These two chambers are connected with the reservoir by three brick inlets , 10 feet in diameter . And thy are to hold water to a depth of 54 feet , yet the outer walls ( which are en- closed in an artificial embankment ) have a uniform thick- ness , from the base to top of but eight feet , while safe construction demands that they should be about four times that thickness at the base . These walls which had been built to a height of fifty
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along the line of the wall for a distance of 150 feet , thus the core wall at its highest end was not built in a narrow trench below the surface of the ground , as is usual in ordinary cases . The bed rock on the north side of the valley is gniess and the structure is founded upon it for a length of 1,200 feet . The character of the rock then changes abruptly from gniess to lime stone and this extends southerly be- yond the end of the structure . Although the contract drawings , upon which the contract was based , indicated " hard rock " at a depth of 75 feet below the bed of the river , yet it was found necessary to excavate the founda- tion pit to a depth of 131 feet before finding a firm foun- dation of hard rock . The character of the stone re- moved varied greatly ; in places it was compact , then di- versified by masses of stone broken up by open , eroded and mud - filled seams , and in places there were belts of disintegrated lime stone . During the progress of the excavation water flowed freely through the seams and several open caves were found in the rock .