DEEP FOUNDATIONSDEEP FOUNDATIONS

Lesson 09 Lesson 09 -- Topic 4Topic 4Drilled ShaftsDrilled Shafts

Learning OutcomesLearning Outcomes

ggAt the end of this session, the At the end of this session, the participant will be able to:participant will be able to:-- Contrast driven piles and drilled shaftsContrast driven piles and drilled shafts-- Compare mobilization of base (tip) and Compare mobilization of base (tip) and

side (shaft) resistanceside (shaft) resistance-- Describe drilled shaft construction Describe drilled shaft construction

processesprocesses-- Discuss the need for quality control for Discuss the need for quality control for

drilled shaft constructiondrilled shaft construction

DefinitionsDefinitions

Figure 9-56

Driven Piles Driven Piles vsvs Drilled ShaftsDrilled Shafts

ggDrilled shaft is installed in a drilled Drilled shaft is installed in a drilled hole, unlike the driven pilehole, unlike the driven pile

ggWet concrete is placed in the drilled Wet concrete is placed in the drilled hole and cures directly against the soil hole and cures directly against the soil forming the walls of the boreholeforming the walls of the borehole-- SideSide--support (casing and/or slurry) may be support (casing and/or slurry) may be

necessary for stabilization of the open necessary for stabilization of the open hole and may be left in placehole and may be left in place

gg Installation method and equipment Installation method and equipment varies with the subsurface conditionsvaries with the subsurface conditions

Advantages of Drilled ShaftsAdvantages of Drilled Shafts

ggConstruction equipment is mobile and Construction equipment is mobile and construction can proceed rapidlyconstruction can proceed rapidly

ggExcavated Excavated geomaterialsgeomaterials can be can be examinedexamined

ggFor endFor end--bearing situations, the soil bearing situations, the soil beneath the tip may be examined or beneath the tip may be examined or probed for weaker materialsprobed for weaker materials

ggChanges in shaft size may be made Changes in shaft size may be made during constructionduring construction

Advantages of Drilled ShaftsAdvantages of Drilled Shafts

ggHeave and settlement at the ground is Heave and settlement at the ground is normally smallnormally small

ggPersonnel, equipment and materials for Personnel, equipment and materials for construction are readily availableconstruction are readily available

ggNoise and vibration level from the Noise and vibration level from the equipment is less than other forms of equipment is less than other forms of construction for deep foundations (e.g., construction for deep foundations (e.g., driven piles)driven piles)

Advantages of Drilled ShaftsAdvantages of Drilled Shafts

ggApplicable to a wide variety of Applicable to a wide variety of subsurface conditions, e.g., can be subsurface conditions, e.g., can be constructed through cobbles and for constructed through cobbles and for many feet into hard rock as well as many feet into hard rock as well as frozen groundfrozen ground

ggUse of a large single drilled shaft Use of a large single drilled shaft (without pile cap) is possible(without pile cap) is possible

ggExtensive data bases documenting Extensive data bases documenting loadload--transfer information are available transfer information are available

Advantages of Drilled ShaftsAdvantages of Drilled Shafts

ggSmaller footprint than a footing and can Smaller footprint than a footing and can thus be constructed near railroad, thus be constructed near railroad, existing structures and in constricted existing structures and in constricted areasareas

ggShafts may be more economical than Shafts may be more economical than spread footing, particularly when the spread footing, particularly when the foundation support layer is deeper than foundation support layer is deeper than 1010--ft below the ground or at water ft below the ground or at water crossingscrossings

Special Considerations for Special Considerations for Drilled ShaftsDrilled ShaftsggConstruction procedures are critical to Construction procedures are critical to

the quality of the drilled shaftthe quality of the drilled shaftggKnowledgeable inspection is requiredKnowledgeable inspection is requiredggNot normally used in deep deposits of Not normally used in deep deposits of

soft clay or in situations where artesian soft clay or in situations where artesian pressures existpressures exist

ggStatic load tests to verify ultimate Static load tests to verify ultimate capacity of large diameter shafts are capacity of large diameter shafts are very costlyvery costly

Effect of Subsurface Conditions Effect of Subsurface Conditions on Drilled Shaftson Drilled ShaftsggCaving soilsCaving soils

-- Temporary casing or other side supportTemporary casing or other side supportggFlowing groundwaterFlowing groundwater

