Buckling of slender piles in soft soils Large scale loading tests - PowerPoint PPT Presentation

Dipl.-Ing. Stefan Vogt Zentrum Geotechnik , Technische Universität München Buckling of slender piles in soft soils – Large scale loading tests and introduction of a simple calculation scheme

Research work at the Zentrum Geotechnik Motivation load foundation slender piles (very) soft weiche Bodenschicht soil layer firm soil layer Has buckling to be expected ?

Research work at the Zentrum Geotechnik Motivation EC 7: „.. check for buckling is not required if c u exceeds 10 kPa..“ Other codes set this limit of undrained shear strength at 15 kPa or 10 kPa (eg. DIN 1054, 2005 or the national technical approvals for micropiles) We asked: � Are the standards requirements save enough?

Research work at the Zentrum Geotechnik Motivation Reviewed papers: Vik (1962), Wenz (1972), Prakash (1987), Wennerstrand&Fredriksson (1988), Meek (1996), Wimmer (2004), Heelis&Pavlovic&West (2004) We asked: � Are the published design methods capable to simulate the interaction between the supporting soil and the pile?

Research work at the Zentrum Geotechnik Introduction � Summary of the results Literature research obtained in the first step Development of a numerical FE-Model � Reported by Prof. N. Vogt at the IWM 2004 in Tokyo Model scaled tests 1.) The standards rules In situ field load test underestimate the possibility of Large scaled loading tests pile buckling Development of a simple 2.) An elastic approach to describe design method the lateral soil support is not appropriate 3.) Most published calculation methods cannot simulate the pile‘s behavior properly

Research work at the Zentrum Geotechnik Introduction Literature research Development of a numerical FE-Model Model scaled tests � Aim: In situ field load test Proofing the obtained expertise with large scaled loading tests on single Large scaled loading tests piles Development of a simple Development of a simple design design method method that can simulate the main effects recognized in the loading tests

Large scaled loading tests Loading of 4 m long single piles Container made up with concrete segments Pile is pinned top and bottom

Large scaled loading tests Loading of 4 m long single piles Container made up with Measuring devices axial concrete segments loading force settlement of the pile head 1,0 m lateral deflection 1,0 m lateral 4,0 m deflection 1,0 m lateral deflection 1,0 m

Large scaled loading tests Mixing up the soil in a liquid consistency Filling the containers by pumping the liquid Draining system soil – following consolidation with the help of the electro osmotic effects surcharge surcharge load load bridge abutment bridge abutment hydraulic hydraulic necessary settlement necessary settlement jack jack Pumping the liquid soil test pile test pile geotextile geotextile drainage drainage rigid rigid foundation foundation

Large scaled loading tests Pile type I: Composite cross section GEWI28 Steel rod d = 28 mm Hardened cement slurry D = 100 mm Pile type II: Aluminum profile Thickness = 40 mm Width = 100 mm

Large scaled loading tests Exemplary illustration of a loading test: Alu-pile surrounded by a supporting soil of c u = 18 kPa

Large scaled loading tests Shear vane tests: Soil support of c u = 18 kPa Statistical analysis: Normal plastic clay TM w = 40,8..42,1 % I c = 0,53..0,48 residual shear Maximum shear resistance c fv strength 2 = 18,7 kN/m Mean value 2 Standard deviation = 2,1 KN/m Residual shear strength c Rv 2 = 12,7 kN/m Mean value 2 Standard deveation = 1,2 KN/m maximum shear strength 0 10 20 30 40 undrained shear strength c u [kPa]

Large scaled loading tests Loading characteristic: Soil support of c u = 18 kPa Settlement of the pile head Versuchsnummer: KFL-FLACH40x100-02 System: 250 20 N u O 225 settlement of the pile head [mm] 200 16 Verschiebung am Pfahlkopf u O [mm] axial pile force [kN] 175 Sudden increase of the pile Pfahlnormalkraft N [kN] head settlement while the axial 150 12 normal force is decreasing Querschnitt: 125 100 8 75 No sign in the characteristic of the A 50 4 measured deformations that showed Pfahlnormalkraft the pile failure in advance! 25 Verschiebung uo 0 0 0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000 6600 7200 7800 8400 9000 time [s] Meßdauer [s]

Large scaled loading tests Lateral deflection: Soil support of c u = 18 kPa N Axial force N deflection w 50 kN 0,4 mm 100 kN 0,9 mm w 212 kN 1,2 mm w 220 kN (ultimate) 9 mm

