Oedometer Cell Research Articles

Effect of Soil Particle Size on the Accuracy of Tactile Pressure Sensors

This paper investigates the effects of soil particle size on the measurement accuracy of single-point type tactile pressure sensors when used for measuring soil contact stresses. Sensors with two different diameters (Dsens) are integrated into an oedometer cell. Granular materials with a wide range of particle diameters (D50) and angularities (from rounded to irregular shaped) are evaluated, resulting in a wide range of ratios of Dsens/D50 from 2.6 to 105.8. The test results show that the soil particle size significantly affects and exponentially correlates with the measurement accuracy of the sensor. A threshold Dsens/D50 of 10.5 is identified for the sensor, beyond which the error is less than 5%. It is also shown that even for ratios of Dsens/D50 as large as 40, an error of approximately 2% can be observed, which is intrinsic to the sensor.

Multi‐scale characterization of granular media by in situ laboratory X‐ray computed tomography

AbstractInvestigations of biphasic monodisperse soft (rubber) and stiff (glass) particle mixtures under hydrostatic conditions show an interesting behavior with regard to the effective stiffness. P‐wave modulus measured by acoustic wave propagation at ultrasonic frequencies showed a significant decline while more soft particles are added, that is, higher rubber volume fractions, due to a change in the microstructure of the granular medium. However, for small volume fractions of soft particles, it could be observed that the P‐wave modulus is increasing. This result cannot be explained by classical mixture rules or effective medium theories. For the understanding of those effects, a detailed insight into the microstructure of the granular medium is necessary. To gain this information and link it later back to the measured effective mechanical properties, high‐resolution micro X‐ray computed tomography (XRCT) imaging is a well‐established tool. With XRCT imaging, the granular microstructure can be visualized in 3D and characterized subsequently. Combining classical effective characterization methods with XRCT imaging can help to solve a variety of multi‐scale problems. Performing the characterization step in situ, meaning inside the laboratory‐based XRCT scanner, has the advantage that exactly the same samples are mechanically characterized and visualized. To address the mentioned observation above, we designed a low X‐ray absorbing oedometer cell with integrated broadband piezoelectric P‐wave transducers which enables this kind of investigation inside a laboratory‐based XRCT scanner. The focus of this contribution is on the general experimental methodology which can be transferred to other multi‐scale problems. It starts with a description of the image acquisition and ends with the post‐processing of the in situ acquired image data. To demonstrate this, cylindrical samples consisting of the same monodisperse rubber and glass particle mixtures that were studied before under hydrostatic stress conditions are considered. Selected results are presented to explain the single steps.

One-dimensional compression creep change under temperature and suction effects

ABSTRACT The impact of temperature and unsaturation on the creep behaviour of bentonite is an important topic for environmental geotechnics as in the case of waste storage and energy geostructures. This paper presents an experimental study on the one-dimensional creep of Maghnia bentonite (MB), using two high-pressure oedometer cells, allowing to impose temperature and suction conditions for an unsaturated medium . The characteristics of these two cells and the protocol developed in the framework of this experimental study are briefly presented. The findings highlight a new perspective on the hypothesis (A) which distinguishes between primary and secondary consolidation, and hypothesis (B), which is governed by structural viscosity. The results indicated that the creep behaviour followed the principle of hypothesis (A) under temperature effect, while it followed hypothesis (B) principle under the suction effect. The void ratio variability under secondary compression depends not only on the vertical stress, but also on the temperature and the first load level . Moreover, as soil suction descreases, the normalized void ratio become higher. The creep coefficient (Cαe) increased as temperature and vertical stress increased, while they decreased as the suction increased.

Estimating the electrical conductivity of clayey soils with varying mineralogy using the index properties of soils

Electrical conductivity assessment provides the valuable information required for comprehensive subsurface characterizations. This study aims to extend the existing modified Archie's equation for capturing the electrical conductivity of soils to various clayey soils in a broad range of plasticity. Five clayey soils that involve two kaolinite-, two illite-, and one montmorillonite-dominant fines were selected, and the bulk electrical conductivity σmix for the five clay samples in slurry state prepared in a modified oedometer cell was measured in a broad range of porosity n = 0.6–1.0 and pore water conductivity σw ≈ 0.013–4.5 S/m. The results showed that surface conduction plays a critical role in the determination of the electrical conductivity of clayey soils when subjected to a relatively low pore water conductivity environment. In this context, detailed analyses revealed that there exist robust correlations between the clay index properties and the unmeasurable input parameters for the modified Archie's equation such as the matrix conductivity λ and m exponent relevant to the tortuosity for pore water conduction. Comparison between the measured and predicted electrical conductivity for the raw clayey sediment used for model validation indicated that the unique global equation extended and verified in this study can be used for the first-order estimates of electrical conductivity of the in-situ clayey sediments.

