Excess Pore Water Pressure Research Articles

An analytical solution for nonlinear consolidation of composite foundations improved by stone columns and vertical drains

The nonlinear compressibility and permeability of soft soils is a key factor in the consolidation of both composite and natural foundations. However, there are few corresponding consolidation theories for vertical drain-stone column composite foundations in which the nonlinear consolidation characteristics of soils can be taken into account. In this study, a nonlinear consolidation model is established to describe the seepage relations between soft soils, vertical drains and stone columns. Then, a consolidation governing equation is derived, where the total excess pore water pressure (EPWP), u¯, is a variable, and the average consolidation degree defined by strain and stress of the composite foundation is obtained, respectively. Numerous calculations are performed to analyze the consolidation behaviors of such composite foundations. The results show that the installation of vertical drains can enhance the consolidation rate effectively; the external load and the initial modulus ratio of column to soil have an influence on the stress concentration effect of stone columns. Finally, the proposed analytical solution is applied to the settlement calculation for an actual project, and the calculated results are in good agreement with the measured data in the early and late stages of consolidation.

Equivalent seismic behaviors of artificial frozen-thawed soft clay with a new reduction factor of shear stress

The artificial ground freezing (AGF) technique is widely used in the construction of underground structures, and the deformation of soft clay is more significant after freezing-thawing cycles. Moreover, many earthquake disasters indicate that the engineering in soft soil areas experience huge risks to seismic fortification. In this paper, the inversion formula of seismic acceleration time history for different peak ground accelerations (PGAs) in the frequency domain was derived. The maximum peak acceleration and maximum dynamic shear stress in the undisturbed and frozen-thawed soft clay sites were analyzed by DEEPSOIL. The reduction factor of dynamic shear stress suitable for the clay field was calculated to provide basic parameters for the cyclic triaxial tests considering different confining pressures, dynamic stress amplitudes and loading frequencies. The ground motion acceleration of soft clay sites has an amplification effect, and that of frozen-thawed soft clay sites is more obvious. The reduction factor of shear stress for the clay site is overestimated by the liquefaction resistance shear stress method. The axial strain of the frozen-thawed clay reaching failure is less than that of the undisturbed clay. Under the same seismic shear stress, the development of the excess pore water pressure in frozen-thawed clay is more significant than that in undisturbed clay.

Nonlinear consolidation analysis for composite ground with combined use of impervious columns and prefabricated vertical drains under time-dependent loading

The technique of the combined use of soil-cement columns (impervious columns) and prefabricated vertical drains (PVDs) has been proposed and widely utilized in ground improvement projects. However, there are few theoretical investigations on the nonlinear consolidation behavior of this combined technique. To fill this gap, the logarithm models of e-lgσ and e-lgk are incorporated to describe the nonlinear consolidation characteristics of soils. Meanwhile, a mathematical model for the nonlinear consolidation of combined composite ground is established by considering the variation of the column-soil compression modulus ratio with consolidation. The governing equations and simplified analytical solutions for four types of loading schemes are obtained in consideration of the smear effects of both impervious columns and PVDs and the well resistance of PVDs. The calculated results of this paper are then compared with the proposed models and the impacts of various parameters on the nonlinear consolidation performance are investigated. The results show that ignoring the nonlinearity will overestimate the consolidation rate when the soil’s compressive indices are greater than the permeability indices. Besides, Cc/Ckv has little influence on the consolidation rate and can be ignored compared with Cc/Ckh. Furthermore, the increase in the stress increment within the ground due to the external loads leads to an acceleration of the consolidation when Cc/Ckh(v) > 1, and the dissipation rate of the excess pore water pressure increases by a maximum of 15 %. Finally, a field case study is conducted to verify the prediction ability of the current model.

