FOUNDATION.–We may now assume that our preliminary studies relating to the location and physical features of the dam site are satisfactory. We must next investigate the foundation upon which the dam is to be built. This investigation is sometimes wholly neglected or else done in such a way as to be practically useless. To merely drive down iron rods feeling for so-called bed rock, or to make only a few bore-holes with an earth auger should in no instance be considered sufficient. Borings may be found necessary at considerable depths below the surface and in certain classes of material, but dug pits or shafts should always be resorted to for moderate depths and whenever practicable. Only by such means may the true character of the strata underlying the surface, and the nature and condition of the bed rock, if it be reached, become known. If a satisfactory stratum of impermeable material be found it is necessary also to learn both its thickness and extent. It may prove to be only a “pocket” of limited volume, or if found to extend entirely across the depression lengthwise of the dam site it may “pinch out” on lines transversely above or below. Shafts and borings made in the reservoir basin and below the dam site will determine its extent in this direction, knowledge of which is very important. Fig. 1, showing a longitudinal section of the site of the Yarrow Dam of the Liverpool Water-Works, England, illustrates the necessity of such investigation. A bore hole at station 2 + 00 met a large boulder which at first was erroneously thought to be bed rock. The hole at station 3 + 50 met a stratum of clay which proved to be only a pocket. The relative elevation of the different strata and of the bed rock formation, referred to one common datum, should always be determined. These elevations will indicate both the dip and strike of the rock formation and are necessary for estimating the quantities of material to be excavated and removed, including estimates of cost. They furnish information of value in determining the rate of percolation or filtration through the different classes of material and the amount of probable seepage, as will appear later. The cost of excavating, draining and preparing the floor or foundation for a dam is often very great, amounting to 20 or 30% of the total cost. Fig. 2 is a transverse section of the Yarrow Dam. This particular dam has been selected as fairly representative of English practice and of typical design. It is one of the most widely known earth dams in existence. [Illustration: FIG. 1.–LONGITUDINAL SECTION OF YARROW DAM SITE.] [Illustration: FIG. 2.–CROSS-SECTION OF YARROW DAM.]