DESIGNING FOR DOWNDRAG ON UNCOATED AND BITUMEN COATED PILES

Transcription

1 DESIGNING FOR DOWNDRAG ON UNCOATED AND BITUMEN COATED PILES Jean-Louis BRIAUD, PhD, PE President of ISSMGE Professor and Holder of the Buchanan Chair Texas A&M University Piling and Deep Foundations Middle East February 2010

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3 Content Outline 1. What is downdrag? 2. Predicting pile behavior subjected to downdrag 3. Design and selection of bitumen 4. Example problem 5. Full scale field test in New Orleans, LA, USA 6. Full scale field test in Edmonton, AL, Canada 7. Conclusions

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5 What is downdrag?

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10 When to Design for Downdrag

11 Design to Avoid these Three Conditions

12 Static Equilibrium Qmax Qt Fn Fp Qp

13 Finding the Depth of Neutral Point ω pile (NP) = ω soil (NP) ω soil (NP)= read on consolidation settlement profile (given) ω pile (NP) = ω point + ω elastic ω point = ω soil (@Z=L) +ω punch ω elastic = elastic compression from bottom to NP

14 Pile Point Behavior punch 4 2 (1 v ) QD p AE s ω punch ν Q p A D E s = Pile point movement = Poisson s ratio = Point resistance = Area of pile point = Diameter of pile point = Soil modulus For clays E s = 100 S u = E pmt For sands E s (kpa) = 800 N = 2 E pmt

15 Pile Point Behavior Q 1 elastic p p 2 F L Z AE ω elastic Q p F p L Z A E = Elastic compression of pile = Point resistance = Positive friction force = Depth of pile point = Location of NP = Area of pile point = Pile modulus

16 Maximum Friction FOR PILES IN CLAY (short term and long term) f max = S u f max = σ ov

17 Maximum Friction FOR PILES IN SAND f max = σ ov f max (kpa) = 5 (N) 0.7 N = SPT blow count

18 Maximum Point Resistance FOR PILES IN SANDS AND IN CLAYS Clay (short term) Clay (long term) q max = 9 S u q max = σ ov N q Sand (short & long term) q max (kpa) = 1000 (N) 0.5 Sand (short & long term) q max = σ ov N q q max (1 v ) 2 max 4 AEs D

19 Downdrag on Pile Groups

20 Downdrag on Pile Groups After Okabe, 1977

21 Reduction Factors for Downdrag in Pile Groups

22 PILENEG - Developed at Texas A&M University - Analyzes axially-loaded single piles under both positive and negative skin friction - Bitumen coated piles can also be analyzed with the program

23 PILENEG Inputs - Top load on pile - Pile data (cross section area of the pile, pile perimeter, embedded pile length, pile modulus) - Soil data (profile of maximum friction, soil settlement profile, soil modulus and Poisson s ratio at point, ultimate bearing pressure at point)

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25 PILENEG Outputs - Top load, top settlement, depth of bitumen coating, depth to neutral point, maximum load, maximum stress and pile point load

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28 WHAT IS BITUMEN? A black viscous substance. The residue left at the end of the refining process of crude oil, made of high molecular weight hydrocarbons WHAT IS A PRIMER? A liquid made of half bitumen and half solvent to liquefy the bitumen for easy application and filling the holes. The layer left is largely the bitumen after drying.

29 Behavior of Bitumen Bitumen is a viscous material τ = η (dγ/dt)

30 τ = η (dγ/dt)

31 τ = η (dγ/dt) η decreases when dγ/dt increases dγ/dt x 10 leads to ~ η / 2 η decreases when T increases T x 2 leads to ~ η / 10 η much more sensitive to T than dγ/dt

32 Bitumen Selection τ = η (dγ/dt) SOIL Storing Driving Downdrag Particle penetration

33 Storage Criterion req( storage ) gtd h 2 ρ = Bitumen mass density g = Acceleration due to gravity t = Max anticipated storage period d = Bitumen coating thickness h = Allowable bitumen flow distance (usually taken as equal to d) Storage requires: η > η req(storage)

34 req( driving, air ) a a Driving Criterion vd ρ = Bitumen mass density v a = Velocity of bitumen in the air (taken as the velocity of the hammer at time of impact) d = Bitumen coating thickness h = Allowable bitumen flow distance (usually taken as equal to d) f max = Maximum friction t = Max anticipated storage period a h 2 req( driving, soil ) s max Driving in air requires: η > η req(driving, air) f h td Driving in soil requires: η > η req(driving, soil)

