Excess Pore Pressure Generation Research Articles

Effect of Micro-Pile Mitigation on Seismic Performance of Liquefiable Ground

Soil liquefaction and its associated ground failures, pose a significant threat, causing damage to engineering structures during earthquakes, and one of the most effective methods used to mitigate liquefaction in liquefied soil is micro-pile (MP) method. Therefore, this study aims to examine the current state of MP method as liquefaction countermeasure in the soil of the Coal Fired Power Station in Central Java, an area with a high liquefaction potential. A three-dimensional finite element analysis, conducted with OpenseesPL software, uses a numerical method to yield information about ground lateral deformation and excess pore pressure generation caused by MP method during seismic shaking. This result examines important design parameters, including diameter, spacing, length of MP, and inclination of ground, to address these issues. MP method increases the stiffness of soil, reducing excessive pore pressure and thereby minimizing liquefaction risks. In general, MP remediation appeared effective for any sloping ground. This study provides valuable information for devising an efficient remediation solution by comparing relevant variables, such as diameter, spacing, MP length, and ground inclination, under the same conditions. Numerical simulation with OpenseesPL yields results such as stress and strain path, acceleration time histories, excess pore pressure, displacement time histories, and maximum lateral displacement, which are then compared with various diameter parameters. The diY6-ameter parameters were compared to test how the additional diameter dimension affects the performance of the micropile provided to the soil. This will be demonstrated based on the results shown on excess pore pressure and maximum lateral displacement. This comparison shows that increasing MP diameter is more effective in reducing the risk of liquefaction.

Experimental Approach for Assessing Dissipated Excess Pore Pressure Induced Settlement

Upon dynamic loading, saturated soil loses its strength and behaves differently with respect to the excess pore pressure variation resulting in volumetric-induced settlements. Traditionally, these settlements have been evaluated using standard charts based on one soil type and its relative density (RD). To assess these settlements, this study established a unique experimental methodology based on two laboratory tests: triaxial simple shear and piezoelectric ring actuator technique. Fifty-seven tests were performed on Ottawa F65 sand under strain-controlled cyclic and post-cyclic conditions. A chart was generated, revealing a relationship between the dissipated energy from cyclic loading and volumetric strain (v), based on the shear wave velocity as a controlling factor. This study was compared with previous studies to verify the compatibility of the proposed approach. Another novelty was revealed by studying v variation with the dissipated pressure. This variation is presented in a post-seismic chart, in which deformations are tracked based on the initial soil state and maximum excess pore pressure generation ratio (Rumax). For each RD, the soil is divided between liquefied and non-liquefied states according to a specific Rumax (Rumax-trigger point). The calculation of the volume compressibility coefficient is proven to serve as a liquefaction-triggering criterion identifying the liquefied state.

Seismic Performance of Drained Piles in Layered Soils.

The provision of drains to geotechnical elements subjected to strong ground motion can reduce the magnitude of shaking-induced excess pore pressure and the corresponding loss of soil stiffness and strength. A series of shaking table tests were conducted within layered soil models to investigate the effectiveness of drained piles to reduce the liquefaction hazard in and near pile-improved ground. The effect of the number of drains per pile and the orientation of the drains relative to the direction of shaking were evaluated in consideration of the volume of porewater discharged, the magnitude of excess pore pressure generated, and the amount of de-amplification in the ground's motion. The following main conclusions can be drawn from this study. Single, isolated piles and a group of drained piles were tested in three series of shake table tests. Relative to conventional piles, the drained piles exhibited improved performance with regard to the generation and dissipation of excess pore pressure and stiffness of the surrounding soil, with increases in performance correlated with increases in the discharge capacity of the drained pile. The acceleration time histories observed within the pile-improved soil indicated a coupling of the rate and magnitude of porewater discharge, excess pore pressure generated, and de-amplification of strong ground motion. The amount of de-amplification reduced with increases in the number of drains per pile and corresponding reductions in excess pore pressure. The improved performance should prove helpful in the presence of sloping ground characterized with low-permeability soil layers that inhibit the dissipation of pore pressure and have demonstrated the significant potential for post-shaking slope deformation.

