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Bearing Capacity and Settlement of Deep Foundations

Summer Term 2015 Hochschule Munchen

Fakultat Bauingenieurwesen

LAYOUT

Bearing Capacity: Theory and Design Table Data

Settlement: Theory and Design Table Data

Theoretical Methods for Bearing Capacity

• Undrained behavior – total stress method

(short time)

Saturated clays

Alfa method

Nc ≈ 9 (≈5≤ Nc ≤ ≈9)

• Drained behavior – effective stress method

(long time)

Unsaturated clays and sands

Beta method

Nq function of ´

• Short Time - Undrained

Poulos et al. 2001

• Factor

Ca = .Cu

• Long Time - Drained

• Factor b

Poulos (1989)

• Factor b

Poulos et al. 2001

• Factor b

Poulos et al. 2001

• Bearing capacity

factor Nq

(Poulos Berezantzev)

• Recomendations from

Prof. H.G. Poulos

sa KK and ´

Practical Usage Tables

• Stas and Kullawhy / and K/K0 Values

• Table 5: Lateral unit friction for driven piles

• Table 6: Lateral unit friction for bored piles

• Table 7.3: ultimate lateral unit friction for driven piles in sand

• Table 7.6: ultimate unit point resistance for driven piles in sand

• Table 9: Unit point resistance

(fs = unit lateral friction.; fb = unit point resistance

• Stas & Kulhawy (1984)

Recognized Brazilian Empirical Methods via SPT

• Aoki & Velloso, 1975 (general)

• Décourt & Quaresma, 1978 & 1996 (general)

• Antunes & Cabral, 1986 (CFA piles)

• Décourt empirical equations for Drilled caissons

Standard Penetration Test (SPT)

• Aoki & Velloso (1975)

Original Values

Some sugestions after publication

• Décourt & Quaresma (1978) & (1996)

Soil Type TYPE OF PILE

Bored Bored

with fluid

CFA Pali

radice

injected

Micropile

injected

Clays 0.80 0.90 1.0 1.5 3.0

Intermediate 0.65 0.75 1.0 1.5 3.0

Sands 0.50 0.60 1.0 1.5 3.0

Soil Type TYPE OF PILE

Bored Bored

with fluid

CFA Pali

radice

injected

Micropile

injected

Clays 0.85 0.85 0.30 0.85 1.0

Intermediate 0.60 0.60 0.30 0.60 1.0

Sands 0.50 0.50 0.30 0.50 1.0

slateralL

pbaseP

1996 modification

• Tropical collapsible soil of Brasília

(few load tests @ experimental site)

Some important remarks from Brazilian practice

• Bored Piles: It is common to allow only 20% of the total capacity of the pile to be resisted by its base

Rbase + Rlateral = Rtotal, Rbase < 20% Rtotal

• Injected Piles: It is common to neglect full base resistance;

• Drilled caissons: It is common to neglect full lateral resistance

• Antunes & Cabral (1986)

......

DNLDNQ SPTSPTult

where: D = pile diameter; L = pile length, NSPT = SPT blow counts; Qult = ultimate bearing capacity of the pile; and b1.NSPT and b2. NSPT are in kPa.

Empirical parameters for Antunes & Cabral (1996) methodology.______________________________________________Soil Type b1 (%) b2 (non dim)______________________________________________Sand 4.0 to 5.0 2.0 to 2.5Silt 2.5 to 3.5 1.0 to 2.0Clay 2.0 to 3.5 1.0 to 1.5_____________________________________________

• Cunha et al. (2006): Tropical soil of the Brazilian Center (pile load tests in Brasília and Goiânia) CFA piles

0 1000 2000 3000

Estimated Ultimate Load (kN)

N) 45º Line

+10% Error Line

-10% Error Line

Aoki-Velloso (Adjusted)

Décourt-Quaresma (Adjusted)

Antunes-Cabral (Adjusted)

Kárez-Rocha (Adjusted)

Gotlieb et al. (Adjusted)

