Principal matters for investigation at this site are the thickness and extent of the glacial deposits, their composition, variability and permeability. The thickness of the deposits was determined by the combined use of diamond drilling, cable-tool drilling, test pits, seismic refraction surveys, and geological mapping.

The glacial deposits appear to rest in part on weathered granite from which they are often difficult to distinguish.

Where exposed in excavations the moraine consists of boulders and pebbles, chiefly of granite, embedded in a matrix usually of unsorted granitic soil or sandy clay. Some lenses of sands and clay occur within this material. The granite boulders have been considerably weathered since deposition, being often completely decomposed to a depth of about six niches. The central cores of the large boulders consist of fresh granite. Drilling this bouldery material for taking samples and for carrying out permeability tests presented considerable difficulties.

Tunnels:

For the 84 miles of major tunnels it is estimated that about 65 per cent. of the tunnelling will be in granitic rocks. Most of the remainder will be in highly folded sedimentary and metamorphosed sedimentary rocks—shale, slate, phyllite, schist, sandstone, quartzite, tuff and some interbedded lavas. These tunnels range in driven diameter from about 10 to 30 feet, All of the major tunnels are pressure tunnels. In addition there are diversion tunnels at dam sites, and aqueduct tunnels which may be free-flow tunnels.

Concrete lining will be required for sections of some of them where the vertical or lateral cover is deficient in thickness or quality to balance the internal pressure.

Determination of the necessity for steel reinforcement of the concrete lining in such places and treatment of the surrounding rock by grouting requires careful study of the structure and properties of the surrounding rock. Tunnels, usually those of the largest sizes, will be lined regardless of the quality of the rock, when it is estimated that the lined tunnel is cheaper than the larger unlined tunnel of the same water-carrying capacity. For the smaller tunnels which would be cheaper if unlined, judgment of the quality of the rock is the factor deciding whether they can be designed and constructed as unlined or lined tunnels.

Practically all the rock types where unweathered and sound are suitable for the construction of unlined tunnels. The kinds of defects encountered are surface weathering where the cover is shallow—both chemical weathering resulting in softening and weakening of the rock, and mechanical weathering resulting in a general loosening of the rock mass caused by opening-up and chemical weathering along joints; deep-seated hydrothermal alteration of the granite causing it to become soft and weak; crushing and weakening of the rocks along and adjacent to faults; and jointing. All rocks contain numerous joints, and an accurate assessment of the character of the jointing is particularly important (but very difficult to make), since it affects large parts of the rock masses as distinct from the other defects which are often only local in their effects, and it is often the dominant factor in determining whether support during construction and lining are required generally or not. It is particularly important to know whether the joints are tightly closed, or open; cemented or uncemented; smooth or rough; whether they are persistent or die out in short distances; and their orientation with respect to the tunnels, since these factors determine the degree to which the joint blocks will remain interlocked in the excavations.