Pore Water Pressure Generation Research Articles

Probabilistic evaluation of the seismic stability of infinite submarine slopes integrating the enhanced Newmark method and random field

For evaluation of the seismic stability of submarine slopes, traditional deterministic methods may not reflect the actual situation due to the uncertainties in input values such as seismic acceleration, soil properties, and hydraulic conditions. In this study, probabilistic analyses were conducted based on the use of an enhanced Newmark method to evaluate the seismic stability of submarine slopes. In the probabilistic workflow, the infinite slope model was used, whereby the spatially varied strengths of marine sediments were simulated by non-stationary random fields discretized by the Karhunen-Loeve expansion. The positions of the potential slip surfaces were searched automatically, rather than being predefined as in the traditional limit equilibrium method. Artificial earthquake accelerations, exhibiting the same spectral characteristics, were simulated as the input ground motions in the probabilistic analysis. The pore water pressure generation and dissipation models were incorporated into the enhanced Newmark method when calculating the permanent displacements of the submarine slopes. Monte Carlo simulations were conducted for statistical characterization of the slope displacements and their failure probabilities. The results of the analysis indicate the superiority of the probabilistic framework and demonstrate the significant effects of pore water pressure and the spatial variability of the soil strength on the displacements and failure probabilities of submarine slopes.

A new effective stress method for nonlinear site response analyses

AbstractA generic loosely coupled effective stress method is presented in this article for one‐, two‐ and three‐ dimensional (1D, 2D and 3D) nonlinear site response analyses. In this method, the 1D non‐Masing hysteretic constitutive model of Chen et al (2020) is extended into 2D and 3D stress conditions, by presenting a clever generalized formulation of equivalent shear strain (γeq). The element‐level simulation tests show that the proposed algorithm of γeq is conceptually simple with high precision to capture the strain reversals under complex multidirectional shakings. The coupling between the cyclic stiffness degradation and the excess pore water pressure (EPWP) generation during irregular cyclic loadings is established using the proposed algorithm of γeq in conjunction with the Chen et al (2019a) EPWP generation model and the extended non‐Masing hysteretic constitutive model. The simulations of the undrained cyclic triaxial tests using the new effective stress method reproduce excellently the observed response of the saturated sand specimens, demonstrating the ability to represent the undrained behavior of liquefiable sands during uniform cyclic loadings. The new effective stress method is then used to simulate the response of a downhole array liquefied site in Japan, which shows an excellent agreement between the simulations and the recordings in both horizontal and vertical components of ground motions at different depths.

Assessing Persistence of Entrapped Gas for Induced Partial Saturation

AbstractInduced partial saturation (IPS) is a novel method to suppress the generation of excess pore-water pressure and increase the liquefaction resistance of loose granular soils. Mechanical bene...

Experimental investigation on static and dynamic characteristics of granulated rubber-sand mixtures as a new railway subgrade filler

Because of their low unit weight, strong linear elastic deformation ability, good durability and high energy dissipation, mixtures of waste tire rubber and sand have been increasingly used as a new type of subgrade filler in railway engineering. To investigate static and dynamic characteristics of granulated rubber-sand mixtures as a new filler, consolidated undrained monotonic and cyclic triaxial tests were carried out. The monotonic triaxial tests reveal that the shear strength of mixtures first increases and then decreases with increasing granulated rubber content and is maximized when the rubber content is approximately 10%. Based on the theory of an equivalent intergranular void ratio, a new equation for estimating the peak deviatoric stress of mixtures is proposed. The cyclic triaxial tests reveal that the addition of granulated rubber can constrain the generation and accumulation of dynamic pore water pressure and improve the liquefaction resistance of mixtures. The change in shear modulus is inversely proportional to the change in granulated rubber content and frequency, and directly proportional to the change in confining pressure. A new equation is also established for calculating the maximum shear modulus considering the effect of the equivalent intergranular void ratio and confining pressure. By comparing the maximum shear modulus of the new railway subgrade filler with that of other subgrade filler, it is concluded that a granulated rubber content of less than 20% can meet engineering requirements related to shear stiffness. The damping ratio is correlated with granulated rubber content, negatively correlated with confining pressure and insensitive to loading frequency. The relationship between the damping ratio and shear strain conforms to the equation proposed by Hardin and Drnevich. Based on the analysis of static and dynamic characteristics of granulated rubber-sand mixtures with different rubber content, the optimum granulated rubber content should be approximately 10%.

