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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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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Figure 2. Cross sections of the Land and Marine Dikes


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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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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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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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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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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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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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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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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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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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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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