02-Loehr PPT Load transfer and failure modes for deep foundation

8/17/2019 02-Loehr PPT Load transfer and failure modes for deep foundation

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Load Transfer and Failure Modes

or eep oun a ons n ar

Slopes

J. Erik Loehr, Ph.D., P.E.

Associate Professor

University of Missouri

Passive slope reinforcement schemes

FillSoil Dowels

Shotcrete

Stiff Clay

Relic Shear Surface

Nails

Railway

2after Bruce and Jewell, 1986

ReticulatedMicropiles

Firm Stratum soil nails

sliding surface


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Mobilization of resistance

Most deep foundations are passive elements

that require relative pile-soil movement to

Relative mobilization of axial and lateral

components depends primarily on

• Orientation of reinforcement

• Stiffness of soil and reinforcement

• Structural and geotechnical limit states

3

Load transfer

S o i l M

o v e m

e n t

4


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Failure modes for piles in slopes

Soil failure

• passive (lateral) failureabove/below sliding

Sliding

SlopeSurface

ReinforcingMember

Sliding

Location after

Sliding

• pullout (axial) failureabove/below slidingsurface

Structural failure• flexural failure

• shear failure

Surface

FailedSoil

RelativeMovement

Initial LocationFailedSoil

RelativeMovement

Sliding

Initial Location

Slidin

Initial Location

• axial failure- compression

- tension Serviceability limits

5

Failure ofmemberin bending

RelativeMovement

li iSurface

Failure ofmemberin Shear

RelativeMovement

Large-scale model tests

6


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Reinforced slope (s/d≈3) w/ capping beam

7

Reinforced slope (s/d≈2) w/ capping beam

8


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Reinforced slope (s/d≈4) w/o capping beam

9

t-z analyses for axial load

axial

Input Profile of Axial Soil Movement

Cap Bearing

Pile AxialStiffness (EA)

Soil ShearResistance (t)

Sliding Surface

Axial Componentof moving soil

10

zSoil EndBearing (Q)

Stable Soil(no soil movement)


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Lateral load transfer – “long” pile

0-400 -200 0 200 400

Lateral Soil Reaction (kip/in)

0-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2

Pile Deflection (in)

5

10

15

20

5

10

15

20

D e p t h ( f t )

sliding surface

limit soil

pressure

13

30

35

40

=0.01 in

=0.1 in =0.3 in

=1.0 in

ℓ

ℓ

ℓ

ℓ

30

35

40

Lateral load transfer – “long pile” mode

0

-400 -200 0 200 400


0

-1.2 -0.6 0.0 0.6 1.2


0

-60 0 60 120 180

Bending Moment (kip-in)

0

-3.0 -1.5 0.0 1.5 3.0

Shear Force (kip)

5

10

15

20

25

5

10

15

20

25

D e p t h ( f t )

5

10

15

20

25

5

10

15

20

25

sliding surface

14

30

35

40

30

35

40

30

35

40

=0.01 in

=0.1 in

=0.3 in

=1.0 in

ℓ

ℓ

ℓ

ℓ

30

35

40

limit soil

pressure


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Lateral load transfer – “short pile” mode

0-12 -6 0 6 12


0-400 -200 0 200 400


=5

10

15

20

D e p t h ( f t )

5

10

15

20

.

=0.5 in

=1.0 in

=10.6 in

ℓ

ℓ

ℓ

ℓ limit soil

pressure

15

30

35

40

30

35

40

sliding surface

Lateral load transfer – “short pile” mode

0

-12 -6 0 6 12


0

-3000-2000-1000 0 1000


0

-30 -15 0 15 30

Shear Force (kip)

0

-400 -200 0 200 400


5

10

15

20

D e p t h ( f t )

5

10

15

20

25

=0.1 in

=0.5 in

=1.0 in

=10.6 in

ℓ

ℓ

ℓ

ℓ

5

10

15

20

5

10

15

20

limit soilpressure

16

30

35

40

30

35

40

30

35

40

30

35

40

sliding

surface


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Lat. load transfer – “intermediate” mode

0-40 -20 0 20 40


0-400 -200 0 200 400


=0.1 in 5

10

15

20

D e p t h ( f t )

5

10

15

20

.

