2001-04-20_REVISION - M1984001 - Laserfiche WebLink

Home Browse Search

~~ ~ a April U, 2001 GCO Job No. 697 hOt 2 :; it ,1 Stability of the Parcel A High Wai! The DMG memo quotes our review of a previous high wall stability assessment published by J.F.T. Agapito & Associates, Inc. (AAI, June 19, 1967) as "We did not identify high wall stability conditions on review of this document', In fad, we did not identify high wall stability concems on review of this document. The AAI report stated i'; "The proposed final wall geometry appears to be more stable than the {previous) -,_; existing condition". We understand the proposed final wall configuration was not achieved. At the time of our site visits, the face of the high wall was rubblized. We do not believe fracture mapping of this area would be representative of the structure because of the face rubblization. If the DMG requires fracture mapping of the high wall area, we request the rubblized near surface zone be removed for viewing of fractures more representative of the high wall structure. Removal of the rubblized Zone would also reduce concems with rock fall. We understand the DMG may request topographic mapping of the high wall area. ~} We requested site topographic mapping be completed in our letter dated November 29, ;, 2000. Topographic mapping was not considered feasible at that time. We agree that monument hubs would be a fairly inexpensive method to monitor (potential) post reclamation movements if a survey is completed. As a part of the investigation completed, we ident~ed the overall height of the high wall to be about 170 feet above and about 370 feet horizontally from Highway 141. Given the initial near vertical wall height of about 110 feet, we believe the identified catchment berms can significantly limit or curtail potential rock fail from the remaining 60 ;: vertical feet over 370 horizontal feet. The DMG memo suggests considering partially backfilling the toe of the high wall with existing talus and generated rock in order to -' provide a buttress to the high wall, We believe the increase in slope stability could be + offset by decrease in factor of safety against rock roil out. We have not identified . concems with slope stability but we have identified concems with potentlal rock fall, We agree rock specimens could be tested for shear strength and analytical stability analysis could be performed to estimate stability. The purpose of this phase of the investigation was to use (in our opinion} reasonable assumptions and analysis to develop an opinion of whether the high wall poses a threat to Highway 141. Our analysis of rock roll out did not indicate impact from potential rock roll out in the high welt area to Highway 141, Due to the relatively large horizontal distance between the . bottom of the near vertical wall and Highway 141, we do not believe slope stability poses a corlcem to the roadway either.