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Static and Dynamic Load Testing of Deep Foundations Expansive and Collapsible Soils Overview of the Section

● Static and Dynamic Testing – Overview of the Basics of Design of Deep Foundations – Static Load Testing of Deep Foundations – Dynamic Load Testing of Deep Foundations ● Expansive and Collapsible Soils – Expansive Soils; Swell Potential – Collapsible Soils CreditsCredits forfor SlidesSlides andand GraphicsGraphics

Some of the slides and graphics for this lecture courtesy of

GRL Goble Raushe Likins & Associates

Pile Dynamics, Inc. Cleveland, Ohio DesignDesign andand SelectionSelection ofof DeepDeep FoundationsFoundations Design Parameters Selection and Types • Service loads and allowable  Type of Foundation is deformations influenced by: • Subsurface investigations  Design Loads • Type of Foundation  Subsurface Conditions • Lateral and Axial Load  Constructibility Capacity  Reliability • Driveability and Ability to  Cost Install  Availability of materials, • Structural Design equipment and expertise • Seismic Design  Local experience and • Verification and Redesign precedent • Integrity Testing DesignDesign ofof DeepDeep FoundationsFoundations  Subsurface  Service Loads and Investigations Allowable  For deep foundations, the subsurface investigations are Deformations generally more intensive and  Most important step expensive  Exploratory borings must  Loads can include extend well below the toe  Axial loads (live or dead, elevation, which may have to tensile or compressive) be altered after initial calculations  Lateral loads (live or dead)  Investigations assist in  Moment loads determining the type of  Should be combined both as foundation that should be used unfactored loads (ASD) or factored loads (LRFD)  Use data from deep foundations that have already  Allowable settlements should been installed in the vicinity be determined by structural (capacity and drivability) engineer LateralLateral andand AxialAxial LoadLoad CapacityCapacity  Axial loads  Lateral Loads  Analytic methods should be  Frequently dictate the diameter performed first, although their (section modulus) of the limitations should be understood foundation, so should be  Analysis should include considered first, if present considerations of allowable  p-y curves are the best way of load, allowable settlement, and determining lateral capacity group effects (including block  Large diameter piles may also failure) be necessary for scour  Ideally, a static load test on test resistance piles should be done; however, if  Batter piles are also a typical economic or other factors make way of dealing with lateral loads, this impractical, other methods but can pose rigidity problems in such as Osterberg Tests, seismic situations CAPWAP, or Statnamic should be considered  Seismic retrofits are a common application of laterally loaded  Blow count specifications using a piles wave equation analysis are acceptable on smaller projects DriveabilityDriveability  Piles that cannot be driven cannot be used to support loads; thus, drivability is an important consideration with driven piles  Wave equation methods are ideal to use for drivability studies  Dynamic formulae should be avoided except for the simplest projects Changes in Soil During Driven Pile Installation • Driven Piles • Clays – Interact with the soils differently – Distortion – the movement of soil than drilled piles (drilled shafts, out of the way during driving auger cast piles, etc.) remoulds the clay and reduces its – This leads to different load strength transfer characteristics; the effect – Compression and Excess Pore of driving must be understood Water Pressures – these are raised – Methods are somewhat different, temporarily during driving and and have been considered thus decrease the driving separately resistance and the load bearing • capacity immediately after driving Sands – Loss of contact between pile and – Sands do not experiences the soils – also weaken the soil-pile kinds of changes that clays do bond along the shaft with driven piles – Sands (especially loose ones) experience localised compaction during driving; this increases the capacity

