DESIGNING FOR DOWNDRAG ON UNCOATED AND BITUMEN COATED PILES Lecture.pdf · DESIGNING FOR DOWNDRAG...

Jean-Louis BRIAUD – Texas A&M University

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 2010

1 February 2010

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

What is downdrag?

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

ωpile(NP) = ωsoil(NP)

ωsoil(NP)= read on

consolidation

settlement profile (given)

ωpile(NP) = ωpoint + ωelastic

ωpoint = ω soil (@Z=L) +ωpunch

ωelastic = elastic compression

from bottom to NP

Pile Point Behavior

2(1 )4

ωpunch = Pile point movement

ν = Poisson’s ratio

Qp = Point resistance

A = Area of pile point

D = Diameter of pile point

Es = Soil modulus

For clays

Es = 100 Su = Epmt

For sands

Es (kPa) = 800 N = 2 Epmt

Pile Point Behavior

2elastic p p

L ZQ F

ωelastic = Elastic compression

of pile

Qp = Point resistance

Fp = Positive friction force

L = Depth of pile point

Z = Location of NP

A = Area of pile point

E = Pile modulus

FOR PILES IN CLAY (short term and long term)

fmax = Su fmax = σ’ov

Maximum Friction

FOR PILES IN SAND fmax = σ’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 = σ’ov Nq

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

Sand (short & long term) qmax = σ’ov Nq

Maximum Point Resistance

max (1 )4 s

Downdrag on Pile Groups

Jean-Louis BRIAUD – Texas A&M UniversityAfter Okabe, 1977

Downdrag on Pile Groups

Reduction

Factors for

Downdrag in Pile

Groups

- 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

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)

- Top load, top settlement, depth of bitumen

coating, depth to neutral point, maximum load,

maximum stress and pile point load

PILENEG Outputs

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.

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

Behavior of Bitumen

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

τ = η (dγ/dt)

• Storing

• Driving

• Downdrag

• Particle penetration

Bitumen Selection

Storage Criterion

( )req storage

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 > ηreq(storage)

Driving Criterion

req driving air

ρ = 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 as

equal to d)

fmax = Maximum friction

t = Max anticipated storage

period

Driving in air requires:

η > ηreq(driving, air)

req driving soil

Driving in soil requires:

η > ηreq(driving, soil)

Downdrag Criterion

bit bit bit

req downdrag

τbit = Allowable shear stress

on bitumen (τbit ~ 0.1 τs,avg)

τs,avg = Soil shear strength

d = Bitumen coating thickness

s = Settlement rate

s1 , s2 = Settlements of the

ground surface at times t1

and t2, respectively (after

placement of an embankment

or other load)

Downdrag requires:

η < ηreq(driving)

Particle Penetration Criterion

FOR DRIVEN PILES: K = 1

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

Bitumen Design Criteria Summary

EXAMPLE:

Find allowable top load for

a top settlement < 14 mm

1) Pile Ult. Capacity

Qu = (25x1.2x30) + 1000

Qu = 900 + 1000 = 1900 kN

EXAMPLE:2) Assume a top load = 500 kN

and find the maximum depth to

500 + X = 1000 + (900 - X)

X = 700 kN or 23.3 m of friction

ωNP(soil) = 35 mm (settlement profile)

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

(This is more than the allowable settlement)

EXAMPLE:3) Assume a top load = 100 kN

and NP = 29 m

Qp = 100 + 870 - 30 = 940 kN

ωp = 4.7 mm

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

= 5.2 mm ~ ωNP(soil)

ωtop = 5 + (535*29000)/(0.09*2E7)

= 13.6 mm

EXAMPLE:4) Use bitumen coating with shear

strength = 2.5 kN/m2 and try a top

load of 500 kN (Assume NP = 29 m)

ωNP(soil) = 5 mm

Qp = 500 + 87 – 30 = 557 kN

ωp = 2.8 mm

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

= 13.8 mm

ωtop = 46 mm ωtop = 14 mm ωtop = 14 mm

with bitumen

Full Scale Field

Alberta,

Edmonton, Canada

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)

• Clean surfaces (free of dust and grease)

• Apply the primer (wait 24 hrs for drying)

Applying the bitumen (10 mm thick)

APPLYING BITUMEN

COATING OUTSIDE IN

THE COLD

BITUMEN BECOMES

BRITTLE DUE TO

COATING OUTDOORS

Handling in the field

Storing the coated piles

In hot weather, store under cover away

from direct sunlight

In cold weather, wait until bitumen

cools down before storing

Driving the piles (splice, temperature)

AVERAGE ANNUAL AIR TEMPERATURE = SOIL TEMPERATURE

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

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

General Observations

Videotape on Bitumen Coating

(email: briaud@tamu.edu)

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

THANK YOU

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

http://www.issmge.org/

Full Scale Field Test:

New Orleans, Louisiana, USA

Full Scale Field

New Orleans,

Louisiana, USA

Full Scale Field

New Orleans,

Louisiana, USA

STEEL WOOD PPC

APPLYING PRIMER

COATING WITH BITUMEN

Full Scale Field

Alberta,

Edmonton, Canada

DESIGNING FOR DOWNDRAG ON UNCOATED AND BITUMEN COATED PILES Lecture.pdf · DESIGNING FOR DOWNDRAG...

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