ugm-ppt

Permanent Deformation Behavior of Unbound Granular

materials(UGM) under cyclic loading

Application: Base and Subbase layers of low traffic (flexible) pavements

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30th January,2012

Kyung Hee University- Geotechnical Engineering Laboratory

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Contents

1. Introduction

2. Mechanical performance tests for Unbounded Granular Materials(UGM)

3. Cyclic Behavior of unbound granular materials

4. Permanent (Plastic) deformation behavior

5. Laboratory testing and Results


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1. Introduction:

Unbound granular materials (UGMs) are continuously graded granular materials, consisting in general of crushed rocks or crushed natural gravels. They usually contain a certain amount of fines (typically 4% to 10 %) and water.

In Pavements, the behavior of unbound granular materials is characterized by the accumulation of permanent deformations, leading to rutting of the pavement.


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2. Mechanical performance tests for unbound granular materials


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2.1 .Stress conditions in unbound pavement layers


Asphalt pavement

UGM

Base

Subbase

Sub-Soil

2.2 . UGM in Pavement base and its grain size

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Grain size distribution curve of UGM used in Laboratory tests


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2. 3. The cyclic load triaxial test- LCPC laboratory - Equipments

View of the LCPC cyclic load triaxial apparatus for unbound granular materials

View of the instrumented specimen

Hall effect local stain transducer were used to measure axial and radial strainsKyung Hee University- Geotechnical

Engineering Laboratory

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2. 4. The cyclic load triaxial test- LCPC laboratory – Equipments (Cont.)

View of the LCPC cyclic load triaxial apparatus for unbound granular materials


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The specimens are compacted using a specific vibro-compression apparatus, which compacts the specimens in one layer, under the simultaneous application of a vertical load and a horizontal vibration

2. 5. The cyclic load triaxial test- LCPC laboratory – Equipments (Cont.)


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Cylindrical specimens are prepared using vibro-compression with density and water content very closed to that of Optimum Proctor :

160 mm Diameter

320 mm Height

15 kg of mass

Specimens were compacted to corresponding site Dry Unit weight value of 2130 kg/m3

2. 6. The cyclic load triaxial test- LCPC laboratory – Specimens preparation


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3. Cyclic( repeated loading) Behavior of Unbound granular Materials(UGM)


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3. 1. Cyclic behavior of unbound granular materials

The cyclic behaviour of unbound granular materials, and the modelling of this behaviour, are based essentially on cyclic triaxial testing . The response of the material is elasto-plastic,


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4. Permanent(Plastic) deformation behavior


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4.1. Performed permanent deformation tests with different stress level

Range A, called the plastic shakedown range, where a complete stabilisation of permanent strains is observed after a finite number of load cycles, and the behaviour becomes entirely resilient (plastic shakedown)


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4.2 Experimental behavior

The most common procedure to study permanent deformations of UGMs using the cyclic triaxial apparatus consists in applying a large number of load cycles

Usually, for stress levels found in granular pavement layers, the permanent deformations increase rapidly during the first few thousand load cycles, and then tend to stabilise (shakedown).


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• Deviatoric and confining stresses have a significant influence on axial permanent deformation based to cycle loading number

• Rotation of Principal stresses. ( Lode angle).

• Water content

• Density

• fine %

• Effect of loading history; when path q/p is constant there is no longer loading history effect

4.3. Factors influencing permanent deformations


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5. Laboratory Testing and Results


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One of the key aspects of the flexible pavement design is to limit the development of permanent (plastic) deformations in unbound granular layers of flexible pavements.

5.1. Determination of mechanical properties


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5.1.1.Study of Long term Behavior- Plastic Deformation

The objective of the study is to evaluate the behavior of the GNT permanent depending on the water content of the specimen.

50000 cycles for each loading level have been applied for long term tests, following different stress paths as shown in following figures.

Ratio q/p = 1; 2 and 2.5.

For example:

Four loading steps have been carried successively for stress path q/p= 2 : ( q= 150, p= 75) then (q=300, p=150) again (q=450, p= 225) and finally (q=600, p= 300).


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Test Loading Sequence

Number of Cycle N

P max (Kpa)

Q max (Kpa)

q/p

MA 0407 1234

50,00050,00050,00050,000

75150225300

150300450300

2222

p1

Program of permanent deformation tests


5.1.1.Study of Long term Behavior- Plastic Deformation (Cont.)

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Program of permanent deformation tests

Testing No

Test Path q/p

MA 0419 Permanent ( Plastic)- 4% 1


MA 0416 Permanent ( Plastic)- 4% 2.5



MA 0411 Permanent ( Plastic)- 5% 2.5







Different samples for different water content and applied stress paths


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5.1.2. The applied stress levels and several loading stages

For the study of plastic or permanent behavior, the following figure shows the stress paths applied successively. These stress paths are compatible with failure line.

For both 2 tests conducted at W= 4% and 5%, the loading is identical.

Palier: is loading sequenceDroite de rupture: is Failure line


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For both 2 tests conducted at W= 6% and 7%, the loading is identical

5.1.2. The applied stress levels and several loading stages(Cont)

Palier is loading sequenceDroite de rupture is Failure line


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5.1.3. Presentation of test results for permanent deformation analyses

The Analysis of the mechanical behavior is based on axial and radial plastic strains

The following figures present the test results (axial plastic strain) corresponding to 4% and 5%. The stress paths are q/p = 1; 2; 2.5

p3

p1

W=4% W=5%


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The figures above show the results of Triaxial tests- cyclic loading on unbound granular material at water content of 4 % and 5%.

The evolution of axial plastic strain, for each step is characterized by a rapid increase during the first loading cycles, followed by a gradual stabilization.

The stabilization is attained for low stress levels (in the first two steps of loading). This stabilization is more important for water content of 4% and 5%.

Observations


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5.1.4. Results obtained for prediction of the experimental results- Gidel Model

These rests have been analyzed and used with a plastic behavior model, developed to describe the permanent deformation behavior of unbound granular material. The model is known as empirical model developed by Gidel in 2001


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Empirical permanent deformation relationships selected for evaluation, and parameters obtained for the two materials, by Gidel 2001


Gidel Model

28Kyung Hee University- Geotechnical Engineering Laboratory

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Calibration of the empirical model on axial plastic deformation

w=5%w=6%

5.1.5. Predictions obtained with the function of variation with the number of load cycles f(N) proposed by Gidel,2001


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Calibration of the empirical model on axial plastic deformation

w=7%

5.1.5. Predictions obtained with the function of variation with the number of load cycles f(N) proposed by Gidel,2001 (Cont)


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5.1.6. Model Parameters values obtained from tests

Tests Correlation Index


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5.1.7. Remarks

The parameter B is set be cause there is a problem of convergence of the model. These convergence problems are related to the non- uniqueness of the pair values of A and B proposed by Hornysh.

Other parameters are in no way influenced by the value of the parameter B. But the Value of A is a direct function of B.

The permanent deformation is growing up until a limit value A as N increases toward infinity.B controls the shape of the plastic strains curve evolution


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Thank you!


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