The upper, bound layers in a pavement are little affected by pavement moisture. “Stripping” can occur in repeated wet weather when the traffic loading causes pulses of pressure of water which has seeped into cracks in the bound materials. Exploiting micro-cracks this water can separate the binder from the aggregate it is supposed to bind, leading to ravelling of the bound material. Similarly, water may cause delamination of one bound layer from another, thereby reducing pavement load-carrying capacity, by exploiting inter-layer cracks.

A whole book could be written on this aspect alone, but this volume only devotes part of Chapter 5 to this topic as its focus is on the lower unbound and subgrade soil layers. These are geotechnical materials and behave according to the basic prin­ciples of soil mechanics as described in many standard text books on the subject. As explained further in Chapter 9 (Section 9.2) mechanical performance depends, largely, on the frictional interaction developed between one particle and the next. When the grains in an aggregate or soil are pushed together, greater friction is de­veloped between the grains. The greater friction leads to improved strength of the subgrade soil or unbound granular pavement material, greater stiffness and greater resistance to rutting.

These inter-particle forces, considered over a large volume of particles, can be treated as a stress, known as the effective stress, o’, which is defined as:

o’ = a — u (11)

where o is the stress applied externally to the volume of particles and u is the pres­sure of water in the soil pores which may be trying to push the particles apart. Thus, well-drained pavements lead to higher values of o’ which means more fric­tion which, in turn, yields a material (and thus a road) that lasts longer and/or is more economic to construct and maintain. For this reason it is the road engi­neer’s task to keep the effective stress high and, from Eq. 1.1, it can be seen that this condition is achieved when the pore pressure is smallest. This is the under­lying reason why drainage is so important for efficient pavement and earthworks structures.

Nevertheless, even if it were possible, a completely dry geotechnical mate­rial is not wanted, instead a partially-saturated condition is often desired. When soil or aggregate is kept relatively (but not totally) dry, matric suctions will de­velop in the pores due to meniscus effects at the water-air interfaces. This suction would be represented in Eq. 1.1 by a negative value of u such that the effective stress, o’, increases as the suction develops additional inter-particle stresses by pulling the soil grains together. The topic of suction is discussed in more detail in Chapter 2.

For these reasons the pavement engineer wants to stop surface water (i. e. rain) from entering the pavement and wants to help any water that is in the pavement to leave as quickly as possible. Sealed layers and sealed lateral trenches may be used as barriers to prevent water from entering into the pavement or earthworks although, in practice, barriers are often not very effective due to defects or flow routes around them. Thus, drains to aid water egress are the primary weapon in the highway engi­neer’s fight against water-induced deterioration. Although there are other techniques than drains that may be employed to stop ingress and aid drainage (discussed fur­ther in Chapter 13), for now it is sufficient to mention drains as interceptors that both cut-off the arrival of groundwater at the pavement and that provide an exit route for water already in the pavement and earthworks. The scope for drainage of pavements is somewhat limited by the need to keep the pavement trafficable – thus steep longitudinal or cross-carriageway slopes cannot be used. For this reason
drainage gradients are, typically, small (^5%) necessitating that highly permeable materials are used that exhibit low suction potential.

High permeability materials are, characteristically, those with open pore struc­tures. In geotechnical terms, the permeability is described using the coefficient of permeability, K, such that

q = -AKi = – AK (1.2)

dl

where q is the volume of water flowing in unit time through an area, A, under a hydraulic gradient, i, and i is defined as the change in head, dh, over a small distance, dl. The negative sign is a mathematical indication that water flows down the hydraulic gradient. Inspecting Eq. 1.2 it is apparent that more effective drainage can be achieved by:

• increasing the area of flow intercepted – e. g. by providing drains with greater face area;

• increasing the hydraulic gradient – e. g. by installing deeper drains or drainage layers with steeper cross-falls; and

• increasing the coefficient of permeability – e. g. by selecting a more open-graded drainage material.

