low-water depth is only about 45 feet, while along the western branch the average mean low-water depth is about 65 feet. The essential features of the structure adopted for estimate in the preliminary project of 1884 are as follows: 1. A solid conical pier of masonry on Avery Ledge, the diameter at top being 30 feet and at bottom 45 feet, and a similar but smaller pier at the western end of the breakwater. 2. A substructure consisting of an enrockment formed by dumping granite grout from barges, the smallest rubble placed in the core, the medium-sized in the inner layer and the largest in the outer covering. This arrangement was intended to increase the volume of the voids, and thus diminish the amount of stone required for the enrockment, and was based upon the experience at Bastia. The upper surface of the substructure was to be 15 feet below extreme low water. This depth was at that time considered to be the limit of the effective action of the sea, the adopted project for the Delaware Breakwater placing the base of the superstructure only 12 feet below mean low water. 3. A superstructure consisting of a masonry wall formed of very large stones laid up dry below water, and above low water a masonry wall laid in cement. The dry wall below water was provisionally adopted, pending further investigation, because of the great facilities for obtaining very large stones in the immediate vicinity, and because of the experience of the local quarrymen in laying large stones under water. Assuming an annual appropriation of $400,000, and the employment of the best plant and the Bastia method, with rubble at 70 cents per ton, this breakwater was estimated by the Board of 1884 to cost $5,000,000. It was proposed to commence the construction of the breakwater by building the pier on Avery Ledge, and to continue the work by extending the substructure and superstructure from that point along the southern branch toward Abner Ledge. This course was adopted because the southern branch of the breakwater is situated in the least depth of water, where beneficial results could soonest be obtained, and because it shelters the anchorage from the most violent northeast storms. The western branch covers the anchorage from the less violent northerly storms, and would be of little or no value until after the construction of the southern branch. Avery Ledge is the most exposed portion of the whole line, and it was considered desirable to determine by actual construction the feasibility of building this most exposed part before committing the Government to heavy expenditures on the rest of the work. Avery Ledge is a great danger to navigation, many vessels having been wrecked thereon. It was considered that a pier constructed upon it would, therefore, be of some immediate value to commerce, would form a base for experiments to determine the character and details of the superstructure, and, finally, would serve to hold in position a spindle necessary as a range in forming the substructure. It was believed that it would be otherwise impossible to maintain such a spindle permanently in this locality. On May 20, 1885, under the orders of the Department, the local engineer submitted a detailed project for the construction of a stone and concrete pier on Avery Ledge, and reported that he had considered a project for a pier consisting of a heavy cylinder of cast iron filled with concrete, which he rejected for reasons stated. On June 25, 1885, the Board of Engineers for Fortifications and River and Harbor Improvements, etc., to whom this project was referred for consideration, reported in favor of the building of the pier "of concrete in situ inside of a cast-iron casing, following the method used in the foundations of the light-house at Duxbury Pier, Massachusetts." The Chief of Engineers, in his indorsement upon this project, dated July 2, 1885, remarked that "it would be less costly to cover Avery Ledge with masses of stone or concrete blocks, forming thus the termination of the breakwater after the portion contiguous to the ledge should have been built." On September 10, 1885, under the orders of the Department, the local engineer submitted a project for the expenditure of the available funds. In conformity with the orders, the project provided for "a structure upon the summit of the ledge," and for the distribution of broken stone upon the bottom along the line of breakwater." By investigations in the locality, and by the study of more recent experience in foreign harbors, it had become apparent that the limit of wave action heretofore adopted-15 feet below extreme low water-was altogether too small, and that it would not be safe to establish the foundation of the superstructure at a higher level than 22 feet below mean low water. Under these circumstances it was considered that the plan for a dry masonry wall must be rejected, and that if any wall was to be built below low water it must be of concrete deposited either in place, as at Quebec and Wicklow, in bags, as at Aberdeen and New Haven, or in sloping blocks, as at Colombo, Réunion and elsewhere. The project repeats the opinion that the establishment upon Avery Ledge of a cast-iron cylinder, to be filled with concrete, is practically impossible; provides for the construction of a portion of the rubble base of the breakwater, the upper surface to be 40 feet in width and 22 feet below mean low water; states that it will probably be found most economical to construct the superstructure of concrete, and finally recommends the placing of two large concrete blocks upon the ledge. The advantages of the proposed structure on the ledge are stated in the project as follows: 1. It will test the durability of concrete exposed to ice and heavy seas in this locality. No wear from floating ice is anticipated, but it is possible that on the occasional quiet days of winter the water in contact with the concrete surface may freeze, and thus that surface may be gradually disintegrated. 