US Army Corps of EngineersBUILDING STRONG®

US Army Corps of Engineers

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Kevin Butler, P.E.Civil EngineerGeotechnical Engineering SectionUSACE Huntington District

6 August 2015

Winfield Locks and Dam: Left Abutment Internal Erosion and Cutoff Wall Remediation

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Project Features

Left Abutment Fishing Access (2008)

•26 ft head differential under normal conditions

AEP-LeasedHydropower Plant

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1937 Left Abutment Plan View

Location of 75 ft long steelsheet pile cutoff

Upstream and downstreamsteel cofferdam cells

Hydropower facility

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1937: Left abutment bank soils excavated during constructionNote 10 to 20 ft thick layer of fine silty sand

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1936: Sandstone blocks used for D/S slope protection

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2008 Fishing Access Reconstruction

Original design plan:► 800-ft long concrete fishing access ramp D/S of

hydropower plant► Stone slope protection below fishing access ramp► Excavation and planting of vegetation above fishing

access ramp► Contractor to work incrementally to avoid slope

failures (no more than 50 ft excavated at a time)

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During 2008 Construction

Contractor cleared/grubbed/excavated 400-ft reach at once Resulted in rapid bank sloughing, localized

slope failures, and piping progression

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2008 Construction

Failed soil mass

Piping of fine silty sand

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2008 Emergency Actions

Subsequent piping and collapse of bank soils

Excavation of failed soilsTrench drain installation

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2008 Emergency Actions

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2008 Emergency Actions

Continued seepage during and following construction

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Seepage Around Left AbutmentFLOW

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Post-2008 Emergency Actions 2008 emergency actions recognized as IRRM

(not long-term solution) USACE hired consultant in 2009 to perform

investigations offer remedial alternatives► Borings and PZs► Dye testing and water quality testing► Weir► Numerical modeling

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Left abutment soil profile

2009 Investigations

•Dye Testing:U/S to D/S connection w/ pool

•Borings & lab testing:20-25 ft thick layer of uniform fine sand

UPPER AQUIFER

LOWER AQUIFER

Cu ranging from 2.3 to 8.1 (median 3.8) – Highly susceptible to piping

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Major findings and recommendations► Continued seepage and soil erosion could eventually

lead to a flanking failure of the left abutment► Suggested increased site monitoring► Four remedial concepts presented

• Seepage cutoff wall• Flatten slope and graded filter• Retaining structure at toe of slope, flatten slope, graded filter• Use walkway as retaining structure and graded filter

2010 Consultant Report

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Observations since 2010

Continued seepage transporting fine silty sand New weir to monitor seepage flows Seepage at weir measured around 22 gpm (not all

seepage captured by weir)

Seepage collection box Transport of fine sand under normal conditions

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Observations since 2010

Enlargement of 3 subsidence features along slope

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Remediation Plan

2014: FERC directed AEP to address left abutment seepage for facility relicensing

AEP hired consultant to design seepage remediation plan – STEEL SHEET PILE CUTOFF WALL PROPOSED

USACE performed reviews (additional analyses) and provided input on proposed plan

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Proposed Cutoff Wall Geometry

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Piping Initiation: Critical Horizontal Gradient Kovacs theory for piping initiation

=

FEM Output

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Piping Progression: Critical Horizontal Gradients

Bligh’s Creep Ratio: For safety against piping, H/L < 1/C, where C is a soil coefficient

For this site: H/L ranging from 0.02 to 0.08 C = 15; 1/C = 0.07 Progressive erosion possible based on creep ratios

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Sellmeijer (2011), Schmertmann (2000), and Hoffmans (2013) theories for piping progression

Piping Progression: Critical Horizontal Gradients

Sellmeijer Equation:

Schmertmann Equation:

Hoffmans Equation:

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Piping Progression: Critical Horizontal Gradients

Summary of Critical Horizontal Gradients for Pipe Advancement

Soil Gradation Band

Evaluation Method

Sellmeijer Schmertmann Hoffmans

iadv iadv iadv

Coarse Band (Cu = 6.13) 0.049 0.046 0.088

Fine Band (Cu = 2.32) 0.022 0.045 0.088

Average Band (Cu = 4.13) 0.033 0.044 0.057

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Calculated Gradients from Site Piezometers

Observed gradients in the range of calculated critical gradients from Sellmeijer, Schmertmann, and Kovacs theories

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Prediction of Required Cutoff Length

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Prediction of Required Cutoff Length

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Selected Plan

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Selected Plan – Completed June 2015

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Post-construction Monitoring

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Post-construction Monitoring

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Additional monitoring:► Lidar scan of bank (enlargement of subsidence

features)► Piezometer trends► Seepage outflow► Dye testing

Post-construction Monitoring

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Conclusions

Provides general awareness of potential problems with seepage through soil abutments

Good case study to compare against current piping progression theory► Agreement between theoretical and observed gradients

Need for more full-scale studies which focus on critical gradients for piping progression

Cutoff completed June 2015► Providing risk reduction based on preliminary data/observations► Continue to monitor site and evaluate data to determine

effectiveness

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Questions?Kevin ButlerGeotechnical Engineering SectionUSACE Huntington District

PH: 304-399-5007Kevin.A.Butler@USACE.Army.Mil