2000-07-31_GENERAL DOCUMENTS - M1978305

• Permit No: M1978305
• IBM Index Class Name: GENERAL DOCUMENTS
• Doc Date: 7/31/2000
• Doc Name: Highwall stability
• From: Allen Sorenson
• To: Bob Oswald
• Email Name: RCO
• Media Type: D
• Archive: No

Dickerson Pit Wall Stability 2 July 31, 2000
ordinary soil slope. If the rock classifies as R2, moderately weak rock, further testing to determine
the shear strength of the rock would be required to decide if it could be benched or terraced for
reclamation. Rock that classifies at R3 or higher is not likely to fail through the intact rock, and
only the fracture orientations would be of concern.
3. Daylighted fracture or joint sets are potentially subject to plane shear sliding failure whenever the
fracture is flatter than the slope angle and steeper than the angle of surface friction along the
fracture. The resistance to sliding along the natural joints must be exceeded by the down dip thrust
of the potential sliding block of rock for a failure to occur. Resistance to sliding along joints is
provided by friction created by the weight of the rock above the joint, the roughness and irregularity
of the joint surfaces, whether the joint is open or closed, and if a mineral infilling heals the joint.
Other factors that influence the degree of resistance to sliding are the dip angle, presence or absence
of water, if there is a clayey or slickenside fracture surface, and the presence, extent, and strength of
intact rock bridges across the fracture surface. The personnel assigned to measure and map the
fractures must make observations of these features and log them in the field notes to make a
complete evaluation of the stability of the proposed highwall configuration. The field
measurements and observations may be used to estimate the proportion of broken and intact rock
along potential joint failure surfaces. Observations and testing may be used to estimate the strength
of intact rock bridges if they are present along adverse fracture orientations.
4. If adverse fracture orientations are present at the Dickerson Pit, it may be necessary to conduct a
physical testing program to determine if the proposed pit wall configuration will be stable in the
long term. A testing program would typically include uniaxial and triaxial testing of representative
samples of the different types of rock exposed in the quarry, as well as direct shear testing of
fracture surfaces to estimate the degree of shearing resistance available along the fractures.
Shearing resistance along adverse fracture orientations can be adjusted to include the influence of
fracture irregularity and intact rock bridges, and can be applied in limiting equilibrium slope
stability analyses. These analyses simply compare the forces tending to cause movement (the force
of gravity on a rock mass above a fracture surface) to the force resisting movement, which are the
frictional forces. A ratio of these forces yields the safety factor for the pit wall. Any safety factor
above one, if the inputs to the ratio are accurate, indicates that the pit wall will be safe.
5. As continued quarrying encroaches upon the final pit wall location, it is important that intact rock
bridges along any adverse fracture orientations are preserved to ensure long term stability.
Production blasting practices may potentially damage or destroy the rock bridges. If adverse
fracture orientations are present at the Dickerson Pit,the Operator may be required to commit to a
perimeter blasting plan. This plan would allow the Operator to employ standard production blasting
practices in the majority of quarry operations. As the final pit configuration is approached, alternate
blasting practices could be implemented to protect the integrity of rock bridges. Such practices
include cushion blasting, precision blasting, smoothblasting, and presplit blasting.