Pore Pressure Development Research Articles

An experimental study on foam infiltration into saturated sand and its consequence for EPB shield tunneling

With reference to the Earth Pressure Balance (EPB) shield tunneling, the pressure infiltration of foam into saturated sand has been investigated in a laboratory set-up that could provide a comparable hydraulic gradient as in field conditions. The focus is to investigate the foam infiltration characteristics in comparison with slurry infiltration. Results suggest that a pressure drop can be realized over the foam infiltrated area during the foam spurt period, suggesting that the supporting pressure can be applied to the soils in front of a tunnel face during standstill. Development of pore pressures shows that the foam infiltrated area functions similarly to an internal cake at the tunnel face of a slurry shield. Verification tests indicate that the permeability of the whole system is determined by the foam infiltrated sand instead of the foam itself, different to the situation in a slurry infiltration. In the meanwhile, further infiltration of foam bubbles in the saturated sand was found in the experiment to be the main process after the foam spurt period. General agreement was found when comparing the experimental results with field measurement data from the Botlek Rail Tunnel.

Effect of confining pressure and loading frequency on dynamic characteristics of Batu Pahat marine clay

This paper presents the laboratory study on the dynamic characteristics of marine clay subject to cyclic load over a long period of time. Remoulded marine clay samples were used to conduct the experiments using a dynamic triaxial machine. Confining pressure and loading frequency were varied while pore pressure development, resilient strains and plastic strain resulting from the application of cyclic loading were observed. The results acquired from this study revealed that as soon as the cyclic loading was introduced, it induced pore pressure that triggered the rapid accumulation of permanent strain and gradual increase in resilient strain. However, after the strain reached a stable value, the rate of accumulation of the permanent deformation started decreasing. The results also showed that an increase in confining pressure improved the marine clay resistance to cyclic loading. Similarly, when the loading frequency increased, it caused a decrease in strain generation and vice versa.

Groundwater erosion of coastal gullies along the Canterbury coast (New Zealand): a rapid and episodic process controlled by rainfall intensity and substrate variability

Abstract. Gully formation has been associated to groundwater seepage in unconsolidated sand- to gravel-sized sediments. Our understanding of gully evolution by groundwater seepage mostly relies on experiments and numerical simulations, and these rarely take into consideration contrasts in lithology and permeability. In addition, process-based observations and detailed instrumental analyses are rare. As a result, we have a poor understanding of the temporal scale of gully formation by groundwater seepage and the influence of geological heterogeneity on their formation. This is particularly the case for coastal gullies, where the role of groundwater in their formation and evolution has rarely been assessed. We address these knowledge gaps along the Canterbury coast of the South Island (New Zealand) by integrating field observations, luminescence dating, multi-temporal unoccupied aerial vehicle and satellite data, time domain electromagnetic data and slope stability modelling. We show that gully formation is a key process shaping the sandy gravel cliffs of the Canterbury coastline. It is an episodic process associated to groundwater flow that occurs once every 227 d on average, when rainfall intensities exceed 40 mm d−1. The majority of the gullies in a study area southeast (SE) of Ashburton have undergone erosion, predominantly by elongation, during the last 11 years, with the most recent episode occurring 3 years ago. Gullies longer than 200 m are relict features formed by higher groundwater flow and surface erosion > 2 ka ago. Gullies can form at rates of up to 30 m d−1 via two processes, namely the formation of alcoves and tunnels by groundwater seepage, followed by retrogressive slope failure due to undermining and a decrease in shear strength driven by excess pore pressure development. The location of gullies is determined by the occurrence of hydraulically conductive zones, such as relict braided river channels and possibly tunnels, and of sand lenses exposed across sandy gravel cliffs. We also show that the gully planform shape is generally geometrically similar at consecutive stages of evolution. These outcomes will facilitate the reconstruction and prediction of a prevalent erosive process and overlooked geohazard along the Canterbury coastline.

