To test or not to test – that is the (design) question

Dr Andrew Bond (Geocentrix)

Eurocode 7 approach to pile design

Eurocode 7 Part 1 §7 ‘Pile foundations’Emphasizes design based on field testingDesign using ground parameters is presented as an ‘alternative’ (yet is main approach in UK)Base and shaft factors vary with method of installation (bored or CFA or driven)

Design from field test resultsDesign resistance related to number of testsCorrelation factors for static load tests, ground tests (e.g. CPTs), and dynamic impact tests

Design using ground parametersUK National Annex adjusts base, shaft, and model factors according to the amount and type of static load testing

Design using ground parametersTo test or not to test – that is the (design) question

Pile design using ground parameters

Resistance factor

Modelfactor

Combinationfactor

Actionfactor

Pile resistance factors for DA1-2 (from UK NA to BS EN 1997-1)

1.3 1.61.3-1.5

1.31.6/1.45

1.5ENV 1997-1: 1994

Shaft (γs)

Base (γb)

1.3

1.5

Shaft (γs)

Base (γb)

1.4

1.6

1.2

2.0As base

1.72.0Default values

Eurocode

UK National Annex to BS EN 1997-1

?EN 1997-1: 2004

1.4

Model factor (γRd)

DrivenBored/CFA Tens-ion (γst)

Total (γt)

1.71.51.7> 1% constructed piles taken to 1.5 x representative load (or settlement no concern)Maintained load test to calculated unfactored ultimate load

Resistance factors from Set R4Static load tests on piles loaded to…

Case study – piles in sand

Ground test results from site in Richmond (courtesy CL Associates, 2004)

4 CPTs in dense (becoming loose gravelly) SANDDesign of 4 x 6m long piles x φ400mm bored piles

ActionsPermanent Gk = 1400 kN + Variable Qk = 500 kN

4-8m1-4m

38-42(say 40)

Angle of shearing resistance φ (°)

36-38(say 37)

2650(assuming

qb/qc ≈0.2)

qb (kPa)

15

27

21

30

1-4m 4-8m

08

05

03

02

CPT number

(15)

9

15

90(assuming Ks = 2.0)

14

qs (kPa)Average qc(MPa)

Verification of ULS (DA1-2) – with no explicit verification of SLS

, ,

333 6784 1686

1.4 2.0 1.4 1.6

b calc s calcd

Rd b Rd s

R RR

kN

γ γ γ γ= +

× ×

⎛ ⎞= + × =⎜ ⎟× ×⎝ ⎠

2 2

,

0.42650 333

4b calc b b

mR q A kPa kN

π⎛ ⎞×= = × =⎜ ⎟

⎝ ⎠

( ), 90 0.4 6 678s calc s sR q A kPa m m kNπ= = × × × =

( )( )1.0 1400 79 1.3 500 2129

d G G G Q QF V W V

kN

γ γ= × + + ×

= × + + × =

Conventional soil mechanics theory

Model factor

Resistance factors from Set R4

Resistance factors

Action factors from Set A2

Action factors

Fd > Rd → ULS NOT verified!

Reliability of calculated resistance depends on amount of testing

With no static pile load testsFactors on base = 1.4 x 2.0 = 2.8Factors on shaft = 1.4 x 1.6 = 2.24Fd = 2129 kN > Rd = 1686 kN (not verified)

With 1% working static pile load testsFactors on base = 1.4 x 1.7 = 2.38Factors on shaft = 1.4 x 1.4 = 1.96Fd = 2129 kN > Rd = 1943 kN (just not verified)

With preliminary static pile load testFactors on base = 1.2 x 1.7 = 2.04Factors on shaft = 1.2 x 1.4 = 1.68Fd = 2129 kN < Rd = 2267 kN (verified)

Traditional lumped factors of safetyGlobal factor of safety for replacement piles

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0% 20% 40% 60% 80% 100%

Qs/Qult

F =

Qul

t / Q

a

Burland et al.

Tomlinson

Lord (for chalk)

Ignore base

Global factor of safety for replacement piles(with 25% variable action)

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0% 20% 40% 60% 80% 100%

Qs/Qult

F =

Qul

t / Q

a

Eurocode 7 vs traditional

No explicit SLS check

Tests on 1% working piles

Preliminary load tests

Design using field test resultsTo test or not to test – that is the (design) question

Pile design from field test results

Resistance factor

Correlationfactor

Combinationfactor

Actionfactor

Correlation factors from UK NA to BS EN 1997-1

(1.18)(1.32)(8)

1.201.337

For structures that can transfer loads from ‘weak’ to ‘strong’piles, ξ1 to ξ4 may be divided by 1.1 provided ξ1 ≥ 1.0 and ξ3 ≥ 1.0

(1.17)(1.31)(9)

1.151.30≥ 10

Figures in (brackets) provide values ‘missing’ from EN 1997†Values chosen to match static load tests

(1.23)(1.34)(6)

1.661.81≥ 201.261.3651.081.35≥ 5

Min.MeanMin.MeanMin.Mean

≥ 15

≥ 10

≥ 5

≥ 2

No

Dynamic impact tests

1.82

1.83

1.85

1.94

ξ5

1.67

1.70

1.76

1.90

ξ6

4

3

2

1

No

Ground tests

1.38

1.42†

1.47†

1.55†

ξ3

1.29

1.33

1.39

ξ4

1.151.384

1.23

1.35

ξ2

1.423

1.472

1.551

ξ1

No

Static load tests

Case study – piles in sand

Data from site in Richmond (courtesy CL Associates, 2004)4 CPTs in dense (becoming loose gravelly) SANDDesign of 4 x 6m long piles x φ400mm bored pilesActions: Gk = 1400 kN + Qk = 500 kN

TotalBaseShaft

3000

2000

3000

2800

pb(kPa)

754

628

754

754

377

251

377

352

15

27

21

30

1-4m 4-8m

08

05

03

02

CPT number

1131120(15)

8791009

113112015

110612014

Calculated resistance (kN)

ps (kPa)Average qc(MPa)

, ,

1062 879770 681 681

1.38 1.29

calc mean calc mink

mean min

R RR or

or or kN

ξ ξ⎛ ⎞ ⎛ ⎞

= ⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠

⎛ ⎞ ⎛ ⎞= = =⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

Verification of ULS (DA1-2)– with 1% working pile load tests

, ,

4 4

251 6294 1851

1.29 1.7 1.29 1.4

b calc s calcd

b s

R RR

kN

ξ γ ξ γ= +

× ×

⎛ ⎞= + × =⎜ ⎟× ×⎝ ⎠

( )( )1.0 1400 79 1.3 500 2129

d G G G Q QF V W V

kN

γ γ= × + + ×

= × + + × =

Correlation factor

Resistance factors from Set R4 (1% working tests)

Resistance factors

Correlation factors for 4 test results

Minimum governs

Fd > Rd → ULS NOT verified!

Action factors from Set A2

ConclusionTo test or not to test – that is the (design) question

Conclusion

“…Much of the design of pile foundations is still dominated by estimation of axial

capacity…where the critical issue is more likely to be magnitude of displacements

under operating conditions…”

Mark Randolph (2003 Rankine Lecture)

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