Category WATER IN ROAD STRUCTURES

Jose Santinho Faisca[28], Jose Baena, Susanne Baltzer, Beata Gajewska, Antero Nousiainen, Ake Hermansson, SigurOur Erlingsson, Mihael Brencic and Andrew Dawson

Abstract This chapter sets out the requirements, possible problems concerning sur­face and subsurface water flow for pavements and offers some technical solutions to control these waters. It presents the general principles for the design and choice of a drainage system, the measures to adopt during construction and maintenance phases and considers the control of surface and subsurface water contamination, in order to minimize the possible detrimental effect to existing aquifers and habitats. This is achieved by a thorough review of available drainage measures, including many illustrations.

Keywords Road drainage ■ sub-soil drainage ■ filter criteria ■ drainage layers ■ trenches

13.1 Introduction

The objective of this chapter is to set out the requirements, possible problems and to give some technical solutions to the control of surface and subsurface water flow for pavements.

The proper management of a pavement is needed to maintain the strength of the road structure, to provide long service life, safe traffic conditions and the environmentally acceptable treatment of pavement water. The increase of moisture in the pavement and in the subsoil or in the pavement foundation can decrease the bearing capacity (as discussed in Chapter 8) and stability, and contributes to physical and chemical phenomena, which modify the pavement’s structure and further may increase erosion, expansion, dilution, cracking, risk of collapse and frost damage (Hall & Correa, 2003).

This chapter presents the general principles for the design and choice of a drainage system, the measures to adopt during construction and maintenance phases, and considers the control of surface and subsurface water contamination, in order to

minimise the possible detrimental effect to existing aquifers and habitats. In partic­ular, the aim should be to guide excess water out of the higher construction layers in such a way that the water beneath the pavement doesn’t weaken the pavement structure or allow subterranean water to enter the structure.

The implementation of mitigation and prevention measures from roads and road traffic should follow the pollutant fate in the environment. Before planning and designing of protection against pollutants from road and road traffic, a conceptual model of the pollutant fate in the particular environment should be established. This should help to estimate potential risks and hazards to water bodies’ pollution. The model usually consist of three main parts that are represented by definition of pollu­tant sources, pathways of pollutants through the environment and targets that receive pollution from the sources in road environment. The concept of pollutant fate in the environment is described in greater detail in Chapter 6.

As in every environmentally-driven decision, care must be taken that the ben­efit to the water body is not offset by an equal or worse disbenefit to another environmental compartment. For example, the higher fuel consumption of cars using a longer road could be evaluated in terms of non-renewable energy impact and

greenhouse gas emissions and these compared with the lower risk achieved to the groundwater. How to make such a comparison is beyond the scope of this book, but interested readers are referred to (Falcocchio, 2004).

It will never be possible to prevent all deleterious impacts of the road on the hydro-environment. However, there are many actions that can be taken to signifi­cantly reduce impacts. In general, the pollution management policy is that protection of the environment should be preformed in a way that the source concentrations of contaminants are reduced as much as possible and to limit, or prevent completely, the appearance of contaminants in the targets. To reach this goal several mitigation approaches and mitigation methods can be adopted (Table 12.1).

Mitigation approaches are divided according to the pollutant fate model: source – pathway – target.

Mitigation at source can be performed with:

• prevention methods;

• avoidance methods; and

• reduction methods.

Prevention methods are in general applied to stop emissions of pollutants in the environment or at particular environmental sensitive areas (e. g. on Natura 2000 areas of sensitive water habitat). A typical general prevention approach is banning of leaded fuel or banning the use of road de-icing agents on the environmental sensitive areas.

Avoidance methods, in general, can be defined as special design proce­dures, mainly connected with road alignment, that avoid crossing envi­ronmentally sensitive areas. They seek to prevent a problem from arising in the first place (or minimize the problem). Often these design options are very costly and they very often interfere with the goals of the road. For example, a road may be longer in order to avoid a particularly sensitive groundwater body leading to greater construction costs and ongoing fuel consumption costs.

