Citizens Coordinating Council Meeting Silver Lake Pilot Study … · 2018-07-14 · 1 101711160.ppt Citizens Coordinating Council Meeting Silver Lake Pilot Study Results December

1 101711160.ppt

Citizens Coordinating Council Meeting Silver Lake Pilot Study Results

December 5, 2007 December 5, 2007 Pittsfield, MassachusettsPittsfield, Massachusetts

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Silver Lake CCC Meeting Summary

December 1, 2004: Pre-Design Investigation Results

March 30, 2006: Capping Bench-Scale Study Results

September 13, 2006: Capping Pilot Study Work Plan

Tonight: Capping Pilot Study Results

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Pilot Study Objectives

• Assess potential for physical mixing of sediments and isolation layer materials as a result of cap placement

• Evaluate constructability issues related to placement of isolation layer materials in thin lifts

• Evaluate effectiveness of employing geotextile in cap configuration

• Assess potential for water quality impacts during cap placement

• Evaluate physical response of soft sediments to cap and armor stone placement

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Pilot Study Location and ComponentsOne acre study area split into contiguous cells with three cap configurations

ISOLATION LAYERONLY

COMPOSITE GEOTEXTILE/

ISOLATION LAYER

NON-WOVEN GEOTEXTILE/

ISOLATION LAYER

ARMOR STONE LAYER

OUTFALL TO HOUSATONIC RIVER

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Pilot Study Cap Configurations

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Pilot Study OperationsIsolation layer materials slurried on shore w/ lake water, and pumped via floating slurry line to spreader barge

FLEXIBLE FLOATINGSLURRY LINE

OUTFALL TO HOUSATONIC RIVER

SLURRY SPREADER BARGE

MATERIALS AND EQUIPMENT STAGING AREA

MATERIALS PREPARATIONAND SLURRYING

OPERATION

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Geofabric Placement• Non-woven geotextile and

geocomposite fabrics installed from barge– adjacent full length rolls sewn

together and loaded onto barge mounted rollers

– anchored to shore and barge moved away from shore

– sand bags and rebar used to weight fabric and secure to lake bottom

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Isolation Layer Material Placement• Spreader-box distributes slurried

isolation layer material over 20-ft span

• Barge speed and slurry delivery rate monitored/adjusted to achieve approximate 1-inch lifts

• Placement rate later increased for 2-to 3-inch/day trials

• Side discharge used in near shore areas (approx. 2- to 4-ft water depths)

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Cap Construction Along Bank• Bank soils excavated in anticipation of

finished cap elevations

• Isolation layer and armor layer placed

• Armor stone layer extends to elevations approximately 2.5 ft above and below the apparent mean water line

• Gravel habitat layer placed on top of armor below water surface

• Remaining bank areas graded and seeded/mulched

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Monitoring Program

Before During Immediately After

6-Months After

X -- X X-- -- X XX X -- --

-- X X X

-- X X X

X X X X-- X X --

-- X X XSediment/Cap Material Collection

Chemical/Physical Coring

Physical Settling PlatesSurface Water Quality Monitoring

Weekly Water SamplingContinuous Turbidity

Sub-Bottom ProfilingSediment Profile Imaging

Geophysical/Consolidation MonitoringVibrating Wire Settlement

Cells

Monitoring Event

Time Relative to Pilot Study Construction Activities

Lake Bottom ImageryBathymetric Survey

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Collection Pan Data

0

1

2

3

4

5

6

0-0.49 0.50-0.74 0.75-0.99 1.00-1.24 1.25-1.49 1.50-1.74 ≥1.75

Thicknes s (in)

# of

Set

tlem

ent P

ans

• Collection pans placed in path of barge travel to represent thickness of one 1-inch/day “pass”

• Data suggests success in placement of approximate 1-inch lifts

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Sediment Profile Imaging

• Collected prior to and at approximate midway point of cap placement (after 7 lifts)

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During-Construction SPI Without

GeofabricWith

Geofabric

• Isolation layer materials appear fairly homogenous

• Mixing appears limited to the first inch of isolation layer materials in non-geofabric areas

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SPI Based Cap Thickness Estimate

• SPI images collected at 20 locations

– suggests achievement of cap thickness goals at mid-point of construction (after 7 lifts)

6”

6”

6”

6”

6.5”

6” 6”

6”

5.5”

7”

7”

7”

7” 7”

8”

8” 8”

8”

8”

8”

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Cap Thickness Probing Results at Settlement Plates

4.8”/13.3”

5.4”/ 14.0”

6.6”/ 21.3”

5.0”/ NA4.2”/

12.3”

6.0”/ 11.6”

4.3”/ 16.5”

6.1”/ 12.5”

4.8”/ 12.8”

• 9 physical settlement plate locations probed by divers to confirm cap thickness and placement rates at interim/immediately after events

• Cap material well distributed using thin lift placement technique

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Bathymetric Surveying

• Performed prior to; immediately after; and 6-months after construction for comparative purposes

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Pre- & 6-Mo Post-Construction Bathymetry –Net Comparison of Surface Elevations

• In general, data suggests final surface is near pre-construction elevations• No indications of significant movement of underlying materials• No apparent difference related to presence/absence of geofabric

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Post-Construction Sub-Bottom Profile

• Acoustic survey capable of identifying unique layers and material interfaces

• Images provide visual information related to cap surface consistency/relief

• Suggests no apparent difference in performance related to presence/ absence of geofabric

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Geophysical/Consolidation Monitoring

