CHANGES AT THE NEW CROTON DAM.

CHANGES AT THE NEW CROTON DAM.

A CRITIC CRITICISED.

Charles S. Gowen, M. Am. Soc. C. E., in a paper read before the American Society of Civil Engineers, criticises adversely the change made in the construction of the new Croton dam, especially the demolition of the core-wall dam. Mr. Gowen was resident engineer in charge of the construction of the dam “from its inception to within a few months, at which time the dam was practically completed,” when he resigned his position. In his pamphlet, as published, the writer joins issue with the board of experts which recommended the substitution of the masonry section for the core-wall section, and, more than incidentally, with the conclusion of former Chief Engineer William R. Hill as to the inadequacy of the limestone rock foundation on which the wall had been built in the beginning. He questions the necessity which, it was claimed, arose for removing the core-wall at all, or for doubting the fitness of the seam or dyke of the limestone, which (he says) “was prepared under the engineer’s direction to receive the core-wall base, and no question can be raised as to any oversight in connection with it.” Yet, almost in the same breath, it is admitted that, “upon the removal of the core-wall the rock showed much softer than it did originally.” Whatever the cause of that deterioration (if deterioration it were), it was there. That being so, it was obviously of obligation on Chief Engineer Hill to obtain a safe foundation for such an enormous superincumbent mass of masonry—even of corewall— if core-wall it had to be—that was to rest upon it. The public safety demanded that not even a suspicion of danger should exist; and. just because such a suspicion did exist, the line adopted by the chief engineer was not only justifiable, but obligatory. As to the substitution of masonry for the core-wall: No one seems to question that the wall and the embankment were solidly built. It is, also, probable that the corewall had stood for several years, showing no signs of settlement, “and that a most careful examination of the wall and its foundations as it was taken down” did not “reveal any trace of settlement.” But it is also admitted that “at the top of the wall at various points along its length, and not confined to the short stretch covered bv the dyke in question, some temperature cracks had shown above the line of refilling.” It is contended, however, that “it was so evident that they were due to changes of temperature and. possibly, to some extent to shrinkage of the setting mortar, that they were not given serious consideration until Mr. Hill’s attention was called to them, and by him they were considered so serious that It’s first report and recommendation that the core-wall be removed were very largely based upon them.” That is to say: The quick, expert eye of the chief engineer at once detected symptoms of danger which had escaped the notice or had been disregarded as of no moment l>y his less skilful, because less experienced subordinates. That fact alone justified Mr. Hill in insisting upon a thorough examination of the whole work. Admiting that the “proper plan and specifications for a reservoir embankment is a subject susceptible of many conflicting opinions,” in the important questions that come under the engineer—those which he alone must decide for himself and for whose solution and its consequences he alone is responsible—namely, the necessity for a core-wall, and, if it is required, its character and dimensions, as well as the height, width, slopes, paving and mode of construction to lw followed in the embankment—he must be “guided entirely by his own judgment, based upon experience and study of similar structures throughout the world.” Mr. Hill, however, knowing how much depended upon the stability of the dam, advised that prominent engineers— William H. Burr, who occupied the chair of Engineering in Columbia University, Nelson P. Lewis, chief engineer of the board of estimate and apportionment of the city of New York— should be appointed by Mayor Low to join him in investigating and reporting on the removal of the core-wall and the extension of the stone dam as unanimously recommended by a committee of the following expert engineers : J. J. Croes, past nrcsident of the American Society of Civil Engineers; Edwin F. Smith. M. Am. Soc. C. E, chief engineer of the Schuylkill Navigation company; and Elnathan Sweet. M. Am Soc. C. E, former engineer of the State of New York. Backed by experts of such a high reputation, Chief Engineer Hill, whose knowledge of and experience in his profession are by no means inferior to that of those already mentioned, was justified in feeling that he .was absolutely right in his judgment, even although that militated against that of the late Alphonse h’teley. In any case, even if the contrary might be asserted to the removal of the core-wall and the extension of the stone dam, the mere fact that the cracks already mentioned had appeared were enough of itself to create uneasiness and suspicion in the public mind, which would never have been content till a further searching investigation of the foundations was made. That was done, and, as Mr. Gowen himself admits, the limestone on which the core-wall rested was found wanting. Thus to what was before only a suspicion that the core-wall was actually or might one day prove unsafe was added the strongest confirmation. Wherefore, since the work of demolition had to take place, and since, besides, Mr. Hill had serious doubts as to the stability of the core-wall and embankment, it was only fitting that a proper foundation should he found and that a stone dam should be substituted for the other. The cost of the whole work was, of course, considerably increased thereby: but at the same time the absolute safety of the structure was put beyond a peradventure. As a means of information as to the construction of the Croton dam, Mr. Gowen’s pamphlet will be found very interesting and useful. As a persuader to side with him in condemning Mr. Hill’s course, however, it is a failure.

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