American Society of Civil Engineers.

Transactions of the American Society of Civil Engineers (Volume 81) online

. (page 77 of 167)
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Run-o!T and spillway discharge, in thousands
of cubic feet per second.



94:2 EECONSTEUCTION OF THE STON'Y RIVEK DAM

Conclusions as to Spillway Provision. — After such studies as the
foregoing, the writer came to the conclusions that:

1. — The spillway capacity provided in the reconstruction of Stony
Eiver Dam is adequate.

2. — Such spillway capacity is not excessive, bearing in mind that:
(a) There is no warrant for assuming that during the life of the
dam the Stony River drainage area will escape rainfall and
resulting run-off such as have occurred in other regions
which are comparable;
(fe) The number of precipitation stations and records, especially
in the Appalachian Moiuitain region, is not sufficient to war-
rant the assimiption that the worst possibilities have been
observed and recorded; and
(c) The provision of adequate spillway capacity in the recon-
struction of the Stony River Dam did not involve unreason-
able cost, though, of course, such spillway capacity could have
been provided more cheaply in the original construction than
in the reconstruction.

3. — It is reasonable to assume that the water level in the reser-
voir will not be higher than Elevation 142.25, under the most severe
conditions within the limits of reason. This elevation of head-water,
therefore, was adopted for design purposes.

4. — Granting the necessity for making provision to pass safely the
greatest flood reasonably to be anticipated, it is also true that such
a flood would be far from normal. Hence, in considering the "nor-
mal maximum load" to which the dam will be subjected, it appeared
proper to assume the head-water at Elevation 140.5. At this eleva-
tion the flash-board supports (on the old and new spillways) are rea-
sonably certain to have failed. The flash-boards and supports having
thus been swept from the main spillway crests, the spillway capacity
would be approximately 10 000 cu. ft. per sec, without taking into
consideration any equalizing effect of the reservoir. In the case of
the original structure, the spillway capacity likewise was approxi-
mately 10 000 cu. ft. per sec. at Elevation 140.5, it being noted, how-
ever, that such head-water elevation could not have existed without
causing the original bulkhead sections (with crest at Elevation 139)
to be overtopped. The corresponding unit spillway capacity is about
875 cu. ft. per sec. per sq. mile of drainage area.



RECONSTRUCTION OF THE STONY RIVER DAM

Rainfall, in inches per hour.



943



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EQUALIZATION EFFECT OF RESERVOIR.
Based on assumptions of
(a) Most severe run-off conditions

reasonably conceivable.
(h) No flash-boards on spillway crests.




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Run-off and spillway discharge in thousands of cubic feet per second .



944 RECONSTRUCTION OF THE STONY RIVER DAM

Type of Structure for New Spillway. — It appeared logical to
reconstruct the new spillway section of the dam according to the
Ambursen, hollow, reinforced concrete type, provided there was no
serious and valid counter consideration. Probably the most serious
questions in regard to the Ambursen type of dam are those dealing
with the protection of the embedded reinforcing steel from corrosion,
and the extent to which the dimensions of the reinforced concrete
members may properly be pared down to theoretical limits. Both
these questions may be answered satisfactorily, so the writer thinks,
by a sufficiently conservative design of the members of the structure.

On the other hand, the advantage of economy lay with the hollow,
rather than with the solid, type, a by no means unimportant factor
being the relatively high cost of the materials composing the concrete.
For equal margins of safety, a solid dam would have involved some-
what greater quantities of concrete, although probably not of excava-
tion. Furthermore, the construction of a solid section at the new spill-
way would have caused difficulties, costly to meet, at Buttresses 10
and 19 by reason of subjecting those buttresses to lateral hydrostatic
pressure for which they were not designed.

In view of its secluded location, the appearance of the structure
is of relatively minor importance. Yet even this consideration argued
in favor of the hollow type, because, at least on the up-stream side,
the solid section would have caused an incongruous break in outline.

Connecting with the foot of the new spillway apron, a channel
mat was constructed which gradually narrowed to a width of 60 ft.
at a distance averaging about 85 ft. down stream, as shown in
Plate XI. The minimum depth of the channel mat varies from 30 in.
on clay (24 in. on rock) at the foot of the spillway apron to 18 in.
at the down-stream end of the mat, where a shallow cut-off wall seals
the mat into bed-rock. Reinforced concrete retaining walls (Figs.
16 and 17) were built along the sides of the channel mat, to support
the hillside on the west and the new fill between the spillways on the
east. Excavation of the new spillway channel was not completed down
stream from the channel mat. Only a small ditch was dug to drain
water from the channel mat into the river, and the first flood practi-
cally completed the remainder of the excavation necessary to furnish
sufficient connection between the new spillway and the river.



