Pore Water Pressure Measurements Research Articles

An alternative method in triaxial tests to obtain cross-anisotropic elastic parameters

A new, alternative approach in a triaxial set-up to fully determine small-strain cross-anisotropic elastic parameters is proposed, which requires neither radial strain sensors nor a radial loading process. This was made possible by additionally measuring pore water pressure responses during undrained axial loading, and inputting them into theoretical expressions derived based on cross-anisotropic elasticity theory. Although the method is intended for saturated soils, slight undersaturation in the system is accounted for by incorporating the pore water pressure coefficient B into the parameter deduction process. Being free from the cumbersome radial sensors and ultra-precise cell pressure controlling devices/operations, the proposed method is applicable even in less specialised laboratories. Four different deduction routes of elastic parameters were applied to results from tests on two reconstituted fine-grained soils. The horizontal Young's modulus obtained with the newly proposed method is consistent not only with those based on radial measurements, but also with those directly measured by axial loading on horizontally cut specimens. A challenge is still open for precisely determining Poisson's ratios, which exhibited only partial matching between different measurement methods. Overall, however, the proposed method is consistent with those adopting full local instrumentation.

A New Suction-Controlled True Triaxial Apparatus for Unsaturated Soil Testing

A new suction-controlled true triaxial apparatus was developed to investigate unsaturated soil behavior under multiaxial stress paths that are not readily achievable in a cylindrical cell. This apparatus is a mixed-boundary type device with a rigid-flexible-flexible boundary that can apply three principal stresses to a cubic specimen via two rigid platens along the vertical direction and four flexible hydraulic bags on the sides. The apparatus was modified based on a previous true triaxial apparatus designed at Xi’an University of Technology, with the addition of four division strips to avoid loading boundary interference. The new apparatus is capable of testing specimens with dimensions of 7 by 7 by 14 cm. The present apparatus was equipped with an independent pore-air pressure control system and pore-water pressure measurement system. Based on the axis-translation technique, a 5-bar high air entry value ceramic disk is seated on the pedestal, and a matric suction state was achieved that remains constant throughout the laboratory tests. Hence, this true triaxial apparatus can be used to conduct suction-controlled experiments. The suitability of this device was validated via two sets of three repeated trial tests and several sets of trial tests performed with remolded loess for different intermediate principal stress parameters under 50, 100, and 200 kPa matric suction values and different net mean pressure conditions. The results presented the stress-strain characteristics and failure loci of remolded loess.

Experimental insights into consolidation rates during one-dimensional loading with special reference to excess pore water pressure

Consolidation rate has significant influence on the settlement of structures founded on soft fine-grained soil. This paper presents the results of a series of small-scale and large-scale Rowe cell consolidation tests with pore water pressure measurements to investigate the factors affecting the consolidation process. Permeability and creep/resistance structure factors were considered as the governing factors. Intact and reconstituted marine clay from the Polish Carpathian Foredeep basin as well as clay–sand mixtures was examined in the present study. The fundamental relationship correlating consolidation degrees based on compression and pore water pressure was assessed to indicate the nonlinear soil behaviour. It was observed that the instantaneous consolidation parameters vary as the process progresses. The instantaneous coefficient of consolidation first drastically increases or decreases with increase in the degree of consolidation and stabilises in the middle stage of the consolidation; it then decreases significantly due to viscoplastic effects occurring in the soil structure. Based on the characteristics of the relationship between coefficient of consolidation and degree of dissipation at the base, the consolidation range that complies with theoretical assumptions was established. Furthermore, the influence of coarser fraction in clay–sand mixtures in controlling the consolidation rates is discussed.

Experimental and numerical investigation of saltwater intrusion dynamics on sloping sandy beach under static seaside boundary condition

Two-dimensional sandbox experiments were conducted to investigate the variable-density circulation and flow patterns in sloping beach configurations. The experiments provide new benchmark results for validating the sandbox models based on quantitative and qualitative measurements. Previous studies have considered density dependent flow in porous media, vertical beach face saltwater boundary, and multilayered hydrogeology ignoring a sloping beach face, which is a much more common phenomenon in real world. The present study considers sloping beach face under both homogeneous and low-permeability strata configurations. The geohydraulic processes encountered were quantified through pore-water pressure measurements and image analysis techniques. Moreover, validations were performed with numerical simulations (FEFLOW). A simple image analysis procedure is proposed with respect to two-dimensional laboratory scale benchmark experiments. Experimental results provided a detailed circulation flow path within and outside the saltwater wedge with sloping beach face. Fingering effect in porous media was also observed for both the experiments during initial time periods. Stability analysis shows the existence of a stationary convective flow pattern followed by gravitational instabilities under the quasi-steady state condition.

