Category WATER IN ROAD STRUCTURES

A trench drain consists of drain wrapped in geotextile, see Figs. 13.13-13.15. The drain is made of a mineral material such as a rounded or crushed aggregate. Origi­nally either no carrier pipes orun-jointed pottery pipes were employed at the bottom of such drains. Nowadays, several materials are used for this type of pipe, from perforated or porous concrete, to PVC and fibreglass, the last ones with grooves or perforations. The pores, joints, perforations or grooves are designed to allow water collection. Typical diameters vary from 150 to 200 mm, with a longitudinal gradient that satisfies the self-cleaning condition (> 0.25%). Whenever these drains reach

their maximum capacity, a lateral pipe with the adequate discharge capability should be placed underneath the drains to take away water from the trench. The geotextile is employed as a filter which prevents migration of fine soil particles into the drain and its silting-up. The water permeability of the geotextile should allow water to flow freely from the surrounding ground into the drain. The characteristic pore opening size, 090, of the geotextile used for the trench drain is selected to prevent mixing of soil with aggregate (see Section 13.3.9). Procedures for selection are provided by (e. g.) Christopher & Holtz (1985), Joint Departments (1995) or Koerner (2005).

2 Readers may like to identify violations of safe working practice which can be seen in this picture. The inclusion of this photograph does not mean that the authors condone such practice!

The cross-sectional area of drain is determined depending on the amount of water which ought to be carried away and on the grain-size distribution of the mineral material in the drain. Determining the dimensions is usually performed accord­ing to empirical procedures as the materials, climate, groundwater conditions and materials all have a pronounced influence upon the water that is to be conveyed and that can be carried by the drain.

The pavement layers must be shaped so as to ensure that water in them moves towards the drain and the base of the trench must be substantially lower than the layer to be drained:

• to ensure a suitable hydraulic gradient towards the drain which will drive drainage action;

• to aid entry across the geosynthetic liner which may require a small head differ­ence before wetting is achieved and water passage possible; and

• to ensure that here is adequate capacity within the drain to hold exceptional water flow events without a risk of water flowing from the drain into the layer.

Thus a distance from the base of carrier pipe to the bottom of the layer to be drained of about 0.5 m is typical. Successfully designed and properly made, trench drains have a lot of advantages. Among other things there are (Uzdalewicz, 2001):

• a lengthy life in which it works effectively;

• low construction and operating costs;

• the possibility of managing the “area above the drain”, for example as a footway. The following are the stages of construction of a trench drain:

i) digging of a narrow trench excavation;

ii) cleaning out the narrow trench;

iii) lining of excavation surfaces with geotextile (along the excavation or cut set across excavation) (e. g. Fig. 13.14);

iv) placing a covering of aggregate at the bottom;

v) installing the drain pipe, if needed;

vi) filling the drain volume with aggregate;

vii) closing the drain and jointing of geotextile edges (e. g. with “U” shaped clamps) (e. g. Fig. 13.15a &b); and

viii) covering the closed drain surface with 3-5 cm (or more) of top soil or other low permeability cover (except where surface runoff is also to be collected – see Section 13.3.5).

The minimal length of overlap for geotextiles should be at least 30 cm (Edel, 2002). The longitudinal direction of overlaps should be consistent with the direction of water flow through the drain. The aggregate filling the trench drain should be compacted (in layers).

13.4.1 Lateral Drains

In order to stop water getting into a pavement foundation and to avoid the con­sequent reduction of its support capability, the construction of ditches or trenches is a common procedure. They are usually filled with highly permeable material, wrapped in geotextile and with a perforated tube or porous material near the bottom. Alternatively a geo-composite based drainage system, known as a “fin” drain, may be used. Fin drains are, typically, only a few centimetres thick. Trenches or ditches are, usually, excavated by digging plant while narrow trenches for geo-synthetic fin drains may be dug or may be saw-cut.

These types of systems, named longitudinal drains, are placed in parallel with the road’s centre line, usually at the edge of the pavement structure, and will lie under the surface water channels or gutters whenever these are permeable.

Lateral drains, according to their function, can be divided in two main groups, interceptor drains and water table lowering drains.

