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The Straight Creek Tunnel, planned as a twin-bore vehicular tunnel, will carry Interstate Highway 70 beneath the Continental Divide about 55 miles west of Denver, Colo. The pilot bore, to be enlarged for the eastbound highway tunnel, is about 8,350 feet long, 13 feet in diameter, and about 11,000 feet above sea level. The final bores will be the highest highway tunnels in the world and the first interstate highway tunnels to cross the Continental Divide. The need for a highway tunnel under the divide has increased in recent years. The Colorado Department of Highways, beginning in 1932, made studies to locate the most feasible tunnel route through the mountains. Independent studies in 1956 and 1960 indicated that the Straight Creek Loveland basin area was the best of several possible tunnel routes. This route is advantageous because it will eliminate a climb of 990 feet, as well as avoid hazardous avalanche areas, and it will shorten by 10 miles the highway distance between Loveland basin and Dillon. The Colorado Department of Highways estimates savings of $7 million (1964) annually to motorists using the tunnel. In June 1962 the U.S. Geological Survey with the cooperation of the Colorado Department of Highways began a detailed geological and geophysical study of the Straight Creek Loveland basin area. Emphasis was placed on the definition of underground conditions for a pilot bore whose location was predetermined. An area of about 6 square miles was mapped geologically at a scale of 1:12,000. Geologic and geophysical logs were made of two diamond-drill holes near the proposed tunnel line, and a seismic survey was made at the surface along the tunnel line. Physical properties tests supplemented the fieldwork. The investigations were concerned mainly with predicting the kinds of conditions, as well as their extent and approximate location, that could be expected at tunnel level. The tunnel is in the Loveland Pass Berthoud Pass shear zone, which is about 2 miles wide, trends north-northeast, and is composed of Precambrian granitic and metasedimentary rocks that are moderately to intensely sheared. The granitic rocks are medium to finegrained and consist of about equal parts of quartz, potassium feldspar, and plagioclase feldspar, and 5 15 percent biotite. The metasedimentary rocks consist of various biotite gneisses. Where the bedrock is not faulted or sheared, its strength depends upon the amount and orientation of biotite and chlorite. In shear zones the granite is more competent than the gneisses. Faults and shear zones are the most important structural features from the standpoint of tunnel driving. By using field and laboratory data, it was possible to construct a statistical model of the geology at pilot-bore level. This model was used to predict rock loads, spacing of sets, amount of lagging and blocking, sections of the tunnel requiring feeler holes, rates of water flows, and amount of grout necessary to seal badly broken ground saturated with water. The Colorado Department of Highways made this information available to prospective bidders. A contract for $1,300,000 was awarded Mid-Valley, Inc., for construction of the pilot bore, and they started construction in October 1963. The pilot bore was holed through in December 1964 at a cost of about $1,400,000. During construction of the pilot bore, geologic, geophysical, hydrologic, and rock-mechanics instrumentation programs were conducted. The purpose of these studies was to determine the relation between, and to evaluate methods of measuring the relation between, geologic conditions and the engineering practices in the design and construction stages of a tunnel. The walls of the pilot bore were mapped geologically at 1 inch to 50 feet. The face, as it was advanced, was mapped once during each shift (8 hours) at a scale of 1 inch to 2 feet. In support of the rock-mechanics instrumentation and geophysical programs, geologic maps were made of one wall or the other at 1 inch to 5 feet for a 50-foot distance on each side of an instrument or geophysical station. Samples were systematically collected during the geologic mapping. Hand samples were collected for petrographic analyses; altered wallrock and fault-gouge samples were collected for determination of swelling pressure and clay mineralogy; chip samples were collected over a 5-foot distance on each side of instrument stations for chipsize and mineral analyses; and blocks of wallrocks about 1 foot in largest dimension were collected and cored in the laboratory for determination, by dynamic and static tests, of elastic properties. As the construction of the pilot bore progressed and the results of the rock-mechanics investigations became available, it became apparent that the distribution and amount of wooden blocking between the steel sets and the wallrock directly influenced the loads supported by the sets. As a result, the wooden blocking around each instrumented set was mapped at 1 inch to 2 feet. Geophysical investigations during construction of the pilot bore consisted of measurements of wallrock temperature, seismic refraction, and electrical resistivity. The wallrock temperatures, which were measured in water-filled drill holes about every 500 feet along the pilot bore, ranged from 4.2°C (39.6°F) to 10.5°C (50.9°F). Seismic-refraction and electrical-resistivity measurements along the walls of the pilot bore indicated that both a low-velocity layer and a highresistivity layer exist in the disturbed rock surrounding the excavation. Seismic measurements were analyzed to obtain the thickness and seismic velocity of rock in the low-velocity layer, the velocity of rock behind this layer, and the amplitude of seismic energy received at the detectors. Electrical-resistivity measurements were analyzed to obtain the thickness and electrical resistivity of the high-resistivity layer and the resistivity of rock behind the layer. The electrical resistivity and the seismic velocity correlated well with the following parameters, all of which are important in tunnel construction: Height of tension arch, stable vertical rock load, rock quality, rate of construction and cost per foot, percentage lagging and blocking, set spacing, and type of steel support required. Results indicated the possibility of predicting, from geophysical measurements, economic and engineering aspects of tunnel construction. Further investigation in other tunnels would determine whether similar correlations exist in different geologic environments. The accuracy of predictions based on surface geophysical measurements was tested by making seismic and resistivity surveys on the surface and in holes drilled from the surface along the line of the pilot bore. Results indicated that reasonably accurate predic-

Engineering geologic, geophysical, hydrologic, and rock-mechanics investigations of the Straight Creek Tunnel site and pilot bore, Colorado