Category WATER IN ROAD STRUCTURES

13.3.6.1 Drainage Layers in the Pavement

The sub-surface drainage system often includes a (permeable) drainage layer in the pavement. Its function is to quickly remove water entering the pavement layers, either through infiltration to the groundwater or to a sub-surface drainage system, before any damage to the road can be initiated. It is common practice to include drainage layers where the groundwater level is high compared to the location of the road, where the subgrade soil has low permeability and on high class roads (Summary of replies to the WATMOVE questionnaire, www. watmove. org).

There are situations where a drainage layer is not considered necessary. Responses to the WATMOVE-questionnaire show that a majority of countries assess the necessity of a drainage layer before deciding to include one. If the groundwa­ter level is low compared to the location of the road, the subgrade soil has a high permeability or if it is a low-class road, a drainage layer may not be included.

There are different national traditions for which layer in the pavement works as the drainage layer, when included. Answers to the questionnaire show that some countries only use one layer as a drainage layer, i. e. the sub-base, whereas other countries mention (e. g.) three different layers. Dependent on the available material and the specific construction, any one or more of these is used as a drainage layer. The questionnaire responses of pavement engineers in many European countries are shown in Fig. 13.7.

An advantage of having the drainage layer just above the subgrade is that it can also act as a capillary break which is highly desirable in cold climate areas to prevent frost-generated water movements from the subgrade into the pavement (see Section 13.3.6). If the drainage layer is placed on the top of subgrade, the

Fig. 13.7 Layer used as primary drainage layer in European pavements. (% of countries indicating that they use this layer. It was possible to indicate more than one layer)
permeability of the granular base and sub-base must be greater than the infiltration rate,[29] so that water can flow freely to the drainage layer.

Dawson (1985) reported that many UK sub-bases (typically containing as much as 10% by mass less than 75 ^m) act more like sponges, absorbing water, than as permeable drainage materials. Jones & Jones (1989) report measurements of coeffi­cient of horizontal permeability in the range 1-60 x 10-3 m/s for typical aggregate sub-bases and Roy & Sayer (1989) report in-situ measurements (by injection) of 2-110 x 10-3 m/s for similar materials. In other granular base course injection tests, Floss & Berner (1989) quote permeability values between 10-6 and 10-4 m/s for sandy gravel aggregates in-situ. Biczysko (1985) tested broadly graded aggregates in the laboratory for their horizontal permeability, obtaining reliable permeability coefficients in the range 2-50 x 10-4 m/s. However, he found that the aggregates with finest gradings (10% of material finer than 75 ^m) were difficult or impossible to saturate – yielding an apparent permeability of 3 x 10-6 m/s – indicating that the lowest values are likely to over-estimate the in-situ behaviour and that substantial

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Bearing capacity measure
ments (e. g. by FWD)

through flow is unlikely to take place. The laboratory values reported by Jones & Jones (1989) and by Biczysko (1985) were both obtained by a specific perme – ameter designed for highway aggregate testing (Department of Transport, 1990) as described Chapter 3, Section 3.1 (Fig. 3.6).

Therefore, it is not sufficient to have a granular base or sub-base layer and assume it will drain. Instead, when the base or sub-base layer is also to act as a drainage layer as in Fig. 13.8, its material must satisfy both the requirements of strength and the permeability requirement for a drainage layer. When the layer is on top of the subgrade, the material may also have to satisfy requirements to act as a capillary break and filter, so that the fine particles of the subgrade soil do not migrate into the drainage layer (see Section 13.3.9).

An overview, resulting from the questionnaire mentioned above, of the require­ments to ensure a successful drainage layer is given in Table 13.1.

Care must be taken if a permeability coefficient and a certain grading envelope are both specified. It would not be difficult to specify one and thereby prevent the other from being achievable. While the relationship between grading and perme­ability can not be precisely defined (see Chapter 2, Section 2.5.1), controlling one will certainly have a large effect on the other.

The performance of the drainage layer does not necessarily stay unchanged with the passage of time. Some countries report that they have noted that the layer might become more or less clogged with time. The fines content might increase caused by degradation of aggregates and/or migration of fines from other layers. This causes decreased permeability and increased frost susceptibility.

