Willmar tribune. [volume] (Willmar, Minn.) 1895-1931, 1912-09-11
GEOL0C|ST Of
TELLS ABOUT
They Gave Much Trouble,
But Never Imperiled the
Great Work.


I'm an article contributed to the En-.
gineering Record Donald F. Mac
Il Donald. geOlOgist to the isthmian
canal commission, says that the
great slides which have occurred in the
CuIebra cut have never for moment
caused the engineering staff to doubt
the ultimate success of the great under
taking. He concludes that when the
proper angle. which will of course vary
with the strength of the rock from al
most perpendicular in the case of
strong lavas to one in five in the case
of the much sheared clay rocks. the
slide problem will be practically solved.
For time the great masses of earth
and rock which broke from the steep.
high slopes of Culebra cut and slid into
| the excavation were, to those who re-
lied on the newspaper reports, a serious
menace to the successful completion OF
the canal, says Mr.. Macdonald. AS a
matter of fact. however, the slides,
vast though they were and are, never
really complicated the engineering
problems of the work. never hindered
the yardage output and never threat
true that the slides have added much
to the necessary excavation and there
fore to the total cost. but it is mis-
take to think that they have in the
past or will in the future put in jeop-
ardy the successful completion and op-
GRAFTON of the canal.
The slides are due in large part to
the geological conditions. The oldest
| rocks are exposed along the canal be-
tween Obispo and Empire. Faulting
shearing and weathering have mixed
this already complex mass so that it
is difficult to analyze it. From Empire
southwark younger rocks are found in
formation estimated to be $50 feet
thick. These grade upward into light
gray lenses of sandstone from three
inches to three feet thick, separated
| from each other by thin beds of shale
Both series of beds show remains OF
marine fauna which indicate that they
were originally laid down as sediment
in the bottom of shallow estuaries OF
the sea, and as the same fossil rela
lions are found all across the isthmus
it is evident that the Atlantic and Pa
cific oceans were joined at that time.
Four Types Of Slide.
There are four distinct types OF
slide. each clearly distinguishable from
the others, yet each aiding and abet
ting the others and all working togeth
er to pull down material from the high
slopes into Culebra cut to squeeze ma
terial up in the bottom Of the cut or to
do both.
These are structural breaks and de-
formations resulting in slides, normal
or gravity slides, fault zone slides and
weathering and surface erosion. The
first class is by far the most important
and troublesome Fortunately they
scarcely occur outside of the Culebra
district. The first manifestation OF
these slides is the appearance of cracks
or assures parallel or somewhat
oblique to the trend Of the edge of the
steep slope of Cu]ebra cut They ex
tend from few yards to a hundred
yards or more back from it and from
each other.
They are usually traceable on the
surface of the ground for several hun
dred yards and gradually widen out
to form perpendicular crevasses sew
eral inches wide. Locally one or more
of these cracks may appear. and they
may develop into the second stage in
a few weeks or in year or two, de-
pending on the surrounding conditions
The second stage of this type OF
slide consists of canalward fitting OF
these blocks. usually accompanied by
reformation or bullying up Of the
rocks in the bottom of the canal oppo
site them. There is a downward and a
canalward movement of all the blocks
which reaches Il maximum OF. say. five
feet to ten feet, at the edge of the cut
There Is also fitting forward toward
the cut on it's own axis of each block.
| Takes Year or More to Settle.
These settling, fitting and formative
movements go on for from few
weeks to a year or more before the
third and last stage Of these slides is
reached. The last stage consists in the
dropping downward of one or more
blocks, due to the failure and squash.
ing out of its base. Then the whole
block disintegrates and soon becomes
a normal gravity earth rock slide, pill
Ing up in the bottom of the cut.
This type of slide Is due to the un
stable geological condition of the rock
formations through which the cut
passes, attributable only to nature and
to the oversteepness and height of the
slopes. blasting and other work attrib
utable only to man. Any excavation
removes support from one side of the
column OF earth or rock which forms
the slopes or walls of that excavation
In an excavation through granite the
slopes or walls may be made perpen
dicular for depths Of several thousand
feet without their crushing in. but II
such excavation were carried down 70,
depth of. say. three miles. perpen
dicular walls could not be maintained
even in granite. for the unbalanced
pressure at the foot Of such walls
would exceed the crushing strength OF