201907_MCD_Sources_Sought_Information.pdf

ALL INFORMATION IS SUBJECT TO CHANGE ONCE THE

PROJECT IS OUT FOR BID.

THIS INFORMATION REPRESENTS A 35% DESIGN

PROJECT LOCATION

OPERATIONAL SCHEMATIC

Presenter Presentation Notes Population at risk Operational schematic

REACHES

Project Office

R ea ch

R ea ch

Control Works/Chena River

FLOODWAY AND DAM

Control Works (1992 Pool of Record)

Presenter Presentation Notes

NEED TO UPDATE

GEOLOGY

• Foundation is gravelly sand/sandy gravel that likely exceeds 1000 ft in thickness with a surficial blanket of silt.

• The transition from sand and gravel to silt includes layers of sand and silty sand, including fine sand with Cu<3.

• The sand and gravel includes gap graded and open work gravel lenses.

• Sloughs from meanders and braids, as well as permafrost, complicate the near surface geology.

Chena River

Tanana River

600 ft

MCD

SLOUGHS AND PERMAFROST LOCATIONS

Mapped Sloughs

Mapped Permafrost

9EMBANKMENT DESIGN

Rock Fill

10GEOTECHNICAL INVESTIGATION – USACE-POA

• 53 additional borings with continuous sampling.

• 5,120 feet drilled.

• Borings ranging from 60 to 130 feet deep.

Results

• Encountered permafrost.

• Fine grained soil layer around 90 to 110 feet below the ground surface.

• Ground truth for Geophysics performed.

• Mix Design samples acquired.

• Better understanding of foundation conditions.

• Encountered zones with cobbles.

11GEOTECHNICAL INVESTIGATION – USACE-POA

Centerline Boreholes

R 3 R4 R5 R 7R6 R8 R9

TYPICAL CENTERLINE BORING

Depth of Cutoff Wall at 464 feet NAVD88

Sands and Gravel

Sand

Water Table

Type III Fill

Semi-Pervious Fill

Type I Fill

Type II Fill

Type II Fill

TYPICAL DOWNSTREAM BORING

Silt

Sand

Sands and Gravel

Depth of Cutoff Wall at 474 feet NAVD88

Water Table

14GEOPHYSICAL INVESTIGATION

Capacitive-Coupled Resistivity (CCR)

• 50 foot depth Electrical Resistivity Tomography (ERT)

• 300 foot depth Ground Penetrating Radar (GPR)

• 50 foot depth

Thawed zone at surface

Permafrost at depth

Station 290+00 Cross-transect

15GEOPHYSICAL INVESTIGATION – ERT

File Name

Station 410+00 to 490+00 (North of Control Works)

Station 230+00 to 390+00 (South of Control Works)

Permafrost

Embankment zone to ~490 feet NAVD88

• High resistivity due to nature of the embankment (Brown to Red) Thawed zone to ~400 feet NAVD88

• Lower resistivity due to thawed coarse grained soils (Blue to Green)

Deeper frozen zone to ~200 feet NAVD88

• High resistivity due to frozen soils

(Brown to Red)

GEOPHYSICAL INVESTIGATION – GPR

GPR

• Harder to distinguish specific features

• Shows excavations and different phases of construction

Results

• Better understanding of permafrost.

• Thawed zone between embankment and deeper permafrost across site.

• See extent of features (Control Works, sloughs, and phases of work).

POTENTIAL FAILURE MODES

24 ft

40 to 80 ft

Silt Layer

Coarse Sand and Gravel Layer

Pooling and Floodway from Chena River

Stability Berm

Upstream

Water Stage <= 525 feet NAVD 88

Continuous Silty sand/fine sand Layer

BoilsBoils

Downstream – City of North Pole

Impervious silt blanket

During periods of high water stages in the Chena River, due to the silty sand and fine sand layer under the silt layer, three modes of failure can occur under the Moose Creek Dam: 1. backwards erosion and piping (horizontal exits), 2. backwards erosion and piping (vertical exits) and 3. contact erosion. These three modes of failure could act independently or together, eventually causing a failure in the dam, leading to flooding and potential risk of loss of life and damage to the cities of North Pole and Fairbanks, and other adjacent communities.

