Evening star. [volume] (Washington, D.C.) 1854-1972, 1931-07-12

Continued From First Page)
the same radius; and It Is designed
on the same general theory that some
concurrent arch action will take place
and thus overcome the limitations Of
the section-a theory widely disputed
by engineers"
Calls Dam too Light for Rock.
The statement. deficient in gravity
Section when uplift is considered" means
the dams weight Is insufficient to keep
IL from sliding with the weight of water
pushing 1t from behind. The "uplift"
Is the upward push of water seeping
unoer the dam, push of tremendous
power when the water Is deep behind
1ne dam. This uplift is at the rate Of
OF ~ pounds square foot of depth. It
Is equivalent to a large reduction in
weight of the DAM
The dams weight Mr.. Gerry gives
as 100000 tons, and the push of
water behind 1t about $500 900 tons.
He goes on to explain why this ap-
paren1y safe weight may not hold the
The natural rock at the SITE he
says, rs said by the Reclamation
Service to have good Notwithstanding
ability. 1t consists of tufts and flows
OF volcanic breccia thard stone having
the appearance Of being filled with
PeooleS) and is of a character usually
regardf> by engineers as irregular and
somewhat uncertain until demonstrated
70 De otherwise by full exposure.
Granting that it will be found to have
simple strength in compression
throughout, this rock is nevertheless
by It's nature permeable to water, and
1t has a low coefflcient of friction"
Fears Dam Ends May Slide.
A low coefhcient of friction" means
tnat the rock Is slippery, that there is
no great amount of friction to keep
the dam from sliding upon it.
Prudence, says Mr.. Gerry, suggests
very conservative structure for such
foundations. The dam might be pro-
foundly inclined to slide, he adds,
either If water flows over its crest, in
earthquake shocks, formation in time
OF colloidal sludge exerting greater
pressure than water. vibrations due to
tunnel discharge and local head
pressure zones developing in the rock
forming the canyon walls.
Figuring the effect of the sliding
factor" Mr.. Gerry says: The end Sec
ton would yield first, as result of
the large loading transmitted to them
by torsion from other parts lower down
and nearer the center of the struc-
ture. The st. Francis Dam yielded in
this way and the Hoover Dam undoubt-
edly would fail initially at the ends,
In similar manner
Strength of Type Unknown.
Obviously, he says, secondary support
against sliding Is intended to be derived
from the arch action of the dam. This
refers to the shape Of the dam, which
is curved inward against the wall Of
water. Concerning this intent he saws:
"There never has been a rational
solution of the problem of intermediate
support for arched gravity dams: nor is
there likely soon to be one, for the
primary reason that the complex dis
tribution of stresses through SO great
a mass is controlled in major extent
by the physical attributes of the con-
crete and rock. the very elements of
which are yet unknown.
AT the present time, only unsup-
ported opinion contends that arch
action may be relied upon with GER
tainty to relieve any considerable def
cheney IN dam Of gravity section.
Declares Remedy Necessary.
THE danger lies not within the
structure Itself, but in the extent and
manner OF support to be derived from
the foundation rocks. 1t Is there that
improvement is essential If adequate
safety is to be obtained.
This situation is avoidable, because
a gravity section can be so designed AZ
to provide, with reasonable certainty,
fixed safe limit to the risk. The addi-
tional cost to secure high degree Of
ssfety may be perhaps as much as
100000. not too much to pay for
security in this case"
NGEAn HITS NAM w ARNVNC