=0.5 in

=10 in

=30 in

ℓ

ℓ

ℓ

sliding surface

limit soilpressure

17

30

35

40

30

35

40

Lat. load transfer – “intermediate” mode

0

-40 -20 0 20 40


0

-1000 0 1000 2000 3000


0

-30 -15 0 15 30

Shear Force (kip)

0

-400 -200 0 200 400


5

10

15

20

D e p t h ( f t )

5

10

15

20

=0.1 in

=0.5 in

=10 in

=30 in

ℓ

ℓ

ℓ

ℓ

5

10

15

20

5

10

15

20

slidingsurface

limit soilpressure

18

30

35

40

30

35

40

30

35

40

30

35

40


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Summary – load transfer and failure modes

Numerous failure modes

Controlling failure mode varies with

Load transfer for deep foundations in slopesis complex• Depends on

- Soil and pile stiffness

- Sliding depth

- Orientation of reinforcement

- Structural and geotechnical limit states

Generally dangerous to assume loaddistribution

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Design Resistance for Deep

Foundations in Earth Slopes

J. Erik Loehr, Ph.D., P.E.

Associate Professor

University of Missouri


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Challenges for predicting resistance

Deformation required to mobilize resistance

Soil provides both load and resistance

Numerous limit states

Must consider compatibility and serviceability

of axial and lateral resistance

21

Limit states for soil reinforcement

Soil failure

• passive (lateral) failure above or below sliding surface

•

Structural failure

• flexural failure

• shear failure

• axial failure

- com ression- tension

Serviceability limits

22


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Prediction of reinforcement resistance

1. Estimate profile of soil movement

2. Resolve soil movement into axial and lateralcom onents

3. Independently predict mobilization of axial andlateral resistance

a. Using “p-y” analyses for lateral load transfer

b. Using “t-z” analyses for axial load transfer

4. Select appropriate axial and lateral resistancew cons era on g ven o compa y anserviceability

23

Soil movement components

Slope Surface

lat.

lat.

axial

24

SlidingSurface

axial

soil


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Lateral resistance from p-y analyses

Use “soil movement” option (L-Pile v4.0M or v5)

For an assumed depth of sliding:1. A l dis lacements in soil above slidin surface

2. Determine response from p-y analyses

3. Mobilized resistance is shear force in member atdepth of sliding

4. Repeat steps 1 through 3 with incrementallyincreasing displacement until a limit state is reached

Shear force at sliding depth when first limit stateis reached taken to be available resistance forthat sliding depth

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Mobilization of lateral resistance

0

-80 -40 0 40 80

Mobilized Shear Force (kip)

=0.1 in

0

0.0 1.0 2.0 3.0 4.0 5.0

Pile Deformation (in)

0

-1500 -750 0 750 1500

Mobilized Bending Moment (kip-in)

10

20

30

=1.0 in

=3.0 in

clay

slide

10

20

30 D e p t h ( f t )

10

20

30

26

40

50

rock

40

50

40

50


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Mobilization of lateral resistance

40

45

50

15

20

25

30

35

M o b i l i z e d S h e a r F o r c e ( k i p )

27

0

5

10

0.0 1.0 2.0 3.0 4.0 5.0 6.0Total Slope Movement (in)

Axial resistance from t-z analyses

For an assumed depth of sliding:

1. Apply displacements in soil above sliding surface

-.

3. Mobilized resistance is axial force in member at depth

of sliding

4. Repeat steps 1 through 3 with incrementally

increasing displacement until a limit state is reached

Axial force at sliding depth when first limit state

is reached taken to be available resistance for

that sliding depth

28


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Mobilization of axial resistance

0

0 20 40 60 80 100 120 140 160

Mobilized Axial Load (kip)

=0.1 in

clay

slide

10

20

30 D e p t h ( f t )

=0.3 in

=0.42 in

=0.5 in

29

rock

40

50

Mobilization of axial resistance

120

140

160

40

60

80

100

M o b i l i z e d A x i a l F o r c e ( k i p )

30

0

20

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Total Slope Movement (in)


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Repeat for other sliding depths…

Result is two resistance functions that describeresistance versus position along reinforcement

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Resistance functions (per member)

0

0 50 100 150

Lateral Resisting Force (kip)

0

0 50 100 150 200 250

Axial Resisting Force (kip)

clay

10

20

30

S

l i d i n g D e p t h ( f t )

d


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Resistance functions (per lineal foot)

00 10 20 30 40 50

Axial Resisting Force (kip/ft)

00 5 10 15 20 25

Lateral Resisting Force (kip/ft)

10

20

30

S l i d i n g D e p t h ( f t )

clay

10

20

30

S l i d i n g D e p t h ( f t )

