Pore Water Pressure Measurements Research Articles

The influence of earthworks construction on pore water pressures in clays and mudstones of the Lias Group

Monitoring the changes in pore water pressure associated with the construction of earthworks can yield information on the stiffness and permeability of the ground, as well as how the natural groundwater regime might be impacted. This paper presents three years of pore water pressure measurements in weathered Lias Group mudstone, obtained from a trial cutting and a trial embankment constructed for the UK's High Speed Two (HS2) railway. The immediate changes in pore water pressure were small in relation to the changes in total stress imposed. This can be explained by the consolidation or swelling during the period of construction, combined with the sensitivity of very stiff clays and mudstones to a very small (0.5%) reduction in the degree of saturation. In the longer-term, pore water pressures reduced across the site owing to the reduction in ground level at the trial cutting. Rates of pore water pressure change were accelerated by more permeable limestone within the ground profile reducing drainage path lengths. It is concluded that construction-induced pore water pressure changes may be smaller, and their rate of dissipation more rapid, in weathered clays and mudstones such as those of the Lias Group than in younger, more compressible clay deposits.

Determination of Constrained Modulus of Granular Soil from In Situ Tests—Part 2 Application

The paper demonstrates how the concepts presented in the companion paper: “Determination of Constrained Modulus of Granular Soil from In Situ Tests—Part 1 Analyses” can be applied in practice. A settlement design based on the tangent modulus method is described. Extensive in situ tests were performed on a well-documented test site consisting of sand with silt and clay layers. The field tests comprised different types of penetration tests, such as the cone penetration test, the flat dilatometer, and the seismic down-hole test. The modulus number and the constrained tangent modulus were derived from the cone penetration test with pore water pressure measurement and the flat dilatometer test. In addition, the shear wave speed was determined from two seismic down-hole tests, from which the small-strain shear modulus could be evaluated. The constrained modulus obtained from the cone penetration test with pore water pressure measurement (CPTU) and the flat dilatometer (DMT) was compared with that from the seismic down-hole tests. The importance of the stress history on the constrained modulus was demonstrated. The range of modulus numbers, derived from different in situ tests, compares favorably with empirical values reported in the literature.

Friction behaviors and flow resistances of rock-ice avalanches

Rock-ice avalanches have high mobility and enormous destructive potential. However, the friction behaviors and flow resistance during the transition of flow regimes of the rock-ice avalanche remains an unsolved problem. Based on a series of measurements of normal, shear stress and pore water pressure at the rotating drum in a temperature-controllable laboratory, we quantify the influence of the ice content, particle size, drum velocity, and meltwater on the friction behaviors. The findings reveal that ice acts as a low-friction particle, leading to a significant reduction in frictional resistance of both particle-particle and particle-base. Furthermore, meltwater lubricates the particles and provides buoyancy, which also increases the mobility of the rock-ice avalanche. In addition, the augmentation of flow resistance is attributed to the increased particle size and drum velocity, thereby intensifying shear motion. In order to propose an unify theoretical model to calculate the flow resistance of rock-ice avalanche in various flow regimes. A heuristic model that relies on the extended dynamics theory is modified and can be well described the flow resistance of the tests involved in this paper. This study in the friction behaviors and flow resistance depending on the internal/external factors may be considered in hazard assessments of rock-ice avalanches in cold high-mountain regions.

Evolution of excess pore water pressure in undrained claystructure interface shear tests

Recent studies focused on the shear behaviour of clay-structure interfaces have shown the importance of the shearing rate on the strength of these interfaces. In normally-consolidated clays, increasing the shearing rate results in a decrease in the interface strength, while the trend is opposite in heavily overconsolidated clays. While analytical and empirical interpretation methods indicate that the generation of shear-induced excess pore pressures are responsible for theaforementioned trends, experiments with pore water pressure measurements at the clay-structure interface are rare. In this paper, we first describe a modified interface shear box testing setup that is equipped with a pore water pressure sensor. For this equipment, the fully rough structural surface was manufactured with a port at the centre of the clay-surface interface to measure the pore water pressure. We present the results of undrained clay-structure interface tests on normally consolidated (NC) and overconsolidated (OC) specimens of kaolin clay. The results agree with the expectations, where the NC specimens generate excess pore pressures with greater magnitudes and heavily OC specimens generate negative excess pore pressures. Measurements of the pore water pressures allow calculating vertical effective stresses, which can be used to investigate the effective stress paths followed by the clay-structure interface during the tests. This paper also provides a comparison of the measured values of beta and adhesion factors with previously published results and relationships used for the design of deep foundations.

