The 30 percent design of the raw water system was based on tapping into an existing reservoir above the normal minimum water surface elevation with twin 4'-0" finished diameter tunnels using micro tunneling techniques. The entrance pit for the TBM was 14'-0" diameter vertical shaft which also served as a wet well to pump raw water from the tunnel to a water treatment plant approximately six miles away. Raw water was conveyed using one 36-inch and one 42-inch cut and cover steel discharge lines. The initial construction of the 36-inch line was followed by the 42-inch line when demand necessitated its construction.
JFSA participated on the VE team by providing tunneling expertise. The VE team recommendations included:
- A single TBM bore of 6'-0" finished diameter. Cost savings: approximately $1,200,000
- Shot creting TBM entrance pit and wet well Cost addition $80,000 Benefit: Prevent surface air slaking of shale following excavation
- Concurred with recommendation to apply coated steel discharge line. Reason: soil contained high concentration of soluble sulfates that may attack concrete.
- Recommended pump station be founded on over excavated and backfilled select material rather than on caissons to minimize the differential settlement between the building and pipes penetrating the building and the sulfate attack on the building foundation. No net cost savings. Benefit includes less risk to building and discharge line.
Reference: Lyle Lallum, PE, Contracting Officer's Representative, 406.247.7644
