The validity of geologic projection, a successful example: the Straight Creek Tunnel pilot bore, Colorado

Projection of details of surface geology to depth prior to construction has met with only limited success in many tunneling operations. in the Straight Creek Tunnel pilot bore good results were obtained by making predictions of the extent and kinds but not the exact locations of conditions that could be expected at tunnel level based on a statistical study of surface features.

Successful predictions were made regarding percentages of rock types, linear feet of faulted and sheared rocks, and attitudes of foliation and fractures, including faults and joints. Predicted rock loads and final swell pressures in gouge and altered rocks agreed well with actual measurements. Ground water flows were encountered in expected amounts, but criteria for estimations proved to be unsound.

Estimates were made of the amount of temporary support, footage of exploratory feeler holes, and amounts of grouting and provided a sound basis for estimating total tunneling coats.