The new French standard for the application of Eurocode 7 for deep foundations. Roger Frank, Ecole des ponts Sébastien Burlon, IFSTTAR

Transcription

1 The new French standard for the application of Eurocode 7 for deep foundations Roger Frank, Ecole des ponts Sébastien Burlon, IFSTTAR

2 Outline Introduction Aspects of pile design following Eurocode 7 (EN ) 1 Table of contents of NF P (July 2012) 2 Bearing capacity from PMT and CPT (assessment of model factors γ R;d ) 3 Displacement under axial loads and under transverse load (PMT "t-z" and «"p-y" methods) - - Conclusions/Prospective

3 Bearing capacity of piles according to Eurocode 7 (compressive/tensile «resistance») The design shall be based on : a) on the results of static load tests b) empirical or analytical calculation methods validated by static load tests in comparable situations c) results of dynamic load tests whose validity has been demonstrated by static load tests in comparable situations d) the observed performance of a comparable pile foundation (to be supported by site investigation and ground testing).

4 Designing pile foundations with Eurocode 7 : * importance of static pile load tests * an innovative approach to pile capacity taking account of number of load tests or number of soil profiles (through the ξ values) * importance of assessing serviceability of structures through displacement calculation

5 Actions due to ground displacement : * the design values of the strength and stiffness of the moving ground should usually be upper values * the ground displacement is treated as an action and an interaction analysis is carried out (e g : transverse thrusts) * an upper bound of the force transmitted by the ground is introduced as the design action (e g : negative friction). or

6 French standard for the Application of Eurocode 7 july 2012

7 NF P : Table of contents 15 sections and 19 annexes (98 pages +108 pages) Main sections: - combinations of actions and action effects - ultimate compressive resistance (bearing capacity) - ultimate tensile resistance - resistance to transverse loadings - structural resistance (concrete and steel properties for piles) - overall stability - verifications of serviceability limit states

8 NF P : Table of contents Normative annexes: - values of partial factors - bearing capacity and tensile resistance: from PMT and CPT results Informative annexes: - types of piles - soil categories - static pile load tests and bearing capacity - negative friction (downdrag) - transverse behaviour - (axial) group effect - horizontal soil displacement - axial stiffness (settlement of piles) - movements of foundations - provision for bridges

9 2. Bearing capacity from PMT and CPT (assessment of model factors γ R;d ) - -

10 Background for establishing a model factor γ Rd Eurocode 7 - Part 1 Geotechnical design General rules (2004), Section 7 : - requires that the validity of calculation methods be demonstrated by static load tests in comparable situation - advocates the introduction of an explicit model factor γ Rd (applied to the calculation model) when designing piles from ground test results 10

11 The database of IFSTTAR (LCPC) 11

12 12

13 13

14 Pile distribution by class and soil type with PMT profiles (Burlon et al, Géotechnique, June 2014 ) Pile class Pile group Soil type Cl 1 Cl 2 Total Cl 3 Cl 4 Cl 5 Cl 6 Cl 7 Cl 8 G1 G2 Silt and clay % CaCo3 < 30% Sand and gravel Chalk Marl and limestonemarl Weathered rocks Total

15 Pressuremeter method : base resistance q p = q o + k p (p le - p o ) 15

16 Pressuremeter rules : shaft resistance q s =α f sol and q s q smax 0,4 α 3,8 and q smax vary according to soil type and pile category Clay and Silt Sand and Gravel Chalk Marl and Calcareous Marl Weathered Rock 125 f sol [kpa] f sol pl* [MPa] 16

17 17

18 N Abréviation Technique de mise en œuvre Argile % CaCO3 < 30% Limon Sols intermédiaires Sols intermédiaires Sable Grave Craie Marne et Calcaire- Marneux Roche altérée ou fragmentée 1 FS ## Foré simple (pieux et barrettes) FB ## Foré boue (pieux et barrettes) FTP Foré tubé (virole perdue) FTR Foré tubé (virole récupérée) FSR, FBR, PU ## Foré simple ou boue avec rainurage ou puits 90 6 FTC, FTCD Foré tarière continue simple rotation ou double rotation VM Vissé moulé VT Vissé tubé BPF**, BPR** 10 BE** Battu béton préfabriqué ou précontraint Battu enrobé (béton mortier coulis) BM** Battu moulé BAF** Battu acier fermé BAO** # Battu acier ouvert HB** # H battu HBi** H battu injecté IGU ou IRS PP** # Palplanches battues M1 Micropieu type I 18 M2 Micropieu type II 19 PIGU, MIGU Pieu ou micropieu injecté (type III) PIRS, MIRS Pieu ou micropieu injecté (type IV)

19 Pressuremeter rules 134 tests (no injected piles; no chalk) Distribution function of limit load (Burlon et al, Géotechnique, June 2014 ) Moy : 0.93 Mediane: 0.95 COV: 0.20 Quantile for 1.15: 88 % γ Rd1 = 1.15 R c;cal /R c;mes

20 Piles distribution by class and soil type for CPT profiles Pile class Pile group Soil type Total Cl 1 Cl 2 Cl 3 Cl 4 Cl 5 Cl 6 Cl 7 Cl 8 G1 G2 Silt and clay % CaCo3 < 30% Sand and gravel Chalk Marl and limestonemarl Weathered rocks Total