2. It forms a convenient central core from which to construct in the future the pier of the breakwater, and it will be useful for this purpose without reference to the method of construction adopted. Thus the final project for the pier is left to be decided upon after the durability of concrete in this locality has been determined by actual experiment. 3. Its spindle forms a permanent range mark for the construction of the rubble base of the breakwater. This project having been approved by the Department, work upon the rubble substructure was commenced under contract in November, 1885. The method of arranging the rubble suggested in the preliminary project was abandoned, the funds available not being sufficient to justify the establishment of the necessary plant for this purpose. Preparations were in progress for the formation of the concrete blocks when Maj. Raymond was relieved from charge of the work on February 4, 1886. On May 14, 1886, the Department modified the plan of construction for the pier at Avery Ledge, authorizing a construction of "concrete in situ inside of a cast iron shell, as suggested by the Board of Engineers for Fortifications and for River and Harbor Improvements, etc., June 25, 1885." This cast-iron and concrete structure was completed in July, 1886; the iron shell was destroyed in less than a month, and Since that time finally the concrete and spindle were swept away. operations on Avery Ledge have been confined to the erection of spindles and their replacement when swept away by collision or storms. (The last spindle, 8 inches in diameter, sunk into the rock 34 inches, has been standing uninjured since January 1, 1889.) Under various appropriations and contracts, work upon the base of the southern branch of the breakwater has been in progress since November, 1885. The Board is informed that at the close of the existing contract the substructure between Avery and Abner ledges, and beyond, a distance of about 5,000 feet, will have been essentially completed, with its upper surface 40 feet wide and 22 feet below mean low water, in accordance with the project. About 538,000 tons of unarranged rubble, at an average price of 675 cents per ton, will have been deposited to form this structure. The Board finds no evidence that any of this rubble has been moved by the action of the sea. The original project of a random stone substructure, carrying a masonry wall, was proposed in order to save the great expense of carrying the rubble mass above high water, which involves the deposit of an enormous amount of material below the limit of the effective action of storms. The execution of such a project is only possible with very large appropriations and the best appliances. If this method of construction is to be followed, the Board is of the opinion that the top width of the substructure should be 40 feet, and it should be 22 feet below mean low water, as now provided for. The Board is further of the opinion that the employment of concrete in place or in bags of large size in the construction of the superstructure is not advisable, and the only method of wall-building which gives reasonable promise of success is the method of sloping concrete blocks adopted at Colombo and Réunion. The Board, however, is not prepared to recommend such a construction at the present time. As stated in the revised project for the Delaware Breakwater (vide Annual Report of the Chief of Engineers for 1891, page 1079), there is no precedent for the construction of an offshore breakwater on the sloping block system. All the breakwaters built on this plan have started from the shore, and this fact has determined the character of the plant employed. Thus at Colombo, Madras, Mormugoa, Manora, Kustendjie, and Réunion, the blocks were placed in position by large and costly overhanging balance cranes, which trayeled on railways along the breakwaters, and could be run on shore during seasons of exposure. The peculiar advantages of such machines would be entirely lost in an offshore construction. The exposure of the site to the violent action of waves and currents, and the necessity of accurate adjustment imposed by the sloping block system, render the use of floating appliances entirely out of the question. A special plant would have to be devised, which would be experimental in character and doubtless very expensive. The leveling of the upper surface of the substructure for the foundation of the wall would have to be done by divers and would be slow, difficult, and costly. The construction of a pier on Avery Ledge, as an abutment for the end of the wall, would be troublesome and expensive, as shown by previous experience. Finally, no experiments have yet been made to determine the durability of concrete in this locality, and the Board is far from satisfied that this material would bear without rapid disintegration the exposure to which it would be subjected. Under existing circumstances the Board is of the opinion that the best method of construction for the southern branch of the break water between Avery and Abner ledges is to form it by the deposit of granite rubble in the manner now adopted