American Society of Civil Engineers.

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

. (page 83 of 167)
Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 83 of 167)
Font size
QR-code for this ebook


4 ft. 6-in. lengths, so as to allow them to project 3 ft. vertically above



EECONSTRUCTION OF THE STONY RIVER DAM 1009

the spillway crests. Seasoned spruce planks were used as flasli-boards,
with an actual net height of 2.78 ft, above the spillway crests.

The final tests, made on pins chosen at random from those pur-
chased for actual service, showed exceedingly satisfactory results. In
no cases where the conditions were equivalent to those of actual
service did the pins fail at heads of less than 4.0 ft., nor did they
longer than momentarily withstand without failure heads of more
than 4.6 ft. above the base of the flash-boards. Moreover, the maximum
temporary deflection above the top of the sockets, with the head-water
at elevations slightly less than that at which failure occurred, was
only about 10° down stream from the vertical. In those instances
where the head-water was allowed to subside to zero after having very
nearly, but not actually, attained the predetermined elevation, the
pins showed a permanent deflection or set of less than 3° from the
vertical. The results of these tests, made in the outlet channel, showed
surprisingly little difference between the effects of turbulent and of
still head-water.

The apparent brittleness of the hardened pins is the more remark-
able in view of the '"qualifying" test to which the pins had been sub-
mitted after they had been hardened, viz., they had been dropped from
the crest of the dam to the concrete flooring of the outlet channel,
a distance of more than 50 ft. In order to make doubly certain that
defective bars were eliminated, each pin was dropped twice in this
manner. Sometimes they fell across previously dropped pins, thus
concentrating the impact at a single point of the pin. Yet very few
which were put through this conclusive test were thereby broken.
This "qualifying" test was applied, not merely to the pins used for
the conflrmatory tests in the outlet channel, but to all pins which
were placed in actual service.

With regard to the sensitiveness of the pins in question, the results
of an earlier test may be of interesT. In that test ly\-in. round
pins, with carbon content of 0.62%, were used at the regular intervals
of 3 ft. 6 in. to support flash-boards about 38 in. high. The head-
water was maintained for 10^ hours at an elevation approximately
3.7 ft. above the base of the flash-boards. At the end of that period
the head was increased to 4.2 ft., and within a few minutes two of the
three pins under test, supporting three panels of flash-boards, snapped
off simultaneously at the top of the sockets.



1010 RECONSTRUCTION OF THE STONY RIVER DAM

Since the reconstruction of the dam has been completed, the head-
water, with the flash-boards in place, has reached Elevation 139.83,
which is about 12| in. over the top, and 3.83 ft. above the base, of
the flash-boards. The supporting pins met the expectations, as none
failed, despite the fact that they were under severe load for several
hours. Subsequently, the flash-boards were taken down for the winter,
and it was found that the pins had suffered no permanent deflection.

Underpinning Problems at tJie New Spillway. — The original footings
of Buttress 10, which was left intact and forms the west abutment
of the new spillway, were about 22 ft. higher than the footings of
the adjacent new Buttress 11, and about 27 ft. higher than the footings
of new Buttress 12. The footings of the latter buttresses rest on
bed-rock, whereas the original footings of Buttress 10 rested on the
clayey over-burden. Evidently, it was necessary to provide secure sup-
port for Buttress 10 as well as to insure the stability of the foundation
soil under the adjacent (up-hill) portion of the original structure.
This was accomplished by underpinning Buttress 10 with a retaining
wall founded on bed-rock. In this manner it was possible, in effect,
to extend the footings of Buttress 10 down to rock and, at the same
time, prevent the lateral displacement of the soil under the adjacent
footings. The retaining wall was extended down stream as a portion
of the new spillway channel, and decreases in height as the channel
swings away from the west bank. It is of reinforced concrete, and is
of substantial proportions for a sufficient distance down stream from
the footings of Buttress 9 to confine adequately the foundation soil
immediately down stream from that portion of the dam, as shown
by Figs. 16 and IT.

