REP17188 - Laserfiche WebLink

Home Browse Search

to 200 ft across. As a result of these anomalies, the strike of the foliation joints varies by 45 degrees either direction from the predominant strike direction and the dip typically varies by 35 degrees or more. 5. Spacing of joints in the primary joint set typically ranges from several inches to approximately three ft. Joint surfaces vary greatly from fresh to highly-weathered. Weathered schist interbeds having a slick and weak character are frequent throughout [he rock mass. At least two secondary joint sets are present, often orthogonal with the primary joint set. These secondary joints typically occur at intervals of one to five ft. In general, secondary joint surfaces are planar, and are fresher and less weathered than the primary joints along foliation and do not exhibit the smooth schist or mica surfaces. The slope of the debris below the newly-excavated benches was measured at a dip of approximately 43 degrees from horizontal. Based on the conditions observed with the present exposures, the rock mass exposed is consistent with the conditions used as the baseline for analyses of the East Highwall. This evaluation may change as additional rock is exposed in this area. The predominant dip angle is slightly steeper than [he inclination of the overall slope and, as a result, the joints typically plunge into the slope but daylight on the vertical and near vertical individual highwalls. One of the primary concerns is that alarge-scale preexisting joint or shear feature exists, which would allow for alarge-scale slope failure. Review of the rock exposures did not reveal any features noted to be of concern which extended more than two benches. How- ever, the most likely problem feamre would be a shallow dipping joint which daylights and could form an unstable wedge near the base of the slope. ]t is likely that any such features would be obscured by the slope debris below the freshly-excavated benches. Localized block and wedge failures of the outside edges of the benches are common in the upper two highwalls: These upper two highwalls are typically in the more weathered rock near the ground surface and were drilled vertically. In areas where the outside of the benches have fallen away, the bench width has decreased by approximately 5 to IS ft. One crack was noted at [he south end of the upper highwall and extending [o the east by up to 10 ft. After careful review, it was determined that this feature is probably limited to the upper highwall and bench and does not extend further downhill. However, this feamre will continue [o be observed as excavation continues. Localized block and wedge failures of the outside edges of the third and fourth benches are common but less prevalent than in the upper nvo benches. This difference in behavior is attributed to the sloped highwalls (from the angle drilling) and from the lesser degree of weathering of the parent rock material resulting in a better quality rock mass. Geometry of the slope is such that the fourth bench will likely have to be less than the planned 50 to 60-ft width. Based on the observed conditions that the rock quality has increased below the upper two benches, a bench width between 40 and 50 ft is expected to G.~l@OO~R09000.300