Excavation

There are several different methods of excavating seabed material. The methods most suitable for typical UCT operations include:

• Suction dredging

• Controlled blasting

Commercial divers sometimes use diesel/ gasoline-driven solids handling pumps and submersible electric-driven solids handling pumps as shown in Figure 2-34. The best method for any particular situation depends on a number of factors including:

• The nature of the seabed material to be excavated: soft or hard, fine grained or coarse grained, maximum size of particle.

• The horizontal distance the excavated material must be moved.

• The vertical distance the excavated material must be lifted.

• The quantity of material to be excavated and the nature of the excavation (i.e., trench for cable or pipeline burial, excavation around the base of existing piles, general excavation, etc.).

• The nature of local currents and wave action.

• The type of topside equipment and the size of construction platform available.

• Limitations on shipping space and weight for the excavation equipment.

Table 2-15 provides some general guidance on the suitability of the various excavation methods for particular circumstances. Of course, not all situations are included, and sound judgment must be exercised in the selection of the optimum method for any particular situation.

A description of each of the excavation methods is provided.

2.6.1 Air Lifting

An air lift can be used underwater to provide the controlled removal of seabed material. The air lift works on a differential density principle. Air is introduced into the lower end of a partially submerged pipe. When air bubbles combine with the water in the pipe, a mixture that is less dense (lighter) than the water outside the pipe forms. The lower density mixture in the pipe rises, causing a suction at the inlet. The amount of material lifted will depend upon the size of the air lift, submerged depth of die pipe, air pressure and volume used, and the discharge head.

The air lift (Figure 2-35) consists of a discharge pipe and a foot piece or air chamber. The size of the discharge pipe can range from 3 to 12 inches in diameter, depending on the work to be done (see Table 2-16). The air chamber should be located 20 to 30 inches from the intake end of the pipe and should be fitted with a control valve that cannot be opened or closed accidentally. A handle attached to the lower end will make it easier for the diver to use. It is sometimes necessary to add weight to the lower end to help keep the intake on the bottom.

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