436 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 5 



posed had never been built, and its great magnitude presented prob- 

 lems which could not be solved; and the river silt would fill the res- 

 ervoir in a short time or would destroy the gates, valves, and tur- 

 bines if allowed to pass through them. 



Geologic examinations, channel drilling and tunneling into the 

 dam abutment during the preliminary investigations had shown the 

 rock to be satisfactory, and the later excavations for construction 

 verified this conclusion. In the following paragraphs, descriptions 

 will be given of the manner in which other questions were answered. 



Primarily on account of its inherent safety features, the dam 

 selected to be built in Black Canyon was one of arch gravity type 

 in which the thrust of the water is taken first by its weight 

 (6,500,000 tons) and, secondly, by the arching action upstream. 

 To provide the necessary reservoir storage with this type of struc- 

 ture, the required dam would have a height of 726 feet (nearly 

 that of the Woolworth Building in New York) ; a base thickness 

 of 660 feet (more than the length of two ordinary residence blocks) ; 

 a crest whose thiclaiess is 45 feet and length 1,282 feet — on which 

 a highway has been constructed; and contain 3,250,000 cubic yards 

 of concrete (a greater volume than that of the largest Egyptian 

 pyramid). 



Owing to the large dimensions of the dam and the enormous 

 volume of concrete to be placed, the problem of temperature stresses 

 within the concrete became one of great importance, and much re- 

 search and study were conducted by Bureau of Reclamation engi- 

 neers before a practical solution was reached. Concrete would be 

 poured in the dam during the summertime at a temperature above 

 100° F. and another approximately 40° F. would be added by the 

 chemical heat of setting. If allowed to cool by natural means, the 

 temperature of the dam would not be lowered to that of the air 

 and water surrounding it for a period of as much as 150 years. 

 During this time, cracking would occur due to contraction of the 

 concrete. 



The dam was, therefore, designed and built as a group of 230 

 interlocking vertical colunms varying in size from 25 to 60 feet 

 square. Steel tubing of 1 inch diameter was installed in the con- 

 crete at approximately 5-foot intervals, both vertically and hori- 

 zontall}^, and water at a temperature as low as 30° F. circulated 

 through the concrete of the dam. Thus the entire mass of the dam 

 was cooled to predetermined temperatures ranging from 43° to 

 72° F. in a period of 19 months. 



As planned, the contraction of the concrete due to cooling caused 

 the columns to separate. These openings were filled with a water- 

 cement grout as soon as cooling was finished. Consequently, as the 



