Engineering geology played an important role in the planning of the 44-mile long West Delaware tunnel. Geologic mapping of the Catskill shale and sandstone through which the tunnel is to be drilled showed that the rocks are of sufficiently favorable and uniform character that only 44 drill holes were required for exploring the site of tunnel and shafts. Seven shafts are planned 4.5 to 7 miles apart. Factors in planning the location are: length of tunnel between shafts shall not exceed 7 miles; shafts should be as equally spaced as topography allows and located close to good highways and to flat areas where rock-disposal piles can be made; shafts sites should be in unimproved areas to minimize cost of acquisition and should be in places where bedrock is found within 50 feet of the surface.

Explorations consist of diamond-drill holes in rock and spoon samples in soft ground at intervals of 5 feet. Holes that penetrated to the level of the tunnel were grouted to prevent leakage during construction.