Application of machine learning to the identification of quick and highly sensitive clays from cone penetration tests

Geotechnical classification is vital for site characterization and geotechnical design. Field tests such as the cone penetration test with pore water pressure measurement (CPTu) are widespread because they represent a faster and cheaper alternative for sample recovery and testing. However, classification schemes based on CPTu measurements are fairly generic because they represent a wide variety of soil conditions and, occasionally, they may fail when used in special soil types like sensitive or quick clays. Quick and highly sensitive clay soils in Norway have unique conditions that make them difficult to be identified through general classification charts. Therefore, new approaches to address this task are required. The following study applies machine learning methods such as logistic regression, Naive Bayes, and hidden Markov models to classify quick and highly sensitive clays at two sites in Norway based on normalized CPTu measurements. Results showed a considerable increase in the classification accuracy despite limited training sets.

Excess Pore Water Pressure along the Friction Sleeve of a Piezocone Penetrating in Clay: Numerical Study

Abstract Excess pore water pressure (EPWP) measurements taken at different positions on a penetrating piezocone into a clayey soil are believed to play an important role in correlating to its chara...

Determination of consolidation parameters based on the excess pore water pressure measurement using a newly developed U-oedometer

In this study, the U-oedometer, a novel modified oedometer cell equipped with tailor-made needle probes, is developed to easily and accurately measure the excess pore water pressure ( $$\Delta u$$ ) during 1D consolidation tests and to determine the coefficient of consolidation ( $$c_{\text{v}}$$ ). The 3D printing technique is applied to make simple yet robust modifications to the conventional oedometer cell for facilitating the installation of the needle probes. The tailor-made needle probes are designed in such a way that the volumetric compliance is lowered to avoid measurement bias. Subsequently, the $$\Delta u$$ -based method is proposed to determine $$c_{\text{v}}$$ , with the target of avoiding the intervention of human judgement and therefore minimizing the degree of subjectivity. The experimental results demonstrate that the measured $$\Delta u$$ matches the theoretical values of the Terzaghi 1D consolidation theory, showing that the estimated $$c_{\text{v}}$$ is sufficiently reliable. In addition to the determination of $$c_{\text{v}}$$ , the U-oedometer allows additional measurements of other soil properties during consolidation, including the coefficient of permeability ( $$k$$ ) and the coefficient of earth pressure at rest ( $$K_{0}$$ ). It is observed that k decreases with the reduction in void volume, due to the increase in the effective vertical stress ( $$\sigma_{\text{v}}^{'}$$ ). Further, the secondary compression seems to be a continuation of the primary consolidation, where the soil sample continues to deform at a relatively slower rate, associated with the slight decrease in $$k$$ . A constant value of $$K_{0}$$ is observed at any value of $$\sigma_{\text{v}}^{'}$$ in the loading path, while during secondary compression, $$K_{0}$$ slightly increases with time.

Measurement of plant xylem water pressure using the high-capacity tensiometer and implications for the modelling of soil–atmosphere interaction

The response of shallow geotechnical structures is affected by interaction with the atmosphere. Since the ground surface is very often vegetated, plant transpiration plays a major role in such an interaction. Transpiration in geotechnical applications is generally modelled by way of a transpiration reduction function (e.g. the Feddes function). However, its parameters are generally borrowed from the agricultural literature, where the focus is on crop species and often loosely compacted organic agricultural soils. For the non-crop species in denser soils typically encountered in geotechnical applications, monitoring of the flow taking place in the soil through the xylem up to the leaves can potentially be exploited to characterise the transpiration reduction function. The main challenge is the measurement of the water pressure in the xylem. Techniques currently used include the pressure chamber and thermocouple psychrometer. The pressure chamber is destructive and thus not suitable for continuous monitoring and/or where a relatively small number of leaves is available (as often occurs in laboratory experiments). The thermocouple psychrometer is not accurate at low water tension, is affected by the presence of solutes in the xylem water and is significantly sensitive to temperature. This paper explores a novel application of the high-capacity tensiometer (HCT), initially developed for pore-water pressure measurement in soils. The HCT was installed on the stem or branch of different trees and its measurement validated against pressure chamber measurements over a range of xylem water pressure down to −1300 kPa. In addition, its measurement was used to investigate the response of the soil–plant continuum. Results show that the HCT is a viable and convenient instrument to use for xylem water pressure measurement and can provide field-based data for the modelling of plant transpiration. Installing HCTs on stems and branches is quite straightforward and this will help achieve a step change in testing and modelling the effect of plant transpiration on the soil water regime in the vadose zone.