StructuralStructural andand SeismicSeismic DesignDesign  Seismic Design  Structural Design  In seismically active areas,  earthquake occurrence must Once the geotechnical be taken into consideration analysis is done, the structural  Liquefaction of soils is a major engineer can proceed with cause of failure during seismic the structural design events   Deep foundations can be Structural design involves both installed beyond liquefying the structural capacity of the soils in some cases deep foundations themselves  Soil improvement is and the pile caps and necessary in others connecting members that are  Most direct seismic loads on above these members foundations are lateral ones so lateral load analysis is  Plans and specifications important for seismic resistance presented for bid should be  Batter piles tend to be too rigid complete and unambiguous for seismic loads OtherOther ConsiderationsConsiderations Scour and Downdrag Verification and Redesign  Scour  Redesign during construction  Scour is an important is to be avoided but is consideration for bridges over sometimes necessary due to rivers or other bodies of water differences in the subsurface with consistent currents conditions between what was  Loss of supporting soil at the estimated and what actually head of the deep foundation takes place must be considered when  designing for scour Dynamic testing (Case Method, CAPWAP) is a useful  Downdrag tool to evaluate the need for  Compression of soil by redesign but must be used overburden around piles creates with good judgement downdrag on piles  Drilled shafts and the cuttings  This is dealt with either by design or by treatment of the surface of from boring should be the pile inspected before and during the placement of concrete Static Load Testing of Deep Foundations Evaluation Method for Deep Foundations  Analytic methods, based on soil properties from laboratory and/or in-situ tests  Load Testing  Full-scale static load tests on test piles  Dynamic Methods, based on dynamics of pile driving or wave propagation ConceptsConcepts toto ReviewReview • Shaft and End Bearing Piles • Resistance to load – Shaft resistance (Qs ) – Toe resistance (Qt) • End-bearing piles, toe resistance predominates • Shaft Friction Piles, shaft resistance predominates • For tension piles, shaft resistance predominates (toe cannot be in tension)  Ultimate vs. Allowable Capacity  Ultimate capacity is the load required to cause failure, whether by excessive settlement or irreversible movement of the pile relative to the soil  For driven piles, one must also consider the resistance to driving, which can be different from the ultimate capacity  Allowable capacity is the ultimate capacity divided by a factor of safety (ASD)  Important to distinguish between the two PredictionPrediction andand VerificationVerification

•• AtAt what what load load will will the the pile pile fail? fail? (Bearing (Bearing Capacity) Capacity) •• HowHow much much will will pile pile deflect deflect under under service service loads? loads? (Settlement) (Settlement) •• TheThe two two concepts concepts are are not not really really separated separated except except by by custom custom and and codecode

PredictionPrediction onon basisbasis VerificationVerification byby ofof sitesite investigation,investigation, somesome methodmethod ofof laboratorylaboratory testingtesting loadload testingtesting andand analyticanalytic modelingmodeling StaticStatic LoadLoad TestsTests  The most precise – if not always the most accurate – method of determining the ultimate upward or downward load capacity of a deep foundation  Static load tests, however, are time consuming and expensive; must be used judiciously  Object of the test is to develop a load- displacement curve, from which the load capacity can be determined Dead Load Test  Considered a reliable static test method  Slow and expensive  Dangerous when done unsafely  No longer commonly used in the US; used where labor costs are lower Reaction Pile Systems  Advantages  Disadvantages  Can be installed with  Reaction piles may pull out same equipment as  If not done properly, reaction pile capacity may result production piles  Flexible system stores energy  Test can be done on during tests inclination (batter) StaticStatic LoadLoad ProcedureProcedure (ASTM(ASTM D1143)D1143)  Procedure  Two categories of tests  Set up reaction stand  Controlled stress tests – the  Apply a test load to the pile most common method  Record the load-settlement  Controlled strain tests history for each load applied  For driven piles, need  Apply the next load to delay static load  Loads are generally applied in increments of 25, 50, 75, 100, test to allow pile set-up 125, 150, 175 and 200% of  Granular soils – 2 days proposed design load  Cohesive soils – 30 days StaticStatic LoadLoad ProcedureProcedure (ASTM(ASTM D1143)D1143)  Load test increments in time  Slow test – maintains load until pile movement is sufficiently small  Quick test – each load increment is held for a predetermined length of time, for 2.5 – 15 minutes  Generally requires 2-5 hours to complete  May be best method for most deep foundations Static Load Tests: Advantages and Disadvantages  Advantages  Disadvantages  Gives reference capacity  Time consuming  Relatively slow loading  Expensive minimises dynamic  Done on specially components designated piles  Can customise to include  Often done carelessly or creep effects inaccurately  Can be instrumented to yield static resistance distribution & end bearing Use of Telltales StaticStatic LoadLoad TestTest InterpretationInterpretation AtAt whatwhat pointpoint ofof thethe measuredmeasured pilepile toptop loadload vsvs deflectiondeflection curvecurve dodo wewe definedefine thethe failurefailure loadload

RRu??

