True Triaxial Test Of Rock Under Stress And Strain Rate Control

The significance of true triaxial tests instead of conventional tests has been enhanced because of the need for generalized constitutive laws for geomaterials. To investigate the behavior of rock masses having restricted conditions for strains, tests of marble under strain rate control as well as stress rate control were performed using a specially designed true triaxial apparatus with rigid platens. As an example of a set of restricted conditions, the condition of the mean principal strain being constant was adopted when the octahedral shear strain was increased. The direction of stress paths obtained under these conditions is discussed, along with the elastic, dilatant, and contractile behavior of marble as clarified by stress controlled tests.

This is the first book ever published on the problems of true triaxial testing of rocks addressing all aspects of true triaxial testing of rocks, including: (i) true triaxial testing techniques and procedures; (ii) test results: strength, deformability, failure mode, permeability, acoustic emission, and elastic wave velocity; (iii) constitutiv

"Although the triaxial compression test is presently the most widely used procedure for determining strength and stress-deformation properties of soils, there have been no books published on triaxial testing since the 1962 second edition of the landmark work The Measurement of Soil Properties in the Triaxial Test by Bishop and Henkel. It is apparent there is a need to document advances made in triaxial testing since publication of Bishop and Henkel's book and to examine the current state of the art in a forum devoted solely to triaxial testing. Because of increasing versatility brought about by recent developments in testing techniques and equipment, it is also important that the geotechnical profession be provided with an up-to-date awareness of potential uses for the triaxial test."--Overview.

A new experimental apparatus for rock-specimen analysis has been developed in Laboratoire 3SR (Grenoble) to characterize the evolution under loading of the deformation mode, placing particular attention on the onset of strain localization and the post-localization regime in rocks. The experimental apparatus can be configured to apply true triaxial or plane strain conditions on prismatic specimens, with a confining pressure of up to 100 MPa. The three principal stresses are independently controlled to apply true triaxial conditions. When applying plane strain conditions, the stress perpendicular to the observed specimen's surface is proportionally varied as a function of the loading applied on the specimen, to satisfy the condition of zero strain along this direction. The prismatic specimen can be observed under loading, because its surface, orthogonal with the direction of the intermediate principal stress, is in contact with a hard window (sapphire), thus allowing pictures to be taken throughout the test, as well as enabling full-field measurement. Photographs are analyzed by digital image correlation (DIC) to measure displacement and strain fields. The new true triaxial apparatus is presented here, first with a description of the experimental setup and mechanical characteristics of the cell. Then, the experimental procedure adopted to apply and assess the speckle pattern on the sample's surface, as required for DIC, is detailed. Finally, selected results from a test performed with the true triaxial device on a Vosges sandstone sample, performed in plane strain compression, are shown. The onset of strain localization and the shear bands' pattern evolution are shown by means of incremental strain fields obtained by DIC.

Engineers wishing to build structures on or in rock use the relatively new discipline known as rock mechanics. Comprehensive Rock Engineering is an up-to-date comprehensive work of reference containing a compilation of knowledge in one coherent publication. Clearly illustrated throughout, this multi-volume publication covers every aspect of rock mechanics and rock engineering. The work is arranged in five volumes under the themes: Fundamentals; Analysis and Design Methods; Rock Testing and Site Characterization; Excavation, Support and Monitoring; and Surface and Underground Project Case Histories, providing information for rock engineering application.

The authors systematically describe the general principles of Kolsky bars, or split Hopkinson bars, which are widely used for obtaining dynamic material properties. Modifications are introduced for obtaining reliable data. Specific experiment design guidelines are provided to subject the specimen to desired testing conditions. Detailed Kolsky-bar examples are given for different classes of materials (brittle, ductile, soft, etc) and for different loading conditions (tension, torsion, triaxial, high/low temperatures, intermediate strain rate, etc). The Kolsky bars used for dynamic structural characterization are briefly introduced. A collection of dynamic properties of various materials under various testing conditions is included which may serve as a reference database. This book assists both beginners and experienced professionals in characterizing high-rate material response with high quality and consistency. Readers who may benefit from this work include university students, instructors, R & D professionals, and scholars/engineers in solid mechanics, aerospace, civil and mechanical engineering, as well as materials science and engineering.

Analysis and Design Methods

Compression tests for investigating the time-dependence of rock under confining pressures are time consuming and require more specimens than do uniaxial compression tests. In this study, a new testing method combining multi-stage confining pressure and alternating loading rate is proposed to investigate the loading-rate dependence of triaxial compressive strength from a small amount of rock sample. In the test, a small-size rock specimen 10 mm in diameter and 20 mm in height was loaded under a strain rate alternating between slow and fast and under a confining pressure increased once just after reaching the peak strength. Strengths corresponding to two strain rates and two confining pressures could be obtained from one specimen. The cohesion, angle of internal friction, and loading-rate dependence of strength from the data were consistent with those from the commonly used triaxial compression tests under a constant strain rate or constant confining pressure. The proposed test with a small amount of rock sample can reduce not only the time and cost but also the environmental load of sampling in situ.