5. afrennslisvandamál og hvernig má komast hjá þeim

5.1 Almennt

Í ROADEX verkefninu hefur afrennslisvandamálum verið skipt í þrjá aðalflokka; a) viðhaldstengd, b) hönnunartengd c) önnur sértæk vandamál. Jarðvegsaðstæður, landslag og veðurfar er mjög breytilegt á norðurjaðrinum en þau vandamál sem upp koma eru yfirleitt hin sömu. Eina minniháttar undantekningin er Skotland og Írland þar sem grasbrúnir eru algengar.

Eftirfarandi kaflar lýsa hverju vandamáli, hvernig á að greina það og lausnir eru settar fram

5.2. Viðhaldstengd afrennslisvandamál

Flokkurinn „viðhaldstengd vandamál“ fjallar um öll þau vandamál sem hægt er að forðast með því að hafa rétta stefnu í viðhaldsmálum og beita áhrifaríkum aðgerðum. Nú til dags er viðhaldsvinna á norðurjaðrinum í höndum verktaka og því hafa útboðsgögn er varða afrennsli orðið mjög mikilvæg.


Lélegt viðhald á afrennsli getur haft mikil áhrif á slitlög og árlegan kostnað vegna viðhalds þeirra. Einnig getur það haft áhrif á umferðaröryggi. Af þessari ástæðu þurfa nákvæmar skilgreiningar og kröfur fyrir hvert viðhaldsverk að fylgja útboðsgögnum og hafa þarf eftirlit með að verktaki fari eftir þeim.


5.2.1. Árstíðarbundin afrennslisstjórnun

Margar aðgerðir í afrennslismálum eru árstíðabundnar, sérstaklega við vetur og vor. Slíkar aðgerðir þarfnast stöðugs eftirlits með veðri og viðhaldsverktaka með nef fyrir tímasetningum aðgerða til þess að varna vegaskemmdum, og lengja þar með líftíma slitlaga. Ef slíkum viðhaldsaðgerðum er ekki sinnt getur það haft meiriháttar áhrif á árlegan viðhaldskostnað vegarins. Gott umbunarkerfi fyrir viðhaldsverktaka sem standa sig vel, sem og stífar sektir fyrir vanefndir, er hagstæð leið fyrir veghaldara þegar upp er staðið. Nokkrum lykil árstíðarbundnum viðhaldsaðgerðum er lýst í næstu köflum. Aðgerðum vegna frosinna ræsa er lýst í kafla 5.2.3 ásamt öðrum viðhaldsaðgerðum vegna ræsa.


Stýring upphleðslu snjós á öxl

Nokkurs konar snjóveggir geta myndast á axlarbrún. Slíkir veggir eru ásættanlegir þegar kalt er í veðri og geta verkað sem frosteinangrun fyrir vegfláann. Þegar hitna fer í veðri bráðna slíkir veggir þiðnað og vatn flæðir inn á veginn sem dregur úr öryggi vegfarenda og burðarþoli vegaxla. Af þessari ástæðu ætti að fjarlægja slíka veggi áður en bráðnum hefst. .


Klakamyndun

Vegir sem liggja í hliðarhalla þar sem kalt er í veðri geta orðið fyrir klakamyndun, sérstaklega ef að vatnsborð ofan við veginn er nálægt yfirborði lands og grunnvatnsflæði er lítið og hægt. Þar sem slíkar aðstæður eiga sér stað flæðir grunnvatnið sem lekur ofan í skurð ekki nógu hratt, vegna hindrunar eða lítils halla, og fer að frjósa í botni skurðarins. Þegar meira vatn bætist svo við fyllist skurðurinn af klaka. Klaki getur einnig myndast í skurði ef að grunnvatnsflæði um veghlotið stöðvast vegna frosts í veginum. Klakamyndun er einnig verið algengt vandamál í bergskeringum í halla.


Eina lausnin við klaka sem myndast óvænt og hratt er að grafa burt ísinn til þess að tryggja að vatn geti runnið frá vegsvæðinu. Hefja ætti slíkar aðgerðir áður en bráðnum hefst. Sýnt hefur verið fram á í ROADEX verkefninu að vatn geti komist inn í veghlot og valdið missigi í veginum og aflögun axla.


