erly be called the history of the work, as in it are described the preliminary steps taken which resulted in a contract made between Mr. A. W. Von Schmidt and the United States, and a description of the operations under that contract. The contractor was not interfered with, and all deviations from the original plan which he thought necessary were consented to by me. My assistant, Lieutenant W. H. Heuer, United States engineers, and myself, acted as supervising engineers from the day the contract was signed until the completion of the work, and this report is the result of our joint labors. With it are submitted eleven sheets of drawings to illustrate it, as without them it would be exceedingly difficult to give an intelligible description of the machinery used, and the operations conducted. As the work was of a novel character,and of considerable magnitude, I think the description of it will prove of great interest to engineers in general. If so, I respectfully request that authority be obtained to have it, with the drawings, published; the form to be in quarto, uniform with No. 15 of the Professional Papers of the Corps of Engineers, so that none of the plates will require folding, except Plate I, which will require one fold. I have the honor to be, very respectfully, your obedient servant, R. S. WILLIAMSON, Major United States Engineers. Brigadier General A. A. HUMPHREYS, REMOVAL OF BLOSSOM ROCK, IN SAN FRANCISCO HARBOR, CALIFORNIA! This rock was situated in San Francisco Bay, directly east of the Golden Gate, or entrance to the bay, due north of the city, on a line between Alcatraz and Yerba Buena Islands, and nearly midway between them. It was distant from the city front about 1,500 yards. It was directly in the course vessels are often compelled to take in entering and leaving the harbor, was in the track of naval vessels passing to and from San Francisco and Mare Island navy-yard, and was also in the way of all passenger steamers and vessels plying between San Francisco and the Sacramento and San Joaquin Rivers. The rock was discovered and named, in 1826, by Captain Beechey, royal navy, F. R. S., who entered San Francisco Harbor in command of Her Britannic Majesty's ship the Blossom. The top of the rock was about 5 feet below the surface of the water at mean low tide. Its greatest length at the depth of 24 feet was 195 feet, and its greatest breadth at the same depth was 105 feet. The quantity of rock to be removed to obtain a depth of 24 feet of water was 5,000 cubic yards. A portion of the top of the rock, about 34 by 22 feet in size, was comparatively level, but two of its opposite sides sloped off quite rapidly into deep water. The place is exposed to southeast gales, which prevail here in the winter months, and the tide whirls over the rock at such a rapid rate that the buoy which the Light-House Department placed there has several times been swept away. CHARACTER OF THE ROCK. The rock was a metamorphic sandstone of a variable degree of hardness, of a brownish-yellow color, of irregular stratification, and in some places contained small beds of gravel cemented together with a bluish substance resembling clay. The specific gravity of the great mass of the rock was 2.64, a cubic foot of it weighing 165 pounds, and was so soft as not to require blasting, but in some places it had a bluish tinge, and was as hard as granite. In August, 1866, my attention was called to Blossom Rock by the following orders : "ENGINEER DEPARTMENT, "Washington, D. C., July 21, 1866. "COLONEL: In addition to your present duties, you are assigned to the charge of th following-named works: . "Survey or examination at San Francisco, California, with the view of the remova of Blossom and Rincon Rocks, by blasting. “You will take measures for the commencement of these works with the least practicable delay, using your best judgment as to the most expeditious method of prosecuting the same. "Respectfully, your obedient servant, "RICHARD DELAFIELD, "Brevet Lieutenant Colonel R. S. WILLIAMSON, U. S. A.. "ENGINEER DEPARTMENT, "Washington, November 27, 1×66. "COLONEL: The Department will direct the insulated wire to be sent you from New York, in order that experiments may be made on Blossom Rock, similar to those made on Rincon Rock. This seems to be necessary in order to establish the character of the former, and to enable you to form an accurate estimate of the amount required for its removal. “Very respectfully, your obedient servant, "A. A. HUMPHREYS, "Chief of Engineers, Brigadier and Brevet Major General, U. S. A. "Brevet Lieutenant Colonel R. S. WILLIAMSON, U. §. A., "Major of Engineers, San Francisco, California." Accordingly, through the kindness of the Superintendent of the United States Coast Survey, the late Edward Cordell, Assistant United States Coast Survey, who was in command of a surveying party on the Coast Survey schooner Marcy, was authorized to make a survey of Blossom Rock. Experiments in blasting were then made on the rock by my assistant, Lieutenant W. H. Heuer, United States Engineers, under my directions, in February and March, 1867. The following is a copy of his report: 'SAN FRANCISCO, March 28, 1867. "SIR: I have the honor to make the following report in regard to the late experiments on Blossom Rock. 66 Blossom Rock is a