-- Leaching of concreteLeaching of concrete-- Use of slurryUse of slurry

ggArtesian water conditionsArtesian water conditions-- Could cause collapse of the shaft Could cause collapse of the shaft

excavationexcavationggCobbles and bouldersCobbles and boulders

-- Sometimes require special toolsSometimes require special tools

Effect of Subsurface Conditions Effect of Subsurface Conditions on Drilled Shaftson Drilled Shaftsgg Presence of existing foundations and Presence of existing foundations and

structuresstructures-- Loss of ground volume into the Loss of ground volume into the exacationexacation

gg Landfill material that cannot be excavatedLandfill material that cannot be excavated-- e.g., an old car bodye.g., an old car body

gg RockRock-- Specialized drilling toolsSpecialized drilling tools

ggWeak stratum below base of shaftWeak stratum below base of shaft-- May need to extend shaft through the weaker May need to extend shaft through the weaker

layerlayer

Estimating Ultimate Axial Estimating Ultimate Axial Capacity of Shafts in SoilsCapacity of Shafts in SoilsggUltimate capacity, Ultimate capacity, QQultult, in compression, in compression

QQultult = Q= QSS + Q+ QTT –– WW

ggUltimate capacity, Ultimate capacity, QQultult, in uplift, in uplift

QQultult ≤≤ 0.7Q0.7QSS + W+ W

Geotechnical Allowable Shaft Geotechnical Allowable Shaft Load, Load, QQallall

QQallall = = QQultult/ FS/ FS

ggFS is the factor of safetyFS is the factor of safetyggUsually FS = 2.5 assuming a normal Usually FS = 2.5 assuming a normal

level of field quality control during shaft level of field quality control during shaft construction. “Normal” is based on the construction. “Normal” is based on the minimum recommendations of FHWAminimum recommendations of FHWA

gg If a static load test is performed, FS=2.0 If a static load test is performed, FS=2.0 may be usedmay be used

Computation of Geotechnical Computation of Geotechnical Axial CapacityAxial CapacityggCohesive soilsCohesive soils

-- Total stress for Total stress for undrainedundrained conditionsconditions•• Similar to Tomlinson method for driven pilesSimilar to Tomlinson method for driven piles

-- Effective stress for drained conditionsEffective stress for drained conditionsggCohesionlessCohesionless soilssoils

-- Effective stress method for drained Effective stress method for drained loading conditionsloading conditions

Cohesive soils Cohesive soils –– Side ResistanceSide Resistance

ggSide resistance (Side resistance (EqEq. 9. 9--36)36)

ggαα is the adhesion factor as follows:is the adhesion factor as follows:

ggUltimate unit side load transferUltimate unit side load transfer

∑=

Δαπ=N

1iiuiiS zSDQ

55.0=α 5.1pSfor au ≤ ( )5.1pS1.055.0 au −−=α for 5.2pS5.1 au ≤≤

fsi = αi Sui

NonNon--contributing zonescontributing zones

Side Side ResistanceResistanceMobilizationMobilizationin Cohesive in Cohesive SoilsSoils

Figure 9-58

Cohesive soils Cohesive soils –– Tip ResistanceTip Resistance

ggTip resistance (Tip resistance (EqEq. 9. 9--39)39)

ggαα is the adhesion factor as follows:is the adhesion factor as follows:

QT = qT AT = NcsutAt

Nc = 6.0[1+0.2(z/D)]; Nc ≤ 9

Unit Tip Resistance in Cohesive Unit Tip Resistance in Cohesive SoilsSoils

qTR = (2.5/[aD/12 + 2.5b]) qT where D is the diameter of shaft in inches, a = 0.0071 + 0.0021 (z/D) with a ≤ 0.015, and b = 0.45(sut)0.5 with 0.5 ≤ b ≤ 1.5

Tip Tip ResistanceResistanceMobilizationMobilizationin Cohesive in Cohesive SoilsSoils

Figure 9-59

CohesionlessCohesionless soils soils –– Side Side ResistanceResistanceggSide resistance (Side resistance (EqEq. 9. 9--44)44)

ggββ is the adhesion factor as follows:is the adhesion factor as follows:

ggUltimate unit side load transfer (Ultimate unit side load transfer (≤≤4 4 ksfksf))

∑=

Δβγπ=N

1iiii

/iS zzDQ

where: ii z135.05.1 −=β with 25.02.1 i >β>

fsi = βi σ/vi

Side Resistance Mobilization in Side Resistance Mobilization in CohesionlessCohesionless SoilsSoils

