Development Of Excess Pore Water Pressure Research Articles

An effective and stabilised ([formula omitted]) SPH framework for large deformation and failure analysis of saturated porous media

Particle-based methods such as SPH have been proven to be powerful numerical tools for addressing challenges in solving coupled large deformation and failure of porous materials. In these applications, explicit time integration schemes are commonly adopted to integrate the coupled pore-water pressure equation. The Courant–Friedrichs–Lewy condition is required and imposes a strong restriction on the time increment, which is inversely proportional to the water bulk modulus, leading to a significant increase in the overall computational costs. This affects successful applications of SPH in solving field-scale fully coupled large deformation and failure of porous media. To address this problem, this study proposes a computationally efficient three-point integration (TPI) scheme that removes the influence of water bulk modulus from the pore-water pressure equation, enabling larger time increments for the time integration and hence saving computational costs for field-scale applications. Furthermore, a stabilised method is proposed to enable SPH to solve coupled flow-deformation of saturated porous media involving negative excess pore-water developments for the first time. The proposed SPH algorithm is verified against analytical and finite element solutions for small deformation ranges. Thereafter, it is applied to predict challenging problems involving large deformation and retrogressive failure of saturated porous materials, where contractive and dilative responses of porous materials can cause significant variations and instabilities in the development of excess pore-water pressure. The results suggest that the proposed SPH algorithm is stable and suitable for handling field-scale applications.

Effect of strain rate on triaxial extension behavior of silty clay

Traditional methods of suction caisson foundation design only consider the soil is under either purely drained or purely undrained conditions and ignore the effects of the excess pore water pressure (EPWP). A series of triaxial extension stress path tests were conducted in this article to evaluate the effects of the loading rate on the development of EPWP and soil strength. The soil specimens were consolidated under pressures of 60, 80, and 100 kPa and then unloaded at seven different loading rates. It is found that the EPWP and soil strength are heavily influenced by the loading rate. The EPWP tends to be negative at small axial strain and then increases to be positive with the increase of axial strain. The normalized deviatoric stress versus loading rate curves are in the backbone curve. The minimum deviatoric strength happens when the normalized loading rate V ref=5 and linearly increases with the increase of consolidation pressure with the slope of −0.715. The magnitude of the slope of the critical state line in p′ – q space is heavily influenced by the loading rate.

Exploring the suitability of using Foam concrete as pore pressure dissipation measure for slope Stability: A state of art review

Landslides are one of the major factors leading to disruption of transportation and urbanization of north-eastern India (NEI). The primary reasons are continuous precipitation and alarming seismicity at frequent intervals. Among various factors which can trigger landslide, most common is the increase of pore water pressure (PWP). This in turn increases the weight of the backfill and subsequently contribute to the overturning moment. Hence, measures to extract the pore water (PW) from the backfill through provision of drainage path is an efficient and widely followed approach to mitigate landslides. This paper presents a detailed overview on various landslides which have happened in the NEI during rainfalls and earthquake events, confirming these as two main triggering factors. Later, the limitations of various conventional landslide prevention techniques is also discussed. Though RWs are commonly adopted slope stabilization techniques, many case studies exist in where even RWs had failed to stabilize the slopes particularly where the slopes with higher groundwater table were subjected to heavy precipitation. The primary reason of failure was development of excess PWP behind RWs. While weep-holes in the RW play a critical role in dissipating PWP of the backfill, these are clogged resulting in the failure of the RWs. The fact cannot be denied that the effectiveness of the weep-hole depends upon its design, construction as well as its maintenance.Recently few researchers have shown that the Foam concrete (FC), a type of lightweight concrete, act as an excellent draining material to facilitate PW discharge for slope stabilization owing to its high permeability. Further, as the density of air voids in FC is more, rate of clogging is expected to be lesser, thus demanding lesser maintenance when compared to conventional weep-holes. This way, the need of regular maintenance of weep-holes can be minimized by use of FC. It should be highlighted here that though FC has lot of potential to be use in aforementioned application, there is not much of research exist in the literature on use of FC for PWP dissipation application. In this light, the present work also presents an in-depth review on permeation characteristics of FC, enumerating the PWP dissipation mechanism with the help of case studies from the literature.

