04-NSF Design and EC7 (Prof Harry Tan)

EC7 for Deep Foundations

(NSF ISSUES)

Prof. Harry Tan

Department of Civil and Environmental Engineering

National University of Singapore

GeoSS/BCA EC7 Seminar

24 April 2015

4/27/2015 1

Motivations of the Lecture

Brief Introduction to pile design based on EC7

Correct understanding of piled foundation design subjected to dragload. Dragload (negative skin friction)

does not diminish pile geotechnical capacity; therefore

the factor of safety will not reduce

Pile design with NSF is a settlement issue rather than capacity issue

Demonstration of dragload cases using Unified Pile Design concept and finite element analysis

4/27/2015 2

Outline

Pile Design using EC7

Problems with BS 8004, CP4, and EC7 on dragload

Design example of dragload using EC7

Unified pile design concept

FE simulation of single pile and groups of piles subjected to dragload

Summary

4/27/2015 3

Pile Design based on EC7 (EN1997-1:2004)

Section 7 Pile Foundations

7.1 General

7.2 Limit states

7.3 Actions and design situations

7.4 Design methods and design

considerations

7.5 Pile load tests

7.6 Axially loaded piles

7.7 Transversely loaded piles

7.8 Structural design of piles

7.9 Supervision of construction

4/27/2015 4

(1)P The following limit states shall be considered and an appropriate list shall

be compiled:

loss of overall stability;

bearing resistance failure of the pile foundation;

uplift or insufficient tensile resistance of the pile foundation;

failure in the ground due to transverse loading of the pile foundation;

structural failure of the pile in compression, tension, bending, buckling or shear;

combined failure in the ground and in the pile foundation;

combined failure in the ground and in the structure;

excessive settlement;

excessive heave;

excessive lateral movement;

unacceptable vibrations.

7.2 Limit States

4/27/2015 5

7.3.1 General

axial load

transverse (horizontal) load (7.3.2.4)

7.3.2 Actions due to ground displacement

consolidation Downdrag (negative skin friction) (7.3.2.2)

downdrag load as an action [7.3.2.2(1)P]

calculated based on upper bound (max. downdrag load) [7.3.2.2(3)]

this is the big issue: Should NSF force be treated as an ACTION or otherwise???

swelling heave (7.3.2.3)

treated as an action

landslides or earthquakes

ground displacement due to adjacent construction

7.3 Actions and design situations

4/27/2015 6

7.4.1 Design methods

(1)P The design shall be based on one of the following approaches

The results of static load tests, which have been demonstrated, by means of calculations or otherwise, to be consistent with other relevant experience;

Empirical or analytical calculation methods whose validity has been demonstrated by static load tests in comparable situations;

The results of dynamic load tests (PDA and CAPWAP) whose validity has been demonstrated by static load tests in comparable situations;

The observed performance of a comparable pile foundation, provided that this approach is supported by the results of site investigation and ground

testing

Other methods

Dynamic impact tests (7.6.2.4); Pile driving formulae (7.6.2.5); wave equation

analysis (7.6.2.6); Re-driving (7.6.2.7)

7.4 Design methods and design considerations

4/27/2015 7

7.6.1.1 Limit state design

(1)P the design shall demonstrate that exceeding the following limit states is sufficiently improbable:

ULS of compressive or tensile resistance failure of a single pile;

ULS of compressive or tensile resistance failure of the pile foundation as a whole (pile group);

ULS of collapse or severe damage to a supported structure caused by excessive displacement or differential displacements of the pile foundation;

SLS in the supported structure caused by displacement of the piles

Ultimate resistance or failure of compression piles [7.6.1.1(4)P]

For piles in compression it is often difficult to define an ultimate limit state from a load settlement plot showing a continuous curvature. In these cases, settlement of the pile top equal to 10% of the pile base diameter should be adopted as the failure criterion.

7.6 Axially loaded piles

Clause 7.6 is the core of the section of EN 1997-1 on pile foundations

4/27/2015 8

Two calculation methods:

Model Pile procedure [clause 7.6.2.3(5)P]

Alternative procedure [clause 7.6.2.3(8)]

7.6.2.3 ULS from ground test results (insitu tests)

4/27/2015 9

Model Pile method the values of the ground test results at each individual tested profile are used to

calculate the compressive resistance of a model pile at the

same location.

