Excess Pore Water Pressure Research Articles

Behavior of group of geosynthetic encased granular piles with tire chips- aggregates mixture under static and cyclic loading – A model study

The current study addresses the dual challenges of improving the performance of soft soil beds subjected to static and cyclic stresses and managing the environmental impact of waste tire disposal. This research contributes valuable insights into the sustainable use of recycled tire chips in granular pile construction, coupled with the efficacy of combi-grid encasement for improved soft ground under static and cyclic loading conditions. A series of laboratory model tests were carried out on a group of granular piles to examine the principal parameters, such as the selection of geosynthetic materials and cyclic loading characteristics, including cyclic loading amplitude (qca) and cyclic loading frequency (f). The granular pile composition consists of (25% tire chips + 75% aggregates). The performance of granular piles on improved ground is assessed based on the settlement reduction ratio (Sc,r), accumulation of excess pore water pressure (Pexc), and stress concentration ratio (n). The key findings from static model tests are that the load-bearing capacity is significantly increased with installing a group of ordinary granular piles (= 58%) and substantially increased with combi-grid encasement (= 335%). The effectiveness of ordinary granular piles (OGP) in enhancing the performance of a soft soil bed becomes greater when subjected to lower cyclic loading frequencies (f) and smaller cyclic loading amplitudes (qca). The incorporation of combi-grid encasement greatly enhanced the cyclic performance of a group of granular piles by substantially minimizing cyclic-induced settlement (Sc) across both principle parameters f and qca. This study also examines the increased cyclic stresses on the improved soft bed, resulting in the accumulation of excess pore water pressure (Pexc) development, which is reduced to a greater extent with the help of combi-grid encasement across both principle parameters.

DEM investigation on flow instability of particulate assemblies under coupling between volumetric and axial strains

AbstractThe discrete element method (DEM) is employed to investigate the impact of coupling between volumetric and axial strains on the flow liquefaction vulnerability of 3D cubic particulate specimens. The virtual testing program conducted here encompasses a wide range of initial states and varying degrees of coupling between volumetric and axial strains. Utilizing data obtained from DEM simulations, the evolution of micro- and macroscale variables, including coordination number, contact fabric anisotropy, redundancy index, strong force networks, invariants of the effective stress tensor, and excess pore-water pressure, is examined. Results from DEM tests indicate that coupling expansive volumetric strain with axial strain leads to a gradual loosening of the load bearing microstructure, a decrease in coordination number, and a faster change in contact anisotropy. DEM simulations demonstrate that the triggering of flow liquefaction instability is followed by a sudden increase in contact fabric anisotropy and abrupt drops in coordination number and redundancy index. Moreover, a detailed analysis of the findings suggests that the stress ratio at the onset of post-peak softening decreases with increasing expansive volumetric strains.

A semi‐analytical model for coupled THM consolidation of saturated clays improved by PVTD considering thermal contraction

AbstractPrevious studies have demonstrated that saturated normally consolidated and lightly over‐consolidated clays undergo contraction when heated due to a reduction in preconsolidation pressure. A linear constitutive model is proposed to describe the thermal contraction, with this model, governing equations are developed for the coupled thermo‐hydro‐mechanical (THM) consolidation induced by a prefabricated vertical thermo‐drain (PVTD). Corresponding semi‐analytical solutions are derived employing the Laplace transform and validated via comparison with experimental results, existing numerical model, and custom finite element method (FEM) model. Subsequently, comprehensive parametric analyses are carried out to investigate the THM coupling consolidation behaviors of the clays. Outcomes show that aside from surcharge load, generation of excess pore water pressure in soils can also be induced by thermal contraction, difference in thermal expansibility between pore water and soil grains, and thermo‐osmosis, where the influence of thermal contraction on the excess pore water pressure is the most prominent among the three factors.

