APPLIED ENGINEERING GEOLOGY CASE HISTORIES FROM VERMONT: FIELD GEOLOGY'S VITAL ROLE IN SOLVING GEOTECHNICAL PROBLEMS

The importance of solid geologic mapping skills at the regional, local and project-area scale is paramount in conveying site condition information for solving geotechnical problems. A strong background in field geology, encompassing surficial, bedrock, geomorphic and hydrogeologic mapping is vital in converting “qualitative” geologic observations to “quantitative” values for geotechnical analysis. These observation skills are first learned in university settings, but are built upon and honed through experience gained by project needs. Three recent engineering geology example projects from Vermont highlight the importance of careful geologic observations used to help solve complex geotechnical problems: 1) Elm Street Rockslide, Montpelier, Vermont; 2) Route 191 Landslide, Newport, Vermont; and 3) New I-91 Bridge over the West River, Brattleboro, Vermont. For these projects, several difficult geotechnical problems could not be solved without gathering field geologic information using several methods, starting with desk-top studies of regional/local surficial and bedrock geology, followed by planning for field studies, and conducting the field studies with the flexibility to modify field programs based on encountered conditions. Collection of the geologic data is only the first step in providing quantitative values of geologic materials; the engineering geologist must always be involved with the geotechnical engineering analysis in selection of appropriate parameters (or more accurately, a range of appropriate parameters) for analytical and numerical models, including sensitivity analyses. While computer analysis is an important, powerful and efficient method to solve geotechnical problems, overreliance can lead to erroneous, and possibly more damaging, results. On any given day, your “gut feel” based on careful, detailed field observations, is more reliable than any computer.