Novel procedure for measuring the saturation of the pore-pressure system in piezocone tips

Uncertainty in the measurement of the cone penetration test with pore-water pressure measurement (i.e. piezocone testing CPTU) can be caused by an equipment malfunction, poor calibration and maintenance of sensors, but also by a lack of saturation of the piezocone. The standards allow for many different options in terms of filters, saturation fluids and methodologies. The novel device described herein aims at measuring in a very simple manner the degree of saturation of the piezocone prior to testing. Because the quality of pore-water pressure measurements can currently only be assessed a posteriori – that is, after the test has been completed, or at best while testing, the methodology proposed herein has the potential to provide quality benchmarks for CPTU. The tool measures a parameter inspired by the well-established Skempton coefficient B, which is routinely used to estimate the sample saturation prior to laboratory testing. Furthermore, by performing the same measurement after a CPTU test, it is possible to assess whether saturation was lost during cone penetration. The paper describes a set of preliminary results that show a clear correlation between the saturation degree of filters and/or of the overall pore-pressure measurement system and the B parameter proposed, thus providing a proof of concept in laboratory settings.

Measurements and Modeling of Pore‐Pressure Gradients in the Swash Zone Under Large‐Scale Laboratory Bichromatic Waves

AbstractThe present work presents physical laboratory measurements of surface elevation and pore water pressures in a fine sand bed under bichromatic waves in a large‐scale laboratory experiment. This was done at three cross‐shore locations in the swash zone, with pressures being measured at different depths in the bed. The measurements show that the pore pressure signal decays and shifts with increased depth. These measurements are used to validate a practical model, based on the theory of Yamamoto et al. (1978, https://doi.org/10.1017/S0022112078003006) and Guest and Hay (2017, https://doi.org/10.1002/2016JC012257). The model corresponds well with the measurements (nRMSE < 0.2 and R2 > 0.95 for most probes) and shows that a frequency‐based model can reproduce the pressures in the bed, despite the bed being exposed during dry periods. Furthermore, the model provides the opportunity to calculate pressure gradients, both throughout the bed and at the bed surface. These modeled pressure gradients at the bed surface show that the vertical pressure gradient can have an important impact on the Shields parameter, thereby influencing sediment transport.

Research on experimental evaluation horizontal coefficient of consolidation Ch on CPTu dissipation test to determine permeability parameters in the field to improvement of soil soft.

In the solution of soft soil improvement by preloading method combined with prefabricated, the horizontal consolidation coefficient Ch is a very important parameter to help predict the settlement speed of soft soil. But when designing the coefficient of horizontal consolidation, it is often assumed that the coefficient of vertical consolidation is quite large. This can lead to settlement over time that will fluctuate in a rather large range of values. In this paper, research and evaluation of the horizontal consolidation coefficient Ch by dissipation test on piezocone penetration device with pore water pressure measurement has been mentioned to determine the permeability parameters in-situ for the soft soil improvement at the projects.