Reduction methods are those that are implemented when emissions from roads and the road environment cannot be stopped. They can be im­plemented by various traffic restrictions such as travel velocity reduc­tions (e. g. on groundwater safe-guard zones) or reduction of traffic flow (e. g. embargo of dangerous goods transport over environmentally sensitive areas). Also, among reduction methods, the proper selection of construc­tion materials can be included (e. g. alternative material use for sub-grade that do not interact with the soil environment).

Mitigation along the pathways can be achieved with

• interception methods; and

• reorientation methods.

Reorientation methods divert water that was polluted at the road surface, or inside the pavement, out of the area sensitive to water pollution perhaps to runoff treatment facilities where water is intercepted and treated. A watertight drainage system that diverts runoff water is a typical example of this.

Interception methods are technical measures that enable interception of pollutant flux. These interceptions can be defined as run-off treatment fa­cilities (e. g. detention ponds) or absorption barriers (e. g. reactive barriers).

Mitigation at the target is achieved when pollutant reaches the target and its dele­terious impact is reduced by

• remediation methods; and

• compensation methods.

Remediation methods are only feasible when some deleterious and adverse effects appear at an environmental target (e. g. damage to local fish habi­tat as the consequence of leakage from alternative material built into a sub-base). A typical remediation method in the pavement and embank­ment domain could be the replacement of contaminated granular base and sub-base materials by earth works. These methods would be extreme and only used in situations when previous mitigation measures were not successful. The use of these methods should not be implemented as an integral method for permanent pollution protection. However, in environ­mentally sensitive areas they could be planned as a part of the intervention measures.

Compensation methods are economic measures or replacement measures. The latter are applied in the case that road construction, and all the conse­quences of it, damage a particular habitat or water body. In this case a new, substitute, habitat or water body is included as part of the construction cost in the area where previously the zone was of lower ecological value. As an economic measure, compensation methods are applied as indem­nity to the owners of the land crossed by the road or who are influenced by it. From the environmental point of view, compensation methods for pollution mitigation should be avoided. This approach implements the principle that loss of environmental values can be compensated by eco­nomic measures. Remediation and compensation mitigation methods are usually applied outside of the embankment zone, so they are not covered in full detail here.

Mitigation methods can be further divided into:

• ex-situ methods. Ex-situ methods are implemented externally as non – technical measures or as technical measures performed in places that are not part of the near-road environment.

• in-situ methods. In-situ methods can be defined as mitigation methods imple­mented on the road or in the near vicinity. These methods are further divided into:

o intervention measures, о non-intervention measures.

Intervention measures are those that involve intervention by human action, either when a problem is detected or on a regular basis (e. g. to maintain a pumping system). Active approaches are the least desirable for a number of reasons:

• Their success depends on continued human attention… which is often dif­ficult to guarantee;

• They continue to require funding after construction, both in terms of pay­ment to the personnel involved and, in many cases, in terms of the running costs of electrical or other energy consuming equipment. In the future there may be pressure on funding and a lack of appreciation of a problem that is in focus at the present time. This can lead to less attention at some future date than is necessary; and

• Detection of a problem is necessary in many cases for the active approach to be implemented. Some problems will be readily detected – e. g. those resulting from a spillage during a traffic accident – but many will not be easily detectable in which case it is difficult to incentivize the search for a problem which could conceivably (but probably doesn’t) exist.

Non-intervention measures rely on the installation of some constructed element that continues to function over a large part, or all, of the life of the project in which it is installed. They are often more costly than active ones if expenditure is only considered over a year or two. However, in the long term the ongoing costs of providing active control will usually make passive approaches seem more economic.

The constructed element is designed to achieve one or more of the following:

• that any actual or potential contamination pathway is blocked;

• that there is a purpose-installed receptor for the any potential or actual contami­nant that will prevent the contaminant from reaching a natural receptor to which it would present a hazard; and

• that the water regime adjacent to the road is maintained in an acceptable manner.