PLATE XI.

TRANS. AM. SOC. CIV. ENQRS.

VOL. LXXXI, No. 1397.

SCHEIDENHELM ON

RECONSTRUCTION OF

STONY RIVER DAM.




NOTE:-The buttress immediately east
(to the left) of any bay determines
the number of that bay.



PLAN OF STONY RIVER DAM
AS RECONSTRUCTED




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;■; i.y/;5-:;i^s^^K6?SB;



EECONSTKUCTION OF THE STONY RIVER DAM 947

Resistance to Sliding.

To the writer it appears that, in the case of many masonry dams,
solid and hollow, the importance of safeguarding the structure against
failure by sliding has been under-rated. It is not unlikely that there
have been more failures of masonry dams by sliding than in any other
manner. The character of the underlying foundation material is
necessarily the primary factor controlling the stability of a given dam
in regard to sliding. The construction of a dam to be founded on
granite, for instance, offers no difficulties in this respect. But, in view
of the foundation material underlying the Stony River site, the
necessity of making the dam safe against sliding presented the most
important problem of the reconstruction.

Uplift pressure, too, is of great importance as affecting the
stability of a dam against sliding. It acts essentially to diminish the
weight of the structure. The effect of a given uplift pressure, how-
ever, depends largely on the coefficient of frictional resistance of the
foundation material at the plane along which sliding may be impend-
ing; that is, the smaller the coefficient of frictional resistance, the less
the net effect of the uplift pressure.

Frictional Resistance. — The term "frictional resistance", as used
in this paper, is intended to express both the effect of roughness and
the effect of adhesion between any two bodies (e. g., of foundation
material) at the surfaces along which sliding is taking place. It is
not intended to express the effect of cohesion within a body; such
cohesion is an additional factor resisting sliding, and, in so far as
resistance to sliding is concerned, must be overcome by shearing the
body in question.* The distinction here made between adhesion and
cohesion does not exist when the material under consideration is
viscous, but, in the writer's opinion, it exists definitely when the
material in question is solidified.

As to the actual coefficients of frictional resistance, there is a
woeful dearth of data. Attention is called first to Table 1 which shows
the assumptions on which the reconstruction design was based. The

* Since the reconstruction was completed, the writer has considered carefully
the paper by William Cain, M. Am. Soc. C. E., entitled "Cohesion in Earth: The
Need for Comprehensive Experimentation to Determine the Coefficients of Cohesion",
Trayisactions, Am. Soc. C. E., Vol. LXXX, p. 1315, and the discussions thereon, but
has been unable to agree that data, sufficient in number and reliability, are available
to warrant the use, especially in the present case, of the analysis set forth in that
paper.



948



KECONSTEUCTION OF THE STONY KIVER DAM



TABLE 1. — Coefficients of Frictional Eesistance Forming the
Basis of Design in the Eeconstruction of Stony Kiver Dam.



Character of foundation
material.


Value

assumed

for design.


Probable

minimum

value.


Ratio.


Clayev overburden - wet


0.33

0.40

0.50


0.25
0.25

0.40


75


Applying: to footings of the original sections
of dam and to probable ■' plaaes of least resist-
ance" in clayey foundation soil.


0.625


Applying to shale near surface of bed-rock;
hence iui-luding shale immediately underlying
new footings at Buttresses 10 to 19, inclusive
(new spillway section.)


0.80


Applying lo shale at probable "planes of least
resistance" at Buttresses 10 to 19, inclusive
(new .spillway section. )





"planes of least resistance" referred to in that table are explained
later.

The pertinent data considered in adopting these assumptions follow.

Frictional Resistance of Clayey Soil. — In the writer's opinion, the
safe coefficient of frictional resistance for the clayey foundation soil
at the dam site when wet is between 0.3 and 0.4. When such soil is
dry or merely moist, the frictional resistance, of course, is greater than
when it is thoroughly wet ; but it is not safe to assume that the founda-
tion soil will always be fairly dry. It is true that the leakage from
the reservoir into the foundation soil has, at this writing, become, and
in all probability will continue to be, a relatively minor factor.
However, ground-water in the soil cannot be disposed of, especially
inasmuch as the dam site receives the ground-water drainage of the
adjoining hillsides. The new drainage system of the dam cannot cause
the foundation soil to become dry. It can merely prevent the accumu-
lation of serious uplift pressure.

