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face consisted also of | by 12-in. boards nailed to 2 by 6-in. studs,
which in turn were nailed to circular boards at regular intervals. This
outside form was held away from the inside form' (down-stream side)
by wooden distance pieces of the proper length, which pieces were
removed just before the concrete reached them.

A 1 : 2 plaster coat of cement mortar i in. thick at the crest, and
increasing to | in. thick 80 ft. below, was put on the up-stream face
with a cement gun.

At the south abutment the two last arches are provided with spill-
way openings. These spillway openings can be closed with loose
flash-boards. This it is proposed to do toward the end of the wet
season, so as to fill the reservoir to within, say 1 ft. of the crest of
the dam, thereby gaining 2 ft. of water over an area of nearly 300
acres. The spillway is shown in detail on Fig. 10.

Cost.
The Gem Lake Dam contains 8 537 cu. yd. of concrete and 82 tons
of reinforcing steel. The contract price was $22 per cu. yd., including
cement, forms, plastering the up-stream face, and all tools and mate-
rials except the reinforcing steel, which was paid for as an extra
at the rate of $110 per ton in place. The excavation, of which there
was only a limited quantity, was also paid for as an extra. The high
cost is explained by the fact that freight rates were high, and, further,
that the distance from the railroad to the power-house site at Silver
Lake, at the foot of the steep mountains, was nearly 60 miles over
desert roads with heavy grades. From the power-house site, a tram-
way, approximately 4 500 ft. long, took all supplies up the mountain
side to Agnew Lake, 1 250 ft. above. At the outlet of this lake, the



MULTIPLE-ARCH DAMS 881

Agnew Dam, a smaller structure, 30 ft. high and 280 ft. long, similar
in design to the Gem Lake Dam, but having only one strut, was
built at the same time and at the same unit prices, in order to increase
the capacity of the natural lake. All material for use in the construc-
tion of the Gem Lake Dam was brought across this lake on a barge,
for a distance of about 2 000 ft., to the foot of another tramway
terminating at the site of the Gem Lake Dam, 550 ft. higher in
elevation. The cost of these tramways was not included in the price
per cubic yard for concrete, but was paid for as an extra. These tram-
ways were also necessary for the construction of the pressure pipes
to the power-house, one from Gem Lake, and one from Agnew Lake,
and would have had to be built independent of the dams.

For the long haul across the desert from the Southern Pacific
Railroad station at Benton, Cal., to the power-house at Silver Lake,
six 75-h.p. C. L. Best tracklayers, burning distillate, were used, each
hauling three trailers. The net load was as close to 20 tons as prac-
ticable, and the time necessary to make one round trip was about 6
days of 12 hours, including loading, unloading, and ordinary delays.
The speed per hour of these tracklayers was 2i miles on high gear,
and li miles on low gear. The contractors found that the cost to
them for hauling this distance was at the rate of $13.50 per ton. The
hauling of the cement and materials for the dams was not paid for
extra, but included in the price for concrete in place.

A rock fill dam on the same site would have had to be built for
$2.15 per cu. yd. (construction cost), including the water-tight face
and hand-laid rock, in order to be on equal terms, as to cost, with
the multiple-arch dam built. In the writer's opinion, this would not
have been possible in this place. In any locality where cement can
be laid down at less cost than in this case, the relative cost of a
multiple-arch dam and a rock fill dam will be still more in favor of
the multiple-arch dam.

The writer furnished the designs and also supervised the con-
struction of the dams.

Mr. C. O. Poole was Chief Engineer for the whole development,
E. J. Waugh, Assoc. M. Am. Soc. C. E., was Resident Engineer,
L. B. Curtis, M. Am. Soc. C. E., Field Engineer, and Mr. F. O.
Dolson, Superintendent of Construction.

Messrs. Duncanson Harrelson Company, of San Francisco, were
the contractors for both dams.



883 DISCUSSION ON MULTIPLE-ARCH DAMS

Disoussioisr



Mr. F. O. Blackwell,* M. Am. Soc. C. E. — This paper is a valuable

Biackweii. (.Qj^tribution to the theory of multiple-arch dam design, particularly
as to the economical spacing of the piers and the thickness of the
arch. The dam sites are on small drainage areas, high in the moun-
tains, where labor and materials are very expensive. It has been statedf
that the cement for these dams cost $7.50 per bbl.

