JEROME PARK RESERVOIR.
CRITICISM ON ITS CONSTRUCTION.
On June 21, 1901, the aqueduct commission in charge of the new Croton dam and the Jerome Park reservoir appointed a board of engineers to consider the plans of the earth embankments at the new Croton dam and Jerome Park reservoir. Under date of November 18, 1901, the board of engineers, comprising J. James R. Croes, Edwin F. Smith, and Elnathan Sweet, made an extended and elaborate report upon the questions involved to the equeduct commissioners. On November 22, 1901, the report of the board of engineers was referred to VV. R. Hill, chief engineer of the aqueduct commission. He, under date of December 4, 1901, reports in substance on the prominent features involved in the report of the commission as follows: “1 concur with the board of engineers in recommending that the plans of construction of the southerly end of the new Croton dam, from the end of the present masonry dam to the gatehouse be modified by the substitution of a masonry structure for the earthen embankment with core-wall.” With reference to the core wall and embankment of Jerome Park reservoir Mr. Hill reports: “In my opinion the building of an embankment and core-wall for a reservoir upon such material and under the conditions as described is a gross violation of the rules of good practice, which prescribes that a core-wall should be built upon rock or upon solid impervious material.” Mr. Hill also takes exception to the method for testing the presence of quick sand under the walls as follows: “It is evident to me that, in order to determine that there is no quicksand under these walls, whose particles are of impalpable fineness, a different method should have been employed in obtaining the samples that were examined otfier than by forcing the material to the surface by jets of water, thus washing away the finer particles and leaving only the coarser ones for examination.” * * * “In conclusion I wish to state that, in my opinion, the material under the embankment and core-wall at the southerly. and easterly side of the reservoir, at station 99, is quicksand, having a maximum depth of about thirty feet under the base of the wall. The sand is unstable and tmeonfined, and it is completely saturated with water, which is now flowing through it. In view o fthe above, I am of the opinion that the embankment and core-wall are utterly unfit for the. purpose for which they were constructed. I, therefore, cannot concur in the conclusions and recommendation of the report of the board of engineers relating to Jerome Park reservoir.”
It is an interesting and important subject for the study of engineers. The character of the investigation on the part of the board of engineers, as outlined in their report, on the proposed alteration of the constructive features of the new Croton dam naturally inclines the thoughtful and studious members of the profession to the consideration of the important factor of safety, as to what its measure may and should be in dam construction. It is well known what arc the infirmities associated with the construction of dams, and this knowledge has cost millions of dollars and the sacrifice of life. In the light of past experience, it is well to be on the safe side, even if it costs more money, than to run any risk, if there is a risk, which does suggest itself, if the dam is completed other than advised by the board of engineers.
Concerning the embankment and core-wall of certain portions of Jerome Park reservoir, it would appear that Engineer Hill dissents from the opinion of the board of engineers solely on the ground of an unreliable condition of foundation, which he asserts exists, owing to certain arbitrary features uncontrolable in a water-bearing stratum of sand, peculiarly, but not unnaturally located in a rock pocket which has an outflow. Under certain conditions and operation of natural law the depth of water in this substraum may prove to be variable. Jt may pass.into another or lower level, thereby impairing the foundation of the core-wall as well as the proposed floor of concrete of the basin of the reservoir by carrying off the fine sand in suspension in the moving water. This assumption on the part of Mr. Hill does not seem to be invalidated by the statement of the board of engineers.
The difference of opinion with regard to this question appears to be based (1) on the statement of the board of engineers in their report as follows: “The rock outside of the reservoir is evidently, from all the borings which have been made, higher than it is under the embankment, and the natural surface of the ground nowhere, within a distance of 500 feet outside of the core-wall, is more than eight feet below the water surface of the full reservoir.” It is based (2) on what Mr. Hill says, as follows: “This is true; but it may be misleading, inasmuch as it might be assumed that the low place, instead of being immediately’ back of the wall, as is the case, might be 400 to 500 feet away.”
The engineer’s report further states; “The character of the topography’ adjoining the reservoir boundary convinces us that no serious leaks need be apprehended under this section of core-wall founded on earth, and that none can possibly occur having sufficient motion to threaten the stability of the embankment in which it stands.” From this it might be inferred that the ground was rising back of the wall, or else what bearing would the character of the topography have upon the subject, or how could it convince any one that no serious leaks need be apprehended? The wall at this place is built across a depression, and the surface of the ground falls away from the reservoir in a valley terminating at the Harlem river.
The examples of reservoirs erected for the Philadelphia waterworks are alluded to by Engineer Smith in a supplementary report; these proved to be failures in point of retaining water and are to be regarded as a warning. Allusion is also made to the defective features in specification requirements. Admitting this to be a fact, there is no good reason for not attempting a course of heroic treatment in the way of now doing what ought to have been done in the beginning. If it transpires that errors of omission have occurred, and that such omission derived its origin from lack of accurate surveys and knowledge of the ground upon which Jerome Park reservoir is erected, some one is to blame for it. The present state of the art of reservoir construction surely enables reservoir embankments and their foundations to be erected under conditions incident to underground considerations which embrace every imaginable problem that can be thought of; consequently, there is no good reason or excuse for approving or adopting defective specifications that fail to embrace every possible emergency likely to arise in construction of reservoir embankment foundations.