Determination of Consolidation Parameters of Geomaterials Using Modified CRS Consolidation Testing System

Nowadays, the constant rate of strain (CRS) consolidation phenomenon has gained more attention as an alternate testing method to evaluate the consolidation characteristics of finegrained soils based on strain rate criterion. In the present study, a modified consolidation cell has been fabricated indigenously and carried out a series of CRS tests using a conventional triaxial loading frame at four different strain rates on reconstituted samples of marine clay at 0.002, 0.003, 0.004 and 0.005 mm/min, black cotton soil at 0.0025, 0.003, 0.005 and 0.0075 mm/min, red mud at 0.10, 0.125, 0.25 and 0.50 mm/min, and Varanasi local soil at 0.05, 0.075, 0.10 and 0.125 mm/min. Based on the pore pressure ratio (PPR) criterion, the best suitable strain rates for marine clay, black cotton soil, red mud, and Varanasi local soil were found to be 0.002, 0.0025, 0.1 and 0.05 mm/min, respectively. Further, the consolidation parameters namely primary compression index (cc), recompression index (cr), coefficient of consolidation (cv), coefficient of axial compressibility (av) and permeability (k) obtained through proposed CRS consolidation method are compared and validated successfully with the parameters obtained through Incremental Loading (IL) tests using both conventional and modified oedometer cells.

Investigation of small- to large-strain moduli correlations of rockfill materials — application to Romaine-2 dam

The establishment of relationships between the shear wave velocity (Vs) and other geotechnical parameters of rockfills under large strains (e.g., oedometer-constrained modulus, Moedo, and tangent elastic modulus, Et) is a significant step toward precise modeling of earth structure stress–strain behavior. In this study, four specimens reconstituted from the rockfill used for Romaine-2 dam construction were investigated experimentally to correlate small-strain to large-strain moduli. The development of Moedoand Vswith consolidation was measured in a laboratory using the piezoelectric ring-actuator technique (P-RAT) incorporated in a large oedometer cell. A correlation between Moedoand the small-strain shear modulus (Go) was proposed. Moreover, numerical simulations were performed using the Duncan–Chang hyperbolic model to correlate Vsand the Duncan–Chang initial elastic modulus (Ei), which depends on the minor principal stress (σ3). Based on the experimental and numerical data, a relationship between the Eiand Vsof the rockfill specimens was established. Verification studies were also performed using in situ measurements obtained from the Romaine-2 dam construction, and the ability of the proposed relationships to predict Eifrom in situ Vsmeasurements was demonstrated. The proposed correlations can help geotechnical designers estimate the deformation characteristics of rockfill materials from in situ Vsmeasurements.

Piezoelectric Ring-Actuator Technique: In-Depth Scrutiny of Interpretation Method

Abstract The piezoelectric ring-actuator technique (P-RAT) is a recent laboratory technique for the measurement of shear wave velocity (Vs) of soils. The measurement is based on transmission of a mechanical signal through the soil specimen with source and receiver transducers capsulated, for instance, in the end platens of an oedometer cell. An interpretative framework of the signals produced in the P-RAT has also been developed to minimize the subjectivity of the process and provide a consistent approach to Vs determination. However, there are some issues that would potentially affect the quality of the signals; consequently, biases of the velocity determination have not yet been explicitly discussed, such as the effects of sample and sensor characteristics as well as the signals used to excite the P-RAT sensors. P-RAT experiments on two different soils using different sensors, input signals, and oedometer cells are implemented in this study. The main purpose of the tests is not to elicit definitive information about the tested materials but to allow the P-RAT interpretation methodology to be revealed. The results suggest that this interpretation method can be beneficially used with other piezoelectric techniques (e.g., BE).

