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s • <br />2. Daylighted fracture of joint sets are potentially subject to plane she r sliding failure <br />whenever the fracture is flatter than the slope angle and steeper th n the angle of <br />surface friction along the fracture. The resistance to sliding along t e natural joints <br />must be exceeded by the down dip thrust of the potential sliding bl ck of rock for a <br />failure to occur. Resistance to sliding along joints is provided by fri tion created by <br />the weight of the rock above the joint, the roughness and irregulari of the joint <br />surfaces, whether the joint is open or closed, and if a mineral infilli g heals the <br />joint. Other factors that influence the degree of resistance to slidin are the dip <br />angle, presence or absence of water, if there is a clayey or slickens de fracture <br />surface, and the presence, extent, and strength of intact rock bride across the <br />fracture surface. The personnel assigned to measure and map the ractures must <br />make observations of these features and log them in the field notes to make a <br />complete evaluation of the stability of the proposed highwall config ration. The <br />field measurements and observations may be used to estimate the roportion of <br />broken and intact rock along potential joint failure surfaces. Obser ations and <br />testing may be used to estimate the strength of intact rock bridges f they are <br />present along adverse fracture orientations. <br />3. If adverse fracture orientations are present at the Surface Rock Pit, f may be <br />necessary to conduct a physical testing program to determine if the existing <br />highwall configuration will be stable in the long term. A testing pro ram would <br />typically include uniaxial and triaxial testing of representative sampl s of the <br />different types of rock exposed in the quarry, as well as direct shea testing of <br />fracture surfaces to estimate the degree of shearing resistance avai ble along the <br />fractures. Shearing resistance along adverse fracture orientations c n be adjusted <br />to include the influence of fracture irregularity and intact rock bridg s, and can be <br />applied in limiting equilibrium slope stability analyses. These analys s simply <br />compare the forces tending to cause movement (the force of gravit on a rock <br />mass above a fracture surface) to the force resisting movement, w ich are the <br />frictional forces. A ratio of these forces yields the safety factor for the highwall. <br />Any safety factor above one, if the inputs to the ratio are accurate, indicates that <br />the highwall will be safe. <br />4. The final configuration of the highwall and pit floor must be capabl of catching <br />falling rock. Call (19861, prepared a series of conservative catch b nch widths that <br />will almost always produce a catch bench that will prevent rock fall from reaching <br />the bottom of the pit. The operator could utilize this work in desig ing rock <br />catching berms along the base of the existing highwall. Please see he attached <br />figure 2. <br />This letter, and the enclosed memo, provides a fairly rigorous discussion o the <br />procedures necessary to demonstrate a safe and stable highwall and mate ial stockpile. <br />However, if a stepwise procedure is followed, the amount of field work, p ysical testing, <br />and analyses required can be minimized. For example, if the operator can demonstrate <br />that there are no adverse fracture orientations dipping out of the quarry, a d that the rock <br />2 <br />