Water table lowering drains allow the lowering of the water level in the pavement structure platform, when close to the pavement’s under-side. They are normally placed at depths varying from 1.2 to 2 m below the pavement surface so as to reach the water table (or to reach the height to which the water table may sometimes rise). Thus they keep the water table low and help to prevent the water from being pulled up to the pavement layers by capillarity.

Interceptor drains are drains of the same type as the previous ones, although they usually go down less far into the ground. They aim to ensure the internal drainage of the pavement and to intercept percolating water.

From a pavement point of view, it is desirable to keep the aggregates in the pavement as dry as reasonably possible – so as to promote strength, stiffness and resistance to deterioration (see Chapters 8-10) – but the pervious pavement concept is directly in opposition to the general principle. Therefore other strategies have to be taken to make sure that the pavement provides sufficient strength, stiffness and durability for traffic loading. Normally this requires particular attention to aggregate quality. While, typically, aggregates to a 4-40 mm grading range, or similar, are required to ensure a suitable pore space for water storage, their requirement for durability may be higher than conventionally. There are greater stresses on particles conse­quent on them having an open structure with fewer contact points (see Chapter 9, Section 9.2). Also, more load spreading should be achieved, if possible, in the over­lying pavements. Furthermore the design must allow for softening of the subgrade caused by the water stored in the pavement. Some design information is given in Woods Ballard & Kellagher, (2007) with more information given in Pratt et al (2002) and, for block pavements, Interpave (2005).

The slow percolation of contaminated runoff into the pavement through a porous aggregate layer to an outlet substantially slows water movements, provides the possibility of filtration and allows water to pass by a large surface area of stone. By these means the water drops the suspended, fine, solids that it is carrying into the pore space of the pavement layers. The contaminants tend to adhere to the surfaces of the porous material’s particles – particularly to the fine fraction as it provides the largest area of fines. Hydrocarbon contaminants also tend to be sorbed to the solids. For these reasons the water that leaves the pavement is substantially

Normally, rain falling on impermeable surfaces, such as a road surface, quickly enters the drainage system, arriving at the outfall to river or stream very soon after falling from the sky. It is estimated (Interpave, 2005) that in a fully forested, lowland catchment only 5% of rainfall will flow across the ground surface, the remainder will be delayed by the vegetation to such an extent that it will soak into the ground. For agricultural land with less vegetation 30% may flow across the surface. However for an urban environment with piped stormwater drainage systems 95% is carried to the surface water bodies. For this reason, in an urban environment the water arrives much more quickly than if it had taken a natural route (movement over vegetated surfaces and by percolation through the ground). Thus the flow pattern in the river or stream rises and falls much more quickly, and reaches higher maximum flow values and lower minimum flows, than it would in a non-built-up area (see Fig. 13.11, explained in more detail below). This “peaky” flow leads to increased frequency of flooding and to reduced irrigation flows in times of drought as there is less water soaked into the ground to provide dry weather seepage supplies to surface water bodies. For these reasons, if a rapidly filling, but slowly emptying store can be provided in the pavement then this undesirable effect will be reduced.

In the example illustrated in Fig. 13.11, three rainfall events occur within a two – day period. It is assumed that this particular pavement can hold 20 mm of rainfall (i. e. 201 per m2). The first storm (0-8 h), which peaks at 5mm/hour, almost causes the pavement to fill with water. The pavement has drained back to half full when

the next storm (18-22 h) arrives. This storm, with peak loading of 8mm/hour does cause the pavement to fill for a short time, whereas by the time the third storm arrives (40-44 h) the pavement has drained further and the 5 mm/h peak intensity storm is just handled by the system. Note how the maximum outflow (egress) is about 1.5 l/m2 when the pavement storage is full (21-23 h) very much less than the peak rainfall of 8 l/m2 which would otherwise arrive into the drainage system and be fed to a stream or river.

This benefit is well illustrated by the performance of car park drainage pavements six years after initial construction reported by Brattebo & Booth (2002). Two of their pavements had grassed unbound surfaces (sand and gravel) held in a plastic grid arrangement while another two used concrete block surfacing with about 40% and 10% open area. Figure 13.12 shows rainfall and run off for a grass-sand and a reference asphalt pavement. Even for the heavy rainfall event (121 mm in 72 h) only 3% of this ran-off the surface of the permeable pavement whereas run-off from the asphalt pavement closely follows rainfall. The other three permeable pavements gave even less run-off.

