Category WATER IN ROAD STRUCTURES

In road rehabilitation, it is important to design the drainage at the same time as the other rehabilitation measures (e. g. strengthening of the structure). The designer should aim to recognise locations where poor drainage is the major cause of road damage. In cold regions the springtime is usually the best time for field studies of the drainage system, since the water level is high, frost damage can be seen, vegetation is low and the bearing capacity and slope stability are lowest. Very wet/soft slopes can indicate a wet structure. There may even be water pressure inside the structure, as in Fig. 13.45 where water is pouring out from the slope of a highway just before a culvert (which is acting as a water barrier).

The appearance of certain vegetation (e. g. rushes and willow trees) on the slopes can also be a sign of excess moisture in the geotechnical structures, appearing typically on low permeability slopes. Video or photos can help to record the conditions. Ground radar surveys may also be used to identify extra moisture on

road structures and in subsoil or on rock surfaces and the level of the groundwater. Bearing capacity measurements can be used to identify weak (moist) locations and rock locations (maybe channelling water) below the road. The designer should anal­yse available observations, measurements and the requirements for both the road and its surroundings.

It is of great importance that the draining system is working properly, hence regular checks (e. g. Fig 13.44) and maintenance are required. Every drainage system should be designed to ensure that inspection and maintenance operations are possible and accessible. Usually, the cleaning of the drainage system should be done at the end of the summer, but inspections could be intensified in periods of high precipitation. However, at least every 5 years it is fundamental that there is a proper inspection of every part of the drainage system.

The problems that practitioners encounter are manifold. In the WATMOVE ques­tionnaire survey (see www. watmove. org) the following issues were mentioned:

• The drainage system becomes clogged with fine materials,

• Crushed pipes,

• Poor outlet conditions, i. e. outlets have negative slopes,

• Root penetration,

• Generation of ferrous hydroxide and calcium carbonate,

• Insufficient capacity,

• Inadequate water velocity,

• The (plastic) cover of the inspection well at the slope may be damaged (some­times due to snow clearance of the road).

An earlier study by Dunnam & Daleiden (1999) revealed similar problems as well as blockages by vegetation and animal nests.

In order for maintenance works to take place and to ensure a long life for the road, it is essential to plan a maintenance programme, based on a series of procedures, measures, actions and practices. The establishment of maintenance actions, of a systematic nature, and emergency actions should be part of that intervention plan.

The systematic plans, i. e. those used in normal conditions, aim to guarantee that the drainage systems remain in a good working condition. These plans will com­prise, at least, inspection actions, vigilance and cleaning (which include the removal of sediments), clearance of channels and ditches and the removal of vegetation.

Inspections could be visual or by video surveillance of pipes, depending on pipe diameter. Close-circuit TV cameras are available, mounted on the end of umbilical cables and incorporating lighting, to achieve down-pipe inspections (Dunnam & Daleiden, 1999; Fleckenstein & Allen, 1996; FHWA website).

The intervention plans, for accident/emergency situations, requires fast inter­vention of maintenance teams, to mitigate the negative impacts on personal safety and environmental contamination. To do so, a sequence of procedures should be established and adapted for different scenarios. These will require prior surveys, covering:

• The boundaries of water areas and environmental compartments;

• aquifer vulnerability;

• sensitivity of each compartment;

• existing drainage systems;

• the assessment of potential hazard sources (including industrial areas); and

• transportation requirements for dangerous substances.

In order for these plans to fully work, one must ensure proper management of the road infrastructure with maintenance programs, specialist human resources and operational and logistical support.

The tests to be performed on site, or in parts of the project, are defined in the testing plan. To verify the characteristics and behaviour of the materials to be used, samples must be taken and tests performed as specified in the contract specifications. These specifications typically define the type and frequency of the tests as illustrated in Table 13.4. The codes used are listed at the end of the Table.

13.7.1 Construction

When construction commences it is necessary to be responsive to the geological and geotechnical conditions encountered and not to adhere to those assumed at the de­sign stage. Therefore the in-situ conditions should be carefully inspected throughout the construction process. Also, care should be taken that the construction activities do not have a deleterious effect on drainage.

The planned drainage systems for a project can only be finalized during the work’s execution, when the local geotechnical conditions are fully understood. Thus, it is important that an adequate specification is produced for the anticipated
type of drainage system and for suitable materials, so that the implementation teams are able to deliver the best solutions.

The many phases which constitute the construction of a road are sometimes de­layed, and this can be drainage related, due to:

• Alteration in design flows;

• Obstruction of the surface and underground water flow path, due to earth moving and material placement;

• Possible surface and underground water contamination, due to earth moving, machine cleaning and associated incidents;

• Increase in the soil’s compaction in the areas where there is flow to or from an aquifer; and

• Alteration in the hydrological regime, as a consequence of the disturbed soil caused by the construction of the road structure.

