Masonry is or should be much sounder and tighter than rock, so the question of a foundation is how much indifferent rock should be replaced by good masonry. The depth to which it will be necessary to excavate the rock has probably been tentatively estimated in advance from information gained from borings. While this estimate may be modified greatly as the excavation proceeds, it is well to start excavation with some idea as to probable depth in order to lay out the work and govern the methods. If a horizontal seam is known to exist, which may be tentatively assumed as the limit of the excavation, care should be taken that drill holes do not extend below it, or if they are drilled below it refill them with sand up to the seam so that the explosive will come above it.
Before building any masonry or finally passing upon any portion of the foundation, a large area should be exposed for inspection in order that one may be sure that excavation of an adjacent section may not demonstrate that the section under consideration should have been carried lower. One may at times, when struggling with water or loose material, be tempted to start masonry upon a small area, but in the bottom of an important foundation this should be carefully guarded against. When a stage has been reached such that the bottom of the dam is built, and the masonry is being extended both ways up the hillsides, this caution need not apply with as much force. In other words, by the time the entire bottom has been rigidly inspected, accepted, built upon and left behind, considerable experience has been acquired regarding the particular foundation. Hence there remains less chance of encountering a surprise, and smaller areas may be accepted at a time. Say 100 ft. square has been exposed in the bottom of the foundation of a big dam, there are uncertainties upon three sides; while for the same area half way up one side there is a less uncertainty on but one side, i.e., the uphill side.