Study of the effects of clay content on loess slope failure mode and loess strength

The particle size of loess in the Loess Plateau varies from northwest to southeast due to wind direction which is predominantly from the northwest. The clay content of loess is significantly different in the loess area of China resulting in different physical characteristics of loess and loess failure process. To determine the failure process triggered by rainfall, a series of studies were conducted via flume and triaxial tests using loess samples with different clay contents. The effects of clay on water content (WC) change, pore-water pressure (PWP) generation, and the failure process of loess slope were analyzed. The changes in WC and PWP response to the failure process of the slope at different positions were different, and the changes in the shallow layer were found to be more prominent than those at other positions. The changes in WC and PWP gradually decreased with increasing clay content. The clay in loess also plays an important role in the failure process of the loess mass, and the failure process changed from regressive overall failure to erosion-shallow sliding failure and then to erosion failure with increasing clay content. Finally, loess strength with different clay contents was tested and discussed. The strength increased and the M value decreased gradually with increasing clay content. Based on the experimental results, it was concluded that clay in loess has a significant impact on the failure process and strength of the loess mass, and the results of the current study would provide support for loess slope mitigation.

Monotonic and cyclic characteristics of K0-Consolidated saturated soft clay under a stress path involving a variable confining pressure

In certain field conditions such as offshore projects under wave loads or embankments under traffic loads, both the vertical and horizontal stresses are variable. However, previous investigations rarely considered the variation in horizontal stress. To better understand the characteristics of natural saturated soft clay, a series of monotonic and cyclic triaxial tests with a K0-consolidation state were carried out under a variable confining pressure (VCP) stress path. The development of axial strain, pore water pressure and effective stress path is analysed. The results show that with the increase in η (the ratio of the variation in the mean effective principal stress to that of the deviatoric stress), the undrained shear strength (qf) decreases continuously. The pore water pressure generation is slightly improved under a stress path with increasing confining pressure. Based on the test results, a unified formula was established to predict the pore water pressure under VCP stress paths. The unique p–q–e relationship of normally consolidated clay in monotonic VCP triaxial tests was also demonstrated. Under VCP stress paths, the amplitude of the pore pressure increases, and the effective stress path tilts more sharply to the right. Moreover, a unified formula was established that can provide a good reference for predicting effective stress paths under cyclic VCP triaxial tests.

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

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

Shear strength characteristics of a sand clay liner

This study investigated shear strength behaviour of compacted sand–clay mixtures used as liners, with 10%, 20%, and 30% clay contents. A natural high-plasticity and highly expansive clay found in the eastern province of Saudi Arabia was used. A series of consolidated undrained triaxial tests and pore water pressure measurements of saturated samples with various clay contents and confining pressures was conducted using a computer-controlled Bishop and Wesley triaxial cell. The unit was equipped with pressure volume controllers and a pressure transducer for measuring sample volume changes and excess pore water pressure. The experimental test results indicate that clay content and confining pressure significantly affect stress strain response curves, pore water pressure generation curves, and steady-state shear strength. Sand–clay mixtures with clay content less than 10% showed a tendency toward contractive behaviour. The failure line slope increased in accordance with clay content increase. The deviator stress versus axial strain of saturated sand–clay mixtures indicated a hyperbolic trend. The stress ratio versus axial strain representation was more informative for the shear strength behaviour assessment. Clay content did not significantly affect critical-state friction angle. Scanning electron microscope images of the sand-clay mixtures with different clay contents are presented.

