The internal fabric and localized deformation patterns of triaxial sand specimens were investigated using Computed Tomography (CT). Three displacement-controlled, conventional, drained axisymmetric (triaxial) experiments were conducted on dry Ottawa sand specimens at very low effective confining stresses (0.05, 0.52, and 1.30 kPa) in a microgravity environment aboard the Space Shuttle during the NASA STS-79 mission. CT scanning was p'erformed on these flight specimens, as well as on an uncompressed specimen and a specimen tested in a terrestrial laboratory at 1.30 kPa effective confining stress. CT demonstrated high accuracy in detecting specimen inhomogeneity and localization patterns. Formation of deformation patterns is dependent on the effective confining stress and gravity. Multiple symmetrical radial shear bands were observed in the specimens tested in a microgravity environment. In the axial direction, two major conical surfaces were developed. Nonsymmetrical spatial deformation was observed in the 1-G specimen. Analysis tools were developed to quantify the spatial density change. Void ratio variation within and outside the shear bands was calculated and discussed.
This paper presents a new suction-temperature controlled isotropic cell that can be used to study the thermo-mechanical behavior of unsaturated expansive clays. The vapor equilibrium technique is used to control the soil suction; the temperature of the cell is controlled using a thermostat bath. The isotropic pressure is applied using a volume/pressure controller that is also used to monitor the volume change of soil specimen. Preliminary experimental results showed good performance of the cell.
A new experimental set-up using a differential pressure transducer was developed, that enables the monitoring of volume changes in cyclic triaxial tests on unsaturated soils. Calibration tests were performed in order to analyze the performance of the set-up, especially in terms of loading frequencies. Based on calibration results, a low frequency of 0.05 Hz was adopted for the tests carried out on the unsaturated loess from northern France. Five water contents were considered in the tests. The obtained results have confirmed the efficiency of the new system for volume change monitoring under cyclic loading. The effect of water content on the cyclic behavior of loess was clearly evidenced. Finally, some suggestions were made to improve the accuracy of the system.
An experimental device was developed to monitor the field soil suction using miniature tensiometer. This device consists of a double tube system that ensures a good contact between the tensiometer and the soil surface at the bottom of the testing borehole. This system also ensures the tensiometer periodical retrieving without disturbing the surrounding soil. This device was used to monitor the soil suction at the site of Boissy-le-Ch\^atel, France. The measurement was performed at two depths (25 and 45 cm) during two months (May and June 2004). The recorded suction data are analyzed by comparing with the volumetric water content data recorded using TDR (Time Domain Reflectometer) probes as well as the meteorological data. A good agreement between these results was observed, showing a satisfactory performance of the developed device.
A completely portable computer-controlled servo-mechanism designed for use with the Cambridge self-boring pressuremeter consists of a computer, a data acquisition system (DAS), and an electric-to-pneumatic (E/P) transducer. The computer obtains information on the current state of a test through the DAS and determines the next step in accordance with prescribed test conditions. An appropriate voltage is then sent to the E/P transducer, which regulates in proportion to the voltage received the pressure for expanding the pressuremeter. At present, four test conditions are programmed: a controlled stress increment test, a controlled strain rate test, a controlled strain test, and a cyclic test. Experience with different soils shows that the mechanism operates to design specifications. Il s'agit d'un servo-mécanisme commandé par ordinateur, entièrement portatif et concu pour être utilisé avec un pénétromètre autoforeur Cambridge. L'appareil comprend un calculateur, un système d'entrée de données (DAS) et un transducteur électro-pneumatique (E/P). L'ordinateur enregistre des données au cours de l'essai grâce au DAS et agit en fonction de celles-cien ajustant la différence de potentiel appliquée au transducteur E/P, ce qui a pour effet d'augmenter proportionnellement la pression qu'exerce le pénétromètre. A présent, il existe quatre programmes d'essais: augmentation de la contrainte par incréments, contrôle de la vitesse de déformation, contrôle de la déformation et essai cyclique. Les expériences effectuées dans différents sols indiquent que le mécanisme se comporte conformément aux spécifications de conception. RES
An apparatus and an experimental arrangement were developed to provide an accurate measurement of the lateral deformation of soft sensitive clays. The apparatus contained light gage points and displacement transducers floated in a cell fluid in such a way that internal strains could be measured directly on the soil specimen with the least amount of disturbance. The performance data obtained from the testing of a soft sensitive clay indicated that the apparatus developed possessed adequate precision and stability for the study of the deformation behavior of clays. L'article porte sur la mise au point d'un appareil et d'un dispositif expérimental destinés à la mesure précise de la déformation latérale dans les argiles tendres et sensibles au remaniement. L'appareil comporte des points de mesure légers et des transducteurs de déplacement en suspension dans un liquide, de manière que les contraintes internes puissent être mesurées directement sur l'échantillon avec un minimum de remaniement. Les données obtenues lors d'essais sur une argile tendre et sensible indiquent que l'appareil mis au point démontre la précision et la stabilité convenant à l'étude de la déformation des argiles. RES
An ultrasonic testing method employing the pulse-echo inspection technique was developed to assess the integrity of annular seals surrounding casings (i.e., instrument tubes or well casings). This nondestructive testing method permits testing a seal from inside a casing without disturbing the casing, seal, or formation. Seals constructed with cement-based and bentonite-based sealants surrounding various types and sizes of casings can be evaluated using the method. An evaluation is conducted by sending and receiving ultrasonic waves using a single piezoelectric transducer and commercially available hardware (a pulser receiver and a waveform analyzer). A probe was designed and constructed for downhole testing. Differences in the ultrasonic responses of the materials in contact with a casing are analyzed to determine the presence of different materials (seal or defects filled with air or water in a seal) outside a casing.