It is conventional to classify pavements according to their construction – flexible (i. e. principally made of asphalt or only granular), rigid (i. e. concrete) and semi-rigid (i. e. made of both concrete and asphalt layers). From the point of view of water movements these classifications are not very relevant. Instead, pavements may be classified by the way in which water enters and moves in the pavement. On this basis the following classification is more appropriate:

A. Impermeable throughout the construction

B. Impermeable surface and structural layers

C. Permeable surface over impermeable structural layers

D. Permeable throughout with water storage capacity within the structure

E. Permeable throughout without water storage capacity

F. Cracked or jointed surface layers over permeable lower layers

Each type of pavement can be constructed on pervious or impervious ground and the ground – (or surface-) water level could be below or above the bottom of the construction. Thus each of the above 6 classes could, in principle, be sub-divided according to these conditions. In fact, except in limited circumstances, only a few of the classes and their sub-divisions have meaning in situations that are at all frequent. Thus circumstances in which the surrounding water is above the base of the con­struction are rare. Normally drainage (e. g. in the form of lateral drains) is provided to avoid this possibility.

Class A constructions are relatively rare. Full-depth asphaltic construction has been used in a few situations, most often in city streets, but its adoption has not been widespread. In this case significant water flows to the subgrade through the pavement layers are not expected but if water does become trapped at the subgrade surface (e. g. where the subgrade is impermeable), then it may be difficult to get it out of the pavement.

Class B pavements are probably the most common in developed countries, typi­cally comprising asphalt or Portland concrete over a granular base or sub-base. The granular layers can act as drainage layers if they are permeable enough and have appropriate falls and outlets.

Class C constructions have become relatively common in recent years. The most common form of these is a flexible asphalt pavement with a porous asphalt surfac­ing. Rain water infiltrates the surface and then runs sub-horizontally within the as­phalt to a drain that must be provided (some more details are included in Chapter 5).

Class D pavements seem, at first an undesirable concept. Storing water in the pavement will be likely to reduce the structural capacity of the construction. However, the chief motivation for this is to reduce runoff rates to surface water bodies (streams, rivers, lakes). With increasing urbanisation and areas of “hard” surfaces, rainfall arrives more rapidly at receiving watercourses than it does in “green” environments where vegetation and partial sorption into soil delay the ar­rival. The consequence is that river hydrographs become more “peaky” and flood­ing more common. Therefore, the provision of water storage within the pavement reverses this trend, delaying arrival of rain to the watercourse. Furthermore, the slowing of water as it percolates through the storage area means that it drops partic­ulates. Also, some sorption of contaminants from the percolating water is achieved. Thus, the water arriving at the water body is also cleaner than it would otherwise have been. They are discussed a little more in Chapter 13.

Class E pavements are usually those with no sealed surface. They are common in parts of Scandinavia and form the minor road networks in many countries. Although unsealed, a well compacted surface of material with sufficient fine particles to block the pores and without potholes and ruts can shed a large proportion of the rain that falls on it. Conversely, distressed pavements of this type rather easily take in water and then tend to rapidly deteriorate further.

In countries with a network of jointed concrete pavements – Class F pavements – entry of water through joints can be significant, especially as the pavement ages and the joint fill compounds become less effective at keeping the water out. Asphaltic pavements that have suffered significant cracking could also be placed in Class F.

Where water does enter the pavement through the upper layers (Classes D, E and F) then the type of subgrade is likely to have more significance than in other pavements. Impermeable subgrades will necessitate horizontal or sub-horizontal egress. Permeable subgrades will allow vertical drainage towards the water table. Impermeable pavement subgrades are typically comprised of clay. When water reaches these it can cause softening and deterioration of the mechanical behaviour of the pavement (see Fig. 1.6).

Porous pavements that are designed to beneficially carry water through their lay­ers (Classes C and D) are liable to deteriorate in their ability to do so, with time, as solid particles block the pore spaces. Porous surfacings are prone to ravelling as direct trafficking and the induced water pressure pulses in the pores and micro­cracks between particles tends to cause particles to separate one from another. This process, known as “stripping”, is common to all asphaltic mixtures to some degree, but is more prevalent in porous asphalts where, of necessity, particles are less firmly fixed together than is conventional in densely-graded materials (see Chapter 5).