CONCRETE DAMS. 109
Fig. 68 is the section of a cofferdam adopted at Limerick in connection
with the rebuilding of a dock wall which had failed, the length being
450 feet.*
Stone Dams are similar in construction to earthwork dams, consisting of
a mound of light stone rubble (such as chalk) deposited and overlaid with
clay to form a watertight skin. This material is also used as filling for the
interior of a cofferdam, as exemplified at Ardrossan harbour t (figs. 69 and
70).
Concrète Dams.—A somewhat novel and ingenious experiment in dam
construction has been successfully tried at Liverpool. A wall was built of
large concrete blocks (each containing 100 cubic feet) bedded in hydraulic
mortar, with a sheet of ordinary brown paper laid between the blocks in
each joint. The paper adapted itself to the surface of the bed and allowed
the blocks to obtain a uniform bearing upon one another, while at the same
Revels of Capinq .
Fig. 68. —Cofferdam at Limerick.
time it prevented any actual adhesion. The stability of the structure
depended, therefore, entirely upon the resistance of the blocks to sliding
friction, which proved to be ample for the purpose. The dam in question
was built upon the outer sill of a lock, 100 feet wide between side walls.
The sill had a straight outer face and a curved inner one for the ultimate
reception of gates. The area of the sill was nearly 400 square yards, with a
minimum width of 25 feet. The front of the dam was a vertical plane, the
back being stepped. The total height above the sill was 42 feet, at which
level a roadway was formed for traffic. High water of ordinary spring tides
* Hall on “Limerick Dock Walls,” Min. Proc. Inst. C.IS., vol. ciii.
+ Robertson on “Ardrossan Harbour Extensions,” Min. Proc. Inst. C.E., vol. cxx.