Numerical modeling of the effect of loading rate on the tensile load capacity of offshore caisson foundations

Suction caissons are skirted foundations frequently used in offshore environments supporting various structures. Among those lie the offshore wind turbines (OWT), which are currently one of the most common renewable energy systems. In this paper we study the tensile capacity of caisson foundations of OWT in dense sands, where the generation of excess pore pressure is analyzed. The tensile capacity of suction buckets under various loading rates is investigated using a series of quasi-static axisymmetric finite element analyses with coupled flow-deformation formulation. DeltaSand model is utilized for the soil, which is an elasto-plastic state-dependent model capturing the response of sands at varying relative densities. Numerical results are validated with available test data of scaled physical model experiments. Particular emphasis is placed on the interaction between suction-bucket and the soil. Parameters governing the soil-caisson interface playing a key role in the uplift response are identified. Lastly, a case study that can be observed in offshore foundations is simulated. Results show that the developed numerical model is applicable to capturing the uplift response of bucket in an ocean environment. The outcome of this study will contribute to closing the gap in the design of suction caissons of OWT in existing standards.

3D Numerical Assessment of Rammed Aggregate Pier Performance under Dynamic Loading in Liquefiable Soils

Ground improvement (GI) techniques have shown promise in effective liquefaction mitigation, but the physical mechanisms governing their three-dimensional (3D) response during dynamic loading are not yet fully understood. To evaluate the 3D performance of one GI technique, the rammed aggregate pier (RAP), in-situ site characterization, full-scale field test data, and calibrated baseline constitutive soil model parameters are combined to model the 3D fully coupled hydromechanical response of natural (unreinforced) and improved (reinforced) soil profiles. To the authors’ knowledge, this contribution represents the first 3D model of a columnar-reinforced soil profile being calibrated using full-scale field testing. The field observations from the comprehensive ground improvement testing (GIT) program in New Zealand and insights from two-dimensional (2D) finite-difference analyses using constitutive model parameters calibrated against the field measurements were used. The developed 3D models were subjected to dynamic loads simulating the excitation generated by a vibroseis truck at one of the in-situ test sites of the GIT program, as well as unidirectional and bidirectional earthquake ground motions from the Canterbury Earthquake Sequence events. The 3D simulations showed that the improved soil profiles experienced reduced excess pore pressures and reduced dynamically induced shear strains compared to the natural, unreinforced soil models. The developed 3D finite-element predictions were compared and validated vis-à-vis the field observations of the GIT program. Compared to 2D analyses, 3D analyses provide a more accurate description of actual field conditions, and, for instance, it was observed that multidirectional shaking has a significant effect on liquefaction triggering, particularly for natural soil profiles. Finally, it was shown that soil densification around the installed pier elements and the lateral earth pressure increase within the densified soil and are the primary ground improvement mechanisms contributing to the reduction of dynamically induced shear deformations and excess pore pressure generation during earthquake shaking. It was also found that the permeability and shear stiffness of the installed RAP piers did not have a significant influence on the pore pressure response and shear strains developed along the centerline of the improved area.

Pore Pressure Generation of Gravelly Soils in Constant Volume Cyclic Simple Shear

Excess pore pressure generation of uniform gravel and gravel-sand mixtures was evaluated in this study. Comparisons were made with existing relationships for pore pressure generation of sands and show that gravel and gravel-sand mixtures can exhibit different pore pressure responses. The influence of liquefaction definition, gravel particle angularity, particle size, relative density, initial vertical effective stress, cyclic stress ratio, and gravel percentage, on the generation of excess pore water pressure of gravel and gravel-sand mixtures was studied. Liquefaction definition, particle size, initial vertical effective stress, and cyclic stress ratio were found to not have a significant effect on the normalized excess pore pressure generation (i.e., ru versus N/NL). Conversely, relative density, particle angularity, and mixture percentage of gravels were found to have a more significant effect on the normalized excess pore pressure generation response (i.e., ru versus N/NL). Additionally, the coefficient of uniformity (Cu) was found to have a strong correlation with increased excess pore pressure generation ratio at values of N/NL less than 0.40, highlighting the influence of grain size distribution on early pore pressure generation response. A new pore pressure model was developed to predict ru based on Cu for gravelly soils.