Study on the dynamic response characteristics of roadbed and pavement under the humidity and season factors in the hilly area of Southwest China

The environment of the roadbed and pavement often has a significant impact on its dynamic performance. The stability of the strata in the Hilly Area is poor, and long-term complex environmental impacts will cause significant damage to the pavement. This article tests the dynamic response characteristics of semi rigid and inverted asphalt pavement through road load tests, and measures the humidity data of the roadbed during on-site rainfall. In addition, the variation of pore water pressure in the transition layer under the coupling effect of humidity and dynamic load was analyzed, revealing the influence of seasonal factors on the dynamic response of the pavement and roadbed. The test results indicate that the humidity inside the roadbed is greatly influenced by seasonal factors, and the humidity conditions of the roadbed and pavement vary significantly due to differences in measurement point depth, season, and rainfall. Graded crushed stone cushion is beneficial for improving the humidity conditions of the roadbed. The pore water pressure of the graded crushed stone transition layer did not show significant pore water reabsorption throughout the entire loading process. Meanwhile, the thickness of the surface layer and the magnitude of the load have a significant impact on the measurement of pore water pressure in the transition layer. The measured values of the dynamic response indicators of the pavement are greatly influenced by seasonal factors. The research in this article will provide theoretical and guiding significance for the dynamic response characteristics of pavement under the influence of multiple factors in the southwestern hilly area.

Triaxial tension and compression tests on saturated lime-treated plastic clay upon consolidated undrained conditions

Lime-treatment of clayey soil significantly increases its shear and tensile strengths. Consequently, the tensile strength of lime-treated soils deserves careful investigation because it may provide an appreciable benefit for the stability of earth structures. This study investigates the tensile and shear strengths of an untreated and lime-treated (3% of lime) plastic clay at different curing times (7 d, 56 d and 300 d), through triaxial tension and compression tests. Triaxial tension tests are performed using “diabolo-shaped” soil samples with reduced central section, such that the central part of the specimen can be under axial tension while both end-sections remain in axial compression. Consolidated undrained (CU) conditions with measurement of pore water pressure allow analyzing the failure conditions through effective stress and total stress approaches. The results of triaxial tension tests reveal that the failure occurs under tensile mode at low confining pressure while extensional shear failure mode is observed under higher confining pressure. Consequently, a classical Mohr-Coulomb shear failure criterion must be combined with a cut-off tensile strength criterion that is not affected by the confining pressure. When comparing shear failure under compression and tension, a slight anisotropy is observed.

Influence of type of drainage boundary on coefficient of horizontal consolidation

Vertical drains are used widely to accelerate the consolidation of soft clay deposits when preloading is used as a ground improvement technique. One of the essential input parameters required in Barron's theory is the coefficient of horizontal consolidation, ch. The values of ch can be determined by the radial consolidation test, using either a central sand drain or a porous plastic peripheral drain. This paper presents the laboratory tests carried out to understand the reason for the difference in values of ch determined from inward and outward radial flow consolidations tests. A 150 mm wide instrumented consolidation cell was used to carry out the inward or outward radial consolidation tests. The total stress measurements during consolidation showed non-uniform stress distribution in clay with higher effective stress values close to the drainage boundary. This stiffening of the clay close to the drain retards the consolidation rate resulting in reduced values of ch. As a result, the ch values determined by radially outward consolidation tests with a larger drainage boundary area are lower to those obtained by inward radial flow tests. The pore water pressure measurements showed significantly higher undissipated pore water pressure away from the drainage boundary for the outward flow tests.

One-Dimensional Consolidation Test with Pore Pressure Measurements — An Accelerated Procedure

Abstract Consolidation properties of cohesive soils are often determined using the controlled-strain loading (CSL) consolidation test. It was well recognized in the literature that the consolidation properties, such as compression index (Cc), coefficient of consolidation (cv), and preconsolidation pressure (σc′), depend on the axial strain rate adopted for conducting the CSL test. Fixing the appropriate axial strain rate is often a challenge in CSL tests. The conventional incremental load (IL) consolidation test suffers from the limitation that it takes long time to complete the test. In the present study, an accelerated IL consolidation testing procedure with pore water pressure measurements is developed in an attempt to overcome the limitations of the CSL and conventional IL consolidation tests. The duration of the IL is controlled such that the subsequent IL is applied once the excess pore pressure dissipates to 15 % of the total applied stress (pore pressure ratio, Ru, is 0.15). The testing procedure is validated by performing a series of experiments on six reconstituted and four intact soil samples. The consolidation parameters obtained from the proposed method compares very well with conventional IL test as well as with the CSL consolidation test, suggesting its validity. The proposed method is about 3 times faster when compared with the standard CSL test. Therefore, the proposed method is a viable alternative to the CSL consolidation test.

