Seasonal changes, frost action and freeze-thaw cycles are the most significant factors affecting gravel road and forest condition of northern cold climate road networks in Europe, Asia and North America. Freeze-thaw processes also cause major problems in high elevated areas in countries with warmer climates, for instance the percentage of the road network sensitive to freeze thaw damage in Romania is 50 %, in Hungary 40 % and in France 20 %. In addition even in tropical countries gravel roads suffer from seasonal changes especially during the rainy season.
Spring thaw weakening causes many types of extra costs to both road owners and road users. In Finland the main extra costs for the forest industry are the capital costs for extra machinery due to delays in timber harvesting (59%) due to weak roads, the costs due to poorer timber quality because of long storage time (13%) and rehabilitation and maintenance costs of forest roads (10%).
The relative amount of severe spring thaw weakening problems have been going down over the last decades in Finland and other Nordic countries. The reason for this could be climate change and warmer winters but, for instance in Finland, there have also been investments in strengthening the roads suffering from spring thaw weakening. Also the awareness that spring thaw weakening is not only the road owner or road user’s problem has been growing and haulage companies understand that it is in nobody’s interest to make the road impassable for weeks by hauling timber, aggregates or other products during the weakest period. Finally the know-how of how to monitor the parameters affecting the severity risk of spring thaw weakening has increased and now road authorities can have much more reliable information for decision-making where and when to place load restrictions, and when to remove them.
10.2. Freeze- thaw cycles, frost action, thaw weakening and spring thaw damage
When managing seasonal changes in cold climate countries it is necessary understand the following factors: freeze-thaw cycles, frost action, thaw weakening and spring thaw damage.
In the seasonal change management of roads, a freeze-thaw cycle means a short duration cycle from one night to less than two weeks when the top part of the road structure freezes and then thaws. During the freezing period cryosuction causes negative pore water pressure to develop and unfrozen water, mainly from layers under the frozen layer, flows to the frozen material forming ice lenses. So when the frozen material finally melts, its water content is always higher than before freezing. In ROADEX areas in Nordic countries freeze-thaw cycles may happen in the fall and in the spring while in Scotland and Ireland freeze-thaw cycles are the main seasonal change problem. In Nordic countries, during the spring thaw weakening time, night to day freeze-thaw cycles can speed up the drying of the road surface because, due to cryosuction, water from the deeper structures and already thawed structures flows to the road surface when it freezes during the night. This water then evaporates from the road surface during the sunny and windy days.
The frost action process in roads consists of a) frost heave and 2) thaw weakening. Frost heave is the upwards movement of the road material or subgrade soil due to the formation of segregation ice lenses or the expansion of material as it freezes. Thaw weakening is weakening of the road material resulting in its saturation as ice in it melts. Spring thaw damage is the damage this process causes to the road structures. The following factors are necessary for the appearance of frost action and spring thaw weakening, and if any one of these factors is absent there is no risk for spring thaw damage.
- The road and/or subgrade soil freezes. This requires minus temperatures but it should be kept in mind that the formation of ice lenses is highest at a temperature of between 0oC and -20C. These temperatures can be found sometimes in almost all the world.
- The material is frost susceptible. However segregation ice can form even in non-frost susceptible materials in favourable conditions, for instance when frozen water from melting snow is flowing into frozen material.
- The freezing front has enough water available.
During the thawing period water, released by melting segregation ice, stays in the road structures or subgrade soils, thus saturating and weakening the structure
- The road is subject to loads during the thawing period. It always has to kept in mind that roads will have hardly any damages if there are no loads on the road during the thaw weakening period.
A central factor in the development of spring thaw damage is excess pore water pressure in the road aggregate or soil caused by dynamic axle loads, which decrease the effective stresses between soil particles. Because the ability of a material to resist deformation under a wheel load depends on the effective stresses between soil particles, the increase in pore water pressure leads to an increase in deformation of the material. The greater the load, the greater will be the degree of permanent deformation. That is why load restrictions are used to protect weak roads spring thaw damage during periods of spring thaw weakening.
10.3. Monitoring and predicting seasonal changes
Because the severity of seasonal changes varies substantially year to year, a gravel or forest management system should have always a system to monitor the status of moisture and frost in the road structures and subgrade soils. In addition, the condition of the road drainage system, as well as winter maintenance practices, can have a great impact on the severity of spring thaw weaking damage or plasticity problems.
Frost depth has been traditionally monitored by installing thermal sensors at different depths in the road structure and subgrade soil. However temperature itself does not indicate how much unfrozen water is likely to be in frozen material and that is why sensors like TDR (time domain reflectometer) or Percosensors, together with temperature sensors, have been used to monitor the status of frozen road layers and subgrade soils. Measuring water content is based on dielectric permittivity measurements. Percosensors also measure electrical conductivity which is a great indicator as to whether the material is totally frozen, or if it is totally plastic.
Percostation data has been used for making forecasts for spring thaw weakening severity in the wintertime to warn hauling companies about the possible load restrictions. This is done based on the fact of how slowly the road and subgrade has been freezing during the fall and early winter, enabling formation of ice lenses close to road surface. And, during the spring thaw, monitoring data is used for guidance as to when load restrictions should be applied and when they should be removed. In spring the focus is on how quickly possible ice lenses close to the road surface are thawing and how much free water is in the road structures.
The use of seasonal change monitoring stations provide in good long term understanding of how different freeze-thaw cycles and different freezing histories happen during the fall when the road is freezing and during the spring thaw weakening period. This information can then used in the recommendations for the load restrictions.
Forecasts for spring thaw and seasonal changes and the need for load restrictions have certain basic rules.
