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analysis of pile load tests

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analysis of pile load tests

  1. 1. A Presentation by: SORABH GUPTA, Vice PresidentSORABH GUPTA, Vice President Cengrs Geotechnica Pvt. Ltd., NoidaCengrs Geotechnica Pvt. Ltd., Noida ISRMISRM -- IGS Delhi ChapterIGS Delhi Chapter National Symposium onNational Symposium on ““ADVANCES IN INSTRUMENTATION, GEOADVANCES IN INSTRUMENTATION, GEO--MONITORING AND VALIDATIONMONITORING AND VALIDATION”” 2323thth July, 2015July, 2015
  2. 2. Piles for High Rise Buildings Cost is often the primary concern, especially inCost is often the primary concern, especially in the unorganized real estate sectorthe unorganized real estate sector Economy and cost saving is paramountEconomy and cost saving is paramount -- thethe builder has to maximize his profitsbuilder has to maximize his profits To meet growing load demands onTo meet growing load demands on foundations, design has to be innovative andfoundations, design has to be innovative and boldbold But risk should be minimalBut risk should be minimal Importance of QA/QC increases proportionatelyImportance of QA/QC increases proportionately
  3. 3. Mixed Land use projectMixed Land use project (commercial, retail,(commercial, retail, residential & studio)residential & studio) Site at prime location inSite at prime location in NoidaNoida Buildings shall have 12 toBuildings shall have 12 to 40 storeys with 240 storeys with 2 basementsbasements Basement floor 8.5 mBasement floor 8.5 m below adjoining road levelbelow adjoining road level Case Study: Project Overview
  4. 4. Initial Static Pile Load Tests 1200 mm dia initial test piles: 4 Nos.1200 mm dia initial test piles: 4 Nos. 1000 mm dia initial test piles: 3 Nos.1000 mm dia initial test piles: 3 Nos. 1000 mm dia working test piles: 1 No.1000 mm dia working test piles: 1 No.
  5. 5. PilePile No.No. PilePile LengthLength belowbelow testtest level, mlevel, m DepthDepth belowbelow COL, mCOL, m Max.Max. appliedapplied test Load,test Load, mm Pile HeadPile Head DisplaDispla-- cement,cement, mmmm InterpreInterpre--tedted SafeSafe Capacity,Capacity, TonnesTonnes Type ofType of Slurry usedSlurry used TP-1 42.2 40 782 131.25 317 BentoniteBentonite TP-2 37.2 35 612 144.80 280 BentoniteBentonite TP-3 36.2 34 1632 127.64 613 CompositeComposite TP-4 36.2 34 1700 21.24 1000 CompositeComposite Composite: Polymud + Alfabond (a thickening agent) and bentoniteComposite: Polymud + Alfabond (a thickening agent) and bentonite Static Load Test Results (1200 mm dia) Poor Construction??
  6. 6. TPTP--1, 2 plunged at1, 2 plunged at 600600--700 MT700 MT –– softsoft toe condition??toe condition?? TPTP--3 indicates about3 indicates about 600 MT safe capacity600 MT safe capacity TPTP--4 gave very good4 gave very good results (~1000 MTresults (~1000 MT safe)safe) Static Load Test Results (1200 mm dia)
  7. 7. Static Load Test Results (1000 mm dia) PilePile No.No. PilePile LengthLength belowbelow testtest level, mlevel, m DepthDepth belowbelow COL,COL, mm Max.Max. appliedapplied testtest Load, mLoad, m PilePile HeadHead DisplaDispla-- cement,cement, mmmm InterpreInterpre-- ted Safeted Safe Capacity,Capacity, TonnesTonnes Type ofType of SlurrySlurry usedused RemarksRemarks TP-1 38 35 856 120.58 353 PolymerPolymer Soft Toe?Soft Toe? TP-2 33 30 1510 35.66 686 PolymerPolymer TP-3 33 30 1510 24.24 860 PolymerPolymer Gravel PadGravel Pad providedprovided at pileat pile bottombottomTP-4 33 30 1334 14.82 >890 PolymerPolymer Poor Construction??
  8. 8. TPTP--1 which is 38 m1 which is 38 m long, gavelong, gave lowestlowest capacitycapacity, matching with, matching with traditional analysistraditional analysis usingusing ““conservativeconservative”” parametersparameters The 33 m long pilesThe 33 m long piles performed significantlyperformed significantly betterbetter Static Load Test Results (1000 mm dia) Soft Toe Condition??
