Pore Water Pressure Transducers Research Articles

Mechanism of a rainfall-induced landslide in a large-scale flume experiment on a weathered granite sand

IntroductionsA large-scale flume experiment was performed to evaluate the mechanism of landslide occurrence due to rainfall using weathered granite sand. The dimensions of the flume were 9 m (length), 1 m (width), and 1 m (depth). The weathered granite sand from the actual landslide site at Da Nang City, Vietnam was used. The pore water pressure was measured by a pore-water pressure transducer at two depths (middle and bottom) to determine the process of rainwater infiltration into the soil. The surface deformation was measured with extensometers at three positions of the slope. The deformation of the entire slope was determined by the 160 cylindrical-shaped makers evenly spaced in the slope and three cameras.ResultsThe results showed that the rainfall infiltrated into the slope process, increasing from negative pore water pressure to approximately 0. The maximum shear strain contour has been plotted in total and in time increments. The shear band was detected from the time increments maximum shear strain contour. The localization in the shear band formed just before failure.ConclusionsTo the best of our knowledge, this is the largest scale laboratory test ever conducted to calculate the shear band. Moreover, it was found that the failure occurred when the sand was in an unsaturated phase. Failure does not seem to depend on the increase in pore water pressure but on the maximum shear strain. This feature can be used to explain the phenomenon of landslides that occur even when the groundwater level does not increase but large deformation occurs.

A systematic approach for conducting and interpreting hydraulic conductivity tests on granular soils under non-isothermal conditions

Although non-isothermal tests are important for many applications in geotechnical engineering, there is no standard for conducting such tests and interpreting the resulting data. This paper describes the development of a temperature-controlled triaxial apparatus, focussing on its thermal performance, and discusses relevant protocols to perform and interpret hydraulic conductivity tests on granular materials at different temperatures. With the aid of thermal cameras, hot spots on the surface of the equipment and instruments were identified. Subsequent modifications to minimise and mitigate heat reaching volume gauges and pore water pressure transducers were introduced. After modifications to reduce the system’s intrinsic head losses, the thermal expansion of the system proved to be significant and needed to be accounted for avoiding overestimation of thermally-induced mechanical strains. The addition of a new probe at the centre of the specimen allowed the characterisation of the temperature field within the system and specimen, as well as assisting with the identification of thermal equilibrium. Significant drops in temperature were flagged by this probe, though these proved to be unimportant in terms of hydraulic conductivity. The use of the average temperature for each pressure step is advised when a specimen probe is available. Alternatively, the use of target temperatures can be chosen, leading to minor underestimations of the intrinsic permeability.

Analysis of Factors Affecting Railway Settlement of Embankment Section due to Liquefaction Using 1-g Shaking Table Model Tests

Herein, 1-g shaking table tests were performed under test by changing the embankment height and non-liquefiable layer thickness below the embankment for the same input seismic load. Accelerometers, pore water pressure transducers, and linear variable differential transformers were embedded inside the foundation ground or installed on the embankment surface to measure their responses to shaking. The testing procedure for analyzing the behavior of a two-dimensional embankment-ground system and well-documented experimental data are presented. The results show that the settlement of the railway embankment placed on the liquefiable sand ground due to ground liquefaction is greatly affected by the embankment height and non-liquefaction layer thickness, which determine the degree of liquefaction and lateral flow of the foundation ground. The embankment settlement increased with the embankment height due to the liquefaction of the ground and increased by twice the settlement of the ground without embankment when the embankment height was approximately 20% of the total liquefiable sand layer. Moreover, the settlement was reduced by over 20% compared to that without a layer when the layer thickness was 50% of the embankment height. Furthermore, the settlement decreased to a negligible level when it was approximately 1.5 times the height of the embankment.

