CEMENT RUBBLE COFFERDAMS.
I give his description of the cement rubble cofferdams, first used in 1808, at the erection of the Bell Rock Lighthouse:—
“At seven o’clock this morning, the tide proving more favourable, the artificers began to work. At nine o’clock the rock was again overflowed, and the boats returned to the tender after two hours’ work. Part of the operations of this morning’s tide consisted in building up the crevices and inequalities of the rock round the margin of the foundation with Pozzolano mortar and the chips produced from excavation, with the view to dam out the water. These little walls varied from six to eighteen inches in height; a small sluice or aperture being formed in one of them, by which the water, during ebb tide, was allowed to drain off.
“It formed part of the writer’s original design to erect a cast iron cofferdam of about five feet in height round the site of the building; but the surface of the rock was so irregular that the difficulty of tightening it, and also of emptying the contained water, so as to get the benefit of it during ebb tide, would have been so great, that taking these circumstances into account, together with the loss of time which would attend the erection of such a preparatory work, the idea of a cofferdam was laid aside, soon after entering upon the actual execution of the work.”