08we Orr Worked Examples Design of Pile Foundations

Eurocodes:Background & ApplicationsGEOTECHNICAL DESIGN with worked examples 13-14 June 2013, Dublin

Worked examples –d i f il design of pile foundationsfoundations

Dr. Trevor OrrTrinity College DublinConvenor SC7/EG3


Worked example 1 –design of pile foundations from load tests

DESIGN SITUATIONdesign of pile foundations from load tests

Eurocodes:Background & Applications

GEOTECHNICAL DESIGN ith k d l 13 14 J D bliGEOTECHNICAL DESIGN with worked examples 13-14 June, Dublin

Design situation for a pile designed from static load test resultsload test results

• Pile foundations are required to support the following loads from a building :building :

• Characteristic permanent vertical load Gk = 6.0 MN

• Characteristic variable vertical load Qk = 3.2 MNCharacteristic variable vertical load Qk 3.2 MN

• It has been decided to use bored piles 1.2m in diameter and 15m long long

• The pile foundation design is to determine how many piles are i drequired

©2013 Trevor Orr. All rights reserved.3



Pile load test results Pile settlementsL d MN

• Load tests have been performed on site on four il f th

00 0,5 1 1,5 2 2,5 3

Load, MN

piles of the same diameter and same length

20

40

Pile 1

• The results of load-settlements curves are plotted in Figure 1

60

80

emen

t, m

m

Pile 1Pile 2Pile 3Pile 4Meanplotted in Figure 1

• Adopt settlement of the pile top equal to10% of

100

120Se

ttle Mean

the pile base diameter as the "failure" criterion (7.6.1.1(3))

140

160


( ( ))


Worked example 1 –design of pile foundations from load tests

SOLUTIONdesign of pile foundations from load tests



Measured pile resistances

• Adopting the pile load at a settlement of the top of the piles equal to 10% of the pile diameter as the ultimate resistance means to 10% of the pile diameter as the ultimate resistance means using the measured resistances at a settlement of:

12.0 x (10/100) x 103 = 120mm

• From the load-settlement graphs for each pile this gives:

Pile 1 Rm = 2.14 MNPile 2 R = 1 96 MNPile 2 Rm = 1.96 MNPile 3 Rm = 1.73 MNPile 4 Rm = 2.33 MN

• Hence the mean and minimum measured pile resistances are :Rm, mean = 2.04 MNR 1 73 MN


Rm, min = 1.73 MN



Characteristic resistance

• The characteristic pile resistance is obtained by dividing the mean and minimum measured pile resistances by the correlation factors p yξ1 and ξ2 and choosing the minimum value:

( ) ( )⎭⎬⎫

⎩⎨⎧

= minmc;meanmc;kc; ;Min

ξξRR

R

• For four load tests, recommended ξ1 and ξ2 values are:ξ1 = 1.1

⎭⎩ 21 ξξ

ξ1 1.1ξ2 = 1.0

• Hence the characteristic pile resistance:

73.1}73.1;85.1{Min0.1

73.1;1.1

04.2Minkc; ==⎭⎬⎫

⎩⎨⎧=R




Design Approach 1 partial factors

• Combinations of sets of partial factorsDA1.C1 A1 “+” M1 “+” R1DA1.C2 A2 “+” M1 or M2 “+” R4

• Partial actions factorsA1 1 35 1 5A1 γG = 1.35 γQ = 1.5A2 γG = 1.0 γQ = 1.3

• Partial material factorsPartial material factorsM1 and M2 not relevant (γφ’ = 1.0 and not used)

• Partial resistance factorsR1 γt = 1.15 (Total/combined compression)

R4 γt = 1.5




Design Approach 1 - pile design

Design equation Fc,d ≤ Rc,d

DA1.C1 Fc,d = 1.35 Gk+1.5 Qk = 1.35x6.0+1.5x3.2 = 12.9 MNDA1.C1 Fc,d = 1.0 Gk+1.3 Qk = 1.0x6.0+1.3x3.2 = 10.2 MN