for the work in progress at the Delaware Breakwater. It is proposed to give the mound a width of 51 feet at mean low water and to form it up to that level by the deposit of random stone, so that the slopes shall be such as may be established by the action of the sea, the heaviest stones to be placed on the exposed face of the structure. It is proposed to give the superstructure, above low water, a height of 18 feet and a top width of 20 feet, with slopes of about 1 on 1 on the sea side and 1 on 73% on the inside, formed by the heaviest stones laid in position, the interior space to be filled with rubble. The cross section provisionally adopted by the Board is shown on the accompanying sketch. It is based upon a study of the storm-developed slopes of the breakwaters at Rockport and in Delaware Bay. Experience during the progress of the work may require the modification of its form and dimensions. As before remarked, it was originally proposed to begin the construction of the southern branch at Avery Ledge, the most exposed point of the line, in order to determine the feasibility of this part of the work before committing the Government to further heavy expenditures. This reason no longer exists, a large sum having been already expended in the formation of the substructure. The Board is, therefore, of the opinion that the work of raising the breakwater to its full dimensions should be commenced at the northern end of the branch, at Abner Ledge, which is the least exposed portion of the line, and should be gradually extended until it reaches Avery Ledge. The experience obtained from this work, and investigations made during its progress, will determine whether the head of the breakwater at Avery Ledge can be formed of large natural or artificial blocks, or which of the other methods proposed should be adopted. For the western branch of the breakwater, the Board, with its present information, is unwilling at this time to recommend any method of construction other than the rubble mound proposed for the southern branch, although the average depth is so great that the structure will be of very great size below the level of sea action. The Board recommends for this branch the provisional adoption of the rubble method, with the cross section shown on the accompanying sketch. During the progress of work on the southern branch a thorough investigation should be made of recent foreign practice, especially at the harbor of Boulogne, with a view to possible economy by the construction of a masonry wall above low water. The project submitted by the Board is based upon the fact that the country in the vicinity of the work is covered with hills of syenite, containing extensive quarries, from which stone can be obtained at a low price. The rubble employed is the refuse of the quarries, and its removal has hitherto been a source of expense to quarry companies. It has been estimated that the four breakwaters built out from the shore near Rockport contain about 1,500,000 tons of this refuse material. In the preliminary project the local engineer estimated the cost of the rubble in the breakwater with the present means of production at 70 cents per ton; but he was of the opinion that if extensive quarries were opened at Gap Head, Gully Point, and Straitsmouth Island, and the rock broken out by large blasts, the work could be greatly cheapened. The actual contract prices paid for stone have been 58.3 cents per ton in 1885, 71 cents per ton in 1886 and 1888, and 734 cents per ton in 1891. The Board believes that if a contract for a very large amount of stone, which would justify the opening of quarries specially for its production, were authorized by Congress, the cost of this material would be greatly reduced. This Board is further of the opinion that the breakwater can be completed with the section recommended, provided liberal appropriations are made by Congress for its continuation, within the estimate submitted by the Board of 1884, $5,000,000. A map accompanies this report. Brig. Gen. THOMAS L. CASEY, WM. P. CRAIGHILL, [First indorsement.] Major, Corps of Engineers. OFFICE CHIEF OF ENGINEERS, Respectfully submitted to the Secretary of War. U. S. ARMY, March 14, 1892. The project submitted in 1884, under which work has heretofore been carried on at this locality, provides for the construction of a continuous breakwater about 9,000 feet long, extending from Avery Ledge northwest to the 86-foot contour off Andrews Point. The estimated cost of this work was $5,000,000. The within project by the Board of Engineers proposing plan for substructure as well as superstructure of the entire breakwater is recommended for approval. It is the opinion of the Board that with liberal appropriations the breakwater can be completed with the section now recommended within the estimate submitted, viz, $5,000,000. THOS. LINCOLN CASEY, Brig. Gen., Chief of Engineers. [Second indorsement.] WAR DEPARTMENT, March 17, 1892. The within project is approved as recommended by the Chief of Engineers. S. B. ELKINS, B 6. IMPROVEMENT OF HARBOR AT GLOUCESTER, MASSACHUSETTS. This is the most important harbor between Boston and Portland, and is the principal resort for all New England fishing vessels. It is situ ated at the southeastern extremity of Cape Ann, 20 miles northeast from Boston. It is easily entered when the dangerous storms of this coast occur, and provides a secure, ample shelter for all classes of vessels, except from south winds, and from these a moderate extent of protected anchorage is afforded in the inner harbor. |