The Sewer Under Beacon Hill

The South sewer tunnel will be finished in a few weeks, and then, with the progress already made in the construction of the main sewers, the city will have the main skeleton of a sewer system to which it can add the lateral sewers as occasion and the growth of population require. The two tunnels, north and south, are the most costly and laborious parts of the system devised by City Engineer W. H. Thomson five years ago. They will carry off the sewage from the interior valleys of the city and from the Lake Union basin, and will ward off the pollution of the great bodies of fresh water bounding the city on the east and north, so that these will always remain resorts for health and recreation.

The annual report of City Engineer W. H. Thomson contains an accurate description of the South tunnel and a history of its construction, of which he has had charge from the beginning, except for a short but exciting interval of one month. This part of the report is as follows:

This tunnel is to serve as the tidewater conduit for the intermediate district, which includes practically all the territory east of the summit of the first hill rising from the shore of Elliott Bay and south of Division street. It is about 4,700 feet long, running under the center of Bay View avenue from the shore line of Elliott Bay, near South Twelfth street, to the Rainier Valley, near Twenty-seventh street. The plan of construction is an oviform brick sewer, the inside dimensions being 48 inches horizontal axis by 72 inches vertical axis. The grade is 1 in 350, from an initial grade point about 18 feet above the datum of mean high tide. This allows for an outfall to be carried out over nearly two miles of tide flats and discharged into tide water at the permanent harbor line.

At the regular election in the spring of 1892 the issuance of $100,000 bonds was authorized for the purpose of constructing this tunnel, that being the estimate of cost made by the city engineer. The bonds were negotiated soon afterward and work on the tunnel was commenced from the bay end August 17, 1892. The progress was good and the excavation free from water, being through a dry blue clay formation for a distance of 2,220 feet. That distance was covered during a period of almost exactly one year. At that distance, on August 22, sand and gravel was encountered and a large stream of water broke in. During the month which followed attempts were made to retimber the walls, but without satisfactory success. At about that time the great defect in the plan of construction was exposed. It was evident that the tunnel was in bad ground and the work was suspended.

While the excavation above described was in progress the brick lining started along close behind. This work began March 1, 1893. At a point 1,200 feet inside the tunnel, a bad ground formation was reached. In August 1893, only 60 feet of brick lining had been completed. To aid the progress of excavation and lining, air was furnished to the workmen by means of a fan driven by water power, forcing air into the tunnel through a nine-inch pipe. A careful study of the situation convinced me that the excavation could be made more rapidly and economically if the tunnel were approached from a shaft sunk from the surface. A shaft was accordingly started at the junction of Nineteenth street and Bay View avenue, 1,225 feet east of the west end of the tunnel, and a point which would give the best grade. Excavation was commenced January 1894 and continued until December, when a depth of 107 feet had been reached. Work was suspended on the shaft about the same time as on the tunnel.

In December 1894 work on the tunnel was resumed, and after driving through about 100 feet of sand and gravel formation hard clay was again encountered. About five months were occupied in excavating through this bad ground, after passing which the work ran rapidly and without interruption.

On April 19, 1894, the sinking of the air shaft was resumed and good progress was made, the 157 feet to the tunnel level (154 feet below the surface) being excavated in thirty days’ work, connection being made to the tunnel on May 18, 1894. At that time the excavation of the tunnel had progressed to a point 205 feet east of the shaft. The shaft was lined with brick to the surface, with foundation entirely independent of the tunnel lining.

The excavation of the tunnel was completed about the middle of October, 1894, the easterly terminus being fixed at the alley between Twenty-sixth and Twenty-seventh streets, the total distance from the bay being 4,662 feet. At this point a shaft was sunk, the roof grade of the tunnel being ten feet below the surface of the ground.

The construction of the brick lining was resumed soon after the excavation. On November 12, 1894, brickwork was commenced at the west end and progressed westward with speed and economy. At this date, January 28, 1895, 1,445 feet of lining in the west section of the tunnel is completed. The work is progressing at the rate of 150 feet per week; the cost per foot for labor and material for the completed 48 by 72 inch double-rowlock brick lining, including excavation of invert, back-filling and all included expenses, has by watchful care and close management been reduced to $5.61 per foot. This is only one-half the cost of exactly similar work done in 1890. In the west 2,200 feet of the tunnel all materials have been delivered from the bay end, a distance varying from 4,700 to 3,300 feet. Wages have been reduced about 20 per cent since the previous construction.

There remains yet to be lined about 1,060 feet between the east and west completed sections and about 150 feet at the extreme west end. At the present rate of progress the tunnel will be completed about April 1, 1895, and at the present cost of construction the total cost of the tunnel will be within the original $100,000 voted in 1892.

The material excavated from the tunnel has been deposited on the beach and a mole thus constructed as far out as the Grant Street bridge. A pile structure extends west about 300 feet to the east channel of the Duwamish River across the tide flats. Over this mole and trestle an outfall conduit will be extended and allowed to discharge into this channel. As the permanent harbor tide-land improvements advance this outfall will be extended out to deep water.

Some interesting details might be added to what Mr. Thomson says of the ventilating shaft sunk from the summit of Beacon Hill. The shaft is elliptical in shape and is bricked all the way up, divided by a brick mid-rib, and is so solidly constructed that it stands entirely independent of any support afforded by the surrounding earth—in fact, it is a chimney sunk in the earth. In sinking the shaft, quicksand and a heavy stream of water were encountered; the water was cut off by filling the shaft with clay. Several bones were encountered, one piece being identified as a shin bone in a fair state of preservation.

The total cost of the tunnel from the beginning of work to January 1, 1895, was $81,444.51. The $100,000 bonds issued for the purpose realized $100,834.42, so that there still remained on that date $19,183.31, which is considered ample for the completion of the work. The main sewers which will drain into this tunnel are the Bush street sewer, which extends from Main street to within half a mile of its eastern end, and the Walla Walla, which is constructed for three-quarters of a mile south of Yesler avenue. Another sewer will have to be constructed down the Rainier valley in extension of the Kenney street sewer, into which these and all the laterals in the intermediate district will drain.