Large scaled loading tests Analysis plastic normal force pile type II 600 Results: 500 - With an increasing soil’s 450 undrained shear strength c u 400 the ultimate bearing capacity rises 350 plastic normal force pile type I 300 - Buckling regularly N [kN] determined the ultimate state 250 of the system, even in soils 200 with an undrained shear strength of c u > 15 kN/m 2 150 100 50 pile type I (E p ·I p = 55 kNm 2 ) 0 0 5 10 15 20 25 pile type II (E p ·I p = 38 kNm 2 ) c u [kN/m 2 ]

Large scaled loading tests Analysis 1000,0 Results: 800,0 No failure due to a limited pile‘s material strength! maximum interaction of 600,0 the internal force N [kN] Interaktionskurve des variables (pile type II) Pfahles FLACH40x100 400,0 c u = 18,7 kN/m 2 200,0 ? c u = 10,5 kN/m 2 Even the backing moment ? c u = 0 kN/m 2 out of the lateral soil support 0,0 is not considered 0,00 2,00 4,00 6,00 M [kNm]

Large scaled loading tests Analysis Results: For lower axial forces the lateral deflections of the pile remain very little (stiff behavior) The failure of the micro piles occurred suddenly (no sign of failure from the measured deformations) The halve waves of the buckling pile‘s bending curve were always smaller than the full pile‘s length (from joint to joint)

Introduction of a simple design method

Introduction of a simple design method Finding a static system N Substituted mechanical system with a buckling length of L Hw z � the length of the effective buckling figure’s half wave L Hw can develop freely for the most conditions in situ at the upper and lower boundaries of the soft soil layer � the large scaled loading tests showed that L Hw the length of the buckling figure’s half waves were smaller than the maximum possible length of 4 m; � an infinite long pile can be assumed for the calculations;

Introduction of a simple design method Finding a static system N All forces acting on the static system with T = P a length of L Hw z z p P M M Lateral soil support L Hw L Hw p(z) T = 0 N Bending moment in the middle w N,M w 0,M

Introduction of a simple design method Derivation N Setting up equilibrium: T = P Condition ∑ M = 0 at the pinned z z p top P   L M M = ⋅  +  − ⋅ M N w Hw P z L Hw   L Hw M N , M p imp   p(z) T = 0 N Force from the lateral soil support is defined piecewise in order to a elastic-plastic soil resistance w N,M w 0,M

Introduction of a simple design method Derivation N Force P from a bi-linear approach of the T = P supporting soil: L z z p = ⋅ ⋅ P k w Hw for: w N,M < w ki l N , M π P L = ⋅ ⋅ P k w Hw for: w N,M ≥ w ki l ki π M M L Hw L Hw supportion force P p(z) T = 0 p f N for a deformation of w N,M > w ki the lateral supporting force is remaining constant k l 1 w N,M w ki w 0,M deformation w N,M

Introduction of a simple design method Derivation Assumption: The pile‘s material Condition ∑ M = 0 at the pinned top remains elastic   L ″ = ⋅  +  − ⋅ = − ⋅ ⋅ M N w Hw P z M E I w   M N , M p M p p N , M imp   defined picewise 2 π 1 2 ⋅ ⋅ ⋅ + ⋅ ⋅ w E I p L N , M p p M Hw 2 π 2 L = Hw N L + Hw w N , M imp

Introduction of a simple design method Presentation π 2 1 N = F (w N,M , E p ·I p , imp, L Hw and 2 ⋅ ⋅ ⋅ + ⋅ ⋅ w E I p L N , M p p M Hw 2 π 2 the soil support: p f and w ki ) L = N Hw L + Hw w N , M imp N perfect bilinear perfect elastically bedded beam bedded beam buckling load according to imperfect elastically ENGESSER bedded beam (elastically bedded beam): imperfect bilinear bedded beam buckling load according to perfect unsupported beam EULER (unsupported beam): imperfect unsupported beam w ki w N,M

Introduction of a simple design method Presentation 2 π 1 2 ⋅ ⋅ ⋅ + ⋅ ⋅ ⋅ w E I w k L L Hw is unknown! ki p p ki l Hw 2 2 π L = N Hw ki L For defined parameters (soil + Hw w ki imp support, imperfection and flexural rigidity) there is one length of L Hw , N ki for which the buckling load N ki is minimal Vary L Hw to find the minimum and therefore effective buckling length! effective N ki effective L Hw L Hw