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**8. Axial Capacity of Single Piles**

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 = e.W.h .**

F (set + tc / 2) Ru= working load, W=weight of the hammer, h= height of the hammer drop (stroke), F=factor of safety tc= elastic (temporary) compression = efficiency F D s tc Ru

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**Static Load Test Load Deflection What is the failure load?**

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

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**Dynamic and Statnamic Testing Methods**

Rapid alternatives to static testing Cheaper Separate dynamic resistance Correlation

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Pu Axial Capacity W Qs Pu = Qb + Qs - W Qb

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**Base Resistance Qb Qb = Ab [cbNc + P’ob(Nq-1) + 0.5gBNg + Pob]**

minus weight of pile, Wp but Wp » Ab.Pob Qb and as L >>B, 0.5gBNg << Wp and for f > 0, Nq - 1 » Nq Qb = Ab [cbNc + P’obNq]

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**Shaft Resistance As Due to cohesion or friction**

Cohesive component : Qsc = As . a . cs Frictional component : Qsf = As .K P’ostan d P’os K.P’os Qs = Qsc + Qsf = As [ a .cs + K P’ostan d ]

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**Total Pile Resistance Qu = Qb + Qs**

Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’otan d ] How do we compute Qu when shaft resistance along the pile is varying?

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

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**Piles in Clay Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’otan d ]**

Qu = AbcbNc + Asa .cs Undrained Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’otan d ] Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’otan d ] Qu = Ab P’obNq + AsK P’ostan d Drained / Effective Qu = Ab P’obNq + AsK P’otan d Qu = AbcbNc + Asa .cs

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Driven Piles in Clay

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Driven Piles in Clay

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**Nc Parameter Nc Compare Skempton’s Nc for shallow foundations**

Nc= 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**

= for cu <215 kPa cu =100 kPa for cu>215 kPa Nc is limited to 9. A reduction factor is applied to account for likely fissuring (I.e., Qb = Ab cb Nc)

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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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**Piles in Sand Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’ostan d ]**

Qu = Ab P’obNq] + AsK P’ostan d ]

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**Overburden Stress P’ob**

Qu = Ab P’obNq] + AsK P’ostan d ] Meyerhof Method : P’ob = g’z Vesic Method : critical depth, zc for z < zc : P’ob = g’z for z > zc : P’ob = g’zc zc/d is a function of f after installation - see graph p. 24

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Critical Depth (zc)

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**Bearing Factor, Nq Qu = Ab P’obNq] + AsK P’ostan d ]**

Nq is a function of : Nq is a function of : friction angle, f Total end bearing may also be limited: Layered soils : Nq may be reduced if penetration insufficient. e.g. Meyerhof (p 21) What affects f ? In-situ density Particle properties Installation procedure Meyerhof : Qb < Ab50Nqtanf Beware if f is pre- or post-installation: Nq determined from graphs appropriate to each particular method

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**Nq factor (Berezantzev’s Method)**

If D/B <4 reduce proportionately to Terzaghi and Peck values

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**Overburden Stress P’os**

Qu = Ab P’obNq] + AsK P’ostan d ] Meyerhof Method : P’os = g’zmid Vesic Method : critical depth, zc for zmid < zc : P’ob = g’z for zmid > zc : P’ob = g’zc zc/d is a function of f after installation - see graph p. 24

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**Lateral stress parameter, K**

A function of Ko 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.tand The K and tand 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, d**

A function of f 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 350mm square**

Uniform dense sand (f = 40o ; g = 21kN/m3) Water table at 1m Pile length 15m Check end bearing with Vesic and Meyerhof Methods Pile is driven on 2m further into a very dense layer f = 44o ; g = 21.7 kN/m3 Compute modified capacity using Meyerhof

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**Example Bored pile 900mm diameter**

Uniform medium dense sand (f = 35o ; g = 19.5kN/m3) Water table at 1m Pile length 20m Check shaft capacity with Vesic and Meyerhof Methods 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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