For a single pileDA1.C1 Rc,d = Rc,k /γt = 1.73 / 1.15 = 1.50 MNDA1.C2 Rc,d = Rc,k /γt = 1.73 / 1.5 = 1.15 MN

Assuming no pile group effect, for n piles, resistance = n x Rc,d

Hence DA1.C1 n ≥ Fc,d /Rc,d = 12.9/1.5 = 8.6DA1.C2 n ≥ Fc,d /Rc,d = 10.2/1.15 = 8.9

Therefore DA1.C2 controls and no. piles required: n = 9





• Combination of sets of partial factorsDA2 A1 “+” M1 “+” R2

• Partial actions factorsA1 γG = 1.35 γQ = 1.5Q

• Partial material factorsM1 not relevant (γφ’ = 1.0 and not used)

• Partial resistance factorR2 γt = 1.1 (Total/combined compression)




Design Approach 2 - pile design

Design equation Fc,d ≤ Rc,d

Fc,d = 1.35 Gk + 1.5 Qk = 1.35·x 6.0 + 1.5·x 3.2 = 12.9 MN

For a single pileRc,d = Rc,k /γt = 1.73 / 1.1 = 1.57 MN

Assuming no pile group effect, for n piles, resistance = n x Rc,d

Hence Fc,d ≤ n Rc,d

h f l d / / ilTherefore no. piles required: n = Fc,d / Rc,d = 12.9 / 1.57 = 9 piles





• Combination of sets of partial factors:DA3 A1 “+” M1 “+” R3

• Partial resistance factor:R3 γt = 1.0 (Total/combined compression)

• Since the R3 partial resistance factor is equal to 1.0, no safety margin is provided if DA3 is used to calculated the design pile margin is provided if DA3 is used to calculated the design pile resistance from pile load test results

Hence piles should not be designed from load test results using • Hence piles should not be designed from load test results using Design Approach 3




Conclusions from pile worked example 1

• The same design number of piles, 9 is obtained for both DA1 and DA2DA2

• Since the partial resistance factor is 1.0 for DA3, this Design Approach should not be used for the design of piles from pile load Approach should not be used for the design of piles from pile load test results



Worked example 2 –design of pile foundations from test profiles

DESIGN SITUATIONdesign of pile foundations from test profiles



Design situation for a pile designed from a CPT test profileCPT test profile

• The piles for a building are each required to support the following loads:loads:

• Characteristic permanent vertical load Gk = 300 kN

• Characteristic variable vertical load Qk = 150 kN

• The ground consists of dense sand beneath loose sand with soft clay and peat to 16.5m as shown in figure on next slide

• It has been decided to use 0.45m diameter bored piles

• The pile foundation design involves determining the length of the lpiles




Borehole log and CPT profile

qc

CPT profile

• 1 CPT was carried outLoose sand, soft clay and some peat• Soil has upper 11m layer of loose

sand, soft clay and some peat over 5.5m of clay with peat seams

peat

Clay with

Cautious average qc = 2.5 MPa

• Stronger layer of medium to dense sand starts at depth of 16 5m

Clay with peat seams

sand starts at depth of 16.5m

Cautious average qc value chosen in Session 4 example Medium to

16.5m

• Assume the soil above 16.5m provides no shaft resistance

dense sand




Unit pile resistances Table D.3Unit base resistance pb of cast in-situ piles in coarse soil

with little or no fines

• The pile resistance is calculated using Tables D.3 and D.4 of EN

Normalised settlement s/Ds;

s/Db

Unit base resistance pb, in MPa,at average cone penetration resistance

qc (CPT) in MPaqc = 10 qc = 15 qc = 20 qc = 25

0 02 0 70 1 05 1 40 1 751997-2 relating average qc values in stronger soil to the unit base and shaft resistances, pb and ps

