Improved Design of Drilled Shafts in Rockdanbrownandassociates.com/wp-content/uploads/2009/02/... · 2009-02-20 · Improved Design of Drilled Shafts in Rock By: ADSC Southeast Chapter,

1

Improved Design of Drilled Improved Design of Drilled

Shafts in RockShafts in Rock

By: ADSC Southeast Chapter, andDan Brown, P.E., Ph.D., Auburn University

Objectives Objectives –– ADSC SE Chapter ResearchADSC SE Chapter Research

Improve design methodology in rockImprove cost-effectiveness of drilled shafts in rock in key Southeastern marketsDemonstrate reliability of drilled shafts founded on rock

2

OutlineOutline

Review of design principals, rock propertiesCase histories in marl, sandstone, shale, PiedmontNashville research in limestoneCost implications of improved designTime permitting: Design for Lateral Loads

Design for Axial LoadingDesign for Axial Loading

Geotechnical Strength Limit State

Plunging failureStructural Strength Limit State

Structural failureServicability Limit State

Settlements or Axial Displacement

3

Characterization of Rock StrengthCharacterization of Rock Strength

Lab Tests of Rock StrengthLab Tests of Rock Strength

4

InIn--Situ Tests of Rock ModulusSitu Tests of Rock Modulus

Rock Mass Rating (RMR)Rock Mass Rating (RMR)

5

Geological Strength IndexGeological Strength Index

Nominal Axial Resistance Nominal Axial Resistance in Shales and Weak Rockin Shales and Weak Rock

a

u

a

SN

pqC

pf

=Side Resistance:

6

Nominal End Bearing ResistanceNominal End Bearing Resistance

Generalized Behavior Under Axial LoadGeneralized Behavior Under Axial Load

7

Dilation at Rock/Shaft InterfaceDilation at Rock/Shaft Interface

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0 1000 2000 3000

Load (kips)

Dis

plac

emen

t (in

ches

)

Example Example –– Tampa, FLTampa, FL

8

What does a strain gauge tell you?

Load = (strain)AE

Note: you need to have a good idea of A and E to get load!

Strain GaugesStrain Gauges

Reducing Test ResultsReducing Test ResultsApplied Load, kips

Note: 1 ft. = 0.305 m1 kip = 4.45kN

0

20

40

60

80

100

0 500 1000 1500 2000

Dep

th (F

t)

9

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0 1000 2000 3000

Load (kips)

Dis

plac

emen

t (in

ches

)

Example Example –– Tampa, FLTampa, FL

-2.0

-1.5

-1.0

-0.5

0.0

0 200 400 600 800 1000 1200Load (kips)

Toe

Dis

plac

emen

t (in

ches

)

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0 1000 2000 3000

Load (kips)

Dis

plac

emen

t (in

ches

)

-2

-1.5

-1

-0.5

0

0 5 10 15 20

Side Shear (ksf)

Segm

ent D

ispl

acem

ent

(inch

es)

Side & Base Side & Base ResistanceResistance

10

Marls and Chalks in SE U.S. Marls and Chalks in SE U.S. -- Test DataTest Data

Drilling & SamplingDrilling & Sampling

Conventional Rock Coring ToolsPitcher Barrel

11

Data Summary (example)Data Summary (example)

12

Nominal Axial Resistance Nominal Axial Resistance in Shales and Weak Rockin Shales and Weak Rock

a

u

a

SN

pqC

pf

=Side Resistance:Horvath & Kenney (1979)Normalized by atm press

Side ResistanceSide Resistance

0

2

4

6

8

10

12

0 10 20 30 40 50

(ksf)

Uni

t Sid

e R

esis

tanc

e (k

sf)

US 80 Test (est)ALChalk (1 test)AL Claystone (Avg)AL Claystone (2 tests)MS Chalk (Avg)MS Chalk (9 tests)SC Cooper Marl (Avg)SC Cooper Marl (7 tests)

qu estimatedN-SPT = 50/3″

equationC=0.65

C=1.0

C=0.85

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Base ResistanceBase Resistance

0

2

4

6

8

10

12

14

0% 1% 2% 3% 4% 5%

Normalized Displacement

Nor

mal

ized

Uni

t Bas

e R

esis

tanc

e

WRT-4 (MS)

LT-8912-2 (MS)

LT-8912-1 (MS)

O'Neil and Reese (1999)

WRT-5-1 (SC)

LT-8904 (AL)

