H 6.

Preservation of Falls of St. Anthony.

MINNEAPOLIS, MINNESOTA, July 22, 1871. COLONEL: In accordance with your orders of the 14th ultimo, I have the honor to submit the following report of works carried on by me under your directions for the preservation of the Falls of St. Anthony, during the year ending June 30, 1871.

The plan which you instructed me to carry out was as follows: To make the coffer-dam around the head of the St. Anthony Falls tunneltight, and then proceed to clear the débris out of gorge in the sandstone through which the water from the river broke into the tunnel, the débris to be cleared out to the bottom, and the sides sloped, and occasional transverse channels to be cut in the sides and across the bottom, and then to be filled with good clay and gravel puddled up to the limestone ledge, forming the bed of the river, and a permanent wall of masonry to be built out from the west bank of Nicollet Island, commencing at a point above the place where the gorge entered the sandstone, and across the gorge near the head; thence along on the river side of the cavity, nearly parallel with the westerly side of Nicollet Island, keeping on the limestone ledge, (see tracing herewith;) to surround the first and second breaches into the tunnel, and join on to Nicollet Island on the easterly side, the wall to be built above high-water mark, and the space between the wall and island to be filled up on top of the puddle until it is above the top of the wall, and then to be covered with riprap.

I commenced on the 9th of August, 1870. From that time to the 20th of September I worked a small force, closing up the leakage in the coffer-dam. In the mean time I was procuring releases from the St. Anthony Falls Water-Power Company, and owners of Nicollet Island, and parties owning interests in Hennepin Island, to permit the United States, by its engineers and employés, to enter upon said island and the banks of the river adjoining, and in the bed of the river, to construct such works as you might authorize me to erect, or any other works deemed necessary by you for the preservation of the Falls of St. Anthony, and to release the United States, its engineers and employés, from every claim for damages which might be supposed to ensue from the construction of aforesaid works, and to permit the United States to have, free of cost, all earth or rock on said islands or elsewhere, near the falls, belonging to any of the aforesaid parties. I then commenced clearing the débris out of that part of the gorge designated on the accompanying plan, (see tracing herewith,) as the first breach into the tunnel. I cleared it out about 8 feet in depth, and found back-water to such an extent as to preclude my going any further in that direction till I had built a coffer-dam, to close the water away from the upper end of the platform of timber which had been built as a temporary protection to keep the water out of the second breach into the tunnel, and to secure a chance to make a drain through the mass of rock and material that had fallen down. I commenced a coffer-dam at the southwesterly angle of the coffer-dam already constructed, and shown on the plan as the old cofferdam, around the first breach into the tunnel, extending it down into the river nearly parallel with the westerly bank of Nicollet Island, leav ing room enough between that and the island to remove the limestone ledge over the cavity caused by the washing out of the sandstone when the water broke through from the river into the tunnel, and also room to build a permanent wall of masonry outside of this cavity, as pro