Assessment on earthquake-induced liquefaction around a hybrid monopile foundation for offshore wind turbines with a transition layer model

Offshore wind turbines are currently being developed to have a large size. A hybrid monopile foundation, which is composed of a single pile and a wheel, is advantageous in terms of bearing capacity and liquefaction resistance in liquefiable soil. In this study, the seismic response of a hybrid monopile foundation was investigated using a verified numerical model. Earthquake-induced liquefaction around the foundation was assessed by considering the influence of the foundation dimensions. In the numerical model, a pile-soil contact element was proposed using a transition layer model that effectively buffered the pile-soil interactions. The soil characteristics were evaluated based on the distribution of excess pore water pressure. The liquefaction resistance of the shallow layer was improved by the wheel, and the pile length had a negligible influence on the seismic response. Larger wheel diameter and thickness demonstrated a more pronounced improvement in the anti-liquefaction ability. The influencing zone was determined based on the liquefiable area and had a conical shape under the wheel. This study quantitatively investigated the liquefaction characteristics of a hybrid monopile foundation and provided insights into the numerical calculation of dynamic pile-soil interactions.

Low amplitude strain accumulation model for natural soft clays below railways

An improved constitutive model for strain accumulation of natural clays under undrained cyclic loading is presented. The proposed model includes a formulation for the non-linear small-strain stiffness in the overconsolidated regime, along with a modified hardening law for cyclic accumulation to improve the tracking of strain accumulation at small stress amplitudes. To calibrate and validate the proposed model, a series of laboratory tests were conducted to study the cyclic response of natural Swedish clays, the effect of loading amplitude and pre-shearing history. Good agreement between predicted and measured accumulated axial strains and excess pore water pressures was obtained with different loading amplitudes. The findings reveal that the undrained pre-shearing has a substantial impact on the rate of accumulated strain, with pre-sheared samples exhibiting lower resistance values. The proposed and validated model opens up possibilities to study the monotonic and non-monotonic quasi-static response of soft clays below railway embankments over the lifetime of the structure, i.e. including the effects of construction, operation and decommissioning.

Incorporation of three different creep models into large-strain consolidation analysis of a clayey tailings deposit

Soft soils and clayey tailings deposits can exhibit substantial creep during consolidation. Creep can strongly influence the consolidation and strength development of tailings deposits but is rarely considered in existing large-strain consolidation analyses for design purposes. This paper presents an analysis of a 13 m high deposit of soft clayey tailings using large-strain consolidation coupled with three alternate creep formulations: the Yin–Graham, Vermeer, and overstress formulations. Predictions of settlement and excess pore water pressure using all three consolidation–creep models have better agreement with the measured data compared to predictions from a large-strain consolidation model without creep. The overstress model is more difficult to calibrate for young soft soils in an initially normally consolidated state, owing to the strong dependency on the initial ratio between an unknown small preconsolidation stress and the initial small effective stress. The models calibrated to the pilot study were then used to extrapolate the full-deposit behavior, and the implications of using such consolidation–creep models and their limitations are discussed.

Two-dimensional plane strain consolidation of unsaturated soils considering the depth-dependent stress

In practical engineering, the total vertical stress in the soil layer is not constant due to stress diffusion, and varies with time and depth. Therefore, the purpose of this paper is to investigate the effect of stress diffusion on the two-dimensional (2D) plane strain consolidation properties of unsaturated soils when the stress varies with time and depth. A series of semi-analytical solutions in terms of excess pore air and water pressures and settlement for 2D plane strain consolidation of unsaturated soils can be derived with the joint use of Laplace transform and Fourier sine series expansion. Then, the inverse Laplace transform of the semi-analytical solution is given in the time domain using a self-programmed code based on Crump's method. The reliability of the obtained solutions is proved by the degeneration. Finally, the 2D plots of excess pore pressures and the curves of settlement varying with time, considering different physical parameters of unsaturated soil stratum and depth-dependent stress, are depicted and analyzed to study the 2D plane strain consolidation properties of unsaturated soils subjected to the depth-dependent stress.

Fractional derivative modelling for rheological characteristics of multilayered saturated porous rock with interfacial thermal contact resistance

AbstractIn this paper, the one‐dimensional rheological characteristics of multilayered saturated porous rock subjected to a ramp‐type heating is investigated. By introducing the fractional order parameter and material parameters, a viscoelastic constitutive model is proposed to describe the rheological characteristics of multilayered saturated porous rock. The general incomplete thermal contact model is established to predict the interfacial thermal contact resistance of multilayered saturated porous rock. Based on the coupled thermo‐hydro‐mechanical theory, the semi‐analytical solutions of the excess pore water pressure, temperature increment and displacement are obtained by using the Laplace transform method. The accuracy of the present solutions is verified by comparing with the classic elastic and viscoelastic models, and existing solutions. In addition, the influence of fractional order parameter, material parameter ratio, thermal contact transfer coefficient and thermal partition coefficient on the excess pore water pressure, temperature increment and displacement of multilayered saturated porous rock are investigated.

