Engineering Geology of Straight Creek Tunnel Site, Colo.

The U. S. Geological Survey, as part of a research program on geologic and geophysical methods of predicting geologic conditions at depth for the construction of underground openings, is conducting in cooperation with the Colorado Department of Highways a study of the site of the pilot bore for the Straight Creek Interstate Highway tunnel under the Continental Divide in Colorado. A statistical model of the geology at the depth of the tunnel was developed as a result of the field and laboratory research. From this model, predictions have been made as to probable rock load, spacing of sets, areas to be tested by feeler holes, the amount of grout required, average flow of water as the heading is advanced, and the maximum initial flow of water that might be expected from broken ground within any interval of the tunnel. During construction of the pilot bore, the accuracy of the predictions can be evaluated, which will lead to better methods of predicting geologic conditions and making engineering estimates for other proposed underground openings.