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8. Axial Capacity of Single Piles CIV4249 ©1998 Dr. J.P. Seidel Modified by J.K. Kodikara, 2001 CIV4249 ©1998 Dr. J.P. Seidel Modified by J.K. Kodikara, 2001

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Methods Pile driving formulae Static load test Dynamic or Statnamic load test Static formulae

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Pile driving formulae e.g. Hiley formula (Energy balance) Q = .W.h. F (set + tc / 2) R u = working load, W=weight of the hammer, h= height of the hammer drop (stroke), F=factor of safety tc= elastic (temporary) compression = efficiency F s tc RuRu

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Static Load Test Plunging failure Load to specified contract requirement What is the failure load? Davisson’s Method Butler and Hoy Chin’s Method Brinch Hanson etc. What is the distribution of resistance? Approximate methods Instrumentation Load Deflection

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Dynamic and Statnamic Testing Methods Rapid alternatives to static testing Cheaper Separate dynamic resistance Correlation

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Axial Capacity W P u QsQs Q b P u = Q b + Q s - W

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Base Resistance Q b = A b [c b N c + P’ ob (N q -1) BN + P ob ] minus weight of pile, W p but W p A b.P ob and as L >>B, 0.5 BN << W p Q b = A b [c b N c + P’ ob N q ] and for > 0, N q - 1 N q QbQb

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Shaft Resistance Due to cohesion or friction Cohesive component : Q sc = A s. . c s Frictional component : Q sf = A s.K P’ os tan P’ os K.P’ os Q s = Q sc + Q sf = A s [ .c s + K P’ os tan ] AsAs

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Total Pile Resistance Q u = Q b + Q s Q u = A b [c b N c +P’ ob N q ] + A s [ .c s +K P’ o tan ] How do we compute Q u when shaft resistance along the pile is varying?

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Mobilization Shaft 2 - 5mm Base % diam Total Settlement Load

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Piles in Clay Q u = A b [c b N c +P’ ob N q ] + A s [ .c s +K P’ o tan ] Q u = A b c b N c + A s .c s Q u = A b [c b N c +P’ ob N q ] + A s [ .c s +K P’ o tan ] Q u = A b P’ ob N q + A s K P’ o tan Q u = A b c b N c + A s .c s Q u = A b P’ ob N q + A s K P’ os tan Undrained Drained / Effective

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Driven Piles in Clay

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N c Parameter NcNc Compare Skempton’s N c for shallow foundations N c = 5(1+0.2B/L)(1+0.2D/ B)

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Adhesion Factor, Aust. Piling Code, AS159 (1978)

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Bored Piles in Clay Skempton’s recommendations for side resistance – =0.45 for c u <215 kPa – c u =100 kPafor c u >215 kPa –N c is limited to 9. –A reduction factor is applied to account for likely fissuring (I.e., Q b = A b c b N c )

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Soil disturbance sampling attempts to establish in-situ strength values soil is failed/remoulded by driving or drilling pile installation causes substantial disturbance –bored piles : potential loosening –driven piles : probable densification

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Scale effects Laboratory samples or in-situ tests involve small volumes of soil Failure of soil around piles involves much larger soil volumes If soil is fissured, the sample may not be representative

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Q u = A b [c b N c +P’ ob N q ] + A s [ .c s +K P’ os tan ] Piles in Sand Q u = A b [c b N c +P’ ob N q ] + A s [ .c s +K P’ os tan ] Q u = A b P’ ob N q ] + A s K P’ os tan ]

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Overburden Stress P’ ob Q u = A b P’ ob N q ] + A s K P’ os tan ] Meyerhof Method : P’ ob = ’z Vesic Method : critical depth, z c for z < z c : P’ ob = ’z for z > z c : P’ ob = ’z c z c /d is a function of after installation - see graph p. 24

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Critical Depth (z c )

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Bearing Factor, N q N q is a function of : friction angle, N q is a function of : Q u = A b P’ ob N q ] + A s K P’ os tan ] What affects ? In-situ density Particle properties Installation procedure N q determined from graphs appropriate to each particular method Total end bearing may also be limited: Meyerhof : Q b < A b 50N q tan Beware if is pre- or post-installation: Layered soils : N q may be reduced if penetration insufficient. e.g. Meyerhof (p 21)

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N q factor (Berezantzev’s Method) If D/B <4 reduce proportionately to Terzaghi and Peck values

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Overburden Stress P’ os Q u = A b P’ ob N q ] + A s K P’ os tan ] Meyerhof Method : P’ os = ’z mid Vesic Method : critical depth, z c for z mid < z c : P’ ob = ’z for z mid > z c : P’ ob = ’z c z c /d is a function of after installation - see graph p. 24

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Lateral stress parameter, K A function of K o –normally consolidated or overconsolidated - see Kulhawy properties manual –see recommendations by Das, Kulhawy (p26) A function of installation –driven piles (full, partial displacement) –bored piles –augercast piles –screwed piles

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K.tan The K and tan values are often combined into a single function see p 28 for Vesic values from Poulos and Davis

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Pile-soil friction angle, A function of See values by Broms and Kulhawy (p26) A function of pile material –steel, concrete, timber A function of pile roughness –precast concrete –Cast-in-place concrete

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Pile-soil friction angle

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Example Driven precast concrete pile Driven precast concrete pile 350mm square 350mm square Uniform dense sand ( = 40 o ; = 21kN/m 3 ) Uniform dense sand ( = 40 o ; = 21kN/m 3 ) Water table at 1m Water table at 1m Pile length 15m Pile length 15m Check end bearing with Vesic and Meyerhof Methods Check end bearing with Vesic and Meyerhof Methods Pile is driven on 2m further into a very dense layer Pile is driven on 2m further into a very dense layer = 44 o ; = 21.7 kN/m 3 = 44 o ; = 21.7 kN/m 3 Compute modified capacity using Meyerhof Compute modified capacity using Meyerhof

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Example Bored pile Bored pile 900mm diameter 900mm diameter Uniform medium dense sand ( = 35 o ; = 19.5kN/m 3 ) Uniform medium dense sand ( = 35 o ; = 19.5kN/m 3 ) Water table at 1m Water table at 1m Pile length 20m Pile length 20m Check shaft capacity with Vesic and Meyerhof Methods Check shaft capacity with Vesic and Meyerhof Methods By comparsion, check capacity of 550mm diameter screwed pile By comparsion, check capacity of 550mm diameter screwed pile

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Lateral load on single pile Calculation of ultimate lateral resistance (refer website/handouts for details) Lateral pile deflection (use use subgrade reaction method, p-y analysis) Rock socketed pile (use rocket, Carter et al method)

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