15 August 2002
handling water percolation depends on the type of enclosure and on the nature of
surrounding and underlying soils. For drydocks constructed in the dry, continued
exclusion of water is most important. (Refer to sectionh 10-2.4.)
Dry Excavation. For hard materials, dry excavation is preferred. Where
dewatering a site must be done by tiers of well points, excavation in the dry is initiated
after the water table in the bank has been lowered. Excavation in the dry may be done
by power shovels, draglines, clamshells, bulldozers, tractor drawn pans, or
Fine Grading. Fine grading is done by hand tools, bulldozers and
scrapers. Replace over-excavation by well-consolidated material or lean concrete.
Cofferdams. Cofferdams usually consist of sheet pile or earth structures,
or both combined. The function of a cofferdam is to surround a site and to cut off, or
minimize, water inflow. Portions of a cofferdam may become permanent parts of the
finished drydock. At the drydock entrance end, a cofferdam must be removable to allow
access for vessels when the drydock is in operation. There are several types of
cofferdams, four of which are described in section 10-2.4.1 through 10-2.4.4.
Excavated Pit Cofferdam. Where most of a drydock is on land, and the
soil in the area is sufficiently impervious and mostly of granular character, a hole may
be dug and dewatered by means of well points or deep wells, or a combination of both.
10-184.108.40.206 Entrance Closure. The entrance closure may be an earth-dike, with or
without a line of sheet pile cutoff, or it may consist of a line of earth filled sheet pile cells.
10-220.127.116.11 Clay and Rock Foundations. The open pit method is generally not
suitable for clays. Refer to American Civil Engineering Practice, Volume II, for
examples of this type. Drydocks built in rock fall into this category, except that water
influx is usually small and can be collected in sumps to be pumped out by conventional
pumps, see (d) of Figure 3-3.
Earth Dike Cofferdam. If suitable foundation and borrow material is
available, a simple earth dike may suffice to exclude water from a site, even where
drydocks will be located well out in the water. Usually, however, a sheet pile cutoff is
provided as a dike core. Where sheet pile cutoffs can penetrate to impervious material,
earth dikes are ideal. Where earth dikes are used, also provide for lowering the water
table by well points and/or deep wells. For an example of a drydock constructed with
this type of cofferdam, see (e) of Figure 3-3.
Cellular Sheet Pile Cofferdam. For particularly unfavorable soil
conditions (for example, where embankment material in place, or available, is very
pervious or otherwise unsuitable), a continuous wall of cellular sheet piles may be used
to enclose an entire drydock site. Normally, do not choose this type of construction
because the cost is relatively high. This method may prove feasible for shipbuilding
docks of semi-permanent character where sheet pile cells become incorporated into the
dock structure. See (a) of Figure 4-1.