LT-8650-1 (SC)

0

1

2

3

4

5

6

7

8

9

0% 1% 2% 3% 4% 5%Normalized Displacement

Nor

mal

ized

Uni

t Bas

e R

esis

tanc

e (q

b/qu

)

LT-8373 (MS)LT-8487 (MS)

US 80 Test (AL) (qu estimated)

O'Neil and Reese (1999)

LT-8194 (MS)

LT-8788 (MS)

LT-8461-2 (MS)

LT-8461-1 (MS)

LT-8661 (SC)

Case Studies in Weak SandstonesCase Studies in Weak Sandstones

Bryant – Denny Stadium, TuscaloosaMN I-35W BridgeWilcox Co., Ala.

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Base Resistance Base Resistance -- TuscaloosaTuscaloosa

Shaft drilled under polymer slurry and base cleaned with bucket

Base Resistance Base Resistance -- TuscaloosaTuscaloosa

270 ksf/in

( )E

qBs

2179.0 νρ −⋅=

For rigid circular footing on elastic half-space:

With ν = ¼ :

With 48” dia, ρ/B ≈ 0.01 at ρ=0.48”

E = 9,600ksf≈ 65 ksi

15

MN IMN I--35W Replacement35W Replacement

MN IMN I--35W Replacement35W Replacement

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MN IMN I--35W Replacement35W Replacement

590

600

610

620

630

640

650

660

670

680

690

0 20 40 60 80 100 120 140 160

Compr Str (tsf), or RQD or Rec (%)

Elev

., ft.

% RecRQDCompr Str, tsf

78” Dia. Socket20’ embedment into decomposed sandstone20’ embedment into soft sandstone (qu ≈ 40tsf = 500psi)

35W 35W –– Side ResistanceSide Resistance

avg

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35W Side Resistance35W Side Resistance

a

u

a

SN

pqC

pf

=

Elev 665-645,C ≈ 2.5 to 2.8

35W Base Resistance35W Base Resistance

135 ksf/in

( )E

qBs

2179.0 νρ −⋅=

For rigid circular footing on elastic half-space:With ν = ¼ :

With 78” dia, ρ/B ≈ 0.01 at ρ=0.78”

E = 7,800ksf≈ 54 ksi

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Wilcox Co. Wilcox Co. –– SR28 over Alabama RiverSR28 over Alabama RiverTest Shaft 1Test Shaft 1

Wilcox Co., Ala. Wilcox Co., Ala. –– Test Shaft 2Test Shaft 2

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Shale: Shale: Some Test Data from Some Test Data from KSKS--MOMO--KYKY AreaArea

Jackson Co., MO Chanute Shale, dry holefs=6ksf, weathered shale w/ qu=14ksffs=9-12ksf, unweathered shale w/ qu=32ksf

Lexington, MO Gray & Black Shale, waterfs=15ksf, qb=144ksf, no strength data

Waverly, MO Clay Shale, polymer slurryfs=6-12ksf, qb=110ksf, no failure, no boring

Topeka, KS, Severy Shale, dryfs=4-40ksf, qb=127ksf, no strength data

Republic Co., KS, Graneros Shale, dryfs=3-4ksf, qb=56ksf, no strength data, RQD >75%

Osborne, KS, Fairport Chalk (gray shale), polymerfs=11ksf, qb=136ksf, no strength data

Des Moine, IA, Soft-Fm Shale, “roughened socket”, polymerfs=4.5-7ksf, qb>40-114ksf, RQD=39-70%, qu=24-170ksf

Owensboro, KY, 3 tests soft gray shale, polymerfs=3-12ksf, qb=140-300ksf, weathered shale w/ qu=28-40ksf

Bond Memorial Bridge, Kansas CityBond Memorial Bridge, Kansas City

alluvium

Shale bedrock

Plan View of Main Pylon Pier

Profile View

Cap

Seal

Drilled Shafts

Steel

Test Shaft. Shown for location only, not a structural member in contact

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Polymer Slurry in Shale Polymer Slurry in Shale at Bond Bridgeat Bond Bridge

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Test Data Test Data –– Bond Memorial BridgeBond Memorial Bridge

Test Data Test Data –– Bond Memorial BridgeBond Memorial Bridge

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Bond Bridge Bond Bridge -- Side ResistanceSide Resistance

a

u

a

SN

pqC

pf

=

Elev 640-610,C ≈ 1

Bond Bridge Bond Bridge -- Base ResistanceBase Resistance

500 ksf/in

( )E

qBs

2179.0 νρ −⋅=

For rigid circular footing on elastic half-space:With ν = ¼ :

With 72” dia, ρ/B ≈ 0.01 at ρ=0.72”

E = 27,000ksf≈ 185 ksi≈ 100 qu

23

Piedmont GeologyPiedmont Geology

Atlanta – Charlotte – RichmondResidual soil over PWR over progressively less weathered rockMajor questions:

Where to stop drilling?Side + base resistance?How to design for imperfect rock?Need for probe hole and hand cleaning?

