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Water jetting

Water jetting (or a mixture of water, bentonite, cement and air) can be used to aid the penetration of a pile into a dense sand or sandy gravel stratum. Jetting is often less effective in firm to stiff clays or soils containing coarse gravel, cobbles, or boulders. Jetting can assist pile installation in several ways. The jetting pressure may loosen (erode) the soil at the tip of the pile. In addition, the flow of the jetting fluid can reduce the shaft friction along the pile. However, the effect of the jetting procedure on the bearing capacity of the pile and sheet pile must be taken into consideration.

It is also possible to use grout as a jetting fluid, which during driving reduces (mainly) the shaft resistance. After pile installation is completed, the hardening grout may increase the bearing capacity of the pile.

Jetting can also be used to reduce ground vibrations during driving in vibration-sensitive areas.

Usually, the jetting tube is attached to the shaft of the pile or sheet pile. If jetting is required to aid penetration of an occasional pile which "hangs-up" in driving, a separate jet pipe is used. This jetting pipe is then moved up and down close to the side of the pile. An angled jet may be used to ensure that the wash water flows to the pile point. In difficult conditions, two or more jet pipes may be used for a single pile. Tube or box piles driven open-ended can be jetted from within the pile, and steel H-section piles can be jetted by sinking the jet pipe down the space between the web and flanges.

An adequate quantity of water is essential for jetting. Suitable quantities of water for jetting a 250-350 mm pile are:

Fine sands

15-25 l/s

Coarse sands

25-40 l/s

Sandy gravels

45-600 l/s

A pressure of at least 5 bar (and in many cases significantly higher pressure) is required. It is sometimes a difficult problem to dispose of the large quantities of water and sand flowing at ground level from around the piles, and great care is needed when jetting near existing foundations or near piles driven to depths shallower than the jetting levels. The escaping water may undermine the pile frame, causing it to collapse. Jetting through sands may be impossible if the sandy strata are overlain by clays, which prevent escape of the jetting water.

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