2023-06-28_ENFORCEMENT - M1982121 ‎(2)‎

• Permit No: M1982121
• IBM Index Class Name: Enforcement
• Doc Date: 6/28/2023
• Doc Name: Rock Failure Analysis and Stability Memo
• From: RMR Aggregates, Inc.
• To: DRMS
• Email Name: ACY
• Email Name: THM
• Media Type: D
• Archive: No

Rockfall Runout Setback
A prescriptive setback was defined from the base of the highwall to the maximum extent of
rock block endpoints across the three East face transects.The 2D sections illustrating the steps,
bounce height and endpoints for the 3,000-block run are provided in Appendix E.The maximum
endpoint block with the longest runout is highlighted. In all three transects, the maximum
runout block was an outlier and considered a conservative estimate for probable rockfall.
Figure 1 represents the setback zone from the base of the highwall that is defined by this
conservative estimate for maximum rockfall runout. No man work shall be performed within
the setback without additional stabilization or barriers. Figure 2 illustrates the rockfall
maximum endpoints and the boundaries of the rockfall setback zone from the toe of the
highwall. Coordinates of the setback and a Google Earth kmz file have been provided to RMRA
to designate the setback.
Rockfall Berm
A rockfall berm was modeled on the three East face transects as a remedial measure to reduce
the size of the setback zone (Figure 2), defined above. The berm size and location were defined
through an iterative modeling process to minimize the size of the berm and decrease the
setback from the highwall toe. Based on computational rockfall modeling, we support using the
equivalent of a berm composed of limestone scree with a height of 15 feet, crest width of 5 feet
and maximum slope angle of 32 degrees. Maximum kinetic energies modeled along the ten
transects are all within that tolerance of maximum allowable impact energy. Rockfall analysis
provided in Appendix E indicates that 100% of simulated rockfall blocks were contained by the
rockfall barrier, in tandem with the catchment basin. Where the rockfall berm is impacted by
larger blocks, the barrier should be repaired. The berm is considered in tandem with a setback
from the highwall toe that will act as a catchment basin. A Rockfall Catchment Area Ditch
(RCAD) is recommended along the entire length of the East face. Parameters contributing to
RCAD effectiveness include 1) slope height and angle, 2) ditch width, depth and shape, 3)
anticipated block size and quantity of rockfall, and 4) effect on rock fall trajectories of slope
irregularities (Wyllie and Mah, 2004). The RCAD will also act as a retention basin for fallen rock
to be cleaned overtime. Rockfall modeling of the RCAD and berm design is effective at reducing
the southern extent of the rockfall setback zone.
Longterm Inspection Program
An effective proactive approach to slope stabilization will require a consistent, long-term
program of inspections and periodic maintenance of the berm and catchment area. Rockfall
blocks should not be permitted to accumulate. Damaged portions of the berm should be
repaired immediately. Periodic inspections of the slope and outcrops by an engineering
geologist or geotechnical engineer will be required over time to investigate natural
deterioration of the stability conditions due to 1) weathering/erosion of the surface rock, 2)
increases in fracture aperture by water causing loosening of surficial blocks, 3) loss of block
interlock or support following minor block failure, and 4) growth of vegetation roots.
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