The type of construction of the underpinning at Buttress 10 is
illustrated in Figs. 25 and 26, which show the underpinning in suc-
cessive stages of construction. To avoid the necessity of temporary
shoring under Buttress 10, relatively narrow excavations were first
made for the 4-ft. wide counterforts, leaving the intermediate soil to
take the weight of the buttress and its overhanging footing (which was
not removed). Due to the character of the soil in question, it was
impracticable to allow the buttress to be supported in this manner
for any considerable length of time. Fortunately, no severe rains
occurred in the meantime, and the counterforts were constructed with-
out mishap up to the bottom of the footings. Then, by means of holes




Fig. 25. — Underpinning at Buttress 10.




Fig. 26. — Underpinning at Buttress 10, Stony River Dam.



EECONSTKUCTION OF THE STONY RIVER DAM 1013

drilled through the footings, grout was forced between the surfaces of
contact. Subsequently, the counterforts took the load without notice-
able sinking on the part of Buttress 10.

As soon as the concrete in the counterforts had set sufficiently, a
reinforced concrete panel, 4 ft. thick, was constructed, bearing against
shoulders which had been left in the counterforts. The concrete in
this panel was poured directly against the clayey bank, so as to
prevent the formation of voids and consequent settlement. All voids
left immediately under the footings were filled with grout, not so much
to enable the west half of the Buttress 10 footings to transmit load
into the foundation soil as to prevent lateral displacement of the soil
imderlying Buttress 9 and its footings. The retaining wall panels
were provided with weep-holes in order to drain adequately the sup-
ported soil.

In view of the fact that Buttress 10 now is founded on rock,
whereas the adjacent westward Buttress 9 rests on clayey soil, it is
probable that the footings of the latter will deflect more than those
of the former in transmitting vertical load. Such inequality in deflec-
tion would cause undesirable stresses in the footings of Bay 10 were
it not for a vertical seam which was cut entirely through the flooring,
along the center line of the bay. Thus the footings of the adjacent
buttresses can act independently as far up stream as the cut-off wall.
The deck of Bay 10 will presumably adjust itself to any such unequal
deflection without ill effects.

At the east side of the new spillway a like problem was encountered
in connection with the footings of Buttress 19. It was treated in a
similar manner, though an additional counterfort was constructed.
Here, too, a retaining wall was extended down stream to form the
east wall of the new spillway channel, supporting the west flank
of the fill which was made immediately down stream from the dam
between the old and new spillways.

Toe Protection at Old Spillway. — The channel down stream
from the old spillway has a concrete mat 9 in. thick, the mat having
been placed on crushed stone which covered the original ground
surface to a depth of 4 in. In the reconstruction it was felt that,
considering the possible effects of frost, in the way of heaving the
channel mat, and of scour at the down-stream end of the mat, such



1014



RECOXSTIU'CTION OF THE STOXY HIVER DAM



construction could hardly be considered permanent, and it appeared
to be wise to make more substantial provision.

This took the form of additional toe protection, rather than of doing
away with the original channel mat and substituting a relatively expen-
sive, heavier mat. The principal features of such toe protection are
shown in Fig. 27. The row of 10 by 10-ft. mat slabs immediately down



EI.llO



Shading- indicates
new work.



EI.80




b^j- 18 (min.)



TOE-WALL AT OLD SPILLWAY
Fig. 27.

stream from the old spillway apron bucket was taken out, and excava-
tion was made for a toe-wall extending to a depth of IT ft. below the
top of the channel mat or hearth. This toe-wall was designed to act as
an anchoring wall, similar in this respect to the anchoring walls pro-
vided for the bulkhead portions of the original structure. However,
it was carried to a greater depth, in order to prevent the undermining



RECOXSTRUCTION OF THE STONY RIVER DAM 1015

of the main footings of the old spillway in ease the channel mat
should be washed away in whole or in part.

Here, again, the toe-wall forms, in effect, an extension of the
original footings. Its load is received through the original
toe of the spillway. However, because the apron of the spill-
way had been cast separately from the footings and the buttresses,
it was impossible to utilize the apron, as originally constructed, to
transmit any load into the toe-wall, except in so far as the weight of
the apron and the fractional resistance, along the surfaces of contact
between the apron and buttresses, were concerned. It was found
advisable to utilize the apron to a greater extent by constructing a
so-called floor-beam, as shown in Fig. 27, and tying the apron into
the floor-beam with the steel dowels there shown. This arrangement
also served the purpose of distributing the load on the base of the
buttress over a greater distance (up and down stream), thus reducing
the unit load on the concrete in proportion.