Soil water retention curve of silty sand – experimental investigation using different laboratory methods

Most of the infrastructure investments and earthworks not requiring deep foundations are usually designed in the vadose zone to avoid excessive groundwater-structure interactions. The mechanical behaviour of soil material, under partially saturated conditions, is greatly influenced by pore-water tension, known as soil suction, and the characteristics of the Soil Water Retention Curve (SWRC). In the present paper, the SWRC of a silty sand was determined using two different experimental methods. In the first method, a modified pore water pressure transducer was used for suction monitoring, while the specimen was allowed to change its moisture content by natural evaporation. For the second method, a modified consolidation cell fitted with a high air entry value ceramic disc on the base pedestal was used. Suction was applied using the axis translation technique by utilising pore air and pore water pressure controllers, while moisture was monitored using a volumetric measurement system. Through the determination of the SWRC for the silty sand, this paper intends to compare the abovementioned testing methods basedon the produced SWRCs and to reveal advantages and limitations.

Centrifugal model test on a riverine landslide in the Three Gorges Reservoir induced by rainfall and water level fluctuation

Frequent soil landslide events are recorded in the Three Gorges Reservoir area, China, making it necessary to investigate the failure mode of such riverside landslides. Geotechnical centrifugal test is considered to be the most realistic laboratory model, which can reconstruct the required geo-stress. In this study, the Liangshuijing landslide in the Three Gorgers Reservoir area is selected for a scaled centrifugal model experiment, and a water pump system is employed to retain the rainfall condition. Using the techniques of digital photography and pore water pressure transducers, water level fluctuation is controlled, and multi-physical data are thus obtained, including the pore water pressure, earth pressure, surface displacement and deep displacement. The analysis results indicate that: Three stages were set in the test (waterflooding stage, rainfall stage and drainage stage). Seven transverse cracks with wide of 1–5 mm appeared during the model test, of which 3 cracks at the toe landslide were caused by reservoir water fluctuation, and the cracks at the middle and rear part were caused by rainfall. During rainfall process, the maximum displacement of landslide model reaches 3 cm. And the maximum deformation of the model exceeds 12 cm at the drainage stage. The failure process of the slope model can be divided into four stages: microcracks appearance and propagation stage, thrust-type failure stage, retrogressive failure stage, and holistic failure stage. When the thrust-type zone caused by rainfall was connected or even overlapped with the retrogressive failure zone caused by the drainage, the landslide would start, which displayed a typical composite failure pattern. The failure mode and deformation mechanism under the coupling actions of water level fluctuation and rainfall are revealed in the model test, which could appropriately guide for the analysis and evaluation of riverside landslides.

Effect of Seismic Loading on Variation of Pore Water Pressure During Pile Pull-Out Tests in Sandy Soils

Experimental model was done for pile model of L / D = 25 installed into a laminar shear box contains different saturation soil densities (loose and dense sand) to evaluate the variation of pore water pressure before and after apply seismic loading. Two pore water pressure transducers placed at position near the middle and bottom of pile model to evaluate the pore water pressure during pullout tests. Seismic loading applied by uniaxial shaking table device, while the pullout tests were conducted through pullout device. The results of changing pore water pressure showed that the variation of pore water pressure near the bottom of pile is more than variation near the middle of pile in all tests. The variation of pore water pressure after apply seismic loading is more than the variation before apply seismic loading near the middle of pile and near the bottom of pile and in loose and dense sand. Variation of pore water pressure after apply seismic loading and uplift force is less than the variation after apply seismic loading in loose sand at middle and bottom of pile.

Effect of initial specimen height and solids concentration on the consolidation parameters of fine-grained coal tailings

ABSTRACT Proper understanding and analysis of the consolidation behaviour and parameters of tailings from a slurry-like to a soil-like state are vital for the design, management, and assessment of a tailings storage facility (TSF). Fine-grained coal tailings slurry can undergo relatively rapid and substantial settlement from a slurried state and can experience relatively slow and limited self-weight consolidation, remaining slurry-like and under-consolidated, rather than soil-like. A large purpose-built slurry consolidometer having a newly developed consolidometer cell, instrumented with load cells and pore water pressure transducers, is well-suited to test tailings from a slurry-like to a soil-like state and enables the constant rate of loading (CRL) to simulate the build-up of tailings. CRL is more representative of the rate of rise of tailings in a TSF than incremental step loading. By conducting consolidation tests on fine-grained coal tailings from a slurry-state under a loading rate of 0.12 kPa/min up to applied stress of 320 kPa, this research investigated the effect of initial specimen height (ISH) and initial solids concentration (ISC) on consolidation parameters and hydraulic conductivity. Using ISH and ISC as independent inputs, a statistical model has been proposed to estimate the consolidation settlement at the end of the slurry consolidation test.

