_The Tabeaud Dam, California._
The Tabeaud Dam, in Amador County, Cal., built under the supervision
of the author for the Standard Electric Co., is an example of the
homogeneous earth dam. A somewhat fuller description and discussion
will be given of this dam than of any other, not on account of its
greater importance or interest, but because it exemplifies certain
principles of construction upon which it is desired to put special
emphasis. This dam was described in Engineering News of July 10, 1902,
to which the reader is referred for more complete information than is
given here.
[Illustration: FIG. 3.–PLAN OF TABEAUD RESERVOIR, WITH CONTOURS.]
[Illustration: FIG. 4.–PLAN OF TABEAUD DAM, SHOWING BED ROCK DRAINAGE
SYSTEM.]
[Illustration: FIG. 5.–DETAILS OF BED ROCK DRAINS AT THE TABEAUD DAM.]
Fig. 3 is a contour map of the Tabeaud Reservoir, showing the relative
locations of the dam, wasteway and outlet tunnel. Fig. 4 shows the bed
rock drainage system and the letters upon the drawing will assist in
following the explanation given in the text. The whole up-stream half
of the dam site was stripped to bed rock. As the work of excavation
advanced pockets of loose alluvial soil were encountered, which were
suggestive of a refill, possibly the result of placer mining operations
during the early mining days of California. In addition to this were
found thin strata of sand and gravel deposited in an unconformable
manner. The slate bed rock near the up-stream toe of the dam was badly
fissured and yielded considerable water. A quartz vein from 1 to 2 ft.
in thickness crossed the dam site about 150 ft. above the axis of the
dam. The slate rock above this vein or fault line was quite variable in
hardness and dipped at an angle of 40 degrees toward the reservoir.
[Illustration: FIG. 6.–VIEW OF BED ROCK TRENCHES, TABEAUD DAM.]
The rear drain terminates at a weir box (Z) outside of the down-stream
slope at a distance of 500 ft. from the axis of the dam. This drain
branches at the down-stream side of the central trench, (Y), one branch
being carried up the hillside to high-water level (W) at the North end
of the dam, and the other to the same elevation at the South end (X).
Fig. 5 shows how these drains were constructed. After the removal of
all surface soil and loose rock, a trench 5 to 10 ft. wide was cut into
the solid rock, the depth of cutting varying with the character of the
bed rock. Upon the floor of this trench a small open drain was made by
notching the bed rock and by means of selected stones of suitable size
and hardness. The stringers and cap-stones were carefully selected and
laid, so that no undue settlement or displacement might occur by reason
of the superincumbent weight of the dam. All crevices were carefully
filled with spawls and the whole overlaid 18 ins. in depth with broken
stone 1 to 3 ins. in diameter. Upon this layer of broken stone and
fine gravel was deposited choice clay puddle, thoroughly wetted and
compacted, refilling the trenches.
[Illustration: FIG. 7.–VIEW OF NORTH TRENCH, TABEAUD DAM.]
[Illustration: FIG. 8.–VIEW OF SOUTH TRENCH, TABEAUD DAM.]
[Illustration: FIG. 9.–VIEW OF MAIN CENTRAL DRAIN, TABEAUD DAM.]
These drains served a useful purpose during construction, in drying
off the surface of the dam after rains. The saturation of the outer
slope of the dam by water creeping along the line of contact should
thus be prevented, and the integrity or freedom from saturation of the
down-stream half should be preserved. It is believed that the puddle
overlying these rock drains will effectually prevent any water from
entering the body of the embankment by upward pressure and that the
drains will thus forever act as efficient safeguards.
The main drain was extended, temporarily during construction, from
the central trench (Fig. 4), to the up-stream toe of the dam. This
was cut 5 or 6 ft. deep into solid rock, below the general level of
the stripped surface. Fig. 6 is reproduced from a photograph of this
trench. An iron pipe 2 ins. in diameter was imbedded in Portland cement
mortar and concrete, and laid near the bottom of the trench.
At the point (B) where the quartz vein (already described) intersected
this drain, two branch drains were made, following the fault well into
the hill on both sides. Figs. 7 and 8 are views of the North and South
trenches, respectively. These trenches were necessary to take care of
the springs issuing along the quartz vein. This water led to a point
(N, Fig. 4) near the up-stream toe, by means of the drain shown in Fig.
9.
