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Chapter 1
How pavements are affected by traffic and weather
Life on our planet has always involved movement, with its various inhabitants seeking food and shelter and partners. On land, many well-used travel routes developed preferentially as they led to common destinations such as fresh water or used convenient ways such as strips of firm ground between swamps or gaps in mountain ranges. Firm is an important term in this book and will imply that, under any likely combinations of temperature and moisture, the layer in question can withstand any realistic construction processes and any traffic-induced forces applied to its upper surface, without limiting its ability to perform all of its intended functions. A very early definition of “firm” was given in an 80 AD description of Roman road construction: The ground must not give way nor must the bedrock or base be at all unreliable when the paving stones are trodden.9
Commonly used routes utilising firm ground often became pronounced paths across the countryside, with hindering vegetation pushed aside and surfaces trampled by the foot passage of their many users. The resulting ways were usually far from ideal, particularly in terms of their location, gradient and surface condition. Thus, as human societies developed, there were increasing pressures to improve the relevance and usefulness of many ways. The demand heightened as humans learnt how to use animals such as oxen and horses to haul loads. The haulage devices were initially sleds and primitive wheeled vehicles. The subsequent widespread availability of useful wheeled vehicles, of large marching armies and of herds of market-bound animals placed significant demands on the surface condition of strategically located routes.
The purpose of this book is to describe how these demands have been met over the ages by improvements to the in situ surfaces and, more commonly, by providing imported pavements to enhance those surfaces. It aims to provide a descriptive history of pavements for a general readership, and so it would be wise to begin by defining what a pavement is. The pavement is the upper portion of a road or path. It is placed on top of the natural ground which may have been prepared for the pavement by clearing, excavating or filling. The pavement thus provides a suitable surface for travellers and avoids the underlying ground being damaged by those travellers or by the weather. The word pavement comes from the Latin pavimentum which means a rammed or beaten floor.
A broad technical definition of a pavement is: 10
A pavement is comprised of a number of horizontal courses, each comprised of a different set of materials. This allows the most appropriate materials to be used for the varying conditions that exist throughout the depth of the pavement structure. These courses are often subdivided into horizontal layers which are portions of a course that can be placed and compacted as single entity.
Pavements are not always needed. In dry climates, traffic could operate over many in situ surfaces without the specific provision of a pavement. Such routes could be destroyed by wind, water or the passage of users. The presence of clay or sound rock could help secure such surfaces, whereas wind could readily remove the surface or cover it with sand dunes. However, the problems caused by water are usually much more substantial and will underlay much of our subsequent discussion.
Given this focus, the book will ignore tasks associated with the realignment of routes, the widening of the right-of-way, the construction of cuttings and embankments to improve gradients, and the building of bridges and fords to cross waterways. These are well discussed elsewhere.11 The crossing of swampy ground, however, does fall within the ambit of this book and is the subject of Chapter 5.
The routes being discussed will all fit within the common perception12 of a road, and the term road will henceforth refer to travelled ways in general. The operational requirements of a road’s pavement are that:
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It contains no unexpected obstacles that would require a vehicle to manoeuvre laterally during its longitudinal passage along the road.
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Its surface is smooth enough to provide an acceptable level of “comfort” to occupants and cargo using the road and not to generate excessive noise or raise a vehicle’s rolling resistance, which typically accounts for about a quarter of a vehicle’s energy consumption.
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Its surface texture is sufficiently rough to allow a vehicle to accelerate, proceed and brake in acceptable and predictable manners and to safely negotiate curves in the road.
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It is sufficiently strong and stiff to carry many vertical traffic loads without suffering unexpected damage.
There are also three management requirements:
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It can be built, cleaned and routinely maintained at an affordable cost.
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It does not require unexpected maintenance expenditure due to wear and tear from expected usage, and
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It can meet all the above requirements without being compromised by expected external factors such as weather and water.
These seven requirements will all be discussed in later chapters.
A pavement meeting these requirements has three basic components. There is a surfacing which directly carries and services the traffic and protects the remainder of the pavement from the weather. The vertical location of the surface is governed by the required alignment of the road. Next, there is a basecourse (or roadbase) which provides the strength and stiffness needed to carry the traffic wheel loads for the intended life of the pavement. It may incorporate the surfacing function. It will commonly be constructed from imported material and may be comprised of a number of horizontal layers. The basecourse sits on a prepared surface known as the grade or formation level. The underlying material below the grade surface is called the subgrade. It is also sometimes called the subsoil or road foundation, but in this book, we will use subgrade.
The subgrade is thus the third component of a pavement and supports the construction of the basecourse and its subsequent operation under traffic. Subgrades occur as a result of three quite different circumstances. There might be pre-existing firm ground, some of which would need to be removed as it was above the grade level. The remaining parts of the firm ground become the subgrade. Or some of the ground below the grade surface might be incapable of supporting the pavement and its future traffic. It would need to be removed and replaced with an adequate thickness of suitable imported material, sometimes called a capping layer. Finally, if the upper surface of the firm ground is below the required grade level, it would need to be covered by an imported topping layer to achieve the required level. In the last two cases, the subgrade is the remaining underlying ground plus the capping or topping layer.
Capping layers are also used to protect the subgrade from water entry or when the subgrade is found to be too weak to adequately support construction of the operating pavement. Such capping layers are sometimes called sub-bases.
Subgrades may be improved by techniques such as rolling, draining and stabilisation – as will be discussed later in the book. Subgrades may be damaged by high loads or by water. Hence, there are two additional structural requirements for a pavement:
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It distributes the loads applied to it by traffic so that the resulting loads do not damage the subgrade, and
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It does not permit damaging water to enter the subgrade.
Thus, a pavement is much more than a road surface as it also provides a road with strength, stiffness and durability. Durability is an important concept as, in any realistic scenario, it is not practical to build and operate a pavement that will last without regular maintenance. Ideally pavement managers should be optimising the distribution of the efforts that they devote to the construction and subsequent long-term maintenance of a pavement.13 This task is complicated by the difficulties of predicting future traffic and local climate.
While vehicle usage is an obvious influence on pavement performance, a commonly under-appreciated fact is that even unused pavements will deteriorate over time. Chapter 3 discusses the elaborate “Military Roads” built by General Wade in Scotland during the 18th century. His expensive roads changed from occasional use to neglect and then to decay over a quarter of a century as essential maintenance work was carried out with waning enthusiasm and decreasing efficiency.14
Traffic and water are the two major damaging influences on the life of a pavement. Traffic damage is typically rutting and wear and tear due to the usage of the pavement and is discussed in more detail in Chapter 19. Water destroys pavements in many ways, and it will be helpful to outline these before proceeding further. Water damages pavements by:
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Washing material away, as illustrated in the Preface.
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Reducing the strength and stiffness of in situ materials, particularly clays and silts. Sometimes even a small amount of water can cause major deleterious changes. However, most paving materials can accommodate some water without serious consequences, and many have an optimum moisture content (see Chapter 4).
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Destroying surfaces by uplift pressures which occur when water fills any internal air voids and pores in a pavement. Any loading pressure on the surface of the water-logged pavement is then carried throughout the pavement by the water acting as an hydraulic system. The resulting hydraulic pressures operate in three dimensions and will push aside any weaknesses in the pavement structure. This process is sometimes called pumping, and the pressures are called pore pressures. Probably the most common example is in pothole formation where the surface of the pavement –...
