Spillway Clearing. Approximately 115 acres, 40 in the spillway and chute and 75 in the emergency spillway area, were cleared of brush and trees. The area below the emergency spillway was not cleared.
Excavation. The three major methods used to excavate the spillway were as follows: bottom-loading scrapers and pushcats, a loader with cats feeding the belt and bottom-dump wagons hauling the material, and two large shovels. In general, the scrapers were used to strip the area to rock. The shovels were used to excavate the rock after it was drilled and shot. The loader was used similarly to the scraper operation, the main difference being that it was possible to work this operation in rougher terrain as up to eight dozers were used to push material to the feeding hopper. A road was graded out below the hopper and the bottom-dump wagons could drive under the hopper to load.
All drilling for blasting was done by percussion type drills mounted on tracks and powered by air. The patterns varied greatly from area to area. The most generally used pattern was 8 by 8 feet; however, patterns ranging all the way from 2 1/2' by 2 1/2 feet to 15 by 15 feet were used depending on the area, type of rock, and excavation objective.
Excavation near structure lines had to be controlled to avoid damage to the rock to be left in place, and 840,000 tons of riprap for Oroville Dam had to be produced.
In part of the emergency spillway, an additional 10 feet of excavation was required to reach acceptable foundation rock, resulting in considerable additional time for excavation and placement of the backfill concrete to subgrade.
Approximately 90% of the chute foundation required blasting to reach grade. The only extra excavation directed was removal of a few clay seams in the foundation and a few areas where slope failures occurred.
The depth of overburden in the approach channel was deeper than estimated and the slopes had to be changed from 1/2 to 1' to 1 1/2 to 1' to prevent sloughing as the excavation reached the final grade.
The slopes in the flood control outlet gate section proved to be of a lower quality rock than anticipated. There were several large seams running parallel with the chute. The planned anchor bars were replaced with grouted rock bolts, pigtail anchors, and chainlink surface covering in that area.