Salt leaching by freshwater and its impact on seafloor stability: An experimental investigation

Offshore freshened groundwater (OFG) has been documented in many continental margins worldwide. OFG systems are dynamic, expanding and contracting with falling and rise sea-levels. OFG has long been thought to be an important geomorphic agent in continental margins, either via active discharge at the seafloor, which can erode depressions, or the generation of excess pore pressure, which can deform sediments and cause slope failure. It has also been proposed that OFG flow can drive the loss of sediment shear strength via salt leaching, when seawater in pores is replaced by freshwater. Here, we measure changes in the geotechnical properties of seafloor clayey silt due to salt leaching using flushing experiments, and assess the implications of these changes on the stability of siliciclastic continental margins with 2D limit equilibrium modelling. We document a ∼ 50% decrease in undrained cohesive strength of seafloor sediment after flushing, as well as a decrease in its shear strength, bulk density, and moisture content, which is similar to that reported for subaerial quick clays undergoing salt leaching. When applied to a theoretical submarine domain 300 m wide by 100 m high, we estimate that salt leaching can trigger slope failure when the thickness of the flushed layer is >3.5 m or when the slope gradient is >3°. Such conditions are primarily satisfied on the continental slope or the shallow seafloor close to the shoreline. Salt leaching by OFG flow merits consideration as a potential mechanism destablising submarine sedimentary slopes.

Cyclic porewater pressure generation in intact silty soils

The results of cyclic strain-controlled, constant volume direct simple shear (CDSS) tests and field shaking tests have been evaluated for intact, natural, low-plastic silts from six different fine-grained soils with 54%–100% fines content, 47%–83% silt content, and plasticity indices (PI) ranging from nonplastic to 16. These tests constitute a subset of a larger archive of CDSS tests performed on silt deposits from the Pacific Northwest, British Columbia, and Alaska collected and analyzed by the co-authors. The cyclic data are presented in this paper for two objectives: (a) to characterize cyclically-induced excess pore pressure generation in intermediate soils with various soil index properties and stress histories, and (b) to provide calibrated Vucetic and Dobry model parameters for simulating excess pore pressure generation in the silt soils based on the data and trends presented in the first objective. The CDSS test results showed that excess pore pressure ratios decrease with PI over the narrow range of PI evaluated and decrease with overconsolidation ratio. The cyclic threshold shear strain amplitude for pore pressure generation extracted from field shaking tests on silts were within the range proposed in the literature, confirming that the cyclic threshold shear strain amplitude is a fundamental soil property. Calibrated Vucetic and Dobry model parameters for these intermediate, fine-grained silts were significantly different than those reported for sands in the literature and were heavily influenced by the overconsolidation ratio. The calibrated parameters obtained in this study can be used as a benchmark in selecting model parameters for silts.

Radial consolidation of prefabricated vertical drain-reinforced soft clays under cyclic loading

Previous studies have revealed that prefabricated vertical drains (PVDs) are effective in stabilizing the soft subgrade soils under traffic-induced cyclic loads. A consolidation model of soft clays under cyclic loading incorporating both radial and vertical drainages, and nonlinear variation of compressibility and permeability is presented in this paper. The prediction of the model matches well with the result from a large-scale cyclic triaxial test conducted on a soil sample installed with a single PVD. Parametric analysis has also been carried out to explore the impacts of cyclic degradation parameter, cyclic stress ratio, nonlinear compressibility and permeability, cyclic loading frequency, drainage condition, and number of loading cycles on the behaviors of the PVD-reinforced soil. Results indicate that radial drainage helps to decelerate the accumulation of excess pore pressures under cyclic loading due to two factors: increased rate of excess pore pressure dissipation and decreased rate of internal excess pore pressure generation. In addition, radial drainage also facilitates the rapid dissipation of excess pore pressures during rest period, leading to a denser soil which is stronger for resisting the subsequent set of cyclic loads. The findings of this study demonstrate the usefulness of PVDs in reinforcing soft subgrades under cyclic loading conditions, e.g., road pavement, railway embankment, etc.

