How to Achieve a Durable Flexible Pavement Structure | Classes of Subgrade

Layered flexible bituminous pavement has been identified as the most common type of pavement used for urban and rural roads around the world. During the design process of these roads, certain design period (identified as structural design period – SDP) is applicable. The design period is defined as the period during which the road is expected to carry traffic at a satisfactory level of service, without requiring major rehabilitation or repair work (though certain maintenance may be carried out during the period to keep the road in good service condition).

The design period is often chosen while considering individual wheel loads, the contact tyre pressure, axle configuration, and load repetition. In Nigeria, the category of roads is also factored in choosing the SDP because of the peculiar functions of each category (see Table 1). However, the period between 10 – 20 years is commonly applied as a design period for most pavements because of the difficulty of projecting traffic beyond the period.

Table 1: Structural design period of roads in Nigeria

Besides the nature of traffic bearing on the pavement, other important factors that should be considered in the drive towards achieving durable pavement include the type of traffic, the climate of the region, the precipitation, the temperature, and the subgrade properties.

A. Type of Traffic

Accurate estimation of the design traffic is necessary to ensure a durable pavement. This begins with the estimation of the average daily number of ESAs (ADE) on one lane at the opening of the new road to traffic, which is then projected and cumulated over the design period to give the design traffic loading. For proper forecast, it is necessary to separate the traffic into three categories:

Normal traffic: Traffic that would pass along the existing road, even if no new pavement was provided.

Diverted Traffic: Traffic that would change from another ruote (or mode of transport) to the project road because of the improved pavement, but still travels between the same origin and destination.

Generated Traffic: Additional traffic that occurs in response to the provision of the improvement of the road.

Using traffic forecast categories outlined above, the pavement designer should ensure that the future traffic loading on the pavement is estimated by accurate consideration of Equivalent Standard Axles (ESAs) growth rate through the consideration of an increase in heavy traffic volume and an increase in loading of heavy vehicles. The designer should also carefully evaluate the facilities that can generate additional heavy vehicle journeys, the economic growth rate expected for the area in the future, existing or probable alternate modes of transportation, possible effect of government legislation in the future, possibility of diversion of traffic to the new route and the amount as well as possibility of negative growth rate. To arrive at the most realistic traffic volume, sensitivity analysis should be carried out while working within the limit of different growth rates of ESA_80s.

B. Climate (Moisture and Temperature)

Climate is defined as the weather conditions prevailing over an area within a long period of time. The climate of an area affects precipitation as well as the temperature and these should be factored in pavement design and construction. It is necessary that the prevailing climate of the ruote location should be known.

The moisture conditions would largely determine the weathering of natural rocks, the durability of the weathered natural road building as well and the stability of the untreated materials in the pavement. The equilibrium moisture content and ambient temperature also affect the stability of the bituminous surfacing and these should be well considered by the designer in extreme cases. In Nigeria for instance, four climatic zones exist and each has their ranges of precipitation (see Table 2). In terms of temperature, Nigeria has a generally moderate to hot temperature range and this should be well understood.

Table 2: Climatic zones in Nigeria, predominant climates and ranges of rainfall

C. Subgrade

Careful evaluation of the subgrade of the pavement route is necessary if durable pavement would be achieved. The subgrade strength generally depends on the geology of the route and the worst-case scenario of the subgrade is evaluated by the soaked CBR condition of the natural soil which is the projected wettest moisture condition likely to occur during the design life of the pavement. CBR can be determined by laboratory tests or the use of the Dynamic Cone Penetration Method. This test should be conducted at minimum acceptable intervals depending on the specification being used and also the minimum number of tests required should be conducted.

Prior to CBR tests on the pavement, compaction tests are carried out to determine the optimum moisture content (OMC) required to carry out the subgrade CBR test. These compaction tests would be carried out depending on the expected field subgrade conditions as outlined in Table 3. The purpose of this is to enable proper classification of the subgrade CBR.

Table 3: Guidance for the testing of subgrade CBR

Note: The conditions above may be subject to adjustments depending on the discretion of the Engineer with regard to deviations to site conditions from what is outlined in Table 3.

Based on the test results obtained from CBR tests based on Table 3, the subgrade can be classified as outlined in Table 4.

Table 4: Subgrade classification based on soaked CBR values

In a situation where variation in results makes classification difficult, it is necessary, first, to check the laboratory test process to ensure there is uniformity and secondly, to test more samples in order to build a more reliable basis for selection. In the case where more samples are tested, a cumulative distribution curve would be plotted with the y-axis representing the percentage of samples less than a given CBR value while the x-axis represents the actual CBR values in percentage. From the plot, a 10% cumulative percentage (percentile) is selected on the basis that 10% of the actual values would be expected to have a lower CBR value than the indicated CBR value.

C1. Pavement Material Depth

Beyond the selection of the CBR class, it is necessary to carefully consider the material depth which is the depth below which soil characteristics of underlying pavement structures do not have an appreciable impact on the pavement behaviour. Based on the different categories of pavement in Nigeria, the material depths are outlined in Table 5.

Table 5: Material depth of different pavement categories in Nigeria

Within the material depth, it is expected that the minimum thickness of the subgrade should be according to the class to which the subgrade belongs (see Table 6).

Table 6: Subgrade classes and minimum thicknesses

Note: In Nigeria, a minimum subgrade thickness of 450 mm is recommended for all subgrade classes except S5 and S6 with higher subgrade thicknesses.

It is also recommended during construction to check the uniformity of the subgrade thickness using the Dynamic Cone Penetrometer (DCP).

C2. Problem Soils

If problem soils are encountered which generally represent in-situ subgrade materials that do not meet minimum strength requirements for a subgrade, or which do not possess other favourable properties, the subgrade material can be removed and replaced with a better material or the following measures outlined HERE can be employed to improve the strength of the in-situ subgrade.

D. Other layers of flexible pavement such as the sub-base course, the base course, and the wearing course also require particular tests, specifications, and treatments to ensure their optimal performance. Subsequent articles will discuss these.