South Queensferry.—On the south shore the water was drawn from the pits of the Dalmeny Shale Works for boilers and other general purposes; but it proved too dirty, and some rough filter-beds of gravel had to be constructed to pass it through. Thence it was forced by pumps to an overhead tank set about 60 ft. above the level of the works and shops. This water was conducted in pipes down the incline to the jetty, and, in various leads, all over the works. For drinking purposes another supply pumped from the sandstone was available; but it proved very intermittent, and at times failed altogether. In the summers of 1886 and 1887 a water famine occurred at South Queensferry, and this was met by the contractors placing large iron tanks at the town harbour and sending a steam barge down the Firth to a place called Starleyburn, seven miles away, where a plentiful supply of water could be got. This was forced into the tanks, and could be drawn upon by all free of charge. In 1887 the united parishes of Kirkliston, Dalmeny, and South Queensferry arranged for a supply of water from the Pentland Hills, and this new supply is both plentiful and of first-rate quality, and has been running since the summer of 1888.
Cement.
The cement used exclusively was Portland cement manufactured on the Medway. This also came by water, but required to be stored for a specified number of days before it could be used. The contractors purchased an old hulk—called while in its prime the Hougomont—in which ten to twelve hundred tons could be readily stored. It was moored off Queensferry, and the ships bringing cement from the south were moored alongside and discharged. From this ship the cement was brought ashore and stored in such quantities as might be required at any time.
Subsequently the Hougomont was moored close to the west side of the South Queensferry jetty, and thus a more direct and speedy communication established. While the caissons were being sunk at Queensferry a large number of foreign workmen were lodged on board this hulk, and when, shortly after new year, 1886, an epidemic of small-pox broke out at Queensferry, the ship was towed round into Port Edgar, and moored in an isolated position, and converted into a small-pox hospital. As such it proved of signal service in speedily stamping out the disease.
Cement Tests.—The cement is described in the specification to the contract as having to be of the best quality and ground so fine that the residue on a sieve containing 50 divisions on the inch, equal to 2500 meshes per square inch, should not exceed 5 per cent, by weight. It had to be kept in a dry store, and was not to be used until a certain number of samples out of every cargo had been tested. Neat cement was not to be set within less than one hour. The weight had to be between 112 lb. and 116 lb. per bushel.
For tests, the cement to be mixed with three times its weight of sand which has been passed through a sieve of 400 meshes but retained upon one of 900 meshes to the square inch (20 divisions and 30 divisions to the lineal inch). About 10 per cent, of water had to be added, and briquettes made, which were to be put into water after twenty-four hours and remain in water twenty-five days, when they had to bear a stress of not less than 170 lb. without breaking. For briquettes of neat cement the breaking stress after four days had to be not less than 300 lb., and after seven days to be not less than 400 lb. per square inch.
A few tons of quick-setting or Roman cement were used in making good the joints of the circular iron cofferdams or caissons for the Inchgarvie north piers. This cement is not very strong, but if the joints are well packed with bags of clay puddle and loaded with stones they answer the purpose of keeping the water out for a while. If all preparations are made beforehand, Roman cement can be used in a plastic state, and is thus very useful to divers working under water; but all the work requires to be done very quickly.
Mortar.—The mortar used in the masonry was in the proportion of one of cement and one of sand in the foundations, where rubble-work was used, and one of cement to two of sand in the piers. The pointing in the joints of the granite blocks was done in pure cement.
Of the other materials used, except the steel, it may be as well to state here that timber in baulk was brought from Grangemouth, ten miles up river, where it was rafted and towed to the works wherever required.
Planks, battens, and boards were brought from the same place in lighters.
Hardwood, such as oak, beech, and ash, used for packings and other temporary purposes, was got in the neighbourhood and dealt with at the saw-mills.
Coal and coke for the three main piers were brought in barges—mostly from Charleston, close by; but for the shops and yards they were brought by rail.
Creosote oil for the Lucigen lamps, and rivet and other furnaces, came in specially constructed tanks by rail.
Commencement of the Permanent Work.
The positions of the four main piers on Fife being fixed at an early date, a beginning was made with the excavation of the rock upon the site of the two north piers. The natural bedrock of whinstone rose here to a level of from 10 ft. to 20 ft. above high water, and excavation had to be made to the level at which the foundation or holding-down bolts started—namely, at 7 ft. below high water. The rock was then levelled with rubble masonry and the building of the granite courses commenced.
_Fig._31,_Page_17.jpg)
Fig. 31. Raft Used for Survey of Foundations.
Excavation was also started upon the site of the north cantilever pier at a level of about 21 ft. above high water, and upon viaduct piers, 10 and 11, at 25 ft. and 22 ft. above high water respectively. These three piers stand on high ground, while piers 12 and 13 were placed at the bottom of the ancient quarry, and were founded at level 7 ft. below high water. The foundations of the abutment upon the hillside was commenced somewhat later at level 92 ft. above high water. All the piers on the Fife side are founded on the solid whinstone rock, with the exception of piers 12 and 13, these being partly on whinstone, partly on freestone. All the viaduct piers being on land no cutwaters were built. The rock after being cleaned and examined, and worked roughly into steps where necessary, was levelled up with concrete and a bed of concrete laid on from 4 ft. to 11 ft. in thickness—this bed projecting all round the granite masonry some 2 ft. Upon this concrete foundation the first course of granite was set, the form of the piers being rectangular and the curved batter of the pier starting from the bed. (See Plate VIII.) Piers 10, 11, 12 and 13 were built to level 37 ft. above high water, and left, pending the construction of the girders.
Generally about Cofferdams.
In the description of the pier foundations, frequent use is made of the terms caisson and cofferdam, and to those not conversant with foundation work in water a short explanation may be acceptable. A cofferdam or caisson may be described as an enclosure in water for the purpose of laying dry the space enclosed, or, at any rate, of preventing a flow of water through it. In soft ground this is done by driving a double row of piles at a distance of from 2 ft. to 4 ft. from each other, and by continuing to drive piles between those already placed until a double timber wall exists all round. Sluice doors or valves are placed so as to allow the tide to flow in and out. The single timber piles are held together by longitudinal timbers being placed on each side and bolted through, and stays or struts are placed between the walls and across the inner space in all directions to give stiffness and resistance to the water pressure from without. The space between the double walls is now cleaned out as well as can be done, and clay puddle is filled into this space and trodden down hard or pressed down by other means, and this is carried on until the whole space is filled up to and beyond half tide or full tide as the case may be. The sluices can then be closed and the water pumped out from the enclosure, and the bottom upon which the foundation is to be placed can be examined, and all necessary excavation made. It depends partly upon the hold the piles have taken of the ground, partly upon the external forces acting upon the timber-walls—namely wind and waves—whether or not the construction of a whole tide dam is advisable. In a whole tide dam, if tight at bottom, when once the water has been pumped out, the work of excavation—of laying the foundation and building a pier or wall—can be carried on without interruption to the end. In the case of a half-tide dam, the space enclosed can only be pumped dry after the tide outside has fallen below the level of the clay puddle wall, and the water requires to be admitted so soon as the tide has turned and is rising again up to that level. Work in a half-tide dam is termed tidal work. When working on rock this mode of forming an enclosed space by driving piles is not admissible, and other means must be found to keep the water