vided for in your plans. I constructed the dam of timber cribs, usually 20 feet long, 16 feet wide, and 14 feet high, and sunk them into the water by loading them with stone, keeping them close to each other while sinking, and in a line as near as possible in the strong current I had to contend with. I extended the dam down stream 353 feet, then turned a right angle, and continued the dam eastward toward the St. Anthony shore, leaving a space of about 160 feet between my dam and the old St. Anthony dam, for the water to flow into the mill-pond from the main channel of the river, my dam crossing the timber-platform over the second breach, and inclosing the upper end of it, which was the part where the principal leak occurred. I extended it in an easterly direction 152 feet, thence extending up stream 217 feet, and joining on to the east side of Nicollet Island. Before I had fully completed this last part of the dam, the St. Anthony saw-mills burned down, thus removing the necessity for drawing water from the main channel of the river, and as they had to construct a temporary dam from the east of Nicollet Island to the St. Anthony shore, thus closing the water out from that side of the island, I continued the dam, from the point where I turned to run easterly, down 166 feet in the same direction as the first part that I had built, and joined it on to the old St. Anthony dam, west of the Tuttle shingle-mill, removing the ballast from that part of the dam I had built out across the second breach, and used it to fill the last section, extending down to the St. Anthony dam. I also built about 4 feet on top of the river-side of the old coffer-dam around the first breach. I completed this part of my work about the 20th of December, making total length of dam built 887 feet in the autumn of 1870, 313 feet of it to be moved on account of change of programme, caused by the burning of the St. Anthony mills, and topping up 246 feet of old dam. About the middle of April I commenced and moved that part of the dam which was extended across the second breach, commencing at the east side of Nicollet Island, and running down stream about 40 feet east of the second breach, and joining it to the old St. Anthony dam, 120 feet east of where the dam, built in the fall previous, joined it, making 430 feet of dam rebuilt this spring. Total length of coffer-dam, inclosing first and second breaches into the tunnel and cavity between, 1,268 linear feet; deducting 264 feet of old dam, which was in place when you took charge of the work, leaves 1,004 lineal feet built, capable of sustaining a head of water from 12 to 16 feet. The dam was built of timber-cribs sunk in from 6 to 12 feet of water, and well loaded with broken stone, planked on the river side with 2-inch plank. I built and rigged three derricks for handling stone, and commenced building the permanent wall around the upper breach into the tunnel, commencing on the westerly bank of Nicollet Island, above the gorge, extending out to within about 25 feet of the gorge, leaving a space across the gorge till I could get the water out so as to clear it out from the bottom. I commenced building, and extended the wall down on the westerly side of the gorge, extending the foundations nearly down to the first coffer-dam extending out from the westerly bank of Nicollet Island to the cofferdam on the outside of the first breach. I also built a barge, and purchased one for bringing clay down the river, with which to make the puddle for filling the gorge. I get the clay on the bank of the river, about two miles above the works. There were 1,100 yards of clay and about the same amount of gravel brought down last fall, using the clay to puddle above the wall extending out from the bank of Nicollet Island to the gorge, and using the gravel to tighten up the coffer-dam. I received your orders of April 12, 1871, on the 14th instant, and com

menced building the coffer-dam from the east side of Nicollet Island to join the old St. Anthony dam 120 feet east of the point where the cofferdam was joined to the same last fall, as described above. I then commenced clearing out the débris from the gorge at the place I had left to cross with my permanent wall of masonry. My pump worked well, and kept the pit clear, and everything worked favorably till I had reached within 2 feet of the bottom of the gorge. On the 21st of May a leak commenced in the pit that I was excavating for my wall, in the soft stratum of sand in the bottom of the gorge, the water coming in under the coffer-dam and passing through among the loose stone in the bed of the river to such an extent as to prevent me from prosecuting my work at that place. I at once filled up the pit I had opened, and commenced filling out on the earth embankment, inclosing the coffer-dam around the head of the gorge, to close the leak. The water was so diffused passing through the broken stone forming the bed of the river, over which the cofferdam and embankment were built, that I was not able to determine the exact place where the water came under the dam. I extended the embankment on the outside into the river and up stream from 60 to 100 feet, letting it lap over on the limestone ledge about 35 feet. On top of the embankment I piled up broken stone 5 feet high, making a very heavy, and, it seemed to me, secure protection against any breaking in of water from the river. I then commenced clearing the débris out of a section 50 feet long, at the lower end of the first breach, intending to go to the bottom at that place and build a wall of masonry across the gorge, and puddle up a sufficient distance to make a secure dam, then go back and commence at the head of the gorge, and make another effort to clear it out and put in my wall of masonry, having the above wall for protection in case of accident. After getting this place cleared out to within about 2 feet of the bottom, the water broke in, coming along on the bot tom, on the westerly side of the gorge. I at once tried to find the place where the leak came in through under the head of the coffer-dam. After hunting about two days, I found a place in the bed of the river, about 20 feet outside of my embankment, inclosing the coffer-dam. The usual depth of the water in the river at that place was 14 feet, while, for a space about 3 feet across, the water was 20 feet deep. I at once ran out an embankment from the one built previously, and, in closing over the deep place, the leak stopped, so that there was much less water running through the tunnel than at any previous time. It appears that this leak occurred by the water coming in under a stratum of the sandstone under the embankment, thus preventing the material in the embankment from falling in, to choke up the place through which the water passed.