Influence of the initial hydrostatic pressure on contact area coefficient under drainage condition

The expression of effective stress proposed by Terzaghi has always been questioned. Many correction formulas are modification of pore pressure term. The pore pressure factor is related to porosity, contact area and other factors. When the particles are in point contact, the expression of the effective stress is that proposed by Terzaghi, while for the surface contact particles, the actual effective stress increases the stress produced by pore pressure passing through the contact surface based on the Terzaghi effective stress. There are many factors that affect the development of contact area and pore pressure, therefore, a series of the drained triaxial tests were carried out on four groups of sand samples with different initial hydrostatic pressures to study the influence of different initial hydrostatic pressures on the effective stress due to the term of contact area (σα). The test results show that the shear strength is increases with the initial hydrostatic pressure under the same effective confining pressure, which indirectly indicates that the initial hydrostatic pressure increases the contact area stress.

Wave-induced residual pore pressure around a buried pipeline

Seabed instability caused by cyclic wave loading is one of the main factors affecting the foundation instability of marine structures, and it is a key problem that needs to be paid attention in the design of marine structures. Based on Biot’s consolidation theory and a semi-empirical formula for calculating residual pore water pressure, the wave-induced residual seabed response around a buried pipeline was investigated by numerical simulation. The correctness of the numerical results is verified by comparing with experimental results. Effect of the self-weight of the buried pipeline on residual pore pressure development and the characteristics of residual pore pressure near the pipeline are discussed.

TURBULENT BORES-INDUCED SCOUR AND PORE PRESSURE VARIATION AROUND A VERTICAL STRUCTURE

Pore pressure and scour formation around coastal structures have been investigated in the past which led to a relationship between the hydrodynamics of tsunami waves and sediment erosion around coastal structures. With the objective of developing a comprehensive model to assess the interaction of hydrodynamic conditions, induced scour, soil pore pressure, and beach slopes, this novel study aims to examine (1) the effect of hydrodynamic forcing factors such as bore depth on scour and pore pressure generation on both horizontal and inclined bed conditions and (2) the effect of bed slope on the scour and pore pressure development and (3) the influence of the soil pore pressure on scour around a square shaped structure.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/dTQ-_GLhtzs

SANISAND-MSf: a sand plasticity model with memory surface and semifluidised state

A new constitutive model for sand is formulated by incorporating two new constitutive ingredients into the platform of a reference critical state compatible bounding surface plasticity model with kinematic hardening, in order to address primarily the undrained cyclic response. The first ingredient is a memory surface for more precisely controlling stiffness affecting the plastic deviatoric and volumetric strains and ensuing excess pore pressure development in the pre-liquefaction stage. The second ingredient is the concept of a semifluidised state and the related formulation of stiffness and dilatancy degradation, aiming at modelling large shear strain development in the post-liquefaction stage. In parallel, a modified flow rule aimed at providing a better description of non-proportional monotonic and cyclic loading is introduced. With a single set of constants, for which a detailed calibration procedure is provided, this new model successfully simulates undrained cyclic torsional and triaxial tests with different cyclic stress ratios, separately for the pre- and post-liquefaction stages, as well as liquefaction strength curves based on [Formula: see text] and shear strain criteria for initial liquefaction. The successful reproduction of the sand element response under undrained cyclic shearing contributes to future applications in realistic and thorough seismic site response analysis.

Effect of Loading Rate on Lateral Pile-Soil Interaction in Sand Considering Partially Drained Condition

Reliable assessment of the lateral pile-soil interaction is of pronounced importance for the design of mono-pile foundations of offshore wind turbines. As the offshore engineering moves to deeper waters, the diameter of mono-piles is getting larger, usually about 5 m and could be up to 8 m, which may lead to partially drained behaviors of sand in the vicinity of the pile and thus imply limitations of conventional design methods in which fully drained conditions were assumed. To shed light on this issue, a fully-coupled finite element model was established using an in-house developed finite element code DBLEAVES, incorporating a cyclic mobility constitutive model that is capable of describing the instantaneous contractive and dilative response of sands simultaneously. Triaxial and centrifuge model tests were conducted to calibrate the constitutive model and validate the pile-soil interaction model respectively. This is followed by a parametric study primarily focusing on the effects of loading rates. The initial stiffness of the p-y curve was found to increase with the loading rate whilst the bearing capacity showed the inverse, and the mechanism behind this phenomenon is examined in detail. Then an explicit model was developed to evaluate the development of excess pore pressure in the pile front upon lateral loading, and an upper boundary of normalized loading rate was identified to distinguish fully and partially drained conditions.