ROADEX hefur einnig sýnt fram á að frosin, stífluð eða ræsi sem einfaldlega vantar getur verið ástæða fyrir klakamyndun. Viðhaldsverktakar ættu því að fullvissa sig um að ræsi á vegköflum sem liggja undir klakamyndum séu opin. Klaki getur einnig myndast ef snjór frá biðstöðvum eða áningarstöðum er fluttur ofan á skurð og samanþjappaður snjórinn byrjar að stífla skurðinn.




Ef klaki myndast hvern vetur á sömu vegköflunum ætti að huga að forvörnum eða endurhugsun á afrennsliskerfi. Góð lausn gæti verið að koma fyrir hitalögn í botni skurðar til þess að ekki frjósi í honum. Staðbundin sólarrafhlaða getur framleitt nægt rafmagn fyrir slíka hitalögn. Djúpafrennsli í efri hluta vegar getur einnig hjálpað til við að sporna gegn klakamyndun. Komast má fyrir klakamyndum í vegrásum og hliðarskurðum með sérstökum afrennslislausnum sem stuðla að afrennsli vatns áður en það kemur að hliðarskurðinum. Slíkar lausnir, sem prófaðar voru í Arctic Road Construction verkefninu í finnska Lapplandi á 9.áratuginum hafa virkað vel eftir 20 ára reynslu. Sjálfbærasta lausnin í flestum tilfellum er þó að koma fyrir nýju þverræsi á þeim stað þar sem vatn lekur in í hliðarskurð og myndar klaka.


Afrennsli og burðarþolstap í þáatíð

Sýnt hefur verið fram á í ROADEX verkefninu að lélegt viðhald á afrennsli vega getur átt þátt í vandamálum sem upp koma í þáatíð, sérstaklega á malarvegum, en einnig á vegum með bundnu slitlagi. Ef snjór er ekki fjarlægður af vegöxlum nógu tímalega, eða ef skurðir eru fullir af vatni, getur vegur orðið fyrir aflögun í styrktarlagi, þ.e. hjólfaramyndun af gerð 1. Ef afrennsli er ekki nægjanlega gott nær vegyfirborð ekki að afvatna sig og verða nægjanlega sterkt til þess að bera þungaumferð þegar frost byrjar að fara úr undirstöðu. Gott afrennsli með nægjanlega djúpum skurðum dregur úr, og gæti komið í veg fyrir slíka hjólfaramyndun af gerð 2.


5.2.2. Viðhaldsvandamál vegna vegrása og lokaðra afrennsliskurða


Blautur jarðvegur umhverfis skurði er óstöðugur og getur skriðið niður í skurðbotn og hækkað botninn. Gróður sækir í raka í botni skurðarins og mun að lokum fylla hann ef að gróðurinn er ekki fjarlægður. Ein afleiðing er að grunnvatnsborð hækkar og rakainnihald veghlotsins eykst. Leiðir þetta til lægra burðarþols og vegurinn mun verða móttækilegri fyrir frostverkunum. Rannsóknir ROADEX hafa sýnt fram á að jafnvel lítil hindrun á vatnsflæði í vegrás eða hliðarskurði getur skilað sér í staðbundinni frostlyftingu í veginum og ennfremur sem aflögun axla og sprungmyndun í slitlagi.


Aðalvandamálið varðandi viðhald skurða og lokaðra afrennslisskurða er að illa er staðið að hreinsun. Stefna ROADEX landanna er að þegar skurðir hafa fyllst upp að vissu marki (hærra en 0.3 m frá skurðbotni) þarf að hreinsa skurðinn. Slíkar aðgerðir eru oft í lágum forgangi og oft er auðveldar að fá fjármagn til þess að leggja nýtt yfirborðslag. Þetta þýðir að tæknimenn eru að eiga við einkenninn en ekki rót vandans með þeim afleiðingum að sömu vandamálin koma upp aftur og aftur. Þörfin fyrir reglulega skurðahreinsun er mest þar sem um er að ræða fínkornóttan jarðveg. Slíkur jarðvegur býr einnig við hátt rakainnihald og lágt burðarþol og þess vegna er nauðsynlegt að afrennsliskerfið virki vel til þess að vegur um slíkt land haldist þurr.