Thus, it is necessary to plan the phases of project to adopt preventative measures so that there is:

• Adequate design flow, taking into account the future plans of the drainage area (land use) as well as current needs;

• Optimization of the programming of the earthworks and drainage activities, tak­ing into account the season in which they are to be performed;

• Adoption of a plan to control erosion and soil sedimentation; and

• A work phasing plan, so that the heavy trucks and machinery do not cross the water courses, and do not affect the infiltration and recharge of the aquifer.

Normally, the supplier is obliged to demonstrate the way in which he establishes, maintains and implements a Quality Management System (QMS) to control the construction. In Europe, this quality management system must comply with the re­quirements of the ISO 9001:2000 standard as well as with any national or European legislation that might be relevant. The system must account for regulations applica­ble to quality assurance as well as for the QMS and should be based on a Quality Plan assembled for the construction project that contains the procedures, inspec­tion and testing plans, work instructions, audit plan, training and information plan, as well as other plans containing the different specialist activities involved in the project.

Some typical design details are presented in the following figures. Figure 13.40 shows the typical details for a drainage channel to be installed in the verge between the pavement and a cutting slope, Fig. 13.41 shows the likely details to be employed

hard shoulder, hardstrip
or carriageway

min. 1.20 m

Fig. 13.40 Cuttings – Standard concrete channel in verge with drain and pipe

Fig. 13.41 Drain for use in conjunction with concrete barrier and linear slot drainage channel

hard shoulder, non-pavement verge made

hardstrip or of impervious material

carriageway

Fig. 13.42 Cutting – Combined surface water and groundwater filter drain and drain pipe

concrete

channel

in the vicinity of a traffic safety barrier such as appears in a central reservation, Fig. 13.42 shows a typical combined drainage system for both surface runoff and subterranean water collection whilst Fig. 13.43 shows a subterranean fin drain ar­rangement for pavement sub-drainage.

In order to estimate the water flow into drainage pipes, one should differentiate between the two distinct situations introduced earlier:

• pipes above the water level (intersection drains); and

• pipes below the water level (groundwater lowering) drains.

When the drainage system is above the water level, the infiltration water from edges, channels and gutters, and from some of the transverse drainage that is covered by permeable surfacing, must also be considered according to the relationship of Eq. 13.2.

qL = R ■ B ■ L (13.2)

where qL is the water flow through the pipe (m3/s), R is the surface runoff water flow (m3/(s. m2)), L is the section’s length (m) (see Eq. 13.3) and B is the width of the section requiring calculation (m) (see Eq. 13.3 as shown in Fig. 13.39).

n

B ■ L = Y, bi x I, (13.3)

i = 1

where b and l are individual widths and lengths, respectively (Fig. 13.39). Other non-runoff flows can be added into Eq. 13.2 by simple addition, provided they are expressed in units of m3/s.

In cases where the drainage system is used not only as an interceptor but also to lower the water level, dimensioning should consider specific calculations for the underground flow into the drain. In this situation the projected flow should be the sum of the aforementioned value and that estimated through the application of Darcy’s Law.

Such a flow estimate and the depth of installation for the drain are based on the assumption of specific tests and sophisticated calculations. Nevertheless, in most

cases they are revealed to be of limited practical relevance because, in the range of commercial diameters, perforated pipes have a considerably larger capacity for in-flow than is strictly required and the depths at which they are installed usually guarantees the lowering of the water level in the zone between drains.

Having said this, and in order to simplify dimensioning, some authors consider that the in-flow to the drain amounts to approximately 35% of the total flow gener­ated as slope runoff with 20% of the surface runoff from the road pavement being added to cater for flows originating in the road platform, i. e.:

qL = 0.35qE + 0.20qp (13.4)

where qL is the water flow to the pipe (m3 /s); qE is the surface runoff water from slopes (m3/s) and qP is the surface runoff water from the platform (m3/s).

Regarding the depth of installation of the drains, one can make a first estimate using the formula:

{IR 05

z = zro + 0.5 ■ b ■ к (13.5)

where z is drain depth (m), zw is the depth at which the groundwater level should stabilize (m), b is the distance between drains (m), IR is the rate of infiltration into soil (m/s) and K is the soil permeability (m/s).

A specific hydro-geological calculation must be done whenever the drainage sys­tem aims to lower the water level. When deciding on the transverse profile to use in a new road’s project, the details of the subterranean drainage, based on tables and criteria, are very important.

