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9. Axial Capacity of Pile Groups

9. Axial Capacity of Pile Groups. CIV4249: Foundation Engineering Monash University. P shaft. Axial Capacity. F u. F u + W = P base + P shaft. W. Shear failure at pile shaft. P base. Bearing failure at the pile base. P shaft,t . Tension Capacity. T u - W = P shaft,t < P shaft,c.

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9. Axial Capacity of Pile Groups

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  1. 9. Axial Capacityof Pile Groups CIV4249: Foundation Engineering Monash University

  2. Pshaft Axial Capacity Fu Fu + W = Pbase + Pshaft W Shear failure at pile shaft Pbase Bearing failure at the pile base

  3. Pshaft,t Tension Capacity Tu - W = Pshaft,t < Pshaft,c Shear failure at pile shaft

  4. Very Large Concentrated Weight Large Distributed Weight Low Weight Applications Soft to Firm Clay Dense Sand Strong Rock

  5. Pug Group Capacity Pile Cap • Overlapping stress fields • Progressive densification • Progressive loosening • Case-by-case basis Pug¹n.Pup Pug = e.n.Pup

  6. Clay Sand d s Rock Efficiency, e Pile Cap n = 5 x 5 = 25 Soil Type Number of Piles, n Spacing/Diameter s/d typically > 2 to 3

  7. Free-standing Groups Capped Groups Flexible Cap Rigid Cap Types of Groups

  8. 13/16 11/16 8/16 û Feld Rule for free-standing piles in clay A B B B A reduce capacity of each pile by 1/16 for each adjoing pile B C C C B e = 1/15 * (4* 13/16 + 8 * 11/16 + 3 * 8/16) = 0.683 A B B B A

  9. n = # cols = 5 e = 1 - q (n-1)m + (m-1)n 90 mn q = tan-1(d/s) m = # rows = 3 d=0.3 s = 0.75 Converse-Labarre Formula for free-standing piles in clay e = 0.645

  10. Flexible Cap D L,B Block Failure PBL = BLcbNc + 2(B+L)Dcs cs Nc incl shape & depth factors cb Pug = min (nPup,PBL)

  11. 1 = 1 + n2P2up e2 P2BL Empirical Modification PBL = BLcbNc + 2(B+L)Dcs Pug = min (nPup,PBL) 1 1 1 P2ug = n2P2up + P2BL nPup n

  12. Block Failure Flexible Cap D = 20m cs = cb = 50 kPa d = 0.3m L = B = 5m

  13. Capped Groups Bc x Lc Rigid Cap Ptotal = Pgroup + Pcap for single pile failure, Pcap = ccapNc [BcLc - nAp] for group block failure, Pcap = ccapNc [BcLc - BL] B x L

  14. Efficiency increases 1.0 72 capped 0.9 0.8 0.7 72 free-standing 0.6 0.5 0.4 • s/d 0.3 1 2 3 4

  15. Piles in Granular Soils • End bearing - little interaction, e = 1 • Shaft - driven • For loose to medium sands, e > 1 • Vesic driven : 1.3 to 2 for s/d = 3 to 2 • Dense/V dense - loosening? • Shaft - bored • Generally minor component, e = 1

  16. Pile Settlement

  17. Elastic Analysis Methods • based on Mindlin’s equations for shear loading within an elastic halfspace • Poulos and Davis (1980) • assumes elasticity - i.e. immediate and reversible • OK for settlement at working loads if reasonable FOS • use small strain modulus

  18. Ap Ep Es As Definitions Pile Stiffness Factor, K Area Ratio, Ap K = RA.Ep/Es RA = Ap / As

  19. % load at the base Pile top settlement d Floating Pile Ep b = boCKCn Es,n L h r = P.IoRKRLRn / Esd Solutions are independent of soil strength and pile capacity. Why? Rigid Stratum

  20. 0.5 Floating pile example P = 1800 kN b = boCKCn r = P.IoRKRLRn / Esd Ep = 35,000 MPa Io = 0.043 RK = 1.4 RL = 0.78 Rn = 0.93 r = 4.5mm bo = 0.038 CK = 0.74 Cn = 0.79 b = .022 Pb = 40 kN 25 32 Effect of : L = 15m db/d = 2 h = 100m n = 0.3 Es = 35 MPa Rigid Stratum

  21. d Pile on a stiffer stratum • % load at the base Ep b = boCKCbCn Es,n L • Pile top settlement r = P.IoRKRbRn / Esd Stiffer Stratum Eb > Es

  22. d Layered Soils Ep E1,n1 L E2,n2 Es = 1 SEi hi L Stiffer Stratum Eb > Es

  23. 0.5 Stiffer base layer example P = 1800 kN b = boCKCbCn r = P.IoRKRbRn / Esd Ep = 35,000 MPa n = 0.3 Io = 0.043 RK = 1.4 Rb = 0.99 Rn = 0.93 r = 4.5 mm bo = 0.038 CK = 0.74 Cn = 0.79 Cb = 2.1 b = .0467 Pb = 84 kN 25 Es = 35 MPa Eb = 70 MPa Effect of: Es = 15 MPa to 15m

  24. Movement Ratios • MR is ratio of settlement to PL/AE • Focht (1967) - suggested in general : 0.5 < MR < 2 • See Poulos and Davis Figs 5.23 and 5.24

  25. Floating Piles End bearing piles Pile group settlment Single pile settlement is computed for average working load per pile

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