A very conservative design might have been prohibitive in cost,
and probably the region is uninhabited and no damage to life or
property would result in case of failure.

The sections appear to be very thin for a 40-ft. span, and the rein-
forcement is extremely light. The use of more concrete would have
added much to the ability of the structure to resist unexpected floods,
ice thrust, changes of temperature, etc.

It is always desirable, too, to have a good margin over theoretical
quantities in order to allow for poor material and workmanship,
which will occasionally be found, no matter how careful the engineeer
may be.

A fair comparison between this dam and other types is difficult
to make, as the standards are quite different, but the multiple-arch
appears to be cheaper than any other kind under the conditions which
existed at the location, except possibly a rock fill with a thin vertical
core-wall. The author states that there was loose rock but no earth
at the sites.

The usual type of solid, retaining-section, gravity dam would have
required about four times as much concrete as was used in the dam
as built, but from 25 to 40% of cheap rock might have been laid
in thick concrete sections, and concrete containing less cement might
have been used. The form work and cost of forms and placing would
also be less in a large mass.

Including engineering and overhead expenses, the multiple-arch
dam probably cost about $30 per cu. yd., so that a gravity dam would
have to be built for $7.50 per cu. yd. in order to equal it in cost.

In an accessible location, and with concrete rubble masonry at from
$3 to $4 per cu. yd., the solid dam, in the speaker's experience, has
always figured out to be cheaper than any multiple-arch or reinforced
design.

Compared with a reinforced concrete slab dam, the multiple-arch
seems to have the advantage in cost. The piers would have to be twice
as many, the deck thicker, and the reinforcing metal in the deck
about ten times as great. ,. ., ,

* New York City.

t In an article in Engineering News.



DISCUSSION ON MULTIPLE-ARCH DAMS 883

One of the principal objections to a multiple-arch is the fact that Mr.
the sections are all dependent on each other for stability. If one ^'^^'^"®"-
pier should settle, the arches on each side of it would break, and, as
there is nothing to take the thrust of the remaining arches, they would
push over their piers, and the whole dam would collapse. The struts
between the piers could not be relied on to save the dam from com-
plete failure, as the movement of any pier would throw them out of
line and cause a resultant thrust, tending to push the pier in the direc-
tion in which it had already started to move. Rigid foundations, such
as the author states exist in this case, are essential.

The Great Western Power Company started to build a multiple-arch
dam at Great Meadows, on the Feather River, California, but aban-
doned it in favor of a rock fill with an earth face sluiced into place
on the up-stream side to make it water-tight and cover the bottom
for a considerable distance up stream. The lava foundations, on being
opened up, were found to contain mud seams, which might have moved
under the piers. As a general principle, a dam should be no better
than its foundations. On a yielding bottom a dam of a flexible type
is required.

There is one important matter that Mr. Jorgensen does not mention :
At an altitude of 9 000 ft. in the Sierras it must be very cold at times,
and thick ice must form in the reservoir. The thin arch is not much
protection against cold, ice must form to a considerable thickness
on the face of the dam, and such cracks as develop will be filled with
ice. There may also be ice thrust against the dam, which could have
serious results. With a mass of ice frozen fast to the arches, the inclina-
tion of the face would not relieve this pressure. Floating masses of
ice may also drift against the arches, thus causing concentrated pres-
sures on parts of the dam, and the arches are not designed to stand
unbalanced pressures. One advantage of the ordinary form of masonry
dam is the weight of its large mass, which will stand considerable
impact without damage.

The spillway does not look very large, and would be likely to be
choked with floating ice, in which case the water would go over the
top of the arches. If this occurred, and the water carried ice with it,
the ice would fall on the struts between the piers and might damage
them. A deck on top of the dam would carry the water so that it
would fall below the dam and not injure the struts and the foundations.
It would also strengthen the top of the arch against ice thrust and
would tie the piers together better.

A pressure of nearly 23 tons per sq. ft. seems to be extreme for a
practically unreinforced and slender pier 90 ft. high.

The coefficient of friction against sliding on the foundations is
0.80 as compared with 0.65 in most solid dam designs. If the rock
were smooth or horizontally stratified this might be unsafe.



