Cross Section Showing Manner of Constructing Lining around Boulder.

Sketch “d”.

Fig. 123.—Sketch Showing Methods of Lining, Milwaukee Water-Works Tunnel.

[Larger illustration]

Upon examining the lake bottom a cavity over 60 ft. deep and 10 ft. in diameter was found directly over the end of the tunnel, which had been caused by the gravel breaking into the tunnel. Having now reached an elevation where it was possible to use compressed air, it was determined to put in double air-locks and use the plenum process. The locks were built, and some 670 cu. yds. of clay were dumped into the hole in the lake bottom. On Aug. 4 the air-locks were tried with 26 lbs. air pressure; but, upon a temporary release of the pressure, the water passed around the locks and back of the tunnel lining for some distance, and even forced through the lining, carrying considerable clay and fine sand with it. Upon sounding the lake bottom it was found that the cavity had again increased to a depth of 65 ft., whereupon an additional 600 cu. yds. of clay were dumped into it.

On account of the water leaking through the brickwork, the only dry place to cut through the brickwork and build in an air-lock was just ahead of the brick bulkhead. This lock was completed Aug. 27, and to avoid encountering the danger of the direct connection with the lake at the end of the drift, it was decided to make another détour to the north. On Aug. 28, therefore, the brick on the north side of the tunnel 12 ft. back from the end of the brickwork was cut through under 25 lbs. air pressure, and work proceeded in good, hard clay. The original air-lock was cut out and a new lock built into this clay about 34 ft. from the last détour, to be used in case of further difficulties. After building the tunnel for about 80 ft. from the détour, the soundings again indicated the approach to gravel and water, and on Oct. 14 the water broke through from the bottom in such volume and with such force that the men ran out, closing every air-lock and the valves of every drain in their haste to escape, until the brick bulkhead was reached. It was with great difficulty that the portion of the tunnel up to the last air-lock was recovered and cleaned out.

It was now recognized that a pressure of from 38 to 40 lbs. of air would be needed to hold this water, and accordingly another compressor was added to the plant. With a pressure of 36 lbs. the water was driven out and the work again started. At this time also an additional 350 cu. yds. of clay were dumped into the hole in the lake bottom. Altogether, 1620 cu. yds. of clay had been put into this hole.

Loose gravel and boulders, some of immense size, were now encountered, and the work became exceedingly difficult on account of the great escape of air. The interstices between the gravel and boulders were not filled with silt or sand, but contained water. Moreover, this material extended upward to the lake bottom, as was shown by the escape of air at the surface of the lake. For an area of several hundred square feet the surface of the water resembled a pot of boiling water. At times the air would escape very rapidly; and again only a few bubbles would show.

It need hardly be said that the work in this gravel was very slow. It was impossible to blast or to tear out the large boulders whole, as so much surface would be exposed that an inrush of water would take place despite the air pressure. The method of procedure was to excavate a heading and build the brick roof arch first, and then to take out the bench and build the invert. [Fig. 123] gives a number of sketches showing how the work was done. A short piece of heading was taken out, the top and face of the bench being meanwhile plastered with clay (Sketches b and c, [Fig. 123]) to reduce the escape of air, and then the roof arch was built and supported on side sills resting on the bench. Bit by bit the roof arch was pushed forward until some little distance had been completed, then the heading was plastered with clay and the bench taken out little by little and the invert built. All the gravel except the small area upon which work was actually in progress was kept thoroughly plastered with clay; and as the air escaped through the completed brickwork very rapidly, water was allowed to cover a portion of the invert (see Sketch c, [Fig. 123]), so as to reduce the area of escape.