Horizontal Exit

Vertical Exit

Allowable Seepage

Backwards erosion

Potential Failure point

Contact Erosion

Piping

LEGEND

Potential Failure in Dam

Fairbanks

City of North Pole

• Backward Erosion and Piping with Vertical Exit

• Backward Erosion and Piping with Horizontal Exit

• Contract Erosion

TENTATIVELY SELECTED PLAN (BARRIER WALL)

Station Wall Depth (feet) Wall Elevation (NAVD88)

240+00 21 509

240+00 243+00 21 56 509 474

243+00 284+00 56 474

284+00 285+00 56 62.5 474 468

285+00 335+00 62.5 468

335+00 336+00 62.5 66.5 468 464

336+00 406+00 66.5 464

407+00 493+00 62 470

LOW POINT DRAIN

20BARRIER WALL (CONTROL WORKS TIE-IN)

21GEOTECHNICAL INVESTIGATION – USACE-POA

Centerline Boreholes with Barrier Wall Extents

R 3 R4 R5 R 7R6 R8 R9

POTENTIAL PERMAFROST AND BEDROCK AREA

Potential Permafrost Potential Bedrock

EXISTING CREST ALLOWABLE CREST HEIGHT

REDUCTION

Existing Crest (ft) Allowable Crest Height Reduction (ft) Crest Reduction 0 5 Working Platform Width 23.5 34 51.5 61 Min Distance from Drain 8 8

MIX DESIGN

FINDINGS TO DATE

Bureau of Reclamation Design Standards No. 13: Embankment Design

(Table 16.7.11-1)

Dam Name Cement Factor (lb/yd3)

Bentonite Content

(% of cement)

W/C Ratio

Coarse/Fine Aggregate

Ratio

28-day UCS (lb/in2)

Island Copper Mine 239 22 2.78 1.0 220

Wister Dam 350 10 1.0 1.39 598

Meeks Cabin 255 15 1.85 1.0 4151

1Based on average for the mix shown.

Golder 2018 Mix Design (with Relief Well Water) FHWA Design Manual: Deep Mixing For Embankment and Foundation Support

(Bench Scale Testing)

Binder Factor Cement Factor (lb/yd3)

Bentonite Content

(% of cement)

W/C Ratio

Coarse/Fine Aggregate

Ratio

28-day UCS (lb/in2)

10 (lb/ft3) (8.16) 187 18.0 2.03 258

20 (lb/ft3) (13.68) 339 9.0 1.38 582

25 (lb/ft3) (16.08) 405 7.2 1.23 934

30 (lb/ft3) (18.27) 466 6.0 1.13 1199

35 (lb/ft3) (20.29) 521 5.1 1.06 1334

Bureau of Reclamation

• Used for comparison of Mix Design

FHWA

• Used Bench Scale Testing to conduct Mix Design

Additional Mix Designs will be performed in the future.

PROPOSED CONSTRUCTION CRITERIA/CONSTRAINTS

Specification

Max Depth 66.5 feet

Min Width 18 inches

Max Width 36 inches

Length 25,300 feet (4.5 Miles)

Permeability 1E-05 – 1E-06 cm/sec

Unconfined Compressive Strength 200 psi @ 28 days (approach 500 psi long term)

Measure of Acceptance Verification Coring Yes

Insitu Permeability Testing Yes

Unconfined Compressive Strength Yes

Permeability on Samples Yes

Homogeneous and Continuity Yes

Connections to Existing Structures Low Point Drain Tie into Sheet Pile

Control Works Tie into Sheet Pile or have positive contact. No embedment.

26ALLOWABLE TECHNOLOGY FOR BARRIER WALL

CONSTRUCTION AT MOOSE CREEK DAM

Cutter Soil Mix Method Trench Remixing and Deep Wall Method Clamshell/Hydromill Method

Cutter Soil Mix (CSM) or Trench Remixing and Deep wall (TRD) methods shall be utilized to mix the embankment and foundation material with Bentonite and Portland Cement in order to construct a Barrier Wall through Moose Creek Dam.

Clamshell/Hydromill or long stick excavator methods with use of a slurry trench will NOT be utilized for barrier wall construction. Foundation and embankment materials will not allow a Bentonite slurry trench. Loss of slurry could result in failure of the embankment into the trench during construction.