Development characteristics of excess pore water pressure in saturated marine coral sand based on shear strain characteristics: An experimental study

Using a hollow-cylinder torsional shear apparatus, we experimentally investigated the development characteristics of excess pore water pressure (EPWP) in saturated marine coral sand. These coral sand specimens were tested under various values of nonplastic fines content (FC), relative density (Dr), and cyclic stress ratio (CSR). A laboratory cyclic torsional shear test under isotropic consolidation showed that the development rate of the EPWP ratio (Ru) versus the number of cycles (N) increased with increasing FC and CSR but decreased with increasing Dr. Additionally, the increase in FC significantly reduced the cyclic resistance ratio (CRR) of marine coral sand. For a given Dr and FC, Ru of the specimens under different CSR was uniquely related to the amplitude of the shear strain (γa). Moreover, a pore pressure evaluation model based on shear strain characteristics was established. The measurements showed that the EPWP model parameter A is a soil-specific constant, and the density-corrected EPWP model parameter B/(Dr)1.5 has a single negative-power relationship with the equivalent skeleton void ratio (esk*.

Liquefaction characteristics of saturated marine coral sand with different non-plastic fines content subjected to complex cyclic stress paths

Coral reefs thrive in complex marine geological environments and are likely to suffer from dynamic marine disasters. The liquefaction characteristics of saturated marine coral sand subjected to dynamic loading, such as ocean waves, submarine earthquakes, storm tides, volcanic activities, and hurricanes, are key factors affecting the safety of coral reefs. A systematic study is conducted through a series of hollow-cylinder torsional shear experiments on the undrained response of saturated marine coral sand with different non-plastic fines contents (FCs) under cyclic linear stress paths with various cyclic loading direction angles (αd). An interesting discovery is that the cyclic linear stress paths with angles αd and 90°−αd have the same stress effect on liquefaction characteristics under isotropic consolidation condition. The test results show that if the near-zero effective stress state is defined as the criterion for initial liquefaction, then the liquefaction resistance of saturated coral sand decreases with increasing αd or FC. The generation of excess pore water pressure (EPWP) presents three modes: (1) “rapid–stable–fast,” (2) “rapid–stable,” and (3) “fast linear-like.” Moreover, an energy-based EPWP prediction method is established. A novel discovery is that the generalized shear strain amplitude (γga) is uniquely related to the EPWP ratio of coral sand for a given relative density and FC. In addition, by introducing unit cyclic stress ratio (USR) as a new index of liquefaction resistance, a common correlation between equivalent skeleton void ratio esk* and USR15 (required for initial liquefaction in 15 cycles) is established for all test cases considered. This is confirmed by the experimental data of five types of terrestrial siliceous sands reported in the literature.

Application of CPT test in the evaluation of the test effect of lime soil compacting pile

In the course of highway construction and use in loess area, subgrade settlement disease is common and has great destructiveness. The cone penetration test (CPT), especially the pore pressure cone penetration test, can provide three continuous readings along the exploration depth cone tip resistance, lateral friction resistance and excess pore water pressure. In this paper, CPT tests are carried out in the losses areas in Shanxi, China. Some data and curves are obtained by using CPT, which are quite different from those obtained by conventional drilling, reflecting the soil layer characteristics of the subgrade section, in order to provide a reference for similar projects. The results showed that CPT has a strong advantage in the analysis of the effect of compacted pile in the treatment of pile length and the improvement and reduction of indicators of different layers along the depth direction. However, the correlation between the static penetration and the compaction coefficient commonly used in the current norms still needs to be further studied to enhance the correlation analysis with detection parameters.