It is always best to attempt to avoid pollution problems rather than to intervene

after the event. Data published by the UK’s Highways Agency (2006) for 5 British

roads reveals the high variability of success of different techniques. Sometimes 99%

reduction in contaminant concentration was achieved, sometimes there was even an increase in concentration after use of a “clean-up” technique due, presumably, to remobilization of previously arrested contaminant.

12.2 Conclusions

No road construction can ever have a zero impact on the environment in which it is placed. The materials of which it is constructed will yield a different response to the hydrological situation than did the soils that they have replaced. The con­struction interrupts the preceding natural flow regime (Fig. 12.3). The traffic on the road generates various pollutants that fall on the road (Chapter 6, Section 6.2). For these reasons the road designer must assess the potential impact of each aspect, compute the risk of unacceptable pollution and put in place mitigation measures that will address each unacceptable risk in a technically and economically satisfactory manner – this is a major challenge, especially as regulatory regimes become more and more demanding.

During the planning, construction, operation and maintenance of roads, economics plays an important role with pollution mitigation measures providing constraints that have a significant influence on the final cost of the road and its operation. Water protection measurements can represent an important proportion of the total road cost. For example, in Slovenia (a country in which groundwater is a very valuable resource) it was estimated that, over the groundwater sensitive areas, the protection costs represent between 10 and 50% of the total road construction costs.

Roads are constructed due to socio-economic demands and local communication needs. They are among the most important infrastructure objects provided by soci­ety’s development. Therefore, it often happens that environmental criteria for their construction and operation take second place to the construction criteria, especially in transition economies. During planning and construction, costs for environmental protection measures are very often treated as direct expenses that cause an unjusti­fied rise in the price of the road. Consequently, it can happen that removing these measures is seen to be a source of savings in the project. Damage to the water environment caused by road construction and operation can be very difficult to eval­uate in terms of cost and revenues. However, experience shows that indirect costs caused by incorrect (or omitted) protection measures, although difficult to measure, are very high with long term consequences that can be very difficult to remediate. Roads across drinking water ‘safe-guard’ zones are a particular example illustrating the high costs or high impact that may occur.

A very important economic dimension of protection measures is their operational cost. These costs can represent a large proportion of the total cost of the ongoing road maintenance. Protection measures have to be properly maintained – especially active ones where the run-off is treated before being released to the wider envi­ronment. The high operational cost of some run-off treatment systems can lead to incorrect or incomplete maintenance and, consequently, in the generation of a new pollution point at the treatment outlet. Therefore, run-off treatment systems should be carefully designed and costed for all the potential problems that could occur during the maintenance processes.

The structure of the traffic greatly influences the road run-off pollution load. As a general rule, pollution on low-density roads is smaller than on high-density roads. However, the relation between pollution and traffic density is not linear and it is very difficult to predict the run-off pollution from road traffic characteristics, al­though these characteristics have a large role in controlling contaminant fluxes. The pollution of road run-off is also highly dependent on the climatic regime.

Traffic characteristics on roads are defined according to the several criteria. The most general parameter is called annual average daily traffic – AADT – however this parameter doesn’t define the structure of the traffic. Usually, this parameter is further defined through the passenger car equivalent – PCE – or by the proportion of the AADT that is heavy commercial vehicles (HCV). These approaches allow consideration of the number of different types of vehicles.

In low volume roads, run-off treatment procedures usually differ from those used with high volume roads. For mechanical reasons, the traffic volume and road design criteria are, of course, connected. Consequently, the pollution potential of all roads is linked not only to the traffic levels, but also to the design criteria and approach adopted to match that traffic level. If the traffic volumes are low, the road’s design is likely to be thin and the attention to detail to handle runoff and seepage waters is likely to be brief or even absent altogether. Thus, while the risks and volumes of contaminants arising may be less than on a heavily trafficked road, there may also be greater opportunity for these pollutants to enter the local water and ground environments.