35 Downdrag Criterion req( downdrag ) d d τ bit = Allowable shear stress on bitumen (τ bit ~ 0.1 τ s,avg ) τ s,avg = Soil shear strength d = Bitumen coating thickness s = Settlement rate s 1, s 2 = Settlements of the ground surface at times t 1 and t 2, respectively (after placement of an embankment or other load) bit bit bit s s s2 s1 t t 2 1 Downdrag requires: η < η req(driving)

36 Particle Penetration Criterion ' H K ' OV FOR DRIVEN PILES: K = 1 FOR BORED PILES: K = K 0 = (1-sinΦ)OCR 0.5

37 Bitumen Design Criteria Summary

38 EXAMPLE: Find allowable top load for a top settlement < 14 mm 1) Pile Ult. Capacity Q u = (25x1.2x30) Q u = = 1900 kn

39 EXAMPLE: 2) Assume a top load = 500 kn and find the maximum depth to NP X = (900 - X) X = 700 kn or 23.3 m of friction ω NP(soil) = 35 mm (settlement profile) ω top = 35+(850*23300)/(0.09*2E7) = 46 mm (This is more than the allowable settlement)

40 EXAMPLE: 3) Assume a top load = 100 kn and NP = 29 m Q p = = 940 kn ω p = 4.7 mm ω NP(pile) = (955*1000)/(0.09*2E7) = 5.2 mm ~ ω NP(soil) ω top = 5 + (535*29000)/(0.09*2E7) = 13.6 mm

41 EXAMPLE: 4) Use bitumen coating with shear strength = 2.5 kn/m 2 and try a top load of 500 kn (Assume NP = 29 m) ω NP(soil) = 5 mm Q p = = 557 kn ω p = 2.8 mm ω top = 5 + (543.5*29)/(0.09*2E7)*(1000) = 13.8 mm

42 ω top = 46 mm ω top = 14 mm ω top = 14 mm with bitumen

43 Full Scale Field Test: Alberta, Edmonton, Canada

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45 CONSTRUCTION AND SPECIFICATIONS Clean surfaces (free of dust and grease) Apply the primer (wait 24 hrs for drying) Apply the bitumen (10 mm thick) Storing (cold, hot) Handling in the field (pad eye, no strap) Drive the piles (splice, temperature)

46 Clean surfaces (free of dust and grease) Apply the primer (wait 24 hrs for drying)

47 Applying the bitumen (10 mm thick)

48 Applying the bitumen (10 mm thick)

49 APPLYING BITUMEN COATING OUTSIDE IN THE COLD BITUMEN BECOMES BRITTLE DUE TO COATING OUTDOORS

50 Handling in the field

51 Storing the coated piles In hot weather, store under cover away from direct sunlight In cold weather, wait until bitumen cools down before storing

52 Driving the piles (splice, temperature)

53 AVERAGE ANNUAL AIR TEMPERATURE = SOIL TEMPERATURE

54 General Observations 1. Downdrag does not reduce the ultimate capacity of a pile 2. Downdrag increases the load in the pile 3. Downdrag increases the settlement of a pile; downdrag is a case where settlement controls the pile design 4. Live loads should not be included in settlement calculations including downdrag

55 General Observations 6. Downdrag makes a pile come out of the ground 7. Bitumen reduces downdrag significantly (90%?) when properly chosen and properly applied 8. Bitumen is most easily applied when the air is colder than the soil (winter) but when the temperature is above freezing 9. Piles located on the inside of a pile group experience less downdrag than the perimeter piles

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57 For more information On Downdrag Videotape on Bitumen Coating ( Briaud J.-L., Tucker L.M., 1998, Design guidelines for downdrag on uncoated and bitumen coated piles, NCHRP Report 393, National Academy of Sciences.

58 THANK YOU

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61 Full Scale Field Test: New Orleans, Louisiana, USA

62 Full Scale Field Test: New Orleans, Louisiana, USA

63 Full Scale Field Test: New Orleans, Louisiana, USA

64 STEEL WOOD PPC

65 APPLYING PRIMER COATING WITH BITUMEN

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67 Full Scale Field Test: Alberta, Edmonton, Canada