Finally, one should add that in order to satisfy the criteria for self-cleansing and guarantee an adequate geometry, the drains should have a minimum longitudinal inclination of 0.5%, which, in exceptional cases, can be reduced to 0.25%. This inclination should not exceed 20%.

There are various computer software codes on the market that can perform cal­culations of flow as described, e. g. CANALIS, HYDRA and MOUSE.

The construction of new roads can cause impacts on the water resources of affected regions, causing irreversible effects in some cases.

Surface and subterranean water resources are finite and irreplaceable natural re­sources for survival, therefore their protection against abnormal flow and against pollution is of great importance, nowadays making their preservation an indispens­able part of a sustainable development policy. For this reason it is fundamental that a drainage system be developed that regulates the flow of effluents from the pavement platform, that controls the subterranean drainage and that minimises the hydrological impacts of the road on the environment.

An example of pavement sealing was seen in Slovenia for highways crossing very highly sensitive aquifers. There, the following requirements are used (Ajdic et al. 1999).

• In the asphalt layer the permeability can be controlled via the air void ratio (see Chapter 5). The wearing course of the asphalt layer should include not more than 5% air voids and the base course not more than 7% air voids.

• A stress absorption membrane should be constructed using a polymer modified bitumen in a layer of 1.5-2.0kg/m2 and appropriate fine aggregate. Junction sealing should be performed with bituminous tape.

An example of the use of a lining system beneath an embankment is Highway A-15 at Botlek in part of Europoort, Rotterdam (in the Netherlands). Approxi­mately 400 000 t of (municipal solid waste) incinerator bottom ash was used in an embankment for this major roadway construction. The ash was covered with a compacted sand-bentonite mixture with a minimum thickness of 20 cm to reduce water infiltration. The formation (founding level) of the embankment was shaped so as to bring any water seeping through the ash to a sampling point at which quan­tity and quality of seeping water could be monitored. The aim of the cover and lining systems was to prevent the contamination of underlying clean groundwater by infiltrating water that would have passed through the ash embankment material, potentially collecting undesirable contaminants on the way. In fact, due to the heavy industrial use of the land in Europoort over many years prior to the embankment’s construction, the natural groundwater is degraded at a regional scale, so use of the ash posed few risks of causing unacceptable contamination (Mank et al. 1992).

At another site in the Netherlands, a wind barrier was built at Caland (Stoelhorst, 1991). This project, built in 1985, used more than 650 0001 of bottom ash in an em­bankment 700 m long and 15 m high. The ashes were covered with a primary cover of 0.5 m of compacted clay with a sand drainage layer (0.5 m thick) and top soil (1m thick) overlaying the clay layer. The slope of the compacted ash was between 40% and 50%. As at the Botlek site, groundwater quality is monitored, in this case on both sides of the embankment.

For pavement seals the following materials are used:

• asphalt layers;

• stress absorption membranes (“SAMIs”) (see Fig. 13.37); and

• junction sealing material.

SAMIs act over an old cracked pavement surface, sealing the cracks against wa­ter ingress. They often provide a small degree of differential horizontal movement between an old and a new pavement layer so that the relative movements of the parts of the old pavement either side of a crack do not cause a stress concentration in the new, overlying, pavement layer at the same point. Without this ability, a crack in the new layer can quickly form immediately above the old crack – a problem known as “reflection cracking”. Water can then, very quickly, re-enter the road and the overlay will not act as an effective seal (e. g. Fig. 13.38).

13.5.1 Sub-Soil Barriers

Sealing systems can be laid during construction to prevent contaminated water from moving in an undesired direction or to keep natural groundwater separate from contaminated road runoff and road construction seepage waters. In many places in which geomembrane barriers could be placed, spillage of petroleum and diesel from vehicles is a possibility. Sealing systems are used for sealing highways and embankments.

During the design of a sealing system the designer should take into account the sensitivity of the area, crossfall and alignment of the road. When the seal is placed on a slope, a very important part of the design procedure is the analysis of the slope stability as the shear strength between the layers of the sealing system may be much less than found between soil layers, thereby significantly reducing the factor of safety against slippage.

Liners are part of the sealing systems that consist of a base, a sealing layer and a protection layer. The base is that part of the construction on which the sealing layer should be placed and it can consist of natural soil or artificial aggregates placed on the natural soil. Materials selected for the granular base should not consist of sharp or large rock blocks that could damage the sealing layer. The base should be stable and compacted to at least 92% of optimum (Proctor) density. An important aspect is to ensure a planar base.

The sealing layer provides the low permeability of the sealing system. The re­quired thickness depends on the sensitivity of the area that is to be protected and on the quality of the material used to make the sealing layer. The material of the sealing layer also depends on the purpose. Materials for sealing the pavement area will be different from those materials used for sealing the slopes of an embankment.