If there is no exact legislative demand for run-off and seepage water treatment, the designer must define the treatment procedure according to the traffic density forecast and to the structure. Some national legislations (e. g. in Central Europe) attempt to define technical protection measures based on traffic characteristics (e. g. Brencic, 2001). Criteria are then related to AADT and PCE. However, these criteria are not very satisfactory, over-simplifying the situation, and should be re­considered with new parameters based on potential pollutant equivalents of different types of vehicles being applied instead. In Slovenia, the technical legislation dealing with road run-off pollution was implemented defining classes according to PCE. The legislation defines classes of PCE according to the aquifer types and surface water bodies that are crossed by roads. If the prescribed limit of PCE is exceeded then road run-off treatment should be performed. On the highly vulnerable karstic aquifers the limit is set to 6,000 PCE, on intergranular aquifers the limit is set to 12,000 PCE and on rocks and sediments with low permeability the limit is set to 40,000 PCE.

Pollutant emissions from roads and traffic present risks and hazards to water bodies where roads are in their recharge area or when they are in direct contact with road environment. Risk is defined as the probability that a particular adverse event will occur during a stated period of time, or it results from a particular challenge (Adams, 1995). Similarly ‘hazard’ is defined as the attribute that is the consequence of the probability of an adverse event and the degree of harm that can happen if this event occurs. A high hazard is present where the potential consequences to water bodies are significant.

Pollution risk depends not on vulnerability but on the existence of pollutant load­ing entering the subsurface environment. It is possible to have high aquifer vulner­ability but no risk of pollution, if there is no pollutant loading; and to have high pollution risk in spite of low vulnerability, if the pollutant loading is exceptional. It is important to make clear the distinction between vulnerability and risk. This is because risk of pollution is determined not only by the intrinsic characteristics of the aquifer, which are relatively static and hardly changeable, but also on the existence of potentially polluting activities, which are dynamic factors which can in principle be changed and controlled.

The hazard of polluting the proximal road environment is the consequence of three types of emissions that are very much related to overall road and traffic char­acteristics:

• permanent emissions;

• incident emissions; and

• seasonal emissions.

Permanent emissions are mainly the consequence of vehicle operation on the road and their interaction with the pavement. This type of emission can also be the conse­quence of the interaction between materials used for road construction, maintenance and their surrounding environment.

Seasonal emissions are the result of the climatic seasonality, which influences circumstances that exist at a particular time on pavement and in the embankment. Typical seasonal emissions are connected with road salting. In northern European countries and Alpine countries during the thawing of snow and ice, a significant amount of chloride is emitted from the roads and their near surroundings. Similar seasonality is connected with higher summer temperatures when pollutants are more bound to asphalt surfaces then during the colder periods of the year.

Accidental spillages of liquids and gases hazardous to water bodies are typical incident emissions on roads. Roads, where the potential hazard of spillage of envi­ronmentally dangerous goods is high, should be treated more rigorously than roads where such potential is small.

Protecting the water environment from different types of emissions requires different mitigation measures. Therefore, it is necessary that during the planning and design of roads, a basic knowledge about potential risk and hazards must be established.

12.3.1 Consideration of Site Sensitivity and Vulnerability

Analysis of the natural conditions of a road course is usually the second stage in the planning of the road construction where the first stage is defined by legislation and socio-economic factors. The protection level of the water and water environment from road influences depends on factors that are connected with:

i) road and traffic characteristics;

ii) natural sensitivity and vulnerability of the existing environment; and

iii) presence of the areas with special public interest (e. g. public water supply re­sources, special areas for vineyards, locations of rare flora or fauna, Natura 2000, etc.).

For example, the protection of the water environment on a low permeable clay stra­tum will need to be totally different from the protection needed on a high yield intergranular aquifer that represents an important source of drinking water. At the same time the protection required against a highly trafficked road will be totally dif­ferent from the protection needed against a low traffic road with only a few vehicles per day.

To establish sound and cost efficient measures for water protection from nega­tive road influences, the natural conditions must be correctly characterised and well understood to enable the proper planning and design of protection measures. The natural conditions of the road corridor are analysed in the field using classical hydro – and hydrogeo-logical methods (O’Flaherty, 2001; DoE, 2005; Brassington, 1998). Results of those investigations are used to determine the level of water protection.