Liquefaction responses of fibre reinforced sand in shaking table tests with a laminated shear stack

Mixing unidimensional tension-resistant elements into cohesionless soils, which is well known as fibre-reinforcement, is effective at strengthening the soils as well as increasing their liquefaction resistance in triaxial tests. In this study shaking table tests were performed on unreinforced and fibre reinforced sand models, contained in a laminated shear stack, to gather evidence as to whether or not the fibre reinforcement technology might reduce liquefaction susceptibility in a loading condition that has the semblance of an earthquake. The test results show that the excess pore pressures generated, and the amount by which the equivalent shear modulus is reduced, are significant in unreinforced and reinforced models when exposed to continuous sinoidal biaxial shaking simultaneously in horizontal and vertical directions. Fibre-reinforcement delays and reduces the build-up of excess pore pressures, slightly. Fibre-reinforcement also decreases slightly the attenuation of the acceleration amplitudes as well as the degradation of stiffness resulting from the generation of excess pore pressures. A constitutive model based on the rule of mixtures, which captures the load sharing between fibres, sand skeleton and pore water in triaxial loading conditions, and accounts for the anisotropy of the fibre orientation distribution, was modified and then used to study the shaking table deformation pattern. The stress the fibres impose on the sand skeleton in the vertical direction was determined and a new pore pressure ratio was used to assess whether or not liquefaction occurred. This enabled a mechanistic understanding of how the fibres alter the sand skeleton stresses and suppress liquefaction development. Liquefaction eventually occurred, or was close to occurring, in the reinforced model at different depths even though the fibres added stresses to the sand skeleton. The earth pressures prevailing in these 1 g shaking table tests were not sufficiently large for optimal interaction between fibres and sand skeleton. The fibres were not sufficiently tensioned as the confining (clamping) stresses provided by the neighbouring sand particles were not large enough. This means that the benefits of this reinforcement technology may be minor in practical situations where the fibre reinforced soil is at shallow depths.

Insight into excess pore pressure generation leading to liquefaction of sand with stress history under saturated and unsaturated conditions

Assessments of the liquefaction resistance of clean sand still involve considerable uncertainties, which are a current research topic in the field of soil liquefaction. The factors considered and discussed in this study include the loading history, degree of saturation, and partial drainage. The effects of each of these factors on pore pressure generation and liquefaction resistance have been studied for decades in the laboratory, and empirical relationships have been derived. In this paper, an attempt is made to explain these effects using the unique index of volumetric strain. A pore pressure generation model is developed which is similar to that of Martin et al. (1975), but based on stress-controlled triaxial tests. The model is verified through comparisons of its results with those of laboratory tests. It is confirmed that the plastic volumetric strain that has accumulated in sand, either by drained or undrained cyclic loading, dominates the increase in the liquefaction resistance of the sand. However, the plastic volumetric strain caused by overconsolidation is less effective in reducing the volumetric strain potential for subsequent cyclic shearing, thus enhancing its resistance to liquefaction. The model provides a better understanding of the physical processes leading to the liquefaction of saturated and unsaturated sand with and without stress history.

Time-Dependent Behavior of Embankment Resting on Soft clay Reinforced with Encased Stone Columns

In this study, the time-dependent behavior of embankment resting on soft clay reinforced with encased stone column (ESC) is examined numerically. A three-dimensional slice model of an embankment resting on reinforced soft clay was developed to examine the influence of the stiffness of encasement material, length of encasement, and floating ESC on the time-dependent response considering various design parameters like generation and dissipation of excess pore pressure, settlement of composite ground, column deformation, and effective stress experienced by columns and surrounding soils. This investigation reveals that the performance of embankment resting on soft clay reinforced with ESC is better than resting on unreinforced soft clay and soft clay reinforced with granular columns without encasement. Apart from strength enhancement, consolidation characteristics of composite ground were also significantly improved by encasing granular columns with geosynthetic material. Moreover, with the increase in the stiffness of encasement, length of ESC, and length of encasement a significant reduction in the generation of excess pore pressure and lateral deformation were noticed. Further, load transfer mechanism, failure pattern, and temporal variation of stress concentration ratio depend on the encasement stiffness, column length, and encasement length.

