Category WATER IN ROAD STRUCTURES

Recycled and alternative materials are finding increased use in the construction of road pavements and embankments. Environmental concerns are leading to con­straints on quarrying of the materials that have, conventionally, been used while tax incentives and legislative limitations are encouraging the uptake of wastes, by-products and recycled elements in their place. These materials do not, nec­essarily, behave in the same manner in the presence of water as many conven­tional materials do. Self-cementation due to pozzolanic activity may help to sta­bilise some, others may exhibit undesirable leaching, yet others may have much higher permeability than the material they replace. As a consequence, it is im­portant for the road designer or manager to understand these characteristics and their implication for the hydrological and environmental performance of a high­way that incorporates such materials. Relying on experience alone is likely to be insufficient.

It is very common for regulators and potential users to express concern about leaching when construction with such materials is proposed – yet this is a concern that often has no basis in fact! Many alternative materials come from an indus­trial process in which some chemical has been involved which no-one would wish to become widely distributed in the environment. Thus, environmental regulators often show particular concerns about materials deriving from metal processing in­dustries – e. g. slags, foundry sands, etc. However, their real impact depends not on the actual content of the chemical of concern in the solid but on its availability to pore fluids, its solubility and its transportability. Many alternative materials have been through a hot process which vitrifies the solids making it extremely difficult for chemicals, now held in a glass-like matrix, to leave the solid phase. Alternatively, the pH level in-situ may render the contaminant essentially non-soluble. Chapters 6 and 12 discuss these issues further.

In the road environment, contaminants are almost entirely moved by water-based or air-based processes. Air-based processes are, principally, by dust and spray, but these are not discussed further, being beyond the scope of this book. In water – based processes, contaminants are carried in and/or by the water through soil or aggregate pores, over the top of the pavement and through drainage systems. As water moves through the sub-surface, water that is fairly pure will have the abil­ity to pick-up chemicals from the soil through which is flowing and to carry these elsewhere while runoff water that arrives from the pavement surface may percolate into the construction carrying impurities with it which are then “dropped” by one mechanism or another into the layer. The interaction between water and contami­nants is the subject of Chapter 6 with methods of measurement being described in Chapter 7.

Movement of contaminants is often considered using a “Source – Pathway – Receptor” framework (Fig. 1.7). In the context of the highway, the “source” would probably be the surface runoff water (although its antecedents of vehicle cargoes, atmospheric pollution, etc. could also be considered as the source) or the pavement

construction. The contaminated water then moves through the pavement and the drainage system which provides the “pathway” for the contaminant to move. Even­tually it arrives at a place where it has a potentially deleterious impact – the “re­ceptor”. In an ideal understanding the receptor is a human, animal or plant that is affected by the contaminant. In practice, policing of impacts on humans would be almost impossible to monitor and, anyway, too late to change an undesirable response. So, instead, it is normal, in most practical circumstances, to treat either the surface water body (river, lake, etc.) or the ultimate groundwater body (e. g. drinking water aquifer) as the receptor. Monitoring their quality is the subject of Chapter 7.

Chapter 12 gives information that, with the help of Chapter 13, can be used to mitigate problems due to contaminant movement in and with the water percolating in the near-pavement environment. Usually, these involve either interruption of the pathway or removal of the source or target.

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).

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.

1.4.1 Definitions

In today’s world almost all traffic runs on an improved surface and not on the nat­ural soil profile… and most users give such improved surfaces little thought. This improved surface is known as a pavement and may be a simple layer of imported aggregate or the structure that comprises a modern expressway. In fact, all but the very simplest pavements comprise a series of carefully designed layers of imported, selected, construction materials placed on the “subgrade” (Fig. 1.4). The subgrade

may either be the natural soil profile encountered at the site (e. g. when the pave­ment construction is built at the same level as the surrounding ground, or when it is built in a cutting) or it may be a “fill” material that has been imported to create an embankment.

This fill is often subgrade soil from some other location, but might also be a by­product or waste material. Even in a cutting, some material may have been imported to form the subgrade as it can aid drainage – and thus improve the performance of the pavement. Material specifically designed to provide an improved subgrade is known as “capping”. It may either be imported or may be achieved by some improvement technique applied to the natural subgrade in-situ (Fig. 1.4).

It is not only the pavement that needs draining; drainage of embankments and cuttings is also important to give soil-based slope stability and longevity of performance.

Finally, what about “moisture” and “water”? Both terms are used, more or less synonymously in this book. In general the direct term “water” is preferred to the more indirect term “moisture”, although “moisture” is often used to describe water in the unsaturated parts of the sub-surface.

The book covers both theory and practice and addresses both sub-surface drainage and water quality issues. Chapter 2 deals with basic flow and suction theory and Chapter 4 with heat transfer, which is implicated in driving water movement by phase change mechanisms (freezing, thawing, evaporation). Chapter 6 establishes a basis for discussing environmental aspects, introducing contaminant transport pro­cesses. Chapters 3 and 7 give an overview of the available techniques to monitor water flow, suction pressures and contaminants in water. After this introductory and descriptive section of the book, Chapters 8-11 aim to explain interaction between water in soil and aggregate and the mechanical response of such materials. This explanation covers theory, field behaviour, laboratory testing and theoretical and numerical modelling. The book ends with two chapters that aim to guide readers to improve the environmental condition, to reduce water pressure and to reduce the volume of water in the pavement and adjacent soils. Aspects only of relevance to environmental matters are covered in Chapter 12 while the much longer Chapter 13 describes the many techniques that can be used to manage water in the pavement and near-pavement soils. Often such management has benefits to both the mechanical and environmental performance of the pavement structure and earthworks.