A book on Water in Road Structures would not be complete without a brief descrip­tion on the use of the pavement as a water store. Pervious pavements (see Chapter 5, Section 5.7) can be taken one step further and not only used to convey water away from the surface, but can also be used to temporarily store the water in the pavement. There are two principal reasons for doing this

• hydrograph attenuation and

• water quality improvement.

Here, the use of pervious pavements may only be briefly discussed, but inter­ested readers may find out much more in the book devoted to the topic by Ferguson (2005). Typically they comprise highly permeable surfaces (e. g. of concrete blocks or stone cobbles) with a coarse, open-graded layer underneath that will act as a water storage and transport layer. It may also retain water that will, ultimately, soak away into the subgrade.

Aggregates and geotextiles employed in drainage systems have to operate next to soil or aggregate that surrounds them. To achieve good performance, they must re­main permeable, retain the surrounding ground or aggregate in place and not clog. These requirements are met by defining specific performance criteria. The first of these is the non-sedimentation criteria, which is usually provided by aggregates with no plasticity and a limited amount of fines (usually no more than 5%).

The following filter criteria should be fulfilled:

• F15 (filter material)/D85 (soil) < 5

• F15 (filter material)AD15 (soil) > 5

• F50 (filter material)/D50 (soil) < 25 where the number, n, following F is the size of the sieve through which n% of a filter soil will pass and the number, m, following D is the size of the sieve through which m% will pass of the soil being drained. This terminology was introduced in Chapter 2, Section 2.4.1.

Typically, literature about geosynthetics also defines the following filter crite­ria (e. g. Bergado et al. (1996) who give a long list of references on the subject, Christopher & Holtz (1985) or Koerner (2005)). Firstly, to ensure that the filter holds the soil in place and doesn’t let it through – the “retention criterion” – the following requirements are, typically, set:

• for fine-grained and erodible soils

• for fine-grained cohesive soils

• for problematic (gap-graded) soils

• for coarse-grained and well graded soils where O90 is the effective opening size – 90% of the openings in the geosynthetic are smaller than this value, and Cu is the Uniformity Coefficient (see Chapter 2, Eq. 2.4).

Next, to ensure that particles of soil don’t enter the filter’s pores and clog it – the “clogging criterion” – the following requirements are, typically, set: • O90 < 5D90 although, for cohesive, soils O90/D90 may be larger.

There will usually be a “permeability criterion”, too, as the geosynthetic should remain at least as permeable as the protected soil throughout its lifetime. For this reason, the permeability of the geosynthetic should be N times greater than that of the soil it is filtering at the outset, where N typically varies between 10 and 100. To achieve this criteria such as the following may be set if permeability values are not obtainable:

• for fine-grained cohesive soils O90 > 0.05 mm

• for coarse-grained and well graded soils O90 > DJ5 and O90 > 0.05 mm.

Where a filter is provided between the soil and a grooved, slotted or perforated medium such as a porous plastic pipe, the relationship between the dimension of the filter material and the grooves or the tube perforation size should be as follows:

• F85 > 1.2 times the grooves’ width, or

• F85 > 2 times the perforations’ diameter.

Where it is difficult to know the best design solution, drains can be filled with aggregate wrapped in a geotextile which has a coefficient of permeability 10 times more than the surrounding soil’s coefficient of permeability.

The characteristics required of geotextiles and geotextile-related products for use in drainage systems is contained in European Standard EN 13252 (2000). InEurope, all geosynthetics used in drainage systems should be CE marked.

Where there are environmentally sensitive areas or high traffic flows, increasing the risks of accidents and generating contamination from wear, water flowing over the surface of the road and the embankment should be collected before it can soak into the ground in an uncontrolled manner. Water seeping through the earthworks and collected by a drainage layer has to be led, by virtue of a fall in the drainage layer and by shaping it, to collection points. At the collection point the water quality can be monitored and, according to the quality measured, it can be fed to a soakaway (Section 13.4.7) or piped away for treatment.

