A Treatise On The Principles And Practice Of Dock Engineering

COFFERDAMS. 105 On grounds of stiffness and strength, whole timber piling is preferable to half timber piling, though a method very commonly adopted is that of driving whole timber guide piles, with intervening bays, or panels, of half timber piles. The guide, or king piles are provided with pointed shoes, but the intermediate piles are shod with wedge-shaped shoes. If an edge or side of each pile foot be splayed, the process of driving will cause it to draw more closely to the adjoining one, and so produce continuous contact. For the same reason it is a good plan to pitch or set a whole bay of piles and slightly drive them all, before proceediug to a conclusion of the process with any one of them. Furthermore, the sides of adjoining piles may be alter- nately tongued and grooved or, alternatively, both grooved, for the reception of a vertical strip of flat iron, say, from 2 to 3 inches wide by | inch in thickness. The former method is of greater service for maintaining the regularity of the piles in driving. Skin dams need not necessarily be piled. A method very successfully praetised at Liverpool (fig. 160) is that of constructing skin dams ashore, in flitches of 100 lineal feet or more. They are then launched from the quay, up-ended with the aid of a floating crane and some iron rail ballast, and inserted in a trench previously dredged to receive them. The dam is finally shored to the wall at uniform intervals, forming bays of from 10 to 12 feet in length. The edges of adjoining piles are rendered a watertight joint by menns of l-inch triangular wooden fillets nailed to the piles and closely cramped together. Torch-wick has also been used as a watertight packing. These flitches proved very successful and were used repeatedly, being trans- ferred from one site to another as occasion required. A length of over 4,500 feet of dock walls was underpinned in this manner. The cost of the flitches, including maintenance and removal, varied between £13 and £18 per lineal foot. A skin dam has been made self-supporting by constructing it in the form of a bottomless box for work which could be carried on in the interior. The outer faces then afford one another mutual support through the medium of cross shores and struts. The method as applied to the construction of a dock wall at Liverpool is shown in fig. 133. It will be noticed that the outer sheeting consists of a series of horizontal timbers, ranging in thickness from 12 inches at the bottom to 3 inches at the top. Water-tightness is effected by means of torch-wick joints. Inside the sheeting there is a continuous row of piles driven down to a rock substratum, and acting as a support for an overhead crane road. The dam in question was 246 feet long, in lö-foot bays. The cost was rather less than £35 per foot run. In all cases the foot of a skin dam has to be amply protected and covered by a thick layer of clay puddle, which will need replenishing from time to time as the clay subsides. Cofferdams consist essentially of two timber faces enclosing a hearting, generally of clay (fig. 66), but occasionally of stone. They are of more solid construction than skin dams, but, at the same time, they offer some risks of