The T-Z method is the common method available to estimate load-displacement behavior of tension piles. Obtaining accurate T-Z curves from laboratory tests has a vital role in better estimation of pile behavior. The available apparatus used to obtain laboratory TZ curves (Coyle and Sulaiman 1967) suffers from a set of limitations. A soil-pile-slip test apparatus has been designed at the University of Nottingham (UK) to obtain better T-Z curves for tension piles. This paper presents the design details and the testing procedure of the soil-pile-slip test apparatus. A set of T-Z curves obtained using this apparatus is analyzed, which provides insight into the mechanism of soil-pile-slip dilation near the pile surface.
This paper describes a new triaxial apparatus for testing the thermo-mechanical behavior of soils. This triaxial apparatus accommodates samples 55 mm in diameter and 110mm in height. The cell is designed to perform compression tests under pressures up to 2MPa and with temperatures ranging from 5°C to 90°C. The calibration of the device, as well as its precision and accuracy, is widely discussed in the paper. The related experimental procedures are also discussed. Finally, the results of tests performed on Kaolin are presented in order to highlight the features of the apparatus and to show some induced thermal effects on soils.
An automated digital image processing technique is presented for the measurement of specimen volume change during triaxial testing. A digital camera is fixed at a constant distance from the triaxial cell. Pictures taken at various intervals are processed (e.g. using contouring techniques) to extract specimen volume and thus volumetric strain. A calibration procedure requiring a rigid specimen is used to correlate specimen image with specimen volume. The accuracy of the method is discussed on the basis of experimental results. This technique is of particular interest for cases where conventional use of pore-water volume exchange cannot be used to extract total volumetric strain, as is the case when testing unsaturated soils.
Methods for determination of shear wave velocity (vs) with bender elements using frequency domain techniques are reviewed. An enhanced method is proposed, which is quicker and provides more information than existing techniques. The method can be implemented using a spectrum analyzer-a common piece of hardware. The validity of the method is demonstrated by means of a series of bench top tests on a reconstituted clay sample. The results are compared both with existing frequency domain practice and with established time-domain techniques. The results reveal that the system comprising benders elements and soil sample is dispersive. The relationship between velocity and frequency is established in detail for several propagation paths through the sample, allowing some insight to be gained into the factors influencing the dispersion characteristics.
The filter paper method is probably the simplest of the methods available for estimating the suction of a clay soil. The method measures soil suction indirectly by simply measuring the moisture content of a filter paper having been brought to the same suction as the soil. Calibration of representative specimens of the filter paper over a range of known suctions defines the suction-moisture content characteristic of the material. The calibration must be carried out in a closely temperature-controlled environment using various apparatus depending on the particular level of suction. There have been a number of impediments to the wider application of the filter paper method. One of these is the need to have access to a closely temperature-controlled laboratory with a range of specialist calibration apparatus. This difficulty has been exacerbated by the belief that to account for the possible variability of filter papers between boxes, even from the same production batch, representative specimens from each box require separate calibration. Based on the results of careful calibration and subsequent analysis, it appears that, provided the boxes are all from the same production batch and are purchased from the same outlet at the same time, the calibration of only one box is necessary.
