Report of Board of Engineers on Buffalo Breakwater.) (See Annual Major D. C. HOUSTON, JNO. G. PARKE, Major of Engineers. Corps of Engineers, Chicago, Illinois. UNITED STATES ENGINEER OFFICE, GENERAL: I have the honor to acknowledge the receipt of your letter of the 16th instant in relation to the modified form of cross-section for the breakwater at Chicago, submitted with my letter of November 29. I could not have been sufficiently explicit or detailed in my letter of that date, for it was not my intention to recommend any radical change in the plan approved for this harbor, or to indorse the principle of the long slope in deep water, in opposition to the vertical face which has been so highly recommended. Assuming the great advantage claimed for the vertical face over the sloping face composed of loose material, the question of foundation becomes of great importance; it is in fact the most essential element to the stability and permanence of the work. In nearly all the harbors under my charge the natural bed for cribs is sand, and it is found that even in the greatest depth of water, when cribs have, been sunk, (as at Marquette, in 28 feet of water,) the sand moves during storms, causing the cribs to settle unevenly, to tilt outwardly, (toward the exposed side,) and in some instances to shift their position. It is, indeed, a rare case that a crib maintains the exact position in which it was first placed. The grillage bottom, which allows a portion of the stone to work through the sand, is a very partial remedy for these evils. The device of placing aprons of loose stone on the outside of the cribs has been resorted to with great benefit, but it is not satisfactory, and in many cases a storm comes up and shifts the crib by the undermining process before the riprapping can be put in. I do not know that this movement of the sand at great depths is evidence that the action of the waves extends so deep in the open lake when no obstruction exists; but our piers and breakwaters are generally placed at an angle with the line of direction of prevailing storms, and a current is produced, the action of which extends to the bottom of the work, and it is generally found that close to the pier the water is deeper than a few yards off. It has been my object to devise some economical plan of foundation for cribs on such bottoms to prevent this universal displacement, and with this view I have consulted freely with my assistants and contractors, who have practical acquaintance with the difficulties to be overcome, as well as all the authorities to whose writings I have had access. In the actual construction of piers during the past season the efficacy of "stone foundations" for cribs has been incidentally demonstrated. At Du Luth, one of the cribs, soon after it was placed and but partially filled with stone, (about 100 cords,) was torn from its position by a heavy northeast sea, leaving the stone on the bottom. A new crib was placed over this, and maintained its position better than any previous crib. It was the sixth crib in the extension of the breakwater. The loss in this instance fell upon the contractors, as they were required to fill the crib to the surface before accepting either the materials or labor. Taking advantage of this experience, a foundation of stone was put in for the next two cribs, which have stood well, although the seventh crib was not completely filled when it was exposed to the most violent gale of the season. It is generally found that, when a pier is extended into the lake, immediately at the head of the pier a hole is made by the action of the waves extending outward some 20 or more feet, and of variable depth. At Sheboygan Harbor, when, in the extension of the piers the past season, cribs 64 feet long were used, immediately at the end of the north pier there were found 20 feet of water, shoaling up to 12 feet, at a distance of 64 feet from the pier-head. It was at first proposed to dredge out a level bed, but the uncertain expense attending the work, on account of the liability to fill up before the crib could be placed, and the necessity of dredging a channel, especially to float out a crib of the required depth of the harbor, caused me to adopt the plan of filling up the hole with rubble-stone. The result was another evidence of the efficiency of the stone foundation in such a bottom. In other instances, also, we were forced to resort to this plan, and hence I directed my assistant, Mr. W. H. Hearding, who has been exclusively employed in the construction of crib-piers for the past four years, to submit his views on this question. Other plans of foundations have also been considered, particularly that of piles sawed off near the bottom of the lake, but have been discarded, owing to the expense and delays incident to all operations in the lakes. I then proceeded to apply the results of these investigations to the foundations of the breakwater at this harbor. Assuming the necessity. for some solid foundation, and the efficiency of rubble-stone for this purpose, the questions were as to the width, the slopes of the sides, how near to the water-surface the top of this foundation should