ABSTRACT
ABSTRACT
This paper describes the planning, implementation and results of the Helms Project's rock mechanics program.
Pacific Gas and Electric Company is currently constructing a 1,050 megawatt underground pumped storage facility
in the Sierra Nevada. The Helms rock mechanics program was defined to include preliminary site evaluation,
in situ testing, geological mapping, field instrumentation and the resulting interpretations. These studies and
field operations were all part of a comprehensive monitoring program which provided pertinent design information,
promoted project safety and documented field conditions.
The rock mechanics program was designed to meet two performance criteria. It had to be (1) practical and
(2) economical. The critical adaptation made was to integrate cost-effective visual monitoring with the more
demanding field instrumentation or field testing. This composite "construction monitoring" program proved
effective and it was concluded that a practical and economical rock mechanics program can be developed for a
large underground project.
Introduction
Pacific Gas and Electric Company's Helms
Project is a three-unit 1,050 megawatt
hydroelectric pumped storage facility
located 50 miles east of Fresno,
California, in the Sierra Nevada. The
main civil contract involves extensive
excavation activities. The major
excavations include almost five miles of
large diameter tunnels, an underground
powerhouse and connecting transformer
vault, two surge chambers, a 1,000-foot
(305 m) elevator shaft and a gate shaft.
Over 1,000,000 cubic yards (750,000 m s )
of material, primarily granite, will be
excavated.
The project will connect two existing
reservoirs. Water from the upper
reservoir, Courtright Lake, El. 8,200
(2,500 m), will be released to generate
power during PGandE's peak electrical
demand periods. During off-peak periods,
the units are reversed and water pumped
from Lake Wishon, El. 6,500 (1,980 m),
back up to Courtright. The project was
first conceptualized in 1971. and it is
scheduled for completion in 1982.
The two large reservoirs, more than
120,000 acre-feet (1.5 x l0 s m s ) each,
were constructed by PGandE in 1956-58.
Their close proximity (Figure 1) and
large elevation difference were primary
factors in its selection as a potential
site for a large pumped storage facility.
A site reconnaissance and review of
aerial photographs confirmed favorable
geologic and topographic conditions.
Preliminary analyses used these factors
in conjunction with hydraulic and environmental
constraints to produce a design
which locates the major facilities
underground.
Description of Project
A plan and profile of the project are
shown in Figure 2. In the generating
mode, water starts its flow at a large
submerged intake-discharge structure
located near the bottom of Courtright
Lake. It then flows through 13,446 feet
(4,098 m) of 27-foot (8.3 m) I.D.
circular concrete lined tunnel and 200
feet (61 m) of 22-foot (6.7 m) diameter
steel-lined tunnel before dropping down
an inclined penstock shaft.
The penstock shaft inclined at 55 ø drops
a vertical distance of 1,330 feet (405 m)
and connects to the penstock manifold.
The penstock manifold bifurcates into
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1980. The University of Missouri Rolla. Permission to Distribute - American Rock Mechanics Association
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