A Study of the Dynamics of Soil Erosion Using Rusle2 Modelling and Geospatial Tool in Edda-Afikpo Mesas, South Eastern Nigeria

Abstract The Revised Universal Soil Loss Equation (RUSLE) was used to study the soil erosion processes in Edda-Afikpo mesas, Lower Cross River watersheds,Nigeria. The mesas occupy an area estimated at 60km2 on a surface relief of about 284m. DEM data, satellite images and basemap of the area were used. Remotely sensed data were ground-truthed through extensive field works. The results show that the process is facilitated by the Trifecta of hill slope hydrology, geology and land use practices. Steep hill Slope of values 78 % at the major hot spots, very fragile, dry and non-plastic sandy soils all aid sediment detachment. Analysis of the index properties which include Liquid Limit(LL) of 25-30,moisture content(w%) of 5.9-7.4,permeability of 1.541x10-3 – 1.636x10-3 cm/s and shear strength of 36-42 KN/m2 predispose the sediments to detachment and erosion. Based on the analysis, the amount of soil loss in the project area is about 1373.79 ton per year. Soil erosivity factor is high at the mesas(5023.83 MJ mm ha−1 h−1 yr−1 - 5069.51 MJ mm ha−1 h−1 yr−1) The sandy layer attain thickness of 50m-60m in places and with high pore pressure development, slope failure are triggered during intense storm events. In terms of vulnerability level in erosion risk, high to very high constitute 4.1% of the watershed which translate to 5.05km2 of the 59km2. The various processes occur simultaneously and are exacerbated by human factors through seasonal bush burning and development along drainage lines. The study reveals that 18.8% of the available land for development is at high to very high risk of erosion. The soil loss model has been validated and the hotspots from the map coincide with the gully sites. The results of this research can therefore be used for conservation and adaptation purposes.

Analysis of the impacts of hydrology, soil properties, and geotechnics on gully propagation on the Edda-Afikpo Mesas of the Lower Cross River watershed (southeastern Nigeria)

The effects of hydrology, soil structure, and geotechnics on soil loss and gully propagation on the Edda-Afikpo Mesas (southeastern Nigeria) were assessed by field studies, laboratory soil analyses, and GIS modeling.The study was carried out to attribute causality and recommend mitigation measures in the fast degrading settlement. Results of field studies indicate that two areas in the study region were of exceptionally negative pore pressure values (−654.03 kPa and −676.89 kPa, respectively) at ground surface level and significantly negative at 50 m below ground level (−163.53 kPa and −186.39 kPa, respectively); both areas aligned in the northeast-southwest direction. These two areas were further characterized by comparatively higher hydraulic heads (158–206 m), seasonal fluctuations of water level, and rapid drainage enhanced by springs flow. Developed models revealed that a slight water level increase in these areas, combined with high surface infiltration as a result of significant permeability values (ranged between 1.522 × 10−3 cm/s and 1.744 × 10−3 cm/s) would translate into positive pore pressure development and seepage flow due to hydraulic gradient towards the slope toes. Thus, the lower part of the slopes on the mesas experiences higher positive pore pressure build-up and particle uplift. This scenario most likely plays out seasonally and could be responsible for the toe failures that are predominant in the area. Results of laboratory analyses further revealed that the soils have geotechnical properties (significant natural moisture content, low to moderate plasticity, marginally low bulk density, and specific gravity and shear strength) that increased their soil loss and gully propagation potentials, exacerbated by hydrology, soil structure, and human factors. This study, therefore, advocates that the solution to erosion and gully menace in the area, and other places with similar geological, topographic, and climatic situations would require an integrated approach.

