The shear strengths of several rocks have been measured using a shear test with either axial or hydrostatic pressure on the rock specimens. Axial pressure produced shear strength-normal stress relationships which correlate closely with Mohr envelopes from triaxial tests. The shear strengths in these tests were slightly higher than those measured by triaxial tests. In the shear test, fractures are forced along definite planes, while in triaxial tests the fractures can choose the weakest paths through the specimens since the stresses on parallel planes are equal. This may account for the difference in the strengths. Hydrostatic fluid pressure on plastic- coated rock specimens produced much smaller increases in shear strength than axial pressure. Apparently, the intermediate stress had some effect on the strength of rock. Friction along rock fracture surfaces decreases to a steady-state value after about 0.01 to 0.02 inches displacement. This steady state friction increases approximately as the 0.4 to 0.8 power of the contact pressure. The coefficient of friction (friction stress/ /normal stress) decreases from about 1.8 to 0.8 as the contact pressure is increased to the compressive strength of the rock.


La resistance au cisaillement a ete mesuree pour differentes roches au moyen d'une epreuve sous poussee axiale ou hydrostatique. Une correlation etroite apparait entre les relations resistance au cisaillement-contrainte normale obtenues sous poussee axiale et les enveloppes de Mohr obtenues par epreuves triaxiales. Ces dernières conduisent cependant à une resistance au cisaillement legèrement inferieure. Cette difference peut être expliquee par Ie fait que dans l'epreuve triaxiale les fractures peuvent se developper suivant les lignes de moindre resistance (puisqu'il y a egalite des contraintes sur des facettes parallèles) alors que dans l'epreuve de cisaillement les fractures sont forcees suivant des plans bien definis. L'augmentation de resistance au cisaillement est beaucoup plus faible par emploi d'une poussee hydrostatique (avec revêtement plastique du specimen) que par emploi d'une poussee axiale II semble que la contrainte intermediaire joue un rôle dans la resistance des roches. Le frottement le long du plan de fracture decroit et atteint une valeur constante après un deplacemeqt de 0,25 a 0,50 mm. Cette valeur limite est sensiblement proportionelle à la puissance 0,4 a 0,8 de la pression de contact. Le coefficient de frottement (rapport de la contrainte de frottement à la contrainte normale) decroit de 2,0 a 0,5 lorsque la pression de contact augmente jusqu'à la resistance de la roche en compression.


Die Schubstarke von mehreren Gesteinen wurde in Scherexperimenten unter axialem oder hydrostatischem Druck ermittelt. Axialer Druck verursacht Schubstarke zu Normalspannung Verhaltnisse, die gut mit von Mohrschen Spannungskreisen erhaltenen Werten uebereinstimmen. Die gemessenen Schubstarken waren etwas höher als in triaxialen Experimenten. In Scherexperimenten verlaufen die Brueche in bestimrnten Ebenen, wahrend sich die Brueche in triaxialen Experimenten entlang der Richtung geringster Starke entwickeln, weil die Spannung in parallelen Ebenen gleich ist. Dies möglicherweise ist der Grund fuer den Unterschied in der Schubstarke. Hydrostatischer Druck auf mit Plastik ueberzogene Gesteinsproben verursachte eine wesentlich geringere Erhöhung der Schubstarke ais axialer Druck. Die mittlere Hauptspannung hat offensichtlich einen Einfluss auf die Starke von Gestein. Die Reibung entlang der Bruchflache fallt nach ungefahr 0,25–0,5 mm Verschiebung auf einen konstanten Wert ab, der seinerseits wie die O,4–0,8-te Potenz der Normalspan- nung zunimmt. Der Reibungskoefficient (Reibungsspannung/ Normalspannung) fallt von ungefahr 1,8 auf 0,8 wenn der Normaldruck auf die compressive Bruchstarke des Gesteins erhöht wird.


The shear strength of rock is an important parameter In predicting rock failure and in designing underground openings. Triaxial tests are normally used to measure Mohr envelopes, but these tests require elaborate equipment and are too time consuming for routine testing. The paper describes a shear test which can be used to obtain these Mohr envelopes on a routine basis. The test Specimens may be subjected to either axial or hydrostatic pressure. This test can also be used to study the friction along shear fracture surfaces, an important factor in geological faulting and other areas of rock mechanics.

Description of Shear Test

The shear tests are made by first applying an axial force, Fa' to the specimens (Fig. 1). Some localized failures were observed at these points, indicating that these stress concentrations do exist. Because of non-linear elastic and plastic deformation that proceeds failure, the exact stress conditions in this shear test cannot be determined. The utility of this test can best be determined by comparing data from this test to data from triaxial compression tests. The results of tests on specimens with thicknesses varying from 1/8 to 1/2 inches showed that the shear strength is independent of specimen thickness. It is concluded that the rock specimens are not weakened by the stress concentrations at the surface. Apparently the localized failures and plastic deformation tended to equalize the shear stresses along the failure planes.

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