The procedure is, in fact, similar to that used with the

results of static load tests, e.g. it involves applying a

correlation factor to the calculated resistance to account for the variability of the pile resistance and obtain the

characteristic compressive resistance.


4/27/2015 10

Design compressive resistance, Rc;d = Rbd + Rs;d

Rb;d = Rb;k/b Rs;d = Rs;k/s

The characteristic value Rb;k and Rs;k shall either be determined by:


4

mincal;c

3

meancal;ccal;ccal;scal;bk;sk;bk;c

R;

RMin

RRRRRR

where 3 and 4 are correlation factors depend on the number of profile

of tests, n, and are applied respectively to:

(Rc;cal)mean = (Rb;cal + Rs;cal)mean = (Rb;cal)mean + (Rs;cal)mean (Rc;cal)min = (Rb;cal + Rs;cal)min

Correlation factors for n ground test results (Singapore NA Table A.NA.10)

For n = 1 2 3 4 5 7 10

3 1.55 1.47 1.42 1.38 1.36 1.33 1.30

4 1.55 1.39 1.33 1.29 1.26 1.20 1.15

4/27/2015 11

Alternative method the ground test results (shear strength, cone resistance, etc) of all tested locations are

brought together before evaluating the characteristic values

of base resistance and shaft resistance in the various strata

based on a cautious assessment of the test results and

without applying the factors.


4/27/2015 12

(8) The characteristic values may be obtained by calculating:

Rb;k = Ab qb;k and Rs;k = As;i qs;i;k

where qb;k and qs;i;k are characteristic value of base resistance and

shaft friction in the various strata, obtained from the values of ground

parameters.

NOTE If this alternative procedure is applied, the values of the partial factors

b and s recommended in Annex A may need to be corrected by a model

factor larger than 1.0 (1.4 or 1.2). The value of the model factor may be set by

the National annex.


This is the most common method for pile design in UK (Singapore)

4/27/2015 13

SS EN 1997-1:2010 Singapore National Annex to Eurocode 7

A model factor is introduced to account for uncertainty of the

calculation results.

Model factor = R;d

The value of the model factor should be 1.4, except that it may be

reduced to 1.2 if the resistance is verified by a maintained load test

taken to the calculated , unfactored ultimate resistance.


4/27/2015 14

Outline


Problems with BS 8004, EC7, and CP4 on dragload

Design example of dragload using EC7



Summary

4/27/2015 15

BS 8004 (CP4) on Dragload

1.2 Definitions

1.2.33 Downdrag (negative skin friction)

A downwards frictional force applied to the shaft of a pile caused by the consolidation of compressible strata, e.g. under recently placed fill

NOTE. Downdrag has the effect of adding load to the pile and reducing the factor of safety

4.5.6 Effect of settling ground and downdrag forces

On sites underlain by recent or lightly over-consolidated clays The drag force should be added to the net additional vertical load applied to the base of the deep foundation in the assessment of allowable bearing pressure caused by downdrag in the bearing capacity of the foundation. Donwdrag can also occur where the groundwater level is substantially lowered or where backfill is placed around the foundation

4/27/2015 16


7.3.6 Negative skin friction

The downdrag drag on the pile may throw enough additional load on the pile point or base to make the total settlement excessive

When piles are driven through sensitive clays the resulting remoulding

may initiate local consolidation. The negative friction force due to this

consolidation may be estimated as the cohesion of the remoulded clay

multiplied by the surface area of the pile shaft.

Where it is expected that the soil around the shafts of end bearing piles

will consolidate, the skin friction exerted by the downdrag moving soil

should be estimated in accordance with the properties of materials. The

downward force will need to be taken into account when the allowable

load on the pile is calculated...

4/27/2015 17


7.5.3 Calculation from soil tests

.

Q = f As + q Ab

The special case of negative skin friction or downdrag has been

mentioned in 7.3.6. Soil strata imposing negative friction forces will

introduce negative components into the fAs term. If all the strata above

the level of the pile base are liable to settlement, the term fAs will be

negative. It should then be treated as part of the design load and not be

divided by the factor of safety.