LoadingLoading methodmethod andand curvecurve interpretationinterpretation cancan makemake significantsignificant differencesdifferences inin result.result. Davisson’s Method

ExampleExample ofof Davisson'sDavisson's MethodMethod 1.51.5 UpliftUplift TestTest Load Transfer Evaluations

EquivalentEquivalent CurveCurve fromfrom OsterbergOsterberg DataData OsterbergOsterberg TestTest AdvantagesAdvantages andand DisadvantagesDisadvantages • Shaft is loaded upward • No reaction load rather than in needed downward direction • Requires jack load only • Tensile vertical strains half of test load near toe will cause cracking in soil • Maximum movement is at the pile toe rather than pile top • Only for specially prepared piles Weaknesses in Static Load Capacity Approach  Uncertainties in soil-pile interface  Multiple interpretations of load-deflection curve  Pile failure and plunging failure not the same  Existence of downdrag in piles  Pile capacity must, in the end, be settlement based OverviewOverview ofof PilePile DynamicsDynamics  Background and the Dynamic Formulae  Development of the Wave Equation  Application of the Wave Equation to Piles  Use of the Wave Equation in field monitoring  Statnamic Testing PilePile BlowBlow CountsCounts DynamicDynamic FormulaeFormulae • The original method of estimating the relationship between the blow count of the hammer and the "capacity" of the pile • Use Newtonian impact mechanics EngineeringEngineering NewsNews FormulaFormula  Variables 2 E r  E = Rated striking Pa= r s+ 0 .1 energy of the hammer, ft-kips  Developed by A.M. Wellington in 1888  s = set of the hammer per blow, in.  The most common dynamic formula  Pa = allowable pile  Assumes a factor of capacity, kips safety of 6 OtherOther DynamicDynamic FormulaeFormulae (after(after Parola,Parola, 1970)1970) Gates Formula Weaknesses of Dynamic Formulae

It is not possible to devise a simple, yet accurate, formula for determining the load bearing capacity of a driven pile and incorporate, therein, allowances for all the modifying conditions that arise. The piles may be long or short, light or heavy, of different cross-sectional areas and materials, and may have been driven with or without a cushion block. Soils very greatly. Diverse styles of pile hammers are employed for driving; some may have, in comparison to the total weight of hammer, or with respect to the weight of the pile that is to be driven, a relatively light ram, others a relatively heavy ram, and may strike few or many blows per minute at widely different velocities. A simple formula cannot take into account the existing ratio of the weight of the striking part (or ram) to the weight of the pile, though both this and the velocity at the moment of impact are factors which greatly affect the efficiency of the blow and, consequently, the bearing capacity of the pile. The effectiveness of the cushioning, elasticity of the pile and quake of the soil are other variable factors of vital importance not possible of inclusion in a simple formula. (Vulcan product literature, 1930’s) The Wave Equation

 When applied to piling, must be  Variables expanded to include dampening and  u = displacement, m spring of shaft resistance  x = distance along the length  Closed form solution possible but of the rod, m limited in application  t = time, seconds  Hyperbolic, second order  Solved numerically for real piling differential equation problems Semi-InfiniteSemi-Infinite PilePile TheoryTheory • From this, • Assumes pile: o Has no resistance of any kind along pile shaft • This relates pile particle o Starts at x = 0 and goes to infinity velocity to pile displacement o Has no reflections back to the pile • Define pile impedance: head • For semi-infinite piles, ModelingModeling thethe PilePile HammerHammer • With semi-infinite pile theory, pile is modeled as a dashpot • Ram and pile top motion solved using methods from dynamics and vibrations • For cushionless ram: ClosedClosed FormForm SolutionSolution FiniteFinite UndampedUndamped PilePile • Simple hammer-pile-soil

