2021-03-05_REVISION - M1977141 - Laserfiche WebLink

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Lyons Quarry �`` 112d Reclamation Permit Operations 77` Technical Revision modeled, although these blocks are deemed highly unlikely to be generated by the quarry slope, providing a factor of safety. The 30-foot long runout area was selected because it corresponds to the eastern end of the northeast dacite highwall where the runout area is narrowest but has an opposite- facing slope at 1H:1V across from the 30-foot runout area. The slope height of 50 feet at this location provides a factor of safety of over five times the necessary slope height of 9 feet to catch all rock sizes simulated with a 30-foot runout and an opposite slope of 1 H:1V. The area with the lowest opposite berm height is the western end of the northeast-facing dacite highwall where the runout area is 130 feet long and the opposite facing berm is 20 feet high, which is well within the limits of the evaluation results. An additional factor of safety can be assumed since the critical section is applied to the entire highwall. With the catchment basin in Quarry 1 as described above, observation of the northeast-facing dacite highwall can only be made from a significant distance. An alternative catchment berm alignment is shown in Figure 6 that would allow closer observation of the dacite highwall. The alternative berm has been placed 30 feet from the toe of the quarry highwall and should be at least 18 feet high based on the evaluation results discussed above, and to provide an adequate factor of safety. This alternative berm was included for consideration on the preliminary design but was not included in the intermediate or pre-final designs. A rockfall evaluation was also conducted for the northwest-facing dacite highwall with the critical Section B shown on Figure 6, to evaluate the necessary height of a catchment berm to be constructed at the highwall toe. The results of the evaluation indicate that with a runout length of 30 feet, an 8-foot high berm constructed with 1 H:1V slopes will catch 100%of all rock sizes modeled, which includes the spherical sandstone blocks up to 12 feet in diameter(which are highly unlikely) providing a factor of safety. The runout length of 30 feet was selected for evaluation as the narrowest available area at the western extent of the highwall. Increasing the berm height to 12 feet will provide a factor of safety of at least 1.5 at the south end where the runout length is shortest and higher along the remaining alignment where the runout length increases from 30 to 60 feet. According to the Kleinfelder report; "Rockfall events could also result in a dangerous condition to structures or people who happen to be located at the base of the slope during a rockfall event. This potential hazard should be effectively mitigated by adequate limitation of access to areas of potential rockfall" (Kleinfelder, 2008). Aggregate Industries proposes to install appropriate land use controls, such as fences and signage, around a buffer zone restricting public access to any areas of potential rockfall or fall hazards. The alignment of this proposed fencing is shown in Attachment B and Chain Link Fence specifications for the land use controls have been developed using Mile High Flood District Technical Specifications. 3.5 Stormwater and Erosion Control Features With the proposed changes to the site grading plan, corresponding revisions to the approved Technical Revision#2 plan for channeling stormwater runoff and controlling erosion will be necessary. As with the original plan, slopes equal to or less than 2H:1V will be stabilized with appropriate Best Management Practices (BMP's) including straw wattles and/or silt fencing. Slopes steeper than 2H:1 V may not be feasible to install BMPs but will be channeled to the rockfall catchment basin and a secondary earthen berm around the scour berm to capture potentially suspended sediments. March 2021 1 12