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PILE DESIGN AND CONSTRUCTION CHALLENGES IN PARTIALLY THAWED ALASKAN SOILS

John D. Thornley, P.E.Golder Associates Anchorage, Alaska

INTRODUCTION

Village of Stebbins on St. Michael Island South side of Norton Sound, near Nome

STEBBINS, AK

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SITE STUDY

SITE MAP

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SITE CHALLENGES

Flood elevation required tanks be 7 feet above existing grade

Thawed and frozen portions of site Construction issues

Location, Schedule, and Access

Climate change and its effect on warm permafrost

Permafrost was “warm” at 30 to 31.8°F

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SUBSURFACE CONDITIONS

FOUNDATION DESIGN

Both pad and pile foundations evaluated Permafrost in area is highly degraded

Design piles for frozen and thawed conditions Thawed condition was most conservative design

Use PDA in thawed region to select appropriate embedment depth

Drive piles in both frozen and unfrozen regions to same depth

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FOUNDATION DESIGN (CONT.)

16-inch dia. piles with ½-inch thick walls

Embedment depth of 45 feet gave design FOS of 2.5 to 3

Cutting shoes Upper portion of pile coated

to reduce frost heave potential

FOUNDATION INSTALLATION & TESTING

Contractor used an APE D25-42 diesel impact hammer on setting 4

PDA testing performed during driving and restrucktwo days later on 4 piles

50-foot embedment achieved FOS greater than 3.5

Deemed appropriate to reduce potential for thaw settlement in future

Blow per foot should stay below 84 to avoid tip damage

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PILE INSTALLATION ISSUES

Piles in thawed portion of site had no problems All less than 30 blows/foot

Piles in frozen portion met refusal at 16 to 35 feet with blow counts over 100

Now what???

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15

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25

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35

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45

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30 30.5 31 31.5 32 32.5 33 33.5 34 34.5 35Depth Below Ground (ft)

Ground Temperatures (°F)

Stebbins AVEC Tank Farm Construction Test Holes

Test Hole 1

Test Hole 2

OPTIONS CONSIDERED

Decrease embedment depth Pre-drill holes and drive piles Pre-drill holes, steam thaw, and drive piles Add driving shoes and drive using less energy

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CONCLUSIONS

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ACKNOWLEDGEMENTS

Brent Petrie and Matt Metcalf of AVEC David Myers of STG Robert Miner of Robert Miner Dynamic Testing

Thank You