Analysis of the thermo-hydro-mechanical coupled dynamic response of sediments under eccentric driving of deep-sea mining vehicle

This study establishes a coupled thermo-hydro-mechanical dynamic model(THMD) for saturated porous sediments under the eccentric motion of a deep-sea mining vehicle, considering the characteristics of deep-sea sediments. We applied the method of complex Fourier series expansion to simplify the coupled equations into ordinary differential equations and obtained the complex Fourier series expansion forms of various physical quantities of sediments during the eccentric movement of the mining vehicle. The undetermined constants are determined based on boundary conditions, yielding a series representation of the solution for THMD of deep-sea sediments. Subsequently, the solution was utilized to investigate the influence of mining vehicle parameters on the dynamic response of deep-sea sediments. This study demonstrates that the vertical displacement, vertical stress, and excess pore water pressure of sediments increase as the speed of the mining vehicle increases. Furthermore, the vertical displacement, vertical stress, and excess pore water pressure increase with the increment of load amplitude, and the differences in the curves at different eccentricities gradually become larger as the load amplitude increases. The proposed model can be applied to geotechnical engineering problems in saturated porous foundations, providing a necessary theoretical basis and analytical approach.

Numerical modeling of LEAP-2017 liquefiable sloping ground centrifuge tests using the ISA-hypoplasticity model

This article evaluates the prediction capabilities and limitations of the ISA-hypoplasticity constitutive model for sands in the simulation of liquefiable sloping ground dynamic centrifuge tests from the LEAP-2017 project. For that purpose, the experimental results of three centrifuge tests performed at different centrifuge facilities are considered. The corresponding dynamic coupled predictions were carried out using the finite element software Abaqus Standard. The material parameters of the adopted constitutive model were determined based on a detailed laboratory characterization of the tested Ottawa F-65 sand, which included drained monotonic triaxial and undrained cyclic triaxial tests considering a wide range of initial conditions and testing characteristics. The comparison between measured and computed sloping ground accelerations and spectral response suggests reasonable predictions by the model. Furthermore, the results suggest that the constitutive model is only able to realistically reproduce the trends of the experimental excess pore water pressure when considering cyclic mobility effects.

Energy-Based Assessment of Liquefaction Behavior of a Non-Plastic Silt Based on Cyclic Triaxial Tests

Earthquakes cause cyclic shear deformations in soil and build-up of excessive pore water pressure as a result of undrained loading, accompanied with rearrangement of soil particles and degradation in stiffness of the soil due to decrease in effective stresses. During loading, the onset of soil liquefaction is defined as a stress state in which the excess pore water pressure is equalized to the total stress. From this point of view, assessment of the pore water pressure development pattern under cyclic loading has been one of the most salient research topics in geotechnical and earthquake engineering. In this study, results of a series of cyclic triaxial tests on non-plastic silt specimens consolidated under 100 kPa effective isotropic consolidation pressure were used to question the modelling ability of pore pressure development models previously proposed for sands. Tests were performed on specimens of 6 different initial relative densities (Dr) ranging between 30-80% and 10 different cyclic stress ratios (CSR). The key parameters of pore water pressure development and shear deformation in the energy-based model used are relative density, cyclic stress ratio and number of cycles. The results revealed that, these energy-based models have a strong potential in evaluation of pore water pressure development pattern of non-plastic silts. Test results also show that the increase in relative density and decrease in CSR causes a ladderlike behavior among pore water pressure and cyclic shear strain, which is relevantly rendered by energy-based models.

Mechanical mechanism of soft soil foundation improved by geosynthetic-reinforced pile-supported technology based on field experiment

Geosynthetic-reinforced pile-supported (GRPS) technology is an effective method for improving soft soil foundations. Conducting field tests is crucial for accurately assessing the mechanical behavior of GRPS technology. Based on the fourth expansion project at Pudong Airport, field tests were conducted to investigate the effects of GRPS technology on soft-foundation treatment. Specifically, the tests examined the changes in the pile and soil settlement, pore water pressure, and pile and soil stress. The results showed that the settlement of the GRPS foundation mainly occurred during the loading period, accounting for approximately 65% of the total foundation settlement. Increasing the pile modulus and reducing the pile spacing decreased the total foundation settlement. The filling load was mainly transferred to the piles through the soil arching effect, resulting in a large excess pore water pressure in the soil layer near the pile tip. In addition, when the filling height was low, the load transfer relied predominantly on the tensile membrane effect of the geogrid, which was more pronounced near the piles. However, as the filling height increased, the soil arching effect surpassed the tensile membrane effect and played a key role in load transfer.

