1129 cofferdam ohio river power.pdf
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Construction of a Cofferdam 1129
CONSTRUCTION OF A 100-FOOT DEEP COFFERDAM IN THE OHIO RIVER
W. James Marold, PE1
ABSTRACT
The American Municipal Power, Inc. (AMP) is in the process of planning, design and
construction of six new hydroelectric projects adjacent to existing USACE Locks andDams on the Ohio River. The projects will deliver about 300 Megawatts of new power to
the grid in the Ohio River Valley. Four of the projects are now under construction with
two others in licensing applications. The four projects under construction are at Locksand Dams at Smithland, Cannelton, and Meldahl all in Kentucky and Willow Island in
West Virginia.
The Cannelton Hydroelectric project was the first to acquire licensing. This paper
describes the construction of the cofferdam that was built to facilitate construction of
Cannelton Hydroelectric project. The cofferdam was completed in September 2010. Thecofferdam for the Cannelton Hydroelectric Project was constructed on the Kentucky side
of the Ohio River just west of the Domtar Paper plant and about 5 miles upstream of
Hawesville, Kentucky. AMP awarded the cofferdam and powerhouse excavationconstruction contract to Kiewit Traylor Constructors A Joint Venture (KTC) on
February 7, 2009. The project was a design-build contract with Mueser RutledgeConsulting Engineers (MRCE) preparing the detailed cofferdam design and KTC
carrying out the construction.
This paper describes the construction of the cofferdam on land and in the Ohio River, the
means and methods including the up to 137 foot deep cement-bentonite slurry wall to top
of bedrock, rock fill placement in the river to form the Marine dike, vibrocompaction of
sand fill material within the rock fill dikes, earthfill, excavation, and emergency floodgate structure. Installation of deep dewatering wells, piezometers, inclinometers and
monitoring points is also discussed. Lessons learned include measures taken during theOhio River flood occurrence in December, 2009.
Figure 1. Site Location Map
1Principal professional engineer and resident engineer for cofferdam contract, MWH Global, Inc.700 West
Wescor Road, Hawsville, Kentucky, 42348, [email protected].
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1130 Innovative Dam and Levee Design and Construction
INTRODUCTION
American Municipal Power, Inc. (AMP) is a nonprofit leader in wholesale power supplyfor municipal electric systems. AMP serves 129 members - 128 member municipal
electric communities in the states of Ohio, Pennsylvania, Michigan, Virginia, Kentucky
and West Virginia and the Delaware Municipal Electric Corporation, a joint actionagency headquartered in Smyrna, Delaware. Combined AMP's member communitiesserve over 565,000 customers. The Cannelton Hydroelectric project has a Federal Energy
Regulatory Commission (FERC) license, and US Army Corps of Engineers 404 Permit.
The project has multiple construction and equipment supply contracts including siteclearing, cofferdam construction, turbine generator equipment, transmission line, gate
equipment, powerhouse cranes, generator step-up transformers, trash rack and log
grabber equipment and reinforcing steel supply. The cofferdam contact was forconstruction of a 100 acre, 108 foot deep cofferdam to allow construction of the
powerhouse in the dry adjacent to the USACE Cannelton Lock and Dam. The project is
subject to FERC, USACE, and a Board of Consultants approval. The normal upstream
pool level is El. 383 and the normal low pool downstream of the USACE dam is El. 358providing an effective 25 feet of hydraulic head for generation of electricity.
The Cannelton cofferdam design includes a 3,641 foot long earthen structure with a
2,400 foot long clay fill dike constructed on stripped natural ground on the landside and a1,250 foot long marine dike composed of zoned rock, sand and clay fill constructed in the
Ohio River founded on alluvial deposits that form the river bottom. The crest of the
cofferdam was set at Elev. 405 Ohio River Datum (OHD), which is 2.6 feet above thecalculated 100 year flood. The bottom of the powerhouse excavation within the
cofferdams interior was excavated to El. 297, 108 feet below the cofferdams crest.
The riverside section of the cofferdam abutted the USACEs dams left abutment and
cellular overflow weir. An existing 60-foot deep sheet pile wall extended 510 feet
southeast into the abutment from the cellular overflow structure. Riprap and derrickstone existed behind this wall to prevent high river flows from cutting a bypass around
the lock and dam.
