AAA 2.

INCREASING THE WATER SUPPLY OF WASHINGTON, DISTRICT OF COLUMBIA.

This work was commenced under an appropriation made by act of Congress approved July 15, 1882.

The plan consisted in raising the dam in the Maryland channel at the Great Falls of the Potomac to an elevation of 148 feet above mean tide at the Washington Navy-yard and its extension at that height across Conns Island and the Virginia channel of the river; extending the Washington Aqueduct from the distributing reservoir above Georgetown to the site selected for the new reservoir near Howard University by a tunnel 20,696.3 feet long; constructing at the tunnel outlet a new reservoir of about 300,000,000 gallons capacity, and connecting this reservoir by a new line of large mains with the existing system of water mains in the city of Washington.

So much of the plan as related to raising and extending the dam at Great Falls was completed during the fiscal year 1887. No operations under the project were carried on during the past fiscal year.

A watchman has been employed during the year at the new reservoir. His duties have included guarding the stone at the mouth of all shafts except the one at Foundry Branch, which is under the care of the watchman at the distributing reservoir.

The following is a list of the appropriations for this work, with the date of act for the same:

The act of Congress approved March 2, 1895, making appropriations for the expenses of the government of the District of Columbia for the fiscal year ending June 30, 1896, and for other purposes, contained the following:

For raising the height of the dam at Great Falls, together with the cost of such other work as may be found necessary in connection therewith, including the cost of strengthening the conduit, and for damages on account of flooding of land, and other damages, one hundred and twenty-five thousand dollars, for which amount and purposes the available balance of the appropriation for increasing the water supply of the city of Washington shall be applicable, subject to all the provisions and restrictions of the act to increase the water supply of the city of Washington, and for other purposes, approved July fifteenth, eighteen hundred and eighty-two, and of the act approved July fifth, eighteen hundred and eighty-four, making appropriations for the expenses of the government of the District of Columbia as to apportionment and settlement between the United States and the District of Columbia, and the refunding thereof.

*

RAISING THE HEIGHT OF THE DAM AT GREAT FALLS.

This dam extended from the Virginia to the Maryland shore, crossing Conns Island. Its total length was 2,877 feet. The width of its crest in the Maryland channel was 7 feet 9 inches, and across Conns Island and in the Virginia channel 8 feet 3 inches. The reference of the crest was 148 feet above datum. (For details, see Sheet A.)

During periods of extreme low water in the Potomac River the reference of the surface of the water at the mouth of the conduit feeder

was but 148.5 feet, while that of the interior of the crown of the conduit itself, if produced to the gatehouse, is 151 feet; consequently at this stage the surface of the river was 2.5 feet below the crown of the 9-foot conduit produced, and the capacity of the conduit was much decreased thereby.

It was therefore proposed to raise the height of the dam from a reference of 148 feet to 150.5 feet, so that at its lowest stage the river would fill the upper part of the 9-foot section of the conduit.

The approved project was to remove the coping stones, raise the height of the dam the desired amount, replace the coping stones, securing them in place by iron bolts and clamps, and strengthen the conduit to withstand the increased pressure caused by raising the dam. The dam was slightly out of level, and it was proposed to correct this as much as practicable while raising the dam, so that when completed its crest would be practically at a uniform height throughout. The project contemplated obtaining all of the stone from the Government quarry at Seneca, Md., which was reopened on March 21, 1895. Quarrying was continued there until December 14, 1895, when it was decided that the depth of "stripping" then necessary (over 30 feet) precluded all idea of obtaining stone from that source economically. Six hundred and fifty-one cubic yards of dimension stone and 17 cubic yards of coping had been quarried at Seneca.

On January 21, 1896, authority was granted by the Chief of Engineers to obtain stone by contract, and after due advertisement the bid of Mr. Asa B. Cook, of Petersburg, Va., for furnishing about 1,100 cubic yards of dimension stone (granite) at $18 per cubic yard, and about 100 cubic yards of coping (granite) at $27 per cubic yard, was accepted, and the contract approved by the Chief of Engineers on March 28, 1896. Time of commencement of contract, April 10, 1896; time of completion, July 1, 1896, extended by authority of the Chief of Engineers to September 15, 1896, and further extended to November 15, 1896. Completed November 7, 1896.

The total length of dam raised was 2,751.1 linear feet, the plan and cross section being as shown on the accompanying tracing, Sheet A.

The first stone was laid on September 30, 1895, and the last stone on November 25, 1896, work having been suspended on account of cold weather from December 14, 1895, to March 23, 1896.

During the working period there were, excluding only Sundays and national holidays, 283 working days, out of which there were lost,

On account of high water in the Potomac River.
For want of stone from contractor....

Days.

84

23

107

Leaving 176 days on which it was possible to work when weather overhead permitted.

The velocity of the water on the crest of the dam was very great, and it was known in advance that whatever the form of cofferdam used, it would be exposed on the upstream side to the pressure due to a head of from 3 to 5 feet. Moreover, the Potomac is subject to large and sudden rises during the working season, and on such occasions carries much driftwood.