Example Example –– Macon AreaMacon Area

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ConstructionConstruction

Conditions from Soil BoringConditions from Soil Boring

340

360

380

400

420

440

460

0 20 40 60 80 100

%Recovery or RQD

Elev

340

360

380

400

420

440

460

0 200 400 600 800

SPT - N

% RecRQDSPT

PWR

Rock

sand

25

Load Test ResultsLoad Test Results

Side ResistanceSide Resistance

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Base ResistanceBase Resistance

220 ksf/inch

( )E

qBs

2179.0 νρ −⋅=

For rigid circular footing on elastic half-space:

With 60” dia, ρ/B ≈ 0.01 at ρ=0.6”

E = 9,800ksf≈ 68 ksi

Limestone Limestone –– Nashville ResearchNashville Research

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Methodology and ScopeMethodology and Scope

Review & evaluate available load test dataPerform select load tests under carefully controlled and well defined conditions, representative of local areaInvolve local practicing engineers to ensure that the tests are representative and meaningful for local practiceEvaluate test data and develop recommendationsOrganize local seminars to transfer research into practice and share local experiences

Test Plan for NashvilleTest Plan for Nashville

Select site with appropriate geologic characteristicsPerform two O-cell tests, with machine-only bottom cleaning and less than ideal rock conditionsAdjust dimensions of 2nd test based on results of 1st test

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Site LocationSite Location

SiteAccording to USGS geologic maps, this site is underlain by Carters Limestone of the Stones River Group, a fine-grained, yellowish-brown limestone with thin beds of bentonite clay. This formation is typical of the Central Basin limestones in the Nashville area.

Rock ConditionsRock Conditions

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11stst TestTest

54” cased hole

48” socket, 15ft deep

36” bearing seat

Top of Rock at ≈ 16ft Isolate 3-4ft to avoid caprock

or weathered zone

34” Dia. O-Cell

Boring Logs (thanks PSI)Boring Logs (thanks PSI)

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BoringsBorings

1

2A

2B

2C

Test Shaft 1

4

3

6

Test Shaft 2

CoresCores

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CoresCores

Core Test DataCore Test Data

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

00 5000 10000 15000 20000 25000 30000 35000 40000

Compr Str, psi

Dep

th B

elow

Top

of S

ocke

t

Test Shaft 2

Test Shaft 1

32

% Recovery% Recovery

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

00 20 40 60 80 100 120

Recovery, %D

epth

Bel

ow T

op o

f Soc

ket

Test Shaft 2 Recovery

Test Shaft 1 Recovery

RQDRQD

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

00 10 20 30 40 50 60 70 80 90

RQD, %

Dep

th B

elow

Top

of S

ocke

t

Test Shaft 2 RQD

Test Shaft 1 RQD

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Test Shaft 1Test Shaft 1

Test Shaft 1Test Shaft 1

39” dia bearing area

48” diasocket

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ConstructionConstruction

Inspection Inspection –– Probe HoleProbe Hole

Test Shaft 1: 51” test hole3” soil seam at 19 inches½” rock¾” soil seam18 inches weathered rock w/ voids & shale

Test Shaft 2: 72” test hole3/8” soil seam at 36 inches

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OO--cell Assemblycell Assembly

BottomPlate

Tell-taleRod

CarrierFrame (rebar cage)

O-cell

Load TestLoad Test

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Load TestLoad Test

Test Results Test Results –– Shaft 1Shaft 1

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Test Results Test Results –– Shaft 1Shaft 1