Experimental Evaluation of Optimizing Sheet Pile Installation Depth and Distance to Mitigate Liquefaction-Induced Foundation Settlements

Liquefaction-induced settlement poses significant challenges to the stability and performance of structures constructed on liquefiable soils during seismic events. To mitigate the adverse effects of liquefaction-induced foundation settlements, sheet piles are utilized to provide lateral confinement to the soil, thereby enhancing its stability. The performance of sheet piles is contingent upon their installation location. Therefore, identifying the optimal installation location for sheet piles remains a topic of interest. This research aims to evaluate the optimal installation location of sheet piles to mitigate liquefaction-induced foundation settlements. The experimental study employs a moderate scale 1g shaking table to replicate seismic events and investigate the behavior of sandy soil under liquefaction conditions. Multiple configurations of sheet piles, varying in terms of distance from the foundation, depth within the dense layer, and effect of thickness are examined. The excess pore water pressure generation and dissipation, and foundation movement (settlement and tilting) are used to determine the optimal location of sheet piles that minimize liquefaction-induced foundation settlements. The results indicate that the liquefaction-induced foundation settlement increased with increasing the distance between the sheet piles and the foundation. Adversely, the settlement decreased with increasing the embedment depth of sheet piles. Additionally, although this was not the focus of this study, the installation of thinner sheet piles surrounding the foundation appeared to be a more effective measure than thicker sheet piles in mitigating liquefaction-induced foundation settlements based on this experiment.

Experimental study of the dynamic characteristics of deep silty clay in Shanghai

To investigate the dynamic properties of deep silty clay, a comprehensive set of cyclic triaxial and resonant column tests were performed on undisturbed samples of silty clay containing sand from depths reaching 54.5 m in Shanghai. This study aimed to characterize the influence of confining pressure on the stress–strain behavior, pore water pressure, and dynamic modulus of the soil. The present findings revealed that the confining pressure, which is correlated with the depth of burial, significantly influenced the soil deformation. Moreover, elevated confining pressures resulted in increased stress within the hysteresis loops, a higher initial shear modulus, and a diminished ratio of excess pore water pressure. Based on the empirical data, a refined model was formulated for the shear modulus and damping ratio as functions of the shear strain for deep silty clay in Shanghai. These insights are intended to offer a robust reference point, facilitating calibration and guiding subsequent investigations into structures situated in the deep subterranean environments of Shanghai.

Influences of strain rate on mechanical behaviors of unsaturated and quasi-saturated loess under varying drainage conditions

Although time-dependent deformation of geomaterials underpins slope-failure prediction models, the influence of strain rate on shearing strength and deformation behavior of loess remains unclear. The consolidated undrained (CU) and drained (CD) triaxial testing elucidated the impact of strain rate (0.005–0.3 mm/min) on strength envelopes, deformation moduli, pore pressures, and dilatancy characteristics of unsaturated and quasi-saturated loess. Under drained conditions with a controlled matric suction of 50 kPa, increasing strain rates from 0.005 mm/min to 0.011 mm/min induced decreases in failure deviatoric stress (qf), initial deformation modulus (Ei), and cohesion (c), while friction angles remained unaffected. Specimens displayed initial contractive volumetric strains transitioning to dilation across varying confining pressures. Higher rates diminished contractive volumetric strains and drainage volumes, indicating reduced densification and strength in the shear zone. Under undrained conditions, both unsaturated and quasi-saturated (pore pressure coefficient B = 0.75) loess exhibited deteriorating mechanical properties with increasing rates from 0.03 mm/min to 0.3 mm/min. For unsaturated loess, reduced contractive volumetric strains at higher rates manifested relatively looser structures in the pre-peak stress phase. The strength decrement in quasi-saturated loess arose from elevated excess porewater pressures diminishing effective stresses. Negative porewater pressures emerged in quasi-saturated loess at lower confining pressures and strain rates. Compared to previous studies, the qf and Ei exhibited rate sensitivity below threshold values before attaining minima with marginal subsequent influence. The underlying mechanism mirrors the transition from creep to accelerated deformation phase of landslides.

