Spreading by barge movement

dredged material released from barges and with material released from

hopper dredges (O'Connor and O'Connor 1983; Morton 1983a, 1987). In

fact, mechanically dredged cohesive sediments often remain in a clumped

condition, reflecting the shape of the dredge bucket. Mounds of such ma-

terial are stable, resist displacement during capping operations, and pre-

sent conditions ideal for subsequent LBC (Sanderson and McKnight

1986). However, these mounds may experience initial surface erosion due

to irregular surface geometry and higher friction coefficients. A concep-

tual illustration showing the use of conventional equipment for capping is

shown in Figure 3.

Figure 3. Conventional open-water placement for capping (after Palermo

1991c)

Spreading by barge movement

A layer of capping material can be spread or gradually built up using

bottom-dump barges if provisions are made for controlled opening or

movement of the barges. This can be accomplished by slowly opening a

conventional split-hull barge over a period of tens of minutes, depending

on the size of the barge and site conditions. Such techniques have been

successfully used for controlled placement of predominantly coarse-grained,

sandy capping materials (Sumeri 1989). The gradual opening of the split-

hull or multicompartmented barges allows the material to be released

slowly from the barge in a sprinkling manner. If tugs are used to slowly

move the barge during the release, the material can be spread in a thin

layer over a large area (Figure 4). Multiple barge loads are necessary to cap

larger areas in an overlapping manner. The gradual release of mechanically

dredged fine-grained silts and clays from barges may not be possible due

to potential "bridging" action; that is, the cohesion of such materials may

cause the entire barge load to "bridge" the split-hull opening until a criti-

cal point is reached at which time the entire barge load is released. If the

water content of fine-grained material is high, the material exits the barge

in a matter of seconds as a dense slurry, even though the barge is only par-

tially opened.

Chapter 5 Equipment and Placement Techniques