The main aim of this book is to increase the knowledge about water in the sub­surface road environment so as to improve highway performance and minimise the leaching of contaminants from roads. Improvement of pavement performance will lead to less road closures, better use of the road network, longer service life and more effective transportation of goods and people.

This aim can be further divided into the following four secondary objectives:

• to describe the most up-to-date understanding of water movements and moisture conditions in unbound pavement layers and subgrades for different types of road constructions in various climatic conditions,

• to explain the relationship between the mechanical behaviour of materials/soils and their permeability[2] and moisture condition,

• to report on advanced modelling of water movement and condition in the sub­surface pavement environment developed from laboratory analysis and field studies,

• to inform about the identification, investigation and control of contaminants leaching from soils, natural aggregates and by-products in the sub-surface layers.

It is important to add that this book is NOT about road surface drainage. There are many books that address this topic, e. g. Ksaibati & Kolkman (2006), whereas this book aims to concentrate on sub-surface water. Inevitably, there is some overlap be­tween the two aspects but, in this book, the information on runoff is only that neces­sary to complete a coverage of the sub-surface condition. Regarding contamination, traffic is, of course, a significant source too (see Chapter 6, especially Fig. 6.1) but the reduction in contaminants from that source is beyond the scope of this book. Rather, the aim is to understand what happens, or should happen, to contaminants in the highway environment.

In Europe road construction may date back as far as 3,500 years ago. These early roads were probably largely for ceremonial purposes, over short distances, and may have carried little, if any, wheeled traffic. It was not until the growth of the Ro­man Empire that a large network of engineered pavements was first constructed (Fig. 1.1). Such was the desire to secure the Empire against enemies and to enhance trade that, at the peak time, about 0.5 km was being built daily. Although foot and hoof traffic probably predominated, those roads were certainly used for wheeled vehicles too.

The engineers responsible for these pavements understood some important truths about pavement drainage – truths which, in practice, sometimes are still not recog­nised today. Figure 1.2 shows a cross section of a high quality Roman road. It illustrates that its designer:

A. R. Dawson

University of Nottingham, Nottingham, UK e-mail: andrew. dawson@nottingham. ac. uk

A. R. Dawson (ed.), Water in Road Structures, DOI 10.1007/978-1-4020-8562-8_1, © Springer Science+Business Media B. V. 2009

Only a small proportion of pavements were built as illustrated, more generally a two layer pavement, comprising nucleus and rudus as explained in the notes to the Figure, was constructed on an embankment (agger – see Fig. 1.2). Despite the econ­omizing on materials, the aim of keeping the construction well-drained remained unaltered.

It was not until the late 1700s and early 1800s that a similar understanding was once again developed. Figure 1.3 shows a cross-section of a main coach pavement as designed by Thomas Telford, the Scottish engineer who worked in the UK and other European countries between about 1800 and 1830. In this design an effort has been made to provide a relatively impermeable surface to prevent water infiltration and a drainable foundation, but the route to lateral drains from these is not well developed.

Despite these early evidences of the understanding that drainage is needed for a well-functioning road, the lesson hasn’t always been fully appreciated. Arthur Cedergren, the American engineer, famously said that “there are three things that a road requires – drainage, drainage and more drainage” (Cedergren, 1974, 1994). He said this many years ago yet, despite many advances in the subject and a huge rise in environmental concerns since he was active, little further has been published in the area. This book is our attempt to redress that omission.

1 The text at the top of the figure reads “POLISH ROAD / Transverse Section of the Road between the City of Warsaw and the Town of Brzesc in Lithuania. / This Road (100 English Miles) was constructed by command of the Emperor Alexander I. and finished in May 1825.” Nowadays, Brzesc is known as Brest, and forms the border crossing town between Poland and present day Belarus. It lies about 215 km East of Warsaw (Warszawa) not “100 English miles” ^ 160 km as indicated in the figure.

In the following sections the modern manifestation of the same objectives – to keep pavements dry by limiting ingress and assisting drainage – are introduced. Alongside this updating of the age-old principle of drainage, the modern pavement engineer or geo-environmentalist has to consider the quality of the draining water. What chemical components does the water contain? Is that a problem? Where will they go? The following sections seek to introduce this modern concern as well.

The following have contributed to this book. Where a name is shown in bold, he or she is a contributor to one of the main chapters. Where a name is shown in bold and italics, the person was a contributor to one of the main chapters but not a member of the COST 351 project. Special thanks is due to these authors for being external contributors to the book. The others listed were members of the COST 351 project team but their contribution has not been separately identified. This doesn’t

mean that it was an unimportant part__ in several cases these people have made

major contributions in editing, providing material, organizing the appendices, etc. Some of those listed only participated in the COST 351 Action for a short period. Particular recognition is due to those who helped establish the direction of the study but were then unable to continue to the final stages of which this book is the principal result.

Andrew Dawson

Abstract This introduction provides a brief review of the history of highway sub­drainage before setting out the aims and organisation of the book of which it forms the first chapter. It gives an overview of the subjects to be covered in the following chapters, introduces the key topics including definitions of subgrade and pavement layers, their classification from a drainage point-of-view together with a brief cover­age of the principle of effective stress, suction, leaching and water movement due to evaporation and frost-heave. It outlines the way in which pavements and the hydro­logical environment interact before introducing the reader to the varieties of climate in which highways and pavements have to operate – a task that is likely to become more onerous in the light of climate change effects.

Keywords Introduction ■ history ■ definition ■ drainage classification of pavements ■ alternative materials ■ drainage systems ■ climate