The protection layer is intended to protect the sealing layer from traffic (e. g. break­through caused by vehicle crashes), damage from the placing of coarse or sharp overlying material and negative climatic influences (e. g. freezing and drying). For this purpose natural materials such as soils, crushed rock and some artificial materi­als, such as concrete materials, are used. If necessary, the surface of the protection layer should also be designed against erosion due to high water flow velocities above it. Figure 13.34 shows an example where altered land use has increased run-off and flow speed to > 1 m/s over a trench so that the existing protection provided by 0/100mm crushed rock is no longer adequate. This could rapidly cut down to an underlying groundwater, damaging its quality. A high performance protection layer would be needed over a sealing layer in such a situation.

Retention tank

Infiltration tank

Fig. 13.32 Environment with extremely high sensitivity. A combination of various types of water treatment is included

For materials in the sealing layer, natural and geosynthetic barrier materials (GBR) can be used. The most common natural material used for sealing is clay, which is sometimes available on the construction site or in clay pits that are posi­tioned in the vicinity. Materials available at the construction site can be enhanced with the addition of clean bentonite clays.

Geosynthetic barriers (GBR) can also be used. They come in various forms:

• polymeric geosynthetic barrier GBR-P;

• bituminous geosynthetic barrier GBR-B; and

• clay geosynthetic barrier GBR-C.

Four types of geosynthetic barriers application may be distinguished (prEN 15382, 2005): [31]

• high GBR on side slopes – where the GBR is installed above the drainage col­lection system as a high laying sealing system and covers the side slope of the road to prevent an overflow of the road surface runoff;

• deep GBR in central reserve – where the GBR is installed under the drainage collection system and covers the section in the central reserve, where sealing is required; and

• high GBR in central reserve: where the GBR is installed above the drainage collection system as a high level sealing system and covers the section in the central reserve where sealing is required.

Polymeric liners are supplied in rolls and must be joined on-site to form continu­ous sheets over large areas. This is a specialist task requiring the use of experienced personnel if one desires a reasonable confidence in achieving an effective water barrier.

Geosynthetic barriers are prone to damage by ultra-violet light and by vermin. The first can be overcome by ensuring that the material is covered in soil or other material rapidly after unrolling. Some are more resistant to vermin than others, but it is always sensible to consider ways of preventing damage from animals (perhaps by providing a light steel mesh cover a little above the placed geomembrane as a vermin barrier).

Clay sealing sheets are also available, especially when clay material is not present on the site. Their advantage compared to on-site materials is their precisely defined properties that allow easy design and construction. Typically, these comprise a thin (circa 1 cm) layer of rather dry bentonite formed between two geo-textile sheets. Supplied as a roll, these liners are unrolled on site and overlapped without seaming. Once buried and in contact with water the bentonite sorbs very strongly, causing significant expansion. This expansion develops an effective seal between the liner and the soils around it and between one roll of liner and another. If punctured, the bentonite expansion means that holes self-seal. Bentonite is an excellent sorbent of many species of heavy metals and some organics. Bentonite clay liners should be properly maintained and they should be prevented from drying out. If this happens, cracks up to some centimetres in width can appear and the sheet will no longer act as a the barrier. In that case bentonite layers can be more permeable than a sub-base.

prEN 15382 (2005) does not advise that geosynthetic barriers be connected to drainage systems when embedded in shoulders or slopes. Figure 13.35 shows a typical application of geosynthetic barriers. Details of technical solutions may be found in prEN 15382 and in RiStWag (2002).

Placing geosynthetic barriers on slopes with a thin cover of soil and lack of suffi­cient overburden to compensate for uplift pressures are elementary misapplications (Fig. 13.36).

The following figures illustrate some options for treatment systems. Figure 13.29 shows a system where little or no treatment is needed. Figures 13.30 and 13.31 show situations in which progressively more treatment is provided, while Figs. 13.32 and 13.33 show situations in which “hard” treatment solutions with settlement, reten­tion and infiltration tanks are provided in some manner. Sometimes settlement and retention tanks can be entirely fabricated from concrete, on other occasions they can be formed of excavations in soil at a location where settlement of solids onto and in the soil is acceptable as the soil has been carefully selected and prepared to prevent

Fig. 13.31 Environment with extremely high sensitivity

long distance movement of the contaminants (e. g. by the use of soils with a high sorptive capacity that have been carefully compacted as a liner).

The potential of wetlands as treatments (Fig. 13.31) was illustrated in Chapter 12 (see Figs. 12.1 and 12.2) and described in Section 13.3.8.