Wireless Mid-height Pore Water Pressure Sensor

Abstract The triaxial apparatus is one of the most widespread testing devices used by geotechnical laboratories for identifying parameters characterizing soil strength and stiffness behavior. In order to do so, the triaxial apparatus has the ability to measure a number of key parameters, among which the measurement of axial force, axial strain, and pore water pressure in the soil play a key role. By default, the measurement of the latter parameter is carried out at the bottom of the sample or, in its more advanced form, at the mid-height of the sample. The second approach, however, introduces the requirement to break the continuity of the membrane surrounding the soil specimen, which can lead to water penetration from the cell into the sample. Nevertheless, this inconvenience can be avoided by using a novel sensor that measures water pressure in the soil pores wirelessly. In addition, as the membrane does not need to be perforated, sample preparation time can be reduced. This article presents the integration of the novel sensor into a triaxial apparatus and its validation in the form of a series of tests performed on several soil types to demonstrate the effectiveness and performance of the wireless water pressure sensor.

Application of Hardening State Parameter Constitutive Model for Prediction of Overconsolidated Soft Clay Behavior Due to Embankment Loading

This paper investigates the possibility of the application of the HArdening State Parameter (HASP) constitutive model for numerical modelling of overconsolidated soft clay under embankment loading. The HASP constitutive model is a critical state soil model with a combined hardening rule that uses a state parameter to determine the dilatancy of overconsolidated clay. The model overcomes some shortcomings of the Modified Cam Clay (MCC) model in the prediction of overconsolidated clay’s behavior, while preserving the simplicity and the same set of five parameters. The HASP model was implemented in finite element software. In order to verify the applicability of the model in predicting the behavior of soft overconsolidated clay due to embankment loading, two examples reported in the literature are analyzed. The numerical predictions of the HASP model are compared with the field measurements of ground settlements and pore water pressures, and with the MCC model’s predictions. The results indicate that the HASP model predicts the development of the settlements of the overconsolidated soft clay deposits with a high accuracy from an engineering point of view. There are certain deviations from the field measurements in predicting the pore pressure development, which is often observed for other models as well. For the embankment settlement assessment, as important serviceability issue, the HASP model has an advantage over more complex models that require a large number of parameters. Since the parameters of the HASP model are obtained from standard laboratory tests, it can be easily applied for routine geotechnical analyses.

Strength of partially frozen sand under triaxial compression

To investigate the effect of the pore matrix (i.e., ice and unfrozen water) on the strength of partially frozen sand, a series of triaxial compression tests with internal pore water pressure (PWP) measurements were performed. Both dense and loose saline sand samples were prepared by rapidly freezing the samples and then slowly warming the samples to, and subsequently shearing at the temperature of −3 °C. Test results indicate the temperature only affects the effective cohesion, not the effective friction angle of sand. The pore ice stress was estimated using Ladanyi and Morel’s 1990 postulate on the internal stresses within frozen soil; the stress (hydrostatic) change in pore ice was reflected by an equal change in PWP in the partially frozen loose sand. Based on Ladanyi and Morel’s 1990 concept of internal confinement in frozen sand, a Mohr–Coulomb model that uses effective failure and residual friction angles from unfrozen sand to estimate the strength of partially frozen sand is presented. The proposed model reflects how the pore matrix contributes to the strength of partially frozen sand. For loose sand, the peak strength is enhanced by the internal confinement and cohesion resulting from the pore water (suction) and pore ice, respectively. For dense sand, only pore ice affects the peak strength by incorporating the internal confinement and cohesion.

