Types of Construction. General types of construction are shown
schematically in Figures 35, 36, 37, and 38. Examples of actual construction
are shown in Figures 39, 40, 41, 42, 43, 44, 45, 46, and 47.
Water Depth. Theoretically, there is no upper limit to the depth
of water in which a gravity structure can be constructed except that the
height of a sheet pile cellular structure is limited by the tensile strength
of the sheets and of the interlocks (bursting strength). However, with modern
high strength materials, high capacity interlocks, and using a clover leaf
form, depths greater than 50 ft (15.2 m)can be accommodated.
Maintenance. Steel sheet piling is subject to corrosion.
Reinforced concrete may develop corrosion of the reinforcement. Timber is
subject to attack by borers. Concrete is subject to sulphate and other forms
of chemical attack. Masonry structures require repointing of joints. In
general, however, concrete and masonry require the least maintenance.
General. Assuming adequate protection of timber (cribs) against
borer attack, the factors cited above (water depth and maintenance) are minor.
Selection is based on minimum first cost, with life-cycle cost an additional
consideration when the use of steel sheet piling is contemplated.
Special Considerations Relative to Filled, Cellular Construction
Cohesive Soils. Generally a filled, cellular structure is a poor
selection if the fill in cells must consist of soft, cohesive soil. The
reason is that high bursting pressures are developed. A second case is where
the sheets are driven through soft, cohesive soils. If such soils are not
removed from inside the cell, again a high bursting pressure is developed, but
if said soil is removed from inside the cells, an inward pressure is developed
which is likely to require a heavy internal bracing system.
Hard or Boundary Soils. Generally a filled, cellular structure is
a poor selection if sheets must penetrate hard or bouldery soils in order to
reach required tip elevations, resulting in a tendency for sheets to drive out
of interlock or to tear.
Detailed Design Considerations
(Refer to MIL-HDBK-1025/6.)
Structural Design Criteria. Deck framing, track beams, or other
members which function as direct supports for crane, railway, or truck loading
shall be designed for Service Classification A. Other elements of support
wherein the impact effect is dissipated into a mass of soil (buried portions
of piling, for example), shall be designed for Service Classification B