Experimental Study on Consolidation Characteristics of Concentrated Full Tailings and Research on Pore Water Relief Methods of Piles

Surface harmless storage of concentrated full tailings (CFTs) involves the technology of adding a curing agent to the tailings slurry discharged from the thickener to realize the modification of the tailings and centralized storage of the tailings on the surface to realize the harmless treatment of the tailings. High water content of tailings is still the key technical problem that restricts the harmless storage of piles at present. Regarding the above problems, we implemented the consolidation test and numerical simulation of seepage-stress coupling consolidation of CFT, clarified the consolidation characteristics and parameters of CFT under different curing ages, and conducted a comparative analysis of pore water pressure in the whole cross-section of piles with different drainage schemes based on the results of the test. In addition, we also clarified the drainage effect of interlayer drainage on reducing the excess pore water pressure of piles and compared the simulation results of the pore water pressure of piles under different permeability coefficients. The results show that as the permeability coefficient of the concentrated tailings material decreases, the pore pressure accumulation inside piles under the same drainage scheme is more serious, and the length of time for consolidation and stabilization becomes longer. Therefore, it is recommended that the excess porous water pressure be relieved by means of increased drainage facilities under a small permeability coefficient.

An experimental investigation on undrained cyclic behaviour of a saturated intact loess

There are growing evidence indicating that earthquakes occur more frequently in the Chinese Loess Plateau, which can possibly cause saturated intact loess liquefaction, leading to geological disasters. However, the cyclic behaviour of saturated intact loess is still much less investigated. In the presented work, typical intact loess was collected to carry out a comprehensive experimental investigation, including both undrained monotonic and cyclic triaxial shear tests with different confining stresses and cyclic stress. The results indicate that the intact loess shows an overall clay-like cyclic behaviour. It shows a strain softening behaviour under undrained monotonic shearing, while under undrained cyclic shearing, a well-defined failure pattern of plastic strain accumulation is observed, and an axial strain of 3 % is determined as the failure criterion. Although different excessive pore water pressure generation patterns and failure criteria are found under monotonic and cyclic conditions, a quantitative relationship is established between the monotonic and cyclic stress-strain behaviour based on the unique relationship found between axial strain and the defined allowable stress. Additional micro-structure observations and analyses further reveal the differences in loess structure destruction caused by different loading conditions. This study helps to further understand the cyclic behaviour and failure mechanism of intact loess, and to provide a useful reference for the evaluation and prevention of possible soil liquefaction failure in loess areas.

Assessment of Axial Resistance of Piles Considering Consolidation Setup and Aging Setup Using Direct Pile Cone Penetration Test Methods

This paper focuses on evaluating the increase in axial pile resistance subjected to both consolidation and aging setups. Consolidation and aging setup models were first developed to estimate the setup parameters based on databases collected from literature, which include 10 instrumented piles for consolidation setup and 26 test piles for long-term aging. The eight top-performing pile cone penetration test (CPT) methods that were evaluated in a previous study were used to estimate the side resistance of soil layers at 14 days after pile driving. The developed consolidation and aging setup models were then used to extrapolate the results to evaluate the side resistance of each soil layer at the end of consolidation and for long-term aging. The estimated side and total resistances were compared with the measurements from pile load tests considering both consolidation and aging setups. The resistances estimated before and after completion of excess pore water pressure dissipation indicates that significant aging takes place after consolidation setup. The value of consolidation setup parameter ( Ac) was 0.53, and, for aging, the setup parameter ( Ag) was 0.23 in clay and 0.16 in sand. The results show that all pile CPT methods with/without using a consolidation setup model tend to underestimate the unit side resistance of clay soil layers. The use of pile CPT methods in combination with an aging model improved the accuracy of pile CPT methods, and this was verified using load test results for five piles subjected to aging. The Philipponnat and University of Florida (UF) methods showed the best performance on estimating the total resistance of piles subjected to aging.