K Hammer Cushion system m Pile Cap • We will use this to analyze the effect of the variation of the pile toe • Pile toe spring stiffness can vary from zero (free end) to infinite (fixed end) and an intermediate condition

Kp Pile Toe Spring

Pile Period: FixedFixed EndEnd ResultsResults IntermediateIntermediate CaseCase ResultsResults FreeFree EndEnd ResultsResults NumericalNumerical SolutionsSolutions  Subsequent Solutions  First developed at  TTI (Texas Raymond Concrete Transportation Institute) – Pile by E.A.L. Smith late 1960's (1960)  Very similar to Smith's solution  Solution was first  GRL/Case – 1970's and done manually, then 1980's computers were  Added adequate modelling of diesel hammers involved  Added convenience features  One of the first  TNO applications of computers to civil engineering NecessityNecessity forfor NumericalNumerical SolutionSolution • Non-uniformity of the pile cross section • Existence of dampening, both at the toe, along along the length of the pile, and in some the shaft, and in all of the physical components cases the pile changes materials. of the system. In theory, inclusion of distributed spring constant and dampening • Slack conditions in the pile. These are along the shaft could be simulated using the created by splices in the pile and also pile Telegrapher’s wave equation, but other factors defects. make this impractical also. • With diesel hammers, the force-time • Non-linear force-displacements along the toe characteristics during combustion are and shaft, and in the cushion material. difficult to simulate in closed form. (It Exceeding the “elastic limit” of the soil is in actually took around fifteen years, until fact one of the central objects of pile driving. the first version of WEAP was released, • Non-uniformity of soils along the pile shaft, both in type of soil and in the intensity of the to do a proper job numerically.) resistance. • Unusual driving conditions, such as • Inextensibililty of many of the interfaces of the driving from the bottom of the pile or use system, including all interfaces of the hammer- of a long follower between the hammer cushion-pile system and the pile toe itself. and the pile head. WaveWave EquationEquation forfor PilesPiles inin PracticalPractical SolutionSolution ““BearingBearing Graph”Graph” ResultResult ofof WaveWave EquationEquation AllowableAllowable AxialAxial StressesStresses inin DrivenDriven PilesPiles TAMWAVE • Originally developed in • Uses method presented 2005; recently extensively earlier to estimate static revised capacity • With simple soil and pile • Uses ALP method for axial input, capable of the load-deflection analysis following for single piles: • Uses CLM 2 method for – Axial load-deflection analysis lateral load-deflection – Lateral load-deflection analysis analysis – Wave Equation Drivability • Hammer database (in Analysis ascending energy order) and initial hammer selection estimate available • Includes estimate of soil set- up in clays

16” Concrete Pile Example

16” Concrete Pile Example

16” Concrete Pile Example

900 3

800 2.5 700

600 2 i s s p k i

500 , k s , s D 1.5 e r R t S 400 S

300 1

200 0.5 100

0 0 0 20 40 60 80 100 120 140 160 180 200 Blows per Foot of Penetration Soil Resistance, kips Maximum Compressive Stress, ksi Maximum Tensile Stress, ksi

BasicBasic StepsSteps inin WaveWave EquationEquation AnalysisAnalysis • Gather information • Construct Analysis o o Run static capacity analysis on pile as Hammer type, ram weight, cushion pile driving resistance data, etc. o Apply setup factor (if necessary) on o Suggested trial energy shown in static capacity chart below (included in program) o Input data for hammer, pile and soil o Pile data, including length, material, resistance profile into wave equation etc. analysis o Soil data; layers, soil types, • Run program properties o Run wave equation analysis for different soil resistances (factoring original static analysis) and (for some wave equation programs) different depths of driving • Analyse Results o Blow counts, tension and compression stresses, driving time SoilSoil ResistanceResistance toto DrivingDriving PilePile SetupSetup inin ClaysClays

soil setup factor: the failure load from a static load test divided by the end-of-drive wave equation capacity PilePile ResistanceResistance ExampleExample Dynamic Testing