Sometimes it is desirable to combine surface water runoff collection and sub-surface seepage water into one, combined, drainage system. Typically, such systems are in the form of lateral trench drains (see Section 13.4.1) that are open to the surface. These are the traditional means of draining roads and continue to be used on lower – trafficked roads where run-off contamination is lower and runoff volumes smaller (because the road is narrower).

Their principal drawbacks are: •

The Highways Agency (2006) publishes a table that indicates when it is sensible to consider a combined system and when not to do so.

Although they may not be considered as part of the drainage system, a pervious type of asphalt treated surface layer, known as porous asphalt, has become common in Europe in recent years. The main advantages attributed to porous asphalt layers are noise reduction, improvement of skid resistance in wet weather, and enhancement of runoff water quality. Asphalt treated drainage layers of this type are discussed further in Chapter 5, Section 5.7.

13.3.4.2 When Drainage is Unnecessary

Some authors suggest that subsurface drainage may not be necessary if:

• annual rainfall is not significant;

• the subgrade has a relatively high permeability value;

• the pavement is structurally adequate without drainage;

• lateral and vertical drainage in the pavement section exceeds infiltration; or

• heavy traffic level is negligible.

For example, Christopher (1998) found that drainage provides no additional benefit if average annual rainfall is less than 400 mm and permeability of the sub­grade exceeds 3.5 x 10-5 m/s, however Dempsey (1988) and Forsyth et al. (1987) suggested different values, 3 x 10-6 m/s and 1.7 x 10-4 m/s, for this parameter.

A rather common subsurface drainage system used to remove the infiltrated/seepage water from the pavement structures is by providing a permeable layer. Permeable layers should be at least 10-15 cm thick and extend under the full width of the roadway. They can be used under both concrete (PCC) and asphalt (AC) pave­ment surfaces. Permeable bases are usually located just above the subgrade and are discussed in more detail in the second part of Section 13.3.6. Permeable unbound granular bases must be separated from high plasticity subgrade soils by mean of geotextiles or impervious materials.

The drainage layer should drain into a longitudinal drainage pipe. In order to encourage the lateral flow of water, a minimum cross-fall should be considered, of, at least, 2%. For curved lengths of road and those with a permeable central reserve, the pavement bed must have a cross-fall of between 2% to 4% inclination, starting 1.0 m away from the paved area (marked with E in Fig. 13.6).

Fig. 13.6 Typical pavement cross-falls. E = position 1 m inwards from the edge of the pavement. Similar cross-falls will exist at road edges

In drainage design, an undamaged asphalt surface is considered almost impervious. However, water can infiltrate into the structure through cracks and joints (see Chap­ter 5, Sections 5.3 and 5.4.2). Also, shoulders and slopes with higher permeability and high water tables can allow significant amounts of water into the structure. Whether the water arrives via cracks in the asphalt and is flowing through the pave­ment according to regime A or B, is threatening to arrive from the margins, or is simply close to the underside of the pavement due to a high phreatic surface, it is then necessary for longitudinal interceptor drains to be provided or other drains that will lower the water table or keep it in a low position. Many types of longitudinal drain are available as described in Section 13.3.9.

In many countries, especially the Mediterranean ones, where roads have open verges and slopes (especially where there are flat areas and hollows) it is customary to construct the verges/slopes with an impermeable surface cover using soils with a percentage of fines typically more than 25% of its weight (D25 < 80 ^m), with a minimum thickness of 20 cm to limit water seepage into the pavement structure.

Subsurface drainage is made up of different parts but all are linked directly with the surface drainage system and all are, fundamentally, taking care of groundwater or water that infiltrates through the pavement surface.

13.3.4.1 Drainage Regime

Part of the rainfall-runoff infiltrates into the ground and continues as subsurface flow. Part of this may, in turn, continue as a sub-horizontal subsurface flow, depend­ing on the permeability of different soil layers. This can lead to increases in the moisture level under the pavement, reducing the bearing capacity. To decrease this phenomenon, the scheme design should note that:

• it is a good practice in embankments in the Mediterranean countries either to keep the thickness between the underside of the pavement and the natural soil to at least 1.0 m, or, if necessary, a drainage layer (see below) as well as other measures should be used, depending of subsoil characteristics; and

• in a cutting, the depth of the lateral drains should allow for an adequate drainage depth to the groundwater level, normally more then 1.0 m.