GEOTECHNICAL CENTRIFUGE TEST OF REINFORCED ROAD EMBANKMENT AGAINST EARTHQUAKE-INDUCED LIQUEFACTION

Liquefaction is one of the causesof ground and structural damages during the earthquake. The occurrence of liquefaction during the earthquake is caused by the contractive behavior of loose sands subjected to cyclic loading. Due to this phenomenon, the soil deposits will lose its strength, causing deformation and settlement. In this study, the seismic response of the road embankment with proposed countermeasures is analyzedby conducting a centrifuge model test. The model was subjected to a gravitational acceleration of 50g to replicate the prototype scale's actual conditions of stress and strain. Two types of proposed countermeasures, gravel mat and gravel mat with geogrid, were used in this study. The study aims to observe the behavior and mechanism of the model subjected to liquefaction. The results show that the proposed countermeasures helped prevent structural failure by reducing the excess pore water pressure development and its dissipation time and reducing the settlement. It also reduces the lateral spreading of the foundation ground based on visual observations.

Behavior of braced wall embedded in saturated liquefiable sand under seismic loading

It is well known that the generation of excess pore water pressure and/or liquefaction in foundation soils during an earthquake often cause structural failures. This paper describes the behavior of a small-scale braced wall embedded in saturated liquefiable sand under dynamic condition. Shake table tests are performed in the laboratory on embedded retaining walls with single bracing. The tests are conducted for different excavation depths and base motions. The influences of the peak magnitude of the ground motions and the excavation depth on the axial forces in the bracing, the lateral displacement and the bending moments in the braced walls are studied. The shake table tests are simulated numerically using FLAC 2D and the results are compared with the corresponding experimental results. The pore water pressures developed in the soil are found to influence the behavior of the braced wall structures during a dynamic event. It is found that the excess pore water pressure development in the soil below the excavation is higher compared to the soil beside the walls. Thus, the soil below the excavation level is more susceptible to the liquefaction compared to the soil beside the walls.

Numerical modeling of soil liquefaction and lateral spreading using the SANISAND-Sf model in the LEAP experiments

Laboratory element and centrifuge tests from LEAP-UCD-2017 and LEAP-Asia-2019 were used for model calibration and evaluation in a dynamic coupled analysis of a saturated and gently sloped deposit of sand subjected to base excitation. The paper focuses on using a recently proposed novel constitutive ingredient for modeling the post-liquefaction large cyclic shear strains in sands. An existing critical state compatible, bounding surface plasticity reference model is used, with and without this new constitutive ingredient, to explore its improved modeling capabilities. The constitutive model was first calibrated based on available laboratory element tests on Ottawa-F65 sand, and compared to the reference model showed significantly improved performance in capturing the strain-based liquefaction strength curve of a series of undrained hollow cylinder cyclic torsional shear tests. The calibrated models were used in Class-C prediction of the slope surface deformation in five centrifuge tests on a mildly sloping liquefiable ground of the same soil subjected to dynamic loading, in the three-dimensional finite difference program FLAC3D. The simulation results were compared with experiments in terms of acceleration history, spectral response, excess pore water pressure development, and horizontal displacement evolution at specified control points. The vectors and contours of displacements at the end of shaking also revealed a pattern of slope deformation consistent with that of a gently sloped liquefiable ground. Following the insights from the performance of the models in simulation of the slope response, the calibration was adjusted to more realistically account for the impact of initial static shear stress on the development of post-liquefaction shear strains. The models were again used for Class-C1 prediction of the slope deformation of the same centrifuge tests. The overall assessment revealed the capabilities and limitations of the models in simulating the soil liquefaction strength and its post-liquefaction response.