Perbandingan Pengujian Konsolidasi Menggunakan Alat Rowe Cell dan Oedometer pada Tanah Lanau Lunak

Soil consolidation testing using an oedometer and rowe cell. Oedometers are often used on clay and soft soils. However, in the development of the rowe cell device, the results of lowering soft soil were better than the oedometer. The advantage of this rowe cell is that it can determine the saturation value of the soil samples tested. The rowe cell tester can measure the pore water pressure at the beginning and end of each consolidation stage. This rowe cell can provide suitable settlement for soft soils. This consolidation test to obtain soil parameters such as Cv and Cc by using the rowe cell tool. After that, from the test results, the two tools were compared.

Compression Index and Small Strain Stiffness of Six Coal Bottom Ashes in South Korea

This study investigates the small strain stiffness (Gmax) of six coal bottom ashes in South Korea. One-dimensional compression tests using an instrumented oedometer cell were performed to measure the shear wave velocity (Vs), and the compression indices (Cc) of the six different bottom ashes were also explored in this study. The results demonstrate that, at low confinement, the Cc of bottom ashes are comparable to that of sand under a similar void ratio. However, at high confinement, the bottom ash shows very high Cc compared to sand due to the crushability of the weak and porous bottom ash particles upon confinement. The variations of α and β, which are Vs parameters (i.e., Vs = α (σ′m/1 kPa)β), with different Cc values of the tested bottom ashes are consistent with the existing empirical relationship. The Gmax of tested bottom ashes can be adequately captured by the existing Gmax estimating formula for angular sand, indicating that the Gmax of bottom ash is highly comparable to that of sand with a similar void ratio and confining stress.

Laboratory Investigation of the Mechanical Properties of a Rubber–Calcareous Sand Mixture: The Effect of Rubber Content

This paper introduces a rubber–calcareous sand mixture as a lightweight building material in offshore engineering. The mechanical properties of mixtures of varying rubber contents were investigated by performing a one-dimensional (1-D) compression test in a modified oedometer cell, as well as a resonant column test. A discussion on the test results, along with detailed interpretations regarding the role of rubber chips in the mixtures, are provided. It was found that the virgin compression curves of the rubber–calcareous sand mixtures tended to converge at a certain stress level, whilst the stress level depended on the rubber content. Moreover, the relative breakage was examined by comparing the particle size distribution curves of the calcareous sand before and after the compression test. It was shown that the grain crushing of calcareous sand was less remarkable with the inclusion of rubber chips. Furthermore, the small strain shear modulus (G0) of the mixtures decreased with the rubber content, yet the modulus reduction and damping curves exhibited little difference for the specimens of varying rubber contents.

An Electrical Resistivity Method of Characterizing Hydromechanical and Structural Properties of Compacted Loess During Constant Rate of Strain Compression.

Hydromechanical and structural properties of compacted loess have a significant impact on the stability and reliability of subbase and subgrade, which needs to be quickly determined in the field and laboratory. Hence, an electrical resistivity method was used to characterize the hydromechanical and structural properties of compacted loess during constant rate of strain compression. In the present work, compacted loess samples with a dry density of 1.7 g/cm3, a diameter of 64 mm, a height of 10 mm and different water content ranging from 5–25% were prepared. The constant rate of strain (CRS) tests were conducted by a developed oedometer cell equipped with a pair of horizontal circular electrodes (diameter of 20 mm) and vertical rectangular electrodes (width of 3.5 mm) to determine the electrical resistivity of compacted loess. The results showed that as average water content increases, plastic compression indices increase from 0.220 to 0.350 and the elastic compression indices increase from 0.0152 to 0.030, but they decrease to 0.167 and 0.010 and yield stress decreases from 381.28 kPa to 72.35 kPa. Moreover, as vertical strain increases, the variation trend of average formation factor and average shape factor for the lower water content decreases but increases for the maximum water content, and the anisotropy index first decrease and then tend to increase slightly, which indicates that the structural properties of unsaturated and saturated samples during compression exhibits different trend and the anisotropy of samples tend to be stable as vertical strain increases. As the water content increases, the average formation factor and average shape factor decrease, but the anisotropy index first decreases then increases, suggesting that water content has a significant impact on these electrical indices. More important, The coefficients of average formation factor decrease from 33.830 to −1.698 and the coefficients of average shape factor decrease from 8.339 to −0.398 as water content increases, whereas there is less variation for the coefficient of anisotropic index with a value of 2.190. An equation correlating average formation factor and water content and vertical strain is regressed to characterize the hydromechanical properties of compacted loess by measuring its impedance, which can be used to evaluate the stability of compacted loessic ground and subgrade.