884 DISCUSSION ON MULTIPLE-ARCH DAMS

Mr. It would be very interesting to hear from the author as to whether

Biackwe . j.]^gj.g ^las been any leakage through the arches, or whether any trouble
has been experienced from ice.
Mr. A. D. Flinn,* M. Am. Soc. C. E. — Minimizing the quantity of

'°"' masonry to be used in a dam naturally occurs to any one who has
given much thought to the design of large masonry dams. The idea
of accomplishing this by arches and buttresses seems to have occurred
to engineers in diiferent parts of the world many years ago, for such
dams have been built in India and Australia, as well as in America.
They have sometimes been called buttressed dams. There appear
to be three important varieties : One, of which the Meer Alum Dam
in India is an example, has the arches vertical, with the buttresses
sloping on their down-stream sides or faces; a second variety is sim-
ilar, but has the spandrel spaces filled with masonry; and the third
variety, to which the dams described in the paper belong, has the
arches inclined, with the down-stream faces of the buttresses vertical,
or nearly so.

Mr. Blackwell has referred to the disadvantages of thin sections
in such structures. The speaker would like to emphasize that point,
on the basis of some experiences with concrete within the last few
years. Possibly, some other engineers have also had experiences show-
ing that Portland cement concrete under certain conditions disin-
tegrates when exposed to weather, and particularly when exposed to
weather and water together. The cause for such disintegration is
not yet fully known, but it seems to be due in part to abuse of the
concrete (or rather of the cement in it) by the use of excessive quan-
tities of water in mixing, and leaving it in the concrete when it
is finally placed in the forms. The speaker knows of one or two
cases in which disintegration, apparently due to this cause, has
gone on rapidly. Therefore, it is quite evident that a structure
like a dam, with walls as thin as 12 to 24 in., would not long resist
such an attack, if it should set in.

Another question — one of economy — has occurred to the speaker.
In the East, dam sites are usually overlaid with glacial drift or other
earthy deposit to considerable depths. Earely does one find the nearly
exposed rock foundations with which the West is favored in many
places. It would seem that digging through a considerable depth of
earth for the arches and the buttresses would be a handicap to the
multiple-arch dam, in comparison with the "solid" masonry dam.

Another thought which occurs to one in looking at dams of this
design, is along a line on which American engineers have not directed
very much attention until within the past 2 or 3 years, and that is,
the vulnerability of structures to attack either by malicious persons
or by an enemy. It seems to the speaker th at a "hollow" dam might

• New York City.



DISCUSSION" ON MULTIPLE-ARCH DAMS 885

be easily wrecked, if not guarded, in time of war, strike, or riot, and Mr.
that a person of malicious intent, in the case of any such trouble,
could readily do a structure of this kind great injury in a few
hours, or even in a briefer time. One of the important considerations
in providing for the guarding of a structure is to have it so arranged
or so strong that it can be easily protected, and by the minimum
number of troops.

In comparing the three forms of multiple-arch dams which have
been mentioned, the question naturally arises: What are the relative
advantages between the arch set vertically with the buttresses sloping
on the down-stream side, and the type of dam described in the paper,
with the arches sloping and the buttresses having vertical faces on
the down-stream side? Is there a distinct advantage in either case,
or is the difference solely due to an endeavor to secure patent rights,
or some other commercial advantage? If any variety of multiple-arch
dam has advantages over the others, will not the author or some other
engineer who has devoted special study to this type, state these
advantages? The speaker is interested as to the readiness or difficulty
of making a dam of this kind, together with its foundations, sufficiently
water-tight, and what standard of water-tightness is accepted. Would
the leakage permitted be greater than that from a well-built "solid"
masonry dam? As time goes by, do the agencies which make for
greater water-tightness prevail over those which tend to produce leaks?

F. W. ScHEiDENHELM,* M. Am. Soc. C. E. — The author has pre- Mr.

sented an interesting and detailed analysis of certain features of ^^^^•^^^ ^™-
multiple-arch dam design in such a manner as to deserve the apprecia-
tion of the members of the Profession interested in dam design and
construction. In addition to the general analysis, he has given an
outline of the special features pertaining to the design and construc-
tion of the two particular multiple-arch dams at Gem Lake and Agnew
Lake. Essentially, however, the paper is general in its character and,
correspondingly, this discussion will also be mainly of a general nature.