sandstone, rather hard; is on a line with Alcatraz and Yerba Buena Islands, and about midway between them. When the tides are lowest, the rock has about 5 feet of water over it. A horizontal section of the rock, 18 feet below the surface of the water at mean low tide, gives the greatest length of the rock 130 feet, by a width of 75 feet; to obtain a depth of 18 feet of water at mean low tide would require the removal of 1,000 cubic yards of stone. "The experiments were made with gunpowder, with the view of estimating the cost of removal of the rock. The charges of powder used were of three sizes, viz, 75 pounds, 125 pounds, and 175 pounds, in order to enable us to ascertain which sized charge would prove most effectual, due regard being had to economy. Each charge was placed in a strong water-tight cask, in the head of which a small hole was bored to admit of the passage of the wire for exploding the charge. The cartridge was placed as near to the middle of the charge as possible. Each cask was inserted in a sack of sail-cloth, which fitted closely, and which was afterward tarred. Two heavy pieces of iron where then tied to each cask, (one on each side,) to prevent the tide from washing the charge off the rock. Soundings were then made to enable us to find advantageous positions for the charge; a small scow was securely anchored over the spot, charge lowered to its place, scow hauled about 200 feet away, the two extremities of the insulated wire placed in contact with the poles of a 12-cup Grove's battery, and the charge immediately exploded; the scow was then hauled back to its first position, and soundings were made, by which we ascertained approximately the effect of the blast. The blasts generally took place at high tide, as then we had the greatest pressure of water on each charge. On the succeeding low water the officers of the Coast Survey (Edward Cordell, Assistant United States Coast Survey, and his assistants) would make an instrumental survey of the rock in the immediate vicinity of the blast, and thus we ascertained quite accurately the results. The officers of the Coast Survey who made the soundings before and after the blasts deserve great credit for the admirable manner in which their work was done, and to their persevering efforts much of the success of these experiments is due. Appended will be found the sketches furnished by Mr. Cordell relative to the experiments. [See Plates II and III.] Owing to the stormy weather, the experiments occupied nearly two months, although advantage was taken of every favorable day during that time, but frequently, after a blast had been made, the bay would be too rough for days, and even weeks, to make an accurate survey, all of which retarded our progress. Before we commenced blasting, an examination of the Coast Survey chart indicated quite a crevice at or near one end of the rock; as it appeared favorable to place a charge there, instructions were given to place a charge of 75 pounds of powder in the crevice. This was on the 29th of January. After they thought they had found the crevice, the charge was exploded in sixteen feet of water. The rock was considerably shattered, but no large masses were broken off, and a sub sequent examination proved that there was no crevice there. The second experiment took place on January 30, when 175 pounds of powder were exploded on top of the rock in 11 feet of water. The explosion shattered the rock, but did not materially increase the depth of water. It was then determined to have an examination of the rock made by a diver in armor. He reported that the rock was broken considerably where the two blasts had taken place, that no large fissures had been made, and that the rock was quite jagged in many places. He brought up an armful of specimens of the rock which we had broken up. "Third experiment, February 9.-One hundred and seventy-five pounds of powder were placed, near the west end of the rock, in 14 feet of water, and exploded. Water was thrown to the height of 40 feet, and the survey indicated that about 11 cubic yards of stone had been removed. "Fourth experiment, February 19.-Two blasts, of 125 pounds each, were exploded in succession in 20 feet of water. About 15 cubic yards of material were removed. "Fifth experiment, February 20.-Two charges, of 125 pounds each, were exploded in succession, one in 18 feet, the other in 21 feet of water. Only about 3 cubic yards of stone were removed. It is believed that these charges were placed on loose rock that had accumulated from previous explosions, which these two blasts broke into smaller fragments, and did much less damage than would have resulted had they been placed on solid rock. "Sixth experiment, March 4 and 5.-Two charges, of 75 pounds each, were exploded in 21 feet of water. Very little damage done. "Seventh experiment, March 6.-Two charges, of 125 pounds each, were exploded in 16 feet of water. Twelve cubic yards of stone were removed. "Eighth experiment, March 7.