Shaking table tests on wall-type gravel and rubber drains as a liquefaction countermeasure in silty sand

In this paper, the seismic performance of wall-type gravel and rubber drains as a liquefaction countermeasure in silty sand has been addressed using a series of 1 g shaking table tests. In these tests, the liquefaction resistance of silty sand was studied by changing soil relative density, silt content, number of walls, and materials of drainage walls. In order to evaluate shear wave propagation, the generation and dissipation of pore water pressure (PWP), and the ground surface settlement, various accelerometers, displacement and PWP transducers were placed. The obtained results indicated that an increase in the relative density and the number of wall-type gravel and rubber drains in liquefiable silty sand reduces the settlement, liquefaction-induced deformations, as well as the excess PWP and eventually improves the liquefaction resistance. Totally, it can be noted that the reinforced silty sand with wall-type gravel drains revealed less excess PWP and settlement than that with wall-type rubber drains.

Design and Development of an In-Flight Actuator for Modeling Dynamic Compaction in a Geotechnical Centrifuge

Abstract The objective of this article is to present the design details and performance of an in-flight actuator for simulating dynamic compaction (DC) at enhanced gravities in a geotechnical centrifuge. The developed actuator is equipped with an automatized lifting and dropping mechanism to induce repeated impacts on the soil surface at high gravities, with prototype energies varying from 50 to 400 t-m. In addition, the developed actuator encompasses a wide range of tamper shapes, tamper diameter, and mass coupled with variable drop heights during centrifuge testing, thereby simulating both low-energy impacts and high-energy DC processes adopted in the field. The various components and working mechanism of the developed actuator are discussed, with particular emphasis on measures taken for ensuring vertical alignment of tamper and for minimizing Coriolis acceleration generated during flight. In this article, the results of four calibration tests and four centrifuge model tests of DC on a typical loose granular deposit are discussed, conducted in a 4.5-m radius large beam centrifuge facility available at IIT Bombay, India. The calibration results indicate that the impact frequency can be regulated remotely by controlling the motor voltage to enable monitoring of pore water pressure build-up in saturated soils during impact and its subsequent dissipation. The average time interval between successive impacts, in this case, is observed to be uniform and approximately 1.4–2.3 min in prototype dimensions. Further, the analysis of data captured by pore water pressure transducers and accelerometers, coupled with GeoPIV-based image analysis, was employed to demonstrate the effectiveness of DC in granular soils using the developed actuator for various tamper energies and saturation levels. The results are interpreted in terms of pore water pressure variations during impacts, crater formations, surface settlements, induced ground velocities and peak accelerations, and increases in relative density of the soil after DC.

Centrifuge Modeling and the Analysis of Ancient Landslides Subjected to Reservoir Water Level Fluctuation

Landslides are among the most severe natural hazards with significant impacts in human life and infrastructure. The Three Gorges Reservoir Area (TGRA) is vulnerable to landslides because of the geological environment and human activities. A centrifuge model test of a landslide with a planar sliding surface in the TGRA was conducted. Based on the multiple monitoring systems composed of a 3D laser scanner, pore water pressure transducers, particle image velocimetry and earth pressure sensors, multiphysical data were obtained. The work described here had the objective of researching the long-term deformation pattern of this kind of landslide that was subjected to periodic fluctuations in the reservoir water level. The results indicated that the failure processes were characterized by progressive retrogression and cracks caused by the reservoir drawdown. Transverse tensile cracks first appeared in the submerged zone of the slope. The front part of the slope was dominated by horizontal displacement, while the consolidation and compaction deformation in the vertical direction dominated at the mid-rear part of the slope. When the water level dropped again, the front part slid down and fell into the river, but the mid-rear part had no obvious deformation and exhibited a phenomenon of self-stabilization. Moreover, the phreatic line is a concave shape directed into the slope during reservoir filling and converts to a convex shape pointing out of the slope during reservoir drawdown. The earth pressures in the slope vary with the failure process of the landslide. Good agreement is obtained for the deformation characteristics between the experimental results and those of prototype landslides.