The estimation of adequate sample size is critical to all laboratory methods of calibration of neutron moisture gauges. Neutron gauge response is a function of the true sample geometry rather than an arbitrarily defined spherical geometry. A two-group solution is obtained for neutron flux in a homogeneous, finite, cylindrical sample. The theoretical flux distributions for this solution and for infinite and spherical samples are compared for three representative media. The results show that significant reductions of sample size are possible for media with low moisture contents if the true cylindrical geometry is considered.
A large-scale probe has been developed for measuring the thermal conductivity of geomaterials. The large probe was designed to conduct tests on materials containing large particles, materials with high heterogeneity, and materials with high stiffness. The probe has dimensions of 680 mm length and 15.9 mm diameter and was constructed of stainless steel tubing. The probe operates on the principle of heating an infinite line source in an infinite medium. Initially, parametric evaluations were conducted to determine the operational and test conditions for the large probe, including power level, heating duration, and zone of heating influence. Then, tests were conducted on five homogeneous materials to calibrate the newly developed probe. Thermal conductivity measurements obtained using the large probe were compared with measurements obtained using a conventional small probe. A calibration curve was established for the large probe. In addition, the performance of the large probe was evaluated in two manufactured heterogeneous materials and a large particle material. The test program indicated that the large probe can be used effectively for determining thermal conductivity of geomaterials. This new probe may be suitable for large-scale laboratory testing and field investigations.
The air-entry suction is an extremely important parameter in the water characteristic curve, but is difficult to identify when using conventional static methods such as the pressure plate apparatus. In the dynamic method described herein, instantaneous measurement of matric suction and water content are acquired from a soil specimen undergoing a slow drying cycle with the aid of a tensiometer and a soil moisture probe, respectively. The method offers a quick and reliable way of quantifying the drying soil-water characteristic curve (SWCC) for matric suctions of up to 80 kPa. The large number of data points acquired make it easy to obtain an accurate estimate of the air-entry suction. It is particularly effective for soils ranging from medium sands to sandy silts where a nearly continuous SWCC is provided
In this paper, possible links between the shrinkage limit and distinct changes in other properties of a clay soil undergoing drying are discussed. These properties include the volumetric air content of the soil, the heat of wetting, the tensile strength, the total suction, and the thermal resistivity of the soil. The distinct changes are of interest in themselves, and possible explanations for them are briefly discussed. For soil under extremely dry conditions, more importance should perhaps be attached to using the shrinkage limit as an index to changes in soil behavior, and this topic warrants further research
Geosynthetic clay liners (GCLs) are often selected as a part of the barrier layer for cover systems in solid waste landfills to prevent infiltration of rainfall and migration of biogas into the atmosphere. To address the ability of GCLs to mitigate gas flow through cover layers, this paper presents an apparatus and testing method to measure the gas permeability of non-saturated and strained GCLs in the laboratory. The test is performed following a transient gas flow regime, and the test results are interpreted using a simple analytical solution. A series of gas permeability tests was performed on a needle-punched GCL with a degree of saturation between 32 % and 47 %. The specimens were tested under a 20 kPa load and 1.3 % strain in the radial direction, whereas the gas flow was induced by a gas differential pressure of less than 4 kPa. The results show that both the apparatus and the testing method can provide a reliable, fast, and simple method to measure the gas permeability of GCLs.
A study was conducted to determine the dynamic properties of compacted clayey soils subjected to low amplitude vibration. A fast and simple ultrasonic pulse transmission method was used. Tests were conducted on three clayey soils with low to high plasticity compacted using standard and modified Proctor effort over a range of water contents. The samples were allowed to dry after compaction and P-wave velocity, S-wave velocity, Poisson's ratio, and shear modulus were determined. The effects of soil type, compaction conditions, and degree of saturation on the dynamic response of the soils were investigated. The elastic wave velocities and the dynamic shear modulus increased as the soil plasticity decreased. The velocities and the modulus increased and the Poisson's ratio decreased as the degree of saturation decreased due to drying. Generally, the velocities and moduli increased significantly at the early stages of drying with the changes becoming more gradual as drying progressed. Variations were high for soils compacted with low energy and high water content. The shear moduli of the soils were also estimated using a common empirical equation. Suction values obtained for the test soils in a different study were used as effective stresses in the estimation. The estimated shear moduli agreed well with the measured values.