On February 17, 2009, AMP awarded the design-build cofferdam and excavation contract
to Kiewit Traylor Constructors (KTC), a Kiewit and Traylor Brothers joint venture.Mueser Rutledge Consulting Engineers (MRCE) from New York City prepared the
detailed design and construction documents. MWH served as AMPs owners engineer
and was responsible for reviewing and approving the design and constructions
documents. These documents were also subject to review and approval by the FERC,USACE and a FERC mandated Board of Consultants.
Construction began on May 18, 2009. The site was cleared, grubbed and stripped prior to
start of work. Photo No. 1shows the project site prior to start of construction. Photo No.
2 shows the project site with the completed cofferdam and orientation with the USACELock and Dam. Figure 2 provides cross sections of the land and marine dikes.
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Construction of a Cofferdam 1131
Photo 1. Cofferdam site prior to start of construction with USACE Dam at left
Photo 2. Completed cofferdam and excavation with USACE Dam on the left
Marine dike
Clay fill dike
Overflow weir
Emergency Flood Structure
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1132 Innovative Dam and Levee Design and Construction
Figure 2. Cross sections of the Land and Marine Dikes
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Construction of a Cofferdam 1133
COFFERDAM CONSTRUCTION
Cofferdam construction on the landside commenced with construction the lower portionof the clay dike up to El. 400 to provide a working platform for construction of the
cofferdams cement-bentonite cut-off wall. Concurrently work began on the marine dike
with the underwater placement of rock fill sections along the dikes interior and exteriortoes. The river bottom had been surveyed by side-scan sonar in 2007. This survey
formed the basis of the bid for the cofferdam contract. Recognizing that river bottom
conditions can vary from season to season, a contract allowance was provided forsupplemental rock fill and removal of unsuitable foundation material should the river
bottom topography be found to be lower and/or softer than anticipated. A 2009 side-scansurvey indicated that the river bottom contours were in fact lower than the 2007 contours
and therefore part of the allowance was used to provide additional rock fill.
The rock fill was a graded limestone quarry rock from 46 inch top size to 3 inch
minimum size with a 50% size of about 22 inches. This material was obtained from
Mulzer Crushed Stone, Inc. (Mulzer) quarry located in Leavenworth, Indiana. The rock
fill was transported by barges from the quarry and off loaded into the river bottom by twocrane barges, one placed material using crane with a three leaf grapple and the other used
a long reach excavator with a large bucket. A visual footprint of the width of the rock fillon the river bottom in the crane cabs was used by crane operators to locate placements of
rock fill. Photo 3 and 4 provide an aerial view of the rock fill dike progress and a land
side view of the barges placing the rock fill. Photo 5 is a close up of the bucket and thegrapple used to make the placements.
The two parallel side by side rock fill sections of the marine cofferdam (one landside andone riverside) were constructed first starting from the rivers edge just downstream of the
existing dams cellular overflow weir and further downstream where the marine
cofferdam reconnects with the landside cofferdam. Photo 3 shows the rock fill sectionsbeing placed in the river from the downstream river bank. The rock fill sections were
about 1,200 feet long with 1.75H: 1.0V exterior side slopes and 1.5H: 1.0V interior side
slopes. The top elevation of the rock fill dikes was El. 365, an elevation selected becauseit was above typical water levels during the summer months.
Once a length of rock fill dike was completed, a three foot thick bedding materialconsisting of 4 inch to inch size graded crushed rock material was placed by clam
bucket on the interior slopes of the rock fill to smooth the interior faces of the section.
Then filter fabric was draped over this transition material to prevent the sand that wouldform the center of the marine dike from migrating into voids in the rock fill. Sand
dredged by Mulzer from the river bottom and sluiced into barges near Troy, Indiana wasthen brought to the site where a clam bucket was used to place the sand between the rockfill sections, taking care not to damage the filter fabric.
Once the sand fill between the rock fill sections reached El. 365, a 70 foot long vibratoryprobe with an eccentric moment of 3400 inch-pounds operating at 90 rpm was used to
compact the sand by probing at a triangular spacing of 17.5 feet on center. Care was
taken to make sure the probing extended no closer than 3 feet away from the filter fabric.
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1134 Innovative Dam and Levee Design and Construction
The specification required that the sand was densified so that the angle of internal friction
was 34 degrees or higher. Densification was verified by cone penetration testing (CPT)probing. Subsequent to underwater sand placement and compaction, sand fill was placed
and compacted in the dry up to El. 390.
