186 middlebrooks. EARTH DAMS [Ch. 10 



where a flat natural divide consisting of soil is used as a spillway. 

 The Mosquito Creek Dam spillway is an example of this type. In 

 this case no formal spillway structure was built. 



Earth Dams 



The two major foundation problems in earth dams are stability and 

 permeability. Settlement of the dam proper is of concern only to 

 the extent that adequate freeboard is maintained. A few consolidation 

 tests, properly correlated with the observed settlement during the early 

 stages of construction, will usually allow the establishment of a satis- 

 factory gross grade. General geology of the site will prove most help- 

 ful in determining the most critical seepage and stability areas. 



Stability of foundation soils is, with few exceptions, the determin- 

 ing factor in the design of the outer slopes of an earth dam. Explora- 

 tions should be planned first to locate and outline the critical regions 

 in the foundations. This can be accomplished adequately by disturbed- 

 drive-sample methods by which satisfactory samples for classifica- 

 tion and moisture determination can be obtained. After the critical 

 areas have been outlined, large borings, 5 inches or more in diameter, 

 should be made to obtain undisturbed samples of the silts and clays 

 for shear and consolidation tests. Consolidated, drained, direct-shear 

 tests are recommended for general use in determining the shearing 

 strength of the materials under the induced loads. Triaxial compres- 

 sion tests are recommended for well-graded coarse-grained soils and 

 for special studies. Consolidation tests are used to estimate the per- 

 centage of consolidation expected during construction. The shearing 

 strength available at the critical period during construction is then 

 determined on the basis of the percentage of the total consolidation 

 that will have occurred. In places where only a small percentage of 

 the total consolidation is expected to occur during construction, un- 

 confined compression tests or unconsolidated undrained (UU) triaxial 

 tests can be employed to determine the shearing strength. In all cases 

 where consolidation is a factor in determining the shearing strength, 

 piezometers should be installed to check the variations in pore pres- 

 sures during construction. Open-ended pipes with a well point or 

 sand pocket at the bottom are satisfactory for most foundations. 

 Where more accurate measurements are required the Bureau of Recla- 

 mation type of hydrostatic pressure gage is recommended. 



Measures for control of underseepage are seldom designed on the 

 basis of adequate information. Natural soils vary widely in character 

 in both vertical and horizontal extent and are often stratified. Exten- 



