Report

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 F • tc= elastic (temporary) compression Ru • e = efficiency tc s D Static Load Test Load Plunging failure What is the distribution of resistance? Approximate methods Instrumentation Load to specified contract requirement What is the failure load? Davisson’s Method Butler and Hoy Chin’s Method Brinch Hanson etc. etc. Deflection Dynamic and Statnamic Testing Methods • • • • Rapid alternatives to static testing Cheaper Separate dynamic resistance Correlation Pu Axial Capacity W Qs Qb Pu = Qb + Qs - W Base Resistance 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 Due to cohesion or friction As 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 Load 10 - 20% diam 2 - 5mm 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 = =A Ab [c [cbN Nc+P’ +P’obN Nq]] + +A As [[ a a .c .cs+K +K P’ P’otan tan dd ]] Q u b b c ob q s s o Qu = Ab P’obNq + AsK P’ostan d Drained / Effective Qu = A + cA+sK tan d Qbu P’ = ob AbNcqbN AsP’ ao.c s Driven Piles in Clay 2.0 1.5 Du vo Average curve for sensitivea marine clay 1.0 Average curve for clays of low-medium sensitivity 0.5 0 10 20 30 r a 40 50 60 Bearing capacity in kN 300 30 200 x 215mm conrete 250 25 (Gothenberg) 200 20 150 15 300 x 150mm tapered timber (Drammen) 100 300 x 125mm I-Beam 150mm (8 in) steel tube (San Francisco) 50 0 (Gothenberg) 1 5 10 50 100 Time after driving in days 500 10 5 1000 Bearing capacity in tons Driven Piles in Clay Nc Parameter Nc Bending capacity factor Nc 10 9 8 7 6 5 0 1 2 3 4 5 L/d B Compare Skempton’s Nc for shallow foundations Nc= 5(1+0.2B/L)(1+0.2D/ B) Adhesion Factor, a Aust. Piling Code, AS159 (1978) 1.0 50 100 150 200 250 Adhesion factor 2.0 1.5 1.0 0.5 Figures denote penetration ratio = Depth of penetration in clay Key: Pile diameter 49 49 Steel tube piles 19 49 56 Precast concrete 13 15 piles 17 27 13 Design curve for 44 39 33 39 44 27 33 penetration ratio > 20 19 17 38 15 0 1000 2000 10 8 40 5 3000 44 35 4000 Undrained shear strength (cu ) lb/ft Reduction Factor , a Undrained shear strength (cu ) kN/m2 0.8 0.6 0.4 0.2 5000 2 0 100 200 Average Undrained Shear Strength, cu , kPa Bored Piles in Clay • Skempton’s recommendations for side resistance – a =0.45 – acu =100 kPa for cu <215 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 [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) 20 15 zc vc zc / d W.T. 10 L 5 d 0 28 33 f 38 43 Bearing Factor, Nq Qu = Ab P’obNq] + AsK P’ostan d ] NTotal function of :may friction f q is aend bearing alsoangle, be limited: Layered soils : Nqaffects may bef reduced if penetration What • In-situ density Meyerhof : Qb?< A 50N tanf b q insufficient. e.g. Meyerhof (p 21) • Particle properties • Installation procedure Beware if f is preor post-installation: Nq determined from graphs appropriate to each particular method Nq factor (Berezantzev’s Method) 1000 If D/B <4 Nq reduce proportionately to Terzaghi and Peck values 100 For driven piles : For bored piles : 10 25 30 35 f 40 45 f ' = 0.75 f '1 +10 f f1 3 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 Das (1990) recommends the following values for K / Ko: Pile Type K / Ko Bored or Jetted piles 1 Low-displacement, driven piles 1 to 1.4 High-displacement, driven piles 1 to 1.8 Kulhawy (1984) makes the following similar recommendations: Pile Type K / Ko Jetted piles 1/2 to 2/3 Drilled shaft, cast-in-place 2/3 to 1 Driven pile, small displacement 3/4 to 5/4 Driven pile, large displacement 1 to 2 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 Broms (1966) suggests the following Pile Material d / f' Steel d Concrete 0.75 Timber 0.66 Kulhawy (1984) Pile Material d / f' Typical analogy Rough concrete 1.0 Cast-in-place Smooth concrete 0.8 to 1.0 Precast Rough steel 0.7 to 0.9 Corrugated Smooth steel 0.5 to 0.7 Coated Timber 0.8 to 0.9 Pressure-treated 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 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. 1992 method)