Niagara Falls 100.000-Hp. Development

Niagara Falls 100,000 Hp. Development [29 timber struts could be placed between the trusses and the piers, thereby transferring the pressure of the top of the cofferdam from the old wall to the new piers. The old wall and the rock between the piers was then blasted and removed. Excavation back of the cof- ferdam was carried down to a depth of about 6 to 8 ft. be- low the bottom of the coffer- dam and within an average of about 2 ft. of the inside line of the cofferdam, and in a few places the rock broke back of this inside line. The con- crete seal which had been placed in the bottom of the pockets could then be seen quite plainly in such places and showed that a good bond with the rock bottom had been ’ FIG 39_________dredging made. As the excavation pro- gressed downward and closer to the cofferdam, extra struts of 12 x 12-in. timbers were placed from the piers to the cofferdam. The view on page 31, which was taken back of the cofferdam, shows the piling of the cofferdam on the right, the piers on the left with the timber trusses above, and the excavation carried up to within about 2 ft. of the cofferdam. At the time when the ex- cavation of the forebay was completed the entire leak- age of the cofferdam and rock did not exceed 4 sec.-ft. On page 30 is shown the completed cofferdam. As some of the piling drove into the bottom farther than was anticipated on account of there being more loose rock on the bottom than was expected, it was necessary to build a light wooden cofferdam along the top in order to take care of any extreme high water. It was necessary to locate the power house as close to the cliff as possible in order to get the foundations and tailraces on solid rock. The top of the cliff here is about 218 ft. above the water level in the gorge. Extending upward about 100 ft. from the water surface is a talus slope which consists of a layer of about ten feet of loose rocks which had fallen from the cliff. Excavation was begun at the toe of the slope using a t-yd. steam shovel, the loose rock being washed down to the shovel with a hydraulic giant. This hydraulic giant had a 4-in. nozzle and the water was supplied to it by a 10-in. pipe line carried from three of the 9-ft. penstocks in old Station No. 3. This gave a work- ing head of about 180 ft. at the nozzle. As soon as sufficient area had been cleaned off with the hydraulic giant, drilling and blasting of the solid rock was begun. Excavation of the solid rock was carried down to about 4 ft. above mean water level. Then a concrete coffer- dam was built on solid rock the length of the proposed power house and about 60 ft. west of the west wall of power house. Excavation was then carried down to 23 ft. below mean water level. The excavation above EL 370 consisted of shale and limestone, from this to the water surface alternate layers of Medina sandstone and red shale, and below the water level was solid Medina sandstone. As soon as the excavation was completed the con- IN A STIFF CURRENT IN THE NIAGARA HYDRAULIC CANAL crete foundations for the turbines were poured. The mixing plant was located at the top of the cliff. The gravel was brought in on cars and unloaded onto a belt conveyor which carried the gravel to a storage pile. It was then conveyed from the storage pile to hoppers over the mixers by a clamshell bucket operated from a derrick. The mixed concrete was conveyed through flexible sheet iron, elephant trunk, chutes sus- pended in cables extending from the top of the cliff to the bottom. The piers and heavy reinforced-concrete floors for supporting the entire units were completed up to the bottom of the wheel cases. Then the wheel cases were set. After the cases were set the concrete piers and power-house floor were poured. FIG. 40—LOW BOOM DREDGE FOR WORK UNDER BRIDGES