The Forth Bridge/Materials of Construction for the Masonry Piers

by Wilhelm Westhofen

Materials of Construction for the Masonry Piers.

It was laid down in the specification to the contract, that all piers, abutments, and arches were to be constructed of masonry consisting of a granite facing, the hearting being of concrete or of rubble masonry. The stipulations as to the quality of the stone, the amount of facing and dressing, and the quality of cement and sand, were of the usual description in contracts of this kind.

Granite.—All the granite for facing—whether rock-faced or dressed—was obtained from Aberdeen, with the exception of the large coping-stones, which weighed between 6 tons and 8½ tons each, and the necking courses immediately below the coping. These were of Cornish granite and were dressed.

The granite is grey in colour generally, and of very handsome appearance, with some slight veins of red granite running through, which, however, rather add than detract from its appearance.

The granite was brought, roughly dressed and squared, in specified courses ranging from 21 in. in thickness down to 16 in., and also specified as headers and stretchers, so as to form proper bond with the hearting of rubble masonry or concrete. Coming by water, the stone could, of course, be delivered at the respective centres at once. The granite coping-stones were 4 ft. 6 in. in depth, not less than 3 ft. in width, and set out alternately as headers and stretchers. The capping—that is, the slightly-curved crown of the piers—was set out after the coping had been built, and the correct measure of every stone was sent to the quarry near Aberdeen, each stone being numbered and marked.

Rubble Work.—For this a very hard flat-bedded and easily-split freestone—in colour from reddish ochre to purple grey—was brought from Arbroath. It was brought in large blocks up to 4 tons each, and in thickness from 3ft. downwards, but it could be split with ease into slabs not more than an inch thick.

Whinstone blocks, roughly levelled on two sides, were also used in rubble work. These were obtained either by quarrying in the open or by the excavations for the piers.

For bond courses in all the viaduct and the cantilever end piers, a hard freestone from quarries in the neighbourhood or extra large whinstone blocks were used.

All the rubble could be obtained at a cheap rate, owing to the inexpensive mode of transport in ships and the facilities of unloading close to the piers.

Concrete.—For making concrete the whinstone found both on Inchgarvie and at North Queensferry was exclusively used. A number of stone-breakers were set up in the last-named locality, where a large quantity of quarry chips were available and close at hand, and the crushers were placed in convenient positions for charging the broken stone into barges or iron skips for transport to the other centres.

The broken metal was passed over screens to obtain the required size for different purposes.

The concrete was mixed dry first, and again after the addition of water, either by hand, or, if required in very large quantities, by a very effective mixer of simple construction.

The concrete used in open foundations, in the caissons and as hearting behind the granite facing, differed somewhat in its composition according to position; but generally speaking the proportion of cement to broken stone was not less than 4+¹⁄₂ cubic feet to the yard of stone, nor more than 6+¹⁄₂ cubic feet to the yard, an equal amount of sand, or a slight excess over and above, being added. These quantities were considered as making up one yard of concrete. Such concrete had a resistance to crushing of 50 tons per square foot, and it weighed about 37 cwt. to the cubic yard.

Sand.—Some difficulty was experienced in obtaining good sharp sand. It was found in large quantities on both shores of the Firth within half a mile, but objections to its removal being raised by the proprietors of the foreshores, it became necessary to send steam barges down the Firth to Kinghorn and Pettycur, a distance of ten to eleven miles, where the tide lay some banks dry at low water. Owing to the barge having to be grounded over a tide, a good deal of time was lost, but there was no charge for the sand.

DETAILS OF SOUTH APPROACH PIERS.