Aðal viðhaldsvandmál þegar kemur að vegrásum og hliðarskurðum er að viðhaldaðgerðir eru ekki framkvæmdar nægjanlega oft. Fjarlægja ætti alla steina, tré og annað sem getur hindrað flæði vatns árlega áður en hindraninar geta valdið miklum vandræðum. Auk þess ætti að grafa upp jarðveg sem skriðið hefur ofan í skurð strax og það uppgötvast. Oft er því haldið fram að slíkar aðgerðir séu ekki hagkvæmar en ROADEX útreikningar hafa sýnt fram á hreinsun skilar ágóða þrátt fyrir að um árlegt verkefni sé að ræða.


Algeng mistök við skurðahreinsun er að jarðefni sem fjarlægt er úr skurðbotni er sett á innri fláa skurðarins, í stað þess að því sé ekið á brott. Reynslan hefur sýnt að slíkt efni getur mjög fljótlega runnið aftur ofan í skurðinn og stíflað hana, og þar með valdið frostlyftingu og aflögun vegar.


Ef að jarðvegur við hliðarskurð er svo óstöðugur að grafa þurfi upp úr honum árlega mætti huga að sérstökum lausnum í afrennsli. Slíkar lausnir eru annaðhvort að a) auka stöðugleika skurðbakka, eða b) fylla skurðinn með gegndræpu efni. Vélræn sprautun (hydroseeding) hefur einnig sýnt fram á aukinn stöðugleika skurðbakka.


Í ROADEX verkefninu hafa ýmsar aðferðir verið prófaðar til þess að bæta ástand hliðarskurða og vegrás. Ein aðferð er að styðja við innri og ytri bakka með grófu efni. Eldra jarðefnið er þá fjarlægt, jarðdúkur er lagður út og gróft malarefni (um 250mm) lagt yfir.


Önnur aðferð sem prófuð var er að fylla skurðinn með mjög gegndræpu efni. Jarðdúkur, drenrör og gróft malarefni er sett í botn skurðar. Rörið og malarefnið er vafið inn í jarðdúk. Lag af grófu malarefni er lagt í skurðfláana.


Sértækt vandamál vegna vegrása um þéttbýli er að landeigendur geta fyllt upp í þessar „ljótu dældir“ fyrir framan húsin þeirra og breytt þeim í fagrar flatir. Slíkt getur litið vel út en valdið meiriháttar vandamálum fyrir veginn. Leysa þarf slík vandamál í hverju tilfelli með samningum milli landeiganda og veghaldara en ein lausn gæti verið að koma fyrir lokaðri vegrás.


One of the biggest difficulties in the maintenance of side ditches is how to monitor the level of the ditch bottom and how to identify objects that are blocking water flow. Major drainage problems can be detected visually, as the drainage analysis description in the next lesson will show, but finding smaller details from a moving car is almost impossible. For this reason ROADEX is recommending the new technology of laser scanning for detecting changes in ditches. With this technique maintenance contractors in the future will be able to react early to developing problems in the ditch and so prevent further damage to the roads.


Maintenance of outlet ditches

Outlet ditches are often the most problematic parts of the routine maintenance of road drainage systems. They are however highly important parts of the system as they lead water away from the road area. If the outlet ditch is not working well any maintenance efforts with the drainage structures closer to the road may be rendered useless. For this reason the condition of outlet ditches should be checked at 1-2 times / 10 year and always at the start of a new drainage maintenance contract.

Outlet ditches are generally located outside the road area and as a result the land under ditches may not be owned by the road administrator. This can cause difficulties, especially when the outlet ditch is clogged and has to be cleared. This cannot be done in most countries without the permission of the landowner. The solution for this is to change legislation to enable cleaning of clogged outlet ditches.


A special circumstance with outlet ditches is where discharge into a lake that floods. This controls the water level in the ditch. In such cases the easiest solution is to raise the grade line of the road instead of trying to drain the ditch and road structures.