Integrated effects of inherent and induced anisotropy on reliquefaction resistance of Toyoura sand with different strain histories

Recent earthquakes in New Zealand and Japan showed that pre-shaking histories significantly affected the reliquefaction resistance of soils. In this study, a series of experimental tests was conducted to elucidate the coupled effects of inherent and induced anisotropy on reliquefaction resistance of Toyoura sand, which have not been studied before. Accordingly, loose and dense Toyoura sands were prepared with two different methods: dry deposition and moist tamping. The specimens were sheared cyclically using a hollow cylinder torsional shear apparatus under various cyclic stress ratios up to different residual shear strains (γres) and reconsolidated at different states. The experimental results were assessed from various perspectives, including stress–strain relationships, failure mechanisms, liquefaction/reliquefaction resistance, excess pore water pressure (EPWP) generation and compressibility in conjunction with micromechanical interpretations. It was shown that fabric evolution affects the reliquefaction characteristics of Toyoura sand substantially. Interestingly, a unique correlation exists between EPWP and shear strain accumulation for all tests. An energy-based model was developed to uniquely correlate the dissipated energy with the cyclic resistance based only on residual shear strains; this model shows great promise to develop a unified, energy-based criterion for quantifying the liquefaction/reliquefaction resistance of soils with different fabrics.

Modelling biogeochemical clogging affecting piezometers in acid sulfate soil terrain

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

EFFECT OF CHANGE IN THE COEFFICIENT OF PERMEABILITY ON CONSOLIDATION CHARACTERISTICS OF CLAYS

The settlement rate and pore water pressure dissipation rate are mainly controlled by the permeability of soil. Both laboratory and field tests show that the permeability is varied during the loading and consolidation process. It is known that consolidation process is accompanied by decrease in void ratio which leads to decrease in the coefficient of permeability. The importance of the decrease of the coefficient of permeability on the time rate of settlement and pore water pressure needs to be investigated.This paper takes into account the change in coefficient of permeability during consolidation and studies its effect on consolidation characteristics of a clay layer. The finite element method is used in the analysis and the package Geo-Slope is adopted through coupling the programs SIGMA/W and SEEP/W. The relationship between the applied pressure and permeability was determined experimentally for three samples.It was concluded that the effect of permeability is clear at later times of consolidation due to decrease in void ratio and hence slower dissipation of pore water pressure. Taking into account variable permeability leads to longer times of consolidation. At later times (after 400 days), the excess pore water pressure predicted for the case of variable permeability is greater thanconventional case by about (10 – 12) %.

A Comparative Case Study on Drainage Consolidation Improvement of Soft Soil under Vacuum Preloading and Surcharge Preloading

Based on an improvement project of soft soil ground in Zhuhai City on the Pearl River Delta, a comparative study on vacuum preloading and surcharge preloading was performed. The ground and stratified settlements, excess pore water pressure, and the degrees of consolidation of soft soil are analyzed, along with the horizontal displacement and soil strength. The results show that surcharge preloading results in smaller secondary consolidation settlements than vacuum preloading. Primary consolidation settlement quickly increases with increasing excess pore water pressure of less than −40 kPa in vacuum preloading, while also increasing between 20 kPa and 25 kPa in surcharge preloading. The sharp increase in the strata permeability coefficient will induce the increase in strata consolidation degree and has little effect on the ground consolidation degree. The surcharge preloading can be given priority to reduce the settlement foundation in the service stage.

Influence of water content on clay deformation characteristics subjected to periodic intermittent loading

The influence of clay's drainage and reconsolidation behavior during the intermittent period on clay's deformation needs to be further studied. Dynamic loads, such as traffic and earthquake, etc., have a certain intermittent period during the loading process. This study examined the deformation behavior of the clay obtained from Nanchang under various cyclic stress ratios (CSR) and water content conditions. The results demonstrated that minor strain rebound occurs in all intermittent periods. Aside from that, drainage reconsolidation during intermittent periods alters the pattern of strain accumulation during subsequent loading periods, and cyclic stress levels and water content conditions can impact the intermittent effects. In contrast, the development of strain in the samples under intermittent loading is more negatively impacted by high CSR and low water content (deviation from optimal water content). The hysteresis curve continued to change throughout the test, the accumulation and dissipation of excess pore water pressure changed, and it was found that the samples' resilient modulus had increased, indicating an improvement in the samples' resistance to deformation.

