Los Angeles effluent outfall bid result 08 Nov 2018

Dragados has emerged as the lowest of five bids received for the 7 mile long x 18ft i.d. (11.25km x 5.5m i.d.) effluent tunnel in Los Angeles. With a bid of US$630.5 million, the bid comes in just narrowly under the second bidder and more than $30 million below the highest bidder (Table 1). The engineer’s current estimate for the design-bid-build contract is updated by the Tunnel Designer Manager at McMillen Jacobs Association to $664 million after being last reported by TunnelTalk in 2015 at approximately $550 million.

Fig 1. Routes of the two existing tunnels and the new tunnel across the Palos Verdes Fault zone

The new 18ft i.d. TBM excavated tunnel is the main element of a long-term plan by the Sanitation Districts of Los Angeles County to provide sufficient capacity and make redundant two existing effluent conveyance tunnels built more than half a century ago. The existing tunnels, connecting the treatment plant in the city of Carson to the sea outfalls on the coast, came into service in 1937 and 1958 and cannot be taken out of service for inspection and maintenance or repair as they now run full constantly.

The new tunnel, following public right of way where possible, is longer than the more direct route of the two existing tunnels, and passes through the seismically active Palos Verdes Fault zone (Fig 1). The full 11.25km length of the new tunnel will be excavated as a single heading drive as there are not opportunities on the route to sink intermediate working shafts. For passage across a number of fault splays within the 75,000ft (2.3km) wide Palos Verdes Fault zone, a 16ft (4.9m) i.d. steel pipe will be back-filled into the primary precast segmental lining.

During certain times, flow in the new tunnel will have to be pumped rather than flowing by gravity and this will subject the tunnel to higher internal pressures and place the liner into a tension situation. Since internal flow pressures will not be consistently high, fitting long sections of secondary steel lining would be costly. As an alternative the tunnel will feature the first use in the USA of a post-tensioned segmental lining. Up to 40% of the alignment will feature post tensioning which will require production of the segments with a tensioning pocket through which a tendon can be fed and both ends joined and tightened to a specified force. This will keep the liner locked in so it can handle the specified internal pressures without the risk of the segments falling apart.

Designed for the owner by Parsons Corporation in association with McMillen Jacobs Associates, the TBM tunnel is likely to take up to 54 months to complete. The type of TBM and its cutter head diameter required to accommodate the 18ft (5.5m) i.d. is being left to the successful contractor.

Within their tenders, the competing bidders were required to price various specific items including hyperbaric interventions for tool changes, the potential for having to free the TBM from squeezing ground, the handling and disposal of contaminated ground and removal of any abandoned oil wells that might be encountered on this Southern Californian drive.