One and forty-two one-hundredths acres of land were also obtained by lease from Ignatius Belt, for one year from July 10, 1893, with the privilege of extending the same for one or two years longer, at the option of the United States, for the purpose of constructing thereon the temporary buildings required.

The preparatory work of this improvement, a description and the plans of which were contained in my last annual report, commenced on July 5. A road was graded extending from the Conduit road at the western foot of Dalecarlia Hill to the mouth of the proposed tunnel near waste weir No. 2 of the Washington Aqueduct, to be used for hauling the compressor and other drilling plant and the materials required for the work. A wooden flume 250 feet long, 4 feet wide, and 12 inches high was constructed from the mouth of the tunnel leading from the waste weir to Little Falls Branch, which flume was made necessary by the leakage from the gates in the dam of the waste weir, which ran directly over the site of the portal of the proposed main drainage tunnel. A magazine of rubblestone was constructed in the valley of Little Falls Branch, above the reservoir, for the storage of dynamite. A blacksmith shop, a storehouse, and other necessary buildings were constructed at points convenient to the work, and a building was hauled from the Champlain avenue shaft of the tunnel leading to the new reservoir near Howard University, to serve as the office of the assistant engineer in local charge of the work. A railway about 500 feet long was constructed for the transportation of material, and a telephone line, connected with this office, was run to the work.

For the purpose of guarding the proposed shaft in the valley of Little Falls Branch and the hoisting machinery in floods, a temporary dam was constructed across the branch at a distance of about 100 feet from the site of the shaft, and the channel of the branch was changed from the west to the east side of the valley. Careful surveys were made along the route of the proposed open channels between East and Mill creeks and between the latter and Little Falls Branch, and borings were made on the sites for the permanent dams across these streams. Contracts were made with the Ingersoll-Sergeant Drill Company, of New York, for furnishing a 40-horse power boiler, an air compressor, and other machinery required for driving machine drills; with McMahan, Porter & Co., of New Cumberland, W. Va., for re-pressed vitrified bricks for the invert, and with the Frederick Brick Works, of Frederick, Md., for common bricks for the main body of the lining of the tunnel and for the shaft; with James H. McGill, of Washington, D. C., for Cumberland hydraulic cement, and with the American Forcite Powder Manufacturing Company, of New York, for dynamite and exploders.

As soon as the water from Waste Weir No. 2 had been turned into the flume and disposed of (July 17), work on the rock excavation of the open cut was commenced with a gang of drillers and laborers. On the 24th another S-hour shift was put on, and on the 29th the excavation of the open cut was completed by the removal of 5381⁄2 cubic yards of rock. Its total length is 70 feet.

On August 1 the excavation of the tunnel was commenced, and from the 3d of that month, when the third shift was put on, until its completion on February 6, it was carried on night and day, except on Sundays and holidays. The excavation, by hand drilling, of the shaft in the valley of Little Falls Branch was commenced on October 12, and was completed to the top of the space to be occupied by a water cushion on January 8. The excavation of the tunnel from its north

erly end at the bottom of the shaft was then commenced, also by hand drilling, and carried on until February 3, when the heading met the heading of the machine drillers working from the southerly end. The excavation for the tunnel was 11 feet in diameter and for the shaft it was 16 feet square. A large portion of the rock through which the Tunnel passed was very seamy and liable to slips, the joints being filled with a material resembling mud, and heavy timbering was necessary to prevent accidents.

The route of the drainage tunnel, passing as it does under Aqueduct Tunnel No. 4, through which passes the entire water supply for Washington, especial precautions were taken not to injure the latter tunnel, which is unlined at the point of crossing, by the dynamite blasts in the former. To this end the drill holes were limited to 3 instead of 6 feet in depth, which was the rule in other parts of the tunnel, and the charges were limited to 12 pounds of 40 per cent dynamite.

The total number of cubic yards of rock excavation was 3,771, the cost of which, including the cost of labor, dynamite, and materials, but excluding the cost of tools and machinery, was, for the tunnel, $6.20 per cubic yard, and for the shaft $6.87 per cubic yard.

About 9,000 pounds of dynamite were used in the work.

Taking into consideration the size and shape of the tunnel, the dangerous material encountered, and the continually increasing quantity of water that poured through the seams of the rock during the sinking of the shaft, the progress made in this work compares very favorably with works of similar character in other parts of the country.

One of the headings was driven a distance of 50 feet in one week, the average weekly distance being 31 feet, and a depth of 8 feet was made in one week in the shaft, the area of the cross section of which was 273 square feet.

