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.