Test Results Test Results –– Shaft 1Shaft 1

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Test Shaft 2Test Shaft 2

Test Shaft 2Test Shaft 2

29” dia bearing surface

48” diasocket

39

Test Results Test Results –– Shaft 2Shaft 2

Test Results Test Results –– Shaft 2Shaft 2

40

Test Results Test Results –– Shaft 2Shaft 2

Summary of Test ConditionsSummary of Test Conditions

Ground Surface 0

Tip of Casing -16.0Top of Conc -17.5

SG Level 1 -26.0

Tip of Shaft -33.5Base of O-cell -33.25

Top of Conc -14.0

Tip of Casing -20.0

SG Level 1 -31.4

Base of O-cell -36.9Tip of Shaft -37.0

Test Shaft 1 Test Shaft 2

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AvgAvg Side Resistance Side Resistance –– Test Shaft 1Test Shaft 1

0

5

10

15

20

25

0 0.2 0.4 0.6

Displacement, inches

Uni

t Sid

e R

esis

tanc

e, k

sfNominal Dia = 48"Nominal Dia = 52.5"

Evaluation of Side ResistanceEvaluation of Side Resistance

( )a

uas

pqpCf ⋅⋅=

For Test Shaft 1:avg qu = 8300psirange = 1660 – 16,110%rec = 74% – 100%RQD = 9% - 65%

Back-calculated C=0.4

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Base ResistanceBase Resistance

-0.75

-0.5

-0.25

00 250 500 750 1000 1250

Bearing Pressure, ksf

Dis

pl, i

nche

stest shaft 1test shaft 2

-2

-1.75

-1.5

-1.25

-1

-0.75

-0.5

-0.25

00 250 500 750 1000 1250

Bearing Pressure, ksf

Dis

pl/D

ia, %

test shaft 1test shaft 2

Evaluation of Base ResistanceEvaluation of Base Resistance

( )E

qBs

2179.0 νρ −⋅=

For rigid circular footing on elastic half-space:

With ν = ¼ and ρ/B = 0.005 to 0.01:

-2

-1.75

-1.5

-1.25

-1

-0.75

-0.5

-0.25

00 250 500 750 1000 1250

Bearing Pressure, ksf

Dis

pl/D

ia, %

test shaft 1test shaft 2

E=536ksi

E=235ksi

E=630ksiE=335ksi

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Characterizing Rock for Base ResistanceCharacterizing Rock for Base Resistance

“Sound Rock”Typical of Test Shaft 2 conditionsErock ≈ 500 to 600 ksiNo more than 2 small seams < ½” thick

“Fair Rock”Typical of Test Shaft 1 conditionsErock ≈ 200 to 300 ksiSoil-filled seams up to 10% base dia., BLocated at Depths > ½ B

Base Resistance in Base Resistance in ““Sound RockSound Rock””

Servicability conditionsLimit base pressure to that causing settlement of 0.005B0.005B = ¼ to 3/8 inch for 4ft to 6ft dia shaft

Service Load Base Pressure ≤ 500ksf (250tsf)F.S. ≥ 2.5 (no bearing failure at 1250ksf)Include load test in design

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Design Requirements for Design Requirements for ““Sound RockSound Rock””

Thorough Site Investigation w/ coring, qu, RQDqu in 10,000 psi range or higher% recovery typically > 90%No significant solution cavities below bearing elevShafts constructed in dry, w/ downhole inspProbe hole to at least 2BNo more than 2 seams and none > ½ “Site specific load test requirement

Base Resistance in Base Resistance in ““Fair RockFair Rock””

Servicability conditionsLimit base pressure to that causing settlement of 0.005B0.005B = ¼ to 3/8 inch for 4ft to 6ft dia shaft

Service Load Base Pressure ≤ 200ksf (100tsf)F.S. ≥ 2.5 (no bearing failure at 500ksf)Include load test in design

45

Design Requirements for Design Requirements for ““Fair RockFair Rock””

Thorough Site Investigation w/ coring, qu, RQDqu in 5,000 psi range or higher% recovery typically > 70%No significant solution cavities below bearing elevInspection to confirm gen’l character of rock from drilling toolsSite specific load test requirement

Side ResistanceSide Resistance

Nominal strength governsUse equation with C=0.4:Rock similar to

Test Shaft 1: Auger refusal in exploratory borings%Rec avg = 85%, range = 20% to 100%RQD avg = 38%, range = 0 to 65%

Use F.S. = 2.5 for service loads

( )a

uas

pqpCf ⋅⋅=

46

Cost ImplicationsCost Implications

Examine several hypothetical examplesPrevious practice allowable base = 100ksfNew practice includes load testEstimated costs:

$400 per cu.yd. in earth$1700 per cu.yd. in rock

Example 1 Heavy Bldg, Small FootprintExample 1 Heavy Bldg, Small Footprint

50 drilled shafts3400 kips / shaft service loadsGeotechnical Conditions:

20ft earth overburden8ft weathered rock8ft “Fair Rock”“Sound Rock”

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Example 1 DesignsExample 1 Designs

A. Previous practice: excavate to sound rock, allowable base resistance = 100ksf

B. “Sound Rock” base resistance onlyC. “Fair Rock” base resistance onlyD. “Fair Rock” base + side resistance

Heavy Bldg Heavy Bldg –– Design A Design A (previous practice)(previous practice)

8ft casing thru 20ft soil7ft shaft through 8ft weathered rock7ft shaft through 8ft rockEnd bearing on sound rock at 100ksf

$15,000 per shaft earth$39,000 per shaft rock50 shafts @ $54,000 = $2,700,000.

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Heavy Bldg Heavy Bldg –– Design B Design B (Strong Rock)(Strong Rock)

4ft casing thru 20ft soil3.5ft shaft through 8ft weathered rock3.5ft shaft through 8ft fair rockEnd bearing on sound rock at 350ksf (175tsf)

$3,750 per shaft earth$9,700 per shaft rockLoad test @ $75,00050 shafts @ $13,500 + $75,000 load test = $750,000.

Heavy Bldg Heavy Bldg –– Design C Design C (Fair Rock)(Fair Rock)

5.5ft casing thru 20ft soil5ft shaft through 8ft weathered rockEnd bearing on fair rock at 175ksf (90tsf)

$7,000 per shaft earth$10,000 per shaft rockLoad test @ $75,00050 shafts @ $17,000 + $75,000 load test = $925,000.

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Heavy Bldg Heavy Bldg –– Design D Design D (Fair Rock + side)(Fair Rock + side)

5ft casing thru 20ft soil4.5ft shaft through 8ft weathered rock at 8ksf side resistance (=900k)End bearing on fair rock at 160ksf (80tsf) (=2500k)

$6,000 per shaft earth$18,000 per shaft rockLoad test @ $75,00050 shafts @ $14,000 + $75,000 load test = $775,000.

Example 1 Cost SummaryExample 1 Cost Summary

A. Previous practice: $2,700,000B. “Sound Rock” base resistance: $750,000C. “Fair Rock” base resistance: $925,000D. “Fair Rock” base + side: $775,000

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Ex 2 Large Structure, Medium LoadsEx 2 Large Structure, Medium Loads

150 drilled shafts1700 kips / shaft service loadsGeotechnical Conditions (same as Ex 1):

20ft earth overburden8ft weathered rock8ft “Fair Rock”“Sound Rock”

Example 2 DesignsExample 2 Designs

A. Previous practice: excavate to sound rock, allowable base resistance = 100ksf

B. “Sound Rock” base resistance onlyC. “Fair Rock” base resistance onlyD. “Fair Rock” base + side resistance

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Large Structure Large Structure –– Moderate LoadsModerate LoadsDesign A Design A (previous practice)(previous practice)

5.5ft casing thru 20ft soil5ft shaft through 8ft weathered rock5ft shaft through 8ft rockEnd bearing on sound rock at 100ksf

$7,000 per shaft earth$20,000 per shaft rock150 shafts @ $27,000 = $4,050,000.

3.5ft casing thru 20ft soil3ft shaft through 8ft weathered rock3ft shaft through 8ft fair rockEnd bearing on sound rock at 240ksf (120tsf)

$2,900 per shaft earth$7,100 per shaft rockLoad test @ $75,000150 shafts @ $10,000 + $75,000 load test = $1,575,000.

Large Structure Large Structure –– Moderate LoadsModerate LoadsDesign B Design B (Sound Rock)(Sound Rock)

52

4ft casing thru 20ft soil3.5ft shaft through 8ft weathered rockEnd bearing on fair rock at 180ksf (90tsf)

$3,800 per shaft earth$4,900 per shaft rockLoad test @ $75,000150 shafts @ $8,700 + $75,000 load test = $1,380,000.

Large Structure Large Structure –– Moderate LoadsModerate LoadsDesign C Design C (Fair Rock)(Fair Rock)

Large Structure Large Structure –– Moderate LoadsModerate LoadsDesign D Design D (Fair Rock + side)(Fair Rock + side)

3.5ft casing thru 20ft soil3ft shaft through 8ft weathered rock at 8ksf side resistance (=600k)End bearing on fair rock at 160ksf (80tsf) (=1100k)

$2,900 per shaft earth$3,600 per shaft rockLoad test @ $75,000150 shafts @ $6,500 + $75,000 load test = $1,050,000.