Coupled smoothed particle hydrodynamics-discrete element method simulations of soil liquefaction and its mitigation using gravel drains

In this paper, a fully Lagrangian particle-based method for coupled fluid-particle interaction is utilized to evaluate liquefaction of saturated granular soils subjected to dynamic base excitations. The discrete element method (DEM) is employed to model the solid particles and the fluid motion is simulated using the smoothed particle hydrodynamics (SPH). A coupled SPH-DEM scheme is achieved through local averaging techniques and well-established semi-empirical formulas for fluid-particle interaction. A key feature of the employed technique is that it does not presume undrained conditions for the granular deposit and allows for spatial fluid movements within the deposit. The responses of loose and dense granular deposits to seismic excitation are first analyzed. As expected, the loose deposit exhibited significant pore pressure development and liquefaction while the dense deposit barely showed any considerable buildup of pore pressure and did not liquefy. Liquefaction of the loose deposit resulted in significant surface settlement while that experienced by the dense deposit was within tolerable limits. A liquefaction mitigation technique through the installation of gravel drains was then introduced to the loose deposit and its effect on mitigating pore pressure buildup was examined. Results of conducted simulations show that the installation of gravel drains effectively reduced pore-pressure buildup and, for the most part, the soil maintained its strength. However, the drains did not reduce the overall surface settlement of the deposit to acceptable levels.

Centrifuge modelling for seismic response of single pile for wind turbine subjected to lateral load

The soil around the pile foundations of offshore wind turbines (OWTs) would suffer from earthquakes which are widely believed to undermine the bearing capacity and serviceability of foundations. The non-intermittent lateral environmental loads in the ocean (e.g., wave, wind, and current) continuously act on OWTs. However, the seismic response of pile foundations and the soil around is less clear when considering the OWTs are subjected to lateral environmental loads. This paper presents two sets of centrifuge modelling tests on single piles, with a prototype diameter (D) of 1.5 m and a first-order natural frequency highly comparable to that of a prototype wind turbine. Two model piles with the same dimension and stiffness, but one with a lateral load on the pile head, were both installed in a dry sand ground and a saturated sand ground. Owing to the fact that the lateral load increased the initial static shear stresses of the soil in front of the pile, the development of excess pore pressure was inhibited and the permanent ground lateral displacement was accumulated during shaking. The existence of the lateral load led to a pile head displacement as much as 34 cm and increased the bending moment along the pile up to 6 MN·m. Moreover, the seismic response of piles and soils were compared between the dry sand ground and saturated sand ground to clarify the influence of excess pore pressure. It was also found that the residual bending moment after unloading the pile was still very large up to 17 MN·m because of the rebalanced soil-pile interaction. These findings highlight the necessity to consider the lateral load effect in the seismic design of pile foundations for OWTs.

Laboratory test and modelling of gas pressure under geomembrane subjected to the rise of groundwater in plain reservoirs

To investigate the development of pore pressures in the air-confined system with a rise of groundwater under a geomembrane, a laboratory test was conducted. Then, a numerical model with the consideration of the water-gas-solid three-phase system was developed based on the hydro-mechanical coupling theory for unsaturated soils. The model was validated by the experimental result and then a numerical study was conducted on the Datun plain reservoir. The result confirms the fact that the rise of groundwater is one of primary driving force for gas pressure under the geomembrane. When there is no surcharge over the geomembrane, air bulge would be produced. As the groundwater under the geomembrane replenished by water infiltration from groundwater around the reservoir, the gas pressure increases from the edge to the center of the reservoir and reaches the same final value with time. The increment of the pore gas pressure is significant during the increase of reservoir water, especially for the soils with larger compression strain. Thus, the coupled deformation is necessary to be considered for the regulating reservoir that undergoes the fluctuation of reservoir water frequently.

