A typical flexible pavement consists of the following 4 layers:

  • soil subcategory
  • sub-base course
  • Basic course
  • surface or wear course

If for some reason, any of the above mentioned layers lose their stability, it will cause the entire pavement to fail.

There are many types of failure of flexible pavement, such as pit formation, ruts, cracks, local depressions etc. When any one type of failure is found on the surface, it is an indication of pavement failure.

Read also: 10 Types of Flexible Pavement Malfunctions

It is therefore essential that each layer of pavement must be carefully designed and constructed to maintain its stability.

The 3 common causes of flexible pavement failure are as follows:

  1. subgrade failure
  2. Sub-base or failure of base course
  3. failure of the surface or wear course

1. subgrade failure

This is the main reason for the failure of flexible pavement. When excessive deformation occurs in subgrade soils, it will result in the failure of the entire pavement. Subgrade soil failure can be traced to the following forms of defects causing unevenness of the pavement surface.

  • excessive bulges and corrugations on the surface
  • depression after heating on the surface
  • Side slip of the sidewall near the side edge of the wheel path

There are two primary causes of subgrade soil failure:

  • insufficient stability
  • extreme stress application
  1. Insufficient stability: Stability is the resistance to deformation under stress. When the soil used to construct the subgrade is of substandard quality, it will not be able to resist the load coming from the wheel and will eventually fail.

Another cause of loss of subgrade soil stability is improper compaction of soil during construction. The presence of excessive moisture at the subgrade level without proper drainage control also affects the stability of the subgrade.

  1. Extreme Tension Application: The thickness of the sidewall should be such as to properly distribute the wheel load. If the pavement thickness falls below the required value, it will result in subgrade failure. Also if the wheel load applied to the pavement exceeds the design value, it will result in subgrade failure.

2. Sub-base or base course failure

There are 5 primary reasons behind the failure of the sub-base or base course as explained below.

  1. Insufficient stability or strength: The role of the sub-base or base course is to convert the wheel load from the surface course or wearing course to the subgrade. Therefore the strength of the sub-base or base course is always higher than that of the subgrade. The strength of a sub-base or base course can be achieved by taking the following measures.
  • Using good quality aggregates
  • proper mix design
  • providing enough thickness
  • proper quality control

If any deviation occurs in any of the above factors, it will lead to pavement failure.

  1. Disadvantages of binding action: When wheel load is applied repeatedly to the road surface, it causes internal movement of particles in the sub-base or base course. This results in relative motion between the surface course and the sub-base or base course. In other words, instead of functioning as a whole, different layers perform different functions. This is the reason for the cracking of the alligator or map on the bituminous surface.

so a layer tack coat or Prime coat The base is fitted on top of the course before the surface course is placed. This leads to a better bonding of these two layers.

  1. Disadvantages of base course material: When there is no wearing course or surface course on the base course, or if the wearing course is completely worn out, this will result in the loss of base course material. This is caused by suction caused by the tire and the exposed base course material. Also the loss of stone aggregates leads to the formation of pits on the surface course.
  2. Inadequate wear course: If the thickness of the wearing course is less, water will find its way into the base course causing damage to it.

It is therefore necessary to consider the type, intensity and volume of traffic before deciding the thickness of the wear course.

  1. Use of substandard material: The material used for the construction of the base course must be chosen in such a way that it can resist wheel load and weathering actions. Substandard material should not be used.

3. failure to wear the course

The wearing course or surface course is a stronger layer than all other pavement layers. This is because the wheel load is applied directly to this layer. Along with the vertical load, it also has to resist the weathering effect of the wheel and the weathering effect of the climate.

Therefore the wearing course should be properly designed and constructed. A distorted layer of wear course can damage all underlying layers. The following measures should be employed during the design and manufacture of wear courses:

  • proper mix design
  • enough thickness
  • good quality of binder
  • proper amount of binder
  • good quality set

High level of quality control should be employed during manufacturing of Wearing Course.

Oxidation or aging of the binder also causes the bituminous surface to become brittle and cracks form on the surface of the pavement. This results in the penetration of moisture into the underlying layers and weakens the layers.

Read also: 10 Types of Flexible Pavement Malfunctions

Read also: What happens with aging of bitumen binders?

Er. Mukesh Kumar

Photo of author
Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.