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

• f a simple calculation scheme

Research work at the Zentrum Geotechnik

Motivation

load firm soil layer slender piles foundation

weiche Bodenschicht

(very) soft soil layer

Has buckling to be expected ?

Are the standards requirements save enough? EC 7: „.. check for buckling is not required if cu 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)

Research work at the Zentrum Geotechnik

Motivation

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

Motivation

Reviewed papers: Vik (1962), Wenz (1972), Prakash (1987), Wennerstrand&Fredriksson (1988), Meek (1996), Wimmer (2004), Heelis&Pavlovic&West (2004) We asked:

2.) An elastic approach to describe the lateral soil support is not appropriate Summary of the results

• btained in the first step

1.) The standards rules underestimate the possibility of pile buckling 3.) Most published calculation methods cannot simulate the pile‘s behavior properly Literature research In situ field load test Model scaled tests Large scaled loading tests Development of a simple design method Development of a numerical FE-Model Reported by Prof. N. Vogt at the IWM 2004 in Tokyo

Research work at the Zentrum Geotechnik

Introduction

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 Literature research In situ field load test Model scaled tests Large scaled loading tests Development of a simple design method Development of a numerical FE-Model

Research work at the Zentrum Geotechnik

Introduction

Large scaled loading tests

Loading of 4 m long single piles

Container made up with concrete segments Pile is pinned top and bottom

axial loading force settlement of the pile head 1,0 m 1,0 m 1,0 m 1,0 m lateral deflection lateral deflection lateral deflection 4,0 m

Measuring devices

Large scaled loading tests

Loading of 4 m long single piles

Container made up with concrete segments

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 Draining system Pumping the liquid soil

Large scaled loading tests

necessary settlement bridge abutment hydraulic jack test pile rigid foundation geotextile drainage surcharge load necessary settlement bridge abutment hydraulic jack test pile rigid foundation geotextile drainage surcharge load

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

Large scaled loading tests

Exemplary illustration of a loading test: Alu-pile surrounded by a supporting soil of cu = 18 kPa

Statistical analysis: maximum shear strength residual shear strength

Large scaled loading tests

Shear vane tests: Soil support of cu = 18 kPa

Maximum shear resistance cfv = 18,7 kN/m

2

= 2,1 KN/m

2

Mean value Standard deviation Residual shear strength cRv = 12,7 kN/m

2

= 1,2 KN/m

2

Mean value Standard deveation Normal plastic clay TM w = 40,8..42,1 % Ic = 0,53..0,48

10 20 30 40

undrained shear strength cu [kPa]

25 50 75 100 125 150 175 200 225 250 600 1200 1800 2400 3000 3600 4200 4800 5400 6000 6600 7200 7800 8400 9000 Meßdauer [s] Pfahlnormalkraft N [kN] 4 8 12 16 20 Verschiebung am Pfahlkopf uO [mm]

Pfahlnormalkraft Verschiebung uo Querschnitt:

Versuchsnummer: KFL-FLACH40x100-02

System: uO N A

Large scaled loading tests

Loading characteristic: Soil support of cu = 18 kPa

Settlement of the pile head Sudden increase of the pile head settlement while the axial normal force is decreasing time [s] settlement of the pile head [mm] axial pile force [kN] No sign in the characteristic of the measured deformations that showed the pile failure in advance!

Large scaled loading tests

Lateral deflection: Soil support of cu = 18 kPa

Axial force N deflection w N 220 kN (ultimate) 9 mm 212 kN 1,2 mm 100 kN 0,9 mm 50 kN 0,4 mm w w

Large scaled loading tests

Analysis

Results:

• With an increasing soil’s

undrained shear strength cu the ultimate bearing capacity rises

50 100 150 200 250 300 350 400 450 500 5 10 15 20 25

cu [kN/m2] N [kN] pile type I (Ep·Ip = 55 kNm2) pile type II (Ep·Ip = 38 kNm2)