4/27/2015 18

EC7 Geotechnical Design Part 1: General Rules

Section 1 General

Section 2 Basic of geotechnical design

Section 3 Geotechnical data

Section 4 Supervision of construction, monitoring and maintenance

Section 5 Fill, dewatering, ground improvement and reinforcement

Section 6 Spread Foundations

Section 7 Pile Foundations

Section 8 Anchorages

Section 9 Retaining Structures

Section 10 Hydraulic failure

Section 11 Overall stability

Section 12 Embankments

Annex A - J

4/27/2015 19

EC7 on Downdrag (actually allow flexibility for

correct analysis of NSF as settlement action)

7.3.2.2 Downdrag (negative skin friction)

(1)P If ultimate limit state design calculations are carried out with the

downdrag load as an action (called the dragload), its value shall be

maximum, which could be generated by the downward movement of the

ground relative to the pile

(2) Calculation of maximum downdrag loads should take account of the shear

resistance at the interface between the soil and the pile shaft and downward

movement of the ground due to self-weight compression and any surface load

around the pile.

(3) An upper bound to the downdrag load on a group of piles may be calculated

from the weight of the surcharge causing the movement and taking into

account any changes in ground-water pressure due to ground-water lowering,

consolidation or pile driving.

(4) Where settlement of the ground after pile installation is expected to be

small, an economic design may be obtained by treating the settlement of

the ground as the action and carrying out an interaction analysis.**

4/27/2015 20

7.3.2.2 Downdrag (negative skin friction)

(5)P The design value of settlement of the ground shall be derived taking

account of material weight densities and compressibility in accordance with

2.4.3. (i.e. use appropriate characteristic values of soil layers to give good

estimates of settlements)

(6) Interaction calculations should take account of the displacement of

the pile relative to the surrounding moving ground, the shear resistance

of the soil along the shaft of the pile, the weight of the soil and the

expected surface loads around each pile, which are the cause of the

downdrag.

(7) Normally, downdrag and transient loading need not be considred

simultaneously in load combinations.

4/27/2015 21

EC7 on Downdrag (actually allow flexibility for

correct analysis of NSF as settlement action)

EC7 on Dragload

7.6.2.2 Ultimate compressive resistance from static load tests

(5)P In the case of a pile foundation subjected to downdrag, the pile resistance

at failure, or at a displacement that equals the criterion for the verification of the

ultimate limit state determined from the load test results, shall be corrected.

The correction shall be achieved by subtracting the measured, or the most

unfavourable, positive shaft resistance in the compressible stratum and in the

strata above, where negative skin friction develops, from the loads measured at

the pile head.

(6) During the load test of a pile subject to downdrag, positive shaft friction will

develop along the total length of the pile and should be considered in

accordance with 7.3.2.2(6). (The maximum test load applied to the working pile

should be in excess of the sum of the design external load plus twice the

downdrag force.)

4/27/2015 22

CP4 on Dragload

7.3.6 Negative skin friction

The allowable geotechnical capacity of a pile subject to negative skin friction in

the long term (Qal) is given by the following general equation:

where Qb is the ultimate end bearing resistance

Qsp is the ultimate positive shaft resistance below the neutral plane

Fs is the geotechnical factor of safety

Pc is the dead load plus sustained load to be carried by each pile

Qsn is the negative skin friction load

is the degree of mobilization typically 0.67, although 1.0 may be

used in specific cases

sncs

spbal QP

F

QQQ

4/27/2015 23

Concluding remarks from BS and EC7

BS 8004, CP4 and EC7 treat dragload as an unfavourable design load that diminishes pile

geotechnical capacity

The pile design can appear to have inadequate safety factor, or, worst, negative capacity

Piled foundation cost will increase significantly and unnecessary

This is grossly incorrect. The codes do not address the issue of dragload holistically.

