1. PURPOSE AND APPROACH
1.1 Purpose
Site characterization in rock engineering projects is an evolutionary process beginning typically with air photo analysis and geologic mapping during prefeasibility studies and progressing through exploratory borings, permeability measurements, laboratory testing, and perhaps large scale in situ rock mechanics tests during the feasibility and design phase of the project. The purpose is to develop an operational concept of the geology of a site, to estimate geometries and material parameters for engineering analysis, and to ensure that details of the site geology that might adversely affect facility performance have been identified. Recognizing that all potential geotechnical problems cannot be identified no matter how extensive the exploration program has been, the site characterization process must continue throughout the construction phase. Construction contract documents include estimates of certain quantities because the nature of some geotechnical features is such as to preclude exact definition prior to the beginning of work. Foundation grouting, rock support, amount of reinforcement in tunnel linings are some examples of construction items requiring definition during construction.
It therefore follows that geotechnical engineers or engineering geologists are essential both during the initial process of site characterization as well as throughout the construction of the project. We are all familiar with projects where there have been excessive cost overruns or where failures have occurred because of a lack of continuity of rock engineering from design throughout construction. It is also generally recognized that budget or time restrictions may prevent a thorough site characterization program and, as such, any significant items must be recognized and resolved during construction with the minimal impact on cost and project schedule.
It is apparent then that there are no clearly established guidelines when attempting to define the extent or scope of a program of site characterization. Certainly methods and procedures vary according to the nature of the project, the complexity of the geology, the background of the engineering company, and the experience of the individual geologists or rock mechanics engineers working in the field.
1.2 Approach
The logic of any program of site characterization requires an understanding of the elements of the project being studied, the possible engineering behavior of the rock mass expected to be encountered, and the influence of the geologic variables inherent in any rock mass. Those in charge of a site exploration program and design will have difficulty in selecting the proper tools and interpreting the results without an understanding of the processes of construction in rock.
The critical variables that must be defined in any program of site characterization involve the strength, permeability, and deformation of the rock mass. Therefore, in most cases, field testing is required where engineering judgement indicates that the rock properties may not satisfy standard design criteria or where the material is so variable as to require detailed examination.
Strength. An assessment of rock strength is generally determined for most projects and usually includes laboratory unconfined compression tests. The results provide a comparative basis for assessing the rock material as regards other projects. It is generally concluded that most rocks have adequate unconfined strengths for most engineering projects.