600

plastic normal force pile type II plastic normal force pile type I

• Buckling regularly

determined the ultimate state

• f the system, even in soils

with an undrained shear strength of cu > 15 kN/m2

0,0 200,0 400,0 600,0 800,0 1000,0 0,00 2,00 4,00 6,00 M [kNm] N [kN]

cu = 0 kN/m2 cu = 10,5 kN/m2 cu = 18,7 kN/m2 Interaktionskurve des Pfahles FLACH40x100

Large scaled loading tests

Analysis

Results: No failure due to a limited pile‘s material strength! maximum interaction of the internal force variables (pile type II) ? ? Even the backing moment

• ut of the lateral soil support

is not considered

Large scaled loading tests

Analysis

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) Results:

Introduction of a simple design method

Substituted mechanical system with a buckling length of LHw an infinite long pile can be assumed for the calculations; N z the length of the effective buckling figure’s half wave LHw can develop freely for the most conditions in situ at the upper and lower boundaries of the soft soil layer LHw

Introduction of a simple design method

Finding a static system

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;

All forces acting on the static system with a length of LHw z

LHw

LHw p(z) P zp Lateral soil support w0,M N N wN,M MM Bending moment in the middle T = P T = 0

Introduction of a simple design method

Finding a static system

Setting up equilibrium: z

LHw

LHw p(z) P zp w0,M N N wN,M MM T = P T = 0 Condition ∑M = 0 at the pinned top

p Hw M , N M

z P imp L w N M ⋅ −         + ⋅ =

Introduction of a simple design method

Derivation

Force from the lateral soil support is defined piecewise in order to a elastic-plastic soil resistance

z

LHw

LHw p(z) P zp w0,M N N wN,M MM T = P T = 0 Force P from a bi-linear approach of the supporting soil:

π ⋅ ⋅ =

Hw M , N l

L w k P

for: wN,M < wki deformation wN,M supportion force P wki kl 1

Introduction of a simple design method

Derivation

for: wN,M ≥ wki

π ⋅ ⋅ =

Hw ki l

L w k P

pf for a deformation of wN,M > wki the lateral supporting force is remaining constant

Condition ∑M = 0 at the pinned top

p Hw M , N M

z P imp L w N M ⋅ −         + ⋅ = ″ ⋅ ⋅ − =

M , N p p M

w I E M

Assumption: The pile‘s material remains elastic

imp L w L p 1 I E L w N

Hw M , N 2 Hw M 2 p p 2 Hw 2 M , N

+ ⋅ ⋅ π + ⋅ ⋅ π ⋅ =

defined picewise

Introduction of a simple design method

Derivation

wN,M N wki buckling load according to ENGESSER (elastically bedded beam): buckling load according to EULER (unsupported beam): imperfect unsupported beam perfect unsupported beam

imp L w L p 1 I E L w N

Hw M , N 2 Hw M 2 p p 2 Hw 2 M , N

+ ⋅ ⋅ π + ⋅ ⋅ π ⋅ =

N = F (wN,M, Ep·Ip, imp, LHw and the soil support: pf and wki)

Introduction of a simple design method

Presentation

imperfect elastically bedded beam perfect elastically bedded beam perfect bilinear bedded beam imperfect bilinear bedded beam

LHw is unknown! For defined parameters (soil support, imperfection and flexural rigidity) there is one length of LHw, for which the buckling load Nki is minimal LHw Nki effective LHw effective Nki

imp L w L k w 1 I E L w N

Hw ki 2 Hw l ki 2 p p 2 Hw 2 ki ki

+ ⋅ ⋅ ⋅ π + ⋅ ⋅ π ⋅ =

Introduction of a simple design method

Presentation

Vary LHw to find the minimum and therefore effective buckling length!