—The remaining charge of 175 pounds of powder was exploded in 13 feet of water. A large volume of water, probably 50 feet in height and nearly as much in diameter, was thrown up. The survey showed that 8.5 cubic yards of stone had been removed. "The experiments just made show that the charges of 175 pounds of powder produced the best results, averaging about 6.17 cubic yards per blast. The 125-pound charges averaged about 5 cubic yards per blast, while the 75-pound charges produced but little effect. It is thought that much loose rock remained where the blasting occurred, which, had it been removed, would have increased the average somewhat, probably 30 per cent. I am also inclined to think that much better results would have been obtained had we used strong iron vessels to contain the powder, instead of wooden casks. "Total expense incurred, including use of surveying vessel Marcy, officers, and crew... $3,417 42 "Average cost of removal per cubic yard "Cost of removing Blossom Rock, at this rate, to a depth of 18 feet of water, (mean low tide) 69 05 69,050 00 "The above amount would be the cost of removing the rock to this depth, if we could continually average a pressure of 18 feet of water on each charge, but when the pressure is less, which necessarily follows as we approach the top of the rock, the expense per cubic yard increases rapidly, and, judging from experiments one and two, I think that twice the above amount, or $138,100, would not be too large an estimate. To obtain a depth of 25 feet of water (mean low tide) would require the removal of 3,345 additional cubic yards of stone, which, at an average cost of $69 05 per cubic yard, would make the cost of the additional 7 feet of depth $230,972 25, or, for the removal of the whole rock to a depth of 25 feet, (mean low tide,) a cost of $300,022 25. "A few suggestions might not be out of place here. If Blossom Rock must be removed, I think it can be done more economically by drilling, and afterward by surface blasting, than by surface blasting alone. But, as the tide is very strong over the rock, drilling might at first be considered rather a difficult matter. This can easily be overcome by building a frame-work of timber and iron, setting it on the rock where we wish to drill, placing a portable 4-inch cylinder engine on top of the frame, drilling with the engine to the required depth, inserting the charge, (nitro-glycerine or powder,) removing the frame, and exploding the charge by means of a battery. After sufficient rock has been removed in this way to enable us to have a depth of 18 feet of water at high tide, then surface blasting can be advantageously used. "Estimated cost of removal of Blossom Rock, in this manner, to a depth of 25 feet of water, "Brevet Lieutenant Colonel R. S. WILLIAMSON, U. S. A., $2,173 50 3,000 00 1.500.00 3,600 00 7,200 00 3,600 00 20,655 00 6,000 00 4,000 00 3,000 00 500 00 1,000 00 56,228 50 "W. H. HEUER, "Lieutenant of Engineers Major of Engineers." In 1868 an allotment of $50,000, from the general appropriation for improvements of rivers and harbors, was made for this work. In October, 1868, Brevet Brigadier General B. S. Alexander, United States Engineers, sent in the following project for the removal of the rock, with letter of transmission: "SAN FRANCISCO, CALIFORNIA, October 3, 1868. "COLONEL: Since the passage of an appropriation by Congress for the removal of Blossom Rock, in this harbor, I have been thinking, from time to time, as to the most practical way of getting rid of that rock; and, having arrived at a conclusion on the subject, which is satisfactory to my own mind, I venture, inasmuch as it differs from the process usually followed, to communicate it to you for your information. "I understand that you are charged with the execution of this work, but, when I assure you that I have no desire to interfere in any manner with your duties, I feel confident that the assurance, taken in connection with our personal relations, will relieve me, in your mind, from any other wish in the matter than to see this work prosecuted to completion in the cheapest and speediest manner. I therefore send you, without apology, a sketch which will explain my ideas on the subject, together with a description of the proposed operations. "Having had considerable experience in such matters, particularly in building a coffer-dam around the site of Fort Richmond, Staten Island, when that fort was commenced, I will add that I have no doubt of the entire practicability of putting the proposed dam on Blossom Rock in the manner proposed, nor have I any doubt but that it may easily be made practically water-tight. "It then becomes a question of sinking a shaft and tunneling under the rock. There will be no trouble in the operation if the rock itself will keep out the water. On this point it is impossible to pronounce judgment before investigation. If the rock is like that at Lime Point, or at Angel Island, or at Alcatraz, Yerba Buena, or Point San José, the leakage into the tunnels will be small, and but little pumping will be required. There can be no doubt if these tunnels can be made without lining, that this will be the cheapest and most expeditious way of removing the rock. "Should it be found, however, (of which I have little fear,) that the tunnels cannot be made as I have designed them, I think you would find such a dam as that proposed of great assistance, whatever plan of operations you might then be forced to adopt. It would give you a fixed point on the rock, which will be the first requisite in enabling holes to be drilled into the rock. If you undertake to