carried out using a three-dimensional finite element programme (PLAXIS3D). A parametric study was conducted to quantify the influence of various factors viz. spacing to diameter ratio (S/D), stiffness of encasement, cohesion of soil, friction angle of stone column and friction angle of embankment on the factor of safety against deep-seated failure. The results show that encasing the stone columns enhances the stability of embankments. Decreasing the column spacing (S/D) enhances the stability, reduces the excess pore pressure development and average settlement. Increase in geotextile encasement stiffness, cohesion of underlain soft soil, friction angle of stone column and friction angle of embankment improves the performance of embankments and also reduces the average settlement of the ground under embankment. Results of the parametric study were used to develop two types of data-driven models viz. multiple linear regression (MLR) and artificial neural networks (ANN) to simplify the evaluation of the factor of safety (FOS) of embankments. Among the two approaches, ANNs were able to predict the factor of safety values more accurately.

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.

Study on the Dynamic Soil-Pile-Structure Interactive Behavior in Liquefiable Sand by 3D Numerical Simulation

The dynamic behavior of structures in liquefiable sand exhibits more complicated characteristics, due to the development of excess pore pressure caused by cyclic loading, than that in dry sand. Therefore, it is crucial to accurately predict the soil–pile structure behavior during liquefaction to prevent damage to the structures. In this study, three-dimensional numerical modeling was performed to predict the dynamic soil–pile behavior during liquefaction. To directly simulate pore pressure generation due to soil shear deformation, the Finn liquefaction model was applied and coupled with the Mohr-Coulomb elasto-plastic model. Soil nonlinearity was considered by applying hysteretic damping, and the interface model was applied to simulate various dynamic phenomena between the soil and pile. Simplified continuum modeling was introduced to prevent reflection wave generation and increase analysis efficiency. The applicability of the proposed numerical model was validated using the experimental results. Thereafter, a parametric study was conducted to provide a better understanding of the dynamic behavior of pile foundation during liquefaction. From a series of parametric studies, several important factors that can affect the dynamic pile responses in liquefiable sand were identified. Also, the characteristics of the dynamic soil–pile structure interactive behavior, which are significantly different from each other in liquefied and dry sand, were analyzed qualitatively and quantitatively.

Ground response during offshore pile driving in a sandy slope

This paper presents an analysis of field measurements obtained during pile installation tests in a submerged sandy slope. The field test was performed in 2016 as part of the construction of a new sea lock in the Netherlands. Ground vibrations and excess pore pressures were to be limited in order to prevent damage to neighbouring locks and maintain slope stability. Geophones and piezometers were installed in the slope, at depth, and at various lateral distances from the pile axis. In the analysis, focus is placed on the installation of a tubular steel pile, which was subjected to both vibratory and impact driving. An assessment is made of some factors, such as driving equipment and pile tip penetration depth, affecting both the magnitude of ground vibration and its spatial and temporal characteristics. A similar analysis is conducted of the pore pressure measurements, with particular attention paid to the part of driving carried out in a homogeneous sand layer. Measured vibrations and excess pore pressures are compared to established attenuation relationships and published data, respectively. An approach is suggested in which the test results and interpretation may be used in assessing the potential implications of excess pore pressure development in the sand layer for slope stability.

Microscopic Mechanism Affecting Shear Strength in Lignin-Treated Loess Samples

In China, engineers have worked to create additional usable land for building construction by flattening the ridges of hills and filling in the adjacent valleys. China’s Loess Plateau comprises a type of soil (loess) with a large pore structure that can collapse and become unstable when exposed to groundwater. Conventional valley fill materials include remolded loess or remolded loess treated with cement, lime, gypsum, or other stabilizing additives. These stabilizers are often detrimental to the surrounding environment. Moreover, loess treated with conventional stabilizers exhibits excessive brittleness, which is not suitable for building foundations. Adequate stability of the building foundations in the filled valleys is required to ensure public safety. In this study, we tested 50 remolded loess samples treated with a lignin polymer compound to determine its potential as a valley fill material. Triaxial tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to study the mechanical characteristics of each sample, determine the effects of the lignin treatment on the loess, and identify the microscopic mechanism affecting shear stress in the lignin-treated loess. The corresponding development of excess pore pressure and volumetric responses under monotonic triaxial testing were also considered. Based on this study’s results, the optimum lignin content in the treated loess samples was 4%; lignin contents exceeding 4% decreased axial stress and increased dilation after saturation. The shear strength and strain-hardening phenomenon of the lignin-treated loess samples increased as the lignin content increased, while the excess pore water pressure decreased. Microscopically, the addition of lignin increased cohesion in the loess samples, while slightly contributing to the internal friction angle. The use of lignin as a stabilizing additive for valley fill material shows potential for controlling building foundation deformation by increasing soil strength and minimizing environmental impacts by maintaining the soil pH and limiting pollutant production.

