Direct Shear Test Device Research Articles

Mechanical characterization of small shale samples subjected to fluid exposure using the inclined direct shear testing device

The mechanical strength, elastic moduli, and other properties of shale are known to be highly sensitive to any variation in pore fluid content and pore fluid chemical composition. To date, all existing rock mechanics testing procedures and studies attempting to quantitatively quantify these changes employ standard methods and techniques, i.e., shale testing under hydrostatic confinement, triaxial tests, or direct shear tests, which require very long fluid circulation time due to the intrinsic low hydraulic permeability and low solute diffusivity of shale rocks and large sample size recommended by these conventional methods. Moreover, standard-size samples for conventional rock testing methods are not always available, and the use of non-conventional size samples is sometimes required. In this work, shale testing and results on rock sample the size and shape of a stack of a few pennies are presented. The inclined direct shear test, an innovative patent-pending testing method for small and thin rock specimens, has been developed to facilitate mechanical strength characterization under variable confining pressure, with tested material exposed to different fluids at any desired exposure time, while dynamic elastic moduli can be simultaneously monitored as functions of applied stress state. This device has been compared to standard triaxial test with excellent results. Practical applications using the inclined direct shear testing device (IDSTD TM) for three shale rocks of different natures are also presented herein.

Analyses of Wellbore Instability in Drilling Through Chemically Active Fractured-Rock Formations

Summary Numerous time-dependent wellbore-instability problems have been reported while drilling through the chemically active fractured-shale formations in the Arabian Gulf. Very often, these shales are characterized by the abundance of not only macroscale bedding planes but also networks of microscale natural fractures. The presence of fractures weakens the shale mechanically and produces higher-permeability fluid-flow paths within the low-permeability rock formation. Because of different fluid-diffusion rates between the fractures and shale matrix, there are two distinct pore-pressure fields in saturated fractured shale. Additionally, in chemically active shale formations, osmotic pressure arises because of the imbalance in mud/shale chemical activity. Practically, it is extremely complex to isolate the fractures from the matrix for analysis or to identify fracture size and fracture density. However, a first-order approach in an attempt to understand the porous fractured-shale behavior is to use dual-porosity and dual-permeability theory of poromechanics in the analytical modeling. In this study, a poromechanical inclined-wellbore solution has been derived that incorporates time dependency, a primary porosity and permeability for the matrix, a secondary porosity and permeability for the fractures, and the chemical effect. The expressions for the stresses and pressure solutions are presented and detailed in Appendix A. These analytical solutions are used to simulate an inclined-wellbore-stability problem in a fractured-shale formation, accounting also for bedding planes in some instances in addition to the microfractures. Additionally, retrieved rock samples of fractured shale were tested by use of an innovative laboratory characterization device, the Inclined Direct Shear Testing Device (IDSTD) (PoroMechanics Institute; Norman, Oklahoma; 2008). This device tests tiny shale specimens (rock volume less than 0.14 in.3) and is capable of measuring cohesion and friction angle while the sample is subjected to in-situ stresses, varying mud pressures, and mud-circulation times.

Shear strength of unsaturated soil interfaces

Unsaturated soil interfaces exist where unsaturated soil is in contact with structures such as foundations, retaining walls, and buried pipes. The unsaturated soil interface can be defined as a layer of unsaturated soil through which stresses are transferred from soil to structure and vice versa. In this paper, the shearing behavior of unsaturated soil interfaces is examined using results of interface direct shear tests conducted on a low-plasticity fine-grained soil. A conventional direct shear test device was modified to conduct direct shear interface tests using matric suction control. Further, the results were used to define failure envelopes for unsaturated soil interfaces having smooth and rough counterfaces. Results of this study indicate that matric suction contributes to the peak shear strength of unsaturated interfaces; however, postpeak shear strength did not appear to vary with changes in matric suction. Variations in net normal stress affected both peak and postpeak shear strength. Failure envelopes developed using the soil-water characteristic curve (SWCC) appeared to capture the nonlinear influence of matric suction on shear strength of soil and interfaces.