53

Example 2 Cost SummaryExample 2 Cost Summary

A. Previous practice: $4,050,000B. “Sound Rock” base resistance: $1,575,000C. “Fair Rock” base resistance: $1,380,000D. “Fair Rock” base + side: $1,050,000

Ex 3 Small Structure, Medium LoadsEx 3 Small Structure, Medium Loads

40 drilled shafts1000 kips / shaft service loadsGeotechnical Conditions:

20ft earth overburden5ft weathered rock5ft “Fair Rock”“Sound Rock”

54

Example 3 DesignsExample 3 Designs

A. Previous practice: excavate to sound rock, allowable base resistance = 100ksf

B. “Sound Rock” base resistance onlyC. “Fair Rock” base resistance onlyD. “Fair Rock” base + side resistance

Small Structure Small Structure –– Moderate LoadsModerate LoadsDesign A Design A (previous practice)(previous practice)

4.5ft casing thru 20ft soil4ft shaft through 5ft weathered rock4ft shaft through 5ft rockEnd bearing on sound rock at 100ksf

$4,700 per shaft earth$8,000 per shaft rock40 shafts @ $12,700 = $508,000.

55

3.5ft casing thru 20ft soil3ft shaft through 5ft weathered rock3ft shaft through 5ft fair rockEnd bearing on sound rock at 150ksf (75tsf)

$2,900 per shaft earth$4,500 per shaft rockLoad test @ $75,00040 shafts @ $7,400 + $75,000 load test = $371,000.

Small Structure Small Structure –– Moderate LoadsModerate LoadsDesign B Design B (Sound Rock)(Sound Rock)

3.5ft casing thru 20ft soil3ft shaft through 5ft weathered rockEnd bearing on fair rock at 150ksf (75tsf)

$2,900 per shaft earth$2,300 per shaft rockLoad test @ $75,00040 shafts @ $5,200 + $75,000 load test = $283,000.

Small Structure Small Structure –– Moderate LoadsModerate LoadsDesign C Design C (Fair Rock)(Fair Rock)

56

Small Structure Small Structure –– Moderate LoadsModerate LoadsDesign D Design D (Fair Rock + side)(Fair Rock + side)

3ft casing thru 20ft soil2.5ft shaft through 5ft weathered rock at 8ksf side resistance (=300k)End bearing on fair rock at 150ksf (75tsf) (=700k)

$2,100 per shaft earth$1,600 per shaft rockLoad test @ $75,00040 shafts @ $3,700 + $75,000 load test = $223,000.

Example 3 Cost SummaryExample 3 Cost Summary

A. Previous practice: $508,000B. “Sound Rock” base resistance: $371,000C. “Fair Rock” base resistance: $283,000D. “Fair Rock” base + side: $223,000

57

ConclusionsConclusions

Unit side resistance >20ksf Extremely high end bearing available in “imperfect” rock bearing stratumMachine base cleaning was sufficientDown-hole inspection delineated between “Fair Rock” and “Sound Rock”Preliminary rock classification for designSubstantial potential cost benefits, even with load testing included

Conclusions Conclusions –– ““Fair RockFair Rock””

Base resistance in “Fair Rock” (or better) exceeds 500ksf at 1% base diameterBase resistance Geotech limit not achievedService load limit resistance at 0.5% base diameter of at least 200ksfBack-calculated modulus for rock of 200 to 300 ksi

58

Conclusions Conclusions –– ““Sound RockSound Rock””

Base resistance in “Sound Rock” (or better) exceeds 1250ksf at 1% base diameterBase resistance Geotech limit not achievedService load limit resistance at 0.5% base diameter of at least 500ksfBack-calculated modulus for rock of 500 to 600 ksi

Discussion TopicsDiscussion Topics

How can we better characterize rock during site investigation?

Will qu, %Rec, RQD be driller-dependent?Other tools? (Goodman jack, PMT, velocity logging)More thorough exploratory program?

Is down-hole inspection justified?Boring per shaft justified?

How much engineering post-bid vs pre-bid?Lateral Load considerations?Other topics?

