On 7 November, some 20 companies submitted a request for pre-qualification for the construction of the nine sections that make up the Lower Northwest Interceptor (LNWI) Programme in Sacramento, US. The scheme includes two large diameter TBM river crossings with an estimated value of US$22M each, to be let as separate contracts, and extensive microtunnelling and directional drilling works.

Both river crossings consist of an approximately 0.6km long, 4.7m diameter bored tunnel with 10m-16m deep launch and reception shafts either side of the Sacramento River. Each bore will pass with about 10m cover to the riverbed and roughly 31m to ground level.

Geotechnical investigations carried out by client, the Sacramento Regional County Sanitation District, indicate varying ground conditions consisting of river deposits such as sands, silts and clays with a high ground water table. Tunnel construction will utilise either EPB or slurry TBM technology, depending on the successful contractor’s choice. The lining will consist of gasketed, precast concrete segments. Both the lining design and the shaft excavation support will be the contractor’s responsibility.

Bill Moler LNWI’s Lead Tunnel Engineer told T&TI: “the two tunnels under the Sacramento River present special challenges with respect to tunnelling and muck handling on the 6% incline/decline slopes, and minimising settlement under the river levees that protect the cities of Sacramento and West Sacramento.”

“The Programme is seeking highly qualified tunnelling contractors with extensive experience in soft ground tunnelling with EPB and/or slurry TBMs below the water table. We are pleased with the response we have had from 17 contractors who have submitted pre-qualification documents for the large diameter tunnels, microtunnels and directional drilled borings.” Moler added.

The pre-qualifiers will be notified on 14 January 2004 in preparation for a construction start in Spring of 2004.

The LNWI is scheduled to come on-line at the end of 2006 and has been designed to convey sewage flows some 6km from the Natomas area to the Sacramento Regional Wastewater Treatment Plant.