Water that arrives at an outlet from a drainage system may need treatment to bring the water quality to an acceptable level. In extreme cases, conventional waste-water treatment systems can be installed, but such a level of treatment is seldom required, except where water has passed through some contaminated soil body. If the water could, following a traffic accident or similar, contain spillages of fuel or cargo from
traffic, then it will be necessary to install a oil/fuel separator. If these are installed then it will usually be sensible to provide a retention pond on the upstream side of the separator. This lagoon can act as a location to temporarily store the spilled fluid from which it can be extracted and taken away for off-site handling. The aim will be to give the road operator or environmental manager sufficient time to respond to the spill before the fluid is allowed to enter a surface water body or allowed to soak down to the groundwater. Storage lagoons also provide a zone in which solids can sediment from water and provide a means of attenuating the peaks of hydrographs.

Man-made wetlands and reed beds can also be provided as water purification areas (see Chapter 12, Section 12.3.1). They are particularly suited to groundwater outlets as the water arrival will be more consistent, at a low flow volume rate, than for surface water runoff and there will be much less need to provide storage for excess water arriving after a rain storm. Reed growth is much more suited to this consistent supply of water, while the area to be occupied by the water purifica­tion planting can remain relatively small. They often have the additional advantage of providing an enhancement to the pre-existing environment. Naturally occurring wetlands are under serious threat in most countries and they should not be used for this purpose.

The WATMOVE questionnaire sent to European road authorities (see www. watmove. org) revealed that settlement of solids and oil separation are the most common treatment methods used in practice, see Fig. 13.10.

Fig. 13.10 An overview of treatment methods used in Europe. The number is the percentage of countries in the survey using the treatment

It is a design objective that snowmelt and rainfall should have fast access to the side of the road and the drainage system. This avoids surface ice formation and skidding, but also protects the structure so that as little water as possible filters through to the pavement. Edges, kerbs, channels and runoff barriers must be kept clear by routine maintenance. Sufficient snow storage (and snowmelt) capacity on the road side must be included in the design. Dry structures and the road bed should be effected less by frost.

During spring-thaw there is considerable increase in the moisture of many un­bound materials. The magnitude of the spring-thaw weakening (on bearing capacity and slope stability) depends, very much, on the functioning of the drainage system.

Due to snow cover on the road side, frost depth is typically deepest in the middle of the road and this will cause uneven frost heave across the section of road. The heave depends on how long the freezing front remains in the frost susceptible soil, the soil characteristics and also how much water there is (see Chapter 4, Section 4.6). If the frost heave differential is big enough, it can cause surface roughness, cracks, breakage of the surfacing and lifting stones (causing first roughness and later on surface cracking).

The permeability of most soils decreases when frozen. In springtime the frost can form an almost impermeable water barrier in the soil below the road since thawing will be fastest from the top and in the middle of the road. In that case the melted water will flow in a longitudinal direction until it can find an exit route, at which point it will flow to the road side or into the subsoil causing localised seepage and/or erosion problems.

Low permeability side-slopes decrease the infiltration of rainfall, but they can also act as flow barriers for the seepage of water out of the pavement structure and thus increase frost damage as well as decrease the strength of the structure. In that case openings with gravel filling at certain intervals and on low-lying locations can help the situation as in Fig. 13.9. Another option is a drain. A drain is the only option if the road structure is deeper than the open ditch at the side of the road.

If there is no outlet ditch or drain available, and the amount of water is small, the water may be soaked into the soil. A transition wedge of coarse material and non water susceptible structure can be used to avoid frost damages at a low lying location where water is periodically accumulated.

In cold climates, where frost heave in winter is a problem, an open-graded layer can be constructed at the bottom of the pavement to prevent water being pulled from the subgrade due to a freezing front in the pavement. This will lower frost heave in the road construction by decreasing the water content compared to pavements with­out a capillary break. However, frost-susceptible layers will always heave to some degree if there are freezing conditions. For example, Hermansson (2004) reports a field study where an extremely well drained soil heaved 80 mm during a period of two months. The open-graded layer will not affect frost heave in a susceptible sub­grade if the freezing front gets that low in winter, but it will significantly reduce the effect on the highway surface by allowing the road to bridge differential subgrade heaves.

More information on drainage of low volume roads in cold climates is given in a report by Berntsen & Saarenketo (2005).