The Effect of Lightweight Expanded Clay Aggregate on the Mitigation of Liquefaction in Shaking Table

Non-conventional materials are often applied to dissipate the energy of dynamic loads and mitigate liquefaction. In this study, the effect of Lightweight Expanded Clay Aggregate (LECA) on the mitigation of liquefaction was examined using shaking table tests. LECA, a common lightweight material, was added to saturated sand in a range of 0–20% to study the variation of excess pore water pressure (EPWP) generation, as well as the acceleration response of the soil. Based on the obtained results, the addition of 5% and 10% LECA to saturated sand resulted in a significant decrease in the generation of EPWP, while higher LECA content led to an increase in the generation of EPWP. The mechanisms behind such behaviors contradict the effects of increasing the damping of the soil mixture and reducing the effective initial stress as a function of overall mixture density. Adding 5 and 10% LECA reduces EPWP because aggregates accelerate the dissipation of EPWP during and after vibration. As the percentage of LECA increases by 15% and 20%, excess pore pressure ratio increases. The cause of this may be the reduction of effective initial stress leading to reduced soil density in higher-LECA contents. 10% LECA is presented as the threshold for changing the liquefaction behavior of sand-LECA mixtures. This study concludes that adding LECA to sand samples increases the rate of dissipation of EPWP, but also leads to a significant decrease in initial effective stress.

Effect of Silt Content on Liquefaction Susceptibility of Fine Saturated River Bed Sands

The effect of silt intrusion on the liquefaction susceptibility of fine saturated sand has been studied here using a series of strain-controlled cyclic triaxial tests on isotropically consolidated soil specimens. The fine sands used in this study were collected from the Ganga and Sone river bed. The samples were prepared with 100% non-plastic silt, 100% sand and different percentage (5%, 10%, 20%, and 30%) of non-plastic silt mixed with fine sand to study the effect of intruded silt on liquefaction susceptibility of sand. It has been found that at the same relative density range (10–25%) and the same percentage of intruded non-plastic silt, the Ganga sand is having higher liquefaction susceptibility than the Sone sand. The outcome of the study also showed that the rate of generation of excess pore water pressure (EPWP) for all three soil specimens was more or less same at higher strain levels (0.66–1.31%). However, the liquefaction potential continues to increase with the increase in silt content at a lower strain rate of 0.13%. A graphical relationship has been proposed for the EPWP development model parameter as a function of non-plastic silt content. This modification in the EPWP model parameter is one of the novel aspects presented here, which can be used for site-specific nonlinear ground response analysis.

A multisurface kinematic hardening model for the behavior of clays under combined static and undrained cyclic loading

AbstractIn dynamic geotechnical problems, soils are often subjected to a combination of sustained static and fast cyclic loading. Under such loading conditions, saturated and normally consolidated clays generally experience a build‐up of excess pore water pressure along with a degradation of stiffness and strength. If the strength of the soil falls below the static stress demand, a self‐driven failure is triggered. In this paper, a constitutive model is presented for the analysis of such problems, based on a general multisurface plasticity framework. The hardening behavior, the initial arrangement of the surfaces, and the nonassociated volumetric flow rule are defined to capture important aspects of cyclic clay behavior. This includes nonlinear hysteretic stress‐strain behavior, the effect of anisotropic consolidation, and the generation of excess pore water pressure during undrained cyclic loading along with a degradation of stiffness and strength. The model requires nine independent parameters, which can be derived from standard laboratory tests. A customized experimental program has been performed to validate the model performance. The model predictions show a good agreement with test results from monotonic and cyclic undrained triaxial tests, in particular with respect to the strain‐softening response and the number of loading cycles to failure. A procedure for a general stress‐space implicit numerical implementation for undrained, total stress‐based finite element analyses is presented, including the derivation of the consistent tangent operator. Finally, a simulation of the seismic response of a submarine slope is shown to illustrate a possible application of the presented model.