This paper describes engineering applications which show the engineering significance of testing the phase composition of soils, including determining the level of water saturation. It further explains why, in order to accurately model in situ conditions in soil deposits, it is necessary to test the phase composition of soil under conditions of horizontal restraint and changing moisture content. The paper reviews the lineal measurement method of Sibley and Williams (1989), which was initially developed to test unrestrained clays. The method has now been adapted for use in characterizing phase relationships in horizontally restrained soil specimens undergoing drying. Results obtained using the new test have drawn attention to the fact that while soils undergoing drying are invariably restrained by naturally occurring mechanisms, the level of this restraint may vary widely. A useful index of the level of restraint applied to a slurried soil specimen undergoing drying is the moisture content at which initially 1-D (vertical) shrinkage behavior becomes isotropic
Coal mine washery wastes comprise two major components whose specific gravities differ markedly. Consequently, their specific gravity is much more variable than that of natural soils and is determined relatively frequently. The specific gravity of coal mine wastes is determined more rapidly and efficiently by gas pycnometry than by water pycnometry. However, the two methods give significantly different results. Results of specific gravity determinations for washery wastes from Goonyella Coal Mine in central Queensland, Australia, by gas and water pycnometry, are compared. The differences between them are shown to arise from the pore structure and particle sizes of the wastes and the interaction between the internal surface of the wastes and the pycnometric fluids.
This paper reports the development of an advanced thermo-hydro-mechanical (THM) oedometer in order to characterize the behavior of soils under combined non-isothermal and unsaturated conditions. The simultaneous control of temperature, suction and stress states within the sample required rigorous calibration. This THM oedometer accommodates samples 80 mm in diameter and 23 mm in height. The applied vertical stresses can reach values up to 1 MPa, the controlled temperature ranges from 20 °C to 80 °C, and the applied suction is up to 500 kPa. The paper thoroughly discusses the calibration of the device and presents some results of tests performed on a sandy silt.
The heat and moisture transfer characteristics of a silt from the Mackenzie River Valley have been determined in a laboratory investigation. A dual gamma-ray scanner was used to measure the changes in density and volumetric moisture content along the soil samples. The coefficients of soil- water diffusivity under isothermal conditions and under temperature gradients have been obtained. The development of a calibration curve for the analysis of gamma-ray readings has been described. The shortcomings of the procedures used for the measurement of transport coefficients have been discussed. Les caractéristiques de transfert de chaleur et d'humidité dans un silt provenant de la vallée du fleuve Mackenzie ont été déterminée en laboratoire. Avec un appareil de balayage à rayons gamma double, on a mesuré les changements de densité et de teneur en humidité volumétrique dans des échantillons de sol. Les coefficients de diffusivité sol-eau dans des conditions isothermiques et dans certains gradients de température ont été établis. La mise au point d'une courbe d'étalonnage pour l'analyse des enregistrements des rayons gamma est décrite. Les points faibles des procédés utilisés pour déterminer les coefficients de transport sont traités. RES
The unsaturated hydraulic conductivity of a soil-like material required in flow models is usually quantified with the aid of predictive models. In this study, an optimum set of input parameters for Campbell's hydraulic conductivity function was estimated. The resulting hydraulic conductivity relationship was compared to that obtained using the van Genuchten model. A flow cell was developed in which transient water contents and matric suctions were monitored while the soil underwent drying and wetting cycles. A one-dimensional numerical model was used to simulate the experimental flow problem for a wide range of input parameters. The sum of the square of the differences (SSD) between the observed and simulated water contents was calculated. The parameters corresponding to the minimum SSD were used to establish three hydraulic conductivity relationships for drying, wetting, and redrying conditions. For wetting conditions, the relative hydraulic conductivity function calibrated against the estimated optimum saturated hydraulic conductivity compared well with the measured unsaturated hydraulic conductivity. A lower-saturated hydraulic conductivity was required to calibrate the hydraulic conductivity function for drying conditions.
A description is presented of a rock-mechanics triaxial test facility capable of coupled testing of medium-sized cylindrical rock specimens at axial loads to 2.67 MN, confining pressures to 140 MPa, pore pressures to 105 MPa, and specimen temperature to 400°C. The test cell of the system is modified to carry out tests on fluid mass and heat transfer through rock joints/fractures and thermomechanical deformation of joints/fractures. Details of test procedures and techniques for conducting the coupled hydro-thermo-mechanical tests are described. Data on both natural joints and artificially induced extension fractures in the Carnmenellis granite are presented and discussed in order to illustrate the multifunctional capability of the testing system to cover the requirement for further understanding of rock joint/fracture and rock masses. The study showed that joint/fracture permeability and mechanical aperture decrease with increasing effective normal stress. The relationship between the changes of mechanical aperture and permeability is influenced mainly by discontinuity surface characteristics. The permeability decreases with increasing temperature while measured mechanical aperture increases. Heat transfer coefficient measured during hydro-thermo-mechanical tests varies from 300 to 1200 W/m2-°C and is affected by flow velocity, temperature, and joint/fracture surface properties.