Method for Estimating Fully Coupled Response of Deep Excavations in Soft Clays

The influence of excavation rate, geometric layout, and hydraulic conductivity of basal and retained soils on the fully coupled solid–fluid behavior of braced excavations in cohesive soils is presented. Fully coupled excavation analyses were performed using the hypoplasticity clay model to reproduce the constitutive soil behavior combined with Biot’s consolidation theory. A method aimed at predicting excavation-induced ground movements using excess pore-water pressure ratios as a function of site-specific groundwater considerations is presented for the determination of the recommended type of excavation analysis (i.e., drained, partially drained, or undrained). Results obtained from the numerical simulations indicated that excavation rate to hydraulic conductivity ratios, ER/k, smaller than 0.1 and larger than 10,000 can be analyzed under drained and undrained conditions, respectively. The proposed method was validated with published case histories, and its ability to estimate excavation performance in terms of excess pore-water pressures and excavation rates is presented in this paper.

Set-up effect of large-diameter open-ended thin-walled pipe piles driven in clay

A finite element method (FEM) model was developed to investigate the distribution and dissipation of the soil excess pore water pressure (EPWP) around large-diameter open-ended thin-wall steel pipe piles (LOTPs) driven by impact loads into saturated clay, in addition to the set-up effect of LOTPs. The initial pore water pressure (PWP) distribution and vertical bearing capacity of LOTPs after EPWP dissipation were analyzed through this numerical model. The EPWP mapped onto the FEM model was compared with previously reported distribution curves, revealing that the numerical results were generally similar to those of previous theoretical models. Following dissipation for 28 days, the load–settlement curves of LOTPs were compared with the results of several specifications; this indicated that the larger the LOTP diameter, the greater the similarity between axial bearing capacities calculated via the FEM model and the specifications. This FEM model could generally satisfy the requirements for analyzing the set-up effect of LOTPs. It was also found that the LOTP capacities were overestimated when the EPWP was neglected. Beyond 28 days, the pile bearing capacity did not increase because the dissipation of the EPWP was almost fully developed. An equation for predicting the set-up effect of LOTPs installed using impact hammers is also presented.

Undrained cyclic behaviour of loess with initial shear stress: A focus on failure mode

Soils in the field conditions, such as in cut slopes or under embankments, often sustains an initial static shear stress prior to the cyclic loadings. Current knowledge on the undrained cyclic behaviour with the presence of initial shear stress has been mainly built on sands. Experimental studies for loess that contains adequate information in this respect is lacking. To fill this gap, this paper presents results from a series of undrained cyclic triaxial tests on the reconsititued and intact loess samples, subjected to different initial static shear stresses. It was found that the existence of initial static shear stress has a significant impact on the failure modes of loess. Three failure modes were observed in the loess samples, including the flow failure, the cyclic mobility and the plastic strain accumulation, among which the flow failure in the reconsitituted loess sample was more destructive because of its sudden nature. On the basis of the experiments, we propose a chart to facilitate the identification of the failure modes of loess by taking into account the influence of initial stress state and the excess pore water pressure (Δu). Specifically, the changes of Δu in the first two cycles are used to distinguish the flow failure and other failure modes. The proposed metod is further validated using the literature data, showing a satifactory predictive peformance. The outcome of this study provides a guideline that is useful not only for evaluating undrained cyclic behaviour of loess, but also for predicting the failure modes without the need to load samples to failure.

Mechanical Characteristics of Underconsolidated Methane Hydrate-Bearing Clayed-Silty Sediments

Natural gas hydrate reserves in the South China Sea are huge, and their commercial prospects have been proved by repeated trial production. It is essential to systematically clarify the mechanical properties of hydrate-bearing sediments for the safe drilling of hydrate reservoirs. To date, most research studies focus on the undrained mechanical behavior of hydrate-bearing sandy or clayed sediments under normal consolidation conditions, but there are few reports on underconsolidated-undrained triaxial shear experiments. In this study, the present research conducts undrained triaxial shear tests on hydrate-bearing clayed-silty sediments. The results show that underconsolidated hydrate-bearing sediments appear to show a strain-hardening phenomenon, which are different from hydrate-bearing sands in Nankai Trough. The excess pore water pressure of hydrate-bearing sediments remains positive during shear. The increasing degree of consolidation will increase the failure strength and the Secant Young’s modulus E50. According to the Mohr–Coulomb criterion, the corresponding predicting formulas are established, and the cohesion increases with the increase in the degree of consolidation, while the internal friction angle is not sensitive to the degree of consolidation. In addition, the failure strength and the Secant Young’s modulus E50 increase with the increase in initial effective confining stress before shearing.