[Illustration: ~Longitudinal Section Showing Method of Construction in Rock Covered with Quicksand.~ ~Sketch “a”.~ ~Section A-B-C-D.~ ~Sketch “c”.~ ~Longitudinal Section of Tunnel.~ ~Sketch “b”.~ ~Cross Section Showing Manner of Constructing Lining around Boulder.~ ~Sketch “d”.~ FIG. 123.--Sketch Showing Methods of Lining, Milwaukee Water-Works Tunnel.] 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.