Impact of beach face slope variation on saltwater intrusion dynamics in unconfined aquifer under tidal boundary condition

Density-dependent saltwater flow in coastal aquifers varies with the seaside tidal and beach slope conditions. This paper presents a set of laboratory-scale saltwater intrusion experiments under different beach slopes (15°, 20°, 25°, and 30°) for unconfined aquifer conditions. Experimental porewater pressure measurements were utilized for quantitative analysis. A new G channel-based image analysis technique is used for experimental image analysis and freshwater–saltwater interface identification. Experimental results were numerically validated using the two-dimensional Finite Element subsurface FLOW (FEFLOW) model. The time-varying analysis revealed that saltwater intrusion occurs rapidly on flatter beach slopes. In the case of a steeper beach slope, saltwater intruded less. Steeper slopes take more time to reach a quasi-steady condition. Tidal oscillations alter hydraulic gradient changes across the beach slope. This gradient change generated clockwise circulating saltwater flow within porous media in the inter-tidal zone. This circulating flow resulted in the formation of the upper saline plume (USP). The USP expanded with time and moved in a downward direction. Finally, a deformed elliptic-shaped USP was observed under quasi-steady-state conditions. Submarine groundwater discharge (SGD) pathways move the saltwater region through the intermediate zone of a saltwater wedge and USP on varying beach slopes. It is evident that SGD particles move along the saltwater–freshwater interface (SWI) zone and rise upward (up to the intersection point). The tracer experiments were started after attaining the quasi-steady state condition and continued till the tracer reached the intersection point of saltwater level and sloping beach face. The experimental data sets can be used as benchmark test cases.

Measurement of Excess Pore-Water Pressure in Frozen Soil at Subfreezing Temperatures Close to 0°C

Response of pore-water pressure has crucial effect on the engineering geological performance of permafrost ground. However, there is a dearth of literature on the measurement of pore-water pressure in frozen soils due to challenges with regard to technical and operational method. To validate the generation of pore-water pressure and to reveal its variation property in frozen soils at subfreezing temperatures close to 0°C, a miniature pressure transducer was utilized to conduct a number of confined compression tests. Double-wall structure avoided damage to the transducer during frozen soil sampling. The transducer at sample bottom perceived a pressure fluctuation as varying air pressure imposed on sample surface, proving the connectivity among the pores in frozen soils with a hysteretic effect on pressure transmission. Pore-water pressure experienced a progressive increase followed by a slow dissipation under invariable load and temperature conditions when water drainage was permitted, while a monotonic increase in pressure was observed for an undrained sample. With decreasing temperature, a smaller peak and a more dilatory dissipation were displayed. When the sample was exposed to a stepwise warming process, a similar performance for the pressure was obtained with abrupt rise at the turning point of temperature.

Simple Shear Strength Analysis of Inherent Anisotropy for a Tropical Alluvial Soil

ABSTRACT Tropical alluvial soils have unusual engineering properties and their behavior is not typical of fine-grained soils. The literature regarding strength properties of alluvial soils is limited. This study presents an experimental evaluation of inherent anisotropy of undisturbed tropical alluvial soil samples by comparing the direct simple shear (DSS) test results of 52 specimens cut in horizontal and vertical orientations. The tests were carried out under consolidated undrained constant volume with excess pore water pressure measurements. The peak shear strength, effective stress paths, and secant shear modulus analyses showed that the difference in properties between horizontally and vertically cut specimens was negligible. Nonetheless, the results for vertically cut specimens presented a higher random error compared to the results for horizontal specimens in all aspects. A good relationship between the undrained shear strength and over-consolidation ratio was found but was not comparable to past DSS results for natural clays. Scanning electron microscopy images revealed flocculated edge-to-edge and edge-to-face particle associations. Directionality analysis of microfabric showed that the alluvial soils had low preferred orientation of particles. The general notion that all alluvial soils are highly anisotropic was not evident, based on the experiments reported in this study. The findings herein are useful for geotechnical engineers to better understand the strength behavior of tropical alluvial soils.