The multiple-arch dam, like the flat-deck dam, belongs to what one
might call the family of hollow dams. The multiple-arch type differs
from the flat-deck type (of which the Ambursen dam is the most
familiar) primarily in the method of transferring the water load to
the supporting buttresses. Both types involve relatively high unit
costs for the concrete work required. Accordingly, they find greatest
favor in situations where it is proposed to build dams of concrete, but
where the cost of cement is high. The dams at Gem and Agnew Lakes
are cases in point, for there the average cost of cement delivered at
the site of the work is reported to have been $7.50 per bbl. Apparently,
only rock-fill dams, but not solid masonry dams, were considered as
alternatives for multiple-arch dams for the sites mentioned. It seems

* New York City.



886 DISCUSSIOX ON MULTIPLE-ARCH DAMS

Mr. evident, however, that, had solid masonry dams been bnilt, the high

Scheidenheim. ^^^^ ^^ cement vs^ould have more than offset the relatively high cost of
the form w^ork required for the multiple-arch dams.

As to hollow dams, there appear to be two principal points which
require careful attention, namely, the protection of reinforcing steel
and the tendency to skimp the sizes of the structural members. In the
first respect, the multiple-arch type has a decided advantage over the
flat-deck type, in that it requires a much smaller quantity of steel.
Consequently, there is so much less steel which might be placed in
danger of corrosion, and it is so much the easier to protect the steel.
Again, the multiple-arch dam permits the use of longer spans, say.
from 20 to 50 ft. between centers of buttresses, as compared with cor-
responding spacings for the flat-deck type of from 15 to 30 ft.

On the other hand, the cost of form work for the multiple-arch dam
is somewhat greater than that for the flat-deck type, and, what is more
important, the multiple-arch dam is more susceptible to difficulties and
damage in case of settlement of the footings. The arches cannot with-
stand as much racking as flat-deck slabs. The author points out that
he provides struts to take care of the unbalanced arch thrust which
would result if one or more arches or bays were to fail. Of course, a
dam is intended not to fail, and, hence, one is concerned more with
precautions against failure than with measures to be taken in case of
failure. Even from the latter point of view, however, the value of the
struts would be problematical were they, in case of failure of the corre-
sponding arches, to be subject to the impact of floating ice, tree trunks,
or other debris. Moreover, it would seem that the struts might become
overloaded due to blocking the passage of such debris, and thus induce
failure in lateral flexure.

After all, it appears to the speaker that, in the case of the multiple-
arch dam, one must insist on unyielding foundations. Fortunately,
there seems to be no deficiency in bearing value on the part of the
foundations of the dams on Rush Creek.

It is not surprising to find that the use of the multiple-arch dam
dates far back of that of the flat-deck type, at least in so far as the
construction of the latter in reinforced concrete is concerned. The
most conspicuous example is that of the Meer Alum multiple-arch dam,
built in India a century or more ago. This dam, referred to by Mr.
Flinn, has a height of about 45 ft., and involves astounding span
lengths, varying from 70 to 147 ft. The arches are plain, and are built
of brick in lime mortar. New South Wales boasts of a dam, the
Belubula, in which the spandrels of the arches are filled in with
masonry, thus making impossible radial action of the water pressure.

In the United States there are, perhaps, a dozen multiple-arch dams,
the most familiar of which heretofore have presumably been those
built under the design and direction of John S. Eastwood, M. Am.
Soc. C. E. The Eastwood multiple-arch dams seem to differ from those




Fig. 19. — Los Verjels Dam, Near Oeoville, Cal.




Fig. 20. — Down-stream View of Los Verjels Dam.




Fig. 21. — Down-stream View of New Big Bear Valley Dam.



^m




.maQ TajoAV flAaa ojH vriVt no waiV



DISCUSSION ON MULTIPLE-ARCH DAMS 889

designed by the author essentially as regards the treatment of the Mr.

arches near the crest. Mr. Jorgensen uses an elliptical, rather than a •'^^''^eidenheim.

circular, form for, say, the upper 15 ft. of each arch or panel, whereas

Mr. Eastwood apparently prefers to retain circular arches, even near

the crest, and, to this end, has made the arch axes vertical for the

upper poi'tions of the panels of certain of his dams. Of this design are

the dams at Hume Lake and Bear "Valley, California.