Maintenance of sub drains

Subdrains can be clogged both mechanically and chemically, and regular inspection and clearing is vital to ensure that they continue to adequately function. This work should be strictly specified in the maintenance contract documents. Subdrains that become chemically clogged are almost impossible to re-open, and clearing should be made too often, rather than too seldom.


5.2.3. Maintenance problems with culverts

The management of the condition of road culverts in maintenance contracts has often been a challenge mainly because the inspection of a culvert needs first of all, the culvert to be found and then the inspector needs to step out of the car in order to check their quality. This makes culvert monitoring both labour-consuming and expensive, and is why the work is easily ignored until the culvert malfunctions and defects appear in the road condition – by which time it is too late. ROADEX research work in following up maintenance contracts in Finland has shown that the role of frozen and clogged culverts in road damages and reduced pavement life time is much greater than earlier expected. A good system of monitoring culverts always pays back, and new innovations to achieve this should be encouraged.


Main road culverts


The stream velocity in main road culverts is an important factor. If the stream velocity in the culvert is lower than upstream, sand and gravel will be deposited in the culvert and it will be an important maintenance operation to clear the culvert when the amount of material deposited has reached a predetermined level. If this is neglected the culvert will not have sufficient capacity and water will flow across the road surface and into the road structure. This is a traffic safety problem and may also cause erosion and wash out problems.


The inlet of a culvert may also become clogged by branches, mud, gravel, rubbish and other things. The consequences of these blockages are the same as described above.



Transverse culverts can also be exposed to frost problems. If ice clogs the culvert water can flow across the road. This is mainly a problem during early spring, and during the winter in mild weather periods with heavy rainfall.


The incorrect installation and sizing of transverse culverts can also produce difficulties. Culverts can be installed too high, or end up too high as a result of the road settling around the culvert if it has been constructed on a better foundation. This results in water lying in the approach ditches and causes siltation in the bottom of the ditches.


If the length of the culvert is too short, it can easily clog. Ends of short culverts disappear quickly and many times GPR surveys have found buried culverts in the road structure that cannot be seen externally. ROADEX drainage analyses in different countries have also found that transverse culverts have been omitted in some low-lying sites where they should have been installed. Culverts can be damage by time, frost, corrosion, etc and should be repaired or replaced when necessary.


Access road culverts

Recent research results in the ROADEX IV project show that missing or poorly performing access road culverts are one of the biggest problems in the functionality of road drainage systems. For instance, many of the differential frost heave locations on main roads have been shown to be a consequence of the performance of adjacent culverts under private access roads. If the access road culvert is missing, or for other reasons is preventing the water flow in the main road ditch, the water level in the ditch will rise, causing sheet ice to form in winter, and the trapped water in the ditches to infiltrate into the road structures causing further damage in the road. In some cases, as shown in earlier lessons, these clogged or missing culverts can cause erosion to the road and traffic safety risks due to water flow on the road.

5239bb

A frost heave video above made based on laser scanner data from road 934. The measurement was carried out twice, in April when frost heave was at its maximum and in June when the frost was almost entirely gone. The blue color represents the lowest frost heave values and the red colour the highest. Note the large frost heaves near the access roads.


Problems with access road culverts can be divided into four different categories; a) missing culverts, b) too small culverts, c) clogged culverts, d) frozen culverts (already described above) and e) culverts that have been poorly constructed. These cannot be defined as purely the maintenance contractor´s responsibility, and at least some of the work has to be fixed in cooperation with the road owner.


If a culvert is missing the only solution is to provide a new one. Different countries have different views on who is responsible for this construction cost. If the current culvert is too small in diameter it should be replaced with a larger one.


Usually the diameters of access road culverts are smaller than main road culverts and this is one reason for these culverts clogging more regularly than main road culverts. The low water flow velocity in access road culverts makes fine materials easily deposited in the culvert, further blocking the water flow. In Nordic countries the shallow placement of access road culverts, and the low flow velocity of the water through them, also make the culverts susceptible to ice and frost. Ice can clog the culvert and lead to severe road drainage problems when the snow melts during the spring thaw. Well working culverts are particularly needed at this time. Frozen culverts are normally opened using steam, but if the culvert is known to freeze every winter a better solution is to install a thermal cable in the culvert, connected to a solar panel, to ensure the culvert is not totally frozen.


