5.1. Subgrade soils and topography
5.1.1. General – Why subgrade soil and topography classification is needed
Information on the subgrade soil type and surrounding topography, including the depth-to-bedrock, is needed initially to allow the designer to determine the most suitable drainage solution and to select the most appropriate rehabilitation structure. The subgrade soil can influence permanent deformation in the following ways:
Some soil types and poor drainage together can affect the permanent deformation that causes Mode 2 rutting and pumping.This is a particular problem in Scandinavia where non-homogeneous soils, together with poor drainage, can cause differential frost heave problems in the road.
Soil type can cause stability problems in road embankments.
Soil type defines the types of special construction materials that can be used in the rehabilitation process. For example, a geotextile in a soil replacement should be used where the subgrade is silt or silty moraine.
The topography classification has been designed to help with the selection of special structural solutions against permanent deformation. In these areas standard solutions and structures might be ineffective. Typical topography classes are: 1) flat and even area, 2) side sloping ground, 3) hummock, 4) wet and low lying valley and 5) bedrock.
5.1.2. Class: Flat and even area
The biggest problem with a road crossing a „flat and even‟ area is that there is usually a significant distance to the nearest natural drainage system and, as such, it is difficult to get rid of water. This problem is most apparent during the spring thaw when the ground is still frozen and there is an abundance of water from melted snow and rain. The subgrade may also have problems with draining the surface water during periods of heavy rain. The extent of the problem will depend on the amount of water and the permeability of the subsoil.
The recommended structural solutions for flat and even areas are, in general, based on the quality of the subgrade conditions. These are:
- Raise the grade line of the carriageway
- Replace frost-susceptible road materials with materials not susceptible to water and frost
- Test if infiltration wells or ditches can be used
- Design long ditches (long outlet ditches) or deep drainage
Clay, silt or peat subgrade
- Raise the grade line of the carriageway. Care should be taken with this however as this solution can increase the loads on the subgrade and increase settlement, especially on peat subgrades. Raising the grade line also helps with winter maintenance as it creates greater space for snow accumulation and snow storage at the side of the road. It can however pose problems where the land for the road is limited and this may make the new inner slopes too steep.
- Use steel reinforcement. This can reduce pumping problems.
- Design long outlet ditches (surface drains or subdrains) to an acceptable natural drainage system
- Consider stabilization if there are no differential frost heave problems
- Consider using new treatment agents if the drainage cannot be improved or the grade line raised.
- Clean the upper ditch and make a support or drainage structure for the upper slope.
- Use a subdrain to lower the groundwater table on the road cut side
- Design additional culverts in places where water is being blocked
- Design edge drainage (especially in non-frost areas)
- Increase the road structure (by soil replacement) on the road cut side, or raise the grade line of the carriageway
- Remove bedrock/impermeable materials that block the groundwater flow
- Provide frost insulation to enable water to flow under the road (expensive)
- Build a new structure on top of the old and make sure that the layer thickness over the bedrock section is at least 40 cm. In the case of a paved road, the old bituminous pavement layers should be milled out first so as not to make a sandwich structure.
- Blast the bedrock to a depth of 1-2 m below the road foundation. This will create cracks in the bedrock and enable water to drain from the road structure
- Blast ditches, or provide deep drainage, at the sides of the road to prevent water from entering the road structure
- Use frost insulation to permit water to flow under the road
- In forest roads install drainage pipes at every location that collects water
5.1.3. Class: Side sloping ground
It is very normal in the Northern Periphery for roads to be 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. Roads on side sloping ground are usually found in areas with frost and water susceptible moraine (glacial till) and sand/silt materials. (With subgrades of clay or peat, the terrain is normally flat.)
In roads on side sloping ground, the groundwater table will normally be nearer to the road surface (and to the wheel load) on the road cut side. One reason for this is that material from the outer slope naturally flows to the bottom of the ditch and this raises the water table in the ditch. The moisture content in a soil is a function of the distance from the groundwater, and thus the rut deformation in the upper wheel track is usually greater on roads on sloping ground. The consequence of this is that rutting on the cut side of the road generally triggers the need for rehabilitation many years earlier than the well-drained embankment side. The lifetime ratio (drained lane / undrained lane) may reach higher than 2.
On sloping ground the groundwater will flow under the road. If there is bedrock or impermeable materials near to the road, these obstacles can block or concentrate the groundwater and thereby increase the potential of the adjacent road to develop frost heaves, spring thaw softening and reduced bearing capacity due to high moisture content.