59

Design for Lateral LoadingDesign for Lateral Loading

Geotechnical Strength Limit StateOverturning failure

Structural Strength Limit StateYield in flexure

Servicability Limit StateLateral Deformations

Applications with Large Lateral LoadsApplications with Large Lateral Loads

Single Column Piers with Monoshaft Foundations

Monoshaft Foundations Used by Caltrans (Caltrans Seismic Design Criteria, Version 1.4, June, 2006)

60

Applications with Large Lateral LoadsApplications with Large Lateral Loads

Shaft

Panel

Lateral LoadLateral LoadDesign ProcessDesign Process

Pushover Analysis

Nominal Moment Capacityvs Maximum Computed

Bending Moments

Lateral Deformations

61

EI (Varies with Moment)

p (F/L)

y (L)

Numerical Solution

Geotechnical InputsGeotechnical Inputs

Soil Layers: c, φ, γ, (total or effective, as appropriate), piezometric surface level, a soil stiffness parameter (ε50 for clay), k (initial p-y modulus).Geomaterial Classifications:- Soft Clay Below Water Table- Stiff Clay Above/Below Water Table- Sand- c-φ Soil- Rock, Weak RockSlope of ground surface or pile batter angle

62

Loading InputsLoading Inputs

Magnitudes of Shear, Moment and Axial Thrust at the Head of the Drilled Shaft Nature of Pile Head Loads

Static or Cyclic LoadingNumber of Significant Load Cycles

Magnitudes of Distributed Loads Along Shaft (Stream Loads, Soil Loads, etc.)

Structural InputsStructural Inputs

Bar Size, Layout, E, and fy ofLongitudinal ReinforcementShaft Diameter(s)f′c of the Concrete

63

φε

Assumed Neutral Axis

Strain Gradient (ε =φε y)Side View

y dy

y σ

dy

Top View

Structural ModelStructural Model

EI = M / φεCoordinates of M and EI Are Plotted for Common Values of Px. Resulting Relations Are Used for Lateral Load Analyses.

EI

M

Px1 (small)

Px2 (large)

Initial cracking Plastic hinges

Gross EI’s

64

Structural Strength Limit StateStructural Strength Limit State

Permissible

“Nominal” Resistance Interaction Diagram

Factored “Ultimate”Resistance Interaction Diagram

NOT TO SCALE

P′= 0.133 f’c Ag

0.9 M (Px = 0) M

Px

.75 Px

.75 M

ExampleExample

40k

800k-ft

Very Stiff Clay Su = 15psi

65

Geotechnical Strength Limit StateGeotechnical Strength Limit State

0

10

20

30

40

50

60

70

0 1 2 3 4 5

Deformation, inches

Shea

r, ki

ps

15 ft long

20 ft long

25 ft long

LPILE analysis of linear elastic shaft with varying embedded length

FactoredDesignLoads

Structural Strength Limit StateStructural Strength Limit State

-20

-15

-10

-5

0

0 200 400 600 800 1000

Moment, k-ft

Dep

th, f

t

20ft Shaft25ft Shaft

Mmax = 873 Mmax = 885

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 0.0001 0.0002 0.0003 0.0004

Curvature

Mom

ent,

k-ft

0.00E+00

2.00E+08

4.00E+08

6.00E+08

8.00E+08

1.00E+09

1.20E+09

0 500 1000 1500 2000

Moment, k-ft

EI, k

-in2

1791

1791

Computed nominal moment resistance = 1791 k-ft

66

ServicabilityServicability Limit StateLimit State

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0-0.2 0 0.2 0.4 0.6

Lateral Deformation, inches

Dep

th, f

eet

Nonlinear EI

Linear EI

Example Sensitivity AnalysisExample Sensitivity Analysis

Soft clay

Weak rock

20ft

Decreased 2%Increased 8%Strong Rock bearing, qu=750psi

Increased 0%Increased 2%Rock qu and Ermreduced 10%

Increased 1%Increased 2%Soft clay strength reduced 10%

Increased 5.5%Increased 22%Soft clay thickness increased 10%

15,722 inch-kips0.116 inchesBase Case (described above)

Maximum MomentDeflectionCase

75k

qu=180psiE = 27ksi

Su=2psi 6’

67

Next Step Next Step –– Piedmont GeologyPiedmont Geology

Find site(s) in Atlanta area with suitable characteristicsDiscussion: Most important issues?

Rock strength below base?Side resistance?

PWR?Rock?

Downhole inspection & seams?

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Opinions.Opinions.””