Newell Creek Dam (NCD) at Loch Lomond Reservoir in the San Lorenzo Valley, California, is an earth fill dam standing 195 ft tall, was completed in 1961. The dam is operated by the City of Santa Cruz Water Department (City) and is under the jurisdiction of the California Department of Water Resources, Department of Safety of Dams (DSOD).
The existing inlet/outlet works are no longer adequate for current DSOD drawdown requirements. To address this, the City decided to replace rather than repair the inlet/outlet works, resulting in the NCD Inlet/Outlet Replacement Project (Project). The project team consists of the City, AECOM as the Designer, Mott MacDonald as the Construction Manager (CM), and Obayashi Corporation as the Contractor; with the State of California as a major funder.
The objectives of the project include:
- Protect the City’s water supply system by addressing existing deficiencies.
- Establish long-term reliable storage of drinking water supply.
- Meet DSOD drawdown requirements.
- Improve overall operational efficiency and system performance.
- Improve inspection and maintenance access.
- Implement a cost-effective project, including operation/maintenance costs.
- Maintain uninterrupted beneficial flow releases during construction.
The construction contract for the NCD Inlet/Outlet Replacement Project was awarded to Obayashi Corporation (Contractor) on March 25, 2020, and the Notice to Proceed was issued on May 4, 2020.
Project components include:
- Three new inlets that control and convey flows.
- New outlet structure with valves and controls to provide for energy dissipation for water releases to the Newell Creek Pipeline (NCP) or beneficial releases to Newell Creek.
- New dam seepage collection and monitoring system.
- 10-14 foot diameter tunnel with 48-in. and 10-in. inlet/outlet pipelines.
- Replacement of approximately 2,000 linear foot segment of the NCP.
- New intake control building on dam crest.
- Access road improvements including a new culvert crossing at spillway plunge pool.
- Decommissioning the existing inlet/outlet works.
As of Sept. 1, 2022, construction was approximately 82% complete. Access improvements to the reservoir, Newell Creek Pipe replacement, box culvert bridge, tunnel excavation, three new inlets in reservoir, and air vent installation are complete while installation of the inlet/outlet conduit, outlet structure, control building and instrumentation and tie ins are currently in progress.
The tunnel is a major component of the project housing the new 48-in. steel inlet/outlet conduit connecting to the three 30-in. inlet shafts in the reservoir and a 10-in. fused PVC conduit that will be used for beneficial release to Newell Creek.
The 1,500-ft long tunnel was excavated under the NCD right abutment. Cover above the tunnel ranged between 40 ft at the tunnel portal increasing to a maximum of 230 ft at the middle of alignment and then decreased to approximately 50 ft of rock cover with 120 ft of water above at the end of the tunnel near the intake shafts. The designed tunnel initial support required a minimum of 5-in. steel fiber reinforced shotcrete with #8 grade 50 rock dowels patterned at 5 ft by 5 ft installed entirely through the tunnel. Drain strips were laid out in between the rock dowel rows to collect ground water prior to placing the shotcrete. A minimum 6 in. concrete invert working slab (1,500 psi) was placed per design to prevent deterioration of the ground that may have occurred as a result of groundwater contact. Supplemental spot bolting, mine mats, drain strips and additional layers of reinforced shotcrete were applied when poorer ground conditions were encountered.
The main technical challenges of the excavation were the size restrictions of the tunnel, the remote location of the project site, and the proximity to the reservoir with limited rock cover.
After shotcrete, the supported tunnel was an 11-ft modified horseshoe with a 30-in. steel ventilation duct, leaving only 8 ft, 6-in. of clearance from invert to ventilation. An Antraquip AQM 150 HR roadheader equipped with a powerful 200-hp cutter boom was used to excavate using full face mining for the entire run and provided the power to efficiently excavate the anticipated rock conditions while narrowly fitting in the allowable tunnel profile to maximize advance rates.
Additionally, the roadheader was equipped with a TUnIS Navigation guidance system by VMT that allowed for minimal survey support and controlled the perimeter excavation to all but eliminate overbreak by providing real time survey to the operator. Not only did this improve excavation efficiency, but also provided a safer work environment for less experienced crew members. Minimizing overbreak was critical as the tunnel will be backfill grouted after conduit is installed, any overbreak would increase either shotcrete initial support or backfill grout quantities.
The largest logistical complication was the shotcrete application used for initial support. Shotcrete was required to be placed after every 10 ft of excavation, which meant that 4CY would need to be applied at the end of every excavation cycle. The closest ready-mix plant was 45 minutes from the jobsite and vendors could not commit to a short load on a variable schedule dictated by the excavation operation. To overcome this obstacle, Sika premixed dry shotcrete supersacks were used allowing on demand application and minimal waste.
As the tunnel advanced into the final 500 ft of excavation, the rock cover continued to shrink as the reservoir water level continued to increase. The end of the tunnel only had 50 ft of rock cover with over 120 ft of water above. To mitigate the potential for water inflow into the tunnel, progressive overlapping grout patterns were drilled ahead of the excavation. Each pattern was grouted, and verification holes were drilled to ensure that the grouting was successful. Any verification hole that encountered water over the allowable limit would require another grout pattern be installed, in some cases up to four iterations were performed at one location to alleviate the water encountered.
Utilizing the correct equipment with the Antraquip AQM 150 HR Roadheader, VMT guidance system, and Sika Dry Shotcrete along with a patient approach to the pre-excavation grouting cycle were key to a successful tunnel excavation operation.
By Maria Chastka, Obayashi Corporation
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