Case Study: Blakely Mountain Dam

CASE STUDY: BLAKELY MOUNTAIN DAM
Ben Emery1
ABSTRACT
Blakely Mountain Dam, a 1,100 foot long, 205 foot high earthen dam, which forms Lake
Ouachita, is located approximately 10 miles NW of Hot Springs, AR. This project was
placed in service in August 1953 and is currently being evaluated for concerns regarding
seepage and piping failure modes of the dam.
Blakely Mountain Dam was built with a gravel drain abutting against the impervious core
for 3,300 square feet along the length of the dam. This gravel drain does not meet filter
criteria for the impervious core materials and therefore may lead to piping of the core
material.
Construction of a 2 million dollar seepage collection system consisting of an impervious
berm with 10 manholes, ditches, and a V-notch weir was completed in October 2009 to
monitor flow through the dam. Three manholes were equipped with turbidity meters to
monitor soil particles movement through the system.
Between November 2010 and January 2011, the collection system had no flow. This
prompted the Vicksburg district to test for possible leaks throughout the dam, which may
cause water and possible piped material to bypass the seepage collection system. In
January 2011, a dye test was performed on the dam. This consisted of pumping highly
concentrated dye into the blanket drain and monitoring for leakage.
To date no evidence of piping has been observed. An Issue Evaluation Study (IES) for
Blakely commenced this past March. This presentation will profile the dam’s history,
results of recent tests, and the outcome of the IES.
INTRODUCTION
All dams have problems; whether its wet spots on the face of the dam, slides, cracking, or
problems with seepage and piping, something can be found on the surface of most dams.
Most of these problems are minor and will never lead to full development of failure, but
some will, which is why constant monitoring of dams is important. Some problems,
however, occur within the dam itself, and can be very difficult to pinpoint, and address.
1
Geotechnical Engineer, USACE, Vicksburg District, Inspection and Investigation Section, 4155 Clay St,
Vicksburg, MS 39183, [email protected]
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Figure
F
1. Blaakely Mounttain Dam
Figuree 2. Layout of
o Blakely M
Mountain Daam
Blakeely Mountain
n Dam is a 1,100
1
foot lo
ong, 205 foott high eartheen dam. It iss located
appro
oximately ten
n miles Nortth West of Hot
H Springs, Arkansas. Blakely Moountain Dam
m
formss Lake Ouacchita which has
h a maxim
mum volume of 617,400 aacre-ft over an area of
48,30
00 acres. Th
his dam was placed in serrvice in Auggust 1953 annd began gennerating
poweer in August 1955. Whille there is ev
vidence of prroblems at B
Blakely Mouuntain Dam,
its peerformance history
h
indicaates otherwiise. Due to cconcerns witth seepage aand piping
relateed failure mo
odes, an issu
ue evaluation
n study of Bllakely Mounntain Dam coommenced in
Marcch 2011. Du
uring this IES
S several faillure modes hhave been diiscussed andd evaluated.
IES team membeers spent hou
urs scouring constructionn photos, as--built drawinngs, and
previious inspections reports to
t try to iden
ntify where tthe problemss in this dam
m lie.
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ative Dam aand Levee D
Design and C
Constructioon
CONSTRUCTION CONCERNS
Blakely Mountain Dam was built with a gravel drain abutting against the impervious core
for 3,300 square feet along the length of the dam. This gravel drain does not meet filter
criteria for the impervious core materials. From looking at the design plans and
construction photos, the dam appeared to be designed correctly, but not constructed
correctly. Blakely Mountain Dam was designed with a three layer filter blanket as seen
in Figure 3a, in a blown up portion of the design plans.
Figure 3a. Design Drawing
Figure 3b. Contract Drawing
Figure 3b is a blown up view of the contract drawing’s drainage blanket detail where
blanket contacts the dam core. Note that the two layers of the drainage blanket are
horizontal in section, and the finer grained top layer does not wrap around the more
course grained bottom layer where they come in contact with the core of the dam. If a
defect (i.e. crack) is present in the core adjacent to the gravel blanket, continuation and
progression of piping of the core material is possible due to the presence of the gravel
drain, yet no evidence of piping is evident.
MONITORING THE PROBLEM
This potential problem prompted the U.S. Army Corps of Engineers to construct a two
million dollar seepage collection system at the toe of Blakely Mountain Dam in October
2009. This system includes an impervious berm, with ten manholes, ditches, and a Vnotch weir. The purpose of this system was to monitor the flow throughout the dam, and
to collect any material that may be piped through the core. Three of these manholes were
equipped with turbidity meters to monitor soil particles movement throughout the system.
The V-notch weir should allow water to pass through the system while solids in the water
drop out and can be collected for analysis. Since this system was placed in service it has
been continuously monitored. The turbidity meters have not shown any indication that
core material is being piped through the blanket drain, and no solid material has been
removed from the V-notch weir. This system also has devices that record the
temperature of the water passing through the dam. This could be a significant source of
information, because a sudden significant drop in temperature could indicate water
directly from the bottom of the lake. To date, no significant changes in temperature have
been noted.