Experimental Study of Modeling Partially Drained Dense Sand Behavior in Monotonic Triaxial Compression Loading Tests

Abstract Sand behavior is significantly affected by the drainage conditions being either fully drained, partially drained, or undrained. Partially drained conditions are often encountered in reality; however, partial drainage effects are not commonly considered at a soil element level. As a common soil element test, triaxial testing is carried out with either fully drained or undrained conditions for the shearing phase, as specified according to the soil type and design application. In this study, a series of monotonic triaxial compression tests were performed under various drainage conditions with a special filter device to investigate partially drained behavior of dense sand. Partially drained behavior resulted in simultaneous excess pore water pressure generation and volumetric strain and behaved more similar to the undrained condition at the beginning of shearing and more similar to the fully drained condition at the large strains. Using three different degrees of drainage induced by three filter devices with different steady-state flow capacity and different shearing strain rates suggest that a ratio of the axial strain rate to the filter flow capacity can be used as a unique parameter to represent degree of drainage in a partially drained test.

Effect of Earthquakes on Slope Stability. (Dept. C)

Failure of slopes usually leads to human and/or environmental disasters. For example, the failure of an earth dam or a riverbank may eradicate villages or even towns, depending on the failure size. Earthquakes propagate from the source fault in the form of waves; these waves induce horizontal and vertical movements which are associated with sudden changes in the vertical, horizontal, principal stresses, and in the generation of excess pore water pressures, leading to failures in soil slopes and dams. Only one earthquake may cause tens of slope failures and landslides cases endangering human beings and structures. Parametric study was performed on a soil embankment in order to assess the response of the slope under the effect of different earthquakes to the geometry of soil slopes, as well as the characteristics of input ground motions. An approach to get the seismic reduction factor for the different slopes under the effect of different earthquakes was developed. The computer program Geo-slope (2007) is used in the analysis. Results showed that just applying the earthquake, even with minor magnitude, causes a significant reduction in the safety factor for the slope, and this reduction rate decreases gradually with increasing the earthquake magnitude.

Investigation on the undrained shear strength of loose sand with added materials at various mean diameter ratios

This study aims to investigate the undrained shear strength of sand with added materials at different mean diameter ratios D50/d50 (D50 and d50 respectively represent the mean diameter of host sand and added materials) in loose state by performing a series of undrained triaxial tests. For this purpose, a very low density index of sand matrix IDmat equaling to 0.00 was employed and the fines of C500 silt, Illite and Speswhite with distinct mean grain sizes were respectively added into the host HN31 sand at different contents Fc (5%, 10% and 15%). For comparison and further analysis, undrained triaxial tests were also carried out on Sand-Sand mixtures, created by adding the HN31 sand into the host HN31 sand at Fc of 5%, 10%, 14% and 18.7%. Results showed that, for low value D50/d50 (e.g., 1 for Sand-Sand), the undrained shear strength Su increased as Fc increased, which might be attributed to the increase of occlusal friction, and in that case the sliding of host sand became difficult. By contrast, in the case of higher value D50/d50 (e.g., 58.33 for Sand-C500, 159.09 for Sand-Illite and 500 for Sand-Speswhite), Su decreased as Fc increased. This was explained by the fact that, with increase of Fc, the quantity of added materials with small size increased, and in that case the sliding of host sand became easy. A critical value of D50/d50 (14.5) was thus deduced, which theoretically verified the negligible effect of Fc on Su. Furthermore, for sand mixed with fines (e.g., C500 silt, Illite and Speswhite clays), Fc has an insignificant effect on the peak value of Δu, while the increase of D50/d50 facilitated the generation of excess pore water pressure after reaching about Δumax. These findings provided useful information for analyzing the mechanical behaviour of sand with added materials in geotechnical practice.

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.

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 r u profiles computed from the two models were compared and analyzed. The r u 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 r u values. The computed r u is also found in line with density of soil and the PGA of the input ground motions where the r u increases with increase in the PGA and decreases with increasing density.

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 r u profiles computed from the two models were compared and analyzed. The r u 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 r u values. The computed r u is also found in line with density of soil and the PGA of the input ground motions where the r u increases with increase in the PGA and decreases with increasing density.

Field Test of Excess Pore Water Pressure at Pile-Soil Interface Caused by PHC Pipe Pile Penetration Based on Silicon Piezoresistive Sensor.