Comparison of cyclic liquefaction behavior of clean and silty sands considering static shear effect

In practical engineering, liquefiable sand deposits in natural or manmade slopes, dams, and embankments may contain a certain amount of silty fines and often sustain a static shear prior to subsequent cyclic loading. To understand the effect of static shear stress on the cyclic liquefaction behavior of silty sand, a small amount of crushed fines was added to clean sand, and a series of triaxial tests considering a wide range of static and cyclic shear stress levels were performed. The results indicate that different stress conditions result in two liquefaction failure patterns for silty sand: cyclic mobility and residual deformation accumulation. A comparison of the liquefaction responses of silty and clean sands with the same relative density confirms that the former typically has a moderately higher cyclic resistance than the latter, which can be explained by the reinforced effect of fines on the soil skeleton. It is also demonstrated that the flow deformation type behavior accompanied by rapid development of pore pressure and severely degraded stiffness is more prominent in clean sand, which is responsible for the continuous decrease in its liquefaction resistance when the extensional static stress increases. Furthermore, a unique correlation of equivalent modulus versus cyclic strain amplitude is observed, which depends significantly on the static shear stress level but relatively insensitive to the addition of fines at least up to the considered fines content.

Long-Term Cyclic Loading Impact on the Creep Deformation Mechanism in Cohesive Materials

Long-term cyclic loading is observed in a wide range of human activities, as well as in nature, such as in the case of ocean waves. Cyclic loading can lead to ratcheting which is defined as progressive accumulation of plastic deformation in a material. Long-term cyclic loading causes a time effect (creep), which is a secondary compression effect. In this article, we conducted 15 triaxial tests on four types of cohesive materials in undrained conditions to evaluate the damage and failure mechanism. To characterize the strain and pore pressure development, we modified the Yanbu resistance concept. On the basis of the static creep tests, we concluded that the stress paths for undrained creep behavior have to take into account the pore pressure developed during long-term cyclic loading. Pore pressure build-up and plastic strain accumulation during long-term cyclic loading are dependent on the number of loading cycles. Finally, we proposed the failure criterion, which was based on the Modified Cam-Clay constitutive model.

Deep-Seated Slope Stability Analysis and Development of Simplistic FOS Evaluation Models for Stone Column-Supported Embankments

Inclusion of stone columns in the underlain soft soil is one of the most prominent methods for improving the stability of embankments. The stone columns are encased with a geosynthetic material to further enhance the stability. The influence of this partial replacement of weak foundation soil with stone columns on the performance of embankments needs to be quantified. In this study, performance of ordinary stone column (OSC) and geosynthetic-encased stone column (GESC)–supported embankments is carried out using a three-dimensional finite element programme (PLAXIS3D). A parametric study was conducted to quantify the influence of various factors viz. spacing to diameter ratio (S/D), stiffness of encasement, cohesion of soil, friction angle of stone column and friction angle of embankment on the factor of safety against deep-seated failure. The results show that encasing the stone columns enhances the stability of embankments. Decreasing the column spacing (S/D) enhances the stability, reduces the excess pore pressure development and average settlement. Increase in geotextile encasement stiffness, cohesion of underlain soft soil, friction angle of stone column and friction angle of embankment improves the performance of embankments and also reduces the average settlement of the ground under embankment. Results of the parametric study were used to develop two types of data-driven models viz. multiple linear regression (MLR) and artificial neural networks (ANN) to simplify the evaluation of the factor of safety (FOS) of embankments. Among the two approaches, ANNs were able to predict the factor of safety values more accurately.

Pore-pressure development near tunnel due to underwater explosion from centrifuge tests

An instrumented 1:70 scale model underground tunnel, located below a free water body, was subjected to explosions in centrifuge model tests. Utilising centrifuge scaling relations, each test at 70g modelled the effects of an explosion due to approximately 0.9 t of trinitrotoluene equivalent. The explosives were located at the ground surface, above the tunnel centreline. The effect of water level on the extent of damage was of primary interest in this study. Instrumentation in each test included pore-water pressure (PWP) transducers and strain gauges. PWP and strain measurements showed instantaneous increase due to the explosion, followed by gradual dissipation. The magnitude of peak pore pressure and strain increased with increase in water level. PWP ratios, calculated based on measured PWPs during the centrifuge tests, indicate that liquefaction occurred near the ground surface, close to the crown of the tunnel in all cases and at the bottom, near the endspan, only when the water level was high. The measured pore-pressure ratios and strains, induced due to the explosion, are useful in understanding the effects of explosion on an underground tunnel below a water body.