1D Effective Stress Site Response Analysis; Using Stress Based Pore Pressure Model and Plasticity Model

The accumulated stress based porewater pressure (PWP) generation model is a simplified model using the concept of damage parameter. The only input of this PWP model is liquefaction resistance curve (CRR-N). The model is very useful since the CSR-N curves can be developed empirically from in-situ penetration tests measurements. In this research work the estimation of excess PWP development during seismic loading by using stress based PWP generation model is compared with a rigorous plasticity model. One dimensional (1D) effective stress nonlinear site response analyses were conducted in DEEPSOIL and Opensees using the stress based PWP model and PressureDependentMultiYield02 (PDMY2) model, respectively. The site response analysis were performed on a sand column 30 m in depth comprises of a low density liquefiable layer in between two dense non-liquefiable layers. Three bed rock outcropping motions with peak ground acceleration (PGA) level of 0.11 g, 0.124 g and 0.357 g were used as input motion in the analysis. The maximum ru profiles computed from the two models were compared and analyzed. The ru time histories at the center of the non-liquefiable layers and liquefiable layer were also compared. The comparisons revealed that the two models used in this study compute most comparable ru values. The computed ru is also found in line with density of soil and the PGA of the input ground motions where the ru increases with increase in the PGA and decreases with increasing density.

Assessment of Dynamic Response of Cohesionless Soil Using Strain-Controlled and Stress-Controlled Cyclic Triaxial Tests

Soils subjected to earthquake motions undergo random variations in stress, strain and frequency during the entire period of shaking. The spatial and temporal complexity of the strong motions is commonly addressed through simplistic strain-controlled or stress-controlled cyclic loading tests as a part of laboratory investigations. Such a methodology is utilized and reported in the present study for assessing the dynamic response of cohesionless sand obtained from River Brahmaputra, India. In order to assess the dynamic response and liquefaction potential of the cohesionless Brahmaputra sand, both stress-controlled and strain-controlled cyclic triaxial tests were performed on reconstituted cylindrical specimens prepared at different relative densities (Dr) ranging from 30 to 90%. The specimens were subjected to varying effective confining pressures (σ′c: 50–200 kPa), shear strain amplitudes (γ: 0.015–4.5%) and cyclic stress ratios (CSR: 0.05–0.3). For all the tests, the frequency of the applied harmonic loading was maintained at 1 Hz. The magnitude of excess pore-water pressure (PWP) generated during successive loading cycles of the strain-controlled tests was found to be considerably lower than that generated in a stress-controlled test. The strain-controlled test reveals a reduction in the development of excess PWP with the increase in confining pressure and relative density, thereby indicating a decrease in liquefaction potential with increasing confining pressure. The stress-controlled test highlighted that based on a particular CSR, an increase in confining pressure results in the requirement of higher deviatoric stress and smaller numbers of cycles are required to initiate liquefaction. Although apparently misleading, it is imperative that during earthquake, same deviatoric stress is applied on the entire soil deposit. Thus, for specimens at larger depths having higher confining pressure, the CSR value is smaller, and thereby successive deeper layers require more number of stress cycles to liquefy. Hence, the study revealed that both stress-controlled and strain-controlled tests can be successfully used to assess the dynamic properties and liquefaction potential of cohesionless specimens. Based on the findings, the developed cyclic shear stress ratio (CSR) = 0.5 and cyclic shear strain amplitude (γ) = 0.5% are considered as the limiting conditions to achieve the onset of liquefaction through strain-controlled and stress-controlled approaches, respectively.