2-D P-SV and SH spectral element modelling of seismic wave propagation in non-linear media with pore-pressure effects

SUMMARYIt has long been recognized that the effects of superficial geological layers, or site effects, can play a major role on the seismic ground motion at the free surface. In this study, we compute wave propagation in a 2-D asymmetrical basin considering both soil non-linearity and pore-pressure effects. Equations of elastodynamics of wave propagation are solved using the spectral element method (SEM). The geometry of the basin gives rise to basin-edge generated waves, that are different for in-plane (P-SV) and out-of-plane (SH) wave propagation and resulting in different non-linear response. Moreover, the excess-pore pressure development in superficial liquefiable layers (effective stress analysis) brings larger deformation and loss of strength than the analysis without pore-pressure effects (total stress analysis). The coupling of vertically propagating waves and the waves specifically generated in 1-D model leads to waves whose amplitude and duration are higher than the 1-D case. This multidimensional effect increases material non-linearity. Such complex wavefield provokes larger deformation and higher pore-pressure rise that cannot be predicted by 1-D modelling. Therefore, our paper suggests the use of multidimensional modelling while studying seismic wave propagation in both linear and non-linear complex media.

Identification of ground motion intensity measure and its application for predicting soil liquefaction potential based on the Bayesian network method

In an attempt to reduce the parameter and model uncertainties of the Bayesian network model for predicting earthquake-induced soil liquefaction, 31 candidate intensity measures were investigated by the analyses of correlation, efficiency, proficiency, and sufficiency based on a large database of historical ground motion records. Two new Bayesian network models were developed using the identified intensity measures by combining the measured liquefaction-related data and the prior knowledge of soil liquefaction based on a large dataset of standard penetration tests. The results reveal that the root-mean-square acceleration is the optimal intensity measure for assessing soil liquefaction, whereas the peak ground acceleration is second best for liquefaction potential evaluation. The two new Bayesian network models with interpretability both in physical mechanisms and mathematics perform better than both the existing Bayesian network model and the Idriss and Boulanger model. The possible bias in this study is also discussed and pinpoints the importance of quantifying excess pore pressure development in the evaluation of soil liquefaction.

Tests on wave-induced dynamic response and instability of silty clay seabeds around a semi-circular breakwater

Model tests aimed at investigating the wave-induced dynamic response and instability of silty clay seabeds around a semi-circular breakwater were conducted in a large wave flume. The pattern of wave forces was measured and analysed. The seabed responses were presented in terms of pore pressure and vertical stress, which were further characterized in terms of their oscillatory and residual values. The wave-induced settlement and instability of the semi-circular breakwater were studied. Test results reveal that the total horizontal wave force was generally greater than the vertical wave force, and a phase difference existed in the total horizontal and vertical wave force patterns. The trends in the oscillatory pore pressure and vertical stress were very similar: they generally developed rapidly in the seabed on the wave side, tended to be larger in the upper silty clay and smaller in the bottom area and increased with increasing wave height. Furthermore, the oscillatory responses mostly reached stable states but were observed to vibrate with cyclic time in the destroyed silty clay. The residual pore pressure and vertical stress mainly developed in the seabed around the breakwater and could be quite different depending on the location. With the exception of the decrease-increase tendency in some seabed areas, the residual pore pressure generally increased with cyclic time beneath the breakwater. The residual vertical stress generally increased with cyclic time beneath the breakwater, except in some regions beneath the toes of the internal rubble bed. The semi-circular breakwater settled substantially into the silty clay seabed, and the settlement was greater on the wave side. This seabed shear failure is attributed to the strength degradation of the silty clay induced by the development of excess pore pressure and the destruction of the soil fabric.

