When a large boulder was reached, which lay partly within and partly without the tunnel section, the lining was built out and around it, as shown in Sketch d, [Fig. 123]. The boulder was then broken and taken out. All through this gravel bed the cross-section of the lining is made irregular by the construction of these pockets in the lining to get around boulders. Sometimes they were on one side and sometimes on the other, or on both, or at the top or bottom. In fact, there was no regularity. Despite the hazard and danger of this work, continual progress was made, though sometimes it was only 4 ft. of completed tunnel per week, working night and day; and, if some cases of caisson disease be excepted, the only mishap occurring was a fire which got into the timber packing behind the lining and caused some trouble. From the gravel the tunnel ran into clay and quicksand, and then into hard, dry clay similar to that encountered near the shore. Some difficulty was had with the quicksand, but it was successfully overcome; and when the hard clay was struck, the trouble, as far as the work from the shore shaft was concerned, was virtually over.
Meanwhile, a different set of afflictions had come upon the engineer and contractors in sinking the lake shaft and driving the heading toward shore. This shaft was intended to be built by sinking a cast-iron cylinder 10 ft. in diameter, made up of sections bolted together. Work was begun July 5, 1892, and the sinking was accomplished first by weighting the cylinder, and afterwards by pumping out the sand and water within it until the pressure from the outside broke through under the cutting edge and forced the sand into the cylinder, allowing it to sink a little. From 10 to 30 cu. yds. of sand were carried into the cylinder each time, and finally it was feared that if the process continued, the crib, which had been previously erected, would be undermined. On Sept. 6, therefore, the contractors were ordered to discontinue this method of work. No change was made, however, until Oct. 1, when the cylinder had reached a depth of 68 ft., and by this time there was quite a large cavity underneath the crib. This was refilled, and the cylinder pumped out, and excavation begun inside of it. On Oct. 11 a 21⁄2-ft. deep ring of brickwork was laid underneath the cutting edge; but in trying to put in another ring beneath the first, two days later, the sand and water broke through the bottom, driving the men out, and filling the cylinder to a depth of 16 ft. with sand. The pumps were started, but the water could not be lowered to a greater depth than 60 ft.
At the request of the contractors, the city engineer had a boring made at the center of the shaft to determine the character of the material to be further penetrated. This boring showed that sand mixed with loam and gravel would be found for a depth of 26 ft., then would come 15 ft. of red clay, and finally a layer of hard clay like that penetrated by the shore end of the tunnel. About the middle of December the contractors made another attempt to pump the shaft, but finding that the water came in at the rate of 25 gallons a minute, abandoned the attempt. In the latter part of February preparations were made to put an air-lock in the shaft and use compressed air. Hardly had the work been begun by this system when, on April 20, 1893, a terrific easterly storm swept the top of the crib bare of the buildings and machinery, and drowned all but one of the 15 men at work there.
This disaster delayed the work for some time, but in June the contractors erected a new building and new machinery, and resumed work. Very little progress was made; and the air escaped so rapidly that it loosened the sand surrounding the shaft and reduced the friction to such an extent that on July 28 the entire cylinder lifted bodily about 6 ft., and sand rushed in, filling the lower part of the cylinder to within 45 ft. of the lake surface. No further work was done by the contractors although they submitted a proposition to sink a steel cylinder inside the cast-iron cylinder and extending from 5 ft. above datum to 100 ft. below datum for $300 per ft. This proposition was refused by the city; and since work on the tunnel proper had been abandoned by the contractors some time before, as had already been described, the city suspended their contract on Oct. 19.
On Oct. 30 a contract was made with Mr. Thos. Murphy of Milwaukee, Wis., to sink a steel cylinder inside the old iron cylinder. The water was first pumped out of the old cylinder, and a timber bulkhead built at the bottom. On this the steel cylinder was built, and then the bulkhead was removed. Air pressure was put on, and the excavation proceeded successfully until the bottom layer of clay was met with, when all chances for trouble ceased.
The cylinder, as it was completed, penetrated 9 ft. into the hard clay, and was underpinned with brickwork for a depth of 29 ft. or more, to a point 4 ft. below the grade line of the tunnel. At the lower end, the section of the shaft was changed from a circle to a square. Later the steel cylinder was lined with brick.
On March 28, 1894, an agreement was made with Mr. Thos. Murphy to construct the tunnel from the lake shaft toward the shore. Except that considerable water was encountered, which, owing to inadequate pumping machinery, filled the tunnel and shaft at two different times, and had to be removed, no very great difficulty was had with this part of the work.
On July 28, 1895, the headings from the lake and shore shafts met. Meanwhile the cast-iron pipe intake, the intake crib, etc., had been completed, and practically all that remained to be done was to clean the tunnel and lift the pumping machinery at the shore shaft. During the cleaning, the air pressure had been kept up on account of the leakage through the brick lining, and, indeed, the pressure was kept up until the last possible moment, and everything made ready for removing the air-locks, bulkheads, pumps, etc., in the least possible time. The pumps were the last to come out.