The complete determination of soil water retention curve requires the sample volume to be measured, in order to calculate its void ratio and saturation degree. During drying in the pressure plate apparatus, cracks often appear in the sample altering its deformation and evaporation patterns. Consequently, this causes a significant scatter in the volume measurement when using volume displacement method. This paper proposes a simple method to avoid cracking, by limiting friction and adhesion boundary effects, to allow for an unrestrained shrinkage of the sample. Such modification of the technique decreases the error of measurements by the factor of three.
Tests were conducted to determine thickness of smooth, nonreinforced geomembranes using three methods: mechanical (according to ASTM and European standards), ultrasonic, and magnetic methods. The mechanical method is the standard procedure used for determining thickness of geomembranes. The ultrasonic and magnetic methods are not commonly used for geomembranes; however, they are used for testing other materials such as metals. Tests were conducted on 15 geomembranes representing five types of polymers (HDPE, LLDPE, PVC, PP, and EPDM). The results of the testing program indicated that the level of pressures applied affected the thickness measurements in mechanical tests. While the low pressures were not sufficient to flatten particularly the rigid geomembranes, the high pressures tended to compress the geomembranes excessively. Both high and low pressures prevented obtaining representative measurements. The measurements obtained using the ASTM method were more reliable than the measurements obtained with the European method, although it is believed that the most reliable measurements can be obtained by the nondestructive methods (ultrasonic and magnetic). These techniques are sensitive only to the thickness of the materials due to the inherent properties of the test procedures, and they work equally well for rigid and flexible geomembranes. Of the two nondestructive methods, ultrasonic testing is better due to several advantages: it allows for testing from the top surface of geoembranes in the laboratory or in the field, and it can be used on coupons of geomembranes as well as on whole sheets without the need for removing test samples. Both nondestructive methods can be improved for application to geomembranes.
Image analysis was used to determine surface topography and thickness of textured geomembranes. Images of cross sections (specimen length × thickness) of geomembranes were obtained at 50× magnification using a digital optical microscope. Thickness was determined as the distance between the top and bottom surfaces along an entire cross section. For surface analysis, profiles of top and bottom surfaces were extracted from the cross section images. Amplitude, spatial, hybrid, and functional texture parameters were determined. Tests were conducted on eleven samples of HDPE and LLDPE geomembranes manufactured by co-extrusion and embossing at varying thicknesses. The geomembranes were classified into three texture categories: high, medium, and low. Anisotropy and directionality were observed for all samples to varying degrees between and within manufacturing directions, respectively. Statistical analysis of the results indicated that surface topography of geomembranes could be determined by analyzing one surface of five specimens with 50 mm length. Comparisons were made between image analysis and mechanical tests for core thickness and asperity height. Image analysis and mechanical measurements were significantly different. Recommendations were made for the redesign of the mechanical devices by sizing the components in relation to the size and spacing of geomembrane texture features.
Measurement of unsaturated soil volumes can be used to monitor drying shrinkage and allows calculation of the volumetric air content/moisture content relationship. From this relationship, the onset of significant desaturation can be established as a bound to the validity of conventional soil testing under saturated conditions and of conventional analysis procedures for the particular soil. Measurement of soil volumes over the full range of drying shrinkage is difficult because the soil is liquid at high-moisture contents, but changes to a deformed, cracked, unsaturated solid when dry. The method of volume measurement should be accurate and consistent, since it is used to calculate small changes in the small volumes of air present in the soil. Ideally, it should not stress the specimen or interfere with drying. Further, the method should be capable of distinguishing any volumes of air in exterior cracks or depressions in the soil surface. Finally, the method should be easily repeatable to enable progressive measurements of soil volume during the drying process. A simple, fast method of soil specimen volume measurement is described, based on measurement of displaced volumes of toluene in a pycnometer-like flask. It is compared with a method of calculating volumes from one-dimensional measurements of specimen length and moisture content during drying
The filter paper method for determining soil suction has been in use for many years and owes its attraction to its simplicity. A major drawback of the method is that the suction range over which it can be applied is limited by the moisture sensitivity of the filter paper or absorbent used. By convention, filter paper has been adopted for use in the method, but other absorbents may well be more suitable, increasing the sensitivity of the method and extending the suction range over which the method may usefully be applied. In this paper the desirable attributes of absorbents for use in determining soil suctions are outlined, and the suitability of a nonconventional absorbent material is assessed. It was found that this material had enhanced sensitivity over the middle suction range.