After a study of the various conditions affecting the problem, it seemed probable that the best results would be obtained by a cofferdam which would protect the workmen and inclose only so much of the dam as could be raised in a single working day, and, after the concrete and mortar had set, could be taken up and used elsewhere.

The proposed plan, with slight modifications to suit the three different cases, was adopted, and proved very efficient and economical. Against a head of from 3 to 5 feet practically no leakage was developed through any of the joints of the three 15-inch courses of masonry laid with the use of the cofferdam. In view of its small cost and great efficiency, it is believed that a detailed description of the cofferdam, the method of using it, and a statement of its cost will not be without interest.

Description of the cofferdam used in raising the height of the United States dam at Great Falls, Md. (See tracing, Sheet B.)-This cofferdam consists of two horizontal movable wooden bulkheads A B C D, each 30 feet long; the 14-inch round iron bars E E, for holding them in position and resisting the pressure of the water, and the sandbag dams FG and H K, for closing the ends. The bulkheads were made by fastening together five 2 by 12-inch by 30-foot Georgia pine planks with three 2 by 12-inch cleats L L L and 40-penny nails. The joints between the boards were then carefully calked with jute. Each bulkhead was also provided with 2 ringbolts M M, so that it could be handled either with the derrick or suspension cable way. After the riprap stone N N N had been placed to shut off a portion of the water, and the old coping had been removed, holes were drilled in the old masonry for the 14-inch iron bars at the angle shown in the section. These holes were put in to a depth of 24 inches, and just inside the upstream building line, so that the bulkheads would rest on the old masonry-which is the same width as the new. The bulkheads were then put in position, and the joint CD between them and the old masonry having been calked with pieces of old bags, and the sandbag dams F G and H K having been built, the cofferdam was ready for use. By the use of an additional row of sandbags on the inside of the cofferdam the headers and stretchers in the bottom course of the downstream face of the new masonry were laid on a bed free from leakage. These sandbags were then removed and the headers and stretchers in the bottom course of the upstream face of the new masonry were laid in water an inch or two deep, but having practically no current. All the new masonry above the bottom course was laid on a bed entirely free from leakage. This style of cofferdam was used in raising the height of the dam in the Maryland channel for a distance of 460 feet after the weather became too cold for the laborers to go into the water. It does not allow the use of any stones in the upstream face of the bottom course that have quarry faces extending beyond the building line, and it requires the spawling of stones at each 14-inch iron bar so as to get the proper width of dam.

In the Maryland channel, until the water became too cold for laborers to enter it with impunity, the bottom of the bulkhead was kept about 6 inches from the upstream face of the dam by means of two figure-8shaped iron braces, through one of the loops of which passed the iron bars E E, while through the other loop passed a similar iron bar, against which the bulkhead rested.

A row of sandbags was carefully placed so that they would be a little below the top of the old masonry and would make a tight joint with it. On top of this row of sandbags the wooden bulkheads were placed, so as to leave between them and the new masonry sufficient space for the passage of the slight leakage through the cofferdam.

This allowed the use, in the upstream face of the bottom course, of stones having quarry faces projecting several inches beyond the building line. Back of the bulkheads, and partly covering the row of sandbags on which they rested, was placed another row of sandbags.

The cofferdam being fixed, the other operations were similar to those described in the first case.

In the Virginia channel there was a 6-inch offset below the third course of masonry (see cross section, Sheet A), and the holes for the round iron bars E E were drilled in this offset 4 inches above the upstream face of the new masonry and to a depth of about 12 inches. The bottom of the bulkhead then rested on the comparatively smooth upper surface of the offset, and the space between the bulkhead and the upstream face of the masonry provided a passage for the escape of the slight leakage through the cofferdam, as in the second case. After the cofferdam was fixed, the other operations were identical with those described in the first case.

The masonry in the portion of the dam across the Virginia channel was much superior to that in the part across the Maryland channel (see cross sections, Sheet A). Owing to this fact and to the existence of the offset already described, operations on this part of the dam were much simplified.

The cofferdam just described was successfully used for raising 2,095 linear feet of dam in the Virginia and Maryland channels, the 656 linear feet across Conns Island requiring no cofferdam.

The entire cost of the cofferdam (wooden bulkheads and sand-bag dams), including all operations connected with setting and calking it, was as follows:

Superintendence
Materials
Labor

Total.....

$184.00

322.06 1,558. 00

2,064.06

Two thousand three hundred and fifty-two cubic yards of stone and concrete were laid behind the cofferdam, which therefore added 88 cents per cubic yard to the cost of these items. As the cofferdam was used for a distance of 2,095 feet, its average cost per linear foot was 984 cents. To strengthen the conduit against the increased pressure which might be developed on account of raising the dam, the embankments in which the conduit rests were widened, as shown in the following table:

This completed the work of strengthening the embankments and makes a total of 9,870 cubic yards of materials used for this purpose. Of this amount, 5,147 cubic yards were put in place during the fiscal year ending June 30, 1896, of which 3,480 cubic yards were charged to the annual appropriation for preservation, maintenance, and repair of