Database on seismic response of instrumented flood control levees

We present a database documenting the seismic response of seven instrumented flood control levee segments in Japan. The database includes (1) maps and cross-sections showing levee geometry and instrument layouts; (2) ground motion time series, and (where available) piezometric recordings; (3) reported field performance from post-earthquake reconnaissance (i.e. permanent displacements, other evidence of ground failure); and (4) geotechnical data, including geologic cross-sections, stratigraphy, penetration resistance, and shear wave velocity ( VS) profiles. The data are complete with respect to these attributes for three sites, with geotechnical data and field performance data missing for two and four sites, respectively. The database contains 150 recordings from 25 events and can be accessed through DOI: 10.21222/C2TC95. Ground motions were processed using standard procedures for Next Generation Attenuation projects. We present example data for a segment damaged by the M9.1 2011 Tohoku-oki earthquake, which includes ground motion and pore water pressure measurements.

Relationship between earthquake-induced excess pore water pressure and strong ground motion observed in a monitored fill slope

The generation of excess pore water pressures during ground shaking can be an important earthquake-induced landslide trigger. We measured pore water pressures and seismic ground motion in filled slopes and examined their relationships obtained from 29 observed earthquakes. We characterized pore water pressure and seismic waveforms by calculating several parameters that described their magnitudes and durations. We demonstrate that whilst the instantaneous magnitude of strong ground motion is responsible for the generation of excess pore water pressure, the magnitude of excess pore water pressure generated is related to the peak ground acceleration (PGA) and Arias Intensity (AI) in the observed PGA range of 0.12–1.9 m/s2. The magnitude of peak ground velocity (PGV) also produced a strong correlation although a few exceptional events where long period component (>1 s) of seismic waves is dominant were observed. Lastly, the time-span over which excess pore water pressure continues to rise is affected by the duration of seismic ground motion. Our study shows how excess pore water pressures can be generated in fill-slopes during seismic events. This new knowledge could be used to improve future earthquake-induced landslide hazard assessment.

Analysis of oedometer and rowe cell consolidation compared to experimental testing

Consolidation settlement in clay soils can be predicted by analyzing the consolidation parameters. Consolidation parameters were obtained from the results of a laboratory consolidation test using an oedometer and rowe cell. The rowe cell can measure pore water pressure, soil saturation, and back pressure as a simulation of field condition while the Oedometer cannot. However, there are significant differences between the results of the analysis with the consolidation settlement occurred in the field caused by drainage conditions, the micro characteristics of the soil structure, and the thickness of the soil layer. Therefore, it is necessary to validate the results of settlement in oedometer and rowe cell consolidation analysis with the value of settlement of experimental results. The method used is a laboratory consolidation test using an oedometer and rowe cell as well as a laboratory-scale experimental consolidation test. The results obtained 79% of differences between experimental and oedometer, while rowe cell had a difference of 37% from experimental. Based on this, analysis using rowe cell consolidation parameters can predict a settlement in consolidation in the field more accurately than oedometer, because it has a smaller difference in value.

Regional assessment of prehistoric earthquake magnitudes in the South Carolina Coastal Plain

The magnitudes of prehistoric earthquakes that created liquefaction features (i.e., sandblows) in the South Carolina Coastal Plain (SCCP) are not exactly known. In this study, in situ geotechnical data, including cone penetration test (CPT) data with pore water pressure measurements, in the vicinity of discovered sandblows at Hollywood, Sampit, Gapway, and Fort Dorchester sites located in the SCCP were used with the cyclic stress method and Ground Motion Prediction Equations (GMPEs) to back-calculate the minimum earthquake magnitude (M) and peak ground acceleration (amax) to form a regional assessment of minimum amax-M in the Charleston area. Results indicate that the minimum earthquake magnitude of the prehistoric earthquakes associated with the Charleston Source ranges from 6.6 to 7.2 for the earthquake that occurred about 546 ± 17 years before present (B.P.) and from 6.2 to 6.7 for the earthquake that occurred about 5038 ± 166 years B.P. Results also show that the minimum earthquake magnitude for the prehistoric earthquakes with a source inferred to the Sawmill Branch fault and occurred at least 3500 years ago, ranges from 5.1 to 5.7.