Effect Rate of Strain on In Situ Horizontal Coefficient of Consolidation from Pressuremeter

The conventional methods for evaluating the values of the coefficient of consolidation of soils usually carried out by Odometer or triaxial testing in laboratory by observing time rates of sample compression or pore water pressure dissipation during the application of a constant pressure. Attaching a small pore water pressure transducer to the membrane of the pressuremeter, enabling the measurement of the horizontal coefficient of consolidation of Glasgow glacial till. The horizontal coefficient of consolidation can be deduced in situ using the pressuremeter from the” holding tests “with the direction of pore water pressure drainage sensibly radial. The conventional method for carrying pressuremeter holding tests for measuring the horizontal coefficient of consolidation is by holding the diameter constant and allow the dissipation of pore water, the author developed a new method for measuring horizontal coefficient of consolidation by holding the pressure constant. Different rates were arranged to expand the membrane. The rate of strain showed distinctive discrepancies on the deduced values of the horizontal coefficient of consolidation during pressuremeter testing for Glasgow till.

Centrifuge model studies on geocomposite reinforced soil walls subjected to seepage

This paper investigates the effect of geocomposite layers (as an internal drainage system) and a chimney sand drain (as an external drainage system) on the performance of reinforced soil walls with panel facing and marginal backfill. Four models subjected to seepage simulating a rising groundwater surface were tested at 40 gravities using a 4.5 m radius large beam centrifuge. In this study, geocomposite layers played a dual function of drainage and reinforcement. The behavior of wall models was monitored using displacement and pore water pressure transducers during centrifuge tests. An image analysis technique was also used to measure displacement and strain fields. A geogrid reinforced soil wall with no drainage system experienced catastrophic failure at the onset of seepage. Provision of geocomposite layers at the bottom portion of the wall improved the wall behavior. Further, including geocomposite layers up to the mid-height of the wall resulted in superior performance compared to geogrid-reinforced soil walls. The model with the chimney sand drain experienced piping failure at the toe of the wall. The performance of reinforced soil walls with geocomposite layers was found to be superior to the geogrid reinforced soil walls with the chimney drain.

Centrifuge modelling on soft soil subjected to underwater vacuum preloading

The performance of soft soil subjected to underwater vacuum preloading was explored using geotechnical centrifuge model tests at different g-levels. A vacuum generator system was developed in a centrifuge to model the behaviour of underwater soft soil subjected to vacuum preloading. Sand drains were partially penetrated in soft soil. Vacuum gauges, pore-water pressure transducers and pressure transducers were used to measure the response of pressures and total stresses in the soil, while linear variable differential transformers and particle image velocimetry analysis techniques were employed to measure and analyse the deformation behaviour of underwater soft soil. Observations showed that the magnitude of reduction of pore-water pressure increased in the soil with an increase in water depth on the membrane. Meanwhile, on application of vacuum, the measured pore-water pressure below the membrane may be larger with an increase in the depth of water above the membrane due to a larger vacuum loss. Inward lateral displacement can be induced by underwater vacuum preloading to a limited extent in the treated and untreated zones. The lateral displacement in the treated zone may be negligible when the distance from the edge is larger than the thickness of clay.

Centrifuge and numerical model studies on the behaviour of geogrid reinforced soil walls with marginal backfills with and without geocomposite layers