1.) Define the parameters of the lateral soil support pf und wki Summary of the calculation sequence: 2.) Define an imperfection and the flexural rigidity of the pile’s cross section 3.) Evaluate the effective buckling half wave's length LHw 4.) Calculate the buckling load Nki 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 www.gb.bv.tum.de

Introduction of a simple design method

Pile type I

50 mm hardened cement: C20/25 GEWI28: BSt 500 S 100 mm

Used: half side cracked cross section (no tension stresses in the hardened cement) Ep·Ip = 55 kNm2

50 100 150 200 250 300 350 400 450 500 5 10 15 20 25

cu [kN/m2] N [kN]

pf = 6 · cu · b kl = 60 · cu imp = 300 pf = 10 · cu · b kl = 100 · cu kl = 100 · cu imp = 600

Introduction of a simple design method

Back-calculation of the large scaled tests

Alu-pile:

40 mm Al Mg Si 0,5 100 mm

Ep·Ip = 38 kNm2

0,0 200,0 400,0 600,0 800,0 1000,0 5 10 15 20 25

kl = 70 · cu pf = 7 · cu · D kl = 100 · cu pf = 7 · cu · D kl = 100 · cu pf = 10 · cu · D

cu [kN/m2] N [kN]

Pile type II

Introduction of a simple design method

Back-calculation of the large scaled tests

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 (wki, pf)
• Viscous influence

creep and relaxation

Thank you for your Attention!

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

Research work at the Zentrum Geotechnik

Introduction

Loading tests on 80 cm long model piles Comparison of the test results with the predicted buckling loads (both numerical FEM and published calculation methods) Varying soil strengths and cross sections

20 40 60 80 100

5 10 15 20 25 30 35

undrainierte Scherfestigkeit cu [kN/m²] Knicklast Nk [kN]

elastic characteristic

• f the springs

buckling load [kN] undrained shear strengh cu [kN/m2] elastic-plastic characteristic

• f the springs

Literature research In situ field load test Model scaled tests Large scaled loading tests Development of a simple calculation scheme Development of a numerical FE-Model

Research work at the Zentrum Geotechnik

Introduction

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

Research work at the Zentrum Geotechnik

Introduction

90

Large scaled loading tests

Results of the loading tests of three unsupported composite piles Buckling load of the unsupported pole (EULER II) 89 kN

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!

10 20 30 40 50 60 70 80 100 20 40 60 80 100 120 140 160 180 200

Lateral deflection in the middle of the pile wN,M [mm] Axial pile force N [kN]

Nu wu Nu wu Nu

Why to use an aluminum pile?

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.

50 mm Cement: C20/25 GEWI28: BSt 500 S 100 mm

Composite cross section, cracked half side Ep·Ip = 55 kNm2 Aluminum profile

40 mm Al Mg Si 0,5 100 mm

Ep·Ip = 38 kNm2

Large scaled loading tests

Test results obtained by loading of an unsupported alu-pile

20 40 60 80 100 120 140 160 180 200 600 1200 time [s] axial pile force N [kN] 20 40 60 80 100 lateral displacement in the middle of the pile w [mm]

Ultimate bearing capacity of the unsupported composite-piles: Nk = 55, 22 und 19 kN Nk = 22 kN Alupile: always

Introduction of a simple design method

Derivation

z

LHw

LHw p(z) P zp w0,M N N wN,M MM T = P T = 0

        ⋅ π ⋅ = z L sin w ) z ( w

Hw M ,

Assumption of sinus shaped bending curves

        ⋅ π ⋅ = z L sin w ) z ( w

Hw M , N N

Assumption of a sinus shaped deformation due to imperfection This yields to a sinus shaped form of the load per unit length due to the lateral soil support

        ⋅ π ⋅ = z L sin p ) z ( p

Hw M

Is the decisive buckling load Nki the ultimate axial load Nu of the micropile? The pile‘s material may yield before the buckling load is

• reached. In this case

the pile‘s material strength governs the ultimate load.

                − ⋅ =

α pl pl

N N 1 M M

                  − ⋅ ⋅ ⋅ π ⋅ =

α pl p p 2 2 Hw pl pl , M

N N 1 I E L M w

Introduction of a simple design method

lateral deformation wN,M axial pile force N

A B D C wM,pl case 1 wM,pl case 2 case 1 case 2