The lateral drains and that portion of the main central drain extending
from their junction (B) to a point (N) about 230 ft. from the axis
of the dam have pieces of angle iron or wooden Y-fluming laid on the
bottom of the trenches immediately over the 2-in. pipe, as shown in
Figs. 7, 8 and 9. These are covered in turn with Portland cement
mortar, concrete, clay puddle and earth fill. The water will naturally
flow along the line of least resistance, and consequently will follow
along the open space between the angle irons and the outside of the
pipe until it reaches the chamber and opening in the pipe, permitting
the water to enter and be conveyed through the imbedded pipe-line to
the rear drain. This point of entry is a small chamber in a solid
cross-wall of rich cement mortar, and is the only point where water
can enter this pipe-line, the two branches entering the wells and the
stand-pipe at their junction (soon to be described) having been closed.
That portion of the foundation between the axis of the dam and the
quartz vein, a distance of about 160 ft., was very satisfactory,
without fissures or springs of water. In this portion the 2-in. pipe
was imbedded in mortar and concrete without angle irons, and the
continuity of the trench broken by numerous cross-trenches cut into
the rock and filled with concrete and puddle. It is believed that no
seepage water will ever pass through this portion of the dam. If any
should ever find its way under the puddle and through the bed rock
formation, the rear drain, with its hillside branches, will carry it
away and prevent the saturation of the lower or down-stream half of the
dam.
[Illustration: FIG. 10.–VIEW OF TABEAUD DAM WHEN ABOUT HALF COMPLETED.]
At the up-stream toe of the embankment, two wells or sumps (shown at
“S” and “K,” Fig. 4) were cut 10 or 12 ft. deeper than the main trench,
which received the water entering the inner toe puddle trench during
construction. This water was disposed of partly by pumping and partly
by means of the 2-in. branch pipes leading into and from these wells.
At their junction (J) a 2-in. stand-pipe was erected, which was carried
vertically up through the embankment, and finally filled with cement.
The branch pipes from the wells were finally capped and the wells
filled with broken stone, as previously mentioned.
EMBANKMENT.–As has been said, the upper surface of the slate bed rock
was found to be badly fissured, especially near the up-stream toe of
the dam, and as the average depth below the surface of the ground was
not very great, it was thought best to lay bare the bed rock over the
entire upper half of the dam site. Had the depth been much greater,
it would have been more economical and possibly sufficient to have
put reliance in a puddle trench, alone, for securing a water-tight
connection between the foundation and the body of the dam.
At the axis of the dam and near the inner toe, where the puddle walls
abutted against the hillsides, the excavation always extended to bed
rock. Vertical steps and offsets were avoided and the cuts were made
large enough for horses to turn in while tramping, these animals being
used, singly and in groups, to mix and compact the puddle and thus
lessen the labor of tamping by hand. In plan, the hillside contact
of natural and artificial surfaces presents a series of corrugated
lines, (as is clearly shown in Fig. 4.) After all loose and porous
materials had been removed, the stripped surface and the slopes of all
excavations were thoroughly wetted from time to time by means of hose
and nozzle, the water being delivered under pressure. Fig. 10 is a view
of the dam taken when it was about half finished and shows the work in
progress.
The face puddle shown in Fig. 11 was used merely to “make assurance
doubly sure” and was not carried entirely up to the top of the dam.
The earth of which the dam was constructed may be described as a
red gravelly clay, and in the judgment of the author is almost ideal
material for the purpose. Physical tests and experiments made with the
materials at different times during construction gave the following
average results:
Pounds.
Weight of 1 cu. ft. earth, dust dry 84.0
“ “ 1 “ saturated earth 101.8
“ “ 1 “ moist loose earth 76.6
“ “ 1 “ loose material
taken from test pits on the dam 80.0
“ “ 1 “ earth in place
taken from the borrow pits 116.5
“ “ 1 “ earth material
taken from test pits on the dam 133.0
Per cent.
Percentage of moisture in natural earth 19
“ “ voids in natural earth 52
“ “ grit and gravel in natural earth 38
“ “ compression on dam over earth at borrow pit 16
“ “ compression on dam over earth in wagons 43
Degrees.
Angle of repose of natural moist earth 44
Angle of repose of earth, dust dry 36
Angle of repose of saturated earth 23
CONSTRUCTION DETAILS.–The materials forming the bulk of the dam were
hauled by four-horse teams, in dump wagons, holding 3 cu. yds. each.
The wagons loaded weighed about six tons and were provided with two
swinging bottom-doors, which the driver could operate with a lever,
enabling the load to be quickly dropped while the team was in motion.
If the material was quite dry, the load could be dumped in a long row
when so desired.