The aim of this paper is to study the effect of geocomposite layers as internal drainage system on the behaviour of geogrid reinforced soil walls with marginal backfills using centrifuge and numerical modelling. A series of centrifuge model tests were carried out using a 4.5 m radius beam centrifuge facility available at IIT Bombay. A seepage condition was imposed to all models to simulate rising ground water condition. Displacement and pore water pressure transducers were used to monitor the performance of all centrifuge models. A geogrid reinforced soil wall without any geocomposite layer experienced catastrophic failure soon after applying seepage due to the development of excess pore water pressure within the reinforced soil zone of the wall. In comparison, reinforced soil wall with two geocomposite layers at the bottom portion of the wall was found to have a good performance at the onset of seepage and by embedding four geocomposite layers up to the mid-height of the wall from bottom as a result of lowering phreatic surface much more effectively. For analysing further the observed behaviour of centrifuge model tests, stability and seepage analysis were conducted using SLOPE/W and SEEP/W software packages. A good agreement was found between the results of numerical analysis and observation made in centrifuge tests. The effect of number of geocomposite layers as well as its transmissivity was further analysed using parametric study. The results of parametric study revealed that the number of geocomposite layers plays a main role on the good performance of the geogrid reinforced soil walls with marginal backfill.

Shear bands in undrained plane strain compression of Norwegian quick clays

This work presents the results of plane strain tests carried out at the Laboratoire 3SR Grenoble (France) on undisturbed samples of very sensitive Norwegian soft clay. Discussion of the results focuses on the observed failure mechanisms and zones of localized deformation. The tested clay is defined as quick clay because it liquefies completely upon remolding. Thus, careful handling was necessary, and original procedures were developed to place samples in the plane strain testing device with minimum disturbance. Results show that excess pore-water pressure had developed during shear loading and shear strength was lost after the peak, which is a typical phenomenon in sensitive soils. The sample was held between two glass plates to enable visual tracking of the deformations. A digital image correlation analysis revealed a complex pattern of emerging shear bands, with one or two bands dominating and providing a final failure mechanism. Two local pore-water pressure transducers were attached to the sample boundary and showed differences in local excess pore-water pressure. It is suggested that excess pore-water pressure depends on the distance from the transducers to the zones of concentrated shear strains.

Physical modeling and analysis of site liquefaction subjected to biaxial dynamic excitations

The paper presents a series of centrifuge tests of level sites consisting of a granular soil deposit, with and without an embedded structure subjected to various biaxial and uniaxial base excitations. The tests were conducted at RPI NEES centrifuge facility to assess the dynamic response characteristics of level deposits and soil-structure interaction (SSI) under multidimensional conditions. Synthetic sinusoidal waves were used as base excitations to test soil models under biaxial and uniaxial shaking. A dense array of accelerometers was used to monitor the deposit response along with pore water pressure transducers. One uniaxial test and two biaxial shaking tests were conducted on three similar soil models, to study the effect of multidirectional shaking on the generation of soil liquefaction for dense deposits. The observed acceleration and pore pressure are used along with non-parametric identification procedures to evaluate the corresponding dynamic shear stress–strain histories. The measured results along with the obtained time histories are used to shed light on the mechanisms of liquefaction occurring through the stratum under multiaxial shaking. It also shows the effect of soil relative density on soil behavior when subjected to biaxial shaking. The soil–structure interaction test showed significant differences in soil behavior at different locations beneath the footing when subjected to biaxial shaking. This difference is associated with coupling in stress–strain in both experiments compared to the more known uniaxial soil behavior.

Centrifuge modelling and analysis of site liquefaction subjected to biaxial dynamic excitations

The paper presents a series of centrifuge tests simulating a level site consisting of granular soil deposits subjected to various biaxial and uniaxial base excitations. The tests were conducted at RPI NEES centrifuge facility to assess the dynamic response characteristics of level deposits under multidirectional shaking. Synthetic sinusoidal waves were used as base excitations to test loose models under biaxial and uniaxial shaking. Dense arrays of accelerometers were used to monitor the deposit response along with pore water pressure transducers. Two uniaxial tests and one biaxial shaking test were conducted on three similar soil models to study the impact of multidirectional shaking on the generation of soil liquefaction. The two uniaxial shaking tests consisted of: (a) a test with input energy content using an Arias intensity similar to that of the biaxial input shaking and (b) a test with 10% increase in one of the components of the biaxial shake amplitude, as commonly done in practice for uniaxial simulation of multidirectional field shaking. The observed acceleration and pore pressure are used along with non-parametric identification procedures to estimate the corresponding dynamic shear stress–strain histories. The measured results along with the stress and strain histories obtained are used for two purposes: first, to shed light on the properties of liquefaction occurring through the stratum under uniaxial shaking; second, to show the difference in soil behaviour when it is subjected to biaxial shaking. The latter purpose is evident in the strain energy generated in the biaxial test compared to that of equivalent and traditional uniaxial tests.