Train induced pore water pressure generation model: numerical comparison

In recent decades, the provisions of new challenges to railway track demanded for a better understanding of the track dynamic, especially to track geotechnical dynamics. For geotechnical problems the presence of water in the void of soils has influence on the behavior of the soil effects such as temporarily development of excess pore water pressure of the soil or even liquefaction and mud pumping can occur. To account these effects considering the soil as two-phase material would be helpful to better understand the saturated railway line failures. Hence, this paper describes the pore fluid analysis of fully saturated railway embankment response subjected to train induced vibrations using three-dimensional finite element package ABAQUS. The modelized 3D track consists of train loads, rail, pads, sleeper, sub-ballast, ballast, saturated sub-grade, saturated subsoil layer and semi-infinite bedrock. The water-saturated soil layers modeled as poroelastic and the infinite bed rock as elastic medium. To this end, the influence of relative density, hydraulic conductivity, un-drained Young’s modulus and depth of embankment on excess pore water pressure development are compressively studied. The predicated results show that hydraulic conductivity, un-drained Young’s modulus and depth of embankment have quite significant effect on the induced excess pore water pressure; while the influence of relative density is very small. The result also shows that positive excess pore water pressure mainly distributes on the soil and this will permit fine grain soils to flow out from interior to exterior region of the model. Hence, mud pumping and liquefaction is developed not in specific type of soil instead can be developed in any type of soil with low hydraulic conductivity, high depth of embankment and low un-drained Young’s modulus.

Field study of pile – prefabricated vertical drain (PVD) interaction in soft clay

Piles and prefabricated vertical drains (PVDs) are two well-established inclusions used by geotechnical practitioners when dealing with soft compressible foundations. Induced movements in highly compressible soil can adversely influence the pile response by inducing additional movements and stresses in the piles. Especially, undesirable soil–pile interaction often leads to the development of excess pore-water pressure during pile installation and negative skin friction caused by the settlement of compressible soil surrounding the piles. Additional drainage by PVDs prior to the installation of a pile could reduce excess pore-water pressure, lateral soil movement, and negative skin friction on the pile. In this paper, full-scale field testing on two trial embankments built on soft soil is reported and the relative behaviour of these two embankments is compared and discussed. Soft soil underneath both embankments was consolidated before one pile was installed at the centre of each embankment. The pore-water pressure, lateral soil movement, surface settlement, and associated strain at the pile shaft were recorded. The pile capacity was tested immediately and 3 h after pile installation. The monitoring and testing results indicated that preconsolidation with PVDs before piling can effectively reduce the excess pore-water pressure, lateral soil movement, and downdrag on the pile.

Influence of Load Frequency on Cohesive Soil Respond

The mechanism of cohesive soils response to cycling loading is less investigated compared to cohesionless soils. Multiple load-unload cycles cause significant changes in the structure of cohesive soils, which result in complex behaviour under the given load. The aim of the paper was to investigate and study the influence of load frequency on cohesive soil reaction. In order to obtain results, tests were conducted using the cyclic triaxial apparatus. Three cyclic tests were carried out, each for different frequency −0.5 Hz, 1.0 Hz, 2.0 Hz and one static triaxial test. The maximal value of deviator stress qmax, used in the cyclic tests, was set to 40 kPa. Afterwards samples were unloaded to qmin = 30 kPa. Cyclic loading triaxial tests were performed in a consolidated-undrained (CU) one-way loading manner, a sinusoidal waves were used. After the cycling loading was completed, a static triaxial shear test was conducted. Changes in the cohesive soil responses depending on cycling load frequency were presented in the paper. Differences in the accumulation of plastic strains were noticed, as well as changes of degradation index values, resilient degradation index values and differences in the excess pore water pressure development.

Effect of dry density on the liquefaction behaviour of Quaternary silt

Quaternary silt is widely distributed in China and easily liquefies during earthquakes. To identify the influence of the dry density on the liquefaction behaviour of Quaternary silt, 40 cyclic triaxial liquefaction tests were performed on loose silt (dry density ρd=1.460 g/cm3) and dense silt (ρd=1.586 g/cm3) under different cyclic stress ratios (CSRs) to obtain liquefaction assessment criteria, determine the liquefaction resistance, improve the excess pore water pressure (EPWP) growth model and clarify the relationship between the shear modulus and damping ratio. The results indicate that the initial liquefaction assessment criteria for the loose and dense silts are a double-amplitude axial strain of 5% and an EPWP ratio of 1. The increase in the anti-liquefaction ability for the dense silt is more significant under lower confining pressures. The CSR of loose silt falls well within the results of the sandy silt and Fraser River silt, and the dense silt exhibits a higher liquefaction resistance than the sand-silt mixture. The relationships between the CSR and loading cycles were obtained at a failure strain of 1%. The EPWP development in the dense and loose silts complies with the “fast-stable” and “fast-gentle-sharp” growth modes, respectively. The power function model can effectively describe the EPWP growth characteristics of the dense silt. Finally, based on the liquefaction behaviour of silt, a suggestion for reinforcing silt slopes or foundations is proposed.