Reliable prediction of structure settlement in Al-Najaf city using laboratory testing

This paper studies the effect of the specimen sizes and shapes on the settlement behaviour of sand soil with gypsum from Al-Najaf city in Iraq. The investigation aimed to verify the impact of the studied parameters of the prediction of the settlement using reliability-based analysis. Three sets of soil samples with different shapes and sizes, 60 mm diameter Oedometer cell, 60 x 60 mm box container and 100 x100 mm box container were remoulded with three different densities related to the maximum dry density, 100%, 95% and 92%. The results reveal that different settlement behaviours were determined at several normal stress levels during the soaking process. The samples exhibited reductions in volume due to the applied stresses, while under the same level of normal stress, the final settlement values due to soaking were higher for the samples with a lower value of initial relative compaction. Further, the test results showed that at any initial relative compaction and applied vertical stress, the higher settlement value is achieved by the larger square specimen (100x100)mm.

Investigation of small- to large-strain moduli correlations of normally consolidated granular soils

The establishment of correlations between the small-strain shear modulus (Go) and other soil parameters (such as the oedometer constrained modulus, Moedo) at large deformations constitutes an important step toward more precise modeling of soil deformation behavior. In this study, the shear wave velocities (Vs) of 22 different granular soils of various physical characteristics were measured experimentally using the piezoelectric ring-actuator technique (P-RAT) incorporated in the conventional oedometer cell. For each sample tested, the development of Moedowith the development of relative density (Id), as well as the void ratio (e), was recorded. Then, the obtained Vsand Moedo/Gotrends were correlated to the physical parameters of the tested granular soils with the development of e and Id. A practical application employing the achievements in geotechnical engineering design was also evaluated. Based on the proposed correlations, geotechnical designers can easily estimate in situ stress–settlement behavior from the predicted Moedoand Idvalues using simple in situ measurements.

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.

Desiccation crack formation beneath the surface

Desiccation cracks affect all mechanical and conduction properties. Previous studies have investigated the formation of surface desiccation cracks in fine-grained sediments in relation to their compressibility and suction potential. This study explores the formation of internal desiccation cracks in saturated kaolinite specimens consolidated inside self-reactive oedometer cells to reproduce the initial effective stress in buried sediments. X-ray tomography is used to monitor internal processes during consolidation and drying. Images capture the evolution of a perimetric fracture, transverse contraction, volumetric changes in entrained gas bubbles and the development of internal desiccation cracks. Entrained gas bubbles act as nucleation sites. They shrink during loading and early stages of desiccation. Eventually, the gas–water interface penetrates the soil at the bubble surface, pushes particles away, enlarges local pores and facilitates further growth of the internal desiccation crack. While air entry is grain-displacive in soft, fine-grained sediments, it becomes pore-invasive in stiff, coarse-grained sediments and internal desiccation cracks are not expected in silts or sands.

Numerical estimation of the one-dimensional consolidation behavior of clay

The consolidation behavior of clayey soils is traditionally evaluated in the laboratory using the one-dimensional consolidometer test. A new oedometer cell design with a ring of 60 mm in height, 75 mm in diameter was made to measure the excess pore-water pressure at the undrained base of the specimen and the friction between the soil and the ring, and to determine the ε – log p curve. This study deals with numerical modeling of the one-dimensional consolidation test and comparing the data obtained from the experimental study with the data from the modeling. In the modeling, the Soft Soil and Soft Soil Creep models were used for the clay proposed in this study. The results show, as a general trend, that the data from the numerical modeling are compatible with those from the experimental study.

Impacts of hydrate on the lateral stress in sediments

The ratio between the horizontal and the vertical effective stresses is defined as the coefficient of earth pressure at rest Ko. Ko in hydrate-bearing sediments is critical in understanding the stress states in hydrate-bearing sediments, yet has not been previously understood. An oedometer cell equipped with vertical and horizontal stress measurement sensors is used to measure the evolution of Ko in tetrahydrofuran hydrate- bearing sands during hydrate formation and dissociation and vertical stress changes. The results show that the response of Ko in hydrate-bearing specimens reflects the combined effects of hydrate cementation, the viscous nature of hydrate crystals, and the stress levels. These results can enhance the understanding of stress anisotropy and geomechanical behaviors of hydrate reservoirs during gas production.