• Load is applied by impact force of ram • Force is measured by strain gages • Velocity is measured by accelerometers with the results integrated in time • This force-time and velocity-time data is used to estimate the static capacity of the pile and monitor pile stresses during driving

TheThe PilePile DrivingDriving AnalyserAnalyser  For Dynamic Pile  For Dynamic Load Monitoring: Test:  Stresses  Hammer Performance  Bearing Capacity at  Pile Integrity time of testing  Separating Dynamic from Total (Static + Dynamic) Soil Resistance  Case Method  CAPWAP-C Data Output of Pile Driving Analyzer

• The two sensors at the pile head give two sets of data – Force-time – Velocity-time, which is generally multiplied by the impedance (in accordance with semi-infinite pile theory) to allow for common data plotting and comparison •

With CAPWAP-C velocity- time history is input into pile model and force-time history is matched, but this can be reversed depending upon the algorithm being used CAPWAPCAPWAP ModelingModeling ofof PilePile ResponseResponse andand CapacityCapacity Case Method for Pile Analysis

● Simple “Back of the Envelope” ● Uses output from Pile Analyzer Method for Estimating Pile ● Definitions Capacity from Dynamic Results – t1 = time of peak impact force (or ● Assumptions: later, depending upon interpretation – The pile resistance is concentrated at method) the pile toe, as was the case with the – t2 = t1 + 2L/c closed form solutions above. – F1 = pile head force at time t1, kN or – The static toe resistance is kips completely plastic, as opposed to the – F2 = pile head force at time t2, kN or purely elastic resistance modelled kips above. (Both the wave equation numerical analysis and CAPWAP – V1 = pile head velocity at time t1, assume an elasto-plastic model for m/sec or ft/sec the static component of the – V2 = pile head velocity at time t2, resistance. m/sec or ft/sec

●

Case Method for Pile Analysis

● Definitions ● Basic Formulas – Z = pile impedance, kN-sec/m or kip-sec/ft – RTL = RD + RS – ZV1 = product of impedance and – velocity at time t1, kN or kips RTL = (F1 + F2)/2 + – ZV2 = product of impedance and (ZV1 – ZV2)/2 velocity at time t2, kN or kips – RD = J(F1 + ZV1 – – RTL = total resistance, kN or kips RTL) – RD = dynamic resistance, kN or kips – RS = RTL - RD – RS = static resistance, kN or kips – J = Case damping constant, dimensionless

Case Method for Pile Analysis

– ● Interpretation Methods RMX Method ● The time t1 is for the RMX method is the peak initial force plus a time shift, generally – RSP Method 30 msec (Fellenius (2009), see below.) The time t2 is still t1 + 2L/c. This time shift ● The t1 for the RSP is to account for the delay caused by the method is the first peak elasticity of the soil. ● The RMX method is best for piles with large point in the force-time toe resistances and large displacement piles with the large toe quakes that curve (see below) accompany them. The quake of the soil is the distance from initial position of the soil- ● Best used for piles with pile interface at which the deformation low displacements and changes from elastic to plastic, see variable “Q”. The toe quake is proportional to the high shaft resistances. size of the pile at the toe.

CaseCase MethodMethod ExampleExample

• Find • Given o Case Method ultimate capacity for o Pile with impedance of 381 kN-sec/ the RSP and RMX methods m o Force-time history as shown at the left • F1 = 1486 kN (shown as FT1) • F2 = 819 kN (shown as FT2) • V1 = 3.93 m/sec (shown as VT1) • Z = 670 kN / 1.75 m/sec = 381 kN-sec/m • ZV1 = (381)(3.93) = 1497.33 kN • V2 = 1.07 m/sec (shown as VT2) • ZV2 = (381)(1.07) = 407.67 kN Case Method Example

