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00The project challenges include:
construction in a dense urban area, challenging subsurface conditions
(soft/mixed ground, presence of cobbles and boulders, groundwater), nearby
existing utilities, fill, and construction under major traffic arterials.
The tunnels are being constructed approximately 15 to 40 feet below the
existing ground surface. Tunneling methods may include pipe ramming, microtunneling,
tunnel boring machines, and conventional open face tunneling. The tunnels
may be supported by steel casing, jacking pipe, liner plate, shotcrete,
and/or steel sets with full perimeter lagging. The final carrier pipe
will be PVC pipe to protect against corrosion from sewer gases when backfilling
is required. The tunnel will be backfilled with cellular grout to prevent
the tunnel from providing a drainage corridor for groundwater. Dewatering
may be required for tunneling some reaches because of the anticipated
cobbles and boulders. When excavation is by microtunneling, a direct jack
carrier pipe will be used.
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