Category WATER IN ROAD STRUCTURES

In recent years, it has been demonstrated that diffuse or irregular sources of pollu­tion, like runoff, constitute significant contamination points in the natural drainage system. In most cases one has to take appropriate measures to control this pollution. In general, the environmental aspect of a road project nowadays constitutes an in­tegral and fundamental part of the project, so that there is a strong interdependency between the various aspects considered in design and in the interventions that have

to be planned. In particular, it will probably be necessary to perform an integrated assessment of the road project itself, the drainage, the water resources, the quality of the water, the geology and the geotechnical aspects, with special focus on hydrol­ogy and landscape studies. Only in this way can one balance need for the road and the impact it will cause and allow the design, dimensioning and implementation of optimal and adequate systems for the conveyance and treatment of water flowing from the road platform.

The choice of treatment system to adopt has to address diverse conditions:

• the type of pollutants to treat (in particular heavy metals);

• the regional climate;

• compatibility with the roads’ drainage project;

• the impacts on the landscape; and, also

• the creation of a passive system, without the need of energy which, thereby, has great reliability and low maintenance needs.

Therefore, once the discharge points have been associated with particular, sensibly- sized, areas of the pavement platform, one can compute the discharge and proceed to study which treatment system should be adopted and its interaction with the drainage system.

The implementation of a project of this kind doesn’t overcome the need for an effective control of the treatment efficiency and this may be achieved through the application of an adequate monitoring system for the site. This should commence operation immediately after the start of the road use. Data can be obtained by sam­pling the pollutant charge in the runoff on first use. It will form a baseline against which to check any eventual flaws in functioning of the recovery and treatment system. Monitoring will allow the operator to detect, and thus rectify, any defects in a timely manner. The successful implementation of a sampling program of this nature will be very useful not only for the scheme in view, but especially for new situations, helping to find better project options which can be implemented in sub­sequent projects.

In parallel one should promote periodic maintenance and conservation action in the drainage network and in the treatment areas. In fact, without adequate mainte­nance the investment made will be likely to prove fruitless.

Water that has been collected from runoff or from sub-surface drainage systems has to be disposed of. The simplest means is to route it to a naturally occurring surface waterbody (stream or lake). Often a retention pond (see Section 13.4.8.1) is interposed between the water coming out of the highway and the outflow into the surface water body. The retention pond reduces peak flow rates (therefore making the outflow easier for the environment to receive without flooding) and may have environmental benefits, too (see Chapter 12, Section 12.4 and Section 13.3.8 in this chapter). However, such surface water aspects are beyond the scope of this book and interested readers are referred to one of the many texts dealing with surface water drainage. Alternatively, the runoff or seepage flows can be introduced into the ground via a soakaway. This is particularly attractive when there would be lit­tle fall between an outlet and the receiving surface water body. Sometimes water can be dispersed into a wetland area. Only purpose-built wetlands (see Chapter 12, Section 12.3.1 and Section 13.3.8 in this chapter) should be used and they should be designed to take this water although, in the past, it has not been unusual for wetland areas to develop around points where sub-surface water seeps to the surface. Areas handling sub-surface seepages should expect low flows over long periods compared with the short, “peaky” hydrographs associated with runoff.

Permitting water to soakaway to the ground is only permissible where regulation, or regulatory authorities, allow. In particular, the use of soakaways in areas where the groundwater is used for drinking water is very restricted or, in some countries, not permitted at all.

Water collected from embankment grips will usually be of acceptable quality for disposing by soakaway as it came from the natural subsurface and is returning to it. The issue is not, therefore, likely to be only one of quality (unless the cutting intercepts an already contaminated groundwater), but maybe more one of volume. Can the disposal soakaway disperse the water supplied to it without surcharging and without causing problems to the receiving groundwater levels? Whether seepage water from the road drainage system can be disposed in the same way is less certain. In many cases the water collected from seepage started as rain that fell onto the road construction, collecting contaminants as it did so. By the time it arrives at the potential disposal route it will have travelled through many pavement layers and a sub-surface drainage system during which sorption and natural attenuation processes may have removed much or all of the contaminant that it once contained (Dawson et al, 2006).