According to Huang (2003) the placement of a drainage layer directly under the asphalt or concrete pavement surface is preferable, because the water in the pavement, either percolating through cracks or entering from the sides, is quickly allowed to move to a lower level from where it can easily be drained. No pore pressures can develop because of the high permeability and rapid dissipation prop­erties of the OGDL eliminating any chance of pumping occurring. Furthermore, it eliminates the final, negative, effects of water or frost. A properly designed and con­structed permeable granular base layer may have a similar structural performance as a conventional base. However, OGDLs have a number of disadvantages:

• the deficiency of fines in the drainage layer may cause stability problems. They are difficult to compact into a stable foundation on which higher pavement lay­ers can be constructed. Perhaps even more problematic is the trafficking of the OGDL by the construction plant that will lay the next pavement layer;

• the water in the sub-base cannot drain readily into the drainage layer;

• if the outlet becomes blocked the drainage layer becomes a reservoir for pore water that can develop pressure pulses under passing traffic thereby causing ero­sion and loss of bearing capacity. This can be a particular problem under jointed pavements where ingress of water along joints may be rather large once joint sealants have failed.

A recent study in Finland showed that more open-graded sub-base is a satisfac­tory means of reducing the moisture content of a granular base course, but that such a sub-base should not be used beneath an open-graded base course due to stability issues.

Typically, open-graded unbound granular materials are used as permeable layers, however the use of cement or asphalt treated permeable bases can add some extra strength and stability to the drainage layer (and, hence, the pavement) if needed. The resulting material is some kind of buried “no-fines” concrete or porous asphalt. Normal OGDLs are relatively expensive to source, due to the wastage of the fine fraction, and expensive to compact due to the stability issue. Treatment only adds to this.

Table 13.2 Example of gradation of unbound granular permeable bases in Spain

Sieve size (mm) 25 20 8 4 2 1 0.500 0.250 0.063

Passing (% by mass) 100 65-100 30-58 14-37 0-15 0-10 0-6 0-4 0-2

Although authors differ over the precise value, pavement layers can be consid­ered as permeable when their coefficient of permeability exceeds approximately 10-5 m/s. Therefore, many of the conventional dense-graded unbound granular layers cannot be considered as permeable. Table 13.2, shows a typical grading of permeable granular layers used in Spain, where material passing the 1 mm size sieve is limited to 10% (by mass). Since sometimes segregation problems have been detected when pavers are used to lay the layer down, a uniformity coefficient, Cu, less than 4 is required. In addition, in order to get enough stability during the con­struction of the layer above, the permeable base must mostly be made of crushed aggregate.

Illinois experimented with the use of thin treated OGDLs (7.5-15 cm thick) beneath both concrete and asphalt surfaced pavements during the late 1980’s and early 1990’s (Winkelman, 2004). Four projects were constructed to monitor the effectiveness of the drainage layer and the performance of the pavement. Five ad­ditional projects were constructed based on the early performance of the moni­tored projects. However, continued monitoring of the initial projects, and additional projects, indicated two of the pavements were quickly deteriorating. Surface pave­ment distress, severe lane to shoulder differential settlement, and high pavement deflections for these two projects indicated a failure of the pavement structure.

The immediate response was to stop using OGDLs, although the non-failing pavements built over OGDLs were not removed. Six of the projects were monitored through from construction until 2003 to ascertain the longer term performance of these pavements and to see whether the OGDL had been beneficial. Some contained cement-treated OGDLs, some asphalt-treated OGDLs.

It was concluded that:

i) The use of an OGDL is more expensive than the use of a standard stabilised base material or lime modified soil;

ii) Some limited benefit due to drainage may be achieved, particularly early dur­ing the life of the pavement. However, the longer term performance of the monitored pavements was not much better than that typical for other pave­ments in the area built without OGDLs;

iii) The intrusion of fines from the subgrade and the aggregate separation layer into the OGDL resulted in settlement, faulting, and eventually premature fail­ure of the pavement, therefore, the use of a geotextile fabric or dense graded aggregate filter under the OGDL to prevent the intrusion of subgrade fines is recommended;

iv) The benefits of using cement-treated OGDLs over asphalt-treated OGDLs, or vice versa, could not clearly be determined;

v) For continuously reinforced concrete pavements, the limited benefits of using an OGDL do not outweigh the increased costs, construction difficulties and maintenance requirements;

vi) Segregation is problematic during construction and careful use of plant is needed to minimize it; and

vii) Careful consideration should be given to subgrade soil analysis, topography and surface drainage, and pavement type. OGDLs are not suitable for all situ­ations.

Most of the pavements were quite heavily trafficked and it may be that different findings would have been obtained on more lightly trafficked pavements.