Sounds unconvincing Well the following notes will, hopefully, convince you that draglod is not a capacity problem but a downdrag (settlement) issue

4/27/2015 24

Outline



Design example using EC7


FE simulation of piled foundation subjected to dragload

Summary

4/27/2015 25

First, Lets look at example for pile subject to dragload based on EC7

(modified from Simpson & Driscoll, 1998)

Pile type Bored pile

Pile diameter 300 mm

Soft clay unit NSF, qD;k

characteristic value 20 kPa

Stiff clay unit shaft resistance, qs;k

characteristic value 50 kPa

Permanent vertical load, Gk 300 kN

(Frank et al., 2005)

Example 1

4/27/2015 26

Pile subject to dragload based on EC7

Characteristic and design value of loads

Permanent load, Gk = 300 kN

Total drag load, FD;k = x 0.3 x 5 x 20 = 94.2 kN

Positive shaft resistance, Rs;k = x 0.3 x LR x 50 = 47.1LR kN

Total design load, Fc;d = GGk + FD;k Design resistance, Rc;d = Rs;k/s + Rb;k/b

DA1 Combination 1: A1 + M1 + R1

Total design load, Fcd = 1.35 x 300 + 1.35 x 94.2 = 532.2 kN

Design resistance, Rc;d = 47.1LR/1.0 = 47.1LR kN

Condition Fc;d Rc;d leads to LR 532.2/47.1 = 11.30 m

DA1 Combination 2: A1 + (M1 or M2) + R4




Note: the correlation

factor, is ignored

Example 1

4/27/2015 27

Pile subject to dragload based on EC7

DA2: A1 + M1 + R2




DA3: (A1 or A2) + M2 + R3




Conclusion

DA-3 requires the longest pile length of the three Design Approaches: LR = 13.87 m, compard with LR

= 11.54m for DA-1 and LR = 12.43m for DA-2. This is due to the fact that for DA-3 the values of the

three partial factors are equal to 1.25 or 1.35. It can also be argued that the application of the

correlation factor to the estimated values of shaft friction qs in DA-1 and DA-2 (see clauses

7.6.2.2(8)P and 7.6.2.3(5)P) would have led to lower values for qs;k than in DA-3 (for which they are

not used).

Example 1

4/27/2015 28

Now Lets consider that dragload does not reduce capacity

DA1 Combination 1: A1 + M1 + R1

Total design load, Fcd = 1.35 x 300 = 405 kN

Design resistance, Rc;d = (94.2 + 47.1LR)/1.0 = 94.2 + 47.1LR kN

Condition Fc;d Rc;d leads to LR 310.8/47.1 = 6.70 m (cf. 11.30m)

DA1 Combination 2: A1 + (M1 or M2) + R4




DA2: A1 + M1 + R2




DA3: (A1 or A2) + M2 + R3




Example 1

4/27/2015 29

Imagine that the project requires 1000 piles, the cost saving will be

1000 x 5.13 m = 5,130 m pile length!

Assuming that the thickness of the soft clay layer is now 15m instead of

5m (in Singapore, typical Marine clay thickness is 10-30 m). The

dragload force becomes 282.7 kN.

Using 13.87m embedded pile length in stiff clay (from DA-3), the pile

design will have a negative capacity (Fc;d = 1.35 x 300 + 1.25 x 282.7 =

758.4 kN cf. Rc;d = 37.7LR = 522.9 kN).

Therefore, in order to satisfy the total design load based on EC7, the

embedment length in stiff clay need to be LR = 758.4/37.7 = 20m.

In order words, to sustain 300 kN permanent load, the total pile length

required is 25 + 20 = 45m.

Now Lets consider that dragload does not reduce capacity

Example 1

4/27/2015 30

Outline






Summary

4/27/2015 31

The Unified Pile Design method

The following slides are extracted from pile

design courses given by Dr. Fellenius.

For more detail information, please refer to

Fellenius B.H. (2012). Basics of Foundation

Design. Available freely from www.fellenius.net

4/27/2015 32

dragload is treated as an unfavourable action

0

5

10

15

20

0 500 1,000 1,500 2,000 2,500

LOAD (KN)

DE

PT

H

(m)

ALLOWABLE

LOAD - (Fs = 2.5)CAPACITY

DRAG LOAD

Drag load must neither be subtracted from

the pile capacity nor from the allowable load

0

5

10

15

20

0 500 1,000 1,500 2,000 2,500

LOAD (KN)

DE

PT

H

(m)

ALLOWABLE LOAD minus

DRAGLOAD*1.0CAPACITY

DRAG LOAD

INCREASE !

Effect of subtracting the drag load

from the allowable load -- only!

If the pile capacity had first been

reduced with the amount of the drag

load, there would have been no room

left for the working load!