  9. 9. Extrapolation of Load Test Results Where test was not carried out till failure,Where test was not carried out till failure, results were mathematically extrapolated usingresults were mathematically extrapolated using thethe Hyperbolic ModelHyperbolic Model IS Code criteriaIS Code criteria 2/3 of load corresponding to displacement of2/3 of load corresponding to displacement of 18 mm18 mm 50% of load corresponding to displacement50% of load corresponding to displacement of 10% of pile diaof 10% of pile dia ' ' ' bSa Q S +=
  10. 10. Regression Extrapolation Extrapolation of Load Test Results
  11. 11. PilePile No.No. QQ18 mm18 mm 2/3 Q2/3 Q18 mm18 mm QQ0.1D0.1D/2/2 Interpreted PileInterpreted Pile Capacity,Capacity, TonnesTonnes TP-1 Test carried out to >100mm settlement.Test carried out to >100mm settlement. Extrapolation not requiredExtrapolation not required 353 TP-2 10301030 687687 12211221 687 TP-3 12781278 852852 14891489 852 TP-4 15231523 10151015 13801380 1015 Extrapolation of Load Test Results Interpreted Safe Pile Capacities from Extrapolated Load Settlement Curves for 1000 mm dia piles Very high numbers!
  12. 12. Results Inconsistent!! No clear trend of increase in capacity with increase in pileNo clear trend of increase in capacity with increase in pile dimensions (dia, length)dimensions (dia, length)-- inconsistent pile qualityinconsistent pile quality Some piles underSome piles under--performedperformed–– structural defects??structural defects?? 1200 mm dia pile generally show poor performance, safe1200 mm dia pile generally show poor performance, safe capacities lower than 1000 mm dia piles of samecapacities lower than 1000 mm dia piles of same lengthslengths-- Reason??Reason?? TPTP--2, 3, 4 of 1000 mm dia indicates very good2, 3, 4 of 1000 mm dia indicates very good performanceperformance Could use of polymer instead of bentonite have made theCould use of polymer instead of bentonite have made the difference?difference? –– contractor opined YEScontractor opined YES Did the use of 150 mm gravel pad at pile bottom haveDid the use of 150 mm gravel pad at pile bottom have any positive influence?any positive influence? –– builder felt it did!!builder felt it did!!
  13. 13. Decision DECISION TAKEN: WORKING PILESDECISION TAKEN: WORKING PILES SHALL BE OF 1000 DIASHALL BE OF 1000 DIA Pile cutPile cut--off level at 10 m depth below EGLoff level at 10 m depth below EGL RCC raft over the pilesRCC raft over the piles Design as hybrid raftDesign as hybrid raft--pile system may bepile system may be consideredconsidered
  14. 14. Unanswered Questions What safe pile capacity should we consider forWhat safe pile capacity should we consider for design?design? GeneralGeneral ““wisdomwisdom”” says 300says 300--400 MT!! Load tests400 MT!! Load tests suggest otherwisesuggest otherwise How do we rationalize the load test data?How do we rationalize the load test data? Load tests indicate anything between 350 MT andLoad tests indicate anything between 350 MT and 1000 MT!! Highly inconsistent data.1000 MT!! Highly inconsistent data.
  15. 15. Analysis Approach
  16. 16. Site Stratigraphy Project area in the IndoProject area in the Indo--Gangetic alluviumGangetic alluvium primarily deposits of the Yamuna & Hindonprimarily deposits of the Yamuna & Hindon riversrivers Fine sand / silty sand with minor discontinuousFine sand / silty sand with minor discontinuous layers of sandy silt to 46 m depthlayers of sandy silt to 46 m depth Groundwater level at 7.5 m below EGLGroundwater level at 7.5 m below EGL
  17. 17. Typical Borehole Data EGL = NGL – 6m
  18. 18. Inadequate Investigation Data Suggested:Suggested: Additional deep boreholesAdditional deep boreholes Pressuremeter TestsPressuremeter Tests Static Cone Penetration TestsStatic Cone Penetration Tests Footing Load Tests (for raft design)Footing Load Tests (for raft design) But client wanted world-class design based on “old class” data
  19. 19. Selected Soil Parameters for Design
  20. 20. Design Profile Depth belowDepth below excavatedexcavated level, mlevel, m Soil ClassificationSoil Classification c KN/m2 φº δº γ KN/m2 K FromFrom ToTo 0 6 Fine Sand 0 30 30 17.0 1.0 6 13.5 Fine Sand 0 36 31 18.0 1.1 13.5 22 Fine Sand 0 38 33 18.5 1.2 22 30 Silty fine Sand 0 40 35 19.0 1.3 30 40 Silty fine Sand 0 45 40 19.5 1.5 40 45 Silty fine Sand 0 45 45 19.5 1.5 To assess pile capacity for best case scenarioTo assess pile capacity for best case scenario φφ higher than normally usedhigher than normally used
  21. 21. Theoretical Pile Capacity (Ultimate) Pile Diameter, mm Effective Pile Length below cut-off level, m Ultimate Pile Capacities Computed Ultimate Pile Shaft Friction (Qs), MT Computed Ultimate Pile End Bearing (Qb), MT Computed Ultimate Pile Capacity, MT 1000 30 658 1404 2062 35 868 1404 2272 1200 30 900 2459 3358 35 1208 2459 3666