Culverts under junctions to shops, petrol stations and other businesses are usually longer than culverts under access road junctions. These long culverts are more exposed to frost and ice and more difficult to clear of fine soils and debris.


Access road culverts are not always installed correctly by their private access road owner. Where they are installed too high, water in the ditch will not be able to flow through the culvert until such time as the upper ditch has great amounts of trapped water. This trapped water causes instability in the ditch slopes, and soil flow that will eventually block the ditch bottom. The installation of plastic culvert requires special care, as if they are installed too close to the surface (i.e. insufficient cover depth), the culvert can be damaged and lose its shape. Very long culverts are also sensitive to damage. Their ends can rise up and prevent water flow through the culvert.


5.2.4. Runoff maintenance, keeping the pavement surface dry

All of the water on the pavement surface should flow away from the road, and not infiltrate to the road structure beneath, to ensure a durable and long lasting pavement. This is achieved by providing a proper crossfall on the pavement surface and by keeping the bituminous pavement water impermeable. In gravel roads the wearing course should be as impermeable as possible, whilst ensuring that it does not turn plastic when it gets wet.


Surface maintenance on a paved road pavement can be done by a) crack sealing and/or b) surface dressing. Crack sealing should be done every year after the spring thaw weakening period when the cracks are still open. Surface dressing is commonly used in Scotland, Ireland and in Norway in maintenance contracts. The surface dressing technique, when properly used, is an excellent method of increasing the lifetime of a pavement, and also ensuring that the pavement friction stays at a good level.



Maintenance contractors cannot do much about changing the crossfall on paved roads, but on gravel roads this can be done by grading the wearing course during the spring and in the fall before the winter. Crossfall on gravel roads can be as high as 5 % and particularly before winter it should be greater than flatter. A good quality gravel road, well compacted and with good crossfalls will ensure that the upper part of the road structures stay dry.


5.2.5. Problems with verges

Verges pose a special problem in the management of pavement crossfall. Verges can be divided into grass verges and soil verges. Grass verges are “man made structures” that continue to be built in some areas in Ireland and Scotland even though ROADEX research has shown that they can have an adverse effect on road performance and pavement lifetime. Soil verges, on the other hand, are “unwanted” soil and grass barriers beside the pavement, that arise due to poor maintenance. These prevent water flow from the pavement to the ditches. They have also negative effect on pavement performance. The new ROADEX drainage analysis includes the analysis of any verges present.


Where high grass verges have been built, water is directed away from the pavement using offlets through the verges at regular intervals. ROADEX drainage analyses have shown that the pavement condition can be much better where the offlets are kept clear, but this is not the case in many roads. If offlets are not cleared regularly, surface water on the road is forced to drain through the pavement structure instead of draining to the ditches. This leads to reduced bearing capacity and deformation.


The most sustainable solution to the presence of grass verges is to remove them and replace them with stronger aggregates such as gravel or ballast. ROADEX tests on the B871 road in the Scottish Highlands have shown a major reduction on pavement failures under heavy traffic on sections where this replacement has been done. Also, drainage analyses in Western Isles and in Ireland have shown that the pavement condition is generally much better and lifetime much longer on those road sections where the grass verges have been removed.


Soil verges can be found in all Northern Periphery countries on roads where the soil or turf on the pavement edge grows higher than the asphalt surface. This prevents the surface water flow from the pavement with consequential loss of bearing capacity and deformation. In order to overcome this the ROADEX countries have created a range of maintenance guidelines and regulations. In Norway the verge material must be removed if it is more than 30 mm above the surface. In Sweden the critical limit is 25 mm. Finland does not have a defined limit value for verges, but the guidelines state that a verge has to be monitored if it is too high and is preventing water flow from road to ditch. In practice the limit value in Finland has been 30 mm as in Norway. If the soil verge is too high it is normally removed with a grader.


Soil verges on gravel roads are often bigger problems than on paved roads as the passage of traffic, and grading operations, easily moves road material to the shoulders. Another reason for build-up of soil verges is permanent deformation during the spring thaw weakening period, i.e. the same process that causes road widening. Soil verges should be removed immediately once they have been noticed by the maintenance contractor.