4/27/2015 33

Similarly for the EC7 and LRFD:

Do not include the drag load when determining the factored resistance!

Drag load not subtracted from the factored resistance Drag load factored and subtracted!

0

5

10

15

20

0 500 1,000 1,500 2,000 2,500

LOAD (KN)

DE

PT

H

(m)

FACTORED RESISTANCE CAPACITY

DRAG LOAD

0

5

10

15

20

0 500 1,000 1,500 2,000 2,500

LOAD (KN)

DE

PT

H

(m)

FACTORED RESISTANCE

minus FACTORED DRAGLOAD

Factors = 0.6 and 1.5, respectively

FACTORED RESISTANCE CAPACITY

DRAG LOAD

4/27/2015 34

Load placed on a pile causes downward movements of the pile head due to:

1. 'Elastic' compression of the pile.

2. Load transfer movement -- the movement response of the soil at the pile toe..

3. Settlement below the pile toe due to the increase of stress in the soil. This is

only of importance for large pile groups, and where the soil layers below the piles

are compressible.

A drag load will only directly cause movement due to Point 1, the

'elastic' compression. While it could be argued that Point 2 also is at

play, because the stiffness of the soil at the pile toe is an important

factor here, it is mostly the downdrag that governs (a) the pile toe

movement, (b) the pile toe load, and (c) the location of the neutral

plane in an interactive "unified" process.

The drag load cannot cause settlement due to Point 3, because there

has been no stress change in the soil below the pile toe.

SETTLEMENT

4/27/2015 35

Therefore, negative-skin-friction/dragload

does not diminish geotechnical capacity.

Drag load (and dead load) is a matter for the pile

structural strength, and

The main question is "will settlement occur around

the pile(s) that can cause downdrag.

The approach is expressed in The Unified Design

Method, which is a method based on the

interaction between forces and movements.

4/27/2015 36

The Unified Design Method is a

three-step approach

1. The dead plus live load must be smaller than the pile capacity divided by an appropriate factor of safety. The drag load is not

included when designing against the bearing capacity.

2. The dead load plus the drag load must be smaller than the structural strength divided with a appropriate factor of safety. The live

load is not included because live load and drag load cannot coexist.

3. The settlement of the pile (pile group) must be smaller than a limiting value. The live load and drag load are not included in this analysis.

(The load from the structure does not normally cause much settlement, but

the settlement due to other causes can be large. The latter is called

downdrag).

4/27/2015 37

Construting the Neutral Plane and

Determining the Allowable Load

"

4/27/2015 38

The distribution of load at the pile cap is governed by the

load-transfer behavior of the piles. The design pile can

be said to be the average pile. However, the loads can

differ considerably between the piles depending on toe

resistance, length of piles.

The location of the neutral plane is the result of Natures

iterations to find the force equilibrium. If the end result

by design or by mistake is that the neutral plane

lies in or above a compressible soil layer, the pile group

will settle even if the total factor of safety appears to be

acceptable.

4/27/2015 39

The principles of the mechanism are illustrated

in the following three diagrams

The mobilized toe resistance, Rt, is a function of the

Net Pile Toe Movement

4/27/2015 40

Pile toe response for where the settlement is small (1)

and where it is large (2)

0

0 1,500

LOAD and RESISTANCE

DE

PT

H

0

0 200

SETTLEMENT

21

1 2

NEUTRAL PLANE 1

NEUTRAL PLANE 2

Utimate

Resistance

Toe Penetrations

Note, the magnitude of settlement affects not only the magnitude

of toe resistance but also the length of the Transition Zone

= Movement into the soil

4/27/2015 41

Pile toe response for where the settlement is small (1)

and where it is large (2), showing toe penetration

Note, the magnitude of settlement affects not only the magnitude of

toe resistance but also the length of the Transition Zone:

0

-500 1,000

LOAD and RESISTANCE

DE

PT

H

0

0 200

SETTLEMENT

2

1

1 2

NEUTRAL PLANE 1

NEUTRAL PLANE 2

Utimate

Resistance

Toe Penetrations

0

0

TOE PENETRATION

TO

E R

ES

IST

AN

CE

C

a b

a b

1

2

Toe Resistances

A B

3

3

c

c

4/27/2015 42

Outline






Summary

4/27/2015 43

FE simulation of piled foundation subjected to

dragload (Single Pile Interaction Analysis)