Pore pressure profiling in Siercza landslide colluvium in the Carpathian flysch using a Cone Penetration Test (CPTU)

This study identifies zones with significant changes in pore water pressure influenced by landslide processes. Measurements were taken in the near-surface strata of the Carpathian flysch, in colluvium of the Siercza landslide, near Krakow. Measurement of pore water pressure in flysch deposits is complicated due to the strongly heterogeneous properties of the medium and by variable water conditions, which are strongly influenced by rainfall intensity. Pore pressure profiling was performed in six series using a cone penetration test with a NOVA Acoustic cone. The tests were carried out in the colluvium to a depth of ~6.0 m under varying water conditions. The cone pore pressure results were compared to results of inclinometer measurements in the research area. Five zones with significant differences in pore pressure have been identified. Changes in both cone pore pressure and inclinometer displacement are evident at a depth range from 1.5 to 2.5 m. Two slip surfaces are likely present in this section. Such information can be used in engineering practice for more reliable assessment of slope stability in the Carpathian flysch

Behavior of Swelling Soil Treated by Grid Geocell Columns under a Tank Footing

Fuel tanks usually induce large settlements due to heavy loading which requires treatment for the underlying soils. In this paper, a method of treating the swelling of expansive soil is presented. The method is simply based on the embedment of a geogrid (or a geomesh) in the soil. The geogrid is extended continuously inside the volume of soil where the swell is required to be controlled and orientated in the direction of swell. Soils with different swelling potentials were employed; bentonite-based Na and kaolinite mixed with bentonite. To investigate the swell, swell- partial shrinkage and other phenomena for untreated soils in comparison with their counterparts of treated soils, laboratory model tests were carried out in 800 × 200 mm box container and 600 mm in height. Swelling and cyclic wetting along with drying tests were carried out on each sample with pore water pressure measurement. Swelling test in the model reveals promising results of the proposed treatment technique. The improvement factors, range between 30% to 60 % depending on the geogrid stiffness, soil plasticity, fill martial (sand or clay) and dry density of sand fill. The geocell has a significant effect on the experimental results. The reduction in swell increases with increasing the geogrid stiffness and when using sand as a fill material, the reduction is apparently due to a strong interference bond which restricts the relative movement between the clay and the grid. The use of geocell filled with the same expansive soil causes a decrease by about 19% and 42 % in the final swelling of soils, whereas filling the cell with sand causes about 35 % and 64 % reduction for the soils, respectively. The treatment method shows the activity of geocell fill material in reducing the matric suction (pore water pressure). When sand is used, a new channel is created for the dissipation of pore water pressure.

Load-indentation testing to assess porewater pressure effects on linear drag and disc cutting of a saturated permeable brittle rock

In this study load-indentation tests with porewater pressure measurement were conducted to explore the unexpected differences owing to water saturation in the linear cutting of a permeable quartzose sandstone using drag and disc type cutters. This was done to evaluate the probable role of porewater pressure in rock fragmentation by drag and disc type cutters. The contrasting response to water saturation of these two types of cutters might be explained by the effects of their different fragmentation mechanisms, such as the relative size of the crushed zone that forms beneath the cutters. The relationship between cutting speed and rock permeability was expected to be a major factor influencing the effective pressure beneath a cutter in saturated rock. However, load-indentation tests with pore pressure measurement at the same speed showed that the pore pressure within the tested sandstone remained too low to affect the rock fracture process. Other possible mechanisms are discussed.

A finite element benchmark study based on experimental modeling of vibratory pile driving in saturated sand

The present study reports the approach and extensive results of FE back-calculations of model tests on vibratory pile installation in saturated sand. The model tests represent a half model setup with a half-pile driven into the soil body along an observation window. Due to this test setup, the soil displacements in vicinity to the penetrating pile have been successfully evaluated by application of high-frequency image recording and Digital Image Correlation (DIC) techniques. Additionally, an extensive instrumentation has enabled the measurement of pile motion, penetration resistance and pore water pressure during the penetration process. An axisymmetric model which incorporates the pile-oscillator system in contact with the soil has been developed in the FE-software Abaqus/Standard for the simulation of the tests. The dynamic analysis of water-saturated soil is based on a u-p formulation using a user-defined element. A hypoplastic constitutive model with intergranular strain has been selected to describe the mechanical behavior of the soil. The various comparison opportunities establish a comprehensive and quantitative validation benchmark of the numerical calculations. The generally good agreement between the results confirms that the pile installation process can be satisfactory reproduced numerically.