Stability Analysis of Cofferdam with Double-Wall Steel Sheet Piles under Wave Action from Storm Surges

Double-wall steel sheet piles (DSSPs) are widely used in large-span cofferdams for docks due to their good performance against wave action during storm surges. This paper describes a study of the dynamic behavior of a DSSP cofferdam under wave action through flume tests and a numerical simulation that combined computational fluid dynamics (CFD) and the finite element method. The influences of the water level and wave height on the DSSP cofferdam were investigated experimentally and numerically. Tall waves in shallow water broke upon and impacted the seaside pile with large dynamic wave pressure, dramatically increasing the stress and displacement of the seaside pile. The overlap of the traveling and reflected waves increased the excess pore water pressure near the seaside pile due to taller overlapped waves and higher wave frequency. The DSSP cofferdam failed under the combined actions of the dynamic wave pressure and erosion of the landside seabed. The leakage and overflow of the breaking waves resulted in significant erosion of the landside seabed and greatly weakened the support of the seabed. The dynamic wave pressure then pushed the DSSP cofferdam until it failed. The simulation with the combined methods of CFD and FEM resulted in trends that were similar to those of the test measurements. Compared to the quasi-static method and pseudo-dynamic method, the results of the simulation via the present method were much closer to the test results because the simulation included the effects of breaking waves. The reinforced measure worked well to prevent the DSSP cofferdam in a sandy seabed foundation from continuous failures of deformation–leakage–erosion–tilting. However, it failed in a clay interlayer seabed foundation due to the large settlement.

Comparison of soil improvement techniques on the development of efficient consolidation response

A comprehensive experimental program including two distinct series of consolidation tests was performed on clay specimens prepared at different dry weight proportions including 0%, 1%, 2.5%, 5%, 7.5%, 10% polypropylene fiber or lime by weight mixed with clayey soil. Fiber inclusion into clay resulted in enhancement of compressive strength characteristics, improvement of hydraulic properties that is an advantage for modification of stability and durability properties of clayey soil under loads. Similarly, the higher hydraulic conductivity of clay resulted that will shorthen duration of consolidation settlement, hence, eventually influence completion of plastic consolidation deformation favorably for soft clays. Lime-treatment on clay specimens showed that the compressibility properties are improved such that the strength of clay against loading enhances, exhibits less consolidation deformation under load owing to increase in lime content. On the other hand, clay becomes highly impermeable, displays substantially larger water-resistant properties because of increased lime mass proportion (i.e. time-extension) in clayey soil that results in prolongation of expulsion of excess porewater pressure from clay due to load application, relevant induced stresses. Fiber-inclusion resulted in exhibiting logarithmic decrement with a mild rate of decline while lime-treatment led to exponential reduction with a sharp rate of drop for compression index (Cc), compressibility coefficient (αv), volume compressibility coefficient (mv). Further, fiber-inclusion stimulated exponential and quadratical increment whereas lime-treatment induced exponential decrement for coefficient of consolidation (cv), hydraulic conductivity (k), respectively. As a result, the Cc, αv, mv enhanced on the order of within 10 at average of 80% to 90% with a minimum of 70% by value for both fiber-reinforcement and lime-stabilization soil-stabilization techniques. The cv, k improved on the order of within 10 at average of 75% to 85% by value for fiber-reinforcement whereas dis-improved on the order of within 10 at average of 70% to 80% by value for lime-stabilization.

Time-dependent performance of large piled raft in clayey soil

In this study, three-dimensional numerical analysis has been performed to comprehend the time-dependent behavior of large piled rafts founded in saturated clayey soil, explicitly for the time of superstructure construction and post-construction up to the end of soil consolidation. The performance under different construction times for the corresponding unpiled raft and the piled raft with uniform pile length configuration of varying pile number, spacing, and length have been examined and then compared with that of non-uniform length configuration for the same total pile length. The dissipation of excess pore water pressure, average settlement, and differential settlement are presented and discussed in detail. A quicker dissipation rate of excess pore pressure is noted with either fewer piles, wider pile spacing, or shorter-length piles. An increase in construction time results in a lesser total average settlement at the end of consolidation. The piled raft with uniform pile length configuration shows a higher differential settlement than the unpiled raft. However, by adopting a suitable non-uniform length configuration, negligible differential settlement, higher load-carrying capacity, and a significant reduction in pile material volume can be achieved. Predictive expressions have been proposed to estimate the average and differential settlements of piled raft with uniform length configuration.

