The Strategy

Abstract

The objective of the site characterization process is to develop an understanding of the geologic conditions (both regional and site-specifi c), which is suffi ciently complete and accurate so that a conceptual geologic model of the subsurface can be created with reasonable confi dence level for the intended project needs. The diffi culty is one of solving the three-dimensional geologic/hydrologic puzzle with many variables and unknowns. The success of a site characterization effort lies in the ability to locate, describe and quantify the natural geological, and hydrological heterogeneity at a site. Heterogeneity is a natural result of the processes that create and modify the geologic setting, and are sometimes further modifi ed by man's activities. The presence of karst typically complicates the process by introducing a higher level of heterogeneity in the three-dimensional geologic puzzle. It can often be the fatal fl aw for a project.Site characterization must focus upon obtaining a thorough and complete understanding of geologic conditions that will impact the site. While uncertainties, assumptions and opinions are a part of any site characterization (Fig. 12.1a ), they should be minimized. Interpretations must be supported in a logical and obvious way by suffi cient data, which have been tested and proven to be correct. A solid base of data enables us to carry out subsequent efforts such as modeling, risk assessment, construction, or remediation with greater confidence and accuracy while minimizing uncertainties, assumptions and opinions ( Fig. 12.1b ) (Benson et al. 1996 ;Yuhr et al. 1996 ). The Detection DilemmaOne of the biggest problems with site characterization efforts has been our ability to effectively locate and map subsurface geologic conditions especially localized anomalous conditions. This is particularly true in fractured rock and karst conditions due to their limited spatial extent, which represent less than 1-5 % of the subsurface area. As a result traditional approaches to investigation, using borings, have a very low probability of intersecting these features. Beginning in the 1950s the development of geophysical instrumentation and its application to characterization of shallow subsurface conditions began. Since then digital technology has resulted in a greatly expended group of geophysical tools for our site characterization tool box (Olhoeft 1994 ). This has provided us with three basic approaches that can be used to improve our ability to detect and evaluate subsurface conditions and geologic variability. They are:In most site characterization efforts all three approaches will be employed to effectively converge upon a complete and accurate conceptual model of site conditions. Direct DetectionDirect detection of geologic conditions can be accomplished in a number of ways such as by observations at outcrops, conventional drilling and sampling, or trenching. This provides a very high level of confi dence in the detailed data from a local area or the area immediately within the bo...