Tower of strength

2 March 2002

Paul Zaman and Norman Holst explain how excavation works were completed successfully at the Diamond Valley reservoir inlet/outlet tower foundation and approach channel in California, US

The Diamond Valley lake (formerly Eastside reservoir), located in Riverside County near Hemet, is the largest reservoir in Southern California, US. It is used to store Colorado river aqueduct water delivered through the San Diego canal, and State Water Project water delivered through the new inland feeder and its extension, the new Eastside pipeline. The water stored here will be released through the same conveyance structures into Southern California's Metropolitan Water District (MWD) distribution system to meet demands during the summer months and periods of drought. The reservoir also serves as a source of emergency supply in the event of a major earthquake damaging the MWD delivery conduits, which cross known active faults, including the San Andreas Fault. The reservoir would provide a six-month emergency supply to Southern California, while the conduits were repaired.

Construction of the Diamond Valley lake was essentially completed in 1999. The project was named by the American Society of Civil Engineers as one of five 2000 Outstanding Civil Engineering Achievement finalists. The award recognises civil engineering projects that contribute to the community well-being, demonstrate resourcefulness in planning and solving design challenges, and use of innovative construction methods.

Two earth and rockfill dams, the East and West dams, impound the 986M m3 reservoir. The West dam rises 86.9m above the original valley floor and is nearly 2.4km long. The East dam stands 56.4m high and 3.2km long. In addition, a 0.8km long, 39.6m tall saddle dam closes a low point in the reservoir north rim. Total embankment volume is over 84M m3.

Water conveyance into and out of the reservoir is controlled by hydraulic structures that include the forebay, the P-1 pumping plant, the P-1 pressure tunnel, the inlet/outlet tower, and the secondary inlet, all located at the west end of the reservoir. The three-story pumping plant contains 12 variable speed, two-stage pumps, each rated at 6032hp to deliver water to the reservoir. The 82.3m tall reinforced concrete tower is the primary structure controlling entry of water into the reservoir and release of water into the MWD distribution system.

As part of the MWD project design team, black-veatch was responsible for reservoir site selection and feasibility studies, design of the hydraulic structures, and start-up support. The company also provided geological mapping and senior geotechnical supervision during construction.

Tower foundation

Since the tower is essential to supply water to Southern California in the event of an earthquake, it must be capable of withstanding a seismic event. It was therefore socketed into the reservoir's rim by making steep cuts around the tower's foundation and in the walls of the adjacent approach channel.

The tower foundation and the 823m long approach channel were excavated through ground with difficult geologic conditions. The channel is 182.9m wide at the bottom and becomes progressively deeper until it reaches 91.4m at its north end.

Pre-construction investigations in the area of the excavation identified potentially unstable slope conditions. Early recognition of this potential and implementation of the following measures helped assure successful completion of the excavation:

• Involvement of design personnel throughout the construction provided continuity and verified the correlation of design assumptions with foundation conditions.

• Continuing investigations and monitoring during construction to evaluate foundation conditions as excavation progressed.

• Contingencies in the plans and specifications to allow adaptation of slope support and foundation remediation.

The tower and channel site is underlain by medium to very thinly interlayered and strongly foliated quartzite, phyllite and schist. The rock is broken by five sets of discontinuities, which include the foliation, and four moderately to widely spaced joint sets. The foliation, dipping 50? to 60? to the northeast and trending parallel to the excavation axis, is the prevalent discontinuity. Shear zones subparallel to the foliation are common. These are expressed as thin clay seams where they parallel the foliation and zones of close fracturing with up to several inches of clay gouge. Kinematic and analytical methods used to evaluate the cut slopes indicated a potential for block slide failure in the foundation west wall, where the foliation dips into the excavation, and for toppling failure in the east wall, primarily due to these foliation shear zones.

Excavation design

The space between the structural concrete walls of the tower and the socket-shaped excavation was to be filled with concrete up to EL1660. To minimise the mass of concrete, it was preferable to use the steepest slopes practicable, consistent with slope stability. However, the potential for slope instability produced by the pervasive rock foliation made this difficult.

The shallow, upper slopes outside the tower footprint and socket, in highly weathered and moderately weathered rock, were designed to have slopes of 1.5H:1V to 1H:1V, requiring no rock reinforcement. The actual footprint of the tower was located in slightly weathered rock with slopes as steep as 1H:10V around the socket. Benches 3-6m wide were provided at each break in the slope. The northeast and northwest walls of the channel in the slightly weathered to fresh rock on either side of the tower foundation were designed at 1H:4V slopes. The channel sidewalls that trend parallel to the rock mass foliation were designed with maximum slopes of 1H:1V, so they would be stable without reinforcement.

Two design features were used to compensate for the adverse dip of the rock foliation in the west tower foundation wall:

• A broad bench was located at EL1660.

• The wall overall slope was reduced by designing it as a series of four segments intercepted by 3m wide benches and by laying the upper three segments back to 1H:4V.

The basic pattern of rock reinforcement for slopes of 1H:4V and 1H:10V consisted of grade 150 rock bolts, 7.6m long and 2.5cm in diameter, installed in a staggered 2.4-by-2.4m pattern. A secondary, or contingent, rock reinforcement design consisting of longer, 4.4cm diameter, grade 150 rock bolts was prepared for the western slope to support an unstable block of the largest size that could be liberated by the foliation and the excavation geometry. The specifications also called for at least 7.6cm of fiber-reinforced shotcrete on the steeper slopes.

The tower foundation design instrumentation consisted of inclinometers and extensometers installed in the socket east north and west slopes. Inclinometers were installed in the vertical cored holes drilled from the 1660, 1680, and 1700 foundation benches. Single point, remote reading extensometers were installed in pairs, each pair consisting of a short and a long instrument, with the lengths selected to straddle the lengths of the installed rock bolts.

Excavation for both the tower foundation and channel began in May 1996. The first blast was made on 12 July. In October 1996, excavation reached the tower socket at EL1660.

The series of planned core borings were drilled from the 1660 bench to assess the condition of the rock and the need for the contingent 4.4cm diameter rock bolts in the west foundation wall. Each core boring intercepted intervals of sheared and broken rock. Many of these appeared to be developed along foliation, and some could be projected as foliation shear zones daylighting near the toe of the future west wall of the tower foundation. The investigation program confirmed the potential for an unstable slide block in the west wall of the foundation, necessitating the use of the contingent rock bolts.

Excavation of the tower foundation below EL1660 began on 16 November 1996 and proceeded concurrently with excavation of the approach channel.

A programme of foundation geologic mapping began with the 1660 bench and included all the slopes and benches below this level. The height of the steep cuts surrounding the tower footprint required special attention to slope stability and safety. After removal of the shot rock from each blast, the cuts were inspected for closely spaced joints or excessive blast damage and the foundation geology was mapped before placement of shotcrete.

Minor modifications to the slope reinforcement included spot bolts and enlarged bearing plates. Major modifications included removing slightly more than half the 1660 bench to form a new bench at 1640 when bench 1660 was discovered to be too closely fractured to provide a bearing surface for the planned 4.4cm rock bolts.

Core holes drilled as the foundation approached final grade determined that the bearing capacity was adequate without remedial treatments. The blast to elevation 1499 was made on 10 June 1997. During the remainder of June and the first half of July, the east portal of the P-1 pressure tunnel was completed. On 24 July 1997, the elevation 1490 sump was shot, resulting in successful completion of the excavation for the tower foundation.

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