Gravity-fed downpipe (tremie)

Figure 10.

Hydraulic barge unloader and sand spreader barge (from Kikegawa 1983)

the slurried sand through a submerged pipeline. A winch and anchoring

system are used to swing the spreader from side to side and forward,

thereby capping a large area.

Gravity-fed downpipe (tremie)

Tremie equipment can be used for submerged discharge of either me-

chanically or hydraulically dredged material. The equipment consists of a

large-diameter conduit extending vertically from the surface through the

water column to some point near or above the bottom. The conduit pro-

vides the desired isolation of the discharge from the upper water column

and improves placement accuracy. However, because the conduit is a

large-diameter straight vertical section, there is little reduction in momen-

tum or impact energy over conventional surface discharge. The weight

and rigid nature of the conduit require a sound structural design and con-

sideration of the forces due to currents and waves.

The Japanese have used tremie technology in the design of specialized

conveyor barges for capping operations (Togashi 1983; Sanderson and

McKnight 1986). This equipment consists of a tremie conduit attached to

a barge equipped with a conveyor (Figure 11). The material is initially

placed in the barge mechanically. The conveyor then mechanically feeds

the material to the tremie conduit. A telescoping feature of the tremie al-

lows placement at depths of up to approximately 40 ft. Anchor and winch

systems are used to swing the barge from side to side and forward so that

larger areas can be capped, similar to the sand spreader barge.

Chapter 5 Equipment and Placement Techniques