15 AUGUST 2005
Sand base. For cell walls on sand, penetration of sheeting must be sufficient
to avoid piping at the interior toe of the wall and to prevent pullout of outboard
Clay base. For cofferdams on clay, penetration of the outboard sheeting is
usually controlled by the pullout requirement; piping is not critical.
Bearing capacity. For cofferdams in either clay or sand, check the bearing
capacity at the inboard toe by methods for shallow foundations.
Cell Deformations. The maximum bulging of cells occurs at
approximately the height above the cofferdam base. The cells tilt 0.02 to 0.03
radians due to the difference in lateral loads on the outboard and inboard faces.
Deflections under the lateral overturning loads are a function of the dimensions, the
foundations support, and the properties of the cell fill.
Cell Fill. Clean, coarse-grained, free-draining soils are preferred for cell
fill. They may be placed hydraulically or dumped through water without any requirement
Materials. Clean, granular fill materials should be used in large and critical
cells. Every alternative should be studied before accepting fine-grained
backfill as fine-grained soils produce high bursting pressures and minimal cell
rigidity. Their use may require the addition of interior berms, increased cell
width, or consolidation (by sand drains or pumping). All soft material trapped
within the cells must be removed before filling.
Drainage. Install weep holes on inboard sheeting to the cell fill. For critical
cells, and/ or those with marginal fill material, wellpoints, or wells between
cells have been used to increase cell stability.
Corrosion retardation. When cofferdams are used in permanent structures,
especially in locations exposed to brackish water or seawater, sever corrosion
occurs from the top of the splash zone to a point just below mean low-water
level. Use a protective coating, corrosion resistant steel and/ or cathodic
protection in those areas.