Figure 9-60

CohesionlessCohesionless soils soils –– Tip Tip ResistanceResistanceggTip resistance (Tip resistance (EqEq. 9. 9--47)47)

ggReduced tip resistance for large size Reduced tip resistance for large size shafts (D is shaft diameter in inches)shafts (D is shaft diameter in inches)

QT = qT AT

qTR = [50/(12D)] qT

For N60 ≤ 75: qT = 1.2N60 in ksf For N60 > 75: qT = 90 ksf

Tip Tip Resistance Resistance Mobilization Mobilization in in CohesionlessCohesionlessSoilsSoils

Figure 9-61

Axial Shaft Capacity in Layered Axial Shaft Capacity in Layered SoilsSoilsggDivide subsurface profile into layersDivide subsurface profile into layersgg In each layer use the appropriate In each layer use the appropriate

methodmethodggSum the resistances from each layerSum the resistances from each layer

Group Action, Group Settlement, Group Action, Group Settlement, DowndragDowndrag and Lateral Loadsand Lateral LoadsggSimilar to driven pilesSimilar to driven pilesggRefer to FHWA (1999) publication for Refer to FHWA (1999) publication for

guidanceguidance

Example 9Example 9--55ggUsing FS=2.5, size a shaft for resisting Using FS=2.5, size a shaft for resisting

170 tons of vertical design load170 tons of vertical design loadN60-values

N60 = 11

N60 = 14

N60 = 14

N60 = 12

N60 = 19N60 = 21

N60 = 37

N60 = 22

Example 9Example 9--55ggFS=2.5FS=2.5ggUltimate axial load = (2.5)(170) = 425 tonsUltimate axial load = (2.5)(170) = 425 tonsggAssume a 3Assume a 3--ft diameter straight shaftft diameter straight shaftggThus, circumference = Thus, circumference = ππd d = 9.42= 9.42--ftftggAssume a shaft length of 60Assume a shaft length of 60--ftftggUse Use ββ formulation as followsformulation as follows

∑=

Δβγπ=N

1iiii

/iS zzDQ

where: ii z135.05.1 −=β with 25.02.1 i >β>

Example 9Example 9--55

ggCompute side resistance with depthCompute side resistance with depthDepth

Interval, Δz, ft

Surface Area per depth interval,

Δz(π)(D), ft2

Avg effective vertical (overburden) stress,

γ/zi, tsf

β

ii z135.05.1 −=β

with 25.02.1 i >β>

ΔQS Tons

0 – 4 37.7 0.115 1.20 5.204 – 30 245.0 0.572 0.94 131.7030 – 60 282.7 1.308 0.59 218.20

QS 355.10

Example 9Example 9--55

gg Compute tip resistanceCompute tip resistancegg At 60At 60--ft, Nft, N6060 = 21= 21gg qqTT = 1.2 N= 1.2 N6060 = 25.2 = 25.2 ksfksf = 12.6 = 12.6 tsftsfgg Tip area, ATip area, ATT = 7.07 sq. ft.= 7.07 sq. ft.gg QQTT = = qqTT AATT = 7.07(12.6) = 89.1 tons= 7.07(12.6) = 89.1 tons

gg Total axial resistance, Total axial resistance, QQultult = Q= QSS + Q+ QTT

gg QQultult = 355.1 tons + 89.1 tons = 440 tons= 355.1 tons + 89.1 tons = 440 tonsgg OkayOkay

Example 9Example 9--66

N60 = 20

N60 = 25

N60 = 50

Axial Capacity in RocksAxial Capacity in Rocks

ggSide resistance (Side resistance (EqEq. 9. 9--50, 950, 9--51)51)

ggUse information in Chapter 5 to Use information in Chapter 5 to evaluate the elastic modulus of rock evaluate the elastic modulus of rock massmass

SRRRSR qLDQ π=

( ) ( ) 5.0apcfap65.05.0

apuqapE65.0SRq ′<α=

Axial Capacity in RocksAxial Capacity in Rocks

ggTip resistance (Tip resistance (EqEq. 9. 9--52, 952, 9--53)53)

TRTTR qAQ = qTR = 2.5 qu

Intermediate Intermediate GeoMaterialsGeoMaterials((IGMsIGMs))ggCohesive IGMCohesive IGM