Centrifuge shaking table tests on effect of vertical drain systems for liquefied soil

It was observed that liquefaction induced by earthquake causes series damages to buildings and threatens the people and their properties. From the past studies, a number of countermeasures were proposed to reduce the build-up of excess pore water pressure and to enhance the stiffness of the soil during earthquake. The vertical drain systems are well known methods and used as remediation against earthquake-induced soil liquefaction for many years. The purpose of the study is to investigate the effect of different vertical drain systems for the liquefiable soil by centrifuge modeling technique. The seismic behavior of liquefiable soil was performed firstly. The free field soil models were then prepared with alternatively arranged drain-belts and gravel-pile drains to investigate the effect of different vertical drain systems on the liquefiable soil. Several arrays of accelerometers, the pore water pressure transducers and displacement transducers were placed to monitor the shear wave propagation, the excitation and dissipation of pore water pressure. Displacement transducers were placed to measure the ground surface settlement. From the test results, it was observed that the vertical drain systems reduce the settlement and excess pore water pressure significantly. In the future, the vertical systems will be applied around the structure and the test results would give engineers suggestions to deal with the arrangements of drain-belts and gravel drains to reduce the damage during and after the earthquake.

Dynamic Response of Saturated Soil - Foundation System Acted upon by Vibration

In this study, the response and behavior of machine foundations resting on dry and saturated sand was investigated experimentally. In order to investigate the response of soil and footing to steady state dynamic loading, a physical model was manufactured to simulate steady state harmonic load at different operating frequencies. Total of 84 physical models were performed. The footing parameters are related to the size of the rectangular footing and depth of embedment. Two sizes of rectangular steel model footing were tested at the surface and at 50 mm depth below model surface. Meanwhile the investigated parameters of the soil condition include dry and saturated sand for two relative densities 30% and 80%. The response of the footing was elaborated by measuring the amplitude of displacement by the vibration meter. The response of the soil to dynamic loading includes measuring the stresses inside the soil using piezoelectric sensors as well as measuring the excess pore water pressure using pore water pressure transducers. It was concluded that the maximum displacement amplitude response of the foundation resting on dry sand models is more than that on the saturated sand by about 5.0–10%. The maximum displacement amplitude of footing is reduced to half when the size of footing is doubled for dry and saturated sand. The final settlement (St) of the foundation increases with increasing the amplitude of dynamic force, operating frequency and degree of saturation. Meanwhile, it is reduced with increasing the relative density of sand, modulus of elasticity, and embedding inside soils. The excess pore water pressure increases with increasing the relative density of the sand, the amplitude of dynamic loading and the operating frequency. In contrast, the rate of dissipation of the excess pore water pressure during dynamic loading is more in the case of loose sand.

The pore water pressure sensor based on Sagnac interferometer with polarization-maintaining photonic crystal fiber for the geotechnical engineering

The pore water pressure sensors with the six-hole suspended-core polarization-maintaining photonic crystal fiber (SC-PM-PCF) and commercial polarization-maintaining photonic crystal fiber (PM-PCF) are designed based Sagnac interferometer and calibrated in the laboratory. According to the theoretical analysis and calibration results, the transmission spectrum is very sensitive to the pore water pressure. It is found that the wavelength of the spectrum has a good linear relationship with variances of the surrounding pore water pressure, and the coefficient of wavelength–pressure of the commercial PM-PCF is 304.41kPa/nm with the length of 35cm as the sensing element while the coefficient of the SC-PM-PCF is 254.75kPa/nm with the length of 100cm. Finally, the two PM-PCF sensors are applied and compared with the conventional Pore water Pressure Transducers (PPTs) in a physical model test. It is found that measurements of the PM-PCF sensors are in good agreement with the results measured by the conventional PPTs.