Verification of unified design pile concept using FE

Hypothetical site with three soil layers: fill, soft clay and dense sand

Simulation of short-term pile load test (undrained situation)

Ground settlement due to surcharge loading at various magnitude (10, 20 and 40 kPa)

Pile load transfer due to dragload at different working load (2000, 4000 and 6000 kN)

Consolidation analysis to simulate the development of dragload as the soils settle with time

Effect of bitumen coating

Results comparison

4/27/2015 44

Hypothetical site 2

5 m

25 m

Fill (3m thick)

s = 20 kN/m3; E50;ref = 10MPa; c = 0; = 30

o

Soft clay (12m thick)

s = 16 kN/m3; Cc = 1.0; Cr = 0.1; eo = 2.0, c = 0; = 20

o

k = 1 x 10-9 m/s

Dense sand (10m thick)

s = 20 kN/m3; E50;ref = 30MPa; c = 0; = 40

o

Surcharge loading

(10, 20 and 40 kPa)

Head load (P)

2, 4 and 6 MN

Axi-symmetric model

Pile diameter, D = 1.128 m

Pile length, L = 20 m

(pile cross-sectional area = 1 m2)

Pile concrete properties:

Concrete modulus = 30GPa

Rinterface = 1.0

Rinterface = 0.10 (with bitumen

coating at fill and soft clay layer)

Dummy plate pile with EA 1E6

times smaller than real pile

Soil constitutive model:

Hardening Soil (HS)

4/27/2015 45

Load movement curve (short term)

Head Load Toe Load Shaft Load Head mvmnt Toe mvmnt Elastic compr

[kN] [kN] [kN] [mm] [mm] [mm]

0 0 0 0 0 0

1000 344 656 3.785 3.135 0.65

2000 595 1405 9.914 8.708 1.206

3000 931 2069 20.273 18.519 1.754

4000 1269 2731 32.066 29.752 2.314

5000 1714 3286 48.3 45.411 2.889

6000 2228 3772 68.757 65.266 3.491

7000 2779 4221 93.178 89.074 4.104

8000 3445 4555 121.227 116.491 4.736

9000 4053 4947 150 144.647 5.353

10000 4752 5248 182.28 176.289 5.991

0

40

80

120

160

200

0 2000 4000 6000 8000 10000 12000

Move

men

t (m

m)

Load (kN)

Head load - Head Mvmnt

Toe load - Toe Mvmnt

Elastic comprs

4/27/2015 46

05

10

15

20

25

0 2000 4000 6000

Dep

th (

m)

Load (kN)

Typical results at 40 kPa surcharge with WL = 4MN

0 200 400 600 800

Settlement (mm)

-100 0 100 200 300

Unit resistance (kPa)

Neutral plane

soil

settlement Pile

Working load

condition

Subjected to

dragload

(1) Initial load

distribution

(2) DRAG LOAD

(1) + (2)

Long-term

load transfer

Net toe

penetration 0

50

100

150

200

0 2000 4000 6000

To

e m

ove

men

t (m

m)

Toe load (kN)

Interdependence between soil

settlement, pile load-movement and

pile load transfer

Drag load does not reduce pile

geotechnical capacity

4/27/2015 47

Pile responses due to various surcharge load (WL=4000kN)

0

5

10

15

20

25

0 2000 4000 6000

Dep

th (

m)

Load (kN) 0 200 400 600 800

Settlement (mm) -100 0 100 200 300


soil settlement at 10, 20 and 40 kPa

pile settlement at 10, 20 and 40 kPa

Working load condition

Unit resistance at 10,20 and 40kPa

Initial load

distribution

Long-term load

transfer for 10,20

and 40 kPa

8

10

12

14

16

0 100 200 300

Dep

th (

m)

Settlement (mm)

Large transition

zone

Small transition

zone

For the same head load, larger soil

settlement results in deeper NP,

larger drag load and larger

mobilized toe resistance. 4/27/2015 48

Pile responses at various level of WL (surcharge 40kPa)

0

5

10

15

20

25

0 2000 4000 6000 8000

Dep

th (

m)

Load (kN)

WL=2000kN

WL=4000kN

WL=6000kN

0 200 400 600 800

Settlement (mm) -100 0 100 200 300


soil

settlement

0

50

100

150

200

0 2000 4000 6000

To

e m

ove

men

t (m

m)

Toe load (kN)

pile settlement at WL=2, 4 and 6 MN

Unit resistance at WL=2, 4 and 6MN

2MN 4MN

6MN

Net toe

penetration

For the same soil settlement, larger

pile head load results in shallower

NP, smaller drag load and larger

mobilized toe resistance.