● RSP Method ● RMX Method – Data points different, see graph below, – Data points given because of the different values of t1 and t2 – F1 = 819 kN – Assume J = 0.4 – F2 = 1486 kN – RTL = (1486 + 819)/2 + – V1 = 1.92 m/sec (1497.33 – 407.67)/2 = – ZV1 = (381)(1.92) = 731.52 kN – 1697 kN VT2 = 0 m/sec – ZV2 = (381)(0) = 0 kN-sec/m – RD = 0.4(1486 + 1497.33 – 1697) = 514 kN – RS = 1697 – 514 = 1183 kN

Case Method Example

● RMX Method ● Substituting and – Value for Case Solving Damping Constant J – RTL = (819 + 1486)/2 ● The Case damping + (731.52 – 0)/2 = constant for the RMX 1518 kN method is generally greater than the one – RD = 0.7(819 + used for RSP, typically 731.52 – 1518) = 16 by +0.2, and should be kN at least 0.4. – RS = 1518 – 16 = ● In this case we will assume J = 0.7. 1498 kN

DeterminingDetermining CaseCase DampingDamping ConstantConstant

The reality is that the Case damping constant is a “job-specific” quantity which can and will change with changes of soil, pile and even pile hammer. These require calibration, either with CAPWAP or theoretically with the wave equation program. The Case Method requires a great deal of experience and judgment in its application to actual pile driving situations. InterpretingInterpreting Force-Force- TimeTime CurvesCurves DynamicDynamic PilePile TestingTesting CritiqueCritique –QuickQuick andand inexpensiveinexpensive –CanCan testtest allall typestypes ofof preformedpreformed pilespiles (concrete,(concrete, steelsteel andand timber)timber) andand drilleddrilled shaftsshafts withwith wellwell defineddefined geometrygeometry –NoNo specialspecial preparationpreparation requiredrequired –StaticStatic capacitycapacity isis interpretedinterpreted ratherrather thanthan measuredmeasured directlydirectly –RequiresRequires experienceexperience forfor correctcorrect interpretationinterpretation StatnamicStatnamic TestsTests

http://www.youtube.com/watch?v=IHnbd-QGdaw StatnamicStatnamic DeviceDevice andand PrinciplesPrinciples  Controlled explosion detonated; loads the pile over longer period of time than impact dynamic testing  Upward thrust transferred to reaction weights  Laser sensor records deflections; load cell records loads TypicalTypical StatnamicStatnamic Force-TimeForce-Time CurvesCurves

0 0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30

-0.01

-50

-0.02

-100 D

e F f o -0.03 l

e r c c t e i o , k n N Load (kN) ,

m

-0.04 -150

-0.05

-200

-0.06 Deflection (m)

-250 -0.07 Time, sec TypicalTypical StatnamicStatnamic Load-DeflectionLoad-Deflection CurvesCurves

50 0 -50 -100 -150 -200 -250 0.010

0.000 Total -0.010 Force

-0.020 D i s p l a c e

Static m -0.030 e n t

Force ( m )

-0.040

-0.050

Total -0.060 Force Unloading Point less -0.070 inertial Load (kN) force StatnamicStatnamic AdvantagesAdvantages andand DisadvantagesDisadvantages  Advantages  Disadvantages  Much faster and  Does not give a clear simpler than static load picture of the testing distribution of capacity  Does not require a pile between the shaft and hammer as high-strain the toe dynamic testing does  Especially applicable  Technique not entirely to drilled shafts and developed for clay other bored piles (high dampening) soils

OverviewOverview ofof CollapsibleCollapsible SoilsSoils DealingDealing WithWith CollapsibleCollapsible SoilsSoils • Solutions for Pavements • Other Foundation o Ponding water over the region of Solutions collapsible soils. o Shallow Foundations—furnish a o Infiltration wells. system of grade beams to distribute o Compaction - conventional with the load and mitigate the effects of heavy vibratory roller for shallow uneven collapse of the underlying depths (within 0.3 or 0.6 m (1 or 2 soils ft)) o Deep foundations—avoid the o Compaction - dynamic or vibratory effects of collapsible soils for deeper deposits of more than altogether by transferring the half a meter (a few feet) (could be structure load to a more stable combined with inundation) stratum o Excavated and replaced. ExpansiveExpansive SoilsSoils Activity Index