I am now engaged in clearing out a place at the head of the gorge, preparing to go down and close it up at that place. As soon as I can get this closed successfully against leakage, I shall push the work of clearing out and filling up the gorge with all the vigor possible.

I have a large quantity of stone quarried and dressed-I estimate sufficient to complete 300 feet of wall; and if I am as successful as I hope to be in closing the head of the gorge at the place where I am now at work, I shall be able, with the present appropriation, to permanently close up the tunnel as far as the foot of Nicollet Island this season. I regret exceedingly that we have met with difficulties of such a nature as to prevent my getting the gorge closed up and permanently sealed 'before this.

Notwithstanding our difficulties and disappointments, I think that we can congratulate ourselves that the precautions taken to prevent the water

breaking into the tunnel, at the time the leaks occurred, have been such as to prevent any damage to the tunnel or adjoining property. I remain, very respectfully, your obedient servant,

FRANKLIN COOK,

Engineer in local charge of work for the preservation of the Falls of St. Anthony.

Colonel JOHN N. MACOMB,

Corps of Engineers, U. S. A., Engineer in charge
of surveys and improvements of the Upper
Mississippi River, &c., Rock Island, Illinois.

SUPPLEMENTARY REPORT.

Preservation of the Falls of St. Anthony.

MINNEAPOLIS, MINNESOTA, July 22, 1871. COLONEL: On the night of the 3d of July, my watchman found the tunnel filling with water. I immediately went to the works and found the water coming in from below all of our works; the gorge was full of water to the top of the ledge forming the bed of the river. I found that it came into the tunnel at some place between the first and second breaches. (See the accompanying tracing.) I could not find the place where the water came in from the river until about five o'clock in the morning. It was found running under the bed-rock in the river on the easterly side of Nicollet Island, near the middle of the channel, immediately under one of the large piers that were put in for a coffer-dam by the St. Anthony Falls Water-Power Company a year ago last spring, at the time they attempted to shut the water out of the tunnel after it had broken through their plug at the foot of the second breach. The pier was placed in the river so that the upper end of it rested on the sand-rock, and the down-stream end on the lime-rock, in place. After planking up the dam to stop the water from passing, it commenced running under this pier, so that it excavated a very deep cavity in the sandstone immediately under the limestone.

This summer, during the low water in the latter part of June, when the saw-mills were running on the new St. Anthony dam, they drew the water down in this part of the river, so that it made a very strong current under these piers, which, no doubt, washed deeper into the exposed sand-rock beneath, thus permitting the water to come in direct contact with a very soft layer in sand-rock strata, which is usually found about 13 feet below the limestone, the water finding its way through this soft stratum into the tunnel at about the same place where the first break from the westerly side entered. I at once collected my men together, and built a temporary dam from the head of Nicollet Island to Boom Island, this being the channel through which the water passed into the channel on the east side of Nicollet Island and the St. Anthony millpond. By the evening of the same day I had completed the dam, turning all the water on the west side of the island, thus leaving the St. Anthony mill-pond dry. I found the chasm through which the water had passed to be 16 feet wide and 8 feet deep from the ledge to the débris in the bottom. Another new feature in the case is, that at the lower end of the old tunnel it became so choked up, with the débris that the water found its way through a new channel into the main river, coming out under the ledge about 50 feet below the apron on the westerly side of Hennepin Island.