 # 8. Axial Capacity of Single Piles

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

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

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

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

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

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

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]

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 ]

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?

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

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

Driven Piles in Clay

Driven Piles in Clay

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

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

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)

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

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

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

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

Critical Depth (zc)

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

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

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

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

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

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

Pile-soil friction angle

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

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