  22. 22. Theoretical Pile Capacity (“Safe”) Pile Diameter, mm Effective Pile Length below cut-off level, m Lower bound Safe Pile Load Carrying Capacity, Tonnes Upper bound Safe Pile Load Carrying Capacity, Tonnes 10001000 3030 330330 890890 3535 434434 996996 12001200 3030 450450 14331433 3535 604604 15871587 Lower Bound: FOS of 2.0 on skin frictionLower Bound: FOS of 2.0 on skin friction Ignore End BearingIgnore End Bearing Upper Bound : FOS of 2.0 on skin frictionUpper Bound : FOS of 2.0 on skin friction FOS of 2.5 on End BearingFOS of 2.5 on End Bearing
  23. 23. Low-Strain Pile Integrity Tests (PIT)
  24. 24. Accelerometer measures response defectdefect Good Bad Small hammer impact device (defect) PIT: Concept
  25. 25. Weak toe response, soft toe below 31.2 m depthWeak toe response, soft toe below 31.2 m depth Explains plunging behavior during load testExplains plunging behavior during load test Necking at 5Necking at 5--6 m depth6 m depth Pile integrity generally okPile integrity generally ok PIT: Results
  26. 26. Pile shaft fairly uniformPile shaft fairly uniform Pile length & integrity okPile length & integrity ok Bulging between 15 and 25 m depth.Bulging between 15 and 25 m depth. Below that, dia is okBelow that, dia is ok Pile length & integrity generally okPile length & integrity generally ok PIT: Results
  27. 27. Instrumented Pile Load Test
  28. 28. High-Strain Dynamic Pile Testing (HSDLT) with the Pile Driving Analyzer®
  29. 29. Motion is measuredMotion is measured by accelerometersby accelerometers strainstrain gaugegauge FF(t)(t) accelerometeraccelerometer vv(t)(t) Load is appliedLoad is applied by impacting ramby impacting ram Load is measured byLoad is measured by strain transducersstrain transducers HSDLT: Concept
  30. 30. HSDLT: On Site
  31. 31. CAPWAP Analysis Shaft Resistance Mobilized: 840.8 T End Bearing Mobilized : 390.2 T Total Resistance Mobilized: 1231 T Max. Estimated Displacement 12.9 mm Interpreted Safe load > 616 T Cracks on pile head, higher impact could not be given
  32. 32. Discussion of Overall Results Safe load for 1000 mm dia 33 m long pileSafe load for 1000 mm dia 33 m long pile ranges from 650 to 852 Tranges from 650 to 852 T To use such high capacity ( >> theoreticallyTo use such high capacity ( >> theoretically computed value), high degree of reliability andcomputed value), high degree of reliability and good, consistent construction quality isgood, consistent construction quality is essentialessential Selection of appropriate value for design isSelection of appropriate value for design is challengingchallenging While builder would like to use highest value,While builder would like to use highest value, caution should be exercised considering thecaution should be exercised considering the consequences of failureconsequences of failure
  33. 33. Testing to Ensure Reliability If capacities higher than IS code values are used,If capacities higher than IS code values are used, testing should also exceed the codal requirementtesting should also exceed the codal requirement Not practical to load test every pileNot practical to load test every pile Plan sufficient testing:Plan sufficient testing: Static load testStatic load test –– min 2min 2--4% of piles4% of piles Low Strain Pile integrity on 100% pilesLow Strain Pile integrity on 100% piles High strain Dynamic Load test on 5High strain Dynamic Load test on 5--10% piles10% piles CrossCross--hole sonic logginghole sonic logging –– say 15say 15--2020 Use integrity tests effectively to decide whichUse integrity tests effectively to decide which piles to testpiles to test
  34. 34. Number of piles to be tested cannot be decidedNumber of piles to be tested cannot be decided beforebefore--handhand As results come in, decision should be carefullyAs results come in, decision should be carefully takentaken If any pile fails or shows compromised integrity,If any pile fails or shows compromised integrity, the number of piles with questionable capacitythe number of piles with questionable capacity should be assessedshould be assessed Number of tests required should be increasedNumber of tests required should be increased appropriatelyappropriately ReRe--design the piling scheme as per the testdesign the piling scheme as per the test resultsresults Testing to Ensure Reliability
  35. 35. Concluding Remarks If load tests show substantially higherIf load tests show substantially higher capacities than computed values, designcapacities than computed values, design capacities may be upgradedcapacities may be upgraded Load test results must be validated byLoad test results must be validated by conventional Geotechnicsconventional Geotechnics Sufficient testing should be done as a backSufficient testing should be done as a back--upup to ensure repeatability and reliabilityto ensure repeatability and reliability
  36. 36. I will be happy to take questions… Now, or Later? Sorabh Gupta Vice President, CENGRS Email me at sorabh@cengrs.com Or Call +91 99108 61118 Visit our Website www.cengrs.com for more information

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