5.3. Topography related drainage problems and solutions

In the previous chapter drainage problems were described by each drainage component and this can be a good system to follow in maintenance contracts. In road rehabilitation projects however the designer can, and must, select the most appropriate drainage solution from the range of options available. In this process the best criteria for the selection is to define the surrounding topography around the road and select and design the drainage solution based on this topography classification.


5.3.1. Sloping ground

The Northern Periphery area has an abundance of mountains, fjells, hills, fjords and river valleys and as a result a great part of the road networks there have been constructed on sloping ground, where one half of the road is situated in a cutting and the other half of the road is situated on an embankment. In such cases, the groundwater table is normally closer to the road surface (and, as such, to the wheel load) on the road cut side. And because water in road structures is a function of the distance from the groundwater table, the bearing capacity of the road on the road cut side can be much lower compared to that of the embankment side, especially if the drainage is not functioning properly.


The ROADEX drainage research results in the Rovaniemi maintenance area of Finland showed that the condition of the road drainage was mainly worse in side sloping ground compared to other road profiles. Deeper rutting and higher road roughness was also measured on the road cut side of these sections. In road Rv-858 in Troms County area in Norway, the average rut depth for the road cut side was 12.9 mm and for the embankment side 7.9 mm. The ROADEX II drainage report showed that the pavement lifetime ratio between drained lane/undrained lane was more than 2. This has a major economical impact on road condition management costs.


On sloping ground, the ground water will flow under the road if at all possible. But if there is bedrock or impermeable materials near the road area, these objects may block or concentrate the ground water to places where there is a high potential for developing frost heaves, spring thaw softening and reduced bearing capacity due to high moisture content.


Design issues

When designing new drainage, or a drainage improvement, for a road section in side sloping ground the main focus should be to make sure that the drainage will work in the upper side of the road. Many times there will be lack of space and it should be ensured that all slopes are stable and that slope materials do not flow or slide to the ditch. A deep drain is also a good option if there is lack of space. Erosion protection should also be considered in the ditches where the road has a steep longitudinal gradient.


5.3.2. Low lying ground

In areas of low ground the natural drainage system for surface water is often poor, or not present, and water has to infiltrate into the subgrade soil. When this soil is frozen, or after a period of heavy rainfall or snowmelt, water can stay on the surface and affect the road. In addition to causing trouble for traffic, the high water table can increase the deterioration of the road. On gravel roads this can soften the road structure and road surface to the extent that the road becomes impassable.


Design issues

The best way to provide drainage to a road on low lying ground is to raise the grade line of the carriageway. How much, depends on the water table and how severe the problem is. The difference in elevation (height) between the road surface and the water table will depend on the materials used in the construction. If dense graded gravel is used the difference should be at least 50-60 cm. If coarse, well-drained materials are used, the difference should be at least 30-40 cm. When raising the grade line in low lying valleys with weak subgrade soils, such as peat and gytja, stability and differential settlement risks should always be assessed.


Infiltration wells or infiltration ditches may be possible where the subgrade soil is glacial till (moraine). The dimensions of these drainage structures will depend on the permeability of the moraine and how much water accumulates in the local area.


5.3.3. Flat areas

Roads crossing flats have similar issues to roads crossing low-lying ground. The long distances to the natural drainage system can make it difficult to get rid of water. This problem is most apparent during the spring thaw when the ground is still frozen and there is a lot of water from melted snow and rainwater. Water that cannot infiltrate into the subsoil can creates large local pools that raise the groundwater table and pose problems for traffic flows.


During periods of heavy rainfall the subsoil may have problems draining the surface water. The extent of the effect will depend on the amount of water involved and the permeability of the subsoil. In any case, the result will be that the groundwater table will rise the consequences of which are described in the previous lessons.


Design issues

Raising the carriageway grade line is always a good way to handle drainage for roads on flat areas, irrespective of the type of subsoil. It is however sensible to use coarse graded materials that are not susceptible to water. Designing and building outlet ditches are also a solution, but due to the terrain these are likely to be long, deep and expensive, and the reason why they have not already been built!


