LOCATED AROUND 18 MILES (29KM) southeast of San Jose, California, US, the Anderson Reservoir (or Anderson Lake) is Santa Clara County’s largest surface water reservoir – an 11km long ribbon of water storing local rainfall-runoff and imported water from the Central Valley Project. With a capacity of nearly 90,000 acre-feet, the reservoir is a critical part of the region’s water supply system, supplying drinking water treatment plants and groundwater recharge schemes. It can also hold enough water to supply around a million people for a year.
But all is not what it seems at the picturesque reservoir. For it will soon be taken out of service, albeit temporarily, due to the discovery of a change in the predicted seismic performance of the Anderson Dam – a 235ft-high, 1,430ft long and 900ft wide earthen-type structure. First, came the discovery of a relatively loose layer of alluvium in the dam’s foundation on both upstream and downstream sides. Second, was the layer of poorly compacted, liquefiable material in the lower rock-fill shell on the downstream side.
According to Valley Water, the local water authority: “These 5-10 feet-thick layers of weak material (located beneath 15-70 feet of rock-fill shells on both the upstream and downstream sides of the dam) are susceptible to a significant reduction in strength when subjected to severe shaking.”
But the problems do not stop there: the dam is located not only near the active Calaveras Fault, but two other faults (the Coyote Creek Fault and the Range Front Fault) run directly under the dam. When these faults were discovered in 1949 – a year before the dam was completed – they were thought to be inactive and of no particular concern. But earthquake science has moved on, and more recent studies have suggested that the faults may be active.
Unfortunately, an outlet tunnel and pipe run beneath the dam (see Figure 1) where it crosses the fault lines. As the outlet pipe tunnel structure was not designed to withstand earthquake-induced displacements, it must be replaced. The new Anderson Dam Tunnel (Figure 2) will form a low level outlet that will release and regulate water flows and – if a major earthquake strikes – allow quick drawdown of the reservoir to ‘deadpool’ status, ie. the point at which the water is so low that it cannot drain under gravity through the existing outlets.
THE CURRENT PLAN
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By GlobalDataGreater understanding of the fault lines prompted the Federal Regulatory Energy Commission (FERG) to order Valley Water to lower water levels in the reservoir so that if an earthquake caused the dam to fail, the risk to the public would be minimal. A study in 2010 concluded that an earthquake of 7.25 magnitude occurring just 1.2 miles away could cause dam failure and send a 35ft wall of water racing down the valley into the town of Morgan Hill, as well as cause severe flooding in San Jose.
To achieve FERG compliance, Valley Water has since October 2020 been slowly lowering the water level in the reservoir. It is doing this by sending most of the water to treatment plants for delivery to water retailers. It is also releasing it into Coyote Creek and percolation ponds to replenish groundwater supplies. By mid-December 2020, the water levels in the reservoir were at just 3% of capacity. This is the lowest achievable that allows proper functioning of the existing outlet tunnel.
Lowering reservoir water to the lowest possible level will allow the construction of a new outlet tunnel (high and low level) and reconstruction of the dam and spillway to meet current standards. Achieving all this to modern-day standards is estimated will take around 10 years. But it could not have come at a worse time: Santa Clara County – like much of California – has been classed as being in a state of ‘extreme drought’.
REPLACING THE DAM
Anderson Dam is an embankment dam of the earth-filled type, which means it is constructed primarily of natural materials and has an impermeable clay core to prevent water infiltration. Such dams rely on their sheer mass to hold back the reservoir forces. In profile, the shape is a symmetrical embankment. According to Valley Water, the replacement dam will also be earth-filled and composed of similar materials to the existing construction but will be compacted to a much higher degree. Reconstructing the dam embankment will require prior removal of all the identified liquefiable material. However, a significant portion of the existing dam’s clay core will be retained.
Throughout the works, the water authority will be monitoring dam condition through two basic parameters: water seepage through the dam and physical movement of the structure. Piezometers measure water pressure and the readings are transmitted to the water district offices via satellite. Movements in the dam will be measured via inclinometers located in the dam and surface monuments.
Valley Water is also building a high-capacity channel that will accommodate the water flow from a bigger outlet valve. From the current capacity of 500cfs, the increased flow will be around 2,500cfs – a 400% increase. This allows Valley Water to leave Coyote Creek intact as it is now, but the creek will continue to flow for the entire 10-years of construction using the pump-station pipe outlet near the base of the dam.
THE TUNNEL
When Valley Water received the emergency order from the FERG in February 2020, the Anderson Dam Seismic Retrofit Project (ADSRP) was divided into two phases of which the Anderson Dam Tunnel Project (ADTP) is the first.
ADSRP will include construction of two new intake and outlet systems to the designs of engineering consultancy AECOM. Stage one entails constructing a diversion tunnel with low-level outlet works (LLOW) for normal operations; stage two involves constructing high-level outlet works (HLOW) to improve high-level evacuation capability.
The tunnel will comprise three sections:
(1) A 400ft-long, 8ft-diameter ‘lake tap’ pipe upstream of the dam (Figure 3), connected to
(2) A 100ft microtunnel boring-machine launch chamber, connected to a 925ft-long, 19ft-diameter reinforced concrete-lined pipe housed within a 24ft-diameter tunnel (Figure 4); connected to
(3) A 375ft-long, 13ft-diameter steel pipeline within an 18ft-diameter horseshoe tunnel (Figure 5).
The three distinct parts of the tunnel built as part of the ADTP will form some of the components of the LLOW and the HLOW during the overall seismic retrofit project (ADSRP).
With the exception of the small length mined by MTBM, the majority of the tunnel will be excavated by roadheader. The lining will be sprayed concrete for initial support followed by a secondary lining of cast-in-place concrete. The tunnel contract is for 980 days (around two years and eight months) with the excavation expected to take around 30 months.
GEOLOGY
The geology along the tunnel alignment is expected to comprise two main types: around 25% of the tunnel will be in Santa Clara Formation, comprising siltstones, sandstones and conglomerates. The remaining 75% or thereabouts will be in the Franciscan Complex which comprises serpentinite, graywacke, greenstone (including basalt and metavolcanics) and mélange.
According to the engineering team: “The geology along the alignment is generally tight and is not anticipated to produce large flows of water, except at locations where the tunnel crosses fault zones where larger flushing flows could occur. The upstream portion of the tunnel is being excavated using a roadheader and is expected to be in the wettest and the weakest rock. It is thought that an assortment of ground conditioning methods will be used in some portions of the tunnel alignment, including probe holes, grouting with micro-fine cement, and tunnel pre-support such as rock dowels, fiberglass face bolts, spiling, or canopy tubes.”
Construction of the tunnel is expected to start in June 2021 and is expected to last for between 24-36 months. During construction of the dam retrofit project – scheduled to start in 2024 – the majority of the newly-built tunnel, with some modifications at the upstream end, will be used for water diversion during construction.
Final outlet works include construction of 78-inch and 33-inch- diameter pipes inside the diversion tunnel and will be the primary means of water release during normal reservoir operations. The 78-inch diameter pipe is designed to accommodate the potential displacements of fault zones located along the tunnel alignment that could occur during a large earthquake, in which case the tunnel could be damaged. The 33-inch diameter pipe is not designed to survive the fault offset elastically, according to Valley Water. The tunnel has been conceived to have a design life of 50 years.
With the design work completed, the project is now at construction stage. Valley Water awarded the construction contract on 27 April 2021 with work expected to start in June or July 2021.