The results of various tests of frictional resistance are of record.
Unfortunately, they are not numerous; nor are the test conditions
sufficiently standardized, nor the character of the material under tests
sufficiently well defined, to allow satisfactory detailed comparison with
each other or with the conditions to which it is attempted to apply
their results. Moreover, it is the rule, rather than the exception, that,
as in the case in point, the foundation material is heterogeneous, thus
increasing the difficulty of drawing comparisons with previous test



EECONSTEUCTION OF THE STOIfY RIVER DAM 949

results and making more necessary the use of judgment in selecting
coefficients for design or other purposes.

Among the recorded data are the experiments of General Morin,
as referred to by Rankine,* which indicate a coefficient of frictional
resistance of 0.31 for wet clay on wet clay, as tested by him. Tests
of the foundation soil in question, however, are more pertinent, and
such tests were made on several kinds of soil by Mr. D. N. Showalter,
Resident Engineer, under the direction of the writer. The results of
these tests are given in Table 2,

In the ordinary case, these tests were made by tamping a given
kind of soil into a box having an open top, the box being a cube of
about 12 in., then capping the boxful of soil with a solid piece of the
same material, so that it protruded above the edge of the box, and
turning the box upside down. The protruding material, after having
been given an approximately horizontal surface and made smooth,
was utilized as the moving element in determining the frictional
resistance on a prepared, smooth, horizontal bed of the same kind of
material in place. The moving element was drawn along by hand over
the bed, the required pull being measured by a spring balance. Ordi-
narily, more than J sq. ft. of the material under test was actually in
contact. The method of making the tests is illustrated by Figs. 8
and 18.

In the case of the tests made in cold weather, the clay bed had
been scraped previously so as to remove all frozen material and any
"mud" formed by melting snow. When not under test, the surface or
bed was protected by a covering of tar-paper. In the last series of
tests, in May, 1915, a galvanized-iron pail was used instead of a
wooden box. The result was to make these experiments appear even
more crude than was really the case; but, inasmuch as the weight
of the moving element and the required pull were not less readily
determinable, such experiments are not believed to be the less worthy
of consideration.

Despite the absence of laboratory refinements, the results obtained
are in all probability reasonably indicative of the actual frictional
resistance, as previously defined. Among the results of any single
group of experiments, the maximum variation from the mean was
30% of the mean, and the average of such maximum variation for all



"A Manual of Applied Mechanics", p. 211.



®m



RECONSTEUCTION OF THE STONY RIVEE DAM



TABLE 2. — Results of Tests of Frictional Resistance of Founda-
tion Soil at Stony River Dam Site.



*->

a


Character of soil under test.


0.2
— «"

' § = s

""


Temperature

range, in

degrees

Fahrenheit.


•9^

11


Pull (while in
motion),
in pounds.


CM


E0.2




1....


White clay, fine-grained


Wet.


35 to 41


160


64


0.40


2....


do.


do.


do.


160


67


0.42


3....


do.


do.


do.


284


80


0.28


4....


do. 1


do.


do.


284


72


0.25


5....


do. ,

1


do.


do.
35 to 41


160
160


55


0.34




Average
75


0.34


6


0.47


r....


do.


do.


do.


160


72


0.45


8....


do. 1


do.


do.


284


118


0.43


9;...


^ do. 1


do.


do.


220


78


35


10....


do-
Yellow clay, containing somegri t .


do.
Fairly wet.


do.
20 to 25


220
149


85
Averagp

120


0.39




0.42


11


0.80


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(2)


(3)


14^


15)


(6)


(7)


■•8)


(9)


(a) Elevation of head-water


140.5


140.5


136.0


136.0


140.5


140.5


139.0


139.0


(b) Ice pressure, in pounds per


















linear foot








20 000


20 000








20 000


20 000


(c) Elevation of lower limit of




equivalent full hydrostatic












i


,■


pressure against dam


89.5


99.0


85.0


95.0


91.0


99.0 ' 85.0


95.0


(d) (1) Uplift pressure negligible.


Yes.


Yes.


No.


No.


Yes.


Yes.


No.


No


(2) Head,infeet, of equivalent


















uplift pressure (acting












1




on one-half of affected












'




area of base) at up-












1




stream edge of struc-
















ture, decreasing uni-


1












formly to zero at down-












1




stream edge of footings,












1




in case of original struc-












1




ture, and at deep drains
















in case of strengthened


















structure






48.0


39.0






54.0


44.0












in re


















sliding.


















42.0


















in re


















over-


(3) Length of base, in feet,
















turning.


over which uplift pres-


















sure is exerted






01.0



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