After plowing the surface of the ground and wasting any objectionable
surface soil, the material was brought to common earth-traps for
loading into wagons, by buck or dragscrapers of the Fresno pattern. In
good material one trap with eight Fresno-scraper teams could fill 25
wagons per hour. The average length of haul for the entire work was
about 1,320 ft.
The original plans and specifications were adhered to throughout, with
the single exception that the central puddle wall was not carried above
elevation 1,160, as shown on Figs. 11 and 12, more attention being
given to the inner face puddle. This modification in the original plans
was made because of the character of the materials available and the
excellent results obtained in securing an homogeneous earthen concrete,
practically impervious.
[Illustration: FIG. 11.–DIMENSION SECTION OF TABEAUD DAM.]
The top of the embankment was maintained basin-shaped during
construction, being lower at the axis than at the outer slopes by 1-10,
to the height below the finished crown. This gave a grade of about 1
in 25 from the edges toward the center, resulting in the following
advantages:
(1) Insuring a more thorough wetting of the central portion of the dam;
any excess of water in this part would be readily taken care of by the
central cross drains.
(2) In wetting the finished surface prior to depositing a new layer
of material, water from the sprinkling wagons would naturally drain
towards the center and insure keeping the surface wet; the layers being
carried, as a rule, progressively outward from the center.
(3) It centralized the maximum earth pressure and enabled the
depositing of material in layers perpendicular to the slopes.
(4) It facilitated rolling and hauling on lines parallel to the axis of
the dam, and discouraged transverse and miscellaneous operations.
(5) It finally insured better compacting by the tramping of teams in
their exertions to overcome the grade.
[Illustration]
[Illustration: FIG. 12.–CROSS AND LONGITUDINAL SECTIONS OF TABEAUD
DAM.]
The specifications stipulated that the body of the dam should be built
up in layers not exceeding 6 ins. in thickness for the first 60 ft.,
and not exceeding 8 ins. above that elevation. The finished layers
after rolling varied slightly in thickness, the daily average per month
being as follows:
April 4 ins.
May 3½ “
June 4 “
July 4½ “
August 5 “
September 6 “
October 7 “
November and December 8 “
During the last few months more than one whole layer constituted the
day’s work, so that a single layer was seldom as thick as the daily
average indicates.
It was stipulated in the specifications that the up-stream half of the
dam was to be made of “selected material” and the lower half of less
choice material, not designated “waste.” “Waste material” was described
as meaning all vegetable humus, light soil, roots, and rock exceeding 5
lbs. in weight, too large to pass through a 4-in. ring.
It may be well to define the expression “selected material,”
so commonly used in specifications for earth dams. In England,
for instance, it is said to refer to materials which insure
_water-tightness_, while in India it refers to those employed to obtain
_stability_. It ought to mean the best material available, selected by
the engineer to suit the requirements of the situation.
The method employed in building the body of the embankment may be
described as follows:
(1) The top surface of every finished layer of material was sprinkled
and harrowed prior to putting on a new layer. The sprinkling wagons
passed over the older finished surface immediately before each
wagon-row was begun. This insured a wetted surface and assisted the
wheels of the loaded wagons, as well as the harrows, to roughen, the
old surface prior to depositing a new layer.
(2) The material was generally deposited in rows parallel to the axis
of the dam. However, along the line of contact, at the margins of the
embankment, the earth was often deposited in rows crosswise of the
dam, permitting a selection of the choicest materials and greatly
facilitating the work of graders and rollers.
(3) Rock pickers with their carts were continually passing along the
rows gathering up all roots, rocks and other waste materials.
[Illustration: FIG. 13.–VIEW OF TABEAUD DAM IMMEDIATELY AFTER
COMPLETION.]
(4) The road-graders drawn by six horses leveled down the tops of
the wagon-loads, and if the material was dry the sprinkling wagons
immediately passed over the rows prior to further grading. When the
material was naturally moist the grader continued the leveling process
until the earth was evenly spread. The depth or thickness of the layer
could be regulated to a nicety by properly spacing the rows and the
individual loads. The grader brought the layer to a smooth surface
and of uniform thickness, and nothing more could be desired for this
operation.
(5) After the graders had finished, the harrows passed over the new
layer to insure the picking out of all roots and rocks, followed
immediately by the sprinkling wagons.
(6) Finally the rollers thoroughly compacted the layer of earth,
generally passing to and fro over it lengthwise of the dam. Along the
line of contact at the ends, however, they passed crosswise. Then again
they frequently went around a portion of the surface until the whole
was hard and solid.
