In advance of any construction work, a thorough knowledge of the character of the foundation is necessary. This may be obtained by a study of surface indications, together with test pits, wash borings and core borings into the rock.
While a casual examination of the surface may indicate that a masonry dam is feasible, still a thorough exploration is necessary in order To Determine:
Whether or not rock exists over the entire site of the dam.
Whether the rock is hard and sound enough to serve as a foundation, and free from seams or joints that would permit an objectionable amount of leakage.
(a) Of earth or loose material overlying the rock.
(b) Of disintegrated or unsatisfactory rock to excavate in order to reach sound, tight bottom.
Answers to Nos. 1 and 2 above answer directly the question whether a masonry dam is possible.
The answer to No. 3 determines the amount of material (excavation and masonry) to be handled, and besides bearing on the design, is a question of cost which in fact may determine the practicability of the structure.
No. 4 is obviously of vital importance as affecting the cost.
As a general proposition a masonry dam should be founded upon rock. Under certain circumstances some slight departures from the general rule are allowable. While somewhat less rigid practice might possibly be permissible in some cases, such an admission would still be a very long way from condoning the construction in several recent cases of dam failures.
This is all prefatory to saying that while the depth and character of the material overlying the rock should be determined by borings, the borings are primarily to determine depth. The character of the material, while most interesting and useful in connection with the questions of cost of excavation, design of temporary works, amount of pumping, etc., is not required for such a primary purpose as judgment of its adequacy as a foundation.
Elevation On A-B.
Fig. 1. Wedges used with clamp to pull casing.