Because of the different types of subgrade and pavement construction, it is important to be able to differentiate between those pavements where water flow will be largely vertical, wetting the subgrade (with the associated loss of subgrade support strength) and those situations where vertical water flow will be arrested by impermeable layers in the sequence, forcing the water sideways and necessitating different drainage measures. Figure 13.4 shows three types of pavements, A B and C, which are now described.

A. Subgrade layer with low permeability – infiltrated water will flow above the sub­grade, at the bottom of granular base/sub-base layers, according to the maximum crossfall;

B. Permeable subgrade layer and impermeable subsoil – infiltrated water flows on the layer between the subgrade and impermeable subsoil;

C. Permeable capping layer and subgrade – the water percolates vertically through every layer.

The basis for choosing whether approach Case A, B or C applies is set out in Fig. 13.5.

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The surface drainage system should remove all flow of rainwater from the road’s sur­face, and from the highway slopes as well as the runoff from adjacent land. Surface drainage systems are also important in the proper management of polluted runoff and in minimizing environmental impacts. The surface drainage can be divided into transverse drainage and longitudinal drainage.

Transverse Drainage – Typically used to allow existing water courses to pass under/over the road which would, otherwise, form a physical barrier. These are normally constructed as aqueducts or culverts (see Chapter 12, Fig. 12.3).

Longitudinal Drainage – The main objective is the fast collection and removal of the rainwater that falls upon the road’s immediate surroundings, and of the water from the adjacent areas, edges, excavation slopes and central reserve. This is funda­mental for maintaining the safety of traffic by eliminating water films and puddles from the road surface (which can result in aquaplaning) at the same time reducing the possibility of water infiltration into the pavement’s layers or foundation, which may reduce its load carrying capability.

Longitudinal surface drainage systems include gutters, channels, ditches, swales, galleries and collectors, complemented by their respective manholes, catchpits and sumps. Surface drainage is not the main topic of this book, so readers should look elsewhere for detailed information on this topic (e. g. Kasibati & Kolkman, 2006).

The drainage system employed in a road construction will depend on factors such as:

• The importance of the road;

• The amount of traffic;

• The zone (rural or populated);

• The sensitivity of the groundwater; and

• The sensitivity of streams, rivers and lakes.

Drainage systems can be classified as follows:

• Surface systems – these involve ditches and open channels in the surface of the ground;

• Subsurface systems – these are not directly accessible from the surface. Water is collected from water in the ground’s pore space and conveyed in trenches and pipes. Subsurface systems can be divided into shallow (interceptor) drains that collect percolating water above the water table and deep (water table lowering) drains.

In a very permeable soil, separate drainage may not be necessary, only provision of some capacity for immediate runoff and for snow and snowmelt.

Open channel drainage systems are favoured because of their low cost (compared with capacity), easy rehabilitation and maintenance. However, deep open ditches and steep inner road slopes can be dangerous for wandering vehicles, especially on a road with high traffic speeds. Steep slopes may also increase cracking along the centre of narrow roads, and, possibly, increase erosion and the need for channel cleaning.

Subsurface systems have advantages in special circumstances and can be efficient tools in road rehabilitation projects, especially when there is limited space available, limited support of slopes or a need to use deep drainage.

The correct depth for the drainage depends on the pavement thickness, the road layout (cutting or embankment), the type of subsoil and the climatic conditions (intensity of runoff, frost depth and snowmelt conditions). The drainage depth is usually the depth of all the layers of the road structure that contribute to the bearing capacity of the pavement.

Fig. 13.3 Drainage economics

The design of drainage is a part of the design of the whole road, so all aspects of the road can have an effect on it. Drainage design is, simply described, the action of finding an optimum balance between the total costs (investment and maintenance costs) compared to the possible advantages and disadvantages (pavement life, traffic safety, easy maintenance and wider environmental aspects) – Fig. 13.3.