drill for blasting, I think you will find a fixed point for your drills absolutely necessary. "Again, the coffer-dam would enable you to build a barrack and blacksmith's shop upon it, where the workmen could live and be always on hand to take advantage of good weather and favorable tides; saving thereby to a great extent the expense of vessels for the transportation of workmen and tools. "I have not attempted to make the sketch or the description in such minute detail as will be necessary before commencing such operations. All I proposed to do was to present a plan in outline, to show the practicability of such an undertaking, with such hints as would serve as an indication of the course to be followed. “I do not propose for you to adopt the plan I have suggested on my responsibility, In an undertaking of this sort, where there is a possibility of failure, it is proper that the mode of proceeding should receive full investigation, and then the Engineer Department should assume the responsibility for whatever plan may be finally adopted. While, therefore, I do not hesitate to say that if I were charged with the removal of Blossom Rock I would urge the department to sanction the plan I have prepared for that purpose, all I can ask of you is that you will forward this plan, with the written description and a copy of this letter, to the Chief of Engineers, for his informa tion. "Very respectfully, your obedient servant "B. S. ALEXANDER, "Lieutenant Colonel of Engineers, Brt. Brig. Gen'l, U. S. A. Colonel R. S. WILLIAMSON, 66 Corps of Engineers, U. S. A., San Francisco, California." The following is the project of General Alexander: 66 BLOSSOM ROCK, "This is a sunken rock in the harbor of San Francisco, with about 5 feet of water upon it at low water. It is situated a little over three-quarters of a mile from the city front, and is in the track of vessels approaching the city from the ocean, or in going to sea, and is directly in the way of vessels running to and from the Sacramento and San Joaquin Rivers. The length of the rock above the level, which would give 24 feet of water over it at low water, is about 180 feet. Its breadth above this level is about 100 feet. The removal of so large a rock by surface blasting will, I fear, be a long as well as an expensive undertaking. After a good deal of reflection on the subject, I have arrived at the conclusion that it may be done in a single season, and at greatly reduced cost, by undermining the rock, making a number of powder chambers under it, and blowing the whole rock to pieces at a single operation. To do this, I propose to inclose a small surface of the rock by a water-tight cofferdam; in this space to sink a rectangular shaft, about 4 feet by 9 feet, which is the size I have seen in coal-mines; from the bottom of this shaft to run tunnels and make powder chambers in such positions that when exploded the whole rock down to the level of 24 feet below the level of low water will be lifted in the air and shivered to pieces. "DESCRIPTION OF THE PROPOSED OPERATIONS. "I propose in the first place to blast off a small portion of the top of the rock, by what we call surface blasting, down to about the reference of 10 feet, or 10 feet below low water. This is for the purpose of furnishing a comparatively level surface upon which to build a coffer-dam. This blasting operation will consist in lowering down charges of from 100 to 300 pounds of gunpowder to the surface of the rock over the part to be removed in water-tight vessels, and exploding them by means of Beardslee's magnetoelectric machine, and afterward removing the shattered portions of the rock by men in armor. "It will not be necessary to try to get a smooth surface. All that is necessary is to get a surface upon which the coffer-dam will stand. (See Figs. 1, 2, and 3, Plate IV.) This operation will be a little tedious, but will not be very expensive, as the quantity of rock to be removed is small, and but few men will be employed in it. There is no doubt of its entire practicability. In fact, it is believed that the whole rock, to any required depth, might be removed by this process, the only objection being the great cost. It is probable that nitro-glycerine or giant powder, owing to their powerful local effects in shattering, would prove a more efficient agent for this surface blasting than gunpowder. This being done, I propose to frame a strong coffer-dam of 12 inches square timber, as shown in Figs. 1, 2, and 3, Plate V. "This may be built on shore and made water-tight. It should be built up so as to be 13 feet high before launching. "It will then, if built of Oregon pine, draw about 10 feet of water, and may be easily towed out and anchored over the spot where it is to be placed. "It will then just ground on the rock at low water, and by the aid of ballast, say bags of sand placed upon platforms or otherwise fastened to the dam, be prevented from floating as the tide rises. "If it does not ground in exactly the right place at the first trial, the bags of sand may be removed, when it will float again, and its position may then be altered at the next low water. A sufficient number of mooring-buoys will have to be placed around the rock during this operation to enable the coffer-dam to be drawn exactly into the required position. |