Soil stability analysis for wave-induced momentary liquefaction beneath porous bonded revetments

The highly porous revetments made of Polyurethane Bonded Aggregates (PBA) are an ecologically friendly alternative to conventional revetments for protection against coastal erosion. No information has yet been reported for these structures on failures from field applications. However, a well-documented collapse of a PBA-revetment observed in large-scale tests (GWK tests) and a first stability analysis were reported by Oumeraci et al., 2010; 2012. Based on these results, a methodology is proposed for stability analysis of the embankment subsoil beneath PBA-revetments against momentary liquefaction, considering the results of the comprehensive parametric study (Alcérreca-Huerta, 2014) using a recently developed CFD-CSD (Computational Fluids Dynamics- Computational Solid Dynamics) model wavePoreGeoFoam (Alcérreca-Huerta and Oumeraci, 2016a; 2016b). It will be shown that the proposed stability analysis is able to reproduce the failure observed in the GWK tests, so that it can be applied for PBA-revetment under field conditions. In this paper, the failure observed in the GWK tests is first briefly reported, followed by a description of the numerical parametric study using the validated wavePoreGeoFoam model in order to extend the conditions tested in GWK. Then, the processes underlying soil liquefaction of PBA-revetments are outlined, showing that the excess pore pressure development in the sand core beneath PBA-revetments is crucial. The latter is therefore examined and a formula to predict excess pore pressures in terms of the wave conditions is developed. Moreover, a methodology for the stability analysis of the soil beneath the revetment against soil liquefaction is proposed and implemented to reproduce the failure observed in the GWK tests. Finally, the main results are summarized and implications for further research are drawn.

Effect of excess pore pressure on the long runout of debris flows over low gradient channels: A case study of the Dongyuege debris flow in Nu River, China

Debris flows with long reaches are one of the major natural hazards to human life and property on alluvial fans, as shown by the debris flow that occurred in the Dongyuege (DYG) Gully in August 18, 2010, and caused 96 deaths. The travel distance and the runout distance of the DYG large-scale tragic debris flow were 11km and 9km, respectively. In particular, the runout distance over the low gradient channel (channel slope<5°) upstream of the depositional fan apex reached up to 3.3km. The build-up and maintenance of excess pore pressure in the debris-flow mass might have played a crucial role in the persistence and long runout of the bouldery viscous debris flow.Experiments to measure pore pressure and pore water escape have been carried out by reconstituting the debris flow bodies with the DYG debris flow deposit. The slurrying of the debris is governed by solid volumetric concentration (SVC), and the difference between the lower SVC limit and the upper SVC limit can be defined as debris flow index (Id). Peak value (Kp) and rate of dissipation (R) of relative excess pore pressure are dependent on SVC. Further, the SVC that gives the lowest rate of dissipation is regarded as the optimum SVC (Cvo). The dissipation response of excess pore pressure can be characterized by the R value under Cvo at a given moment (i.e., 0.5h, 1h or 2h later after peak time). The results reveal that a relatively high level of excess pore pressure developed within the DYG debris-flow mass and had a strong persistence capability.Further research shows that the development, peak value and dissipation of excess pore pressure in a mixture of sediment and water are related to the maximum grain size (MGS), gradation and mineralogy of clay-size particles of the sediment. The layer-lattice silicates in clay particles can be the typical clay minerals, including kaolinite, montmorillonite and illite, and also the unrepresentative clay minerals such as muscovite and chlorite. Moreover, small woody debris can also contribute to the slurrying of sediments and maintenance of debris flows in well vegetated mountainous areas and the boulders suspended in debris flows can elevate excess pore pressure and extend debris-flow mobility.The parameters, including Id, Kp, R and etc., are affected by the intrinsic properties of debris. They, therefore, can reflect the slurrying susceptibility of sediments, and can also be applied to the research on the occurrence mechanisms and risk assessment of other debris flows.

Simulation of Blast Induced Liquefaction Susceptibility of Subsurface Fill Mass

Tailings backfill, which is a subsurface fill mass, has been extensively utilized worldwide in underground mines to fill mined-out cavities for the purposes of ground control and tailings disposal. Just after placement, very early-age backfill which commonly contains a large volume of water exhibits little or no interparticle bonding, and is subjected to the risk of liquefaction induced by routine mine blasting. In this study, a modified total-stress viscoplastic cap model is developed to investigate the blast-induced liquefaction susceptibility of very early-age fill mass under various practical backfilling and field conditions. The developed model well represents the strain-rate and fluid-compressibility dependence of nonlinear material behavior under such dynamic conditions, and also captures the development of excess pore pressure due to irrecoverable volume changes. The model is validated against a series of blast and impact tests on saturated natural soils (sand and silt) and tailings fill masses, and a good agreement is found between the experimental and simulated results. Subsequently, the model is applied to investigate the effects of drainage conditions, distance from detonation, stope size, location of retaining structure, and blast sequence on the liquefaction susceptibility of early-age fill mass after mine blasting. The results obtained from the study will provide practical insight into the blast liquefaction potential of backfill mass in field conditions.