A geogrid composite, consisting of filter fabric and reinforcing geogrid was placed on theexterior surface of the marine dikes compacted sand fill and covered with two foot thick
24 to 1 graded riprap. The cement-bentonite cut-off wall though the marine dike wasconstructed from El. 390 though the sand fill and sandy alluvial river bottom foundation
material down to the top of rock. Once the cut-off wall was complete, the final 15 feet of
the marine embankment was constructed of compacted clay fill. The exterior side of theclay fill was overlain by geogrid composite and riprap. The slopes of the compacted sand
and clay fills were 2.5H: 1.0V on both exterior and interior slopes.
Photo 3. Rock fill sections placed in river for the cofferdams marine dike
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Construction of a Cofferdam 1135
Photo 4. View of excavator and grapple barges placing rock fill on river bottom
Photo 5. View of bucket (left) and grapple (right) used to place rock fill from barge
CUT-OFF WALL CONSTRUCTION
The cofferdams cement-bentonite slurry cut-off wall was required to be at least 31.5
inches thick with an average coefficient of permeability of 110-6
centimeters per second
and an unconfined compressive strength of at least 20 psi after the slurry sets up at about14 days. The cut-off wall was required to extend to the top of rock, which was present at
a depth of approximately 130 and 140 feet below the riverside and landside work
platforms, respectively. A key into rock was not required. However, excavation ofweathered rock and incidental scarifying the top of rock was anticipated to occur while
excavating and cleaning the bottom of the slurry trench.
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1136 Innovative Dam and Levee Design and Construction
The cut-off wall was constructed in panels using a hydromill and a crane-mounted 20 ton
clam bucket. Photos 6, 7 and 8 show the hydromill and clam bucket used. The hydromillconstructed the 10 foot wide primary panels and the clam bucket constructed 12 foot
wide secondary panels. The clam bucket excavated at least 6 inches into each of the
adjoining primary panels to obtain a good connection between panels. Control of the
panel locations for the hydromill was by use of a Defined Measurement System (DMS)that measured verticality and rotational deviation in the x and y axis. Verticality and
rotational deviation of the clam bucket was measured using a Teraban device. The
orientation of each panel was plotted in three dimensions. The plots of adjoining panelswere overlaid to verify overall continuity of the wall. Continuity between the primary
and secondary panels was defined as maintaining a minimum thickness of 10 inches in
any portion of the wall.
The cut-off wall was constructed from a working platform about 40 feet wide on thelandside and about 80 feet wide on the riverside. KTC placed concrete guide walls along
the entire length of the wall to serve as a guide for the hydromill and the clam bucket.
The guide walls were about 2.5 and 3.5 feet deep. In advance of panel construction with
the hydromill and clam bucket, an excavator was used to excavate the upper 20 feet undercement-bentonite slurry. Throughout slurry wall construction, the trench was kept full of
slurry to within 2 feet of the ground surface and at least 3 feet above groundwater at all
times.
Photo 6. Hydromill used to excavate primary cut-off wall panels
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Construction of a Cofferdam 1137
Photo 7. View of the cutting teeth of the Hydromill
Photo 8. Liberhh Crane and 20 ton clam bucket used to excavate secondary panels
To verify that the primary panels had reached the top of rock, the slurry pumped to the
de-sander was monitored as the hydromill was approaching the expected top of rock
excavation. Top of rock was determined to have been reached when continued scrapingat the base of the trench with the clam produced no significant rock pieces, just rock
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1138 Innovative Dam and Levee Design and Construction
chips. Depth soundings were taken at each panel excavated to provide an accurate record
of the cut-off wall depth. The depth of adjoining panels was compared to confirm noabrupt changes occurred in elevation along the bottom of the wall.
The cut-off walls cement-bentonite slurry was mixed at an on-site batch plant that could
mix two 6 cubic yard batches concurrently. Photo 9 shows the batch plant used. A sixcubic yard mix consisted of 1,150 gallons of water, 480 pounds of bentonite, 960 pounds
of slag cement, and 0.64 gallons of Aquafix, a proprietary chemical provided by Liquid
Earth that controlled the set time of the self-hardening slurry mix. The unit weight of themix was between 66 to 68 pounds per cubic foot. Samples were taken at one third, two
thirds and near the bottom depth points for each day panels were constructed. An on-sitelaboratory tested samples retrieved from the trench for coefficient of permeability and
compressive strength. The average k value was 6.0210-7
cm/sec based on 125 samples
tested at an average age of 102 days. The average unconfined compressive strength was43.9 psi based on 541 samples tested at an average age of 59 days.
