Figure 196. --Spring Creek Power Conduit gate shaft--Plan, profile, and sections.

DELETED GENERAL PLAN. ADDED LURGE TO MIDT DEGE.
BELOLATER FENCE, ADDED PENCE DETAIL

THIS DRAWING SHEETS I THRU

SUPERSEDES

The designs for the siphon were similar to previous Bureau designs for this type of structure. Conditions and criteria which governed the designs are given in some detail in paragraphs that follow.

With approximately 14 feet difference in elevation between the outlet portal of Tunnel 1 (elevation 986.09) and the inlet portal of Tunnel 2 (elevation 999.80), the siphon alinement was established to avoid the development of unnecessary pressure head (fig. 199). This required some extra length of siphon, but resulted in the low point on the siphon (which was set to accommodate storm drainage) being no more than about 1 foot below the outlet elevation of Tunnel 1. A few cuts, up to 40 feet in depth above invert level, were required in order to secure this favorable vertical alinement.

Previous Bureau experience with thin steel liners in siphon barrels had indicated their tendency to buckle inward if subjected to an external loading such as might be developed by ground water when the siphon was drained. To eliminate the possibility of developing ground-water pressures on the outside of the shell, the typical siphon section called for backfill to be placed only up to a point about 3-1/2 feet below the horizontal centerline of the conduit. Where it was necessary to backfill over the top of the siphon, as at a road crossing or near the tunnel portal cuts, the siphon section was developed to carry the greater external load and longitudinal drains were provided on each side of the section to prevent the accumulation of ground water. Alinement and typical details for the siphon are shown on figures 199, 202 and 203.

Stresses in the siphon section were computed by using coefficients from the Bureau's Engineering Monograph No. 61/ for moment, shear, and thrust. Support on the bottom of the siphon was taken to be along a 90° arc. The siphon throughout its entire length was designed to carry the maximum bursting head at normal working stresses. In addition to bursting, the siphon shell was reinforced (fig. 204) to carry the bending stresses induced by the dead-load weight of the shell full of water and, where necessary, the weight of superimposed backfill. An allowance for earthfill to a height of 10 feet above the top of the shell was considered for the design of the shell adjacent to the tunnel portals and for the county road crossing over the siphon. The normal working stresses, in pounds per square inch, applied to the design of the siphon shell were as follows:

In those locations where the siphon was designed to be without earth cover, the effect of inside and outside temperatures on the siphon shell was computed by assuming a 50° F. temperature differential across the shell. Moments derived from this assumption were added to the moments in the shell due to other loads. At the top of the siphon shell, the effect of combining the moment due to temperature differential with deadload moments was to increase the computed reinforcement stress due to dead-load moments by about 18 percent. Because of reinforcement selection, however, the stress at the top of the section due to combining all moments was still less than 1 percent in excess of the normal allowable design stress of 20,000 pounds per square inch. The increase in reinforcement stresses caused by the horizontal bend at station 118+06.89 was kept within allowable limits by the use of a sufficiently large radius on the bend (fig. 199). The radius used for the bend at this location was dictated practically by the requirement for structural stability with the siphon under internal head.

In order to allow for possible temperature effects in a longitudinal direction, transverse joints are located along the siphon at intervals not to exceed about 150 feet. These joints are of a type previously used by the Bureau, and will accommodate longitudinal expansion or contraction of the siphon. Where the siphon barrel is not restrained by external forces, the total change in 150 feet of length was computed to be 0.54 inch for 50° F. change in temperature. One of the joints can accept such a movement. Details for the joint are shown on figure 202.

Access to the inside of the siphon is provided by two manholes, one located at station 99+55 and one at station 124+50. A 12-inch-diameter blowoff is also located at station 99+55 to facilitate unwatering the siphon. As the major portion of the water in the conduit will be drained through the powerplant at a rate of about 300 second-feet, the 12-inch-diameter blowoff will be required to drain only the water in the siphon which is not available, due to the inverse grade, for draining through the turbines. A pump sump was also furnished in the invert of the siphon at station 99+55 so that a pump might be installed to expedite the final unwatering.

Turnout facilities for domestic water supply to the city of Redding, Calif., were included in the designs for the siphon. These consist of a 48- by 36-inch steel cone nozzle out of the siphon, a 36-inch-diameter motor-operated steel gate valve, a 36- by 48-inch steel cone expander section, an automatic air-inlet and air-release valve for the turnout pipe, and concrete housing for these items.

A system of drainage channels and dikes alongside the siphon, together with two cross-drainage culverts and outlet channels, were provided to care for the runoff from storm waters. In addition to these facilities, an 8-inch-diameter draintile is laid with open joints and a covering envelope of gravel on each side of the siphon for its entire length. The purpose of these parallel draintiles is to reduce surface flow of water adjacent to the siphon so that erosion will not develop. Details of the tile drains are shown with the siphon section on figure 202, and details and locations of channels and dikes are shown on figure 203.

1/Olander, H. C., "Stress Analysis of Concrete Pipe, " Engineering Monograph No. 6, Bureau of Reclamation, October 1950.