In addition to the deep toe-wall, wing-walls were extended down
stream, as shown in plan on Plate XI. The inner end of each wing-
wall extends to the same depth as the toe-wall, but the outer ends are
approximately 11 ft. deep below the top of the mat. The wing-wall
at the west end of the old spillway was capped by the concrete of a
wasteway, which allows any excessive head of water in the channel
immediately down stream from the bays of maximum height to relieve
itself into the old spillway channel. The new fill between spillways
has been protected on its east flank along the old spillway mat by
the construction of a wall of sandstone boulders laid in cement mortar.

Anchoring Wall at Heel of Gate Sections. — At Bays 30 and 31
the openings from the reservoir to the outlet gates prevented the
construction of the standard type of "heel" shown on Plate XII. The
upper cantilever member of the anchoring wall would have interfered
with the opening. Therefore, a modified method of construction was
used, as shown on Plate XIII. In this case the anchoring wall extends
from the top of the concrete protection mat, immediately up stream
from the gate openings, to a depth of about 15 ft. below the original
footings. The cantilever arms are not so long as in the standard
"heel", for the reason that the tie-steel enters the anchoring wall at
the middle, inclining downward from the original footings. The
steel bars find embedment in the new concrete immediately adjacent



1016 EECONSTRUCTION OF THE STONY EIVER DAM

to, and bonded into, the lower portions of Buttresses 29, 30, and 31,
as showai in Section B-B of Plate XIII. The bars were encased entirely
in concrete to protect them against corrosion in the foundation soil.

In order to place the tie-steel, holes were opened by blasting through
the original cut-off wall, and tunnels were carried upward under the
footings to connect with openings which had been cut down through
the footings. The bars were then placed and the concrete poured
from the interior of Bays 30 and 31. The special construction at
these bays was simpler, in so far as the anchoring wall itself was
concerned, but was more expensive than the standard type because
of the necessity of placing the tie-steel in the manner described.

Outlet Channel. — The outlet channel has been referred to and
partly described in preceding portions of the paper. Its purpose i-s
essentially to prevent any scour which might result either from dis-
charge from the sluice-gates or from water flowing over the inter-
mediate spillway which extends the length of the bulkhead section
between the old and new main spillways. The sides of the channel
are constructed as retaining walls to hold the down-stream fill. Lon-
gitudinal and transverse cross-sections of the outlet channel are shown
in Fig. 24, and Plate XI shows it in plan. The channel extends a
total distance of 64 ft. down stream from the toe of the original struc-
ture, and is protected by a cut-off, about 12 ft. deep, penetrating a
bed of heavy boulders. The details of this structure are not such as
to warrant further description.

A connecting channel was necessarily constructed to lead from
the outlet channel directly down stream to the old river-bed. This
change of stream channel has a further advantage in that it protects
the lower end of the old spillway channel mat from the scour, due
to sluice-gate discharge, to which it was subjected under the original
construction.

Materials of Construction.
Cement. — The cement for the work of reconstruction, as also that
for the original construction, was furnished by the Alpha Portland
Cement Company, and all the cement for the reconstruction came
from its Lehigh Valley mills. The cement was required to meet the
Specifications of the American Society for Testing Materials plus those
specifications of the United States Government (Bureau of Standards)



PLATE XIII.

TRANS. AM. SOC. CIV. ENGRS.

VOL. LXXXI, No. 1397.

SCHEIDENHELM ON

RECONSTRUCTION OF

STONY RIVER DAM.




■v:-r , ■ :;^.^- , - - ^. !^v.v,^v.r.w■■f■■:vjr;^-^■^V , , .
^-^^^ ^ -P'- 8S=S3 -p- ^ ~-^.
- Clayey-SoiP |_ V _^— ^ J^ofd weep^oleJ"

v_. ^. l^"— ^ - — ■ v..^ ^_"^ ^ V. Rluergea-wich

^ 1 confcrete

Section at Center of BaJ 31



■^




yiX y-^ "" f'CSnstructio!)
y X% .-^J^.iJoint
^


^






1

CO






Allowable




03
Si


Minimum

embedment,

surface of

concrete to

center of

bar, in

inches.


At 16 000 lb. per
sq. in.


At 18 000 lb.
per sq. in.


a


b


range of
spacing
between
centers of
bars, in
inches.


M.2








u
o


Bond stress, in pounds per
square inch.







Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 83 of 167)