Buckling of slender piles in soft soils. Large scale loading tests and introduction of a simple calculation scheme

Size: px

Start display at page:

Download "Buckling of slender piles in soft soils. Large scale loading tests and introduction of a simple calculation scheme"

Aron Baldwin
6 years ago
Views:

1 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

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

3 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. DI 1054, 2005 or the national technical approvals for micropiles) We asked: Are the standards requirements save enough?

4 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?

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

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

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

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

electro osmotic effects surcharge load bridge abutment Draining system necessary

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

10 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

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

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

13 Large scaled loading tests Loading characteristic: Soil support of c u = 18 kpa Versuchsnummer: Settlement KFL-FLACH40x of the pile head System: axial pile force [k] Pfahlnormalkraft [k] Sudden increase of the pile head settlement while the axial normal force is decreasing o sign in the characteristic of the measured deformations that showed the pile failure in advance! Meßdauer [s] time [s] settlement of the pile head [mm] Verschiebung am Pfahlkopf u O [mm] u O Querschnitt: A Pfahlnormalkraft Verschiebung uo

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

15 Large scaled loading tests Analysis Results: - With an increasing soil s undrained shear strength c u the ultimate bearing capacity rises - Buckling regularly determined the ultimate state of the system, even in soils with an undrained shear strength of c u > 15 k/m 2 [k] plastic normal force pile type II plastic normal force pile type I pile type I (E p I p = 55 km 2 ) pile type II (E p I p = 38 km 2 ) c u [k/m 2 ]

16 Large scaled loading tests Analysis Results: 1000,0 o failure due to a limited pile s material strength! [k] 800,0 600,0 400,0 maximum interaction of the internal force variables (pile type II) Interaktionskurve des Pfahles FLACH40x100 Even the backing moment out of the lateral soil support is not considered 200,0 0,0? c u = 18,7 k/m 2? c u = 10,5 k/m 2 c u = 0 k/m 2 0,00 2,00 4,00 6,00 M [km]

17 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)

18 Introduction of a simple design method

19 Introduction of a simple design method Finding a static system Substituted mechanical system with a buckling length of L Hw 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 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; z L Hw

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

21 Introduction of a simple design method Derivation Setting up equilibrium: T = P Condition M = 0 at the pinned top z p z P M M = w,m + LHw imp P z p p(z) T = 0 M M L HwLHw Force from the lateral soil support is defined piecewise in order to a elastic-plastic soil resistance w,m w 0,M

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

23 Introduction of a simple design method Derivation Condition M = 0 at the pinned top Assumption: The pile s material remains elastic M M = w,m + LHw imp P z p M M = Ep Ip w, M = w,m π 2 L 2 Hw E w p,m 1 Ip + π LHw + imp 2 p M L defined picewise 2 Hw

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

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

26 Introduction of a simple design method Summary of the calculation sequence: 1.) Define the parameters of the lateral soil support p f und w ki 2.) Define an imperfection and the flexural rigidity of the pile s cross section 3.) Evaluate the effective buckling half wave's length L Hw 4.) Calculate the buckling load ki 5.) Check if the pile s material strength governs the maximum bearing capacity (this means: does the pile s material yield before the buckling load is reached ) You may download an Excel-Sheet at

27 Introduction of a simple design method Back-calculation of the large scaled tests Pile type I p f = 10 c u b k l = 100 c u imp = 600 Used: half side cracked cross section (no tension stresses in the hardened cement) [k] mm hardened cement: C20/ p f = 6 c u b k l = 60 c u imp = c u [k/m 2 ] 100 mm E p I p = 55 km 2 GEWI28: BSt 500 S

28 Introduction of a simple design method Back-calculation of the large scaled tests Pile type II 1000,0 800,0 [k] 600,0 400,0 k l = 100 c u p f = 7 c u D k l = 100 c u p f = 10 c u D Alu-pile: 200,0 0,0 k l = 70 c u p f = 7 c u D c u [k/m 2 ] Al Mg Si 0,5 40 mm 100 mm E p I p = 38 km 2