Laboratory Investigation and Modelling of the Thermal-Mechanical Properties of Soil in Shallow Mineralized Groundwater Area

The capillary rise of shallow mineralized groundwater can contribute to the salinization of the soil layers. The excessive salt amounts adversely affect soil physical and mechanical properties, as well as the heat transfer performance, all of which are key factors with regard to the design of geothermal-related earth structures such as geothermal energy piles (GEP), ground source heat pumps (GSHP), and earth-air tunnel heat exchangers (EATHE). Therefore, in this study, the thermal-mechanical properties of saline soils are systematically investigated. A series of thermal and mechanical response tests were carried out under different salinity conditions, and the shear wave velocity-stress behavior of saline soil was measured using a modified oedometric cell coupled with an anchored bender element pair. Experimental results showed that saline soils generally have higher dry density and lower optimum moisture content at higher salt contents. The shear strength of saline soil increased about 5% while the salt concentrations of bulk solution increased from 0 mol/kg to 6 mol/kg, and the shear wave velocity increased by 50% to 83% when the normal load increased from 12.5 kPa to 250 kPa for sodium chloride- (NaCl-) treated soil and 39% to 52% for calcium chloride- (CaCl2-) treated soil. In addition, the thermal conductivity decreased by 0.121 W m-1 K-1 for NaCl-treated soil and 0.129 W m-1 K-1 for CaCl2-treated soil on average when the salt concentration increased from 0 mol/kg to 6 mol/kg. Finally, an elastic shear modulus (G0) model and a thermal conductivity (K) model were formulated for saline soil for the first time, and the effectiveness and feasibility of the proposed models were validated by comparisons of the model predicted values and experimental data.

Creep and consolidation of a stiff clay under saturated and unsaturated conditions

In this paper, the one-dimensional (1D) time-dependent behaviour of natural and reconstituted London Clay samples under saturated and unsaturated conditions is studied. For this purpose, a set of 1D consolidation tests including multi-staged loading (MSL) oedometer tests and single-staged loading (SSL) long-term oedometer creep tests were carried out on saturated and unsaturated specimens. Conventional oedometer cells were used for tests on saturated specimens, whereas a newly designed unsaturated oedometer cell, equipped with two high-capacity tensiometers (HCTs) for suction measurements, was used for unsaturated tests. The tests results revealed stress dependency and suction dependency of primary and secondary consolidation responses of the soil samples. Furthermore, counter to formerly acknowledged suggestions of independency of the slope of normal consolidation line to suction changes, it was observed that an increase in suction results in a decrease of the slope of compression curve (Cc) and the creep index (Cαe) values and an increase in yield vertical net stress (σp). Moreover, the Cαe/Ccratio for London Clay was found to be stress- and suction-dependent, unlike the previously suggested hypotheses.

Particle crushing in hydrate-bearing sands

The depressurization of natural hydrate deposits to produce methane gas can increase the effective stress in hydrate-bearing sediments, which may induce sand particle crushing. Sand crushing may become detrimental to gas production, because the breakage of sand particles can cause sediment contraction, reduction in permeability, and the generation of finer particles exacerbating solids migration and sand production. This study experimentally investigates particle crushing in tetrahydrofuran hydrate-bearing sands. Sandy specimens with various hydrate saturation are loaded to different stresses (up to 40 MPa) in an instrumented oedometer cell to induce sand crushing. The results show that the presence of hydrate crystals inhibits sand particle crushing. More pronounced sand crushing occurs in specimens with lower hydrate saturation and under higher maximum effective stress. Hydrate crystals can cement neighboring sand particles and constrain the particles from rotating and rearranging during loading. Relatively larger particles in hydrate-bearing sediments experience surface grinding with no significant decreases in particle sizes, and shear-off asperities of the larger particles contribute to an increase in the proportion of smaller particles. The total volumetric strain in hydrate-bearing sediments due to loading can be estimated using the known hydrate saturation and the volumetric strain of hydrate-free sediments subjected to an identical load. The changes in hydraulic conductivity due to sand particle crushing are correlated to the breakage index. The results highlight that gas production from hydrate deposits using depressurization increases the effective stress and the loss of hydrate crystals can exacerbate sand crushing.