COMPLETE ROCK CUT. CHICAGO DRAINAGE CANAL.
(Depth 35 feet.)
Unlike most canals, the Suez canal has no locks. The original plan of the Panama canal did not include locks, but the revised plan provided for them, in order to save excessive cutting. The Nicaragua canal scheme necessarily includes locks. The water for the Suez canal comes directly from the seas which are connected. A canal with locks necessarily requires an ample water supply from some river or fresh-water lake. If the Suez canal had been constructed at a higher level than the Mediterranean and Red seas, had been supplied with water from the Nile, and had, therefore, been constructed with suitable locks at each end (as was actually recommended by some engineers), the cost of construction, as well as the perpetual expense of maintenance, would have been greatly in excess of its actual cost. And so the fact that it was possible to construct the canal without locks, and without providing for a supply of water, was a great advantage that facilitated the promotion of the enterprise.
Manchester Canal.—This canal, having a total length of only thirty-five and one half miles, has transformed the city of Manchester, England, from an inland city to a seaport. Actual excavation was begun in November, 1887, and just six years afterwards the whole canal was filled with water. It has a depth of 26 feet, and a width at the bottom of from 120 to 170 feet, thus giving a greater capacity than the Suez canal or the proposed Panama canal. Some of the greatest difficulties involved arose from the necessity of providing for the existing canals and railroads with which that busy portion of England is so crowded. Perhaps the most interesting feat of engineering was the drawbridge carrying the Duke of Bridgewater’s canal at Barton. This small canal, having originally a depth of only four and one half feet, here crosses the River Irwell. It was justly considered a great feat of engineering when James Brindley constructed the canal, during the eighteenth century, so that it crossed the river on a viaduct. A waterway crossing a waterway on a viaduct was then a new idea. But this old canal was constructed considerably above the desired level of the Manchester canal, and yet, of course, not so high that a masted ship might pass under it. Therefore a draw became necessary. To add to the complication, the water supply of the small canal being somewhat limited, it was considered very undesirable to lose a troughful of water (roughly, 200,000 gallons) each time the draw was opened. To allow this water to flow into a tank and then pump it back would consume too much time, to say nothing of the expense. Therefore the bridge must swing with the trough full of water. That required gates at each end of the draw, as well as at the ends of the canal on each abutment. These gates were comparatively simple; but the difficult problem was to ensure a water-tight joint between the ends of the draw trough and the corresponding ends of the canal. Temperature changes, as well as many other considerations, would preclude the possibility of making even a fairly tight joint by swinging the draw to a close fit with the abutments. The desired result was accomplished by placing at each end of the draw a very short U-shaped structure, having the same cross section as the cross section of the trough, and having beveled ends fitting corresponding bevels on the ends of the trough. These beveled ends are faced with rubber. To open the draw the gates are closed, the water between the gates at each end (a comparatively small amount) is drained off and wasted, the U-shaped wedges are raised, and the draw is then free to turn. The wedges are operated by hydraulic rams.
AN “ATLAS” POWDER BLAST UNDER A TRAVELING CABLEWAY. CHICAGO DRAINAGE CANAL.
Chicago Drainage Canal.—It will probably be a surprise to many people to learn that this “drainage” canal has a greater cross section throughout the “earth-work” sections than any ship canal in existence, and is only exceeded through the rock sections by the Manchester canal. The city of Chicago obtains its water supply from Lake Michigan. The “intake” pipe was at first located comparatively near the shore. As the population of the city grew and the volume of its sewage increased, it was observed that the water supply was becoming contaminated. The Chicago River, into which the sewage was emptied, became so foul that the odor was intolerable. The very evident fact of this odor probably had more to do with the promotion and accomplishment of the means of relief adopted than the far less evident but very dangerous pollution of the water supply. An extension of the intake pipe to a point several miles from shore by means of a tunnel (which was in itself a notable feat of engineering) only deferred the time when the water supply would again be fatally contaminated if the sewage continued to flow into the lake. It was accordingly determined to dispose of the sewage by discharging it into an artificial channel where it might become diluted with water from Lake Michigan, and thence pass from the watershed of the Great Lakes to the watershed of the Mississippi. The level of Lake Michigan is so high that there was no trouble about obtaining the requisite grade, and the divide between the watersheds is so low that the depth of the required cutting at the summit was not forbidding. But why have such a large canal? It was required that the sewage should be diluted, so as not to become offensive to the inhabitants of the region through which the canal must pass. The law under which the work was authorized required that the flow should be 600,000 cubic feet per minute, and that the minimum width at the bottom of the channel must be 160 feet. According to the well-known laws of hydraulics, it was seen that a deep canal would have a greater capacity per unit of excavation than a very wide shallow canal. This is especially true through the sections of deepest cut, since excavation above the water line adds nothing whatever to the capacity for flow. The sections adopted called for a depth of water of 22 feet. The side walls in rock are practically vertical, the width of channel being 160 feet at the bottom and 162 feet at the top. In earthwork the cross section is larger than in rock, thus reducing the velocity of flow and danger of scouring the banks. The width of channel at the bottom is 202 feet, the width at the water surface being 290 feet, and the side slopes 2 horizontal to 1 vertical.
A very expensive feature of this great work was the necessity for constructing a diversion channel for the Desplaines River throughout that portion of the river valley occupied by the canal. Lack of space forbids a further discussion of this feature. The canal drains into the Desplaines River at a point where the slope of the river is so great that there will never be danger that a strong west wind or an unusual lowering of the level of Lake Michigan can possibly cause the current to flow eastward.
Work on the canal was commenced only after many years of discussion, planning, legislation, litigation, and bitter opposition by the varied interests which considered themselves more or less injured. But the work was actually commenced in July, 1892. The estimated excavation was approximately 40,000,000 cubic yards—about one half that of the Suez canal; but the length is only 29 miles, compared with 101 miles for the Suez canal. The total cost was estimated at something over $27,000,000. On August 22, 1900, the Congressional River and Harbor Committee approved the work as far as completed.