ROCK ZONE DAMAGE STUDY

The region in which the rock mass is disturbed is generally known as the Rock Damage Zone (RDZ), the zone where the rock is physically damaged (mechanical properties are permanently affected). Tensile micro-cracking, exacerbated by low confinement conditions near excavations, leads to a unique failure process (slabbing in the extreme) that is inconsistent with conventional shear based failure criteria. Rock mass strength near underground excavations is controlled by damage initiation mechanisms that are relatively insensitive to confinement and by fracture propagation (extension) mechanisms that dominate at low confinement. Small amounts of confinement are required to mobilize frictional resistance on joint surfaces such that wedges or blocks become stable under gravity loading. Loss of this confinement is responsible for delayed failure of structurally defined rock masses. Loss of confinement in laminated ground leads to increased gravity induced displacements and failure. Rock Damage Zone (RDZ) studied here by selecting six Egyptian rock types such that Basalt, Granite, Oyster, Marble, Limestone and Sandstone, where cylindrical samples ( 5.4cm. in diameter 10.8cm. in length) prepared from that rocks were tested in tri-axial tests at different confining stress levels ( 1, 2, 3, 4, 5, 8, and 10 MPa,) in order to construct ( σn(tau)- t ) curves for all rock types. Based on the tangent to the shear strength curves, the Mohr-Coloumb rock parameters { σn- (tau)-c - and φ } were determined at σmin. and σmax-, also the IIoek-Brown parameters (m - G.S.I.} were determined . Two approaches were used here to establish the upper bound ( no damage), the lower bound ( heavy damage ) and the based case to estimate the values of σCm and c using the mechanical properties of the selected rock types. Finally, a statistical relation was developed using the multi regression analysis to identify the variations of rock properties { c, φ, m and GSI. } at different values of confining stresses.