Niagara Falls 100.000-Hp. Development

28] Niagara Falls 100,000 Hp. Development t-yd. steam shovel. The rock was drilled and blasted in the dry down to about 2 ft. below the water surface by leaving a narrow ridge of solid rock as a cofferdam. The blasted rock was removed into dump wagons by the t-yd. steam shovel. The next 11 to 13 ft. down was drilled and blasted in the dry and removed with a dipper dredge. The drill- ing was done with tripod drills, holes being placed about 4 ft. apart each way and along the side it was close drilled. These holes were 6 to 8 in. apart. It has been found that this close drilling in this class of rock gives a comparatively smooth wall for the side of the cut. In the excavation above water 0.8 lb. of 60 per cent dynamite per yard was used. The lower 9 ft. of excavation for the basin enlarge- ment was drilled and blasted from the drill boat and removed by the dipper dredge along with the deepening of the rest of the basin and in the same manner as the canal excavation. Much of this basin enlarge- ment was done in the summer and fall of 1918 when it was imperative on account of the war to keep all the old power plants running- to their utmost capacity. The blasting up of the bottom of the basin, the ridge formed in front of the dipper dredge and the dredge with scow restricted the cross- sectional area so as to increase the velocity of the water very much. These conditions are shown in the view on p. 29, which was taken Oct. 19, 1918. The work was done in strips the length of the basin and sufficiently wide for the con- venient operation of the dredge. Each strip was com- pleted through before begin- ning another so as to reduce the cross-section as little as possible. The forebay was built in rather cramped quarters, being separated from the basin by the basin road and between two large flour mills on the north and south, while on the west were busy switchtracks. The old west wall of the canal basin, which is shown on p. 29, is built of concrete upon limestone. The top of the rock is at about water level. Under the roadway the inlets to the forebay have been built. In the construction of the forebay this west wall with a wider strip of rock under it was at first used as a natural cofferdam for the excavation. As soon as the basin had been widened and deepened enough by dredg- ing to allow a cofferdam to be built in it and also permit about 7000 sec.-ft. of water to pass to old Station No. 3, a cofferdam was built. It was quite important that this cofferdam should restrict the flow as little as possible. With this in mind a narrow coffer- dam was built of Lackawanna 12| x f-in. straight web steel sheet piling in water varying in depth from 16 to 24 ft. The cofferdam consisted of 18 oval-shaped pockets. The size of the typical pocket in plan is 14 ft. 4 in. long, 9 ft. 10 in. wide at the mid-section, and I ^-TELESCOPIC PIPE Fasteninq I j 4''Diam. 6'long 6 ft. 5 in. at the end where it joins the adjacent pocket. The outside row is composed of piles 24 ft. long and the inside row of 20-ft. piles. In the construction, timber trusses were first built in lengths 28 ft. long and 8 ft. deep, using 12 x 12-in. timbers for the chord members and 8 x 8’s for the diagonal members. These were built on the bank along side of the basin, launched, and floated into their place which was along the old concrete basin wall. Upon the cofferdam side of the truss were fastened timbers shaped to fit the outside of the pockets. After the truss was in place templets made of two thick- nesses of 2-in. plank and the shape of the inside of the pocket were floated into place and the sheet piling placed around these templets and against the truss. Work was begun at the upstream end so as to take advantage of the current. As soon as the piling for 5'long FIG. 38—DRILL BOAT USED IN CANAL EXCAVATION one pocket was placed they were driven with a light piledriver so as to drive them through any loose rock which might be on the bottom of the basin and cause them to fit as closely as possible to the solid rock. After this a diver went down into the pocket and closed up any openings that were found between the piling and the rock, using bags filled with concrete. Good rich concrete was then poured in the bottom of the pocket for a depth of about 2 to 3 ft. so as to properly seal the bottom and thoroughly fasten the cofferdam to the rock. The pocket was then filled up to the top with ordinary gravel, the pressure of this gravel causing the pockets to bulge slightly and completely tightening the joints. This made a very- narrow cofferdam but it was thoroughly fastened to the rock at the bottom by concrete and at the top supported by two stiff wooden trusses. In excavating the rock back of this old basin wall as much solid rock was left as possible between the piers to act as pilasters for supporting the old concrete wall which was receiving the load thrust upon it through the trusses from the top of the cofferdam. As soon as the piers were completed the old concrete wall was removed directly in front of the piers so that