= -

33

40

50 rock

40

50

Member resistance divided by member spacing

Summary – design resistance

Predicting load transfer is the difficult part

Must consider all limit states, including

compa y an serv cea y

Resistance changes with location of sliding

surface

Process is presently tedious

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A Few Issues

35

Issues

Mode of soil movement

Drained vs. undrained loading

Factors of safety

p-y and t-z models

• influence of member size and spacing

• influence of member inclination

• drained models for cla s Cap/group/network effects

36


19/25

Drained vs. undrained analyses

Check short- and long-term conditions

• Possible to have drained loading of slope, but

-

• Prudent to evaluate both & use least resistance

Question whether can use “sand” models to

evaluate drained loading for clays

37

Factors of Safety

Factor of safety against slope instability just like

other slope applications

reinforcement resistance is subject of debate

• Slope analyses already includes margin of safety

• Applying factor of safety to non-controlling limit states

has no benefit

• It is not appropriate to apply factor of safety toreinforcement resistance alone

• I personally believe should not apply factor of safety to

reinforcement resistance

38


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p-y and t-z models

Approach is at least semi-empirical

Not a lot of good data available

• - -

- Brown and Chancellor, 1997

- Liang, 2000

- Hasenkamp and Turner, 2000

• Large-scale laboratory model tests

- Boeckmann, 2006

- ex or,

- Bozok, 2009

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p-y and t-z models – limit soil pressure

0.0

0 100 200 300 400 500 600 700 800 900 1000

Limit Soil Pressure (psf)

1.0

2.0

3.0

4.0

D e p t h ( f t )

DeBeer & Carpentier Ito & MatsuiReeseFlemingBromsPoulos

40

.

6.0

7.0

8.0


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Brown and Chancellor, 1997

41

Bending moments – Littleville

0

-40 -20 0 20 40

Bending Moment (in-kips)

redicted

0

-40 -20 0 20 40

Bending Moment (in-kips)

redicted

10

20

30 D e p t h ( f t )

measured (2+70U)

measured (1+70U)

downslope

p mod = 0.2

10

20

30 D e p t h ( f t )

measured (2+70U)

measured (1+70U)

upslope

p mod = 0.2

42

40

50 tot = 0.31-in

40

50 tot = 0.39-in


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Axial resistance – Littleville

0-60 -40 -20 0 20 40 6

Axial Load T, kip (+=tension)

0-60 -40 -20 0 20 40 60

Axial Load T, kip (+=tension)

10

20

30 D e p t h , z ( f t . )

downslope

10

20

30 D e p t h , z ( f t . ) upslope

= 0.3

z ult = 0.06-in

43

40

50predictedmeasured (2+70U)

measured (1+70U) tot = 0.24-in

.

z ult = 0.06-in40

50

predicted

measured (2+70U)

measured (1+70U)

tot = 0.34-in

Predicted mobilization – Littleville

80

100

120

Upslope Micropile

Sliding Depth = 33-ft

140

16.0

18.0

Upslope Micropile

Slide Depth = 33-ft

-

-40

-20

0

20

40

60

M o b i l i z e d A x i a l F o r c e ( k i p )

6.0

8.0

10.0

12.0

.

M o b i l i z e d S h e a r F o r c e ( k i p )

44

-120

-100

-80

0.0 1.0 2.0 3.0 4.0


pre c on

prediction A*

calibration points

0.0

2.0

4.0

0 5 10 15 20


s c ay mo e

API sand model

alternate

calibration points


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Model tests – no cap

25

30

o m )

44 (2.8) 25

30

o m )

10

15

20

n A l o n g P i l e ( i n .

f r o m b

o t LPile (2.8)

10

15

20


f r o m b

o t 44 (2.8)

t-z (2.8)

45

0

5

-300 -200 -100 0 100 200 300

Induced Bending Moment (lb-in)

P o s i t i

0

5

-1000 -500 0 500 1000

Induced Axial Load (lb)

P o s i t i

TC

Model tests – with cap

40

45

o m )

44 (1.9)40

45

o m )

44 (1.9)

15

20

25

30

35

n

A l o n g P i l e ( i n .

f r o m b

o t t t-z (1.9)

15

20

25

30

35


f r o m b

o t e .

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0

5

10

-1000 -500 0 500 1000Induced Axial Load (lb)

P o s i t i

TC

0

5

-1500 -500 500 1500

Induced Bending Moment (lb-in)

P o s i t i


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Results

2.0

Kubo's tests

Awoshika's tests

0.5

1.0

.

p - m u l t i p l i e r

Recommended by

Reese's et al., 2006

0.0-35 -25 -15 -5 5 15 25 35

Batter angle (Degree)

Recommended by

Bozok, 2009

Summary – a few issues

Performance depends on soil movement

Check short- and long-term conditions

ac or o sa e y app e o s ope s a y

Uncoupled method suitable when no cap or

when cap influence is limited

Influence of cap difficult to model with current

tools

Work needed on p-y and t-z models

• Some evidence that current models appropriate

• Some evidence that modified models needed

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02-Loehr PPT Load transfer and failure modes for deep foundation

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