Geologic Techniques in Civil Engineering 1149 



The sediments in some places sometimes possess natural-moisture contents 

 that exceed 50 percent. Yet, after they are oven dried, no more than 

 25 percent of moisture can be added to them before they lose all solidity 

 and assume a semiliquid consistency. How was the initial water retained? 

 The answer, when obtained, will doubtless also explain an interesting 

 phenomenon observed in the field: the natural material will stand in- 

 definitely on vertical cuts if undisturbed and can be observed to have done 

 so in the scarps of high terraces; yet, if dug with a power shovel or other- 

 wise drastically disturbed, it will flow and assume an angle of repose of 

 two or three degrees. This disturbance apparently releases the excess water 

 which, through the action of time and pressure, it has bound to itself, and 

 it becomes a semiliquid slurry. Research continues in the hope of unravel- 

 ing facts fundamental to the explanation of the natural mechanisms of 

 consolidation of sediments and to the factors involved in the stability, in 

 the engineering sense, of sediments in general. 



Questions 



1. What are the responsibilities of the engineering geologist to the 

 engineer? 



2. Why are physical properties of rocks and materials important? 



3. Why must subsurface exploration be conducted at most engineering 

 sites? 



4. What might be the significance of a zone of rock where core recov- 

 ery was very poor? 



5. What methods of subsurface exploration are employed in engineer- 

 ing geologic examinations? 



6. Why are large diameter cores desirable, particularly in unconsoli- 

 dated or poorly consolidated material? 



7. In what way does petrography serve the engineer? 



8. Why is soils mechanics considered an engineering geologic tool? 



9. What is the normal sequence for conducting engineering geologic 

 examinations? 



10. What geologic features of a site should be included on an engineer- 

 ing geologic map? 



