Roads form the lifeline of any nation, and in India as well as globally, Flexible Pavements are the most common type of road because they are economical, faster to construct, and provide a smooth riding surface. However, their layered structure—subgrade, sub-base, base, and surface—makes them more prone to failures if not designed, constructed, or maintained properly.

Understanding the types of failures in flexible pavements, their causes, and solutions helps engineers, contractors, and authorities extend pavement life and reduce maintenance costs.

What are Flexible Pavements?

Flexible Pavements are multilayered road structures where traffic loads are transferred gradually from the top bituminous layer down to the natural soil (subgrade). The key layers are:

1. Subgrade – the natural soil foundation.
2. Sub-base course – provides support and acts as a drainage layer.
3. Base course – the main load distribution layer, usually made of crushed aggregates.
4. Surface/Wearing course – the bituminous layer directly in contact with traffic.

Failures can occur in any of these layers. They are generally classified into subgrade failure, base failure, and surface failure.

1. Subgrade Failure

The subgrade is the foundation of the pavement. If it is weak, the entire pavement structure becomes unstable.

Causes:

• Inadequate compaction during construction.
• Expansive or weak clayey soils.
• Poor drainage or high groundwater table.
• Excessive axle loads not considered in design.

Effects:

• Rutting – depressions along wheel tracks.
• Settlement – uneven surface and ride discomfort.
• Cracks in the surface layers due to lack of support.

Remedies:

• Stabilising soil with lime, cement, or fly ash.
• Proper field compaction to 95% of Modified Proctor density (IS:2720 standards).
• Subgrade thickness and CBR value checked as per IRC:37-2018 guidelines.
• Providing adequate drainage, such as longitudinal and cross drains.

2. Base Failure

The base course is the strength-providing layer above the sub-base. If it fails, the load transfer mechanism breaks down, leading to visible surface damage.

Causes:

• Use of poor-quality or ungraded aggregates.
• Thin base layer contrary to design.
• Lack of compaction.
• Water infiltration through surface cracks.

Effects:

• Shear failure of base aggregates.
• Potholes – bowl-shaped cavities caused by weakened base material.
• Corrugation or shoving – wave-like deformation, especially at intersections.
• Edge failures – cracking and breaking along road shoulders.

Remedies:

• Use of well-graded aggregates as per MoRTH specifications (5th Revision, 2013).
• Providing water-proofing with prime coat/tack coat.
• Proper thickness design using IRC:37 pavement design charts.
• Ensuring compaction to 98% of Modified Proctor density for base materials.

3. Surface (Wearing Course) Failure

The surface or wearing course is the most visible part of the pavement. Failures here directly affect safety and driving comfort.

Causes:

• Ageing and oxidation of bitumen.
• Poor mix design or low binder content.
• Overloading and heavy traffic.
• Poor workmanship during laying.

Common Failures in Surface Layer:

• Cracking
  
  • Fatigue (Alligator) Cracks – interlinked cracks like crocodile skin due to repeated traffic loading.
  • Longitudinal Cracks – parallel to the road centreline, often due to poor joint construction.
  • Transverse Cracks – perpendicular to traffic, caused by temperature changes.
  • Block Cracks – rectangular cracks due to hardening of bitumen.
• Raveling – dislodging of aggregates, leading to rough texture.
• Bleeding/Flushing – excess binder on the surface, creating slippery shine.
• Polishing of Aggregates – smooth surface reducing skid resistance.
• Potholes – further development of cracks and water damage.

Remedies:

• Using correct bitumen grade as per IS:73-2013 (Paving Bitumen specifications).
• Crack sealing with hot bitumen or polymer-modified binders.
• Applying seal coat, slurry seal, or micro-surfacing for preventive maintenance.
• Overlaying with new bituminous layers as per IRC:81-1997 (Maintenance of Bituminous Roads).

General Causes of Failures in Flexible Pavements

• Inadequate pavement thickness.
• Overloaded commercial vehicles.
• Poor drainage and water infiltration.
• Unsuitable construction materials.
• Lack of preventive maintenance.
• Harsh environmental effects (rain, heat, freeze-thaw cycles).

Preventive Measures

• Pavement design strictly as per IRC:37-2018 considering projected traffic and soil CBR.
• Quality control during construction – compaction, material testing, and bitumen content.
• Effective drainage design to avoid water stagnation.
• Regular inspections and maintenance.
• Enforcing axle load regulations to prevent overloading.

Importance of Timely Maintenance

Timely maintenance extends the life of pavements and saves costs. For example:

• Small cracks sealed early prevent water entry and pothole formation.
• Surface dressing improves skid resistance and prevents raveling.
• Regular overlays keep the pavement structurally strong for decades.

Preventive maintenance is always cheaper and more effective than full-scale rehabilitation.

Conclusion

Flexible Pavements are the backbone of modern road networks, but they are vulnerable to failures if not designed and maintained properly. Subgrade failure leads to rutting and settlement, base failure results in potholes and shear cracks, while surface failure causes cracking, raveling, and bleeding. By following IRC and IS standards, ensuring quality construction, and adopting preventive maintenance, engineers and contractors can build safer, durable, and cost-effective roads that truly serve their purpose for years.

FAQs on Flexible Pavements

Q1. What is the most common failure in flexible pavements?
Fatigue cracking (alligator cracking) is the most common.

Q2. How can rutting be reduced?
By using adequate base thickness, proper compaction, and controlling vehicle overloading.

Q3. Why do potholes appear after rains?
Because water infiltrates through cracks, weakens the base, and traffic removes the loosened material.

Q4. Which code is used for flexible pavement design in India?
IRC:37-2018 is the latest guideline for pavement design using CBR method and mechanistic-empirical design.

Q5. How long can flexible pavements last?
With proper design and maintenance, they can last 15–20 years before requiring major rehabilitation.