The speaker has not had the privilege of visiting either of the dams
described by the author, but has had the opportunity of inspecting the
60-ft. high multiple-arch dam of the Los Yerjels Land and Water
Company, near Oroville, Cal. This dam is illustrated in Figs. 19 and
20. The former is a view along the spillway crest. Incidentally, it
emphasizes the lack of adequate spillway capacity. This is shown by
the erosion which is apparent at the opposite bank.

Fig. 20, a view from down stream, shows the buttresses and bracing
to be relatively slim, to such an extent, in fact, as to afford to the eye
no impression of stability.

The Bear Valley Dam, Fig. 21, makes quite a contrary impression,
and, though the product of the same designer, governed as he apparently
was by the financial ability of the respective clients, is of pleasing
appearance and proportions. Even in this case, the speaker would
prefer a system of bracing between buttresses consisting of relatively
deep (vertically) beams or walls. Such deeper beams would certainly
afford greater stiffness, and, moreover, would tend toward a better dis-
tribution of the load on the footings. In other words, with the buttresses
well braced, any deflection or settlement of the different footings would,
of necessity, be practically equal, and hence the firmer, less yielding,
foundation material would, as is desirable, take more load per unit of
area than the less firm portions. The last-mentioned advantage of the
deeper bracing beams is, to be sure, of little value in foundations where
the deflection under load is inappreciable.

Turning now to the details of the paper, the speaker is inclined to
think that the multiple-arch dam may properly be applied even more
widely than the author claims, for he, by implication, limits its utility
to locations where "the foundation is solid rock." Nevertheless, the
multiple-arch dam is not entirely free from the ills of other dams.
More particularly the speaker cannot agree with the general statement
of the author that "there is no hydrostatic uplift to amount to any-
thing acting on a dam of this type." Although not obsessed with such
a fear of uplift pressure as is shown by some engineers, the speaker
insists that the possibility, and often the certainty, of uplift pressure
must be recognized.

In the absence of more definite information, the allowance made
for uplift pressure must be, of necessity, a matter of judgment. Cer-
tainly, however, in the case of a foundation material which, where



890 DISCUSSION ON MULTIPLE-ARCH DAMS

Mr. unbroken, is impervious, but contains approximately horizontal lamina-

c eiden e m. ^j-j^j^g^ -^ must be admitted that uplift pressure may exist in such lamina-
tions or seams. Under such circumstances, the effect of the uplift
pressure would be practically the same as if the footings of the but-
tressed hollow dam were continuous, with uplift pressure existing in
the plane of contact of the footings with the surface of the foundation
material. Manifestly, with rock of the characteristics previously sug-
gested, a buttressed hollow dam, such as the multiple-arch dam, has no
advantage over the solid masonry dam, except to the extent that, by
reason of the details of construction, it may be a simpler matter ade-
quately to drain the underlying foundation material and thus relieve
any uplift pressure.

As regards sliding, also, the speaker feels that it is unwise, and, in
some instances, has been fatal, to consider a dam apart from its imder-
lying foundation material. The foimdation material, or substructure,
must act as a unit with the dam body, or superstructure, if the desired
object of constructing a safe dam is to be attained. The speaker does
not intimate that either of the multiple-arch dams described by the
author is in the least unsafe as regards either uplift pressure or slid-
ing. However, he does feel, for instance, that it is not sufficient to aim
for a safe shearing resistance at, or near, the base of a buttress, but
believes that it is fully as important, and in many cases even more diffi-
cult, to design so as to obtain safe resistance to shear or to sliding
within the foundation material itself at planes or laminations just
below the base of the footings.

That these considerations are not far-fetched is shown by the fail-
ures of such dams as those at Austin, Tex.; Lock 26, Ohio River, and
Austin, Pa. In the first two, at least, sliding took place within, and not
on, the foundation rock.

The speaker has not found in the paper any specific description of
the foundation material, but the author characterizes it as "bed-rock
worn clean by glacial action", and this statement in itself leads one to
believe that the foundation conditions are fortunate, and that probably
no harmful laminations exist.

Finally, the speaker believes it would be interesting to others, as
well as to himself, if the author were to share with his fellow engineers,
somewhat more specifically and in more detail, his views as to the effect



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