Influences of root concentration and suction on Chrysopogon zizanioides reinforcement of soil

The influences of root concentration and suction were investigated on Chrysopogon zizanioides (vetiver grass) root-reinforcement of clayey sand, using suction-monitored direct shear tests at four different suctions ranging from zero to 50 kPa. The vetiver grass specimens were grown in containers for about a year until they showed various root concentrations prior to testing. Of all suctions, soil samples with greater root concentrations had a higher peak strength and were more ductile than those without roots. Unsaturated root-reinforced soils contracted while the non-reinforced soils dilated during shearing. The rate of increase in root cohesion with root concentrations were highest at the suctions around 20 kPa. The rates of increase in shear strength with suction were similar for non-reinforced soils and reinforced soils up to a threshold in root concentration beyond which the rate became slightly smaller. The obtained relationships between root area ratio and strength were used in the infinite slope stability calculation of an instrumented slope based on field pore-water pressure and mini-rhizotron measurements. The analysis showed that in unsaturated condition, the stabilizing effect of suction was greater than that of roots, while in saturated condition, the stabilizing effect of root reinforcement was more important especially for shallower depth.

Seasonal influence on cone penetration test: An unsaturated soil site example

Interpretation of electric cone penetration test (CPT) based pore water pressure measurement (CPTu) is well established for soils with behavior that follows classical soil mechanics. The literature on the interpretation of these tests performed on unsaturated tropical soils is limited, and little is known about the influence of soil suction on in situ test data. In this context, the CPT data are presented and discussed to illustrate the seasonal variability in an unsaturated tropical soil site. The test data show that soil suction significantly influenced CPT data up to a depth of 4 m at the study site. It shows the importance of considering seasonal variability in unsaturated soil sites caused by soil suction, which was related to water content through a soil-water retention curve (SWRC). It is also important to consider this aspect in the interpretation of CPT data from these soils.

Field measurement of pore pressures and liquid-gas distribution using drilling and ERT in a high food waste content MSW landfill in Guangzhou, China

Systematic field measurements were carried out at the sliding area of the Xingfeng Landfill, which is a large high food waste content municipal solid waste (HFWC-MSW) landfill. Sixteen boreholes were drilled and pore pressure transducers, gas pressure pipes and inclinometer pipes were installed to measure pore water pressures, pore gas pressures and waste displacements, respectively. Two measuring lines of electrical resistivity tomography (ERT) were arranged to analyze the liquid-gas distribution in the waste mass. In addition, a stability analysis was conducted on the basis of the measured results. The main findings are as follows: (i) In the regions at the depths of 5–15 m in the waste mass, the saturation level of the waste was relatively high, and high gas pressures were not evident. However, in the regions at depths blow 15–20 m, the waste was unsaturated, and there were obvious local high gas pressures, (ii) The low-water-content and high-water-content zones obtained by ERT corresponded to the low-saturation and high-saturation zones inferred by the measurements of the water and gas pressures, respectively, indicating the consistency between the results of ERT and drilling, and (iii) By field measurement and stability analysis, the correlation between high gas pressures and sliding was confirmed, indicating high gas pressures may be the critical cause for the frequent waste sliding. In addition, the possible causes for the liquid-gas distribution and high gas pressures in the waste mass were discussed by considering the biodegradation characteristics of HFWC-MSW. This study provides the references for field measurement and stability control in similar landfills.

Experimental evidence about misconception of Terzaghi's 1-D consolidation theory in terms of degree of consolidation

Terzaghi's 1-D consolidation theory is commonly used for prediction of embankment settlement in soft fine-grained soil. Application of this theory is not always effective due to the simplifications of assumed soil behavior under load. This paper examines relationship between the degrees of consolidation determined by pore water pressure measured at the base of the sample and one-dimensional strain. Theoretically, this relationship should be unique, however experimental relationships correlating the degrees of consolidation are found to be non-linear. The article presents the results of multistage consolidation tests with pore water pressure measurements, carried out on intact and reconstituted deep marine clay. Consolidation tests were conducted with uniform stress distribution and single-sided drainage conditions. It was observed that the relationship between degrees of consolidation for intact and reconstituted samples largely divergent from theoretical unique line.