1124 Subsurface Geologic Methods 



the stratigraphy at a particular site. This is not necessarily done for any 

 great depth below the site but for the most part on the near-surface 

 formations. 



One of the most difficult tasks that an engineering geologist faces is 

 convincing the engineer of the need for drilling at least one or two deep 

 test holes in a construction area. Naturally the engineer is only concerned 

 with the first few feet into bedrock. Several cases have developed where 

 objectionable material has later been found at relatively minor depths 

 below the original exploration. A knowledge of the general stratigraphy 

 of the area will permit the engineering geologist intelligently to specify 

 the depth to which exploration must be carried to delineate fully the 

 subsurface geologic problems in the area. 



Geomorphology 



A knowledge of physiography and geomorphology is helpful in that 

 they are indicative of particular processes of erosion. Land forms, when 

 identified, may give a clue to the underlying rock or the depth of surficial 

 material. Examples might be pediment surfaces, river terraces, alluvial 

 fans, landslides, or the more common forms of erosion such as may be 

 found in areas where outcrops of sandstone or shale are found. One of 

 the first indications that an engineer uses in selecting the location of a 

 particular site is the topographic relief of the area in which he is inter- 

 ested. Oftentimes this topographic relief is the direct result of some land 

 form that may be explained by geologic processes. Thus, on the initial 

 reconnaissance investigation, geologists may be helpful in selecting a site 

 that does not have some particularly objectionable subsurface character- 

 istic. 



Structural Geology 



The geologic structure, i.e., faults, folds, joints, and dipping strata, 

 has a decided influence on engineering features. A clear picture of the 

 geologic structure will permit the engineer to design his structure with 

 due allowance both for good or bad conditions. 



Faulted areas are usually zones of weakness or potential leakage and 

 are to be avoided if possible. Careful investigation is necessary fully to 

 determine the character of each fault encountered. The age of the fault 

 and the possibility of renewed movement must be determined. The dip 

 and strike of the fault plane with relation to the intended structure have 

 an important bearing on strength. A fault dipping in the same direction 

 as the ground slope provides a potential slide plane, but one dipping in 

 the opposite direction has little detrimental effect on the foundation 

 strength. The leakage problem in faulted areas may be influenced by the 

 following factors: the type of formation through which the fault plane 

 passes, the age of the fault, and the amount of movement. For example, 

 ancient faults in limestone are apt to be recemented and offer little trouble. 



