Name and Location
Boone dam seepage remediation project, Tennessee, US.

Client
Tennessee Valley Authority (TVA).

Project Description
The TVA's Boone dam is a multipurpose dam on the South Fork Holston river, on the border between Sullivan and Washington counties, in Tennessee.

In October 2014, a small sinkhole and seepage were discovered at the base of the dam, which indicated a potential risk to the integrity of a section of the dam’s earthen embankment.

The project involves remediating the seepage of water and sediment under Boone dam by building a composite seepage barrier in stages from the crest of the dam embankment downward into the foundation soils, weathered bedrock, and underlying bedrock beneath
the dam.

Stage 1 will involve using low-mobility grouting to install a grout curtain into foundation soils and the epikarst beneath the dam.
Grout will be injected under controlled pressures and flow rates into numerous holes drilled in multiple lines along the crest of the embankment.

The drill holes, spaced a few feet apart, will supply grout to the voids in the karst bedrock to depths potentially up to 91 m or more below the crest of the Boone dam.

Stage 2 of grouting activities will involve the installation of a high-mobility grout curtain deeper into the bedrock and additional grouting support of the epikarst. The working platform will be widened to between 21 m and 30 m, and will be built by placing fill and supportive structures on the upstream and downstream sides of the dam.

The drill holes, spaced a few feet apart, will supply grout to the voids and fractures in the karst bedrock potentially up to 91 m or more below the crest of the dam to fill the voids and thin fractures in the rock beneath the future diaphragm wall.

The grout curtain will be formed as part of stages 1 and 2 and is about 335 m long, although the exact alignment is yet to be determined.

These activities will be initiated in early 2016 and are expected to occur over the first 12 to 24 months of the project.

The third stage of the seepage barrier construction involves the construction of a concrete diaphragm wall.

The TVA will install the concrete wall component through the dam and epikarst, terminating in the underlying competent bedrock. The wall will be built by excavating deep trenches into the dam’s embankment, into which concrete will be poured. The wall will reduce movement of water through the dam’s foundation and epikarst. Construction of the wall may begin before grouting activities have concluded on other portions of the embankment.

The final stage of the project includes restoration of the crest, including possibly removing or covering a portion of the work platform.

Further, rock and/or soil stability berms could be included on the upstream and/or downstream side of the dam as part of the seepage remediation. The TVA will assess the effectiveness of the seepage barrier after it has been installed to determine whether the berm is necessary.

If a berm is built, portions of the fill materials that were used to create the work platform on the dam’s crest might be left in place as part of the berm, rather than removing them.

Value
Initial cost estimates to repair the dam are at between $200-million and $300-million.

Duration
Repairs to the Boone dam will take about five to seven years to complete.

Latest Developments
A draft of the environmental-impact assessment was posted for public comment in October.

Key Contracts and Suppliers
None stated.

On Budget and on Time?
Not stated.

Contact Details for Project Information
TVA public relations, Boone dam, Mary Ellen Miller, tel +1 423 574 8101 or email tvainfo@tva.gov.