The most common type of failure that occurs on a flexible pavement is Alligator (map) cracking. Alligator cracking is characterized by a series of interconnected cracks that resemble the pattern of an alligator's skin or a map. This type of cracking is a significant distress that can compromise the structural integrity of the pavement and lead to further deterioration if not addressed in a timely manner.

There are several reasons why alligator cracking is the most common type of failure on flexible pavements:

1. Fatigue Failure: Alligator cracking is primarily caused by fatigue failure, which is the result of repeated loading and unloading of the pavement surface. Over time, this repetitive loading causes the asphalt binder to weaken and eventually crack, resulting in the characteristic pattern of alligator cracking.

2. Aging and Oxidation: As asphalt pavements age, they become more susceptible to alligator cracking. The binder in the asphalt mixture oxidizes and loses its flexibility, making it more prone to cracking. This aging process is accelerated by exposure to sunlight, heat, and moisture.

3. Insufficient Thickness: Alligator cracking is more likely to occur when the pavement is not adequately designed or constructed. If the asphalt layer is too thin to support the traffic loads or if the underlying layers are weakened, the pavement is more susceptible to fatigue failure and subsequent alligator cracking.

4. Heavy Traffic Loads: Alligator cracking tends to be more prevalent in areas with heavy traffic loads, such as highways, intersections, and trucking routes. The repeated loading from large vehicles increases the stress on the pavement surface and accelerates the development of cracks.

5. Poor Maintenance: Lack of proper maintenance can also contribute to the development of alligator cracking. If minor cracks are not sealed and repaired promptly, they can propagate and lead to more extensive alligator cracking. Additionally, inadequate drainage or improper pavement repairs can exacerbate the problem by allowing water to infiltrate the pavement layers and weaken the subgrade.

In conclusion, alligator cracking is the most common type of failure on flexible pavements due to the combined effects of fatigue failure, aging and oxidation, insufficient thickness, heavy traffic loads, and poor maintenance. It is crucial to identify and address alligator cracking early to prevent further deterioration and ensure the longevity of the pavement. Regular maintenance practices, such as crack sealing, patching, and proper drainage, are essential to mitigate the occurrence and progression of alligator cracking.