GEOLOGICAL STRUCTURE OF THE SITE

As already mentioned, the area in the vicinity of the power station is intersected by a series of overthrust faults. These trend northeast and generally dip 30°–45° SE.

The closest large fault to the power station intersects the headrace surge tank, about 1000 feet in elevation above the machine hall. The dip ranges from 30° to 45°, and the thickness of the fault zone ranges between 50 and 90 feet. It consists mainly of very closely jointed granite but contains several seams composed of crushed rock and clay 1 - 5 feet thick. A narrow basalt dike intersected by this fault shows overthrust displacement of a maximum distance of 15 feet within the fault zone, but almost no total displacement of one side of the zone with respect to the other.

Another wide fault zone was intersected by the tailrace tunnel, 4000 feet downstream of the station. This fault strikes N. 60° E. and dips 20°–35° SE. About 300 feet of gneiss is very closely jointed and softened and contains many thin crushed zones and clay seams, several of which are up to 1 foot in width.

The block of rock in which the power station is located is thus between the surge-tank fault above and the tailrace fault below, and although the site is free from major faults, the rock mass shows much evidence of strain in the form of slickensides along very many joints and by the presence of minor faults related to these major faults.

Two small but persistent minor faults intersect the machine hall. One is over the full length of the roof (Fig.4, A). It strikes N. 40° E. to N. 60° E. and dips approximately 35° SE.

In the granite it is seen as one or a group of several persistent fracture planes with ½ to 1 inch of crushed granite containing a little clay along the planes, or as a zone of close jointing. In the gneiss it is represented by a zone of close jointing 5–10 feet wide, with joints spaced 2–6 inches apart. These joints are usually smooth, coated with chlorite but not clay, slickensided, and lightly closed.

The second small fault has a strike of N. 30° E. and dips 50°–70° W. It cuts across the tailrace surge chamber, draft tubes, and the western end of the machine hall (Fig. 4 B). In the granite it usually consists of one or two fracture planes with ¼ of 1 inch of clay and crushed granite along the planes. As the contact with the gneiss is approached it becomes less distinct and splits into several parallel clay-coated joints, many without crushed rock, and continues in the gneiss as a group of clay-coated joints.

No displacement can be seen along fault A or B because of the uniformity of the rock and consequent lack of marker beds. They are assumed to be faults from the character of their surfaces.

In addition to these minor faults, there is a very persistent zone of close fracturing a few inches in width, which occurs in the lower part of the power-station walls, and in the walls of the transformer hall (Fig, 4, D). This zone has been found only in the granite. It is remarkably undulating but overall has a roughly north-south strike and a gentle dip to the east. The granite is irregularly and closely fractured and is characteristically very heavily stained reddish brown with limonite. The main fracture commonly has a coating of ½ to 1 inch of calcite and quartz and is gaping open as much as ½ inch. Some fracture planes have a little crushed rock and clay on them. Ground-water flows and seepages sometimes occurred in this zone; in contrast, the faults A and B were quite dry.