According to answers to the WATMOVE questionnaire (see www. watmove. org), approximately half the countries in Europe that use soakaways have requirements concerning the water that flows from pavements into them. Often the water has to go through some kind of treatment, i. e. as a minimum sedimentation and oil separation, or a minimum vertical infiltration/percolation time must be ensured by the design. Soakaways need to be positioned to meet several criteria:

• that they encounter a substantial zone of porous ground. For this reason they cannot be used in clay soils. In moderate permeability soils, soakaways may be made more efficient by providing radiating “branches” from a central water store so that slow seepage over a wider area can provide sufficient rate of seepage;

• that they have sufficient capacity to hold most[30] of the water supplied to them through the connected drainage system until it has percolated into the ground. For this reason a soakaway must have void space that is above the maximum groundwater level; and

• that the soakaway space is kept open – either by filling the soakaway hole with coarse aggregate that has a high voids content and lining the sides to prevent wash in of fines, or by supplying a solid wall with openings (see Fig. 13.28).

For a successful design the volume of water to be drained; the return period for this amount of water to come into the soakaway again; and the percolation rate of the soil/sub-soil must all be assessed. Then a water balance calculation can be

Fig. 13.28 Schematic of a walled soakaway

performed – for example by calculating the volume of water arriving at the soak­away in each hour, m, (from a knowledge of the rainfall intensity, P (mm/h), and the area being drained to the soakaway, A (m2), and calculating the amount soaking into the ground, IR (m3/h/m2) (Eq. 13.1). The excess generated during the storm of length n (h) must never accumulate to a volume larger than the storage capacity, V (litres), of the construction.

m=n

(Pm – IRm) X A < V (13.1)

m=1

Swales (see Figs. 1.10 and 1.11) are a form of linear soakaway, with water being able to soak into the ground but, hopefully, leaving contaminants behind in the lining and vegetation of the swale. As a system for dealing with surface runoff they are beyond the scope of this book, but the water that they introduce into the ground does need to be considered by the designer of drainage of subterranean waters. They should not be so positioned that they will raise the groundwater levels in areas where this would decrease the stability of the surrounding slopes or where it would feed water into the pavement foundation, thereby reducing the bearing capacity of the pavement. For these reasons, the positioning of a swale for disposal of surface runoff presents the designer with a dilemma – too close to the pavement and it is likely to result in reduced pavement performance, too far away and it will not be easy to make it useful for its primary purpose.

In colder climates, deep drains (“cut-off’ drains) are used to reduce local frost damage by intercepting the flow of groundwater and seepage water under the road structure, usually where there is a crossfall (see Section 13.4.4). The depth is usually at least the design frost depth (e. g. in Finland, this is between 1.5 and 2.2 m). Lesser depths may be used if there is very low permeability soil below. Deep drains can be installed beneath open ditches but then some gravel cuts may be needed to connect the drain and the structure. However, to ensure the fastest drainage of the road struc­ture during the thawing period, the best location is connected to the structure under the inner slope, see Fig. 13.27. A particular benefit over an open trench is that such a drain doesn’t get clogged with ice. In most cases the need for a deep drainage or cut-off drain is due to longitudinal variation in the permeability of the subsoil. For example, a rock or belt of clay may change the flow direction to flow under the road bed (see also Section 13.3.7).

In road rehabilitation, the trench for a deep drain is made with a narrow bucket at the toe of the road embankment slope, a drain pipe is laid near the bottom (filter fabric first if necessary), initially filled with drain gravel or crushed stone #5-10 mm and followed by a top filling with coarse gravel or similar material, or crushed rock. The sides should have a low permeability lining. Narrow channels should be blasted through rock thresholds. Inspection wells can be made, for example, from plastic culvert material at 400 mm diameter with a cover on the top and some silt storage at the bottom, at about 50 m intervals. A cover flap should be used at the drain outlet opening. The water from this outlet is led to a lateral ditch, to a diversion ditch or into a rainwater sewer. A wide gravel outlet can ensure a safe discharge in the case of a local blockage of the outlet especially if the drain opening is located in a low gradient area. A sunny position for the outlet also decreases the risk of freezing. If the collected water has to cross the road, a separate pipe is usually constructed, instead of a culvert, to avoid freezing problems.

