NEGATIVE SKIN FRICTION DOWNDRAG DRAGLOAD

January 10, 2016 Best Practices in Numerical Modeling – TRB 2016 Slide 1

NEGATIVE SKIN FRICTIONDOWNDRAGDRAGLOAD

General Framework and back analysis of a real case

Augusto Lucarelli, Derrick Blanksma, Ryan Peterson

Terminology and general framework

Is it a bearing capacity problem?

What happens when Q+QNSF > QP+QPSF ?

Applied Load Qc at the top of the pile

Drag load: the difference between the max axial force and the load at the top of thepile. It is maximum when Qc is zero and goes to zero when geotechnical capacity isreached.

So…….

Negative skin friction is a soil-structure interactionproblem.• It doesn't change the geotechnical bearing capacity;• It changes the pile stiffness and produces settlements

(downdrag);• It changes the axial load distribution along the pile shaft

(dragforce). Check structural capacity of the pile.

From a geotechnical point of view, it is a Service LimitState issue. It might be a structural capacity problem atthe neutral plan elevation although usually there is nosignificant bending moment.

The ultimate bearing capacity is around 10 MN

Let’s work out an example….Drilled Shaft, D=1.0 m; L=30m

Layer 1: soft claytlim = 25 kPa

Layer 2: medium sandtlim = 70 kPa

Layer 3: dense sandtlim = 110 kPa

Base:qblim = 5000 kPa

Ground level

Settlement profile

200 mm

Pile element

Displacements imposed on the non-liner springs

Pile-Soil interaction…

Pile-soil interaction (along the shaft and at the base) is accounted for by means of non-linear t-z springs. The effect of negative skin friction is evaluated by imposing boundary displacements to the spring.

Base curve:displacementat full capacityfor drilledpiles 0.25-0.30D

Curve at 5 mdepth withnegative skinfriction

t-z curve at 5 m depth

Soil-pile relative displacements100

Curve at 27m depthwithoutnegative skinfriction. Itgoes throughthe origin

t-z curve at 27 m depth

Soil-pile relative displacements

Displacements at the base

Load curve without NSF

Load Curve with NSF

Load settlement curve at the head of the pile

NSF doesn’t effect the ultimate bearing capacity of the pile-soil system.NSF does effect the stiffness of the pile-soil system and axial loaddistribution along the shaft.

Let’s consider a Service Load of 4000 kN: without negativeskin friction the settlement would be around 5 mm. With skinfriction the settlement would be around 15 mm. If the lastvalue is not tolerable, the Service Load must be reduced.

Failure Load: 10000 kNA

Displacements [mm]

5 1540 100

Axial force distribution along the pile

Downdrag force 1800 kN

Sand SandDe

Axial Force [kN]

Qc = 0.0Qc = 2000 kN

4000200020001000

Axial Force [kN]

Qc = 5000 kN Qc = 8000 kN

Axial Force [kN]

5000 8000

Axial Force [kN] Axial Force [kN]D

Sand20 20

Qc = 10000 kN

Downdrag force 0All NSF has become PSF

Qc = 2000Qc = 5000

Qc = 8000

Qc = 0 Qc = 10000

4000 10000

BACK ANALYSIS OF A REAL CASE USING FLAC3D

Steele County Highway 7 in Owatona

Bridge 74551

Project Site

ShapeAccelArray (SAA) Profile

Deformation profile measured by the SAA after the bridge deck was placed.

South Abut.

North Abut.

0 20 40 60 80 100 120Length [ft]

ximate D

isplace

Maximum SAA Deflection = 3.6 in (91 mm)

Approximate Length [m]0 6 12 18 24 30 36

Plan Location

Elevation Location

SAA Time History

• Surcharge loads induced roughly 1.7 inches of vertical displacement.

• Surcharge removal resulted in 0.9 in of rebound.• Construction loads resulted in a little over 2 in of

vertical displacement.

South Abut.

North Abut.

-4Time (August, 2010 – June, 2014)

ximate D

isplace

Plan Location

Elevation LocationSettlements of the soil around the pile

Soil Stratigraphy

Model Setup: single pile interaction

FILL 9 m

SOIL 15 m

BEDROCK

Simplified model: only one embedded pile. The objective is to simulate the local interaction with soil considering the main construction phases.