Final adjusted empirical parameters for Décourt & Quaresma and Antunes & Cabral methodologies.______________________________________________Soil Type b1 (%) b2 D bD

______________________________________________Sand 4.4 2.2 -- --Silt 2.9 1.4 -- --Clay 2.6 1.2 -- --Any Soil Type -- -- 0.21 1.13_____________________________________________

Qallowable at base = qbf x Abase (soil)

Qallowable at base = Ab x qba, where qba ≤ 0.33 fck (rock)

or qba ≤ 1/5 to 1/8 qu (rock)

• Drilled Caissons

Décourt et al, (1996)

• Décourt (1992): Generalized usage: quc = NSPT/40 (MPa)

Décourt (1995): Saturated clays, not laterized

quc = 0,08NSPT (MPa)

where quc = ultimate unit point resistance

NSPT = Blow counts from Standard Penetration Test (SPT)

Piles under Traction

• Unit values of resistance

fb = 0 (base), 0.7 ≤ fs-traction/fs-compression ≤ 1.0

pile weight must be included

• RLt/RLc (De Nicola e Randolph (1993)

End Bearing via CPT Test 32/57

Pile Group • Efficiency

Clays: η ≈ 1 for large center to center spacings (floating and end bearing piles)

η < 1 for low center to center spacings (floating piles)

Sands: η ≈ ≥ 1 floating and end bearing piles

• Empirical Formulae

• Floating Pile in Clay

Group capacity is the lower result of:

• ∑ individual capacities

• Capacity of a “composite block”

of soil and piles

More realistically:

• Floating pile in Sand Driven Piles: η = 1 in design (η maximum for s/d between 1.3 & 2) Bored Piles: η ≈ 0.7 • Summary:

Definition of Settlement

• Total • Differential • Angular distortion

Types of Settlement

• Immediate, drained or Short Time

Sands, drained soils and unsaturated soils

• Undrained or Long Time

Saturated Clays, undrained soils

Poulos et al. 2001

Day, 2000

Load Transference Mechanism

• Elastic Shortening

• Bulb of Pressure (Martins, 1945)

(Bowles, 1988):

FPelTOTAL

Friction Base

• Elastic method of Poulos & Davis (1990): Mindlin´s Equation

a) Equations & general procedure

b) Load transferred to base

Theoretical graphics

c) Settlement of isolated pile

Gibsian or stratified soils

General remarks

• Major part of settlement is IMMEDIATE settlement, typically > 80 %;

• Effect of pile compressibility is most important for long slender piles;

• For long compressible piles, settlement is not influenced by the nature of the bearing stratum tip;

• For piles of normal proportions, the load - settlement behaviour is substantially linear at normal working loads;

• Nonlinearity becomes important for piles which derive much of their load capacity from base resistance eg large diameter drilled piers.

Parameters for the Elastic Method

• Poulos & Davis (1990)

and Poulos (1999)

Method of the Equivalent Raft

Method of the Equivalent Pile

REFERENCES

• Velloso, D.A. & Lopes, R.F. (2002). Fundações-Volume 2 (Fundações Profundas). Editora COPPE-UFRJ. 472 p.

• Day, R.W. (2006). Foundation Engineering Handbook. ASCE Press

• Bowles, J.E. (1988). Foundation Analysis and Design. McGraw Hill Company. New York. 4th. Edition. 1004 p.

• Hachich et al. (1996). Fundações: Teoria e Prática. Editora PINI Ltda. São Paulo, 1ª. Versão. 751 p.

• Poulos, H.G. (1999). Analysis and Design of Pile Foundations. Handouts from the Foundation Course – Univ. of Sydney

• Poulos, H.G. & Davis, E. H. (1990) Pile Foundation Analysis and Design. Robert E. Krieker Pub. Company, Inc., Florida, 397 p.

• Cunha, R.P. (1996). Foundations. Handouts from the Foundation Course - Universidade de Brasília, 477 p.

• Poulos, H.G, Carter, J.P., Small, J.C. (2001). Foundations and Retaining Structures. XV ICSMGE, Instanbul, State of Art

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