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.
Model Pile procedure [clause 7.6.2.3(5)P]
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:
Model Pile procedure [clause 7.6.2.3(5)P]
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.
Alternative procedure [clause 7.6.2.8(8)]
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.
Alternative procedure [clause 7.6.2.3(8)]
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.
Alternative procedure [clause 7.6.2.3(8)]
4/27/2015 14
Outline
Pile Design using EC7
Problems with BS 8004, EC7, and CP4 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 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
BS 8004 (CP4) on Dragload
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
BS 8004 (CP4) on Dragload
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
Pile Design using EC7
Problems with BS 8004, EC7, and CP4 on dragload
Design example using EC7
Unified pile design concept
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
Total design load, Fcd = 1.0 x 300 + 1.25 x 94.2 = 417.8 kN
Design resistance, Rc;d = 47.1LR/1.3 = 36.2LR kN
Condition Fc;d Rc;d leads to LR 417.8/36.2 = 11.54 m
Note: the correlation
factor, is ignored
Example 1
4/27/2015 27
Pile subject to dragload based on EC7
DA2: A1 + M1 + R2
Total design load, Fcd = 1.35 x 300 + 1.35 x 94.2 = 532.2 kN
Design resistance, Rc;d = 47.1LR/1.1 = 42.8LR kN
Condition Fc;d Rc;d leads to LR 532.2/42.8 = 12.43 m
DA3: (A1 or A2) + M2 + R3
Total design load, Fcd = 1.35 x 300 + 1.25 x 94.2 = 522.8 kN
Design resistance, Rc;d = 47.1LR/1.25 = 37.7LR kN
Condition Fc;d Rc;d leads to LR 417.8/37.7 = 13.87 m
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
Total design load, Fcd = 1.0 x 300 = 300 kN
Design resistance, Rc;d = (94.2 + 47.1LR)/1.3 = 72.5 + 36.2LR kN
Condition Fc;d Rc;d leads to LR 227.5/36.2 = 6.28 m (cf. 11.54m)
DA2: A1 + M1 + R2
Total design load, Fcd = 1.35 x 300 = 405 kN
Design resistance, Rc;d = (94.2 + 47.1LR)/1.1 = 85.6 + 42.8LR kN
Condition Fc;d Rc;d leads to LR 319.4/42.8 = 7.46 m (cf. 12.43m)
DA3: (A1 or A2) + M2 + R3
Total design load, Fcd = 1.35 x 300 = 405 kN
Design resistance, Rc;d = (94.2 + 47.1LR)/1.25 = 75.4 + 37.7LR kN
Condition Fc;d Rc;d leads to LR 329.6/37.7 = 8.74 m (cf. 13.87m)
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
Pile Design using EC7
Problems with BS 8004, EC7, and CP4 on dragload
Design example using EC7
Unified pile design concept
FE simulation of single pile and groups of piles subjected to dragload
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
Pile Design using EC7
Problems with BS 8004, EC7, and CP4 on dragload
Design example using EC7
Unified pile design concept
FE simulation of single pile and groups of piles subjected to dragload
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
Unit resistance (kPa)
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
Unit resistance (kPa)
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
Unit resistance (kPa)
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
Unit resistance (kPa)
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
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 (15m thick)
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
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 (15m thick)
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
Pile Design using EC7
Problems with BS 8004, EC7, and CP4 on dragload
Design example using EC7
Unified pile design concept
FE simulation of single pile and groups of piles subjected to dragload
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)