Construction geology; As-built conditions
Most of the historic milestones in engineering geology resulted from the construction of important, large-scale projects, as described in Kiersch (this volume, Chapter 1). The construction phase of a project follows final design and precedes operation and maintenance. Careful documentation of geologic conditions exposed in significant excavations and modification of designs to cope with “changed conditions” are routine elements of major construction projects such as dams, tunnels, power plants, and highways. Unless required by a local ordinance, as-built geological mapping on smaller projects, such as industrial developments and residential subdivisions, usually is not requested by a project owner. However, the increasing attention to economic considerations and the contemporary legal atmosphere are contributing to the expanded use of geological documentation during construction on smaller projects. Roberts (1973, p. 145) analyzed more than a thousand project reports and case histories and concluded that “the comparatively small investment required to obtain a foundation ‘as-built’ report provides a very substantial return.” He notes that preparation of foundation “as-built” reports assures: (1) the owner received what was paid for; (2) the design engineer that construction was in accordance with design assumptions, or any modifications necessary were made in a timely and economical manner; and (3) a record is provided to guide any subsequent structural modifications, remedial treatment of failures, or design of nearby new construction.
The initial involvement of geologists on construction projects most likely occurred in response to problems or failures caused by an inability of the engineer’s conservatism or “factor of safety” to compensate for the geologic conditions, which at most sites are unique and cannot necessarily be treated as a random variable with some average value.
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A review of milestones and changes in geological theory and practice from which modern engineering geology in North America has developed. Five chapters discuss historical events and the contributions of early scientists and engineers; nine chapters review the state of knowledge of dominant geologic processes, phenomena, and specialized principles critical to modern practice; and three chapters discuss geologic environs and the properties of construction materials. Four chapters are devoted to geoscience investigations and related techniques for: initial regional-areal evaluation of conceptual candidate sites (Phase I); selection of preferred-designated sites and design (Phase II); typical kinds of investigations used during project construction (Phase III); and as-built documentation and explorations of the operating or rehabilitation phases. Closing chapters focus on the geoscientist's responsibilities relative to engineering failures, errors of judgment that impact works, litigation, and forensic geoscience. The 34 contributors present extensive case histories applicable worldwide.