When designing technical measures for groundwater protection, the goal of proper protection of hydro-environments from a road’s influence will, usually, only be achieved by developing, defining and adopting a classification scheme. Such a scheme must be based on a conceptual model of the road’s course above water bodies and a conceptual model of the hydro-logical system’s source-pathway-target arrangements. These conceptual models can be divided into two main groups; those for surface water bodies and those for groundwater bodies.

The degree of protection required for a surface water body from negative road influences depends on its ecological, qualitative (chemical) and quantitative status. The most important relation in designing protection measures for surface water bod­ies is the ratio between run-off water discharged from the paved surface of road and the discharge of flowing water in the receptor river/stream bed. In general, the principle is that the quality of the receiving water should not be deteriorated by the arriving water. This is minimised by ensuring that the road-water: receiving-water ratio should be strongly in favour of the receiving surface water body.

Special care is needed when water from a road is discharged into a standing water body. In this case the mass of contaminant entering the static water body becomes more important rather than concentration (which is the most important in stream and river situations). A mass-balance calculation will typically be required to deter­mine the amount of contaminant that can be handled by the static water body. This requirement is illustrated in Fig. 12.1 for a man-made wetland environment com­prising an, essentially, static water body with small and intermittent surface water inflows and outflows. Special care is also needed when dispersal of road run-off is planned. Dispersal is possible only in the case when average daily traffic is relatively low. A less effective, but more controllable solution is provided by “underground” wetlands (Fig. 12.2).

When a road crosses a water catchment area (especially small ones) road con­structors are often tempted to concentrate the small upstream watercourses in a single water structure (e. g. a pipe) in order to cross the road. Such a concentration of upstream catchment area run-off modifies the normal flow-rate of the downstream part of the watercourse, leading to an increased erosion over a variable distance unless the stream bed is adapted to the new hydraulic conditions (SETRA, 1993). Upstream, in case of runoff exceeding the discharge capacity of the pipe (e. g. after a

Man-Made Wetlands

(Organic Soil, Microbial Fauna,

Algae, Plants, Microorganisms)

Fig. 12.1 Contaminant mass transfer considerations required for a man-made wetland (Van Deuren et al., 2002). Reproduced by permission of U. S. Army Environmental Command

storm), the road body may temporary act as a dam, inducing flood and stagnant waters. Lastly, all the downstream parts of watercourses that were diverted up­stream, toward the main course, are no longer fed by the catchment area. All these hydraulic perturbations induce impacts on aquatic habitats and organisms. They can be avoided thanks to an improvement of the road structure’s hydraulic transparency, by means of sufficient upstream-downstream connections across the road structure (Fig. 12.3).

To define the influence of road pollution on groundwater bodies, the groundwater vulnerability concept can be applied. The most useful definition of vulnerability is one that refers to the intrinsic characteristics of the aquifer, which are relatively static and mostly beyond human control. It is proposed, therefore, that the ground­water vulnerability to pollution is defined as the sensitivity of groundwater quality to an imposed contaminant load, which is determined by the intrinsic character­istics of the aquifer. Thus defined, vulnerability is distinct from pollution risk. It is important to recognise that the vulnerability of an aquifer will be different for

different pollutants. For example, groundwater quality may be highly vulnerable to the loading of heavy metals from road runoff and, and yet be less vulnerable to the loading of pathogens originating from the same source. In view of this reality, it is scientifically most sound to evaluate vulnerability to pollution in relation to a partic­ular class of pollutant. This point of view can be expressed as specific vulnerability (COST 620, 1998).

An example to illustrate this approach is a conceptual model adopted in road construction in Slovenia (Brencic, 2006). For groundwater bodies, protection is de­fined by a simple conceptual model of an aquifer that is divided into unsaturated and saturated parts. Protection requirements follow from:

• the estimation of transit time of the probable pollutant as it takes its route from the source on the road through the unsaturated zone; and

• on the interaction of the probable pollutant with the soil and water through which it passes en-route to the groundwater.