1 2 3 4A

B

CDE

F

G

• 28 individual vibrating wire settlement cells and 9 physical settlement plates installed on top of sediment (above geofabric)

• Monitored during and after construction to assess sediment response (i.e., consolidation) to cap placement

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Vibrating Wire Settlement Cells-10

0

10

20

30

40

50

60

7010.19.06 10.29.06 11.08.06 11.18.06 11.28.06 12.08.06 12.18.06 12.28.06

Set

tlem

ent (

in)

D4

E4

F4

10/25 - Firs t lift; da te of tra ile r movement

11/8-9 - 7 lifts ; approximate ha lf-way point; ha lt placement for SPI and probing

11/13 - Initia tion of placement from bank

11/21 - Fina l lift placed

11/17 - Increased placement ra te

• Confirms general patterns of settlement as indicated by bathymetric mapping and conventional survey at physical settlement plates

• Provided insight into sediment response to initial shoreline capping approach• No apparent difference related to presence/absence of geofabric

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Cap Coring and Sample Collection Program

COMPOSITE GEOTEXTILE& ISOLATION LAYER

ISOLATION LAYERONLY

NON-WOVEN GEOTEXTILE& ISOLATION LAYER

• Performed immediately after, and 6 months after construction• Cores visually observed to evaluate cap thickness and extent of mixing• Cap materials analyzed for PCBs and TOC

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Cap Coring PCB Results –6-Months After

ND

A

NDND

NA

0.14

ND

B

NDND

NA

0.15

ND

C

NDND

ND

2.91

ND

D

NANA

NA

0.15

0.43

K

NDND

ND

ND

0.33

1482.49NDND

0.66

H

ND

1780.15NDND

ND

G

ND

L

NDND

ND

0.07

ND

M

NDND

NA

0.27

ND

88.80.16NDND

NA

FTOP

REM

4-6”2-4”0-2”SED

• 43 of 55 total samples contained no detectable PCBs• 9 of 13 cores have no detectable PCBs below the surface increment• Excluding TOP and 0-2” sample intervals, only one core has a detection in the

remaining interior intervals

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Cap Coring TOC Results

REMAINDER

2- to 4-inch

0- to 2-inch

4- to 6-inch

TOP

• Dry isolation layer material samples pre-characterized for TOC:

– Average: 1.1%

• Sedimentation pans collected outside study area:– Average TOC ~ 4.0%

• Depth weighted average TOC ~ 0.5 %

0.47%

0.51%

0.44%

0.44%

0.58%

Average TOC

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Water Quality Monitoring Program

PILOT STUDY TEST AREA

WATER MONITORINGLOCATION

SILT CURTAIN LOCATION

OUTFALL TO HOUSATONIC RIVER

MON-1

MON-2

MON-3

• Weekly collection at all three locations (PCBs, TSS)• Continuous turbidity monitoring performed• Turbidity >50 NTU at MON-2 or MON-3 triggered sample collection at all three

locations (PCB, TSS)

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Turbidity Results

0

10

20

30

40

50

60

70

80

90

100

110

120

10.10.06 10.20.06 10.30.06 11.9.06 11.19.06 11.29.06 12.9.06 12.19.06 12.29.06 1.8.07

Turb

idity

(NTU

)

0

50

100

150

200

250

300

350

Dai

ly C

ap V

olum

e (c

y)

Daily Isola tion Layer Volume (cy)15-Min. Moving Avg. (Mon-3)15-Min. Moving Avg (Mon-1)15-Min Moving Avg. (Mon-2)

10/25 - Firs t Lift

11/21 - Fina l Lift

11/1 - Mate ria ls Switch

11/14 – Increased Placement Rate

• Few exceedances of 50 NTUs observed at outfall location • Turbidity appears to approach pre-construction levels within 1 month of

completion of placement activities

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Surface Water PCB, TSS Results

11/14 – Increased Placement Rate

Turb

idity

(NTU

) &

TS

S (m

g/l)

PC

B (m

g/l)

0

10

20

30

40

50

60

70

80

10.10.06 10.20.06 10.30.06 11.9.06 11.19.06 11.29.06 12.9.06 12.19.06 12.29.06 1.8.070

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

MON-3 TSS (ppm)

MON-3 PCB (ppm)

15-Min. Moving Avg. (Mon-3)

10/25 - Firs t Lift

11/21 - Fina l Lift

11/1 - Materia ls Switch

• PCBs concentrations in surface water were lower during the pilot study than PCB concentrations found before the study.

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Pilot Study Objectives & Conclusions• Assess potential for physical mixing of sediments and

isolation layer materials as a result of cap placement

– Minimal mixing observed at the sediment/cap interface only• appears limited to the first 1- to 2-inches of isolation layer

material

• where detected, PCB concentrations 1 to 3 orders of magnitude below that of underlying sediment

• Evaluate effectiveness of employing geotextile in cap configuration

– No significant differences noted between geofabric and non-geofabric areas based on physical or analytical data

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Pilot Study Objectives & Conclusions (cont’d)

• Evaluate constructability issues related to placement of isolation layer materials in thin lifts– Generally successful from barge with fabricated spreader-box

– Important to use appropriate near-shore placement methods

• Evaluate physical response of soft sediments to cap and armor stone placement– With exception of near-shore areas, settling observed to be fairly

uniform in time and space

– Majority of locations exhibited settlement within 1- to 2-ft range

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Pilot Study Objectives & Conclusions (cont’d)

• Assess potential for water quality impacts during cap placement

– Short-term increased turbidity observations related to isolation layer placement

– No increase in surface water PCB concentrations observed

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Questions ?