downdrag lecture

7/30/2019 Downdrag Lecture

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Jean-Louis BRIAUD Texas A&M University

DESIGNING FOR DOWNDRAG ON UNCOATED AND

BITUMEN COATED PILES

Jean-Louis BRIAUD, PhD, PE

President of ISSMGEProfessor and Holder of the Buchanan Chair

Texas A&M University

Piling and Deep Foundations Middle East 2010

1 February 2010


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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

Content Outline


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What is downdrag?


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7/67Jean-Louis BRIAUD Texas A&M University



When to Design for Downdrag



Design to Avoid these Three Conditions



Static Equilibrium

max t n p pQ Q F F Q



Finding the Depth of Neutral Point

pile(NP) = soil(NP)

soil(NP)= read on

consolidation

settlement profile (given)

pile(NP) = point + elastic

point = soil (@Z=L) +punch

elastic = elastic compression

from bottom to NP



Pile Point Behavior

2

(1 )4

p

punch

s

Q D

v AE

punch = Pile point movement

= Poissons ratio

Qp = Point resistance

A = Area of pile point

D = Diameter of pile point

Es = Soil modulus

For clays

Es = 100 Su = EpmtFor sands

Es (kPa) = 800 N = 2 Epmt



Pile Point Behavior

12

elastic p pL ZQ F

AE

elastic

= Elastic compression

of pile

Qp = Point resistance

Fp = Positive friction force

L = Depth of pile point

Z = Location of NPA = Area of pile point

E = Pile modulus



FOR PILES IN CLAY (short term and long term)

fmax = Su fmax = ov

Maximum Friction



FOR PILES IN SAND fmax = ov

fmax (kPa) = 5 (N)0.7

N = SPT blow count

Maximum Friction



FOR PILES IN SANDS AND IN CLAYS

Clay (short term) qmax = 9 Su

Clay (long term) qmax = ov Nq

Sand (short & long term) qmax (kPa) = 1000 (N)0.5

Sand (short & long term) qmax = ov Nq

Maximum Point Resistance

2 max

max(1 )

4s

q Dv

AE



Downdrag on Pile Groups


20/67Jean-Louis BRIAUD Texas A&M UniversityAfter Okabe 1977

Downdrag on Pile Groups



Reduction

Factors for

Downdrag in PileGroups


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- 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

PILENEG

http://ceprofs.tamu.edu/briaud/pileneg/pileneg.zip


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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 Poissons

ratio at point, ultimate bearing pressure at point)


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- Top load, top settlement, depth of bitumen

coating, depth to neutral point, maximum load,

maximum stress and pile point load

PILENEG Outputs


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WHAT IS BITUMEN?

A black viscous substance. The residue left at theend 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.


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Bitumen is a viscous material = (d/dt)

Behavior of Bitumen


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= (d/dt)


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= (d/dt)

decreases when d/dt increases

d/dt x 10 leads to ~ / 2

decreases when T increases

T x 2 leads to ~ / 10

much more sensitive to T than d/dt


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= (d/dt)

Storing

Driving

Downdrag

Particle penetration

SOIL

Bitumen Selection


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Storage Criterion

2

( )req storage gtdh

= Bitumen mass density

g = Acceleration due togravity

t = Max anticipated storage

period

d = Bitumen coatingthickness

h = Allowable bitumen flow

distance (usually taken as

equal to d)

Storage requires:

> req(storage)


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Driving Criterion

2

( , )

a a

req driving air

a

v d

h

= Bitumen mass density

va = 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 asequal to d)

fmax = Maximum friction

t = Max anticipated storage

period

Driving in air requires:

> req(driving, air)

max

( , )

s

req driving soil

f td

h

Driving in soil requires: > req(driving, soil)


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Downdrag Criterion

( )

2 1

2 1

bit bit bit

req downdrag

s

d d

s s st t

bit = Allowable shear stress

on bitumen (bit ~ 0.1 s,avg)s,avg = Soil shear strength

d = Bitumen coating thickness

s = Settlement rate

s1 , s2 = Settlements of theground surface at times t1and t2, respectively (after

placement of an embankment

or other load)

Downdrag requires:

< req(driving)


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Particle Penetration Criterion

' '

H OVK

FOR DRIVEN PILES: K = 1

FOR BORED PILES: K = K0 = (1-sin)OCR0.5


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Bitumen Design Criteria Summary


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EXAMPLE:

Find allowable top load fora top settlement < 14 mm

1) Pile Ult. Capacity

Qu = (25x1.2x30) + 1000

Qu = 900 + 1000 = 1900 kN


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EXAMPLE:2) Assume a top load = 500 kN

and find the maximum depth toNP

500 + X = 1000 + (900 - X)

X = 700 kN or 23.3 m of friction

NP(soil)

= 35 mm (settlement profile)

top = 35+(850*23300)/(0.09*2E7) = 46 mm

(This is more than the allowable settlement)


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EXAMPLE:3) Assume a top load = 100 kN

and NP = 29 m

Qp = 100 + 870 - 30 = 940 kN

p = 4.7 mm

NP(pile) = 4.7 + (955*1000)/(0.09*2E7)

= 5.2 mm ~ NP(soil)

top = 5 + (535*29000)/(0.09*2E7)

= 13.6 mm


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EXAMPLE:4) Use bitumen coating with shear

strength = 2.5 kN/m2 and try a topload of 500 kN (Assume NP = 29 m)

NP(soil) = 5 mm

Qp = 500 + 87 30 = 557 kNp = 2.8 mm

top

= 5 + (543.5*29)/(0.09*2E7)*(1000)

= 13.8 mm


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top = 46 mm top = 14 mm top = 14 mm

with bitumen


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Full Scale Field

Test:

Alberta,Edmonton, Canada


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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)



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Apply the primer (wait 24 hrs for drying)

Applying the bitumen (10 mm thick)


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APPLYING BITUMEN

COATING OUTSIDE INTHE COLD

BITUMEN BECOMES

BRITTLE DUE TO

COATING OUTDOORS

Handling in the field


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Handling in the field


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Storing the coated piles

In hot weather, store under cover away

from direct sunlight

In cold weather, wait until bitumen

cools down before storing


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Driving the piles (splice, temperature)

AVERAGE ANNUAL AIR TEMPERATURE = SOIL TEMPERATURE


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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

General Observations


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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 groupexperience less downdrag than the perimeter

piles

General Observations


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F i f i


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Videotape on Bitumen Coating

(email: [email protected])

Briaud J.-L., Tucker L.M., 1998, Design

guidelines for downdrag on uncoated and

bitumen coated piles, NCHRP Report 393,

National Academy of Sciences.

For more information

On Downdrag

https://ceprofs.tamu.edu/briaud/NCHRP_Downdrag.pdf


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THANK YOU

http://ceprofs.civil.tamu.edu/briaud/

http://www.issmge.org/


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Full Scale Field Test:


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Full Scale Field Test:

New Orleans, Louisiana, USA


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Full Scale Field

Test:

New Orleans,Louisiana, USA


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Full Scale Field

Test:

New Orleans,Louisiana, USA


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STEEL WOOD PPC


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APPLYING PRIMER

COATING WITH BITUMEN


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Full Scale Field

Test:

Alberta,Edmonton, Canada

downdrag lecture

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