Pile foundations

Design approaches according to Eurocode 7

Structural Engineering Master program

Section 7 – Eurocode 7

is addressing to:

type of the load-transfer characteristic for the pile of interest -end-bearing piles (piloţi purtători pe vârf), shaft-bearing piles(piloţi flotanţi), piles subjected to tension (piloţi solicitaţi latracţiune), and transversaly loaded piles (piloţi încărcaţitransversal),

type of the piling technology intended to perform on site -driven, jacking or screw piles (piloţi puşi în operă prin batere,presare sau înşurubare), CFA or bored piles (piloţi foraţi cu şneccontinuu sau forare clasică), with or without injections (cu saufără injectări).

Pile foundation failure issues Failure by reaching one of the following limit states:

ULS – Ultimate Limit States Overall stability (pierderea stabilităţii generale)

Bearing resistance failure (epuizarea capacităţii portante a fundaţiei pe piloţi)

Up-lift or tensile resistance failure (ridicarea sau rezistenţa la tracţiune insuficientă a fundaţiei pe piloţi)

Transverse failure in the ground (cedarea terenului datorită încărcării transversale a fundaţiei pe piloţi)

Structural failure of the pile in compression, tension, bending, buckling or shear (cedarea structurală a pilotului la compresiune, întindere, încovoiere, flambaj sau forţă tăietoare)

SLS – Service Limit States

Excessive settlement (tasare excesivă)

Excessive heave (ridicare excessivă)

Excessive lateral movement (deplasare laterală excesivă)

Vibrations (vibraţii inacceptabile)

pile load tests

When decision over performing pile load tests ismandatory?

A pile type or a piling technology is intended that lackscomparable experience;

Piles are subjected to loads for which both theory andexperience are not providing enough confidence for that specificproject;

During piling, the resulting pile is strongly devianting and totallyunfavorable from the anticipated behaviour.

Pile resistance - R

can be established from:

Ground test results (in situ tests, laboratory tests),

Pile load tests (Static Load Test – SLT, Dynamic Load Test – DLT),

Rk,d – resistance as characteristic, design value for ULS onaxially loaded piles - compressive or tensileresistance failure;

Rtr,k,d – resistance as characteristic, design value for anyULS on transversally loaded piles

Pile load tests represent valid input data for assessing pile resistances

when subjected to various load configuration;

confirm or disagree the results from the analyticalcalculations;

performed on trial piles, individually;

on one or several piles from the group – piles aresubjected to the design load, and potentially increasedup to 30%;

single pile – outside the pile group area – this pile issubjected to load increments until failure occurs;

Intended results from pile load tests

Evaluation over the benefits of using a specific pilingtechnology;

To establish the response for a representative pile wheninteracting with the surrounding soil, regarding values ofboth settlement and limit load;

Evaluate the entire pile foundation behaviour.

Decision over the number of piles to be tested, selectionof their location and the appropriate moment to start thetest.

Static load tests – SLTthe bench-mark of pile performance

the direct measurement of pile headdisplacement in the response to a physicallyapplied test load in various configurations(compression, lateral, tension);

http://www.loadtest.co.uk

axially loaded pile

laterally loaded

pile

Dynamic Load Test

30 tons hammer with a fewseparated impacts;

forces and motions are recorded byPile Driving Analyzer (PDA) → pile

capacity as pile resistance;

Pile test– single pile

Pile load test

Geotechnical restriction

Fd (Ftr,d ) <= Rd (Rtr,d) Fd = γF * Fk

Md = Mk / γM

Rd = Rk / γR

Parameters with their real values, without safety factors, arenamed characteristic values with an index k: Fk – of theapplied load, Rk - resistances.

Parameters with safety factors are named design values with

an index d: Fd – of the applied load, Rd – resistances.

Characteristic value of compressive or tensile resistance Rk – derived

from values R (measured values Rm or calculated values Rcal)

Rk = R/ξ, ξ as the correlation factor

Rk = Min {Rm, mean/ξ1 ; Rm,min/ ξ2}

from static load tests

Rk = R/ξ, ξ as the correlation factor

Rk = Rbk+Rsk = (Rb,cal+Rs,cal)/ ξ = Rcal/ ξ= Min {Rcal, mean/ξ3 ; Rcal,min/ ξ4}

from ground test results and calculation rules

Rbk = qbk Ab and Rsk = Σqsik Asi

an alternative procedure using base resistance qbk and shaft friction qsik from values of ground parameters

Characteristic values

from dynamic load tests

dynamic impact tests;

pile driving formulae;

wave equation analysis;

Rk = Min {Rm,mean/ξ5 ; Rm,min/ ξ6}

For the ULS, the three possible design approaches

use different sets of partial safety factors:

Design approach 1 with two combinations: for piles: Combination 1: A1 + M1 (= 1) + R1 Combination 2: A2 (= 1) + (M1 (= 1) or M2) + R4 (M1 for pile resistance, M2 for unfavorable actions like negative skin

friction or transversal loads)

Design approach 2: A1 + M1 (= 1) + R2 → safety factors on loads and resistances

Design approach 3: A2 (= 1) + M2 + R3 (= 1) (A1 for loads from the structure without influence of soil material

parameters)

