The Princeton union. [volume] (Princeton, Minn.) 1876-1976, 1912-11-07

The weaker the rock the less deep
need be the excavation and less steep
the walls in order to cause crushing
and reformation, and the critical depth
and steepness for the rocks involved
have locally been exceeded in the cut
near Culebra. In the first estimate of
excavation for Culebra cut this weak
rock factor was not sufficiently con-
sidered. The earth vibrations set up
by deep. heavy blasting near slopes al
ready under great strain have had a
considerable tendency to develop slides.
Only One Known Remedy.
For this type of slide there is only
one remedy and that is now being ap-
plied-namely, to make the slopes less
steep by removing material from their
upper portions so that the unbalanced
pressure toward the floor of the steep.
high cut banks shall be less than the
crushing strength of the rocks involv-
ed. To do this several steam shovels
have been put up on top of the banks
on each side of the cut, and these are
now terracing the upper part of the
slopes. making them less steep and re-
lieving somewhat their strained condi-
ton.
Also each block as it brushes down
generally leaves behind it not grad
ual slope. but the nearly perpendicular
face of another block. so that when
the first slide is shoveled out the ten.
dency for the next block to crush
down is not removed and in most
cases is but little lessened. Another
factor which enters into the cause of
these large slides and determinations is
| time. Some of these movements run
their course in few weeks or months.
but others have shown cracks for
many months or even years and have
not yet slid.
These cracks and assures are sources
| of weakness which sooner or later. es-
pecially after the cut shall have been
deepened, will give trouble unless
remedied by reduction of Slopes
These slides also involve the additional
expense and trouble of keeping the
| cut drainage open and of shifting and
| adjusting the railroad tracks in the
| cut so that the dirt trains may not be
interrupted.
1t is fortunate that of the highest
and steepest parts of Culebra cut
namely, Gold hill and Contractors
hill-will not deform and crush into
the cut. This is due to the fact that
their foundations are relatively strong
rocks Of volcanic origin. which extend
down more than 1000 feet into the
earth. Locally, especially in the upper
| portions. these masses in their forma-
| tive stage had mushroomed out some
what Rock. loosened by jointing. by
Weathering and by former heavy
blasting or by all three causes. will
from time to time drop from these
| steep places. even to the extent of
what might be called good sized rock
slides. but the hill masses themselves
will stand immovable.
Another Large Erosion Remedy.
Another large erosion problem is
that which will result along the wa
ters edge from the wash of steamers
going through the canal. The wash
from the many steamers that will pass
through Culebra cut will certainly be
sufficient to very considerably erode
the softer rocks along the waters edge
unless they be protected.
The turning Of the water into the
canal will remedy the slides only in so
Tar as it will provide cheaper methods
OF excavation and removal of them by
dredging. The counterbalancing ef-
feet of the water against the slopes of
the canal will be an almost negligible
quantity. so far as the slides are con-
cerned, for the following reasons: The
maximum depth of water in Culebra
cut will be forty-five feet, and the
height of the slopes where sliding
might occur is from Is to 300 feet.
The sliding material has an average
specific gravity of almost two and a
half; hence forty-6ve feet of water
would balance only about eighteen feet
of slide if the pressure were evenly
distributed.
Considering the more or less wedge
shaped fronts and the back pressures
of many Of the slides, it is estimated
that the forty feet of water would
be equivalent to only about ten feet of
slide. Other considerations are that,
though the water will protect the low
er part of the canal from oxidation
and weathering, thus removing a small
source of weakness. it will cause wa
ter to permeate the cracks and inter
stices at the foot of the slope and thus
be an added source of weakness.
though not a serious one. to the slides.
On the whole, the water in the canal
will slightly increase the tendency to
slide, but this will be more than offset
by the cheaper methods Of excavation
and transportation which can then be
used.
With the slopes reduced to the prop
er angle the slide problem will be soly-
ed. With slope pressures thus finally
adjusted and protective mantle of
vegetation minimizing erosion on the
banks. Mr.. Mcdonald saws thpre is M