Embedded pile

Embedded pile: lateral interaction with the soil

Yield Criteria: effective stress approach

BEDROCK

knfnlim

𝑓𝑠𝑙𝑖𝑚

ҧ𝑙= 𝜏𝑠

𝑙𝑖𝑚𝑃

𝜎𝑧𝑧′ is the effective vertical stress

𝛽∗ factor ….function of soil type, installation method…

𝑘𝑠 =𝑓𝑠𝑙𝑖𝑚

𝛿𝑠𝑙𝑖𝑚

𝜏𝑠𝑙𝑖𝑚 = 𝜎𝑧𝑧

′ 𝛽∗ 𝑘𝑃𝑎

Linear elastic beam element, EA, EJ…

ҧ𝑙= 𝜏𝑠

𝑙𝑖𝑚𝑃

P is the perimeter of the pile

ҧ𝑙 is unit length along the pile

ҧ𝑙

𝑓𝑠ҧ𝑙

𝑘𝑠 =𝑓𝑠𝑙𝑖𝑚

𝛿𝑠𝑙𝑖𝑚

𝐸𝑙𝑎𝑠𝑡𝑖𝑐 − 𝑝𝑒𝑟𝑓𝑒𝑐𝑡𝑙𝑦 𝑝𝑙𝑎𝑠𝑡𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑖𝑡𝑢𝑡𝑖𝑣𝑒 𝑏𝑒ℎ𝑎𝑣𝑖𝑜𝑟…𝑓𝑟𝑜 𝑛𝑜𝑤

𝑆ℎ𝑒𝑎𝑟 𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠

Shear Response along the shaft of the pile

𝛿𝑠𝑙𝑖𝑚 = 1 − 5𝑚𝑚 𝛿𝑠

𝑘𝑁/ ҧ𝑙

Loading

BACKFILL

BEDROCK

(1) Load induced by backfilling was modeled with a density “ramping” procedure of the back fill.

(2) Additional loading (Pile cap, beams, etc…) was simulated by applying an axial force directly to the pile head.

SURCHARGE

(3) Additional surcharge loading after the deck was placed, was simulated by increasing the density in the zones above the pile.

Soil Profile & Properties

BACK FILL

A) CLAY

B) SANDY CLAY

C) CLAYEY SAND

D) SANDY CLAY

E) SAND

F) SANDY CLAY

G) BEDROCK

STRATA BF A B C D E F G

Bottom elev. [m] 9.0 10.0 11.5 13.5 20.5 22.0 24.0 30.0

Young’s modulus, [MPa]

35 35 35 35 52.5 65 65 100

Poisson ratio [] 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

Angle of Friction [°] 26 26 26 26 26 26 26 30

Cohesion [MPa] - - - - - - - 520

Density [g/cc] 1.8 1.7 1.7 1.7 1.8 1.8 1.8 2.5

Settlement

4 cmCalibrated with the SAA

Initial Shear Response to Loading

′ 𝛽∗

BACKFILL

BEDROCK

Final Shear Response to Loading

′ 𝛽∗

BACKFILL

BEDROCK

Loading Process

Load History

• Case 3: 100 MPa Base Layer

Place Fill

Load Pile

Additional Load After Deck Placed

Sensitivity analysis: Cases

• Five cases were run to simulate the response to changing base layer stiffness– Case 1: The base layer is 10 GPa representing bedrock– Case 2: The base layer is reduced to 1 GPa, representing weathered

bedrock– Case 3: The base layer is reduced to 100 MPa, representing gravel or

very soft bedrock– Case 4: The base layer is reduced to 10 MPa, representing loose

sand– Case 5: The base layer is reduced to 1 MPa, representing soft clay

• In all cases, the mean soil modulus was kept constant at 52.5 MPa

Axial Force as a function of bedrock stiffness

(Case 2)

(Case 3)

(Case 4)

(Case 5)

Mean soil modulus = 52.5 MPa

Base layer modulus varies by case

Case 3 (100 MPa base layer) correlates fairly well to the strain gage data.

(Case 1)

h = Ln/Ls = 10/15 = 0.67

BEDROCK

Neutral plane:

relative displ.

is zero

Bottom of fill

Relative displacements

Relative displ. = soil displ. – pile displ.

Results from other real cases around the world

Neutral plan position – end bearing in clay

Neutral plan position – end bearing in sand & rockIn our case Nspt>50….but is an H pileand there is a sand layer just on top ofthe bedrock….100 Mpa is not a verystiff bedrock.

