Geologic Techniques in Civil Engineering 1141 



engineers, not only of the local areas of possible sites, but also of the sur- 

 rounding regions, to detect, if possible, any features such as faults and 

 contacts that are obscured at the site but are capable of being projected 

 into it. No detailed mapping is included in stage A exploration. Sufficient 

 time should be allowed that a thorough examination can be made and no 

 essential data overlooked. A reconnaissance geologic report should be 

 as complete as possible and should set forth all that is known of the ge- 

 ology and engineering aspects of prospective sites. In the case of a dam, 

 the feasibility of the reservoir site should also be discussed. Reports 

 should include maps and photographs from other sources when available. 



Stage B: Selection 



When there is any doubt as to the choice of site after the recon- 

 naissance examination, sufl&cient additional work should be done to select 

 the site to be used. Selection may depend upon (a) a comparison of two 

 or more sites in rough cost estimates; (6) a comparison of the details 

 of the geology as to bedrock areas, the type and quality of the rock for 

 foundation material, the areas covered by overburden, the thickness of 

 overburden, and its adaptability for use in construction in areas of 

 required excavation; and (c) a comparison of the availability and quality 

 of the construction materials. 



When additional exploration is necessary to resolve the choice of 

 site, it is desirable that the stage A data be expanded and supplemented 

 by the following additional work: (1) Detailed geologic maps covering 

 the alternative sites should be made. These will allow comparison of the 

 geology of the sites and will serve as a guide to necessary drilling and 

 other testing, if such are needed in the choice of sites. Detailed geologic 

 maps of small areas such as dam sites constitute one of the least expensive 

 steps in the exploration program and one of the most useful in all sub- 

 sequent investigations. If stage B is not necessary, the map can be rele- 

 gated to stage C, confined only to one site after the completion of stage 

 A. (2) Surface profiles or rough topography for comparison of quantities 

 should be submitted. (3) Limited test sampling and laboratory testing 

 should be done as necessary for comparing the suitability, quantity, and 

 costs of construction materials. (4) Limited core drillings, exploration 

 tunnels, and the like are necessary for the comparison of foundation 

 conditions, bedrock profiles, and the layout of appurtenant structures. 

 (5) Geophysical surveys should be conducted, when pertinent, to deter- 

 mine the depth to bedrock and to obtain a rough classification of materials, 

 especially if the site is covered by overburden that conceals all or most 

 of the bedrock. 



The stage B geologic report may be appended to and added as a 

 supplement to the reconnaissance report or as a revision thereof. The 

 data obtained from this exploration stage should be formulated into an 



