basin at Torresdale and the pump wells at Lardner's Point will be as follows: Ft. Per Second From the clear water basin at Torresdale to Shaft No. I... .5.92 The total loss of head between the clear water basin at Torresdale and pump wells at Lardner's Point, when the conduit is delivering 300,000,000 gallons per day, is estimated at 9.25 feet. To prevent the tunnel from becoming air bound it is graded upwards nine inches per 1,000 feet from Shaft No. 1, Torresdale, to Shaft No. 11, Lardner's Point. Air which may be carried down Shaft No. I will either be carried into the tunnel or rise through the water in the shaft. Such air as may be carried into the tunnel will flow with the water and be vented at Shaft No. II, the upward inclination of the tunnel from the influent to the effluent shaft preventing the accumulation of air, which might be a cause of interference with the operation of a horizontal tunnel. Should there ever be any indications of air sticking, as it were, in the conduit, it is believed that it can readily be removed by increasing the speed of the pumping machinery at Lardner's Point, lowering the level of water temporarily in the pump wells, and creating an increased velocity of flow through the conduit until the air is removed. The conditions under which the water is conducted to the influent shaft, and the upward gradient of the tunnel from the influent to the effluent shafts, are thought to be effectual safeguards against the introduction of any considerable quantity of air into the conduit, or of any reduction of its capacity by the accumulation of air at any point along the roof. CHARACTER OF MATERIALS ON THE LINE of Work. The only deep rock operations from which information could be obtained upon the probable character of the material to be encountered in driving the shafts and headings for the northerly end of the conduit are found near the county prison, north of Pennypack Creek. Here the rock excavation in the deep quarry is very hard, with few seams or fissures, and a small amount of water, and the typical sections showing excavation for the conduit in hard rock were partially based on this information. The experience with the conduit confirmed the diamond drill borings, which showed that the character of the rock in the quarry is not maintained for the whole length of the work, and that a great change in the hariness and stability of material takes place in going from the north to the south end. The treacherous material, however, is not continuous, but occurs in reaches of the work, sometimes being abreast of and sometimes crossing the line of the tunnel diagonally. The diamond drill borings which preceded the preparation of the detail plan indicated the varying character of the rock, but the rapidity with which some of the material would deteriorate upon contact with the air in the tunnel was not fully suspected in advance. Operations on the conduit were conducted from nine working and two permanent (end) shafts. All shafts were lined to the solid rock. The working, as well as the permanent, shafts were constructed with steel shells sunk into hard rock and sealed, the interior being lined with an eighteen ring of hard-burned stretcher brick laid in cement mortar in the permanent shafts, and a nine inch ring in the working shafts. The sinking and lining of the steel shells for the end shafts was much more carefully executed that it was for the working shafts, because the latter, upon completion of the work, were closed with brick arches continuous with the arch of the tunnel, above which were constructed in each shaft two relieving arches to take a part of the weight of the backfill of the shaft off the arch of the conduit. From the arch of the conduit to ground level the working shafts were solidly backfilled with selected materials. About ninety-one per cent. of the entire tunnel excavation was in hornblendic and biotite gneiss rock, which required no timber to support the roof. In the north end the rock was very good, but the flow of water, as will be noticed later, was large enough to render it a matter of concern to the contractor and interfered with the lining of the tunnel. Between Shafts Nos. 4 and 7, the rock was of excellent quality, and the flow of water not enough to hinder the work. Between Shafts Nos. 8 and 9, and in the north heading of Shaft No.8, the micacous rock was very rotten and treacherous, and required heavy and close timbering to prevent dangerous falls. In all such localities the thickness of the arch was increased to four or five rings of brickwork. Excepting where falls in the roof occurred, the rough diameter of the excavation, heading and bench, is about fourteen. feet. For several hundred feet in the north heading of Shaft No. 9, under the Disston Saw Works, the collar beams and posts supporting the roof abut against each other to prevent dangerous falls and possible injury to valuable surface structures. The placing of this timber not only delayed the progress of the excavation, but naturally occasioned great loss of time in its removal in part, and in placing the brick arch and the concrete packing above and around it. In carrying forward the arch, for nearly sixty per cent. of the total length enough water was encountered dripping from the rock to make it almost impossible to insure proper setting of the mortar joints excepting the water was wholly excluded from the back of the brickwork, and in order to prevent this, wherever wet roof was met, tar paper in two layers from springing line to springing line of the arch was used to exclude the water until the mortar had set. It was not thought that the tar paper would be a permanent protection in this respect, but experience demonstrated that it could be relied upon to exclude the water for many days, during which time the mortar in the brickwork had sufficient time to harden. In strengthening Shaft No. 7 as originally sunk and lined, while the contractor insisted that it was perfectly secure for his work, it was not thought that it was entirely safe, either for the contractor's men or for the employees of the City, and in order to guard against any risk from inflows of water from the Delaware River, which, aside from the possible accidents to men employed in the shaft or in the headings driven, would have occasioned a very serious delay in recovering the shaft or in driving a new one, it was decided to reinforce with a ring of concrete from twelve to eighteen inches thick, a portion of the shaft, beginning eleven feet above the shoe of the steel shell, and continuing down thirty-eight feet six inches into the excavated rock. |