Cyclic Simple Shear Response of Sand–Rubber Tire Chip Mixtures

The dynamic properties of mixtures of poorly graded sand and the fine rubber tire chips are assessed under undrained, strain-controlled cyclic simple shear (CSS) conditions. In this study, CSS equipment with a confining pressure system is used. The CSS tests are performed on the mixtures with rubber content of 0%, 10%, 30%, 50%, and 100% by weight prepared at relative densities in the range of 65%–80%. Tests are carried out at high shear strain amplitudes in the range of 0.15%–3% at various total vertical stress levels (50–200 kPa). The material and loading parameters such as rubber content, shear strain, frequency, total vertical stress, influencing the dynamic properties and the pore pressure responses of the mixtures are investigated. It was found that the mixture with 10% rubber content has better dynamic properties than other mixtures. In addition, the pore pressure development can be retarded by the use of the sand–rubber tire chip mixtures.

Cyclic Properties of Artificially Cemented Gravel-Silty Clay Mixed Soils

The aim of this paper is to provide an insight into the effect of fine content (FC) on the artificially cemented gravel-silty clay mixed soils (CMS) in terms of strength, deformation, effective stress path and pore pressure response by undrained cyclic triaxial tests. In addition, some cyclic triaxial tests were also conducted on remolded gravel-silty clay mixed soils (RMS) under the same test conditions in order to evaluate the effect of cementation provided by the cementitious material. The corresponding cyclic responses, such as the generation and accumulation of axial strain, pore pressure and effective stress path, are compared across a range of CMS and RMS. A constant 5% percentage (by weight) of calcium oxide in mixed soils with four different ratios of fine content (13%, 30%, 50% and 70%) are tested under four types of confining pressures, 50 kPa, 100 kPa, 200 kPa and 300 kPa respectively. The results demonstrate that: an increase in fine content leads to decrease in both strength and liquefaction resistance, but accelerate the strain accumulation and pore pressure development. The cyclic responses of CMS exhibit “hysteresis” significantly compared with those of RMS due to the bonding strength between soil particles.

An effective stress analysis for predicting the evolution of SCR–seabed stiffness accounting for consolidation

Steel catenary risers (SCRs) are an efficient solution to transfer hydrocarbons from deep-water seabeds to floating facilities. SCR design requires an assessment of the fatigue life in the touchdown zone, where the riser interacts with the seabed, which relies on reliable estimates of the SCR–seabed stiffness over the design life. Current models for SCR–seabed stiffness consider only undrained conditions, neglecting the development and dissipation of excess pore pressures that occur over the life of the SCR. This consolidation process alters the seabed strength and consequently the SCR–seabed stiffness. This paper summarises experimental data that show that long-term cyclic vertical motion of an SCR at the touchdown zone leads to a reduction in seabed strength due to remoulding and water entrainment, but that this degradation is eclipsed by the regain in soil strength during consolidation. The main focus of this paper is on prediction of the temporal changes in seabed strength and stiffness due to long-term cyclic shearing and consolidation, to support calculations of SCR–seabed interaction. The predictions are obtained using a framework that considers the change in effective stress and hence soil strength using critical state concepts, and that considers the soil domain as a one-dimensional column of elements. The merit of the model is assessed by way of simulations of SCR centrifuge model tests with over 3000 cycles of repeated undrained vertical cycles in normally consolidated kaolin clay. Comparisons of the simulated and measured profiles of SCR penetration resistance reveal that the model can capture accurately the observed changes in SCR–seabed stiffness. Example simulations show the merit of the model as a tool to assess the timescale in field conditions over which this order of magnitude change in seabed stiffness occurs. It is concluded that current design practice may underestimate the seabed stiffness significantly, but the new approach allows rapid checking of this for particular combinations of SCR and soil conditions.