INTRODUCTION
In several national and international engineering conferences since 1970, it has been eloquently stressed that there is an urgent need for more accurate subsurface information particularly for the design and construction of underground facilities. It has been hypothesized that reliable subsurface information is necessary for improved, more efficient designs and for less expensive construction. This hypothesis appears especially true for the construction of rock tunnels.
One should realize that rock is usually heterogeneous and that the behavior of the rock mass is independent of the physical properties of the solid rock material that constitutes the mass. The location of faults, the nature of discontinuities, their spacing, their strike and dip, the properties and thickness of gouge material and, in general, all the characteristics of the discontinuities affect the rock mass behavior.
The rock mass behavior is the most important factor in the design and construction of a tunnel. Subsurface exploration methods that can reliably predict the rock mass behavior and can identify the type of material that will be encountered during tunneling would prove to be indispensable aids in the design and construction of tunnels.
SITE CHARACTERIZATION
Site Characterization is the classification of the conditions at a site, both on the surface and below, as they relate to and influence the unique requirements of a particular construction project. Site characterization should indicate difficult material and ground behavior conditions that are likely to affect the design and construction of an engineering project.
What happens if one improperly characterizes a site for a specific project? Possibly an engineering failure and even a disaster may result from an improper site characterization. Let us examine two case histories: the Yacambu-Quibor Irrigation Tunnel (Engineering News Record, 1981) in the Andean highlands of Venezuela and the Stillwater Tunnel in Northern Utah (Civil Engineering, 1981).
In Venezuela, the construction of the irrigation tunnel began in 1972 with a project cost of forty-one million dollars. To date the two international contracting groups who originally started the work have given up the job and allegedly have suffered financial losses. The tunnel is now expected to cost one hundred sixty-four million dollars and some experts express doubts that it will ever be completed.
The main reason for the problem is attributed to encountering unanticipated material and unanticipated rock behavior. The inundation of the tunnel by mud flows was not anticipated. Also, the three fault zones through which the tunnel has passed were not expected.
Problems at the Stillwater Tunnel were caused by unexpected rock movement of 178mm (7") horizontally inward and 250mm (10") vertically which resulted in uneven pressures on the lining segments, severely damaging them.
Post analysis of the Stillwater Tunnel indicates that subsurface exploration was never performed for the 10,200m (33,600ft) section where problems occurred.
Both the previous examples have one common factor: unexpected site conditions caused significant cost overruns and resulted in engineering problems. How could this have happened? Was it because of inadequate subsurface exploration techniques? Do we have the capability, the knowledge, and tools to accurately determine subsurface condition?