Among the possible benefits of deep drainage are:

• decreased growth of vegetation and thus some lower maintenance costs;

• better support of the slopes and pavement edges;

• improved traffic safety since open drains can be shallow and the slope gradi­ents low;

• reduced probability of cracking at pavement joints, especially for narrow roads; and

• sheet ice should be eliminated.

Because deep drainage may also lower the groundwater table in the long term, the risks and disadvantages for the environment have to be evaluated separately.

In excavated areas and cuttings where the longitudinal slope is more than 3%, a longitudinal water flow may appear fed by water from under the pavement that is

separate from the flow in channels, gutters and gully’s. In these cases, the inclusion of pavement underdrains (Fig. 13.26), installed transversally under the pavement, can be used, in order to collect all subsurface waters. These kinds of drain are best constructed in transition areas and, in areas of excavation or fill, placed centrally to improve the rapid flow of the infiltrated water. In sandy soils they should be placed with a spacing between 20 and 25 m while, in very clayey soil, these distances should be reduced to about 5 m.

It is advisable that these transverse drains be constructed in a trench, filled with drainage material wrapped in geotextile, or by synthetic filters, constructed right

down below the level to be drained. In cold regions, the dimensions and depth of the network may be greater than in non-frost affected regions.

Drains can be placed as a trench or fin at the toe of a cutting, between the cutting and the pavement construction (Fig. 13.23). These act to lower the water in the cutting both increasing the stability of the slope and also reducing the water arriving at, and the pore water pressure in, the foundation of the pavement. Typically these cut-off drains also perform the function of lateral pavement drains (see Section 13.4.1). However, they differ from lateral pavement drains because they are designed to collect water from both sides of the trench or fin.

In porous ground where water tables are high, or high rainfall is anticipated, the drain must be designed to carry relatively large, continuous flows unlike nor­mal lateral pavement sub-surface drains. This may have implications for the design dimensions of the carrier pipe at the bottom of the trench, for the regularity of points at which the trench is emptied to a surface water body or other outlet, and for the accessibility for maintenance. The last point deserves emphasis – if a normally op­erative cut-off drain ceases to function, water pressures will quickly rise in the toe of the cutting slope and in the pavement foundation, leading to rapid pavement dete­rioration and reduced cutting slope stability. In general, fin drains are less desirable when large flows have to be carried away as their capacity is normally less than that of a comparable trench drain.

Longitudinal cut-off drains may also be installed in sidelong earthworks so as to prevent water from ever arriving at the road’s construction. Figures 13.24 and 13.25 give examples.

Drainage masks provide control of emerging water (e. g. a spring line) on a slope’s face with a geotextile covered by hand-placed rock (Fig. 13.21). The material used for a drainage mask should be angular and should comprise 100-500 mm sized stone. The rock provides improved slope stability both by allowing the reduction of pore water pressures without erosion and by adding mass to the slope.

13.4.4.2 Drainage Spurs

Drainage spurs have the double function of draining and reinforcing the excavated slopes. Spurs are constructed as stone-filled trenches excavated perpendicular to the slope face (Fig. 13.22) to provide both drainage and buttress support. The material to be used in drainage spurs should be coarse and angular (e. g. broken rock in the size range 100-200 mm) and should be contained within some geosynthetic wrap to ensure soil does not migrate into the large void space. A heavy-gauge geosynthetic is indicated given the abrasive nature of the fill.

13.4.4.3 Wells

Wells are vertical shafts of sufficient diameter in order to lower the water level. They are usually associated with a pumping system and, hence, require constant maintenance.