Design values of the pile resistance:

Rd = Rk/γt or

Rd = Rbk/ γb + Rsk/γs

Design value of the applied compression/tension load:

Fd = γt Fk

Design example 1 – pile foundationsexample from the workshop on Eurocode 7 – Roger Frank, Cermes-ENPC

bored pile, 600mm diameter

ULS → compressive resistance failure

design load on an individual pile

Fd <= Rd

Fd = γG Gk + γQ Qk = γG 1200 + γQ 200

design compressive resistance

Rd = Rb,d + Rs,d = Rb,k/ γb + Rs,k/ γs

characteristic and design base and shaft resistance

Rb,k = Ab qb,k and Rs,k = As qs,k

Since characteristic values qb,k , qs,k are not provided from ground profiles from in situ tests but from one characteristic value Φ’k → values of γb , γs need to be corrected

by a model factor γR = 1.5

partial safety factors on Actions - A

Actions A1 A2

Permanent loads, unfavorable 1.35 1.00

Permanent loads, favorable 1.00 1.00

Variable loads, unfavorable 1.50 1.30

Variable loads, favorable 0.00 0.00

DA1 – C1 Fd = 1.35 x 1200 + 1,5 x 200 = 1920 kN

DA1 – C2 Fd = 1.00 x 1200 + 1,3 x 200 = 1460 kN

DA2 Fd = 1.35 x 1200 + 1,5 x 200 = 1920 kN

DA3 Fd = 1.35 x 1200 + 1,5 x 200 = 1920 kN

partial safety factors on Materials - M

Materials M1 M2

Angle of internal friction tan(ϕ) 1.00 1.25

Cohesion c 1.00 1.25

Undrained cohesion cu 1.00 1.40

Unit weight γ 1.00 1.00

Φ’k = 350 → Φ’d = tan-1 [(tanΦ’k)/γM] = tan-1[(tan350)/1.25] =

partial safety factors on Resistances - R

Characteristic values of qbk and qsk

qbk = σv0’ x Nq

Nq = eπ x tanφ’tan2(π/4 + φ'/2) σv0’ = 2 γ + (L-2)(γ- γw) qsk = Σσ’h tanδ = ΣK0 σ’v0 tanδ = 0.5(1-sinφ’) σ’v0 tanφ’

→ L = ?

Design lengths for different Design Approaches

Design Approach pile length (m) OFS = Rk/Fk

Drainedcondition

DA1 (Combination 1)

DA1 (Combination 2)

DA2

DA3

Design situation

pile group – piles are located to work as single piles (no group effect considered);

precast piles, driven into the ground;

pile diameter 0.4m, pile length 15m;

two pile load tests performed beyond 0.1 pile diameter;

the allowable settlement is 10mm;

actions from the structure above the ground level: permanent vertical load 20,000kN, variable vertical load 5,000 kN

Geotechnical restriction

Fd <= Rd Fd = γF * Fk

Md = Mk / γM

Rd = Rk / γR

Parameters with their real values, without safety factors, arenamed characteristic values with an index k: Fk – of theapplied load, Rk - resistances.

Parameters with safety factors are named design values with

an index d: Fd – of the applied load, Rd – resistances.

actions

Fd = γk Gk+ γQ Qk = γk 20,000+ γQ 5,000

Actions A1 A2

Permanent loads, unfavorable 1.35 1.00

Permanent loads, favorable 1.00 1.00

Variable loads, unfavorable 1.50 1.30

Variable loads, favorable 0.00 0.00

DA1 – C1 Fd = 1.35 x 20,000 + 1.5 x 5,000 = …….. kN

DA1 – C2 Fd = 1.00 x 20,000 + 1.3 x 5,000 = ……… kN

DA2 Fd = 1.35 x 20,000 + 1.5 x 5,000 = ……… kN

DA3 Fd = 1.35 x 20,000 + 1.5 x 5,000 = ……… kN

Design example 2

example from the workshop on Eurocode 7 – T.L.L. Orr, Trinitiy College, Dublin

Pile group resistance

Rk = R/ξ, ξ as the correlation factor

Rk = Min {Rm, mean/ξ1 ; Rm,min/ ξ2}

from static load tests

Rk,single pile =?

Rk,pile group = N x Rk, single pile/γR

For the ULS, the three possible design approaches

use different sets of partial safety factors:

Design approach 1 with two combinations: for piles: Combination 1: A1 + M1 (= 1) + R1 (= 1) Combination 2: A2 (= 1) + (M1 (= 1) or M2) + R4 (M1 for pile resistance, M2 for unfavorable actions like negative skin

friction or transversal loads)

Design approach 2: A1 + M1 (= 1) + R2 → safety factors on loads and resistances

Design approach 3: A2 (= 1) + M2 + R3 (= 1) (A1 for loads from the structure without influence of soil material

parameters)

No. of piles for different Design Approaches

Design Approach ULS no. of piles

DA1 (Combination 1)

DA1 (Combination 2)

DA2

DA3

Design Approach SLS