Axial Force and Neutral Plane Position

Neutral Plane

Case 1Axial Force [kN]

As the base layer stiffness is decreased, the neutral plane position moves from the bottom of the pile (Case 1) up towards the top (Case 5).

Sensitivity AnalysisCase 1 Case 2

Case 3

Case 4Case 5

Case 1Case 2

Case 3

Case 4

Case 5

Case 1 Case 2Case 3

Case 4

Case 5

• Relative stiffness is calculated as the ratio of the mean elastic modulus of the soil to the elastic modulus of the base layer.

• The soil modulus was kept constantat 52.5 MPa and the base layermodulus was decreased by an orderof magnitude for each case. Theinitial base layer modulus was 1,000MPa.

• With a relatively stiff base layer, downdrag forces increase, the neutral plane is near the bottom of the pile and pile displacement is minimal.

Comparison of Axial Forces and Neutral Plane Depth

Case 5

Case 4

Case 3

Case 2Case 1

Case 5

Case 4

Case 3

Case 2Case 1

Considerations• Relative stiffness between the soil and base layer

influences the amount of dragload, the axial force distribution and the position of the neutral plane.

• At relative stiffness below 0.1 (very stiff base layer), the neutral plane is at the bottom of the pile and maximum possible dragload forces are realized.

• At a relative stiffness above 10 (very soft base layer), the neutral plane is near the top of the pile and the drag load forces are minimal.

• Between a relative stiffness of 0.1 and 10, The neutral plane position and drag load forces are functions of several factors.

• This region is a transition zone where 1) the drag load increases with increasing base layer stiffness and 2) the position of the neutral plane decreases with increasing base layer stiffness.

Case 1 Case 2

Case 3

Case 4Case 5

Case 1Case 2

Case 3

Case 4

Case 5

Downdrag Force Matrix (kN)

0.00525 0.0525 0.525(Base Case)

5.25 52.5

0 630 610 500 405 365

275 538 506 390 246 192

550(Base Case)

430 385 225 40 0

825 320 263 65 0 0

RSPileLoad (kN)

RS is relative stiffness i.e., the ratio of the mean soil modulus to the base layer modulus.

Pile Load is applied directly to the top of the pile.

The base case is the scenario calibrated to the field data i.e., RS = 0.525 and Pile Load = 550 kN

Relative to Max Downdrag Force Matrix

0.00525 0.0525 0.525(Base Case)

5.25 52.5

0 1.0 1.0 1.0 1.0 1.0

275 0.85 0.83 0.78 0.61 0.53

550(Base Case)

0.68 0.63 0.45 0.01 0

825 0.51 0.43 0.13 0 0

RSPileLoad (kN)

RS is relative stiffness i.e., the ratio of the mean soil modulus to the base layer modulus.

Pile Load is applied directly to the top of the pile.

The base case is the scenario calibrated to the field data i.e., RS = 0.525 and Pile Load = 550 kN

Downdrag Force Contour

Downdrag

force [kN]

Increasing base stiffness

Downdrag

Base case

Load test simulation: effect of NSF

250 kN Applied load 500 kN Applied load

750 kN Applied load 1000 kN Applied load

1250 kN Applied Load 1500 kN Applied Load

As the axial load applied atthe pile head increases, themaximum axial movestoward the pile headvanishing the effect ofnegative skin friction. All theavailable friction along thepile becomes positive.

Future Developments: pile group

Project overview

Application of FLAC3D on a non-standard foundation design

A4 Arc Bridge and its interferences with other bridgesIssues:1. Narrow piles at the perimeter2. Mutual interaction between foundation

piles of neighboring structures

Expo ViaductAbutment

A4 Arc: Plinth #2

A4 Arc: Plinth #1

Stephenson ViaductAbutment

A4 Arc: Plinth #4

Expo Viaduct Pier #1

A4 Arc: Plinths #1 and #2 A4 Arc: Plinths #3 and #4

Application of FLAC3D on a non-standard foundation design

A4 Arc, Plinth n.4: Vertical interaction between adjacent foundations

Expo Viaduct Pier #1

A4 Arc: Plinth #4

Tensional axial stress piles,subjected to negative skin friction

NEGATIVE SKIN FRICTION DOWNDRAG DRAGLOAD

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