TRACE

Figure 235. --Spring Creek Powerplant storm drainage bypass structure--General plan and sections.

41-D 1074

1,110 feet long, and approximately 190 feet above the bed of Spring Creek. The dam has a maximum base width of 1,040 feet. The embankment is constructed of two zones, a selected weathered rock zone upstream and a miscellaneous fill zone downstream. A 10-foot layer of selected miscellaneous material underlies the downstream zone. The upstream slope is protected by 3 feet of riprap on 18 inches of bedding, and the downstream slope is covered with a 12-inch-thick blanket of rock above elevation 770. (See also sec. 305.)

303. Reservoir. (a) Storage Allocations. --Storage space in the reservoir is allocated as follows:

This allocation is based on initial reservoir capacity and will require adjustment as the reservoir fills with sediments and the sill of the intake structure is raised.

(b) Releases. --Releases from the reservoir will be made through the outlet works, and it is intended that they will be equally divided between the two regulating gates. The plan is to release all water as rapidly as possible consistent with a dilution ratio of 1:45 with Sacramento River water to prevent fish kill.

304. Geology. Quartz-diorite (granite) underlies the entire foundation area for Spring Creek Debris Dam. It is a light greenish-gray, predominantly medium coarse-grained, and mostly very closely and irregularly jointed rock. Severe to moderate weathering increases to depths of 50 feet or more up the slopes, while fresh to almost fresh rock is found at river level.

Smelter waste material and rubbly fill mantle the right abutment to depths of about 10 feet. The smelter waste material is composed of roasted and untreated pyrite, slag, and rubble to depths of 4 feet and is highly acidic. The remaining part of the foundation is covered by colluvium and young and old terrace gravels to about 17 feet in depth. Unconsolidated gravel, with cobbles and boulders to 3 feet in size, comprise the river alluvium which is up to 12 feet thick in depth.

Joints dip predominantly 50° or steeper and strike principally northeasterly. A number of faults cut the foundation; several are from 5 to 10 feet in width. The major fault pattern is steeply dipping and northeasterly trending. See chapter II for detailed geology of the general area.

305. Earth Dam Details. See section 302 for a general description of the dam.

(a) Zoning. --The embankment was built of disintegrated and decomposed granite from required excavation and from the borrow area. By careful selection, some of the excavation required for the spillway and outlet works was used in the zone 1 portion of the embankment; however, the major portion of the structure excavation was utilized as zone 2 material.

The upstream face is protected from the erosion of wave action by 3 feet of riprap on 18 inches of bedding. The downstream face is protected from erosion by zone 2 material. The larger and harder of the materials placed in the zone 2 portion of the dam embankment were deposited near the downstream slopes. In addition, a 1-foot-thick layer of riprap quarry undersize rock was placed on the downstream 1-1/2 to 1 and 2 to 1 slopes of the dam embankment and the spillway channel walls. The need for this cover had become apparent when earlier rains caused a considerable amount of erosion along these surfaces.

(b) Specially Compacted Earthfill. --Special compaction was provided in areas where it was not practicable to use the normal tamping roller. These areas included zone 1 material surrounding the spillway and outlet works within the limits of the dam embankment. Also, the specifications quantity included an allowance for 24 inches of specially compacted earthfill on the bottom of the cutoff trench.

(c) Crest and Camber Details. --Slightly more than 6 feet of freeboard is provided to protect against wave action and overtopping of the embankment. A camber of 2 feet is provided between stations 14+00 and 16+00 with diminishing amounts toward both ends of the dam. This amount of camber will assure that the freeboard will not be diminished by foundation settlement and embankment consolidation.

A 6-inch layer of zone 2 material was placed as a surfacing on the crest to support the occasional maintenance and operational vehicles and to protect the crest from erosion. The surface of the crest is sloped to direct runoff from rainfall onto the upstream slope of the embankment.

(d) Drainage. --A minimum of 10 feet of selected material was placed under the zone 2 portion of the embankment to provide an exit for any seepage that may collect in the downstream portion of the embankment.

(e) Stability Analyses. --Stability analyses for Spring Creek Debris Dam were performed according to the modified Swedish slip-circle method in which all of the body forces and the neutral stresses are considered. Six selected trial arcs were studied. These trials included the limiting stress conditions occurring during construction, during high-level steady state (maximum reservoir water surface elevation), and drawdown.