29 Summary In (very) soft soils pile buckling should always be verified! With the help of the presented design method the main effects of the loading tests can be considered in basic. The insecurities upon the design method is based and which are recognizable comparing the theoretical results with the data form the pile load tests must be covered by partial safety factors on the structural part and the soil resistance. - Compound effects steel-concrete - Soil resistance (w ki, p f ) - Viscous influence creep and relaxation

30 Thank you for your Attention!

Research work at the Zentrum Geotechnik Introduction Literature research beam supported by springs Development of a numerical FE-Model Model scaled tests In situ field load test

32 Research work at the Zentrum Geotechnik Introduction Literature research beam supported by springs Development of a numerical FE-Model Model scaled tests In situ field load test Large scaled loading tests Development of a simple calculation scheme characteristic of the lateral reaction forces elastisch or elastisch-plastisch supporting force lateral deflection

33 Research work at the Zentrum Geotechnik Introduction Literature research Development of a numerical FE-Model Model scaled tests In situ field load test Large scaled loading tests Development of a simple calculation scheme buckling Knicklast load k [k] Loading tests on 80 cm long model piles Varying soil strengths and cross sections Comparison of the test results with the predicted buckling loads (both numerical FEM and published calculation methods) elastic characteristic of the springs elastic-plastic characteristic of the springs undrainierte Scherfestigkeit cu [k/m²] undrained shear strengh c u [k/m 2 ]

34 Research work at the Zentrum Geotechnik Introduction Literature research Development of a numerical FE-Model Model scaled tests In situ field load test Large scaled loading tests Development of a simple calculation scheme Loading test of a GEWIpile in soft, organic soil Sudden pile failure at a load very little above the design load

35 Large scaled loading tests Results of the loading tests of three unsupported composite piles Why to use an aluminum pile? k90 Buckling load of the unsupported pole (EULER II) Axial pile force [k] u Loading an pile with such an inelastic behavior due to its cross section material (unpredictable crack propagation of the concrete) is improper to qualify the lateral soil support! u 20 u 10 0 w u w u Lateral deflection in the middle of the pile w,m [mm]

36 Large scaled loading tests Pile type II: Aluminum profile Some pile is needed which behaves elastically over a wide range of lateral displacements and which reproduces the buckling load according to EULER in the unsupported case. Solution: A aluminum pile that has a similar flexural rigidity compared to the cracked composite cross section. Composite cross section, cracked half side 50 mm 100 mm E p I p = 55 km 2 Aluminum profile Cement: C20/25 GEWI28: BSt 500 S Al Mg Si 0,5 40 mm 100 mm E p I p = 38 km 2

37 Large scaled loading tests Test results obtained by loading of an unsupported alu-pile Ultimate bearing capacity of the 200 unsupported composite-piles: 180 k = 55, 22 und 19 k axial pile force [k] Alupile: always k = 22 k lateral displacement in the middle of the pile w [mm] time [s]

38 Introduction of a simple design method Derivation Assumption of a sinus shaped deformation due to imperfection w 0 π (z) = w z 0,M sin LHw P Assumption of sinus shaped bending curves w π (z) = w,m sin z LHw This yields to a sinus shaped form of the load per unit length due to the lateral soil support π p(z) = p z M sin LHw p(z) z p T = 0 w,m M M z w 0,M T = P L HwLHw

39 Introduction of a simple design method Is the decisive buckling load ki the ultimate axial load u of the micropile? C w M,pl = M π pl 2 L E 2 Hw p I p 1 pl α D The pile s material may yield before the buckling load is reached. In this case the pile s material strength governs the ultimate load. axial pile force case 2 case 1 B w M,pl case 2 w M,pl case 1 M= M pl 1 pl α A lateral deformation w,m

Critical Load columns buckling critical load

Critical Load columns buckling critical load Buckling of Columns Buckling of Columns Critical Load Some member may be subjected to compressive loadings, and if these members are long enough to cause the member to deflect laterally or sideway. To