Modern pavements normally comprise one or more bound layers overlying one or more unbound aggregate layers which, in turn, rest on the subgrade. In almost all cases the uppermost layers are bound by bitumen or cement. In the case of an embankment the subgrade is comprised of imported fill. In the case of a cutting it will often be the natural rock or soil at that location. Figure 1.5 provides two typical pavement profiles. Considering these from the bottom upwards, the following layers are, typically encountered:
• The pavement foundation consists of the natural ground (subgrade), and often a capping layer, the role of which is to improve the levelling, homogeneity and bearing capacity of the subgrade, and often also to ensure frost protection.
• The sub-base layer is normally comprised of an aggregate layer which acts as a platform for construction and compaction of the higher pavement layers and continues to function during the life of the pavement as an intermediate distributor
Thin asphaltic ‘chip-seal’ (<30mm) Asphaltic or concrete Unbound aggregate layer(s) surfacing & base layers
= Base & Sub-base Unbound aggregate Sub-base
Soil improvement layer (Capping)
Natural or imported (fill) subgrade
of stress from the higher layers of the pavement down to the foundation. It may also have a frost protection role.
• The pavement base is usually comprised of treated materials in high-traffic pavements or may be untreated in low traffic pavements. These layers provide the pavement with the mechanical strength to withstand the loads due to traffic and distribute these loads to the weaker lower pavement layers.
• The surface course (and possibly a binder course below) is the top layer of the pavement, exposed to the effects of traffic and climate. It must resist traffic wear and also protect the structural layers, in particular against infiltration of water.
These are described more fully in Chapter 8. The higher volume cross-section is typical of those found in major highways. It is critical that it continues to function well, so drainage of these pavements is provided, principally, to maximise the life of the pavement structure and thus to minimise the cost. Although longevity is also an issue for the low volume pavement, it also needs drainage to perform at critical times — e. g. after heavy rain storms or during spring-thaw (see Section 1.8) — when it would otherwise become impassable.
The pavement construction is there to provide an almost fixed, plane surface on which tyred vehicles may pass without difficulty. To meet this requirement the surface:
• must not deflect much transiently — otherwise vehicles will be travelling in a depression of their own making and using excess fuel in a vain attempt to climb out of it;
• must not deform plastically — otherwise ruts will form, hindering steerage, leading to increased fuel and tyre costs due to a greater contact with the tyre and tending to feed rainwater to the wheel path thereby promoting aquaplaning;
• must provide adequate skidding resistance — to enhance safety; and
• must continue to meet these requirements for a long time so that the pavement is economic and so that users are not unduly affected by pavement rehabilitation needs.
As far as the lower unbound layers and subgrades are concerned, they have to provide the necessary support to the upper layers so that those layers do not flex too much under trafficking as this could lead to those upper, bound, layers failing prematurely by fatigue. The upper layers need to be thick enough so that they spread the traffic loading so that the lower layers are not over-stressed and can provide their function successfully (Fig. 1.6). Successful pavement design is all about satisfying
Fig. 1.6 Malfunction of the lower pavement layers. A depression in an impermeable sub-grade allows water in the aggregate layer to collect there, leading to subgrade softening and consequent rutting of the whole pavement
these two needs in the most efficient manner given the properties of the available materials.