Interface Bonding between Hot-Mix Asphalt and Various Portland Cement Concrete Surfaces

Interface bonding between hot-mix asphalt (HMA) overlays and portland cement concrete (PCC) pavements can be one of the most significant factors affecting overlay service life. In this study, a direct shear test device was built and used to investigate the characteristics of the HMA–PCC interface and to determine the interface shear strength. Several parameters affecting the interface performance were included in this study, such as HMA type, tack coat type, tack coat application rate, PCC surface texture, and temperature. The test results showed that the use of SS-1hP emulsion resulted in a greater interface bonding strength than the use of RC-70 cutback. Surface mix SM-9.5 was found to have a better interface strength than binder mix IM-19.0. Among the four residual application rates considered in this study, 0.05 gal/yd2 was found to be the optimum rate, independent of test parameters. The direction of tining in the PCC surface was found to have no effect on interface shear strength at 20°C. At the optimum tack coat application rate, the PCC smooth surface showed a higher interface shear strength than tined surfaces. When tests were performed at the intermediate temperature range, as the temperature increased, the interface bonding strength decreased.

Anisotropy-Based Failure Criterion for Interphase Systems

This paper presents a methodology for estimating the shear strength of interphase systems composed of granular materials and planar inclusions having various degrees of roughness. Existing empirical and semiempirical relationships between strength and surface roughness do not appear to be general and are unable to account for surface-particle interactions at the appropriate scales. The proposed method is based on the contact force anisotropy of those particles that touch the inclusion surface. It was developed using two-dimensional discrete element method simulations of interphase systems constructed within a direct interface shear test device. Particles consist of polydisperse and monodisperse spheres of constant median grain diameter. Surface roughness was varied by using profiles with regular and random asperities, and profiles of manufactured surfaces. Results indicate that the magnitude and direction of average contact total force at the interface controls strength. A bilinear relationship, independent of particle to surface friction coefficient, exists between the principal direction of contact total force anisotropy and strength. Results using the proposed criterion are in good agreement with laboratory results using spheres and subrounded sand.

PH—Postharvest Technology: Friction of Wheat on Corrugated and Smooth Galvanized Steel Surfaces

A new modified direct shear device was constructed and experiments performed to estimate the coefficient of friction of soft red winter wheat on corrugated and smooth galvanized steel surfaces. The modified direct shear test device is slightly different from most conventional direct shear testers in that the metal test specimen is pulled to create a shear plane as it is moved within stagnant granular material. In most conventional direct shear test devices, the granular material is sheared across the metal test specimen by a force applied laterally to the grain compartment. The effects of steel sample size, sliding speed and normal pressure on the coefficient of friction were studied for the corrugated sample using the modified direct shear device. Four speeds of 0·05, 0·5, 5, and 50 mm/min were tested. Tests were conducted at three different normal pressures of 6·9, 27·6, and 48 kPa. Using the modified shear tester the coefficient of friction of wheat on the corrugated surface increased with an increase in speed and decreased with an increase in normal pressure. ‘Slip-stick’ behaviour was observed for tests with sliding speeds lower than 2 mm/min. Coefficients of friction determined with the shortest and/or the narrowest steel sample were significantly higher (α=0·05) than the values obtained with larger steel samples.The test device was also adapted to enable measurements of the friction force of wheat on smooth galvanised steel. Friction was measured for two sliding speeds (0·5 and 5 mm/min) and three normal pressures (6·9, 27·6 and 48 kPa) At low speeds, slip-stick was observed that ceased after the speed increased to a critical value of 0·2 mm/min. Coefficients of friction were found to decrease with an increase in vertical pressure.Friction was measured using a tilting table test method on the same steel samples which had been used for the tests with the modified shearing device. The coefficient of friction had a tendency to decrease with an increase in normal pressure. Values of the coefficient of friction determined using the tilting table test method were in the same range as those determined for test conditions using the modified shearing device at the lowest normal pressures and sliding speeds.Results from either the tilting table method or the modified shearing method can be applied to design conditions at low pressures and low sliding speeds. For higher normal pressures and greater sliding speeds, the modified shearing method should be applied. For practical applications the modified shearing method should be used because it most closely smulates the actual conditions at the frictional interface in a grain bin when grain slides down the wall.