Comparison of behaviour of basal reinforced piled embankment with two layer of reinforcement

Interest to basal reinforced piled embankments is increasing recently due to their rapid construction and reliability. A comprehensive parametric study is conducted to determine effects of pile properties, reinforcement stiffness, embankment properties and soft soil properties into settlements, pressures and excess pore water pressure development and dissipations. Results which are obtained by using one-layer reinforcement during construction are compared with the results obtained by using two-layer reinforcement during construction. Finite element method is used during the parametric study. Second layer of reinforcement is placed

2D Numerical Modeling of the Cyclic Simple Shear Test Using Opensees

Simple Shear (SS) test is considered to simulate the in-situ stress conditions more accurately than any other testing system. Undrained Cyclic Simple Shear test is extensively performed to evaluate the cyclic shear strength and liquefaction potential of soil. In this study, the numerical modeling of the SS testing was undertaken using a finite element method (FEM) in the Opensees (Open System for Earthquake Engineering Simulation). The study showed that FEM simulation can efficiently capture the cyclic shear behavior and the excess pore water pressure development. The analysis results were also compared with the laboratory measurements and shown to satisfactorily match the measurements.

2D Numerical Modeling of the Cyclic Simple Shear Test Using Opensees

Simple Shear (SS) test is considered to simulate the in-situ stress conditions more accurately than any other testing system. Undrained Cyclic Simple Shear test is extensively performed to evaluate the cyclic shear strength and liquefaction potential of soil. In this study, the numerical modeling of the SS testing was undertaken using a finite element method (FEM) in the Opensees (Open System for Earthquake Engineering Simulation). The study showed that FEM simulation can efficiently capture the cyclic shear behavior and the excess pore water pressure development. The analysis results were also compared with the laboratory measurements and shown to satisfactorily match the measurements.

The catastrophic landfill flowslide at Hongao dumpsite on 20 December 2015 in Shenzhen, China

Abstract. A catastrophic flowslide occurred at the Hongao dumpsite on 20 December 2015 in the Guangming New District of Shenzhen, China. The flowslide caused 77 causalities and damaged 33 buildings. In the absence of extreme weather conditions and seismic activity, the causes of the failure were analyzed on the basis of multi-temporal remote-sensing images, site investigation, in situ tests, laboratory tests, and numerical analyses. Site investigations showed that the volume of the displaced material was 2.32 × 106 m3 and the volume of the pre-failure waste filling was 6.27 × 106 m3. The flowslide was characterized by high travel velocity and long runout distance. The displaced material was primarily a mixture of silty soil and construction and demolition waste with water content of 17.3–42.4 %. The primary causes of the failure were concluded to be the following: (1) groundwater flow had stagnated in the dumpsite due to drainage system failure and the underlying impermeable granite stratum; (2) the accumulation rate and total volume of the waste filling was in exceedance of the design capacity. The flowslide may be ascribed to the development of excess pore-water pressure as evidence of liquefaction was observed at several locations, and it is postulated that such phenomena were related to the surcharge loads imposed by the unregulated disposal activities.

