field testing to determine realistic geotechnical parameters · 2015. 12. 29. · realistic...

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Field Testing to Determine Realistic Geotechnical Parameters

J. Ryan ShaferNichols Consulting Engineers

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EERI TechnicalSeminar Series

Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations

Agenda

Why direct measurement methods might be appropriate?Background on plate load test (PLT)Some limitations to the PLTA case history on the UC Berkeley Campus for use of the Footing Load Test (FLT)Concluding remarks

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Applicability

Projects increasingly using more demanding modeling that require greater accuracy.Highly variable subsurface conditions can make it difficult to estimate foundation stiffness and bearing capacities.

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Plate Load Test

Plate Size Typically 1’X1’

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Good Applications of PLT

Fairly uniform subsurface conditions within the influences of planned foundations (2B to 3B)Plate is seated at expected foundation levelsProgram should include variable plate sizes and depths for extrapolation to larger footing sizes and different depthsEconomical approach for direct measurement method

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Limitations of PLT

Limited stress influence due to small plate sizesSites with variable or stratified subsurface conditions (i.e. compressible layer just below stress influence of PLT)Does not fully capture real footing behavior

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Example of Stress Influence

0

5

10

15

20

0 1000 2000 3000 4000 5000 6000

Stress (psf)D

epth

(ft)

5X5 FTG

1X1 FTG

2B

2B

STIFF CLAY

ROCK

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Case History

UC Berkeley Stanley Hall Biosciences Facility

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Seismic Setting

Hayward Fault

Liquefaction

Landslides

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Stanley Hall

UC Berkeley Campus

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Stanley Hall Looking East

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Site Conditions

Fill and colluvium over Franciscan Bedrock (shale and graywacke sandstone)Shale bedrock predominantly in the east, very fractured and weathered, and generally soft rockGraywacke sandstone in the west becoming hard and more competent with depthOld Strawberry Creek stream channelShallow groundwaterAbout 500 feet to Hayward Fault

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Geologic Mapping of Bottom of Basements

?

?

?

? ? ?

S O U T H W A L L

DONNER LAB

GRAYWACKESANDSTONE

BRECCIATED

MUDSTONE

AND SHALE

INTERBEDDEDSANDSTONE AND SHALE

CARBONACEOUS

SHEARED AND

SERPENTINITE

SHALE WITH

Colluvium

Legend:

Mudstone/Shale

GraywackeSandstone

Fill

South Wall

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Geologic Mapping

T

P

390.00

380.00

370.00

360.00

350.00

340.00

330.00

320.00

310.00

300.00

T

T

T

T

T

T

P

T

T

T

T

T T

T

T

T

T

T

T

ELEVATIONIN FEET

SOUTH WALL ELEVATION

ColluviumLegend: Mudstone/Shale GraywackeSandstone

Fill

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Geotechnical Uncertainties -Franciscan Rock

Difficult to estimate engineering properties of highly heterogeneous rock based on traditional boring data and laboratory tests

Subgrade modulus (foundation stiffness)Bearing capacity

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Foundation Stiffness Relationships

)5.1

5.0(1

mm

Bkk v

s+

= 21 )

21(

BBkk vs+

=Cohesive Soil Cohesionless Soil

Relationships Used (NFEC Design Manual 7.02)

FEMA 356 Relationship

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Bearing Capacities

Conservatively based on historical, presumptive, and/or empirical valuesDifficult to assign material properties to Franciscan sandstone and shale that can be incorporated into traditional bearing capacity relationships

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Implications?

What is a conservative subgrade modulus?Actual modulus is stiffer may result in damage to shear wall elementsActual modulus is softer may result in damage to building frame

Conservative bearing capacities that will likely perform well but may be costing the project with larger footings with increasing seismic demands

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Why the Footing Load Test (FLT) was conceived?

Large seismic demandsDifficult to determine the stiffness and bearing capacity for heterogeneous Franciscan sandstone and shale Many buildings on campus are founded in Franciscan rockImprove design for future campus buildings with direct measure of stiffness and bearing capacity of full size footings founded in Franciscan rock

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The Plan

Construct full size square footings (2.5, 4, and 6 feet) founded in sandstone and shale in basement levels of the active excavation of Stanley Hall

Perform load tests to cyclically impact footings using a Pile Load Tester (PLT) with loads up to 800 kips

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FLT Location Plan

Western Footings

(Sandstone)

Eastern Footings (Shale)

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FLTs Amidst Construction

25Test Apparatus

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25,000kg mass on heavy coiled

springs

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6’X6’4’X4’

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Load Versus Displacement (2.5' x 2.5')

-5.5

-4.5

-3.5

-2.5

-1.5

-0.5

0 100 200 300 400 500 600 700 800Load (Kips)

Dis

plac

emen

t (in

ch)

Maximum Displacement

Residual Displacement

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Load Versus Displacement (4' x 4')

-1.5

-1

-0.5

00 100 200 300 400 500 600 700 800

Load (Kips)D

ispl

acem

ent (

inch

)



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Load Versus Displacement (6' x 6')

-0.5

-0.4

-0.3

-0.2

-0.1

00 100 200 300 400 500 600 700 800

Load (Kips)D

ispl

acem

ent (

inch

)



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Summary of Load Tests

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

00 20 40 60 80 100

Bearing Pressure (ksf)

Max

imum

Dis

plac

emnt

(inc

hes)

FLT TEST 2.5x2.5 Footing

FLT TEST 4x4 Footing

FLT TEST 6x6 Footing

Original Recommendation: Kb =(900 / B) kcf for 2.5x2.5 Footing

Original Recommendation: Kb =(900 / B) kcf for 4x4 Footing

Original Recommendation: Kb =(900 / B) kcf for 6x6 Footing

New Recommendation: Kv1=3000kcf, Disp.<0.5in; Kv1=750 kcf,0.5<Disp.<1.5; for 2.5x2.5 Footing

New Recommendation: Kv1=3000kcf, Disp.<0.5in; Kv1=750 kcf,0.5<Disp.<1.5; for 4x4 Footing

New Recommendation: Kv1=3000kcf, Disp.<0.5in; Kv1=750 kcf,0.5<Disp.<1.5; for 6x6 Footing

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What Was Gained?

Original Recommendation: Kv1=900 kcfAfter FLT: Kv1=3000 kcf (<1/2 inch), 750 kcf(1/2 to 1.5 inch), plastic after 1.5 inchOriginal Recommendation: Ultimate Bearing Capacity (UBC) = 26,000 psfAfter FLT: UBC = 30,000 to 70,000 psfCost Savings to Next Campus Building

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Concluding Remarks

As seismic modeling and demands increase there is a shift towards ultimate load vs. displacement.Provide more realistic geotechnical recommendations because often it is not clear what is conservative.Direct measurements methods such as the PLT or FLT can help achieve these goals.

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THE END

field testing to determine realistic geotechnical parameters · 2015. 12. 29. · realistic...

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