21 CPT rules : base resistance q p = q o + k c q ce Values of k c (for D e /B > 5) 21

22 CPT rules: shaft resistance q s =α f sol and q s q smax 0,2 α 2,25 and q smax vary according to soil type and pile category Clay and Silt Intermediate Soil Sand and Gravel 125 fsol [kpa] q c [MPa] 22

23 23

24 N Abréviation Technique de mise en œuvre Argile % CaCO3 < 30% Limon Sols intermédiaires Sols intermédiaires Sable Grave Craie Marne et Calcaire- Marneux Roche altérée ou fragmentée 1 FS ## Foré simple (pieux et barrettes) FB ## Foré boue (pieux et barrettes) FTP Foré tubé (virole perdue) FTR Foré tubé (virole récupérée) FSR, FBR, PU ## Foré simple ou boue avec rainurage ou puits 90 6 FTC, FTCD Foré tarière continue simple rotation ou double rotation VM Vissé moulé VT Vissé tubé BPF**, BPR** 10 BE** Battu béton préfabriqué ou précontraint Battu enrobé (béton mortier coulis) BM** Battu moulé BAF** Battu acier fermé BAO** # Battu acier ouvert HB** # H battu (identical to pressuremeter) 15 HBi** H battu injecté IGU ou IRS PP** # Palplanches battues M1 Micropieu type I 18 M2 Micropieu type II 19 PIGU, MIGU Pieu ou micropieu injecté (type III) PIRS, MIRS Pieu ou micropieu injecté (type IV)

25 CPT rules 31 tests (no injected piles; no chalk) γ Rd1 = 1.18 F62TV - 31 essais DTU - 31 essais Moy : 0.86 Mediane: 0.80 COV: 0.34 Quantile for 1.18: 85 % B&G - 31 essais - Etape R ; c R c m

26 Design value of the compressive (or tensile) capacity from ground test results Model pile procedure : R d = R k /γ R = R mod /(γ R;d1* ξ * γ R ) with ξ a correlation factor Alternative procedure ( ground model ) : with γ R;d2 = 1,1 R d = R k /γ R = R mod /(γ R;d1* γ R;d2* γ R ) Resistance factor (Design Approach 2 - all cases, in compression) : γ R = 1,1

27 Model coefficient γ Rd;1 for PMT method (and γ Rd;2 = 1,0 or 1,1) γ R ;d1 Compression γ R ;d1 Tension All piles, excepted injected piles and piles embedded in chalk Piles embedded in chalk, excepted injected piles 1,15 1,4 1,4 1,7 Injected piles 2,0 2,0

28 Model coefficient γ Rd;1 for CPT method (and γ Rd;2 = 1,0 or 1,1) γ R ;d1 Compression γ R ;d1 Tension All piles, excepted injected piles and piles embedded in chalk Piles embedded in chalk, excepted injected piles 1,18 1,45 1,45 1,75 Injected piles 2,0 2,0

29 3. Displacement under axial loads (PMT "t-z" method ) t-z curves from Ménard pressuremeter modulus E M (Frank & Zhao, 1982) for fine grained soils : k τ = 2.0 E M /B and k q = 11.0 E M /B for granular soils : kt = 0.8 E M /B and k q = 4.8 E M /B

30 1st example : cased screw pile in Ypresian clay (Belgium)

31 31

32 2nd example : CFA pile in silt and clay (Northern France) B = 0.5 m ; D = 12 m 0 E M (MPa) p LM (MPa) 0,0 0,5 1,0 1,5 2,0 2 EM moyen 2 4 EM-Forage100 4 z(m) EM-Forage-10 EM-Forage20 z(m) plm moyen plm-forage100 plm-forage10 plm-forage20

33 0 Charge en tête / vertical load (kn) déplacement / settlement (mm) Pieu test/pile tested (Bustamante M. et Gianeselli L.) Antoinet et al. Bahar et al. Robas et Kuder Said et al. Mecsi Monnet 50

34 Behaviour under transverse loads ("p-y"method ) Examples of "p-y" curves from PMT results Short duration Long duration with K f = 2E SM E sm = E M or E M (2,65 B/B o ) α B o /B + 3α 4(2,65) α + 3α E sm = Ménard subgrade reaction modulus

35 g(z) method transverse thrusts d 4 y EI P(y - g) = 0 dz 4

36 Pile of Provins (1974)

37 (Bigot, Bourges and Frank, 1982)

38

39 Conclusions/Summary The recent PMT rules for piles have just elaborated in France (new standard for piles, compatible with Eurocode 7, 2012). They have been fully calibrated against the databank of more than 170 full scale static load tests on instrumented piles. These full scale tests were mainly conducted at the occasion of real projects/case studies. The corresponding CPT rules have also been calibrated (but there are much less data, of course). 39

40 Conclusions/Prospective l In accordance with Eurocode 7 the calculation methods are based on the results of full scale load tests on piles l The important role of displacements of foundations of structures is fully recognised (more than the safety with regard to soil failure? ) l Are we ready to base our SLS verifications solely on displacement assessments? and is the structural engineer ready himself/herself?

41 Thank you for your attention! Acknowledgments: