Alcan Tahtsa Lake-—Kemano Tunnel

This grey granodiorite which is more than 14 miles wide at Siffleur Lake
narrows perceptibly east of the lake and pinches out before reaching Tahtsa
Lake. These intrusions have caused a good deal of shearing and fracturing
of the older rocks, a condition that presented some difficulties in the construc-
tion of the tunnel. The granodiorite itself is not fractured and little difficulty
Was experienced in driving the tunnel through it.

That part of the tunnel examined by the writer at the Tahtsa Lake end
in 1952 was entirely within the greenstone complex. At the portal and for
about 3,000 feet along the tunnel the greenstones are cut by dykes and
cupolas of red granite. This granite is itself cut by narrow dark green to
black basaltic dykes. Much of the granite is sheared and broken and traversed
by numerous faults. Each granite-greenstone contact is marked by a mud
seam or filled with rock gouge. The numerous cross-cutting faults are also
gouge-filled. This broken condition made overbreaking in the tunnel a serious
problem at the Tahtsa Lake end and much timbering and rock support were
required. Guniting of the walls proved satisfactory in some cases in pre-
venting rock spall along the tunnel walls. In some cases the strike of the
faults was the same as that of the tunnel direction. Where this was
encountered timber or steel supports were necessary.

At Horetzky Creek the line of the tunnel ran close to the contact of
the quartz diorite-granodiorite tongue with the layered series, and much
broken ground was encountered in this section. Support was accomplished
by lining the tunnel with cement and guniting the walls.

At the western end the big powerhouse excavations are mainly in
granodiorite and quartz diorite which proved to be a much more massive
rock. A few faults close to the walls caused a weakening of the structure, in
some instances, but broken conditions there were much less severe than in
the layered sequence and greenstone complex.

All the powerhouse excavations and the main tunnel inverts are in the
granodiorite-quartz diorite mass. Of the 53,050 feet of tunnel (R. A. Stuart,
personal communication) 28,025 feet are in the granodiorite-quartz diorite,
7.800 feet are in the younger layered complex and 17,225 feet are in the
greenstone complex. Lining and support in the tunnel were as follows:
(Mathias, 1954, p. 1400) “The main tunnel invert was concrete full length
and concrete lining was placed in the 15,422 feet that required steel supports
while driving, or was unreliable. The equivalent of about 20,000 feet of
tunnel received gunite to protect against the unravelling of the live, but
broken, rock which contained fine seams filled with clay or with decomposed
rock. The remainder of the tunnel was left without lining, but some per-
manent rock bolting was done to tie exposed blocky, but sound rock, deep
into the walls and arch of the tunnel.”

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