An ultrasonic testing procedure was developed to evaluate geomembranes. The pulse-echo inspection technique was used on the surface of geomembranes without disturbing the material. The equipment required consists of a P-wave transducer, a pulser-receiver, and a signal acquisition system. Travel time of ultrasonic waves and waveform energy are measured to evaluate the condition of geomembranes. Laboratory tests were conducted to assess the effectiveness of the method. Defects that simulate installation damage and various in-service degradation conditions were induced on samples. The defects were identified with the ultrasonic method at a success rate higher than 98%. Changes in the thickness or the microstructure of the geomembranes were identified. Surficial and internal defects were located. Defects that were not visible from the measurement surfaces were identified. This method appears promising to monitor the condition of geomembranes in the laboratory or in the field and to assess in situ damage to geomembranes.
This paper evaluates the results of triaxial tests carried out over a very wide range of effective stresses using custom-built triaxial systems specially developed for testing at low, medium, and high pressures. These tests were performed on samples of resedimented Boston Blue Clay, and good reproducibility is demonstrated across testing devices. A novel approach was taken to evaluate the effect of apparatus compressibility of the drainage system on the measured undrained shear behavior, as this issue becomes increasingly important for triaxial testing at high stresses. The effect of apparatus compressibility on triaxial shear results was evaluated by comparing the volume of pore fluid necessary to develop the drained strength of a specimen to the volume of flow caused by apparatus compressibility. This method is more illustrative and intuitive for undrained triaxial testing of saturated specimens than the traditional approach of using a specimen's B-value. Triaxial test results show a consistent decrease in both undrained strength ratio and critical state friction angle over the effective stress range of 0.1 to 100 MPa for each overconsolidation ratio investigated. The conventional assumption made in soil models that fine-grained soils exhibit constant normalized strength properties is therefore shown to be invalid when these properties are viewed over a significant range of effective stresses.
Broad-band permittivity data enable the determination of micro- and macro-scale material characteristics and the monitoring of geo-processes. While high-frequency (> ≈ 100 MHz) permittivity measurements can be performed readily in the laboratory, low-frequency (< ≈ 10 MHz) measurements are more difficult to conduct. This paper describes two low-frequency techniques and presents broadband permittivity data for various soil-water mixtures. Low-frequency data were gathered with an impedance analyzer in conjunction with two-terminal and four-terminal measurement systems. The two-terminal cell consisted of two copper electrodes; its range was restricted at low frequencies due to electrode polarization. The four-terminal system used separate current and voltage electrodes; its accuracy at low frequencies was limited by the ability of the equipment to resolve small phase angles. High-frequency data obtained with a coaxial termination probe and low-frequency data obtained with these two cells are presented. Soils of very different specific surface were tested at water contents ranging from air-dry to saturated. It is shown that the conductivity of the specimen controls not only the low-frequency measurement limit (i.e., electrode polarization and phase resolution), but also the high-frequency limit due to stray inductances.
In this study, the economical FlexiForce sensor, which is thin and flexible, was used to measure Ko and the excess pore water pressure during a 1D consolidation test on kaolinite clay samples. Before the sensor could be used in the measurement, a special waterproof coating was applied, after which sensor calibration was carried out. The coated FlexiForce sensor was incorporated into two types of tailor-made needle probes for the pore water pressure measurement. The experimental results demonstrate that the two types of needle probes can well capture the evolution of the induced excess pore water pressure during consolidation. Upon a loading increment, the measured excess pore water pressure at the middle of the sample continues to increase and reaches a maximum value at t = ∼4 min and subsequently start to decrease. There is a distinct change in the decreasing trend of the measured excess pore water pressure; the trend becomes mild after t = ∼480min, indicating the end of primary consolidation. During secondary compression, the excess pore water pressure continues to decrease due to the continuous settlement. A constant Ko value, which is comparable to the prediction by Jaky's equation, is measured by the sensors in the loading path; during secondary compression, Ko increases with time.