Variability of in situ sediment strength and pore pressure behavior of tidal estuary surface sediments

Tidal estuaries feature spatially and temporally varying sediment dynamics and characteristics. Particularly, the variability of geotechnical sediment parameters is still poorly understood, limiting the prediction of long-term sediment stability and dynamics. This paper presents results from an in situ investigation of surficial sediments (≤50 cm) in a tidal estuary in New Hampshire (USA), using a portable free fall penetrometer. The aim is to investigate variations in sediment strength and pore pressure behavior with regard to sediment type and seabed morphology. The study also provides a detailed analysis of high velocity impact pore pressure data to derive information about sediment type and permeability. The penetrometer was deployed 227 times, and the findings are correlated to 78 sediment samples. Differences in sediment strength and type were found when transitioning from tidal flats to the deeper channels. Finer-grained sediments located predominantly on the tidal flats appeared well consolidated with noticeable and spatially consistent sediment strength (reflected in an estimate of quasi-static bearing capacity qsbcmax ~10 kPa). Sediments with higher sand content (>75%) showed more variations in strength relating to differences in gradation, and likely represent loose and poorly consolidated sands (qsbcmax ~10–55 kPa). The rate at which the recorded excess pore pressures approached equilibrium after penetration was classified and related to sediment type. The data indicate that the development of excess pore pressures upon impact and during penetration may provide additional insight into the nature and layering of bed material, such as identifying a desiccated or over-consolidated dilative surficial layer. In summary, with varying sediment grain size distributions, bulk densities and morphology, sediment strength and pore pressure behavior can vary significantly within a tidal estuary.

Dynamic instabilities under isotropic drained compression of idealized granular materials

The compressibility behaviour of loose and contracting granular assemblies, normally consolidated and overconsolidated, under isotropic drained compression is investigated in this paper. Short cylindrical samples of water-saturated monodisperse glass beads, initially assembled in loose state by moist-tamping technique, are isotropically compressed in a classical axisymmetric triaxial machine. Very loose glass bead samples experience numerous unexpected events, sometimes cascading, under undetermined triggered effective isotropic stress in loading and in unloading, while the classical compressibility behaviour of granular material is recovered once these events ignored. Each event, resembling the stick–slip instability during shear in triaxial compression, is characterized by a transient dynamic phase I with very fast drop of effective isotropic stress \(\sigma ^{'}\) due to an excess pore pressure development at nearly constant volume and constant axial strain, followed by a quasi-static phase II with gradual increase in axial \(\varepsilon _\mathrm{a}\) (contraction) and volumetric \(\varepsilon _\mathrm{v}\) (compaction) strain, and a full progressive recovery of \(\sigma ^{'}\) to the previous level before event. A short-lived liquefaction with null \(\sigma ^{'}\) measured in the first phase I results in a local collapse state. Collapse events also happen on unsaturated moist and dry states. Rare events even occur during the unloading of subsequent isotropic compression cycles. The effects of triggered isotropic stress are discussed, the instability characteristics measured, the comparison with stick–slip instability made and the hypothesis of micro-structural instability with local collapse of contact networks and rapid micro-structural rearrangement argued.

Rayleigh Waves Transformation in Liquefying Water-saturated Sands

Abstract The behaviour of a water-saturated sand deposit subjected to dynamic loads induced by the propagation of Rayleigh surface waves is analysed. Cyclic shearing of the saturated sand matrix due to ground motions results in the development of excess pore pressures in the soil and its subsequent liquefaction. The phenomena of pore pressure generation and soil liquefaction are investigated within the framework of a compaction theory for saturated granular media. The results of calculations, carried out by a finite-element method, illustrate the evolution of pore pressures and the development of liquefaction zones in the soil, and show the variation of surface wave parameters with the progressive degradation of the strength of the subsoil.

The influence of overpressure and focused fluid flow on subaquatic slope stability in a formerly glaciated basin: Lake Villarrica (South-Central Chile)