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Figure 4. Construction of Seepage Collection System
Between November 2010 and January 2011, the USACE, Vicksburg District noticed a
significant change in the weir flow at Blakely Mountain Dam. The flow in the collection
system went from a steady 25-30 gallons per minute to no recordable flow during this
time period. This reduction in flow could have been due to the fact that Lake Ouachita
and Lake Hamilton, which flows from the dam discharge into, were both at low water
elevations, or it could mean that there was a significant seepage problem somewhere else.
If water was escaping the seepage collection system at another location, core material
could be being piped there as well. To test for a seepage problem, the Corps of
Engineers decided to run a dye test to try and find out where the water seepage through
the dam was exiting.
DYE TEST
The dye chosen for this test was a highly concentrated red rhodamine dye. Figure 5,
(shown below), shows five cups with varying ratios of the rhodamine dye to water
solutions. While the rhodamine can be easily seen in some of these cups, it is hard to tell
there is any rhodamine in the others. This meant to detect rhodamine coming through the
dam a fluorometer would be needed.
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Figure 5. Illustrating Several Different Dye Concentrations
Figure 6. Buoy Placement for Dye Testing Locations.
Twelve buoys were placed just below the seepage berm in the outlet channel. These
would be the places in the downstream channel where the USACE Vicksburg District
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would check for traces of rhodamine with the fluorometer, along with each manhole in
the seepage collection system, and an on-going visual inspection around the dam. Three
gallons of the rhodamine dye were injected into the blanket drain via piezometer 1-70.
This piezometer is located just upstream from the toe and ties directly into the blanket
drain. Approximately, 1.5 million gallons of diluted dye was pumped into the
piezometer, which would move it throughout the blanket drain. For the next three days,
USACE Vicksburg District engineers monitored the area for evidence of seepage. When
three days of inspection resulted in no dye found, the engineers concluded the dye had
either escaped from an unmonitored location or the dye had remained in the blanket
drain. Pumping the manholes would allow the USACE Vicksburg District to see if the
dye had remained in the blanket drain.
PUMP TEST
The first day of the pump test involved Vicksburg District setting up pumps into
manholes 4, 8, and 10. These manholes were pumped until the water level in them was
equal to the tailwater elevation. This was done to achieve a steady state condition for the
pump test. Below are pictures of the setup used to perform this pump test.
Figure 7. Location of Manholes Used during Pump Test (View Looking Downstream
from Dam Crest).
After the elevation in the manholes had been reduced to the tail water elevation, the
pumps in manholes 4 and 8 were turned off. Manhole 10 continued to be pumped into
the weir, so the Vicksburg District could measure how much water was being pumped
from the drainage blanket). The 150 gal/min was the rate at which the steady state
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conditions were achieved in the blanket drain and was determined to be the amount of
seepage entering the blanket. Manhole 10 was pumped for 5 days. Dye was seen after
approximately 5 hours of pumping. A peak concentration of 103 µg/l of rhodamine was
seen approximately 36 hours after starting pumping.
The seepage appears to be contained in the blanket drain until it reaches a certain
elevation, and then exits through the seepage collection system. The 150 gal/min seepage
entering the blanket does not all exit through the weir, leaving some of the seepage to
bypass the collection system through small cracks/fissures in the rock foundation. This
quantity of seepage is so minor that the dye was not pulled through these leaks and
remained within the blanket. By combining the results of the dye test and the results of
the pump test, the Vicksburg District felt confident that Blakely Mountain Dam does not
have major seepage problem.
HYDRAULIC FRACTURE
Another factor driving the concern for seepage and piping of this dam is the suspicion
that Blakely has a hydraulic fracture in the core. The dam was originally constructed
with a closed piezometer system. Over the years this piezometer system became
unreliable, and in 1976 ten open tube piezometers were installed in the dam. These
piezometers indicated a safe saturation profile through the dam. In 1980 falling head
tests were performed on these piezometers. Some of the piezometers started to react
slowly to the falling head test and they were flushed by running a small hose to the
bottom of the piezometer and pumping water through the small hose to flush the clogging
material out of the top of the piezometers. The day after flushing piezometer 6, which has
a depth of 230 feet and is founded at the embankment-foundation contact, it stabilized
approximately 55 feet higher than it originally was. Apparently hydraulic fracturing had
occurred during cleaning.
To try to demonstrate that it was a localized increase in pressure, piezometer 6-A was
installed 20 feet away. This piezometer indicated the same pressure that the old
piezometer indicated prior to flushing. Piezometer 6 still indicates the high pressure that
it indicated after flushing. In 1990 an obstruction was found in piezometer 6-A, and it
was replaced with piezometer 6-B in 1991. Both of these piezometers have always
indicated pressures similar to piezometer 6 before the rehabilitation efforts.