Numerical modeling of contaminant advection impact on hydrodynamic diffusion in a deformable medium

The self-weight of solid waste or machine-rolled compaction can induce or trigger contaminant migration in the landfill. Although the consolidation-induced hydraulic gradient driving solution transport has been extensively investigated, little attention has been paid to ion migration caused by its concentration gradient variation. It is necessary to more precisely predict the multi-stage contaminant transports in deforming porous material. Based on the modified Cam-clay model, the proposed fluid-solid coupled model can simulate the elastoplastic deformation behavior of layered kaolinite and KBr solution transport/sorption, and its modeling results were validated by published laboratory data. The solid-fluid interactions were analyzed by comparing various transport manners of K + and Br − from excess pore pressure generation to dissipation. Results reveal that the consolidation process can accelerate KBr solute advection from the contaminated layer into the uncontaminated layer, and then affects the subsequent diffusion, mechanical dispersion and sorption for K + and Br − . The simulations also indicate that consolidation-induced solute transport is time-dependent, and therefore the ion diffusion and mechanical dispersion should receive more attention.

A viscoplastic recoverable sensitivity model for fine-grained soils

Full-flow penetrometer testing in fine-grained soils indicates that undrained shear strength decreases due to remoulding induced generation of excess pore pressures and increases due to strain rate effects and dissipation of excess pore pressures. This paper presents a viscoplastic strain-softening–hardening constitutive model that captures this duality of behaviour via non-local regularisation, strain rate dependency, and consolidation induced recovery of sensitivity. The model is based on Structured Modified Cam Clay and Perzyna’s overstress framework. Validation of the implementation is first demonstrated through simulation of constant rate of strain triaxial and oedometer compression tests performed on different clays. The model is then applied in large deformation finite element analyses of monotonic and variable rate T-bar penetration to demonstrate successful numerical implementation and its ability to simulate strain rate effects. Finally, the model is applied in the simulation of undrained cycles of penetration of a T-bar penetrometer in kaolin clay and a carbonate silt and compared with data from geotechnical centrifuge experiments. The results show that the model without sensitivity recovery significantly underestimates the consolidated-undrained resistance in both soils, suggesting that recoverable sensitivity is an aspect of behaviour that ought to be considered in the development of constitutive models for soft sensitive soils.

Static liquefaction mechanisms in loose sand fill slopes

To investigate potential triggering and failure mechanisms of loose fill slopes, two centrifuge model tests were conducted on loose sand with a contractive tendency andhigh liquefaction potential. The behaviour ofloose sand fill slopes subjected to rising groundwater or rainfall was simulated using a geotechnical centrifuge. Moreover, a three-dimensional back-analysis and parametric study were conducted to investigate the role of soil nails in preventing the triggering of static liquefaction in loose fill slopes. Contrary to the rainfall test where only excessive settlements are observed, arapid, and brittle fluidised flowslide in the slope can be identified whensubjected to rising groundwater. The fluidised flowslide is triggered by a localised drained surface failure in the form of a gully at the crest of the slope. The initial failure at the crest induces the generation of significant excess pore pressures within the slope, causing a significant reduction in effective stress andloss of shear strength due to static liquefaction. For an identical slope, numerical simulations indicate that a fluidised flowslide can be prevented by soil nailing. Soil nailing can prevent the peak shear strength of the soil from being mobilised and limits the excessive strains required to trigger static liquefaction.

Development of soil–pile interactions and failure mechanisms in a pile-reinforced landslide

The reinforcement effect of stabilizing piles is significant for ensuring the stability of major landslide mitigation projects. To investigate the work performance of piles under different boundary conditions, we introduce two physical modelings of a pile-reinforced landslide with the same pile parameters and soil conditions but different load conditions and analyze the consequent differences in the responses of the soil and piles and their interactions in these two physical modelings. Results show that piles in the same landslide/pile systems may have different reinforcement effects; this phenomenon has been explained by quantitatively describing the role of piles and analyzing the processes of soil–pile interactions in the physical modelings. Further, the dynamic responses between the excess pore pressure and the soil–pile interaction are investigated and the excess pore pressure-dependent mechanisms of soil–pile interactions are revealed. The generation of excess pore pressure and its accumulation around piles leads internally to a mechanism in which the arching effect is weakened and externally to a phenomenon in which the progressive failure of pile-reinforced landslides is accelerated. Eventually, we calculate the effective stress in the soil around piles, and the results reveal that a sharp reduction in effective stress in the soil around piles causes failure of the pile reinforcement. In addition, for a pile-reinforced landslide with a strong soil–pile interaction, an imbalance of soil arch footholds is the main failure macro-mechanism. Based on the analysis results, we present some feasible suggestions for the installation of rowed pile and their monitoring to ensure the long-term stability of a pile-reinforced landslide.

