The oedometer is one of the simplest and most reliable laboratory tests. Its main drawback is that stress state is not fully known. Over the years, a number of enhancements were intended to overcome this limitation. This work presents the development of a large size oedometer for testing gravels made of crushed rock under varying moisture conditions. Load application is done by air pressure acting on the upper end of the specimen (Rowe type oedometer), and the lateral stress is measured using a strain-gage-instrumented oedometer ring. Special features were included in the ring geometry for enhancing the lateral stress measurement signal, while keeping the deformation of the ring to acceptably low levels. The lateral friction between specimen and oedometer wall is computed as the difference between the applied load and the reaction force at the specimen base, measured by means of three load cells. The performance of the oedometer is studied by means of a finite element model of the oedometer ring. The deformability of the ring and the production of spurious readings of lateral stress due to varying operation conditions are evaluated and found to be satisfactorily low. Some experimental results on a rockfill type material are presented. The lateral stress and friction data are analysed in terms of the lateral stress ratio and in the '-q space. The observed behavior is found to be stress- and suction-dependent. The results are consistent with triaxial tests of the same material published elsewhere.
This paper describes the capabilities of a novel technique to investigate crack formation and propagation in drying soils. The technique is a relatively simple, non-destructive indirect technique using a ground-penetrating-radar (GPR) system to detect cracks that form and propagate inside a soil specimen during desiccation. Although GPR devices have been used for multiple applications, their use in soils for the detection of small desiccation cracks has not been demonstrated yet. The experiment and the methodology used to test the accuracy of a small compact commercial GPR device for crack identification are described. The main objective was to identify what type of signals and what crack width and separation between them can be detected using the GPR device. The results indicate that cracks of 1 or 2mm wide can be detected depending on its position and shape, whereas sub-millimeter cracks are undetectable with the currently existing devices in the market. Regardless of this limitation, the GPR method can be useful to find time-related bounds of when the cracks appear, to point at their location and sometimes at the separation between two of them. Detection of cracks with origin at the bottom or within the specimen was accomplished with this system. Distances of 5 cm or more between cracks can be detected and measured, as well, with accuracy.
This paper presents the development of a new high-pressure triaxial apparatus specifically prepared for inducing and tracking the degradation of clayey rocks. Total suction—used to induce the hydraulic degradation—is applied with vapor transfer technique by controlling the relative humidity of air in contact with the material. The evolution of the degradation process along different hydro-mechanical stress paths is continuously tracked with bender elements, as well as with air or water permeability measurements on partially saturated or saturated states, respectively. Relevant test results on a low porosity clayey rock (Lilla claystone, Spain) are presented to evaluate the main capabilities of the new equipment. The results bring up the high sensitivity to water of the material, which is evidenced by the important reduction of shear wave velocity induced on first wetting and drying at low confining stress, as well as by the significant increase in the air permeability of the degraded material (around four orders of magnitude larger than the intact material). Test results also showed clear differences in the volume change and shear strength behavior of undisturbed, saturated, and degraded samples, highlighting the relevance of degradation on macroscopic behavioral features.
This paper describes the development of a new triaxial cell specifically built to test coarse granular material under partially saturated conditions. The partial saturation is achieved by imposing a total suction that is correlated with the relative humidity. Relative humidity control was achieved by water vapor transfer using a forced convection system, which is driven by an air pump transporting the air humidity to the sample, and it is controlled by saturated saline solutions placed in a vessel.A double-wall cell was designed to monitor global volume changes of the material on compression and shearing. In addition, the cell uses a novel technique to measure local axial and radial deformations. The use of both global and local measurements allows detecting experimental problems such as membrane penetration on isotropic compression and shearing, as well as membrane
sliding on shearing at low confining stresses. Selected test results are presented to show the capability of the cell during isotropic compression and shearing made with strain and stress control. The results of the test with strain control show a nonlinear increase in strength and dilatancy of the material in relation to the decrease of the relative humidity. Also they have a tendency to reach the critical state, and it can be seen that such a critical
state line is a function of the relative humidity. Under a constant deviator stress, a collapse test of the rockfill was carried out and the results showed that the deformation depends strongly on the time. Furthermore, it is larger than that produced by making a test under saturated conditions, commonly taken as final reference.
The paper presents an innovative oedometer cell (EIT oedometer), accomplishing for monitoring the spatial and temporal evolution of different physical quantities inside soil samples through seismic and electric non-destructive measurements. The technical solutions implemented to perform correct electrical measurements are reported together with the results of benchmark tests demonstrating the potentialities and the limits of the 3D electrical resistivity tomography in detecting both pre-existing and induced sample heterogeneities. It is shown that resistivity imaging can offer a powerful tool for the investigation of soil heterogeneities not detected by external measurements. The relationship between electrical resistivity and soil properties makes this application potentially useful for monitoring the evolution of transient processes as for instance those related to the diffusion of chemical species in clay soils and associated coupled chemo-mechanical processes, whereas the information gathered by classical oedometer measurements and by seismic waves propagation could be used to explore the associated macroscopic phenomena.
An apparatus for performing tests under different environmental conditions was designed and constructed. Basically, the equipment consists of an automatic system for the observation of the hydromechanical behavior of porous media and it has three components: physical parts, an electronic interface, and a control system with an automatic register. It is possible to use it for tests in saturated or unsaturated conditions with different porous materials. In this study the apparatus is used to develop a study of flow in saturated and unsaturated conditions and the flow and transport of solutes in saturated conditions. In order to demonstrate results obtained with our equipment independent tests were carried out with other samples of metallurgical waste. The experimental results demonstrate that the apparatus is capable of measuring surface suction, vertical shrinkage, temperature, relative humidity, change in the water content at different points in the sample, and evaporation. The performance of the equipment under different conditions is proved to be excellent, showing that it is possible to increase or reduce the size of the sample and it is also possible to change different parameters according to the aim of tests. Data from flow and transport studies show that desiccation cracks reduce the resident time of water and solute in porous media.