lesson 09 - part 4 - deep foundations - university 667 geotech design/lesson 09-chapt… · hole,...
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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?
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