Where the subsoil is moraine, it is possible to use infiltration ditches or infiltration wells. These will help infiltration in periods when the ground is frozen or when the surface cannot drain the water down to the groundwater table. When the subsoil is peat or soil with low permeability, it is not possible to use infiltration. If excavating ditches is not possible, an alternative solution is to raise the carriageway grade line. Any subsoil materials that are susceptible to settlements and increased load may however result in an uneven finished road surface.


5.3.4. Bedrock cuttings

Drainage issues in road cuttings through bedrock relate mainly to differential frost heave and permanent deformation. These may appear if the bedrock cutting does not permit water to drain or if water collects in locations in the cut. Depressions in the bedrock surface can collect water and if there are frost-susceptible materials in the road structure segregation ice and uneven bumps can form in the road surface.


The best solution for drainage in Nordic countries has been to be breaking the bedrock by blasting to a depth of 1-2 m below the top of the blasted bedrock formation. This creates cracks in the bedrock allowing water to drain from the road structure. In colder areas, where the frost front goes deeper, the blasting depth should be deeper.

A good solution has also been to blast ditches, and/or make deep drainage structures, that prevent the water from entering the road structure.


5.4. Special design related problems

5.4.1. Design of road structures

In addition to design problems described earlier there are a few drainage related road design issues, that road engineers often have a tendency to forgot or neglect, that can cause major problems later with road condition. A brief description of these is made in the following:

Inner curves

Permanent deformation problems often occur in the inner curves of road sections. When the road structure is wet and under traffic load, the road structure will soften. A possible reason for this is that roads are dimensioned on the centre line thickness and often the excavated shape is horizontal, with the result that the structure is thinner on the inner curve than elsewhere.


Solution

One solution is to dimension the structure at the thinnest point (i.e. the inner curve). This way the road structure will be thick enough everywhere. Another solution is to excavate the formation parallel to the road surface to ensure a constant structure thickness.


Widened roads

Road widenings bring their own considerations. Usually the widenings are constructed incorrectly, such as the bottom of the road structure being excavated horizontal, which causes additional issues for the drainage of the road and road structure.


Solution

The road widening should be constructed in such way that the new layers continue the same crossfall angle as the original structure.


Another option to improve the durability of a widened section is to ensure that a layer of free draining material is included within the new structure to provide an effective drainage mechanism for the widening.


Moisture traps

Moisture traps, known also as “sandwich structures” can arise on paved roads where the road has been rehabilitated by laying unbound aggregates directly on top of the old bound layers. In these cases any water that penetrates through the asphalt surface, or unpaved road shoulder or ditch (periods when snow is thawing), will be trapped between the new and old bound layers. The net effect of this will be that the moisture content within the “trap” will increase more than it would for a normal road structure and will be moister for a longer time due to the lack of drainage possibilities. If the moisture content then gets close to the saturation level, dynamic loads from the pavement can cause high hydraulic pressures to develop within the trap causing the pavement above to break.



There are a few solutions to avoid sandwich structures. The old bound layers can be milled and crushed before laying the new unbound course. Experiences in Finland and Sweden have shown that the old pavement should be always be broken if it is closer than 40 cm from the pavement surface.

5.4.2. Slope stability problems

Slope instability and erosion are very common problems on roads, especially in road cuts. Erosion can be caused by surface water, ground water or surface slides. Sensitive soil types for erosion are silt, silt moraine and sand.


Materials from unstable slopes can flow into the roadside ditches and block the water flow and cause the ground water level to rise.


Instability problems can be solved partly by improving the drainage outside the road structure, especially in sections with an abrupt outer slope. One solution is to dig a back drain. Usually the back drain is provided above the cutting slope with the aim of intercepting surface and ground water before it can cause erosion. Water from these ditches must be directed to the natural drainage system.

Reference: Teiden suunnittelu IV, Tien rakenne 4, kuivatus


Another option is to support the slope to make it less unstable. This can be done for example with a gabion wall, rock support or with “hydroseeding”. Hydroseeding and supporting ditch slopes have been discussed earlier in section 5.2.2 “Maintenance problems with side ditches”.