After trimming the tunnel for the lining and laying of the concrete foundation for the vitrified brick invert, the brick lining of the tunnel was commenced on the 19th of March. It and the portal wall were completed on June 20. The lining of the shaft, which is 10 feet interior diameter, was completed to the coping on June 19. The arched lining of the tunnel is 7 feet interior diameter, and from the center of the shaft in the valley of Little Falls Branch to the portal on the southerly side of Dalecarlia Hill, its length is 965.6 feet. From the bottom of the invert to a height of 2 feet on the sides, the arch is laid with vitrified brick to take the wear of the gravel and sand that will pass through the tunnel. The depth of the shaft from the surface of the ground to the bottom of the tunnel is 62 feet, and to the bottom of the water cushion it is 70 feet.

For a distance of 10 feet from the shaft the tunnel arch is constructed entirely of vitrified brick, and, as it supports the lining of the shaft, it is, above the springing line, 4 rings thick; for a distance of 165 feet southerly the arch above the vitrified brick invert is 3 rings thick; for a farther distance of 89.5 feet it is 2 rings thick; for a farther distance of 20 feet (this is under Aqueduct Tunnel No. 4) it is 3 rings thick, and thence to the south end of the tunnel it is 2 rings thick. The spaces between the arches of the tunnel and shaft and the rock were packed very carefully and solidly with rubblestone laid in cement mortar.

There were used for lining the tunnel and shaft 107,878 vitrified bricks, 231,979 common bricks, and 2,231 barrels of cement.

Much credit is due Mr. F. W. Johnston, my assistant engineer in immediate charge at the work, for zealous and skillful supervision and very correct instrumental work, the center lines run from the two ends of

B

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the tunnel meeting within one-fourth of an inch, and the levels for grade as run from the two ends differing by only of a foot; also to Mr. Peter McAtee, tunnel foreman, for the very efficient performance of his.duties.

The oversight of the work, both by Mr. Johnston and Mr. McAtee, was so careful that there was no loss of life, and no man was injured from the beginning to the end of the work. This is a record of the use of high explosives in tunnel work that has rarely, if ever, been equaled. I am also indebted to Mr. John T. Ensor, U. S. attorney for the district of Maryland, for his prompt, efficient, and zealous assistance in examining and passing upon the titles to the lands that I was required to purchase for the work.

The main drainage tunnel under Dalecarlia Hill and the shaft in the valley of Little Falls Branch, the most important and difficult parts of the project for the improvement of the Dalecarlia receiving reservoir, are now complete except the coping of the shaft and the retaining wall in the side of the hill in rear of the shaft, and the appropriation has been exhausted.

There remains to be done for the completion of the project a short tunnel through the hill on the easterly side of Little Falls Branch; the permanent dams across the valleys of Little Falls Branch, Mill Creek, and East Creek, and the open channels between these streams that are to conduct all the polluted waters of the watershed of the reservoir into Little Falls Branch, from which they will pass into the shaft and thence by the main drainage tunnel around the reservoir and into the Potomac. When this has been done, the water of the reservoir will be drawn off, the reservoir will be filled with Potomac water from Great Falls, and the passage through the reservoir of this water, for which purpose it was originally constructed, will be renewed. The work can be completed in the next fiscal year if the necessary appropriation be made by Congress.

Accompanying this report will be found plats showing the portal of the main drainage tunnel and the shaft in the valley of Little Falls Branch; also the short tunnel through the hill east of Little Falls Branch yet to be excavated.

Respecting the latter, and also the open channels between the tunnel and Mill Creek and between Mill Creek and East Creek, of which a plan was shown in my last annual report, I have to remark as follows: The greatest quantity of water (see my last annual report) that is likely to pass through this tunnel in the heaviest rainfall, as found by

the Burkli-Ziegler formula, Q=f. r{ $), is 418 cubic feet per second.

A

If we make the interior diameter of the tunnel 7 feet, which is the diameter of the main drainage tunnel just completed, we can have the economical advantage of being able to use the "centers" constructed for the latter tunnel. The velocity with the tunnel running full would then be 10.86 feet per second. That with this velocity the invert would not be abraded by sand and pebbles carried along by the water, is shown by the fact that in Washington there has not been found any abrasion of inverts of sewers, when made of vitrified brick, from velocities as high even as 16 feet per second.

By the Kutter formula V

(

assuming 0.013 as the coefficient of the roughness of the brick lining of