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Evaluation of Rock Socket Design Evaluation of Rock Socket Design from Load Tests in the Southeastern U.S. W. Robert Thompson, III, P.E., D.GE W. Robert Thompson, III, P.E., D.GE Dan Brown and Associates, PC Dan Brown and Associates, PC M t Al b M t Al b Montgomery, Alabama Montgomery, Alabama

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Evaluation of Rock Socket DesignEvaluation of Rock Socket Design from Load Tests in the Southeastern U.S.

W. Robert Thompson, III, P.E., D.GEW. Robert Thompson, III, P.E., D.GEDan Brown and Associates, PCDan Brown and Associates, PC

M t Al bM t Al bMontgomery, AlabamaMontgomery, Alabama

Introduction

• Research project sponsored by ADSC SE Chapter

• Rock‐socketed drilled shafts

• Aim to improve:• Design methods

• Cost‐efficiency 

• Involve designers, contractors, owners (DOTs)

Current Practice in Southeast• Allowable Unit Base Resistance of 60 to 120ksf (maybe up to 200ksf)

• Design for base resistance only 

• Inspected by probe hole drilled in base of socket

• If seams found, excavation continues until inspector satisfied “sound rock” is below shaft

• Lawrenceville – Rock defined as Rock Auger Refusal = 2in/5min with LLDH rig2in/5min with LLDH rig

Test Program

• Two sites tested to date

• Nashville, Tennessee• Hard Limestone (sometimes solutioned)

• Birmingham, Knoxville, Chattanooga

• Lawrenceville, Georgia (Metro Atlanta)M t hi k (G i S hi t t )• Metamorphic rock (Gneiss, Schist, etc.)

• Piedmont Formation – Georgia to Virginia

Test Program

• Thorough Subsurface Explorations

• Two test shafts at each site

• O‐Cell testing device

• Inspected by local geotechs

• Conducted field day with local ASCE/G‐I Technical GroupGroup

Nashville Site

00 10 20 30 40 50 60 70 80 90

RQD, % Rock Core RQD

-15

-10

-5

Sock

et

Test Shaft 2 RQD

Test Shaft 1 RQD

RQD ‐Nashville

30

-25

-20

ow T

op o

f S

-40

-35

-30

Dep

th B

el

qu = 5,000 to 20,000 psi

-50

-45, p

b l l k ( f )Construction ‐ Nashville

• 48in core barrel to excavate limestone sockets (16ft)

• Mechanical cleaning only

• Inspectors consensus:

• TS 1 had 3in to 4in soil seam 19in below base• TS 1 had 3in to 4in soil seam 19in below base

• Shaft should have been extended

• TS 2 no significant seams• TS 2 no significant seams

• Typical conditions sought

B h h f d d ddi i l l i• Both shafts needed additional cleaning

• Significant concrete overrun in TS 2

Unit Side Resistance ‐ Nashville

• Test Shaft 1 best for side resistance• Fully mobilized side resistance vs Test Shaft 2

• No overrun on concrete to complicate interpretation

• Fully mobilized at small displacement: 1 % of dia. (~0 2in)( 0.2in)

Unit Side Resistance ‐ Nashville

25

Unit Side Resistance vs Normalized Upward O-Cell Displacement

~ 0.2in

20

ce, k

sf

Test Shaft 1

10

15

e R

esis

tanc Nominal Dia = 48" Nominal Dia = 52.5"

5

Uni

t Sid

e

00.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2%

Displacement/Diameter, %

Unit Base Resistance ‐ Nashville

• Displacements about 1% of loaded area(B 39i f TS 1 B 29i f TS 2)(B = 39in for TS 1; B = 29in for TS 2)

I ti i di t d il 19i b l TS 1• Inspections indicated soil seam 19in below TS 1(10% of B approx. 0.5B below shaft base )

• Maximum unit base resistanceTS 1 500 k f• TS 1 = 500 ksf

• TS 2 = 1250 ksf

Unit Base Resistance ‐ Nashville

Bearing Pressure, ksf

Unit Base Resistance vs Normalized Downward O-Cell Displacement

-0.25

00 250 500 750 1000 1250

Test Shaft 1

Test Shaft 2

1

-0.75

-0.5

pl/D

ia, %

Test Shaft 2

E=630ksi (0.5%)E=335ksi (0.5%)

-1.5

-1.25

-1

Dis

p

E=536ksi (1%)

E 235k i (1%)~ 0.5in (D =39in) ~ 0.3in (D =29in)

-2

-1.75E=235ksi (1%) 0.5in (D 39in)

( )qBs

2179.0 νρ −⋅=

Soil seam beneath TS 1?