-- SSuu value of 2.5 to 25 value of 2.5 to 25 tsftsfggCohesionlessCohesionless IGMIGM

-- NN6060 values > 50 blows/ftvalues > 50 blows/ft

ggRefer to FHWA (1999) publication for Refer to FHWA (1999) publication for further information and design further information and design procedures for shafts in procedures for shafts in IGMsIGMs

Construction MethodsConstruction Methods

ggDry methodDry methodggWet methodWet methodggCasing methodCasing method

ggCleaning of the shaft excavation is the Cleaning of the shaft excavation is the most important step in construction of most important step in construction of drilled shaftsdrilled shafts

Dry MethodDry Method

DrillDrill CleanClean PositionPosition PlacePlaceCageCage ConcreteConcrete

Wet MethodWet Method

DrillDrill Slurry CleanSlurry Clean PositionPosition PlacePlaceCage ConcreteCage Concrete

Casing MethodCasing Method

DrillDrill Case Clean Position PlaceCase Clean Position PlaceCage ConcreteCage Concrete

Effect of Shaft Cleaning During Effect of Shaft Cleaning During ConstructionConstruction

Quality Assurance and Integrity Quality Assurance and Integrity TestingTestingggDrilled shafts are “manufactured” at the Drilled shafts are “manufactured” at the

sitesiteggOften anomalies develop during Often anomalies develop during

constructionconstructionggAn anomaly is deviation from an An anomaly is deviation from an

assumed geometry of the shaft and/or assumed geometry of the shaft and/or shaft properties (e.g., homogeneity)shaft properties (e.g., homogeneity)

ggNHI 132070 2.5NHI 132070 2.5--day courseday course

Types of Anomalies in Drilled Types of Anomalies in Drilled ShaftShaftgg NeckingNeckinggg BulbingBulbinggg SoftSoft--bottombottomgg Voids or soil intrusionsVoids or soil intrusionsgg Poor quality concretePoor quality concretegg DebondingDebondinggg Lack of concrete cover Lack of concrete cover

over reinforcementover reinforcementgg HoneyHoney--combingcombing

Non Destructive Tests (Non Destructive Tests (NDTsNDTs) for ) for Detection of AnomaliesDetection of AnomaliesggNDTsNDTs are geophysical testsare geophysical testsggExternalExternal

-- Sonic echoSonic echo-- Impulse responseImpulse response-- UltraUltra--seismicseismic

gg InternalInternal-- CrossholeCrosshole Sonic Logging (CSL)Sonic Logging (CSL)-- Gamma Density Logging (GDL)Gamma Density Logging (GDL)-- CSL Tomography (CSLT)CSL Tomography (CSLT)-- Perimeter Sonic Logging (PSL)Perimeter Sonic Logging (PSL)-- Neutron Moisture Logging (NML)Neutron Moisture Logging (NML)

CrossholeCrossholeSonic Sonic LoggingLogging

Gamma Gamma Density Density LoggingLogging

Load Testing of Drilled ShaftsLoad Testing of Drilled Shafts

ggStatic Load TestsStatic Load Tests-- Similar to driven pilesSimilar to driven piles-- OsterbergOsterberg Load Cell testLoad Cell test

ggStatnamicStatnamic testtest

ggMust perform caliper logging and Must perform caliper logging and NDTsNDTsbefore load testingbefore load testing

OsterbergOsterberg Load Cell TestLoad Cell Test

OsterbergOsterberg CellCell

ggTable 9Table 9--11, Table 911, Table 9--1212

O-cells between two steel plates

CSL tubes

Cage Centralizers Instrumentation

(strain gages)

StatnamicStatnamic Load TestLoad Test

StatnamicStatnamic Load TestsLoad Tests

Learning OutcomesLearning Outcomes

ggAt the end of this session, the At the end of this session, the participant will be able to:participant will be able to:-- Contrast driven piles and drilled shaftsContrast driven piles and drilled shafts-- Compare mobilization of base (tip) and Compare mobilization of base (tip) and

side (shaft) resistanceside (shaft) resistance-- Describe drilled shaft construction Describe drilled shaft construction

processesprocesses-- Discuss the need for quality control for Discuss the need for quality control for

drilled shaft constructiondrilled shaft construction

Any Questions?Any Questions?

THE ROAD TOUNDERSTANDING

SOILSAND

FOUNDATIONS

Inspector Qualification CoursesInspector Qualification Courses