Analytical Solution for Electroosmotic Consolidation Considering Nonlinear Variation of Soil Parameters

AbstractElectroosmotic consolidation can be used as an efficient technique for soft soil improvement. Considering the limitation in previous theories that soil parameters keep constant during electroosmotic consolidation, the nonlinear relationships between soil compressibility, hydraulic and electroosmosis conductivities, and void ratio are incorporated in a one-dimensional model in the present study. The analytical solutions for the ultimate excess pore-water pressure and surface settlement are derived. A comparison between the proposed analytical solutions and traditional theory indicates that the nonlinear variation of hydraulic conductivity results in a larger ultimate excess pore-water pressure, whereas the nonlinear variation of electroosmosis conductivity leads to a smaller one. The effects are more significant for soils with higher compressibility. The nonlinear variation of soil compressibility exhibits remarkable impact on the development of excess pore-water pressure when the nonlinear variati...

Evaluating pile installation and subsequent thixotropic and consolidation effects on setup by numerical simulation for full-scale pile load tests

During pile installation, stresses and void ratios in the surrounding soils change significantly, creating large displacements, large strains, soil disturbance, and development of excess pore-water pressures. The surrounding disturbed soil tends to regain its strength with time due to both consolidation and thixotropic effects. In this paper, the pile installation process and subsequent consolidation, thixotropy, and load tests conducted at different times after end of driving (EOD) were modeled for test piles at the Bayou Laccassine Bridge site, Louisiana. In the finite element (FE) model, the pile was considered as an elastic material and the anisotropic modified Cam-clay model (AMCCM) was used to describe the behavior of the surrounding clayey soils. Pile installation was modeled by applying prescribed radial and vertical displacements on the nodes at the soil–pile interface (volumetric cavity expansion), followed by vertical deformation to activate the soil–pile interface friction and simulate static load tests. The thixotropic effect was incorporated by applying a time-dependent reduction parameter, β, which affects both interface friction and material properties. Results from the FE numerical simulation include the development of excess pore-water pressure during pile installation and its dissipation with time, the increase in effective lateral stress at the pile–soil interface, changes in stress state of the surrounding soil, and setup attributed to both the soil consolidation and thixotropy at different times. FE results are compared with measured values obtained from full-scale instrumented pile load tests, which show good agreement between measured and FE-predicted results.

Numerical Simulation of Consolidation Settlement of Pervious Concrete Pile Composite Foundation under Road Embankment

Abstract Having the advantages of high permeability and high strength, pervious concrete is suitable for improving ground-bearing capacity. In the Yellow River Delta, a pervious concrete pile (PCP) composite foundation has been constructed to reduce settlement of an expressway embankment. To study the working mechanism of PCPs, a numerical model was constructed based on the finite-difference method and Biot’s consolidation theory, which was validated by data from in situ tests. The excess pore-water pressure, pile–soil stress ratio, lateral displacement, and settlement of the PCP composite foundation under the loading of the road embankment were numerically calculated and compared with those of gravel pile and low-grade concrete pile composite foundations. Comparisons show that the dissipation of excess pore-water pressure in the PCP composite foundation was fastest, which implied that PCPs can significantly mitigate the development of excess pore-water pressure and thus enhance subsoil strength. Furtherm...

Effects of density on cyclic behaviour of anisotropically consolidated Ottawa sand under undrained torsional loading

The undrained response of Ottawa sand to cyclic torsional loading in a hollow cylinder apparatus is assessed at various densities. The cyclic resistance of Ottawa sand is strongly affected by anisotropic consolidation. It is less than half of the cyclic strength of isotropically consolidated material at the loose state; this trend reverses at the dense state. Cyclic strength resistance increases with density irrespective of consolidation path. Anisotropically consolidated specimens develop at all densities a terminal excess pore-water pressure which is less than 40% of the initial effective stress, and fail due to strain rather than pore-water pressure accumulation, contrary to isotropically consolidated specimens reaching pore pressure ratios higher than 70%. The development of excess pore-water pressure is uniquely related to the shear work imparted to the specimen; at the same excess pore-water pressure ratio shear work increases with density, whereas it is not affected by the applied cyclic shear stress amplitude. The phase transformation line is not affected by the consolidation path and appears to be the same under monotonic and cyclic loading conditions.