Figure 5.2 is a cross-section view of a typical dike. The core is 
made of a substance more impervious than the surrounding material. The 
armor stone provides protection from incoming waves and the possible 
effects of currents. A properly designed and constructed dike of this 
type will last a very long time with little need of repair. 
There is a considerable amount of information available on the 
design and construction of a dike. Most dikes have been designed 
conservatitvely to avoid disaster due to wave attack, storm surges, 
leakage, and similar phenomena. For a polder to be used as the MOBS 
construction site, it appears highly likely that a less conservative 
design may be acceptable because the useful life of the MOBS dike is 
many times shorter than that of a dike built for long-term protection. 
Settlement and/or shrinkage of the dike may be tolerated and leakage 
can be combated by installing pumps. Also, it may not be necessary to 
use armor stone if there is little wave action in the waters surrounding 
the construction site. 
There appear to be a large number of estuaries, lagoons, and 
similar bodies of water near the ocean which, with little effort, could 
be used as a construction site. If a suitable location of this type 
can be found, it may be possible to decrease site preparation costs 
considerably. A small dike could be built to block-off the sea, the 
area pumped dry and, if necessary, expanded employing conventional earth- 
moving equipment. 
An obvious advantage of the dike and polder approach is the pos- 
sibility of using standard concrete mixing and placing equipment in their 
design environment. Also, there will most likely be sufficient space 
for equipment and material storage. Another important advantage is 
the lack of constraints on the size of the module in plan; even full- 
scale, shallow-draft platforms could be fabricated and launched. But, 
the constraint of most concern for the dike and polder is depth: plat- 
forms with excessive draft might make this approach economically 
impractical. 
A number of concrete structures have been successfully fabricated 
behind dikes. The designers of the La Fontaine Tunnel in Canada used 
a polder excavated in the St. Lawrence River to construction prestressed 
concrete sections 120 feet wide, 15 feet high, and appproximately 300 
feet long. The dike extended over a half mile into the waterway. All 
of the tunnel sections were constructed before a portion of the dike was 
broken and the construction area flooded. The sections were then towed 
to the tunnel site and lowered into a trench in the river bottom. 
Another example of a project employing this construction technique 
is the large oil tank fabricated in a shallow pit on a beach in the 
Arabian Gulf (Qcean Industry, Sept 1969). Shaped like an inverted funnel, 
the tank is 207 feet in diameter at its widest section. It weighs 
15,000 tons, had an 8-foot draft when floating, and required two tug- 
boats for towing to its final resting place 58 miles offshore. 
a 
“PCI Journal, August 1965 and Engineering News Record, 1 July 1965. 
323) 