Prestressed high-strength concrete (PHC) pipe pile with the static press-in method has been widely used in recent years. The generation and dissipation of excess pore water pressure at the pile–soil interface during pile jacking have an important influence on the pile’s mechanical characteristics and bearing capacity. In addition, this can cause uncontrolled concrete damage. Monitoring the change in excess pore water pressure at the pile–soil interface during pile jacking is a plan that many researchers hope to implement. In this paper, field tests of two full-footjacked piles were carried out in a viscous soil foundation, the laws of generation and dissipation of excess pore water pressure at the pile–soil interface during pile jacking were monitored in real time, and the laws of variation in excess pore water pressure at the pile–soil interface with the burial depth and time were analyzed. As can be seen from the test results, the excess pore water pressure at the pile–soil interface increased to the peak and then began to decline, but the excess pore water pressure after the decline was still relatively large. Test pile S1 decreased from 201.4 to 86.3 kPa, while test pile S2 decreased from 374.1 to 114.3 kPa after pile jacking. The excess pore water pressure at the pile–soil interface rose first at the initial stage of consolidation and dissipated only after the hydraulic gradient between the pile–soil interface and the soil surrounding the pile disappeared. The dissipation degree of excess pore water pressure reached about 75–85%. The excess pore water pressure at the pile–soil interface increased with the increase in buried depth and finally tended to stabilize.

On the deep soil mixing method in the mitigation of liquefaction-induced bearing capacity degradation of shallow foundations

ABSTRACT Bearing capacity degradation in shallow foundations due to subsoil liquefaction is not a new challenge in earthquake geotechnical engineering. Thus far, limited mitigation measures against this problem were suggested in the literature. In this paper, mitigation of liquefaction-induced bearing capacity degradation of shallow foundations using Deep Soil Mixing (DSM) method was investigated numerically. A 3D finite difference model has been developed via FLAC3D. It was explored whether liquefaction effects on static and seismic bearing capacity of shallow foundations were diminished, and may be effectively overlooked, for DSM column diameter and length and also shallow foundation width variation. It was found that an artificial non-liquefiable soil layer made by DSM method improves the seismic bearing capacity of shallow foundations remarkably. Therefore, in terms of excess pore water pressure generation decreases and bearing capacity ratio increases. It was also concluded that by increasing the diameter and length of the DSM columns, the seismic bearing capacity of shallow foundations increases. However, as shallow foundation width increases, along with an increase in DSM column diameter and length, the performance of DSM columns and accordingly shallow foundation decreases.

Bearing capacity of shallow foundation under cyclic load on cohesive soil

Abstract The bearing capacity of shallow foundations under static loads has been widely studied by various authors. In contrast, the bearing capacity under dynamic loads has been solved indirectly by adopting an equivalent pseudostatic approach or by means of reduction coefficients. The study of the problems related to cyclic loads has focused mainly on the behavior of granular soils. However, in the case of cohesive soils, these began to be studied in detail as a result of the Northridge, Kocaeli and Chi-Chi earthquakes that took place in the 90s. The object of the present work is to evaluate the influence of pore pressure in the calculation of bearing capacity of shallow foundation on cohesive soil and dynamically requested. This research is based on the results of cyclic simple shear tests carried out with samples from the Prat port (Barcelona). A pore-water pressure generation equation is proposed that depends on the effective vertical stress “in situ” (σ’ov), static shear stress (τo), cyclic shear stress (τc), and void ratio (e0). The formulation was implemented to the finite difference software FLAC2D, by which the bearing capacity of foundations can be determined. The proposed formulation can calculate the maximum cyclic load (CL) that a cohesive soil can resist before failure by cyclic softening, as a consequence of cyclic loads transferred by a shallow foundation. The practical application of this research is to provide a chart to calculate the bearing capacity of cohesive soils requested by cyclic loading (CL). This application takes into account the properties of the ground, the static load capacity (Phe) and the effective load outside the foundation ( q ). In addition, the formulation is validated in the application of a real case.