- If the fall was wet, there had been freeze-thaw cycles and the road materials were wet when they froze the forecast for the surface thaw weakening is bad and load restrictions are recommended to be applied immediately when the frozen road surface starts to thaw. But if the road structures had been freezing quickly and they had been dry, then early spring restrictions are needed only if the weather is very rainy during the surface thawing weakening season.
- During the main road structure thawing weakening season later in the spring attention should be paid as to whether the top structures have dried enough and created a “dry crust” on the top of thawing unbound road structures. This is especially critical if there are ice lenses at the range of 0.3-0.5 m. Load restrictions should be immediately applied of there are signs that the road surface is becoming plastic again.
- During the subgrade thaw weakening season the snow has already melted and the worst frost bumps can be seen on the road because the thawing of the biggest ice lenses take more time. If the road surface has stayed dry, and there are no indications of major plastic deformations, load restrictions can be removed. But if the road structures are still wet load restrictions should be kept in place.
If there are not any frost or spring thaw monitoring data available – and even if that data is available – visual field data is always needed. The basic moisture and spring thaw weakening monitoring technique is visual monitoring when driving the road with car. An experienced engineer or technician can make a rough evaluation of the status of the road surface and its risk for different damages. Problems come when the frost thaws deeper into the structure and it is difficult to estimate how thick is the dry crust and how wet is the material beneath it.
Bearing capacity surveys of gravel roads and forest roads do not always show the sections with spring thaw weakening or seasonal change problems. In Finland spring thaw weakening sections are mainly located on roads on wet moraine or silty subgrades, and especially on side sloping ground. But many times spring thaw problems are located on road sections where bedrock is close to the surface or the subgrade is peat. The problem with FWD surveys during the spring thaw is that the E2 values and the maximum deflection values are strongly dependent on the fact whether there is frost in the subgrade and in this case these values are much too good. Also many times FWD bearing capacity results can be also very good during the summer or fall on the sections suffering from spring thaw weakening.
The ROADEX project has also tested the Dynamic Cone Penetrometer (DCP) technique and found it to be a very promising method in spring thaw weakening monitoring. The DCP results can be used in detecting the depth of the thawed and unfrozen ground interface and also the bearing capacity during the spring thaw.
The bearing capacity development of a gravel road in Tohmo, Finland in spring 2004. Top graph is from late April, when the frost was still around 0.5 m from the road surface, and bottom graph from early July when the frost was totally thawed and the road structure bearing capacity has returned to its summer level. This data also shows well the exposure of the road towards sunshine. The left side of the road is the southern side and frost melts earlier on that side.
An additional potential method for seasonal change monitoring is GPR as it provides a longitudinal profile over the roads. However this method remains quite costly for routine monitoring.
10.4. Traffic management and seasonal changes
Traffic management on gravel road network suffering from potential seasonal changes requires a good cooperation between road owners, maintenance contractors and the haulage industry. That is why a key element in the successful management of spring thaw weakening is how to inform road users of the load restrictions, and about weak and dangerous roads. The forest industry, and especially haulage companies, require reliable information early enough to be able to make a harvesting and transportation plan that will ensure the continuity of raw material supply. They also need real time information concerning the status of thaw weakening and load restrictions. In most ROADEX countries this has been carried out by public websites.
The best option for ensuring proper road condition over the long term is to avoid using the road during the weakest period in spring. Since most of the timber is first transported over weak forest roads, that are not passable during the thaw period, there will always be a need to make special arrangements for timber haulage in order to assure a continuous supply of raw material for the forest industry.
The most popular method to deal with this issue has been to haul timber from road networks in areas suffering from spring thaw problems to temporary stockpiles in the vicinity of better roads with no load restrictions. If the roads are very weak, this can be an economical solution and,for instance in Finland, some road regions have built these timber storage areas for the forest industry.
10.5. Road friendly vehicles
A good solution for timber transportation problems during the spring thaw period is to control the tyre types and the tyre pressure. The smaller the tyre footprint the higher is the stress on the gravel and forest road structure, and the higher is the deformation in the top part of the road structure. That is why most gravel and forest road friendly trucks are the “classic” truck configurations equipped with dual tyres. According to a study in Scotland the rutting impact of timber trucks with super single tyres is more than two times higher compared to dual tyre trucks.
An alternative way of reducing the stresses on a road is to amend the contact pressure of the vehicle’s tyres on the road by reducing the pressure in the tyres. “Central Tyre Inflation” (CTI), is the generally accepted terminology for the onboard automated system of tyre pressure control that permits the driver of a vehicle to adjust the pressure of the tyres on the vehicle whilst the vehicle is in motion. A number of automated systems are available worldwide. Using a greatly reduced tyre pressure increases the contact area by almost 60 % compared with the normal tyre air pressure. However CTI systems are very effective only on the stress levels on the top part of the road structure but not on the subgrade. So CTI systems do not help on gravel or forest roads road on weak subgrade soils.
In the future the modern sensor and ITS technology will also enable trucks to report their total weights and if they are using CTI when driving on gravel roads during the spring thaw weakening period. In addition the road owner will be able to report real time to the hauling companies and to the trucks about the conditions on specific roads and if load restrictions are used or if the use of CTI systems is required.
10.6. Emergency repair techniques
Spring thaw weakening repair should be always made preventatively and if there are signs of localized plastic deformation they should be immediately fixed by spreading coarse aggregates on the top, or load restrictions applied to the road section. However hauling the repair aggregates with a truck might trigger new problems in other places, so all the pros and cons of the maintenance decisions should be considered seriously. A bigger problem will arise if the road structure turns totally plastic and makes the road impassable. In this case road closures should be considered if possible. But if that cannot be done then the best solution is coarsening the material by placing ballast on the top of the aggregates. Also some cases with plastic deformation have used ‘alternative treatment agents’ that quickly stabilize the defective material.