0,02 0,70 1,05 1,40 1,750,03 0,90 1,35 1,80 2,25

0,10 (= sg) 2,00 3,00 3,50 4,00NOTE Intermediate values may be interpolated linearly.In the case of cast in-situ piles with pile base enlargement, the values shall be multiplied by 0 75

• Assume settlement of the pile at the ULS, sg so that the normalised

shall be multiplied by 0,75.s is the normalised pile head settlementDs is the diameter of the pile shaftDb is the diameter of the pile basesg is the ultimate settlement of pile head

settlement is 0.1

• Interpret linearly between relevant Average cone penetration (C )

Unit shaft resistance ps

Table D.4Unit shaft resistance ps of cast in-situ piles in coarse soil

with little or no fines

qc values to obtain pb and ps from these tables

resistance qc (CPT)MPa MPa

0 05 0,040

10 0,080> 15 0 120


> 15 0,120NOTE Intermediate values may be interpolated linearly



Pile design

Pile diameter D = 0.45mPile base cross sectional area Ab = π x 0.452 / 4 = 0.159 m2

b

Pile shaft area per metre length As = π x 0.45 = 1.414 m2 /mLength of pile in stronger layer providing shaft resistance = Ls

Calculate compressive pile resistance for the one profile of test results from Calculate compressive pile resistance for the one profile of test results from equation:

Rc,cal = Rb;cal + Rs;cal = Ab x pb + As x Ls x ps

Apply recommended correlation factors ξ3 and ξ4 from EN 1997-1 for one profile of test results to obtain the characteristic base and shaft compressive pile resistancespile resistances

Use the recommended partial factors for a particular Design Approach and hence determine the design length of the pile



Worked example 3 –design of pile foundations from soil parameters

DESIGN SITUATIONdesign of pile foundations from soil parameters



Design situation for a pile designed from soil parametersparameters

• The piles for a proposed building in Dublin are each required to support the following loads:

• Characteristic permanent vertical load Gk = 600 kN

• Characteristic variable vertical load Qk = 300 kNk

• The ground consists of about 3m Brown Dublin Boulder Clay over Black Dublin Clay to great depth

• It has been decided to use 0.45m diameter driven piles

• The pile foundation design involves determining the length of the piles




Characteristic undrained shear strength

Brown DBC

shear strength

• Figure shows plot of SPT N values obtained plotted against depth Black DBCobtained plotted against depth

• Shaft resistance in Brown Dublin Boulder Clay is ignored

l l k bl ld

Average N = 57

Cautious average N value = 45• Average N value in Black Dublin Boulder

Clay = 57

• A cautious average N value = 45

Cautious average N value 45

• PI of the Dublin Boulder Clay = 14%

• From Stroud and Butler plot of f1 vs. N

Ad t f 6

cu = f1 x N

Adopt f1 = 6

• Hence calculate the characteristic undrained shear strength cu;k = f1 x N




Pile designPile diameter D = 0 45mPile diameter D = 0.45mPile base cross sectional area Ab = π x 0.452 / 4 = 0.159 m2

Pile shaft area per metre length As = π x 0.45 = 1.414 m2 /mLength of pile in Black D blin Cla p o iding shaft esistance LLength of pile in Black Dublin Clay providing shaft resistance = Ls

The unit pile base and shaft resistances, qb;k and qs;k are obtained as follows:qb;k = Nq x cu;k

q D c 0 4 0 45 cqs;k = α x π x D x cu;k = 0.4 x π x 0.45 x cu;k

Assume Nq = 9 and α = 0.4

Hence calculate the characteristic base and shaft resistancesRb;k = Ab x qb;k

Rs;k = As x Ls x qs;k

Use the recommended partial factors for a particular Design Approach and h d h d l h f h lhence determine the design length of the pile

Since the building is being constructed in Dublin, the Irish NA must be used, which requires a model factor of 1.75 on γb and γs


Compare with a pile designed in Germany to DA2 with a model factor of 1.27


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08we Orr Worked Examples Design of Pile Foundations