NP:

3436KN

NP:

5324KN

NP:

7224KN

4/27/2015 49

Consolidation analysis at WL=4MN and surcharge 40kPa

0

5

10

15

20

25

0 2000 4000 6000

Dep

th (

m)

Load (kN)

Initial

1 year

5 year

15 year

Fully Drained

0 200 400 600 800

Settlement (mm) -100 0 100 200 300


final soil

settlement

1 yr 5 yr

15 yr Working load

condition

Unit resistance at 1, 5, 15 yr

consolidation and fully drained

Neutral Plane

4/27/2015 50

Effect of bitumen coating (R=0.1 for fill and soft clay layers)

0

5

10

15

20

25

30

0 2000 4000 6000

Dep

th (

m)

Load (kN) 0 200 400 600 800

Settlement (mm) -100 0 100 200 300


without bitumen

N=5324 kN

with bitumen

NP=4285 kN

Short-term load

transfer without

bitumen

Neutral

Plane

soil

settlement

without

bitumen

with

bitumen

without

bitumen

with

bitumen

Working load condition without bitumen

Bitumen coating of pile shaft reduces drag load

significantly and smaller settlement. However, at the same

time, pile capacity also reduces.

4/27/2015 51

Variable working load cases

Head Load Toe Load Toe Penetration

[kN] [kN] [mm]

2000 1180 27.99

4000 2241 65.27

6000 3431 111.58

Variable surcharge load cases

Surcharge Toe Load Toe Penetration

[kPa] [kN] [mm]

10 kPa 1704 44.38

20 kPa 1951 53.42

40 kPa 2241 65.27

Importance of toe load toe penetration curve

0

40

80

120

160

200

0 1000 2000 3000 4000 5000 6000

To

e m

ove

men

t (m

m)

Toe load (kN)

Toe load - Toe Mvmnt

variable working load cases (with dragload)

variable surcharge load cases (with dragload)

4/27/2015 52

Force and settlement (downdrag) interactive design.

The unified pile design for capacity, drag load, settlement, and downdrag

0

5

10

15

20

25

30

0 50 100 150 200

SETTLEMENT (mm)

DE

PT

H (

m)

0

5

10

15

20

25

30

0 2,000 4,000 6,000

LOAD (KN)

DE

PT

H (

m)

O-cell

Silt

Sand

Clay

Till

0

1,000

2,000

3,000

4,000

0 50 100

TO

E L

OA

D

(KN

)

Qd

Pile toe load in the load distribution diagram must

match the toe load induced by the toe movement

(penetration), which match is achieved by a trial-

and-error procedure. PILE TOE PENETRATION (mm)

Pile Cap Settlement

Soil Settlement

q-z relation

The final solution is based on three "knowns": The shaft resistance distribution, the toe load-movement response, and the overall settlement distribution. Which all comes from basic site and project knowledge. 4/27/2015 53

Simulated Load Tests Results

54

30

10

Pile movement [mm]

50

LOAD [kN] 2000 4000 6000

SUMMARY RESULTS NSF do not affect Ultimate Pile Resistance (about 6500 kN in above cases) Soil settlements (So) produce drag-loads (NSF) on piles Larger So showed softer pile response; and larger pile settlements 4/27/2015

Results of load tests on bitumen coated piles

55

30

10

Pile movement [mm]

50

LOAD [kN] 2000 4000 6000

Bitumen Coating reduces total resistance (geotechnical capacity) of pile from 6500 kN to 5300 kN But the external ground settlements influence on pile movement is almost insignificant compared to uncoated pile

Uncoated Pile

Bitumen coated piles

4/27/2015

FE analysis of groups of piles Interaction Analysis

Hypothetical cases (1, 2, 4, 9 and 36 piles) as per Fellenius (2012)

6 x 6 m

(36 piles)

3 x 3 m

(9 piles)

