15 August 2002
Internally Braced Sheet Pile Cofferdam. The use of this type depends
on special foundation conditions. The bottom must be relatively impervious and must
have the strength to resist a blow. One example of this type of bottom is firm clay into
which sheet piles can be driven to a substantial depth for cutoff. The clay must be firm
enough so that the depth of material surrounded by the sheet piles has sufficient
strength to resist uplift. Design as follows:
Refer to DM-7.02 Foundations and Earth Structures for design data on
internally braced cofferdams.
Sheet piles of the Z-type are usually used because of the high ratio of
Internal bracing must be placed in two directions and in vertical tiers
spaced to accommodate the strength of sheet piles. Wales and struts
may be of wood or steel or a combination of both. Steel is usually
required below the top tier.
Foundation Piles. Piles may be required to help support drydock
structures constructed on weak soil types. They may also be used to help hold down a
drydock floor slab against uplift pressures. Piles may be needed for track and capstan
Types. Piles may be of wood, steel, or concrete. Wood piles, not
completely covered, must be treated. Steel piles are not generally used, unless each
remains embedded for its entire length at all times after installation.
Driving. Piles may be driven either in the dry or underwater. Steel piles
placed underwater are usually driven to a predetermined top elevation. Wood piles
driven underwater may be cut to a predetermined top elevation by an underwater saw
operated by a barge mounted motor and guide frame.
Length. Because piles will vary in length at any given location, the
lengths ordered must be sufficient to allow for possible variations. Piles may or may not
penetrate into a structural foundation slab, depending on design anchorage
Locations. The accurate positioning of each pile is important to maintain
the validity of design assumptions and make possible the installation of prefabricated
Precautions in Driving and Handling. Be sure that piles are
undamaged by handling prior to driving, and substitute sound piles for those damaged
in driving. Creosoted timber piles and precast concrete piles are particularly susceptible
to handling and driving damage.
Foundation Course. To stabilize soft bottom material, place a foundation
course consisting of a layer of gravel or crushed rock several feet thick. For drydocks