Considering all these aspects, there are a number of steps which, in general terms can be considered fundamental. They are listed in the Portuguese Road Drainage manual as follows (IEP, 2001):

1st Step – Gather all the relevant information:

• Road importance (traffic flow values, status and whether a “lifeline” road);

• Geometric characteristics of the road (layout and profiles);

• Drainage areas and existing drainage systems;

• Geology;

• Meteorological data (precipitation, temperature, frost, etc.);

• Hydrological and hydrogeological conditions in the area surrounding the road (ground-water conditions); and

• Identification of specific constraints (technical, social, economic or envi­ronmental);

2nd Step – Identify critical/sensitive areas:

• Vulnerable areas with particular conditions, geological, environmental or ecological;

• Areas with a high frost formation probability;

• Specific areas of the road, such as high and low level points;

• Extreme gradient and cross fall situations; and

• Cutting/embankment transition areas.

3rd Step – Adopt standard/typical layouts, where possible, for road segments with similar characteristics.

4th Step – Define the basic data for the water flows in each layout, using existing methods, tables and software to perform necessary calculations.

5th Step – Analyze the possible and adoptable solutions, based on standard drawings and typical dimensions used in each region or country.

6th Step – Perform the hydraulic calculations so as to obtain drainage sizes. If the estimated amount of water is small, exact hydraulic calculations may not be needed.

7th Step – Consider the location of the discharge points, as well as the need to design retention and/or treatment basins, which may be associated with individual drainage systems.

Roads are normally constructed with two types of drainage systems, the surface and the subsurface drainage systems, each taking care of their separate sources of water and moisture.

13.3.1 Road Alignment and Routing

For new roads, before drainage can be considered, the routing of the road must be fixed. Road routing is a complex procedure that involves very many factors including social, economic, engineering and environmental criteria as well as pub­lic acceptance. However, proper selection of the road corridor is one of the most important factors in the protection of water bodies. Technical measures aimed at mitigating a problem provide a poor substitute for efficient routing and alignment which could have avoided the occurrence of the problem altogether. Nevertheless, in those cases where conflicts between roads and water bodies cannot be avoided,

design of the road’s elements, together with technical measures, plays an important role. The design of the road’s alignment has a major influence on the inclination of the road surface. Although the main criterion for road surface dewatering is usually traffic safety (e. g. to prevent aquaplaning) water protection criteria must also be included into design practice. The selection of the longitudinal and transversal cross fall of the road can

• Direct the water from different parts of the highway surface and sub-surface to different drainage outlets, thus allowing “isolation” of spillage-affected run-off to localised collection points rather than allowing the contaminant to affect an entire drainage system,

• Lead water to less sensitive ground – or surface-water zones where it is more easily handled,

• Be an influence on the recharge area and, at the same time, on the total pollutant loads flowing from the particular section of the road.

Horizontal alignments can be selected to take a road away from a place where it may be expected to have an undesirable impact. Vertical alignments can permit the drainage of water collected from above the pavement’s formation level (e. g. water deriving from cutting slopes and from pavement runoff) to be led out of a local catchment where it might be undesirable, to another location where it can be handled with less risk to the environment (Fig. 13.1). Less obviously, but more practical in many situations, horizontal curves may be introduced into a road’s alignment in order that the pavement has to built with some super-elevation. This will then provide a cross-fall across the pavement to the margin on the inside of the bend (Fig. 13.2). By carefully choosing the road’s horizontal and vertical alignments, both surface and sub-surface waters can be moved to a desired outlet point where the impacts will be minimised.

There are two aspects, which must be addressed in answering the question: “Why is road drainage so important?”. They are:

• A road’s infrastructure is an engineering work, aimed at the establishment of a platform on which vehicle circulation is possible under safe conditions, with proper traffic flow, utility, and economy, independent of the region’s climate con­ditions; and

• Water, along with heavy traffic, is one of the greatest causes of road deterioration. As previous chapters have shown, even relatively small increases in water con­tent can often result in significant reductions in the mechanical properties of the aggregate and soil layers in and under the road, thereby speeding up pavement failure.

The overall objective is, therefore, to keep the pavement and the subsoil dry enough to avoid any potentially harmful effects of the water and to control the envi­ronmental effects of that drainage water. This is done by decreasing the infiltration through the pavement platform’s surface by adopting integrated solutions. These should simultaneously

• permit the re-establishment of natural groundwater patterns,

• avoid the access of runoff water from nearby land areas to the road platform,

• reduce the risk of erosion due to surface and subsurface flow on/in the nearby slopes, and

• provide preventive measures against soil and/or the aquifer’s contamination ei­ther as a consequence of an accident or due to regular traffic and the construction.