Gilpin observer., 1913-07-03
GEOLOGY OF THE
jim PEAK Tunnel

wrote a very interesting article for
the current Issue of the Mining Sci
ence, outlining the work to be done
in the boring of the proposed Mof-
fat tunnel, the formation of rock that
will be encountered, and other in
formation that we'll interest the peo-
pie of this county, through which
half of the tunnel will pass. The ar
ticle follows:
In digging long tunnels, knowledge
trated is Of the utmost importance
i'm estimating the cost of the work
and the time required to complete
the bore.
Gross Understanding of the nee
essary time and money has occurred
i'm several tunnels and in every such
case that has come within my no.
tice, the greater part Of the excess
cost in time and money was due to
encoultering bad ground, which, be-
ing interpreted, means that the ge-
ology of the section had not been
thoroughly investigated beforehand.
made, the bad ground would have
been indicated on the surface and
provision made in advance lin time
money and methods) to handle the
bad ground.
Under modern engineering prac-
tice, a tunnel can be made through
almost any substance that is found
in the earths crust.
Very hard rock is really the least
Objectionable of the many difficult
ties that tunnel builders encounter.
Hard rock means more power, hard
er steel and stronger powder. That
is all. What with vanadium steel,
blasting gelatin and drill sharpeners
run by power, the bugbear Of hard
rock has become a teddy bear; it
has been defeated.
Rock that Is not only hard, but
tough, such as some OF the serpen-
tines and peridotites (iron-a,umina-
magnetism siIcates) are still men,
ace to rapid progress; but luckily,
the builders of the Motfat tunnel will
have no such rocks to penetrate.
Strange as 1t may seem, very soft
rock is the worst obstacle to over
come in tunnel building. Very soft
rock is generally saturated with wat
er and under the enormous pressure
QXfeffnVr f,,n nd u.
ings soft rock will often give way
and run like soft butte. The Moffat
tunnel will attain a maximum depth
of $500 feet Compared to the AL
pine tunnels. this is no depth at all,
but at the same time 1t is depth
enough to cause great difficulty in
penetrating soft strata of water-soak-
ed rock, If any exist.
The ideal rock for tunnel build-
ing is one that drills easily. breaks
long and stands safely without tim
bering or lining.
With moderately soft rock, the
tunnel bore is driven ahead just as
though the rock were expected to re-
main standing indefinitely, and the
ttmbermen to follow along behind the
heading, catching up the ground us
fast as is convenient. With this sys-
tenn a little Of the roof will have fall
en- out before the timber or artificial
arching is in place. but. inasmuch
as broken rock fills more space than
solid rock the voids are soon filled
up and the ground arches over the
lining without much weight really
resting on the latter.
when the ground Is very soft. the
timbers must be carried right up to
the breast or working space. Under
driving spiling' is resorted to. By
this method a cantilever projection
OF the wooden roof or back ttmbers
ds CARRIED forward even beyond where
the ground is broken. AS soon as
enough ground rs broken and remov-
ed, q new set OF timbers ds put in
and spihng again driven ahead. So
with great care and expense, the face
is advanced.
I'm direct proportion as the ground
ds soft and soaked with water, the
expense of tunneang increases. In
moderately soft ground. spihng is
driven only on the roof. In softer
ground sp5ling Is driven on roof and
sides. In still softer ground, the
breast itself will have to be held
back with temporary tagging.
when the time comes to advance
the breast tagging in soft ground,
there Is always moment Of s,is-
pense. have seen 60 feet Of a
tunnel fiI with clayey material 70