RegionsRegions ofof ExpansiveExpansive SoilsSoils DamageDamage DueDue toto ExpansiveExpansive SoilsSoils Swell Potential

Swell Potential

DeterminationDetermination ofof ActualActual SoilSoil ExpansionExpansion BasisBasis forfor CalculationsCalculations VanVan derder MerweMerwe MethodMethod Example of Swell Calculation • Given • Solution – Clay soil, PI = 40, CF = 55 – Compute Activity Indices • Find • A = PI/CF = 40/55 = 0.72 • Am = PI/(CF-5) = 40/50 = 0.8 – Swell potential – Obtain swell potential – Amount of vertical • From Figure 5-27, High, movement assuming only 1 approx. 10% m of soil is significant • From van der Merwe’s chart, Very High – Compute actual swell using van der Merwe’s Method w/ 1 layer 0.5 − ΔH DF (1)(10 6 ) ΔHH = = = 0.069m=69mm PE 12 FoundationsFoundations forfor ExpansiveExpansive SoilsSoils ShallowShallow FoundationsFoundations forfor ExpansiveExpansive SoilsSoils UseUse ofof DrainageDrainage TechniquesTechniques BelledBelled BasesBases forfor DrilledDrilled ShaftsShafts  Use of an enlarged base, in conjunction with the shaft resistance of the straight portion, is a common way of dealing with expansive soils for major structures  Enlarged bases offer additional uplift capacity  Only applicable to clays, since sands would usually collapse  Difficult to quantify UpwardUpward LoadLoad CapacityCapacity ofof BelledBelled ShaftsShafts  Variables for uplift capacity 2 2  (Pupward)a = ultimate/unfactored π ( Bb−Bs ) upward load capacity ( Pupward )a= su N u 4  su = average undrained shear strength of the cohesive soil between the base of the bell and 2B above the base Unfissured Clay: b

 σzD = total stress at the bottom of the 7 Db base N u= ⩽9  B = diameter of the enlarged base 2 Bb b  B = diameter of the shaft Fissured Clay: s  Db = depth of embedment of enlarged base into bearing stratum 7 Db N u= ⩽9  Uplift of base only 10 Bb ExampleExample ofof UpliftUplift CapacityCapacity  Given  Drilled Shaft as shown  Find  Uplift capacity of shaft  Assume  Factor of safety = 5  Very stiff clay at toe is fissured  Include weight of foundation ComputeCompute ShaftShaft FrictionFriction andand NNuu

Layer Layer Start Layer End Thickness, ft. su, psf alpha fs, psf As, sq. ft. fsAs, kips

1 0 5 5 1600 0 0 31.42 0.00

2 5 12 7 1600 0.55 880 43.98 38.70

3 12 37 25 1400 0.55 770 157.08 120.95

4 37 52 15 4000 0.48 1920 94.25 180.96

5 52 62 10 4000 0 0 62.83 0.00

Total Shaft Resistance 340.61 Shaft Diameter 2 feet Perimeter 6.28 feet Different from compression Top Segment to Equal to twice the diameter of the be Ignored 5 feet belled toe Toe Segment to be Ignored 10 feet

Db 22.5 Nu=0.7 =0 .7 =3.29≤9 Bb 5 ComputeCompute UpliftUplift CapacityCapacity • Compute Ultimate Uplift • Compute Weight of Capacity of Bell Foundation and Total Uplift Capacity

π B2−B2 2 ( b s ) 2+5 ( Pupward )a= N c su 4 2 π 2 2 π 2 ( 2 ) π ( 5b−2s ) ( P ) = (4)(3.29) W f = 58 .5 + 1.5 0 .150 upward a 4 ( 4 4 ) ( Pupward )a=217 kips W f =34.5 kips 34.5+340.6×0.75+217 ( P ) = =101 .4 kips upward a 5 Note that we use FS=5 for uplift QuestionsQuestions