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Q. Explain the typical flexible pavement failure with neat sketches. May 17, 10 Marks.

Q. Explain typical flexible pavement failure with neat sketches. Dec, 17, 10 Marks.

[1] Sub-grade failure.

Failure of flexible pavement is caused by the localized settlement of any one layer of pavement. Hence each layers should be properly laid and there by stability of the pavement structure should be maintained as a whole by making each layer stable. Due to this complete pavement maintain its stability. Fig. 7.2.1 shows the flexible pavement failure in subgrade.

It has been observed from the Fig 7.2.1, 7.2.2 and 7.2.3, that the failure occurs in subgrade or surface or base due to surface deformation.

• The failure of subgrade can be caused by the basic reasons.

1. Excessive load application or excessive stress application.

2. Inadequate stability.

• The failure in sub base or base course can be caused by the following reasons.

1. Inadequate stability.

2. Loss of base course material.

3. Loss of binding action.

4. Use of inferior materials and crushing of base course materials.

5. Inadequate wearing course.

6. Inadequate strength.

• The wearing course failure are caused by improper mix design, improper gradation of aggregates of aggregates, inadequate binder content and inferior type of binder result in a poor bituminous surfacing.

Improper compaction of subgrade and pavement layers is one of cause for flexible pavement failure as shown in Fig. 7.2.4.

[2] Shear failure.

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• When shear resistance is low due to insufficient stability or excessively heavy loading and this failure are caused by the inherent weakness of the pavement mixture.

Fig. 7.2.5 shows the shear failure showing fracture or cracking in flexible pavement.

[3] Alligator cracking or map cracking.

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• This type of failure is most common in bituminous surfacing and caused by relative movement of pavement layer materials by repeated loading of heavy wheel.

• Fig. 7.2.6 shows the alligator cracking.

[4] Frost heaving.

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• In case of shear failure, the upheaval of portion of pavement is followed with a depression, whereas in case of frost heaving, there is a localized heaving up movement caused by the ground water and climatic condition.

• Failure cause by frost heave is as shown in Fig. 7.2.7.

[5] Reflection cracking.

(MU - Dec. 13)

• This is a different type of failure seen in bituminous overlays provided over existing cement concrete pavement.

• The crack which are developed in cement concrete pavement are reflected on bituminous surfacing as shown in Fig. 7.2.8.

• This type of failure result in mud pumping due to seepage of surface water through reflection cracks.