The main focus in the design of the structural solution should be to make sure that the drainage system proposed will work effectively over the life time of the pavement – and not just for a few years. Once the drainage has been fixed the structural solution can be designed, and often this can prove difficult due to the limited width of the road. Some possible options are:
5.1.4. Class: Hummock
Roads going over, or crossing, moraine hummocks often have special permanent deformation problems, particularly in frost areas.
Damage at these hummock sites is generally located on the top of hill or in the transition zone where the road leaves the embankment and the structural layers become thinner.
Unlike side sloping ground, softening of the road surface on a hummock takes place throughout the cross section.
Differential frost heave bumps are also often found in these transition zones during the winter.
Soil replacement structures are the most suitable solutions for these sites because they do not need the grade line to be raised. If the road is in a road cut on the top part of the hummock it quite often collects snow (and water)
and ditches get easily filled. For this reason these sections should receive special attention when designing drainage maintenance measures.
5.1.5 Class: Wet and low lying valley
A wet and low lying valley is a special case, and a place where the problem of permanent deformation is often only one of many problems to be considered. Other problems which may arise include settlements, differential frost heaves (especially around culverts) and winter maintenance problems. In addition, road user interviews in the ROADEX project have also shown that drivers of heavy trucks feel that problems in these places have the potential to cause major traffic safety risks. View the report: “User Perspective to ROADEX II Test Areas’ Road Network Service Level”
A wet and low lying valley, or hollow, differs from a „flat and even‟ area in that the usual and recommended rehabilitation solution (with a few exceptions) is to raise the grade line by building a new structure on the top of the old road.
A higher grade line provides numerous advantages and benefits. A thicker structure above the groundwater level will increase the bearing capacity of the road, even though the drainage in the valley cannot be improved. A thicker structure will also provide better resistance against frost heave, and reduce the risk of getting frost bumps around culverts that could cause problems to heavy traffic.
A higher grade line can also help winter maintenance. Low lying valleys often have problems with drifting snow, and a higher grade line can leave more space for snow accumulation.
Finally an improvement of the vertical geometry will improve traffic safety, especially for heavy traffic in slippery driving conditions in winter, as well as saving their fuel consumption.
The quality of the subgrade soil has to be assessed however before a decision can be made for a higher grade line. If the subgrade soil is peat, compressive clay or gyttja, the risk for uneven settlement has also to be assessed, view the report “Dealing with Bearing Capacity Problems on Low Volume Roads Constructed on Peat ”. Another problem, regularly faced, is that the improvement of the vertical alignment in a valley requires more width for the road and this may need further land from adjacent landowners. This can be reduced in some situations by using guard rails, but these can cause other problems.
5.1.6. Class: Bedrock
Low volume roads constructed on a bedrock cutting, or where the bedrock is close to the surface, are always special cases in design as they pose special problems. The rehabilitation of these types of road sections can be very expensive.
Bedrock prevents water draining from the road structure and this can lead to a reduced bearing capacity within the road. During the frost season, ice lenses can form on top of the bedrock and cause uneven bumps to be formed in the road surface. Any depressions in the bedrock surface will collect water and if there are frost susceptible materials in the road structure, segregation ice will form.
Bedrock may also block the flow of water under the road and/or cause water flows into the road structure from the roadside. Bedrock and boulders can dam the water flow over long distances and this can cause differential frost heave.
Bedrock overlain by thin unbound layers (< 30-40 cm) can also act as a “sandwich structure”. When these layers are saturated with water, heavy traffic can produce high hydraulic pressures that can rupture the pavement. This can happen particularly during spring when snow is melting
There are several solutions to bedrock related problems and the final selection is normally based on the road class and the resources available. Some of the most common solutions are:
Make a soil replacement structure down to the bedrock surface and replace the old material with a coarse grained non-frost susceptible material, always remembering to construct transition wedges at the changes in structures
Drainage is possibly the most important factor to influence the development of permanent deformation but poor drainage can also create traffic safety risks. Permanent deformation hardly ever exists if excess water is not present in the road structures and subgrade soils. Results from the early ROADEX projects clearly show that keeping the road drainage system in a good condition is the most profitable maintenance action possible on low volume roads. And for this process an effective drainage monitoring system is essential. This is considered, together with detailed descriptions of classifications, analysis and diagnoses, in the ROADEX “Drainage” eLearning package.