This kind of drainage is installed with the aim of controlling waters emerging from earthworks, which includes not only the water that appears at the base of the exca­vations but also the flow coming from excavated slopes. Five types of systems can be used:

13.4.4.1 Drainage Layers

These consist (see Sections 13.3.4 and 13.3.6) of a layer of granular material with constant thickness (normally between 0.40 and 0.60 m) that is spread at the base of the excavation along the formation, or, for an embankment situation, at its foun­dation (Fig. 13.19). This layer is placed between geotextiles having separation and filter functions.

Fig. 13.19 Drainage layers

At the base of large embankments crossing deep valleys, the main aim may be to keep the water table at its original position in the old valley bottom. For this purpose it may be more desirable to use a series of trenches let into the original valley profile which lead water away before it can rise into the base of the embankment. Due to their appearance in plan these are colloquially known as “Christmas Tree” drainage systems (Fig. 13.20).

Transverse drains are similar in construction to lateral drains, except that they run perpendicular or slightly skewed to the centreline of the carriageway (Kasibati & Kolkman, 2006). They are mostly used to drain water that may infiltrate into the road bases and sub-bases at joints, see Fig. 13.18 (FHWA, 1980).

It is important to provide drains at joints in rigid pavements as the joints’ seal­ing can deteriorate with time allowing water to flow into the pavement. Perforated pipes are usually used as transverse drains and they may empty directly into the side ditches. Pipes as transverse drains are classified as passive drains, meaning that they do not constrain the water movement but they are placed in the hope that water will find its way into the drain. However, some of the water may move past the drain and cause some problems. Transverse drains should be used with caution in frost heave prone areas as differential frost action may damage the pavement structure.

Outlet Pip*

Coarae Filter Perforated Collector Pipe

—^Traneveree ‘ Interceptor Drain tatlet Pipe

■ Longitudinal Collector Pipe

Legend

Traneveree Draine on Superelevated Curve (16)

So-called “Californian Drains” are sometimes used. These consist of parallel and closely-spaced tubes, arranged vertically or sub-horizontally. The tubes can be perforated or grooved and are installed into natural ground or fill. The main objective of such drains is the reduction of pore water pressure in a certain area, in order to lower the water level or treat a retained water pocket. A typical application, to stabilise a slope with high pore water pressures, is illustrated inFig. 13.17.

13.4.2 Drainage Layers for Rigid Pavements

In a rigid, concrete, pavement structure, water filters mainly through

• open joints between the concrete slabs;

• open pavement-shoulder joints; and

• and the areas between the verges and the pavement.

Unless transverse drains are used (see Section 13.4.5), this filter water is collected by the drainage layers and is directed to longitudinal drainage pipes. The drainage layer should be constructed of a non-sedimentary granular material, placed between the pavement’s aggregate base and the subsoil. It is also advisable to place a geo­textile to separate the different layers. The minimum thickness of the drainable base should be approximately 0.15-0.20 m.

Fin drains or drainage screens are also longitudinal drains, manufactured from com­posite materials. Their essential make-up is of two geotextile faces that provide a filter function between the surrounding ground and a rigid plastic core that is sandwiched between the geotextile faces – see Fig. 13.16c. The so-called “drainage core” is, typically, formed of a high-density polyethylene, HDPE, structure. Often this feeds into an integral collector at the bottom. The core permits the water to flow in the plane of the geocomposite (compared with most simple geosynthetics

in which only cross-flow can readily take place). These drains are usually placed at the pavement’s edge, allowing the collector of percolating water (Fig. 13.16a & b). Their main purpose is not to lower the groundwater level. The advantage of fin drains is their narrow thickness, allowing the construction of narrow trenches. This is particularly advantageous in improvement works. The disadvantage is that they are less able to carry high volumes of water and, thus, are less suitable where groundwater lowering of permeable soils is to be attempted.

A fin drain is normally supplied in the form of a roll of constant width. Thus, its height in the ground is constant, too. Care must, therefore, be taken that the base of the slot in which the drain is to be placed will fall evenly towards an outlet rather than following the vertical profile of the pavement alongside which it lies (Highways Agency, 2006).