The state herald., 1890-08-01


IN the construction Of water storage
dam there 10 an element OF inseour-
ity 70 be guarded against In some
csses, which does not seem to have
been publicly noticed. John D. En-
ersys In the Mining and Scientific
Pred. referring to the swelling u the
ground under or near to the dam. con-
siders 1t source Of danger.
A valley or wide rayiue with a
slight descent, and having side hills
coming near to each other at Its lower
end, 10 economically favorable for
water impounding purposes, provid' d
that the collecting surfaces above IRE
large enough to insure the supply re-
Quirsd. In the arid regions such a
valley Is usually so dry that, on the
side hills at least. the general water
Lowell Gun only be reached by deep
sinking. II solid primary rock, with
little permcability. is available In
founding the dam, its bulk. when
submerged, will not increase; but II
dependence Is placed on a stratltied
formation containing layers OF clay,
Ialc snd shale, Its expansion when OK
posed to presumed water must cer-
tainly be expected. Every old miner
has had trouble with swelling or
"creeping" ground, and bull ,ers OF
escarpment walls are aware how hard
Il il TO keep some kinds OF rock In
place during wet weather.
Assuming that dam has been built
IN an unstable foundation OF the kind
described, what will the effect be
when pressure OF 90, 20 or 100 feet
,f water comes upon itp Tho whole
country rock" above tho dam will,
In t1v.center Of the raviue especially,
both underneath and outside OF tho
dam building, DO saturated to great
depth. Under tho abutments on tho
cohvcrglngs sido hills tho pres
sure will be less. yet every pore and
lnterstice will be filled. Should there
be the slightest tendency Of this water
charged rock to expand. either later
ally or vertically, 1t is easy to under
stand how even dam In itself we'll
planned and carefully built may in
time give way. owing to such expan
sion.
The sapping aud weakening effects
Of water percolation under high pres
sure may go on for years without be-
Ing noticed. but II the dam erection is
ultimately, though 1t may be imper-
ceptibly, lifted or compressed by the
slow swelling OF the ravine or hillside
formations. so that cracks and veinlets
are formed In or beneath it, increased
pressure may suddenly destroy it.
The wearing or mechanical effects
resulting from sweating process go
ing on In dam. or the rock under
lying IL, is not the only evil which IL
to be feared. The air acting on wet
surfaces promotes chemical changes
which are followed by disintergation
OF the affected rocks, and thus slowly
yet surely there may be destructive
agencies Mt wor.; where least expected.
Should there he veins OF porous
rock dipping under dam from its up
per side, the passage OF water through
such veins may OF itself prove hid
den cause OF disaster The escape
may be small at first, but softcning
and wiueniug work going on for years
cannot fail to weaken Il heavy dam
building not very far above il.
II am right, continues the author,
In assuming from reasons stated above
that the building OF dams on some
kinds OF straiiiied rocks renders them
unsafe, trust by calling attention TO
the subject TO encourage investigation
md the adoption Of adequate engi-
neerlng remedies. 1t would be some
satisfaction TO know whether the
Johnsiown md Walnut Grove dams
Wefc built on strntified rocks. II they
were. arording evidence long before
they collapsed, which they did not
give when first In use. that cracks
had been opened In them, 1t 10 reason
able TO assume that they had been In
jured by the expansion Of the ioundn-
ton and hillside rocks.