2 x 2 m

(4 piles)

2 x 1 m

(2 piles)

1 x 1 m

(1 pile)

Driven concrete pile, D = 318 mm (circumference area, A = 1 m2/m), pile length, L = 20 m

Each pile in the group has a 1.0m2 portion of the total group area.

c/c spacing = 1.0 m, i.e., 3.14D

Soil layers are similar to that of previous slides

No working load applied, 40 kPa surcharge

Pile cap thickness= 1m, except for 36 piles (2m thick) 4/27/2015 56

FE analysis of groups of piles

6 x 6 m

(36 piles)

Fill (3m thick)

s = 20 kN/m3; E50;ref = 10MPa; c = 0; = 30

o


s = 16 kN/m3; Cc = 1.0; Cr = 0.1; eo = 2.0, c = 0; = 20

o

k = 1 x 10-9 m/s


s = 20 kN/m3; E50;ref = 30MPa; c = 0; = 40

o

Surcharge 40 kPa Pile cap thickness = 2m

centre

corner

side

interior

4/27/2015 57

FE analysis of groups of piles

3 x 3 m

(9 piles)

2 x 2 m

(4 piles)

1 x 1 m

(1 pile)

2 x 1 m

(2 piles)

Fill (3m thick)

s = 20 kN/m3; E50;ref = 10MPa; c = 0; = 30

o


s = 16 kN/m3; Cc = 1.0; Cr = 0.1; eo = 2.0, c = 0; = 20

o

k = 1 x 10-9 m/s


s = 20 kN/m3; E50;ref = 30MPa; c = 0; = 40

o

30 m

30 m

Surcharge 40 kPa

Pile cap thickness = 1m for all groups

centre

corner

single

side

4/27/2015 58

Group of 36 piles results

0

5

10

15

20

25

0 200 400 600

Dep

th (

m)

Load (kN)

Single pile

corner

Side

Interior

Centre

0

5

10

15

20

25

0 200 400 600 800

Dep

th (

m)

Settlement (mm)

soil

settlement

single

corner side

centre Interior

Neutral

Plane

Pile

settlement

12

14

16

18

0 5 10 15 20

Dep

th (

m)

Settlement (mm)

soil

single piles in group

Group of piles is beneficiary in reducing drag load. The innermost

piles see smaller drag load. For group of piles to settle uniformly, the

group must have the same neutral plane location.

4/27/2015 59

Group of 2, 4, and 9 piles results

0

5

10

15

20

25

0 200 400 600

Dep

th (

m)

Load (kN)

Single pile

9 piles group (corner)

9 piles group (centre)

9 piles group (side)

0

5

10

15

20

25

0 200 400 600

Dep

th (

m)

Load (kN)

Single pile

2 piles group

4 piles group

single

single

centre side

corner

Group of piles is beneficiary in reducing drag load.

Therefore, designing piled foundation using single pile case

is quite conservative 4/27/2015 60

Measured group response Okabe Field Experiments (1973)

61

62

Centrifuge Experiments

Outline






Summary

4/27/2015 63

Summary

Pile design according to EC7 design approaches has been presented

EC7, BS8004 and CP4 do not address the dragload (or NSF) correctly.

It has been shown here using FE analysis of single pile and groups of piles that dragload does not reduce pile geotechnical capacity.

The key point in pile design is settlement not capacity.

FE analysis can easily predict the location of NP with no iterations required.

Group of piles connecting to a rigid pile cap has a beneficiary effect in reducing the dragload.

Pile design subjected to dragload using single pile scenario is quite conservative.

4/27/2015 64

EC7 Provision for NSF Design

4/27/2015 65

In essence EC7 do allow us to do specialized FEM analysis to design for NSF

This will enable us to take advantage of the actual expected NSF force over the period of design life

It will also allow us to include pile group effects where much reduced NSF will be observed in the inner piles of large pile groups or piled-raft foundations

4/27/2015 66

EC7 ALLOWS FOR INNOVATIVE DESIGN FOR NSF BY USING GOOD FEM PILE-SOIL INTERACTION ANALYSIS TO ACCOUNT FOR CORRECT CONSOLIDATION SETTLEMENTS (TREATED AS ACTION)

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04-NSF Design and EC7 (Prof Harry Tan)

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