Large deformation coupled analysis of embedded pipeline – Soil lateral interaction

Subsea pipelines buried in the seabed may undergo large lateral displacement under environmental, operational, and accidental loads at different interaction rates and hence different drainage conditions. The undrained shear strength is commonly used in practice to assess the pipe-soil interaction assuming a sufficiently high displacement rate. This approach neglects consolidation effects and the rate-dependent response of the soil and may significantly underestimates the lateral resistance for a pipeline moving slowly relative to the ground. In this study, a coupled large deformation finite element (LDFE) framework is developed via a remeshing and interpolation technique with small strain (RITSS). A Modified Cam-Clay (MCC) model with efficient numerical integration is used. The proposed coupled LDFE framework is verified against selected physical model tests. Effects of the interaction rate and hence drainage condition on the p-y curve, excess pore pressure generation and dissipation, and failure mechanisms are discussed. An empirical relationship between the ultimate resistance and the normalized velocity of the pipe (denoting the drainage condition) is proposed, which may be applied for the integrity and safety analysis of buried pipes in landslide or fault-crossing regions.

Numerical investigation of liquefaction mitigation potential with vibroflotation

Vibroflotation is one type of ground improvement technique used for liquefaction mitigation in saturated sand. In order to study the feasibility of vibroflotation for liquefaction mitigation, two numerical models were created in the finite element framework. A nonlinear, coupled, hypoplastic u-p formulation was developed to simulate the behaviour of saturated sand. Numerical results were validated against on-field cone penetration test measurements after vibroflotation and centrifuge test simulating seismic site response of saturated sand. Seismic site response of saturated sand before and after vibroflotation was analysed. Saturated loose sand with fines and lower permeability underwent excess pore pressure generation, softening, and possibly liquefaction under seismic loading before vibroflotation. Sands with higher permeability were found to respond better to vibroflotation. The most effective degree of compaction was achieved in dry sands. Vibroflotation of saturated sand with fines ensured improved resistance against liquefaction whereas for coarse sands it led to complete eradication of the liquefaction.

Seismic performance of pervious concrete column improved ground in mitigating liquefaction

This paper investigates the performance of pervious concrete column improved ground subjected to seismic loading. The seismic performance of pervious concrete column improved ground is also compared with conventional stone column improved ground in responses of lateral displacement, excess pore pressure, ground surface acceleration, shear stress-strain behaviour and stress path. A fully saturated mildly sloping sand strata is considered as unimproved ground. OpenSeesPL software is used to analyse soil models representing unimproved ground and improved ground with stone column and pervious concrete column inclusions. It is found that the pervious concrete column improved ground has better seismic performance than stone column improved ground. The lateral displacement of ground is found to be significantly reduced while using pervious concrete column. Also the use of pervious concrete column has reduced excess pore pressure generation than stone column indicating that the improved ground with pervious concrete column inclusion is efficient in mitigating liquefaction than conventional stone column improved ground.

Coupled analysis of full flow penetration problems in soft sensitive clays

This study describes the development and implementation of a numerical procedure for the analysis of coupled geotechnical problems involving finite deformations, changing boundary conditions, multiphase porous media and softening behaviour. The numerical scheme is first validated through a benchmark problem concerning the laying of an on-bottom offshore pipeline, and then some further highlights of the effects of soil softening are presented. The scheme is then specifically employed to study the penetration process of a full flow cyclic T-bar test in soft sensitive clay. An advanced soil constitutive model has been implemented to capture the effects of soil structure and its progressive de-structuring as well as soil fabric anisotropy. Accordingly, the changing soil resistance due to the combined effects of soil softening, remoulding and reconsolidation is illustrated, by considering a T-bar undergoing large amplitude cyclic sequences interspersed with consolidation periods. The results from the numerical simulations reveal the effects of soil softening on the soil deformation pattern, the evolution of shear bands, the generation of excess pore pressures and the process of soil reconsolidation.