As the rock excavation proceeds toward a possible satisfactory foundation, increasing care should be used that the rock is not unnecessarily shattered. The amount of explosive should be limited, and, if possible, black powder should be used instead of dynamite. A common specification is that the final 2 ft. of rock excavation shall be made by picking, barring and wedging, and without using any explosive whatever. The desired result is obviously that rock shall not be shattered or tight seams opened below what might otherwise prove to be an acceptable foundation. While this result might be accomplished by 6 in. of barring and wedging in some kinds of rock, and require several feet of barring and wedging in some other kinds, the final foundation should show no traces of the effect of explosive. It will assist in the proper appreciation of a foundation if one conceives of it, not as a rock foundation, but as a foundation composed of an aggregation of rocks more or less intimately associated. With this conception in mind, aim so to prepare the foundation that it will consist of the smallest number of rocks within reason. Seams varying from tight to wide open, occur all through it; they may be empty, may carry water or may be so filled with clay or other loose material as to be classed as mud seams and be tight in the sense of not permitting leakage.
N.B. - The term "tight seam" may be used in two senses:
(a) - A seam so thin that some force or shock must be applied to separate the rock on the two sides of it.
(b) - A seam of any width or thickness so filled with clay as to be tight against leakage.
Generally the seams, whether with or across the stratification, become fewer and tighter as the rock is penetrated, in other words, the rock becomes more massive. From the foundation should be removed all rock that has been moved from original position, that sounds hollow on being struck with a pick or bar; or small pieces that would serve simply to keep the masonry from contact with more massive rock below. Reduce as far as practicable the number of lin. ft. of seam that will come in contact with the masonry, thus reducing the likelihood of an uplift pressure.
Soft rock does not by any means indicate rock unsuitable for a foundation; it may apparently be quite soft and still be entirely able to bear the pressure of the proposed structure. It is also likely to contain less objectionable seams than a harder rock. In building upon rock whose stratification is horizontal or nearly so, it may be well to go into it to such a depth that there will be a large mass of rock in front of the toe of the dam, in order that there may be a considerable resistance to any tendency toward sliding, either at the bottom of the masonry or at a still lower plane. In the case of high masonry dams it is usual to excavate in the foundation a so-called cut-off trench, parallel to and under the dam near its upstream side; later filling it with masonry as part of the dam. The purpose of this trench, aside from a possible small value as a bond with the rock, is to cut off possible leakage under the dam. Obviously a trench say 20 ft. deep below the general foundation will get into tighter rock, and cut off any horizontal seams as effectively as if the whole foundation had been carried down. In order to avoid shattering the sides of the cut-off trench while excavating it, the sides should be channeled or treated to obtain the same effect. At the Wachusett and several other dams, a line of 3-in. diameter holes 3 in. apart in the clear, was drilled on each side of the trench, forming planes of cleavage, to which the rock readily broke when shooting out the mass between the lines. The side lines of holes were not loaded.
The rock in the foundation has long existed at a relatively even temperature, protected from disintegrating influences, and many kinds may crumble, scale or otherwise deteriorate rapidly on exposure to sun, air, or a different and fluctuating temperature. Hence it may be found necessary to go over the bottom for a final preparation just in advance of the masonry, even if the foundation had appeared perfectly satisfactory but a short time before. All seams that are wide enough should be raked and washed out so that the mortar or concrete may be forced into them for some depth.
For the final cleaning stiff wire brooms should be employed; also jets of water under considerable pressure. Indeed, if proper jets are used they may, in some kinds of rock, accomplish a final and highly desirable step in the excavation. A good jet has a most admirable way of searching out and removing superfluous or weakly bedded fragments The final foundation should be absolutely clean from everything but microbes.