A Treatise On The Principles And Practice Of Dock Engineering

LOCK FOUNDATIONS. 249 2. Where the discharge has been greater, but sufficiently moderate not to interfere with work in the vicinity, it has been allowed to find its way into the pumping well in the ordinary manner (i.e., by open “ grip ” or drain pipe), and any sand, which accompanied it, filtered by laying straw, bags, strips of canvas, &c., over the source and weishting them down. 3, Where the discharge has been more rapid still, proceeding from a definite “ blow, with a tendency to diffusion, it has been concentrated into a special iron pipe which led temporarily to a pumping well, or in another case was carried up to a height equal to the head of discliarge. There is a danger, however, attached to this method of repressing the flow by a counteracting head. The general pressure is in no way relieved, and there is every likelihood of the blow re-asserting itself at another weak spot, so that the horizontal duet is a preferable course to adopt. At a later period, the pipes referred to were grouted with cement under pressure, and in due time, after the cement had set, the projecting portions were cut off. The connection between the pipe and the blowhole necessitated care- ful and ample packing with rubble and clay or cement in bags, so as to secure a thoroughly watertiglit joint. In one instance a hollow hemi- spherical casting was employed to collect the outflow. It was 3 feet in diameter, with an upper flanged connection for a 6-inch pipe, and sat upon a ledge surrounding the hole, below the foundation level, in which position it was concreted. 4. Where the discharge has been so great as threaten to overpower the pumps, it has been deenied advisable to block the holes, temporarily, with rubble and clay puddle, tipped in large quantities, until the affected area could be isolated by an enclosure of whole timber sheet piling. One hole, treated in this way, is recorded as liaving absorbed several hundred cubic yards of puddle. 5. Finally, where the pumps have actually been overpowered, the water has been allowed to rise to its natural head and the excavation completed by dredging. The foundation was then piled and the pile-heads cut off by divers to one uniform level. A covering of jute cloth was next laid over them and fastened there while concrète was deposited upon the site by skips opening at the bottom. When the concrete had reached a height sufficient to counteract the under-pressure, the area was pumped out again and the work resumed in the open. One essential feature stands out prominently as the outcome of niuch experience the necessity for adequate pumping power. It is, in fact, wiser to provide it in excess of all anticipated requirements, rather than to run the risk of a stoppage of the works at some critical and momentous period. At the same time, it must be borne in mind that there is a judicious limit to be observed. If the water be continuously and heavily charged with silt or sand, which cannot be checked, it is evident that a void is being formed somewhere, and that settlement of the foundation will be the