Two rollers were in use constantly, each drawn by six horses. One
weighed five tons and the other eight tons, giving respectively 166 and
200 lbs. pressure per lin. in. They were not grooved, but the smooth
surface left by the rollers was always harrowed and cut up more or
less by the loaded wagons passing over the surface previously wetted.
The wagons when loaded gave 750 lbs. pressure per lin. in., and the
heavy teams traveling wherever they could do the most effective work
compacted the materials better even than the rollers.
Several test pits which were dug into the dam during construction
showed that there were no distinct lines traceable between the layers
and no loose or dry spots, but that the whole mass was solid and
homogeneous.
A careful record is being kept of the amount of settlement of the
Tabeaud Dam. It will be of interest to record here the fact that just
one year after date of completion the settlement amounted to 0.2 ft.,
with 90 ft. depth of water in the reservoir.
Water was first turned into the reservoir five months after the dam was
finished. The very small amount of settlement here shown emphasizes
more eloquently than words the author’s concluding remarks relating to
the importance of thorough consolidation, by artificial means, of the
embankment. (See p. 64, Secs. 6 to 8.)
OUTLET TUNNEL.–The outlet for the reservoir is a tunnel 2,903 ft. in
length, through a ridge of solid slate rock formation, which was very
hard and refractory. At the north or reservoir end of the tunnel, there
is an open cut 350 ft. long, with a maximum depth of 26 ft.
Near the south portal of the tunnel and in the line of pressure pipes
connecting the “petty reservoir” above with the power-house below,
is placed a receiver, connected with the tunnel by means of a short
pipe-line, 60 ins. in diameter.
A water-tight bulkhead of brick and concrete masonry is placed in the
tunnel, at a point about 175 ft. distant from the receiver. In the line
of 60-in. riveted steel pipe, which connects the reservoir and tunnel
with the receiver, there is placed a cast iron chamber for entrapping
silt or sand, with a branch pipe 16 ins. in diameter leading into a
side ravine through which sand or silt thus collected can be wasted or
washed out. By the design of construction thus described, it will be
seen that all controlling devices, screens, gates, etc., are at the
south end of the tunnel and easily accessible.
WASTEWAY.–The wasteway for the reservoir is an open cut through its
rim, 48 ft. in width and 300 ft. long. The sill of the spillway is 10
ft. below the crown of the dam. The reservoir having less than two
square miles of catchment area, and the feeding canals being under
complete control, the dam can never be over-topped by a flood. Fig.
3 shows the relative location of the dam, outlet tunnel and wasteway
channel.
Almost the whole of the embankment forming the Tabeaud Dam, not
included in the foundation work, was built in less than eight months.
The contractor’s outfit was the best for the purpose the writer has
ever seen. After increasing his force from time to time he finally had
the following equipment:
1 steam shovel (1½ yds. capacity),
37 patent dump wagons,
11 stick-wagons and rock-carts,
39 buck-scrapers (Fresno pattern),
21 wheel scrapers,
3 road-graders,
3 sprinkling wagons,
2 harrows,
2 rollers (5 and 8-ton),
233 men,
416 horses and mules,
8 road and hillside plows.
STATISTICS.–The following data relating to the Tabeaud Dam Reservoir
will conclude this description:
DAM.
Length at crown 636 ft.
Length at base crossing ravine 50 to 100 “
Height to top of crown (El. 1,258.) 120 “
“ at ends above bedrock 117 “
“ at up-stream toe 100 “
“ at down-stream toe 123 “
Effective head 115 “
Width at crown 20 “
Width at base 620 “
Slopes, 2½ on 1 with rock-fill 3 to 1.
Excavation for foundations 40,000 cu. yds.
Refill by company 40,000 “
Embankment built by contractor 330,350 “
Total volume of dam 370,350 “
Total weight 664,778 tons.
Length of wasteway (width) 48 ft.
Depth of spillway sill below crown 10 “
Depth of spillway sill below ends 7 “
Height of stop-planks in wasteway 2 “
Maximum depth of water in reservoir 92 “
Area to be faced with stone 1,933 sq. yds.
RESERVOIR.
Catchment area (approximate) 2 sq. miles.
Area of water surface 36.75 acres.
Silt storage capacity below outlet tunnel 1,091,470 cu. ft.
Available water storage capacity 46,612,405 “
Elevation of outlet tunnel 1,180 ft.
“ “ high-water surface 1,250 “
“ “ crown of dam 1,258 “
Fig. 13 is a view of the finished dam, taken immediately after
completion.