. Canadian engineer. lled to within 1 ft. of the arch con-crete, this space being underpinned with brickwork afterthe concrete had set for at least 48 hours to allow forany shrinkage. When ihese alternate lengths werefinished throughout the drift the intervening portions weretreated in the same way. By this method of placing theinvert the arch was never without support for any greaterlength than 15 ft., for when the alternate sections werebeing taken out the intervening sections carried the arch,and when the intervening sections were removed the inverthad the load. In concreting the arch, stee
. Canadian engineer. lled to within 1 ft. of the arch con-crete, this space being underpinned with brickwork afterthe concrete had set for at least 48 hours to allow forany shrinkage. When ihese alternate lengths werefinished throughout the drift the intervening portions weretreated in the same way. By this method of placing theinvert the arch was never without support for any greaterlength than 15 ft., for when the alternate sections werebeing taken out the intervening sections carried the arch,and when the intervening sections were removed the inverthad the load. In concreting the arch, steel circular ribs of iron, with 2-in. wooden lagging, dressed on threesides, were used for the form work. These steel ribs didnot require any bracing (Fig. 4) and thereby allowed thecars to be taken right up to the face of the work. In theinvert, wooden ribs with i-in. sheeting were used for formwork, allowance being made in placing them for the onering of brick that was to be laid. When all the concreting. Fig. 9.—Showing Method of Excavating and Nature ofMaterial, Section 2. had been completed the trestlework and tracks were re-moved, all the surplus concrete was trimmed off and thebrick invert laid. The concrete in the arch was mixedrather wet until the centre of the arch was nearly reached,when it was stiffened up and rammed back on the formswith hoes. In the invert the concrete was made as wetas possible to assure that no voids would be left, as it wasbeing poured in a rather awkward place and tamping wasalmost impossiljle.
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