Ch. 10] CONSTRUCTION MATERIALS 187 



sive explorations are usually necessary to arrive at a reasonable esti- 

 mate of permeability values. Positive cutoffs should be employed 

 wherever feasible; therefore the exploration should first determine 

 whether a cutoff is feasible along the center line or upstream of the cen- 

 ter line. Borings made for this purpose should extend a sufficient depth 

 into rock and be tested for water loss to determine if grouting of the 

 rock will be required. If a cutoff is not practical, extensive explorations 

 must be made upstream of the dam site to determine the extent and 

 thickness of natural blanketing material and downstream to obtain 

 foundation data for design of the required seepage-control measures. 

 A row of deep borings at the downstream toe is required from which 

 the effective size or relative permeability of each strata should be 

 determined as a basis for relief-well design. In addition, borings 

 should be made on several sections in order that continuity and extent 

 of the pervious strata shall be outlined in the direction of flow. 



CONSTRUCTION MATERIALS 



The soils and geological survey of the site and surrounding area 

 should develop the availability of (1) impervious and pervious ma- 

 terials for the embankment, (2) sand and gravel for drains and filter 

 blankets, and (3) stone for riprap. In order to appraise properly the 

 suitability of the local material, the soils engineer and the geologist 

 should know how these materials will be used in the embankment. In 

 the design of dams the terms impervious and -pervious are employed in 

 a rather broad sense to designate materials which are relatively imper- 

 vious or pervious when compared with other materials in the dam and 

 foundation. In the classification of different soils with regard to 

 permeability, the following table may be used: 



Impervious K less than 0.01 X 10 -4 cm. per sec. 



Semi-impervious K from 0.01-1.0 X 10~ 4 cm. per sec. 



Semi-pervious K from 1.0-50 X 10~ 4 cm. per sec. 



Pervious K from 50-500 X 10~ 4 cm. per sec. 



Very pervious K greater than 500 X 10 -4 cm. per sec. 



A brief discussion of embankment sections and the use of these ma- 

 terials therein are given in subsequent paragraphs. 



Depending on the availability of the relatively impervious material, 

 the embankment may have a narrow central core such as employed on 

 Sardis (Fig. 7), Mariposa (Fig. 8), and Franklin Falls (Fig. 12) dams, 

 or a full impervious section as on Hulah Dam (Fig. 11). If there is 



