Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 189.—A very large surface void appeared directly above a testmicrotunneling installation when the operator stopped advancement of themachine and briefly allowed the slurry pump to continue to circulate thedrilling mud. The void appeared in backfill surrounding some testinstruments, some of which appeared to support the adjacent soils.the design engineer, the proposed procedure and materials prior to implementation.Worster, et al. (2002, p. 9) state:Many of the contractors doing this work may be accustomed to applications whereseepage along the outside of the carrier pipe, or between the carrier and liner pipe, isnot as significant of a concern. The focus on grouting and pipe placementrequirements may be unusual for these contractors. For this reason, requirements forgrouting the carrier pipe/embankment contact, placement of the liner pipe, andgrouting the liner pipe/carrier pipe annulus should be specified in detail. A contractorsubmittal on the means and methods to accomplish these items should be required forapproval, to ensure that all specification requirements are met and proper proceduresare followed during construction.Microtunneling with wet recovery of the tunnel boring machine is a relatively newand rapidly developing method of installing conduits into a reservoir partially or fullyfilled with water. For a wet recovery of the tunnel boring machine, a recovery pit isexcavated into the reservoir bottom. The tunnel is advanced from the downstreamside of the embankment dam in the direction of the pit. Once the tunnel hasadvanced to a point near the pit and where the advancing face of the tunnel is stillstable, the tunneling is stopped and the hydraulic lines are disconnected from thetunneling machine. A temporary bulkhead is placed inside of the tunnel downstreamof the boring machine. The boring machine and tunnel liner are then jacked for thefinal length of the tunnel, until the boring machine is pushed into the recovery pit.340