Accession Number : ADA019145

Title :   Dynamic Strength of Rocks.

Descriptive Note : Final rept. for Period 1 Sep 72-31 Aug 75,


Personal Author(s) : Handin,John

Report Date : 15 NOV 1975

Pagination or Media Count : 51

Abstract : Experimental work was done on four subprojects with potential rock-mechanics applications ranging from earthquake prediction and control to advanced excavation technology. Measurements of the rate of flow of air through porous Navajo Sandstone with simulated fractures show that as confining (overburden) pressure increases, the fractures (1) have a greater fractional rate of permeability loss than does the rock as a whole, (2) experience the greater inelastic deformation, and (3) become effectively sealed when their permeability approaches that of the whole rock. A cause of the dilatancy that is premonitory to certain earthquakes may be the feather fracturing, associated with frictional sliding on pre-existing faults. In Coconino Sandstone specimens sliding on a sawcut, extension microfractures develop near the 'fault zone.' Their abundance increases with increasing normal stress and shear displacement, and they dilate the rock. Frictional sliding of Tennessee Sandstone under cyclic loading has been investigated in an electrohydraulic, servocontrolled testing machine. At confining pressures in the range of 0.5 to 0.75 kb, no stick-slip occurs at low shear-displacement rates of the order of .0001 m/ sec, but it does occur at .001. The static coefficient of friction is about 0.8, independently of normal stress and displacement rate over the narrow ranges of these tests. Stick-slip is favored, when the testing machine is effectively soft. The stress drop occurs in less than 30 microseconds so that no machine could react quickly enough to arrest the event.

Descriptors :   *Rock, *Rock mechanics, Fracture(Mechanics), Cracking(Fracturing), Strength(Mechanics), Earthquakes, Mathematical prediction, Control, Excavation, Simulation, Sandstone, Stresses, Strain(Mechanics), Stress strain relations, Compressive properties

Subject Categories : Geology, Geochemistry and Mineralogy

Distribution Statement : APPROVED FOR PUBLIC RELEASE