be, and the manner of placing the stone so as to secure a level bearing for the crib. The consideration of these questions led to the cross-section submitted in my letter of November 29. I consider it probable that I have placed the top of the foundation too near the surface of the water for the present work, viz, 4 feet, but I proposed at first to give the stone for the foundation a narrow base, as shown by the black dotted lines, and leave this exposed to the action of the waves until it should be practically determined at what depth the stone composing the foundation would remain undisturbed. In practical construction I would make the first base as narrow as possible, so as to bring the stone up to the height indicated, and also narrowing the top. The main point, however, which I intended to submit for your approval, was the importance of these stone foundations, which.in this locality can be made as broad and deep as may be desired without increasing the present cost of the work, owing to the cheapness of stone and the facilities for handling it. It is asserted by English authorities, upon extensive evidence, that rubble-stone of the size we use will be undisturbed by the waves in the most exposed situations at a depth of from 10 to 15 feet. From my observations at this harbor, I think they will be undisturbed at a much less depth. This can readily be determined by experiment. I inclose a sketch (Fig. 5) giving a cross-section with a foundation, the top of which is placed 12 feet below the water-level. The width of the crib-work is reduced to 20 feet, which is that given to all piers in 12 feet of water, and which experience has shown to be ample when there is a good foundation. Now, if this cross-section should be adopted, the question as to the depth at which the stone are disturbed by the waves could be determined without cost, by first placing the stone with the cross-section, shown by the lines A B, for a length of 100 feet, and allowing them to be exposed for a year. The result would enable us to decide what depth could be considered safe in the most extreme cases. I have not the report of the board of 1853 with me, but my recollection is, that the crib-work was mainly adopted on account of economy, and that the portion of the work above water should eventually be replaced by masonry. The cross-section of a crib is necessarily rectangu lar, or nearly so, and its mass is calculated to withstand the shock of the waves in the same manner as enormous blocks of stone. Hence, the question of vertical or sloping exposure to the sea does not assume the same importance as in the case of harbors where stone only is used, either in the form of masonry or rubble. The form of cross-section, other things being equal, should be determined by the cost of the work. While a vertical wall may, in theory, be the most suitable form for a breakwater, there are questions of locality, character of the bottom, exposure to storms, and accessibility of materials for construction, which would decide in favor of the other form of work on the score of economy. The board of engineers convened in New York in 1867, of which Brevet Brigadier-General Hartman Bache was president, for the consideration of the breakwater at Block Island, was composed of members most of whom, I think, formed the theory of vertical exposure to the sea, yet, in the absence of any precedent for the construction of a breakwater with a vertical face in such an exposed locality, and with sandy bottom, they were constrained to recommend a cross-section similar to the Delaware breakwater. If it is assumed that the timber and iron used in the construction of cribs in lake-harbors are sufficiently durable to warrant the ultimate construction of a masonry superstructure, it is of the first importance that the cribs should be so placed as to be in close contact and good alignment. This cannot be accomplished without a good foundation. A board of engineers has recommended an outer harbor at Michigan City, Indiana. From my observations at that harbor, I am satisfied that it would not be practicable to construct a breakwater without extra precautions as to foundation. During the present month the end crib on the west pier, which was placed in the summer of 1870, has moved from its position, but we have not yet been able to ascertain the extent of the displacement, as the pier is covered with ice. A breakwater at that place will, I think, require a broad base of stone, rising to within 15 feet of the surface, and allowed to remain at least one season, before placing the cribs. The place is very much exposed, and the material composing the bottom easily moved. It is not my intention in this letter to urge the adoption of the crosssection submitted in my letter of November 29, but to ask your approval of the stone foundations, as shown on the accompanying sketch, or up to such a height as may be thought expedient. It is simply an extension of the system aimed at by the grillage bottom and riprapping, as heretofore practiced. I have never been able to satisfy myself as to the correctness of the arguments used in favor of the upright breakwater, and I take the |