Pore fluid pressure is a key parameter for subaquatic slope stability and has been put forward to explain the development of many submarine landslides, especially when occurring on very gentle slopes or in areas with high sedimentation rate. Due to the sparse availability of in-situ pore pressure data, a profound understanding and quantification of excess pore pressure development and its influence on subaquatic landslide processes is still missing. In this study, we use glacigenic Lake Villarrica (Chile) as a model basin, in which we document in-situ excess pore pressure, focused fluid escape features and subaquatic slope failures at relatively shallow subsurface depth (<10m). A subaquatic slope was characterized in great detail by a dense network of high-resolution seismic-reflection profiles, multibeam bathymetry and sediment cores. In-situ undrained shear strength and formation pore fluid pressures were documented using free-fall piezocone penetrometer (CPTU) and subsequent pore pressure dissipation tests. We show four independent lines of evidence for overpressure of variable magnitude within the sedimentary slopes: i) pockmarks and other fluid escape features on seismic profiles and bathymetric maps, ii) estimation of ambient in-situ pore pressure via CPTU dissipation tests, iii) underconsolidation and downward decrease in in-situ undrained shear strength within units of uniform lithology, and iv) hydrofractured glacio-lacustrine sediments. Locally, overpressure reaches about 80–90% of the hydrostatic vertical effective stress. We hypothesize that glacier-proximal sediments are strongly underconsolidated and overpressured due to rapid deposition and/or subglacial meltwater pumping. Postglacial consolidation of a thick glacier-proximal unit leads to the upward expulsion of excess pore water. A permeability barrier is formed by the overlying glacio-lacustrine clays and fine silts, focusing these migrating fluids to topographic highs. There, very high overpressure ratios facilitate the development of hydro-fractures, repeated fluid escape and pockmarks on the lake bottom. Overpressure may decrease slope stability in Lake Villarrica in different manners: by focused fluid escape weakening sediments, by generating underconsolidated sequences of low undrained shear strength, and by facilitating static failure. Based on high-quality in-situ data, this study confirms that high overpressure ratios in shallow sediments may be a typical feature at formerly-glaciated marine and lacustrine environments and highlights the major role of pore fluid overpressure for preconditioning subaquatic slopes to failure.

Load-Settlement Mechanism of Shallow Foundations Rested on Saturated Sand with Upward Seepage

AbstractIn this study, the ultimate bearing capacity and settlement of square and strip foundations in the presence of upward seepage were assessed using physical modeling. The experiments were conducted in two series with different loading values of excess pore-water-pressure paths. The experimental results demonstrate significant degradation of bearing capacity resulting from excess pore-pressure development; however, a remarkable amount of soil strength was observed when the excess pore-pressure ratio was unity in the free-field ground. A correction coefficient (fγ) for the bearing-capacity factor Nγ was estimated based on the results. This factor is a function of upward-seepage magnitude. Based on the experimental results, dimensionless charts are presented to predict the settlement of square and strip footings rested on saturated sand with varied amounts of excess pore-water pressure. Results show strong dependency of settlement on the bearing-capacity safety factor presumed for the foundation design.

Centrifuge testing to evaluate the liquefaction response of air-injected partially saturated soils beneath shallow foundations

Earthquake-induced liquefaction of saturated soils continues to cause severe damage to structures with shallow foundations. In recent years, artificially reducing the degree of saturation and forming partially saturated zones within saturated soils has been proposed as a liquefaction mitigation technique. This study experimentally investigates the liquefaction response of air-injected partially saturated soils beneath shallow foundations. A series of centrifuge tests were conducted on the shallow foundations with different bearing pressures. The results of the tests show that the generation of excess pore pressures and consequent liquefaction-induced settlements of shallow foundations were a strong function of the degree of saturation. Forming spatially distributed partially saturated zones in the liquefiable soils limited the development of high excess pore pressures and liquefaction susceptibility of soils, particularly at the higher confining stresses. The reduction in the degree of saturation of soils decreased the depth of liquefied soil layer, and increased the resistance of soil to the bearing capacity failure. On the other hand, the decrease in the degree of saturation of liquefiable soils led the larger accelerations to be transmitted to the foundations through unliquefied soil zones. It is therefore concluded that use of air-injection as a liquefaction mitigation measure does reduce structural settlements, but will have the consequence of larger structural accelerations.

PORE PRESSURE CHARACTERISTICS IN ISOTROPIC CONSOLIDATED SATURATED CLAY UNDER UNLOADING CONDITIONS

When foundations, such as caissons, spudcans, and mudmats, are pulled out from the saturated soft clay foundation, negative excess pore pressure will develop in the interface between the base and the soil. This phenomenon will significantly increase the uplift force. Therefore, estimating the developed negative excess pore pressure in the interface under unloading conditions is important. The development of excess pore pressure was investigated based on the triaxial extension test under isotropic consolidation condition. The method of using the modified Cambridge model to predict the excess pore pressure was programmed and has been proven to be feasible. By associating Henkel’s theory in p-q space, the generation mechanism of negative pore pressure was revealed. The research indicates that excess pore pressure is initially negative and then becomes positive in triaxial extension test under isotropic consolidation. The generated excess pore pressure can be divided into two parts. The first part generated from the change in the mean total stress is negative, whereas the second part generated from the change in deviatoric stress is positive. However, these two parts are unbalanced. Research results provide theoretical basis for the development of excess pore pressure in soft clay to analyze the pull-out of structure and the excavation of foundation pit.