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<br />6.2 Shear Strength -Unaltered Materials <br />An approach developed by Barton and Choubey(1977) was employed which relates the peak <br />friction angle for a material to the surtace roughness, the base friction angle, the strength of the <br />rock and the applied normal stress on the failure plane. The following equation is employed: <br />F ~ o = o tan (ub +JRC log JCS) <br />a <br /> O~ <br />' where, <br /> o is the shear strength along the discontinuity <br /> o the normal stress acting across the joint <br />o <br /> m <br />b the base friction angle for rock <br /> JRC the joint roughness coefficient <br />' JCS the uniaxial strength of the joint wall rock <br />' <br />i A value of 25 degrees is generally assumed for the base friction angle for unaltered materials <br /> (Einstein et al, 1979). Employing a surface roughness of JRC = 10 (moderate) and a JCS of 9900 <br />' psi with a normal loading stress o} 60 psi, results in a friction angle of 47 degrees. However, for <br /> purposes of design a value of 35 degrees was considered as the upper bound. NVhere altered <br /> materials occur this value may reduce to 20 degrees, which is generally considered to be the lower <br />' bound for highly oxidized materials. (Einstein et al, 1979). <br /> 6.3 Shear Strength -Weak Materials <br /> Figure 12 shows the results of shear strength tests, as recorded in the laboratory, for nhe weak rock <br />' mass materials. The testing irnolved shearing an intact sample followed by pertornning a staged <br /> shear test on the surtace formed. This will determine the intact cohesive strength for the material <br />' and the joint residual properties. This was performed under moist and dry conditions for the Santa <br />' Fe, Pink Gneiss and the Green Clay for SRMR ratings between 10 - 1596. Remoulded samples of <br /> Santa Fe and Pink Gneiss were tested (SRMR = 10%) in order to enable a lower bound strength to <br />' be assessed. <br /> This resulted in three strength curves to be determined: <br />1 hi <br />f <br />i <br />h <br />h <br />h <br />h <br />h <br />i <br /> ear strengt <br />1) Upper bound <br />ntact s <br />curve w <br />c <br />re <br />ers to t <br />e strengt <br />of the mater <br />al that <br /> must be broken prior to undergoing movement <br />' <br /> 2) Residual shear strength curve which refers to the strength parameters after movement has <br />' occurred along a surface <br /> 3) Lower bound remoulded shear strength curve which assumes that the material has no <br />' inter-particle bond strength <br />1 <br /> <br />