Geologic Techniques in Civil Engineering 1135 



The permeability of a foundation of an engineering structure has 

 several major implications. The first is that of structural stability. If the 

 flow of water through the foundation actually dissolves the foundation 

 rock, causes removal of the finer rock particles, or causes uplift pressures 

 to be built up, structural failure may result. Another major considera- 

 tion is one of economies; that is, how much water can be lost at a certain 

 structure before it becomes economically unfeasible. 



In the drilling program for most foundations a requirement is made 

 that all holes be water-tested at various suitable pressures at successive 

 depths in the drill hole. Careful measurements are made of the water 

 loss, the length of time of the test, and the pressure used so that calcula- 

 tions may ultimately be made as to the permeability of the material tested. 

 It can be seen that joints, cracks, or other structural weaknseses in a rock 

 formation may greatly influence the permeability as measured from pres- 

 sure tests in bore holes. These weaknesses are usually sealed off^ or filled 

 during construction operations by high-pressure grouting. 



Geophysics 



In the investigation of a dam site and in many other problems of 

 engineering geology, certain types of information can often be obtained 

 by geophysical methods at less cost and more quickly than by drilling or 

 other means; for example, such data as the depth to firm bedrock or the 

 extent of a buried gravel deposit. Geophysical exxploration is a blend of 

 physics and geology, for it involves measurements that are interpretable 

 in terms of local subsurface geology, which are made with suitable in- 

 struments at the ground surface. Although the methods and equipment 

 used in the present-day geophysical explorations are extremely accurate, 

 the interpretation of the data obtained by these methods may be compli- 

 cated by the diversity of subsurface conditions that are usually encoun- 

 tered. However, geophysical methods are rapid and economical and, 

 under proper conditions of control, can be used to great advantage in 

 locating rock profiles quickly and inexpensively during preliminary in- 

 vestigations and in establishing the continuity of strata between borings 

 during more detailed explorations. 



Reconnaissance of a site should be made by experienced personnel to 

 determine the geophysical procedure best adapted to the local conditions 

 and the specific problems to be investigated. Since identical subsurface 

 conditions are seldom encountered over wide areas, and since each site pre- 

 sents special problems, a stereotyped program cannot be applied. It is very 

 important that the geologic and engineering problems involved at a given 

 site be thoroughly understood by those undertaking the geophysical survey. 

 Most geophysical explorations in connection with engineering geologic 

 problems pertain to the near-surface material, which normally is cor- 

 rected for or eliminated from standard geophysical explorations as con- 

 ducted in the petroleum industry. For this reason, as the areas under 



