Tunneling: A Practical Treatise.

by Charles Prelini

A brick bulkhead was built into the end of the brickwork of the tunnel, and provided with a timber door for passage, and two 10-in. pipes for the outlet of the water. With these openings closed, the flow was checked sufficiently to allow the placing of pumps at the bottom of the shore shaft. Meanwhile the pressure of the water against the bulkhead caused dangerous leakage, and so after the pumps were in position the 10-in. pipes were opened, relieving the pressure and allowing the water its normal rate of flow. Trouble with the pumps now arose, and after various stoppages and breaks the discharge pipe finally fell, disabling the whole plant. It became necessary to close the 10-in. pipes in the bulkhead and draw up the pumps. This allowed the tunnel to again fill with water.

After thoroughly overhauling the pumping machinery, the contractor again laid the tunnel dry on March 19; and after the pumps had been permanently placed so as to take care of the water, an examination of the work was made. It was found that the water was coming from the north, and with the hope of avoiding the difficulties of the old heading, it was decided to make a détour of the south. On April 16 work was begun at a point about 90 ft. back from the face, and deflecting the line about 38° toward the south. About 38 ft. from the angle of junction a brick bulkhead with two 8-in. openings was built into the new bore. The work progressed successfully for about 75 ft., when water was again encountered; and upon pushing forward the heading, gravel and sand came in such quantities that it was found impracticable to continue the work further. On June 1 the bulkhead was permanently closed, and the work in this direction was abandoned.

A further and closer examination was now made of the heading first abandoned. Upon breaking through the rock-like clay it was found that the water came from an underground stream flowing from the north through a well defined channel in red clay. This channel was about 13 ft. above the grade of the tunnel; and above it in every direction visible was a bed of hard, dry, red clay, while immediately in front of the face of the work was a bank of coarse gravel. [Fig. 122] is a sketch of the channel and stream where they entered the work. In this last drawing the photograph has been followed exactly, no particular being exaggerated in the slightest. The water from this stream was clear and pure; and a chemical analysis showed that it was not lake water, but must come from some separate source.

Fig. 122.—Sketch Showing Underground Stream, Milwaukee Water-Works Tunnel.

While the engineer did not consider the difficulty of proceeding along the old line insurmountable, it was decided to be less difficult on the whole to go back from 150 ft. to 175 ft. and deflect the line to the north and upward, so as to pass over the underground entrance. Instead of allowing the water to flow at its normal rate and take care of it by pumping, the contractors desired to reduce the pumping, and to this end they constructed a bulkhead just west of the deflection toward the south with a view of shutting off the water. The water, however, accumulated with a pressure of some 50 lbs. per sq. in. and penetrated the filling around the brick lining of the tunnel, preventing the cutting through of the lining for the new line. A second bulkhead was then built about 20 ft. west of the first, but with not much better results, for upon closing it the water was found to leak through the brickwork for a long distance west. Finally on Aug. 2, 1892, the contractors lifted their pumps and allowed the tunnel to fill again with water.

No further work was done on the tunnel by the contractors, although they continued work on the lake shaft for some months. Difficulties had, however, arisen here, which will be described [further on]; and finally a disagreement arose between the contractors and the city over the delay in prosecuting the tunnel work and over one or two other questions, which resulted in the City Council suspending their contract and ordering the Board of Public Works to go ahead with the work.

The first step to be taken by the engineer was to purchase adequate pumping machinery and empty the tunnel. This was effected Jan. 17, 1894; and as soon as practicable thereafter the two bulkheads were removed and the tunnel cleaned, tram-car tracks laid, and everything prepared for work. It was now determined to go ahead on the original line of the tunnel if possible, and the bulkhead here was removed and work begun. Meanwhile, a safety bulkhead had been built to replace the first one torn away. This was provided with a door and drainage pipes. Work was begun on the original heading, but had proceeded only a little way when the water broke in, driving out the workmen. This was removed three or four times, when the flow suddenly increased to 3000 gallons per minute. An examination of the lake bottom above the break showed that it had settled down, indicating that the new break connected with the lake bottom, and making further work along the original line out of the question.

The question now arose what it was best to do. It was impracticable to use a shield, as the material ahead of the break required blasting, and the pressure from above was enormous. On account of its expense and difficulty of application the freezing process did not seem advisable, and the plenum process was likewise out of the question on account of the great pressure which would be required at this depth. The détour to the south which had been made by the contractor had been unsuccessful, and had left the ground in a treacherous condition. To depress the tunnel was not advisable, for it was not by any means certain that the bed of gravel could be avoided in that way; and, moreover, it would be necessary to ascend again further on, and thus leave a trap which would effectually cut off escape to those at work on the face if water again broke into the tunnel.

It was finally decided that the old plan of deflecting the line toward the north and upward so as to pass over the underground stream should be tried. A hole was therefore cut through the tunnel lining 1433 ft. from the shore, and work was begun on a détour of 20° toward the north and an upward grade of 10%. Fair progress was made on this new line, gradually ascending into solid rock, until May 10, when the test borings, which were constantly made in every direction from the face, showed that sand was being approached. A brick bulkhead was therefore built into the masonry as a safeguard, should it happen that water was encountered in large quantities. As the borings seemed to indicate that the top surface of the rock underlying the sand was nearly level, the lower half of the tunnel was first excavated, leaving about 18 ins. of the rock to serve as a roof (Sketch a, [Fig. 123]), and the brick invert was built for a distance of 52 ft. The rock roof was then carefully broken through for short distances at a time, and short sheeting driven ahead into the sand, which proved to be a very fine quicksand flowing through the smallest openings. Extreme care had to be taken in this work, but little by little the brickwork was pushed ahead until at a distance of 90 ft. from the point where the sand was first met, and 208 ft. from the old tunnel, the sand stopped and the heading entered a hard clay.