Photo 9. Batch Plant used to mix the Cement Bentonite Slurry Wall
CUT-OFF WALL PRODUCTION
Work on the cofferdam cofferdams cement-bentonite cut-off wall began on August 31,
2009, starting on the landside. KTC worked on 8 hour day shift from August 31 toSeptember 8 and then began two12 hour shifts 7 days per week on September 9, 2009. A
half day was utilized each week for maintenance of the equipment. Cut-off wallconstruction was completed on December 30, 2009. The production was about 23 panels
(230 lineal feet) per week.
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Construction of a Cofferdam 1139
DEWATERING WELLS AND INSTRUMENTATION
KTCs dewatering subcontractor Moretrench, Inc. installed 20 deep dewatering wells.
The wells were spaced in a ring around the inside of the cofferdams crest. Photo No. 10
shows the installation rig. The 12-inch diameter wells were taken to the top of rock,
which was found between about Elev. 249 and 260. The pumps were 15 HP, 250 gpmcapacity stainless steel submersible pumps capable of pumping 140 feet with a 4-inch
diameter Schedule 80 PVC pipe connected by a 4-inch diameter flexible hose to an 18-inch diameter HDPE header pipe. Each well has a Rossum Sand Tester port, a check
valve, flow meter with totalizer, and an isolation butterfly valve. Backup generators are
on-site in the event of a power loss.
Photo 10. Drilling dewatering well
Cofferdam instrumentation consists of 18 vibrating wire piezometers installed in holes
drilled to the top of rock. A one inch diameter standpipe was included in each hole
drilled to allow manual measurements with a water level indicator if needed.
All piezometers had solar panels and are connected to an automated data acquisitionsystem that uploads data to a secured website where it can be assessed by project staff
and regulators. The piezometers are positioned to provide piezometric pressure both
inside and outside the cofferdams cut-off wall. The piezometers indicate that there is ahead drop across the cut-off wall of between 70 to 85 feet. This indicates that the cut-off
wall is performing as intended.
Other instrumentation included inclinometers and survey monuments on the USACE
dam, along the cofferdams crest and the USACEs overflow weir. These readings have
indicated horizontal and vertical movements of up to about 1 inch during the constructionof the cofferdam,
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Construction of a Cofferdam 1141
EXCAVATION
The excavation for the powerhouse was staged with cofferdam construction to enable
excavation of impervious material for use as cofferdam fill, while maintaining the
continuity of the dams water retaining elements and cutoff through the left abutment
until the cofferdam was in place and they were redundant. Initially, excavation wasallowed within two areas within the cofferdams interior, one upstream of the USACE
dams existing 550 foot long sheet pile cut-off wall and one downstream of the cut-offwall. Until the cofferdams cut-off wall was complete, the excavation in these two areas
was limited in depth and excavation was prohibited along the dams sheet pile cut-off.
Once the cut-off wall was completed around the upstream side of the cofferdam, the
USACE sheet pile wall could be removed and the powerhouse excavation could proceed
across the entire area of the cofferdam. Concurrently, the installation of the dewateringwells and instrumentation was ongoing, the cofferdams marine dike was completed and
the flood control structure was constructed. The entire cofferdam was completed to El.
405 by April 2010, and the dewatering system was activated and dewatering wellpumping began. At that time, the mass excavation also began with eight CAT double pan
scrapers pulled by Challenger MT tractors working two 10-hour shifts. The excavation
was completed by June 15, 2010. The excavated material was spoiled on-site.Cofferdam interior excavation slopes were 2.5H: 1.0V except at the upstream and
downstream sides where 6.0H: 1.0V slopes were excavated that will ultimately form the
invert of the approach and tailrace channels. A 40 foot wide haul road was alsoincorporated into the downstream slope for access by the powerhouse and appurtenances
contractor.
CUT-OFF WALL PERFORMANCE
The 20 dewatering wells drilled within the cofferdam pumped out infiltration through thecofferdams cement-bentonite cut-off wall and infiltration from the rock foundation. Thewells were installed at or near the top of rock. The wells have the capacity to pump out
5,000 gpm. Dewatering of the interior of the cofferdam began on March 15, 2010 with
the wells pumping 4,100 gpm. On April 8, 2010, the dewatering wells were pumpingabout 600 gpm and the groundwater table had reduced to Elev. 290 to 300. Currently the
wells are pumping a total flow of 90 gpm and the piezometers are indicating El. 276 to
278 below the bottom of the powerhouse excavation. While some seepage may be
making its way through bedrock and the cement-bentonite cut-off wall, these amounts areminimal. The results indicate that the cofferdams cut-off wall met and exceeded the
design expectations.