The greater the spreading of pollutant in the vertical direction, the higher is the vulnerability of the water resource. On the basis of the estimation of the pollutant spread and progression in the direction of the water resource e. g. borehole, spring, etc., the classification of arrival times from the spill location, or permanent pollution point due to the road operation, to the water resource is defined. Based on the cross­classification between vertical and horizontal spreading velocity of the pollutant, mitigation guidelines for the selection of suitable technical protection measures of the groundwater from negative highway influences were defined (Brencic, 2006).

The level of pollution originating form roads depends on several factors that can also have an influence on pollution mitigation and prevention. These factors can be divided into two general groups:

• natural factors that depend on the environmental characteristics of the road’s surroundings; and

• technical factors that are connected with road design, construction and traffic characteristics.

Together with air, water is the main transport media for pollution dispersal from roads to the environment. Water is a very efficient solvent and, during its path through the road construction and surrounding environment, it dissolves and trans­ports many pollutants – some of them in large quantities and over long distances. Pollutants are transported in water by the mechanisms described in Chapter 6 with solution and suspension of solids being important. Preventing pollution impacts through technical measures represents a considerable challenge to planners, design­ers and operators of roads.

Road construction is an activity that very directly affects the hydrologic and geo­environment. Due to their linear nature, roads characteristically divide the hydro­logic and geo-environment into two or more parts. The degree to which these parts are separated from one another depends on

• the road category;

• the road’s topography related to the surrounding area;

• the kind and density of traffic; and

• the existence of connecting structures between the two sides of the road (hy­draulic or for biota).

The influences of road operation on water bodies may be classified as direct and indirect. Direct influences include water pollution that can be treated by various treatment methods. Also, the road construction may disrupt sub-surface and/or surface water flow paths. Most of these direct influences are measurable if one is prepared to monitor the impact of the road. Indirect influences are not usually de­tectable at first sight being mainly connected with activities that are induced by the road operation – for example, new industrial zones development or new residential areas. Ultimately, these consequences can be more severe than direct pollution of water body from road run-off. Nevertheless, we are addressing only the former, the hydrological impact of new developments being far beyond the scope of this book.

During planning, construction, operation and maintenance of roads, protection and mitigation measures for the water environment play an important role. These activities have to be established on the basis of conceptual models that enable the proper implementation of the appropriate technical measures. Natural conditions such as sensitivity and vulnerability of water bodies are among the most important influences to be considered by these conceptual models.

Various constraints can influence the selection of procedures to mitigate the threat of pollution from roads and traffic to many of the environmental compartments. For example, water bodies are very important as they represent an important habitat, may provide a water resource for public water supply and last, but not least, wa­ter is one of the most important transport agents for spreading pollutants. These constraints are the consequence of natural site characteristics, usually defined as the sensitivity and vulnerability of water bodies, they depend on geotechnical and hydro-technical conditions in the environment and on road traffic characteristics, es­pecially the share of heavy vehicles in the total daily traffic flow. Legislation is another important constraint to pollution mitigation. To protect the environment as much as possible, there is an increasing trend for laws and other legal instruments to impose upon planners, designers, operators and users of roads many obligations that should be carefully studied and implemented. The knowledge of all constraints can help establish proper criteria for preventing and mitigating harm from road pollution.

In the past extensive monitoring of road infrastructure and its influence on various environmental compartments has been performed. Among other results of monitor­ing that have been extensively reported are those affecting the hydro – and hydro-geo­logical environment (e. g. Hamilton & Harrison, 1991; Bruen et al., 2006). The most profound and known effects of roads on the water environment, especially in north­ern European countries, are the influences of winter maintenance practice where salts and some other agents are used as de-icing agents (Amrhein et al., 1992; Oberg et al., 1991). Also reported are the influences of heavy metal contamination on soil micro-structure and, consequently, on the physical and chemical conditions of soils and water percolated through the unsaturated zone in the groundwater (Folkeson 2000). Literature reports on oil spills on Slovene karstic areas reveal that spilt oil sinks very fast into the karstified underground and very soon appears in springs used for water supply (Kogovsek, 1995 & 2007).