Measurement of Physical Parameters for Transient Seepage Assessment of Levees

Abstract An embankment was instrumented and monitored as part of a multiyear project to assess appropriate pore-water pressure distributions and soil water retention curves for use as initial conditions in transient seepage analyses. The increased use of unsaturated soil mechanics and transient seepage has led to the need for practical methods of assigning accurate properties and initial conditions in these types of analyses. Monitoring results were validated against published pore-water pressure measurements collected from multiple embankment studies. The collected distributions showed nonlinear behavior in near-surface embankment soils due to infiltration and evapotranspiration processes. Seasonal variations in pore-water pressure and the soil moisture deficit were shown to correlate at the monitoring site. The seasonal response of the embankments exhibited a large variation in pressures between summer and winter. Assessment of flood occurrence showed the distribution of pore pressures during flood loading was typically near −10 kPa at a depth of 2 m at the onset of the flood. Laboratory soil water retention testing of undisturbed samples collected from the embankment showed that when compared with field measurements, they overestimated the field response for the considered initial water content and dry density values. Also, comparisons between hydrostatic pore pressures above the phreatic surface showed that at soil moisture deficits greater than 20 mm, the difference between the measured and hydrostatic pore pressures greatly increased. The laboratory and monitoring results were used to provide practical methods of assigning representative pore-water pressure distributions and material properties necessary for performing transient seepage analyses. These results will aid in more accurate transient seepage analyses of flood control embankments.

Possible Effects of Multiphase Methane Evolution During a Glacial Cycle on Underpressure Development in Sedimentary Basins: An Analysis With Application to the Northeast Michigan Basin

In low-permeability systems, groundwater may be accompanied by separate-phase fluids, and measured pore water pressures may deviate from those expected in steady-state, single-phase systems. These same systems may be of interest for storage of nuclear waste in Deep Geologic Repositories. Therefore, it is important to understand the relationship between the presence of a separate phase and anomalous pressure development. At the Bruce site in Southern Ontario, a significant underpressure was observed, and there is evidence for the presence of gas-phase methane in situ. This study used a one-dimensional (vertical) numerical model of the subsurface down to a depth of 844 m beneath the Bruce site to evaluate possible effects of hydromechanical coupling with multiphase flow on pressure evolution during glacial loading and unloading. The simulated pressure conditions were affected strongly by the amount of methane initially present in the system, and the maximum simulated underpressure varied nonmonotonically with increasing initial methane content. When the initial methane content was below the solubility limit, exsolution led to underpressures that briefly exceeded those that formed in the single-phase case. At intermediate initial methane contents (sufficient to produce an immobile gas phase), the gas phase dampened the hydromechanical effects of the glacial cycle. At large initial methane contents (when a mobile gas phase was present), gas migration caused a large decrease in relative liquid permeability, which further contributed to underpressure development in the pore water. Multiple scenarios that spanned a range of initial methane contents yielded underpressures like those observed at the Bruce site.

Analysis of Load Transfer Law of Short Pile-Net Composite Foundation for High-Speed Railway

Although pile-net composite foundation has been widely used in the construction of high-speed railway, load transfer characteristics of its structural system during the filling of roadbed are seldom studied, so field studies are conducted in the Gan-Long (Ganzhou-Longyan) high-speed railway DK86 + 998.0–DK87 + 191.2 test section, aiming to explore the bearing characteristics of a short pile-net composite foundation over low to medium compressible soil. The study encompasses a field measurement of Earth pressure and pore water pressure which lasted almost two years, and a systematic analysis of variation law of soil pressure between piles and their tops and that of pore pressure in reinforcement zone and retaining layer in medium and low compressibility soil area. The results suggest that the soil in the test section bears the characteristic of a low water content, low porosity ratio, and high liquid limit, with compression factor being approximately 0.25, and test values of the natural soil pressure and the lateral distribution are close to the theoretical values. Soil pressure at the top of the piles is higher than that between the piles by approximately 2.4 times because of the differences in pile-net rigidity. Besides, the soil pressure of the pile-net composite foundation follows the pattern of “jagged” in the transverse direction of the roadbed. The filling load of the composite foundation soil arch is 80 kPa and the composite foundation reaction coefficient is 6.4 kN/m3 when the filling is stable. The pile-soil stress ratio at the shoulder and the center of the line is 3.2 and 2.8, respectively. The change of the hole pressure at the pile end point is larger than that at the reinforcement area, and the side verification short pile can effectively transmit the additional stress of the upper part to the pile end soil layer.