Es 79.0ρ

Implications ‐ Nashville

• “Sound Rock”: conditions similar to Test Shaft 21 2 ll ½ i h thi k• 1 or 2 small seams <  ½ inch thick

• Allowable unit base resistance = 500ksf

• “Fair Rock”: conditions similar to Test Shaft 1• soil‐filled seams up to 10%B, at depths > ½ Bsoil filled seams up to 10%B, at depths > ½ B

• Allowable unit base resistance = 200 ksf

• Displacement approx. 0.5%B

(1/4 to 3/8 inch for B = 4 to 6 ft)( / / )

Implications ‐ Nashville

• Side resistance not factor in “Sound Rock” for typical designs (when socket not needed for lateral)designs (when socket not needed for lateral)

• Utilizing side resistance in “Fair Rock” conditions may• Utilizing side resistance in  Fair Rock  conditions may be prudent • When socket > 10ft to “find” base resistanceWhen socket > 10ft to  find  base resistance

• Use lower base resistance + side resistance

Lawrenceville Site

Construction ‐ Lawrenceville

• TS 1 – Test side and base resistance of 40.5in rock socketsocket• Started with core barrel, completed with rock auger

• Penetration = 4 to 6 5in/min – not “Rock Auger Refusal”Penetration = 4 to 6.5in/min  not  Rock Auger Refusal

• TS 2 – Test base resistance at “Rock Auger Refusal”TS 2  Test base resistance at  Rock Auger Refusal  with 66in socket and 19in O‐cell• Drilled with rock auger

• Two distinct zones of PWR

• Penetration = 3in/5min – not “Rock Auger Refusal”

Rock Core RQD ‐ LawrencevilleRQD %

5

100 10 20 30 40 50 60 70 80 90 100

haft

1)

RQD, %

Test Shaft 1 Test Shaft 2

5

0

5

of S

ocke

t (S

haft

2), (

ft)

15

-10

-5

Belo

w T

op o

or P

WR

(S

25

-20

-15

Dept

h

-25

qu = 7,000 to 11,000 psi

Test Shaft 1 ‐ Lawrenceville

Test Shaft 2 ‐ Lawrenceville

Unit Side Resistance ‐ Lawrenceville

50

55

Unit Side Resistance vs Normalized Upward O-Cell Displacement

Sh ft 1

35

40

45

stan

ce, k

sf

PWR - Shaf t 1 - Nominal Dia = 42"

Shaft 1 ~0.2in

Shaft 1 ~0.6in

20

25

30

nit S

ide

Res

is ROCK - Shaft 1 - Nominal Dia = 40.5"

PWR - Shaf t 2 (19-35f t) - Nominal Dia = 66"

PWR - Shaf t 2 (35-42.5) - Nominal Dia = 66"

5

10

15

20

Un

0

5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Displacement/Diameter, %Shaft 2Shaft 2 ~0.3in

U it Sid R i t N li d U d O C ll Di l t

Unit Side Resistance ‐ Lawrenceville

50

55

Unit Side Resistance vs Normalized Upward O-Cell Displacement

35

40

45

stan

ce, k

sf

PWR - Shaf t 1 - Nominal Dia = 42"

ROCK Shaft 1 Nominal Dia = 40 5"

20

25

30

nit S

ide

Res

is ROCK - Shaft 1 - Nominal Dia = 40.5

PWR - Shaf t 2 (19-35f t) - Nominal Dia = 66"

PWR - Shaf t 2 (35-42.5) - Nominal Dia = 66"

5

10

15

Un

0.3in

0

5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Displacement/Diameter, %

Unit Base Resistance ‐ Lawrenceville

~ 1.6in

~ 2in( )E

qBs

2179.0 νρ −⋅=

TS 2 in softer rock

Implications ‐ Lawrenceville

• Terminated in material that did not meet local criteria for rock: “rock auger refusal”criteria for rock:  rock auger refusal .

N i l/ lti t it b i t i ifi tl• Nominal/ultimate unit base resistance significantly greater than current design limits.

• High nominal/ultimate unit side resistance can be achievedachieved.

Implications ‐ Lawrenceville

• Formed committeeAtl t titi ADSC S th t Ch t d• Atlanta area practitioners, ADSC Southeast Chapter, and the researchers

• Reviewed results local practiceReviewed results, local practice

• Identified key considerations for applying results

• Developed recommended design values• Developed recommended design values

• Developed specific criteria to be met• RQD penetration rate inspection criteria• RQD, penetration rate, inspection criteria

Conclusion

• Tests demonstrated:

• High nominal base and side resistance• High nominal base and side resistance

• Higher design values can easily be achieved

L th “ f t” diti d t• Less than “perfect” conditions exceed current design values

• Design guidelines are suggested to provide more economical use of drilled shaft foundations in theeconomical use of drilled shaft foundations in the two markets.  

Conclusion

• Site‐specific criteria are provided to apply test results.

• ALWAYS have a thorough site investigation

• Inspection program to confirm the findings of the site investigation. 

Conclusion

• Reports available:E P di• Expo Proceedings

• http://danbrownandassociates.com/publications

• Many thanks go out to:• ADSC Member Firms and Suppliers• ADSC Member Firms and Suppliers

• Participating Geotechnical Firms

• Loadtest, Inc.Loadtest, Inc.

• Individuals

Questions?