Some case studies recording the impact of alternative road construction materi­als on groundwater are reported in the literature. As an example of this, a road was constructed on a new alignment in Cumbria, Lake District, UK in 1975 over a some­what acidic bog. Soft marshy material was removed and the area was back-filled with approximately 300,000 tones of Blast Furnace Slag from iron production. The slag was also used below the water table. The slag material contained large quantities of calcium sulphide. Soon after construction, sulphur related pollution began to be observed in surrounding water courses, apparently as a consequence of groundwater seepages from the construction. Sulphide has a directly toxic effect on many life forms but its oxidation to sulphate can be a more serious problem as this can cause severe oxygen depletion, effectively asphyxiating many life-forms within the affected water bodies. In winter the problem was not so severe since there was plenty of water in the local receiving streams to enable dilution to occur. However, as groundwater continued to flow into receiving streams whose flow rates had decreased during the summer, and as water temperature increased, excessive algal blooms occurred. After construction was finished, the records indicate that samples of water discharging from drains from the roadway were found to be con­taminated with sulphides and hydrogen sulphide. Biological and fisheries investi­gations showed that there was a marked impact on the invertebrate fauna and fish populations in the two streams receiving the drainage from the new road (Taylor, 2004).

Many alternative materials are often mentioned by reference to their leaching potential as a potential risk regarding the pollution of surface and groundwater. However, this risk must also be considered regarding some natural materials. Acid drainage is one of the phenomena that can appear in some natural materials. As an example we can refer to the construction of a capping layer consist of hornfels metamorphic rock aggregates that resulted in percolating waters with a pH between 3 and 4 (Odie, 2003). From the construction phase fish-kill was observed in the neighbourhood of the road. Such an acidification was induced by the oxidation of solid and dissolved pyrite (FeS2) originally present in the aggregate, and resulted in the production of H2 SO4 and Fe(OH)3. The presence of the latter at the outlet of the drain was testified by a dense orange colour.

Mihael Brencic% Andrew Dawson, Lennart Folkeson, Denis Francois and Teresa Leitao

Abstract There is often a risk of pollution entering or moving in the road environ­ment. This may give rise to problems of various severities dependant on the local environment around and under the pavement. Therefore the risks have, first, to be assessed and then appropriate action taken to minimise the movements and/or the impacts. This chapter describes the criteria to be applied when considering pollution mitigation schemes and the constraints that must be taken into account. Both traffic considerations (which often form the driver for pollution supply) and economic con­siderations are included in the coverage of the chapter together with some comments on site sensitivity. In particular, the chapter provides a framework for considering alternative mitigation strategies against a background of the benefits and limitations of each. Pollution mitigation measures are only mentioned where they are identifi – ably different from conventional drainage measures which are covered more fully in Chapter 13.

Keywords Pollution control ■ impact mitigation ■ flow disruption ■ site sensitivity

12.1 Introduction

Roads and road traffic can act as serious sources ofvarious types ofpollution. Pollu­tants spread to the environment through different pathways, with different transport agents and mechanisms. Once pollutants are transported away from the road and traffic sources they can reach various environmental compartments where they can have detrimental effects. Pollution from roads and traffic must be managed and its harmful affects prevented at all stages, especially in environmentally sensitive areas.

The objective of this chapter is to describe general principles of prevention and mitigation of pollution that originate from road and traffic operation and that can influence the water environment. Consideration was taken mainly of mitigation of [27] deleterious effects caused by seepage water. Pollution prevention and mitigation is associated with several constraints that can be classified in five major groups; site sensitivity and vulnerability, risk and hazard to pollution, traffic characteristics, economic and legislation constraints.

In the second part of the chapter general principles of mitigation methods are described. A new classification of mitigation approaches based on the pollutant fate model that consist of the chain sources – pathways – targets is described. Classifi­cation is described based on the ex-situ and in-situ mitigation methods and descrip­tions of intervention and non-intervention mitigation measures are also introduced.

The content of this chapter is very much connected with the next chapter “Rec­ommendation for the control of pavement water” where design and technical mea­sures for pollution prevention are described.