Effect of advance rate on excess pore water pressure induced by TBM tunnelling in saturated sand

The stability of tunnel face can be significantly influenced by the excess pore water pressure induced by tunnel boring machine (TBM) tunnelling in saturated sand. This article aims to investigate the effect of TBM advance rate on the variation of such excess pore water pressure. Numerical and analytical solutions are given for the situations of TBM advance rate faster and slower than the slurry infiltration velocity, respectively. It appears that the excess pore water pressure at the tunnel face maintains constant with time when the TBM advance rate exceeds the slurry infiltration velocity and the gradient close to the TBM decreases with distance. In the opposite situation, there is a temporal decrease in excess pore water pressure at the tunnel face, accompanied by a more gradual spatial drop in pressure. In addition, the gradient close to the TBM remains constant. Notably, a larger extent of excess pore water pressures is present in the latter situation, augmenting the risk of a blow-out during tunnelling. Comparative analysis of field measurements and theoretical calculations indicates that the conditions at Green Heart Tunnel align with the latter situation. Furthermore, for the latter situation, a slower TBM advance rate will result in a higher discharge and thus a higher pore water pressure.

Large‐strain consolidation of vacuum preloading combined with partially penetrating prefabricated vertical drains

AbstractA system of vacuum preloading combined with partially penetrating prefabricated vertical drains (PP‐PVDs) is an effective solution for promoting the consolidation of the dredged marine clay. However, a significant and traditionally challenging‐to‐predict amount of deformation or settlement occurs. Therefore, it is necessary to introduce a three‐dimensional large‐strain consolidation model to consider the length of the vertical drain to determine the consolidation time and degree of consolidation (DoC), and associated settlement. The predictions using the proposed analytical model provide fair agreements with the field data and those in the literature. Parametric studies reveal that to achieve a 90% DoC within 100 days in soft soil, the optimal penetration depth for PP‐PVDs would be 0.7 times the depth of the soil layer. With the increase of DoC, the ratio of excess pore water pressure to applied vacuum pressure (u/P) in the whole soil layer moves toward 1.0. With the increase of PVD penetration ratio (H1/H), more DoC is required to dissipate the excess pore water pressure in the top improved soil layer.

Failure mechanisms of saturated sand under different loading frequencies: Experimental observation and constitutive modelling

In this paper, we report on cyclic hollow cylinder tests on Fujian sand and Nanjing sand subjected to different loading frequencies. Depending on the loading frequencies we observed two distinct failure modes, i.e. cyclic mobility and cyclic instability. We proceed to develop a plasticity model to capture the features of our observations. The model employed a new flow rule for the effect of loading frequency on shear dilation and contraction characteristics to better reflect the changes in excess pore water pressure, deformation and strength of the saturated sand under cyclic loading. The model was validated by comparing numerical simulations with experimental results.

Analysis of Offshore Wind Turbine by Considering Soil-Pile-Structure Interaction: Effect of Sea-Wave Load Duration

ABSTRACT Offshore Wind Turbines (OWTs) confront different types of environmental loads during their lifetime. One of the most significant loads is the cyclic sea-wave load which affects the OWT system during the approximate 25 years of design life. This lateral cyclic load can influence the response of the OWT system because of accumulated permanent displacement and excess pore water pressure generation in soil. This procedure can change pile-soil stiffness. However, the behavior of OWT and its foundations is mostly studied under short-term cyclic loading, and the effects of duration of the cyclic loads on pile-soil interaction and performance of the foundation are not well understood and documented. Therefore, there is a lack of guidance in codes for the duration effects of cyclic loads on the structural and geotechnical response of OWTs. In this regard, the current study considers the effects of duration of the cyclic loads on serviceability and performance of the OWT system by considering soil-foundation-structure interaction using a 2-D finite element method. The behavior of the OWT system is evaluated based on the internal forces and deformation of the monopile foundation, shear strain, and excess pore water pressure ratio in the surrounding soil. Besides, liquefaction susceptibility in the sandy soil layer at the vicinity of the monopile and its effect on the performance of the foundation is investigated. Finally, the results can provide guidance on estimation of the dynamic performance of the OWT system during long-term cyclic loads. They can be used to specify the need for consideration of the duration of cyclic lateral loads for the design of OWT structures.