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Figure 8. Increase in Piezometric Grade Line in Piezometer 6.
Figure 8 is a plot of piezometric grade lines through the dam as measured 14 January
1988. The dashed line indicates the change in pressure at piezometer P-6 that resulted
after the attempt to clean the tip of the piezometer by flushing material from the tip up
and out the riser top.
In 2010 automated pressure transducers were placed in the piezometers, giving the
Vicksburg District several readings per day. Normally these piezometers were read on a
quarterly basis. This gave the Vicksburg District a much better way of tracking how
these piezometers react to changes in pool and tail water conditions. Below is a plot of
piezometers 5 and 6. The one-foot spikes noted in the data for piezometer 6 are a result
of the power conduit being pressurized. The power conduit is located in the right
downstream abutment of the dam and is almost 500 feet away from piezometer 6. This
could be a result of the hydro-fracture, yet, there is no way to test this theory, as
piezometers were never read when this tunnel was pressurized. The right downstream
abutment is categorized as being highly weather shale and sandstone. No other
piezometers reacted to the power tunnel being pressurized. This could indicate that the
hydraulic fracture runs parallel to the dam core rather than perpendicular to the lake.
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Figure 9. Plot of Piezometric Gradients in 5 and 6.
WET SPOTS
Lastly there are three wet spots that appear on the downstream face of the dam. These
were originally discovered in 1976. They appear around elevation 493.0, with top of dam
being at elevation 616.0 and dam toe at an approximate elevation of 410.0. Descriptions
of these wet spots have been almost identical in every periodic inspection, with the
largest wet spot estimated as producing less than a cup a minute. These wet spots have
been monitored on a regular basis since they were discovered and have never been
suspected of moving any type of material. It has been suspected that these wet spots
occur due to rainwater being trapped in rip rap and exiting at these points. However, the
wet spots were considered in potential failure modes analysis as a potential defect during
the IES.
ISSUE EVALUATION STUDY
To date, the Issue Evaluation Study has not been completed, so final findings from this
IES are currently unavailable. One major finding that was brought up during the IES
process was the fact that Blakely Mountain Dam is founded on rock that has a very
favorable strike and dip. As seen in the picture below, the rock has a strike and dip
parallel to the dam. This means that any seepage and piping that may occur under the
dam will have to travel a much longer path to exit the dam.
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Figure 10. Geological Conditions
Another benefit is that the zoning of the embankment material would allow for “selfhealing” if a major seepage path existed. Finally, good performance history was a major
indicator that the dam did not likely have serious problems. Since being placed in service
Blakely Mountain dam has been loaded at 90% of its designed PMF. Normal pool
loading is at 85%. The dam has a crest elevation of 616. The pool of record occurred at
elevation 590.1, and the normal pool elevation is at elevation 578.
CONCLUSION
Although there are potential problems with Blakely Mountain Dam, current studies and
investigations indicate that seepage and piping failure modes are unlikely for this dam.
However, additional geotechnical subsurface exploration and hydraulics analysis may be
needed to verify assumptions that were made during study. This dam has been in service
for 58 years, with a very good performance history. The suspected hydro-fracture
occurred over 30 years ago, and other than elevated readings in piezometer 6, no other
issues have been noted from this deficiency. Monitoring the problem has given the
Vicksburg District a better understanding of how the dam is performing. The pump test
indicated that the seepage rate entering the blanket is very similar to the rates measured
just after construction. This indicated that even though there may be a hydraulic fracture
in the dam, it does not travel completely through the core. The IES was a helpful process
to consider potential failure modes that could affect this dam, and to indentify initiating
mechanisms.
ACKNOWLEDGEMENTS
I would like to give special thanks to Noah Vroman, for his input on several key issues
discussed in this paper. The efforts by the Lake Staff and risk cadre team were
invaluable during this process and are greatly appreciated.
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REFERENCES
Blakely Mountain Dam, Ouachita River, Arkansas; Drawings for Construction of
Embankment and Intake Structure, 1950; Spec. No. CIVENG-22-052-50-54, File No. O9/23, Date: 23 DEC 1949.
Department of the Army, Corps of Engineers, Office of Division Engineer, Lower
Mississippi Valley Division, Vicksburg Mississippi; Ouachita River Basin-Ouachita
River Arkansas, Blakely Mountain Dam Drawings, Spec. No. CIVENG-22-052-50-84,
file no. 0-9/23, Date 23 Dec. 1949.
Lake Ouachita, Blakely Mountain Dam, Volume I of II, Periodic Inspection Report No.
9, February 2010.
Waterways Experiment Station, “Blakely Mountain Dam embankment, foundation, and
borrow areas, interim report no. 2”, July 1949.
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