The transport model of pollutant leaching from the secondary road construction material was developed by the Environmental Research Group from the University of New Hampshire, USA (Apul et al., 2003). Water flow in a Minnesota highway embankment was modelled in one dimension for several rain events and calibrated to the field condition (Fig. 11.20). The test facility consists of 40 and 152m-long

hot mix asphalt and Portland cement concrete test sections with varying structural designs. Each test section is instrumented to monitor strength and hydraulic proper­ties. The hydraulic properties of the embankment were predicted from water content measurements made in the embankment, a Portland cement concrete pavement with an asphalt shoulder. The hypothetical leaching of Cadmium from coal fly ash was probabilistically simulated in a scenario where the top 0.50 m of the embankment was replaced by coal fly ash. The groundwater table was set at 1.9 m below ground level (b. g.l.), which is within the range (1.3-4.6m b. g.l.) observed at test site. An entire year’s precipitation data repeated 10 times was input as the variable flux boundary condition. The molecular diffusion coefficient of Cadmium in free water

was input in the model as a constant (6.2 x 10-5 m2/day) and tortuosity factor was calculated within the finite element code, HYDRUS2D, as a function of the water content.

The probability distributions of unsaturated hydraulic properties of the em­bankment were determined from parameter posterior probabilities obtained from embankment infiltration simulations. The probability distributions were used to fit to the four parameters of the van Genuchten SWCC model (see Chapter 2, Section 2.7.3). Weighted moment equations were applied to calculate the means and standard deviations for the normal distributions. Saturated permeability and saturated water content were assigned joint log-normal distributions. To account for the variability of partition coefficient, kd, uniform distributions were assigned. In the study kd was considered as lumped parameter. The temporal and spatial variability of kd that would be expected in the field was incorporated in the modelling approach by probabilistically varying kd values of the subgrade and the coal fly ash for each simulation.

The average percentage of initial available mass leached after 10 years, as ob­served 0.01 m below ash, is presented, idealised, in Fig. 11.21, for a point a short way into the ash layer (point marked on Fig. 11.20). No significant Cadmium fluxes were observed 0.25 m below the coal fly ash or at the groundwater table depth. After 10 years, the fraction of initial available mass leached was 5 x 10-6 percent at 0.25 m below the coal fly ash, and 0% at the groundwater table depth (at the 90th percentile of uncertainty). The cumulative release at the 90th percentile of uncertainty and the appropriate probability distributions, was 2.65 x 10-3 mg Cd/kg ash after 10 years. The mean of the release estimate was 1.15 x 10-3 mg Cd/kg ash. Further details

are available from Apul et al. (2005) but it is clear that infinitesimal leaching occurs in real pavement/earthworks sections arranged in a similar way to the construction studied in Minnesota.

11.5 Conclusions

The partial differential equations that govern solid mechanics, water transfers, heat transfers and pollutant transfers have been restated. The specificities of the finite element method when dedicated to such non-linear phenomena and their coupling have been summarised. Then a number of numerical simulations have been pre­sented. They cover moisture transfers, freezing, mechanical strains and permanent deformations. It appears that, for the most part, realistic numerical modelling is today available, at least for advanced research teams. But progress is still needed, for example to couple changes of moisture level and changes of the mechanical behaviour.

A first series of ORNI calculations was performed considering only the rutting of the UGM layer, and assuming different temperatures in the bituminous wearing course (between 15° and 35°, corresponding approximately to the range of temperatures measured in-situ). The results are presented on Fig. 11.18. It can be seen that the temperature in the bituminous wearing course has a large influence on the permanent deformations of the UGM (the temperature affects the modulus of the bituminous material and, therefore, the stresses transmitted to the granular base).

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Figure 11.19 presents the results of a second series of calculations where the rutting of the subgrade soil was also taken into account. The contribution of the subgrade to the total rutting is important, representing about 40% of the total rut depth. The final rut depths obtained after 1.5 million loads, with the contribution of the subgrade, are close to the experimental measurements, especially for the temperatures of 23° and 27°, which are close to the average in-situ temperatures. However, the model predicts a too rapid stabilisation of the permanent strains in comparison with the experimental measurements.