CONSTRUCTION ISSUES
The project site previously served as a staging area during construction of the USACEs
lock and dam, which was construction in 1965. The exploration program performed did
not reveal the extent of material buried or left embedded at the site. During thecofferdam and excavation contract for the Cannelton Hydro electric project, sheet piling,
concrete, pipes, and other buried debris was uncovered. These obstructions required two
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1142 Innovative Dam and Levee Design and Construction
realignments of the cofferdams cut-off wall, pre-probing and removal of obstructions,
which required the reallocation of resources intended for other work.
The guide walls constructed to align the equipment for the cut-off wall sloughed into the
slurry on a few occasions shortly after cut-off wall activities commenced. This was
thought to have resulted from slurry levels not being maintained through the eveninghours. In one instance, guide walls and adjacent sand sloughed in along a 20 feet length
of the slurry trench. There was a concern that this may result in a sand window withinthe cut-off wall. Subsequently, a continuous sample drill rig was used to obtain samples
of the panel where the settlement and slippage occurred. The drill was not able to stay
within the slurry wall but a subsequent three dimensional probe did verify that the wallwas intact for at least 75 feet before going outside the wall. As noted above, the wall has
performed well with no areas of elevated piezometric levels or seepage.
The cofferdam construction proceeded on schedule until January 25, 2009 when the Ohio
River was in flood stage. The low area of the cofferdam construction along the riverside
where it was at about El. 390 (15 feet below the design crest of El. 405.0) and the floodcontrol structure had yet to be finished. The National Weather Service and USACE
prediction indicating that the river would rise to El. 389 by January 29, 2009 with
unknown fluctuations thereafter. After evaluation of the risk, KTC and MWH decided toconstruct a temporary 5-foot high clay dike at a reduced width along the riverside
cofferdams low area to protect the cofferdam and prevent flooding of the cofferdams
interior. This emergency dike was constructed in four days. Winter weather conditionsand the schedule prevented drying of the clay, which was well above optimum moisture
content. These constraints minimized the ability to compact this material. Ultimately,
this temporary dike was only subject to about 1 foot of head and performed satisfactory.
LESSONS LEARNED
The following lessons were learned during the construction of the cofferdam:
During the placement of the grouted rip rap for the emergency flood controlstructures spillway chute, the grout with -inch aggregate was used initially with
small vibrators to get the grout completely around the large stone to the bottom of
the filter fabric placed below the rip rap. Subsequently, inch aggregate wassubstituted because there was concern that the grout was not getting through to
the bottom of the riprap.
When it was imperative that the marine section of the clay dike continue duringwinter months when drying of the impervious fill could not be accomplished viaspreading and disking, KTC chose to add fly ash to the impervious fill. A StehrSBF 24/16 Soil Stabilizer mixed the fly ash into each spread lift of the clay prior
to compaction. Standard Proctor tests were performed on the mixed material to
provide a compaction curve for verification of compaction with field density tests.
The design of the dewatering wells provided pumps that would remove up to5,000 gpm. At the beginning of the dewatering, about 4,000 gpm was being
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Construction of a Cofferdam 1143
pumped but within three weeks this reduced to about 500 gpm. Subsequently
with the extremely lower (90 gpm) flows, it became necessary to operate fewerpumps and monitor very closely those pumps operating to assure that the pumps
did not run out of water and seize the motors. The low infiltration rate from the
marine cofferdam may be partially the result of the hydromill excavating into
bedrock for some time before rock chips reached the desander through the hosethat was several hundred feet long from the hydromill. Rock chips were used to
identify when the hydromill was at the top of bedrock and the panel was
complete.
ACKNOWLEDGEMENTS
The author would like to acknowledge AMP and their member municipalities for their
initiative to develop this and other similar projects in the Ohio River valley and for theirvision to undertake this work for renewable energy. I would also like to thank KTC and
MRCE for their efforts including the dedication of resources, equipment and manpower
to the project to complete the project on time in spite of several issues which affected theschedule. KTCs timely and successful installation of the cut-off wall was a big factor
that allowed the project to reach completion on time and ready for the powerhouse
construction contract.
REFERENCES
Kiewit Traylor Constructors A Joint Venture, Final Specifications, June 24, 2010.
Kiewit Traylor Constructors A Joint Venture, Final As-Built Drawings, July 6, 2010.
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