5. Roads on weak subgrade

5.1. General

Most gravel and forest roads are heavy haulage routes, ie they are regularly used by heavy trucks. On the other hand these roads often have major limitations on funding for design and this leads to challenges with the structures on weak subgrade soil sections. That is why most gravel roads and forest roads have been built floating on peat which brings consequent geotechnical issues in the management of these road sections.

In this chapter we will mainly use the material that has already been published in the ROADEX eLearning package “Roads on Peat”. This chapter is will focus mainly on the maintenance management and rehabilitation techniques of gravel and forest roads resting on peat.

5.2 Compressive soils

Compressible soils can be classified those soils which decrease in volume under mechanical load. The process is called soil consolidation. Even though clay is a compressive soil, the main soil types with compressibility problems on gravel and forest roads in the Northern Periphery are peat soils and gyttja containing high amount of organic material. The third organic soil in the Northern Periphery is Dy, which is a a sediment formed in nutrient-poor water bodies from colloidal suspensions. All these soils have very high water content that can vary from 400% to 2000% and under external load the compression takes place when water is flowing aways from the material. Dry densities of peat soils typically vary from 60 kg/m³ to 120 kg/m³. Higher values are possible when the deposit has a high mineral content.

5.3. Behaviour of peat, geotechnical risk management

A peat soil under load can consolidate, compress and settle in two ways:
a) Slowly, with gradual consolidation and compression allowing time for the peat mass to respond to the load. This is the desired method for constructing a road on peat and allows time for the peat to improve its strength and bearing capacity.

b) Rapidly, without a change in volume, with rapid spread and shear of the peat causing failure. Peat is highly vulnerable to ‘shear overstress’ and loadings need to be carefully controlled to keep stresses within the available strength

When road is constructed over peat there is normally three phases where consolidation, compression and settlement taken place: “instantaneous settlement, “primary consolidation” and “secondary consolidation”. Instantaneous settlement takes place immediately when load is applied on the peat. Primary consolidation or settlement under controlled load is normally large and takes place normally a few days. This settlement is normally about 50% of the total settlement over time. Secondary consolidation is normally a linear function as a logarithm of time and can take up to 30 years. In addition seasonal frost and permafrost can bring extra unknowns to the construction behavior.

When discussing the condition management of an old gravel or forest road resting on peat the most important fact that everyone should be aware of is that if a new static load is applied on to the peat this will lead also to new compression and further settlement.

Another important fact with gravel and forest roads floating on peat is that their behaviour under a moving load is never elastic but mainly viscoplastic and a small unrecoverable deformation always takes place when a heavy truck passes over the site. When the truck passes, the displacement of its front axle does not recover before the next axle, or tandem axle passes the point, which means that the displacement is incremental, and the aggregated displacement of the road structure bottom can be more than 100 mm deeper under the last axle of a timber truck. This results in the road structure starting to push water in front of the truck until it hits non-permeable soil. This forces water up the road surface, or to the sides, causing road failure. Another option is that a pumping affect appears when water and mud is pumped up through the road structure causing weakening of the structure.

5.4. Survey and monitoring techniques

This section sets out brief summaries of possible survey and monitoring techniques suitable for gravel and forest roads including desk study, visual inspections, sampling and probing, GPR, laser scanner, 3d accelerometer and video techniques.

Desk study and site visit

Desk study is normally made by reviewing old design documents if they are available. In addition, geological soil maps can provide useful information about the peat and its thickness. For instance the new soil maps in Finland classify peat cover in three classes: shallow paludified peat cover 0 – 0.3 m, 0.3 – 0.6 m thin peat and >0.6 m thick peat. Also maintenance histories from the local maintenance staff can provide valuable information. In addition a site visit is essential to get a better view of the local conditions,

Probing and sampling

Simple probing can be used to establish the depth of a peat layer. This is usually carried out with steel rods approximately 1m long with bayoneted or threaded connections to permit the rods to be assembled to a length to suit the depth of peat. Where the peat overlies a hard surface such as moraine or rock a probing survey can produce a good indication of the thickness of the peat.
Some investigation of the properties of the peat should always be carried out to get a notion of the type of peat involved. This can be as simple as a Von Post classification of the peat, and an estimation of water content if settlement calculations are to be attempted. Undisturbed peat samples can be difficult to obtain however especially in peats with a high water content.

GPR and laser scanner surveys

The Ground Penetrating Radar technique has proven to be a very useful technique in the surveys of roads constructed on peat. The greatest benefit of GPR is that it provides a continuous thickness profile of the peat deposit and of the road structure, and in many cases both of them can have substantial variations in longitudinal and transverse directions. A laser scanner survey can support the analysis by providing accurate surface shape of both the road body and the surrounding peat. When this is known it is possible to make an evaluation, for instance, on how much the existing embankment has been settling into the peat deposit under the survey.

The GPR signal attenuation can be quite high in compacted peat under the road however resulting in the reflected GPR signal from the peat bottom being difficult to detect, and only the bottom of the road body. If this is the case then it is recommended that the peat thickness is surveyed with a low frequency GPR antenna (100 or 200 MHz) and the peat thickness under the road defined based on these two survey results. The best information for road and embankment structures can be detected normally using a 400 MHz GPR antenna. Peat under the road embankment can be detected much easier if the survey is conducted during wintertime when the road structures and subgrade soil is frozen. At that time the GPR signal reflection is very wide and clear because of the high contrast between dielectric values of frozen and unfrozen peat. In interpreting peat thickness from the GPR profile it should be always kept in mind that the y- thickness scale of peat is totally different compared to road structures and dry soils. The dielectric value of peat under road structure is normally around 40 whereas road structure dielectric values are 6-7.

One benefit of using GPR on gravel and forest road surveys is that GPR provides in most cases good information where the peat subgrade changes to mineral soil. This is important because these roads suffer often pumping problems on these sections and, for instance, a steel grid structure can be installed in the correct place based on GPR and FWD data analysis.

Falling Weight Deflectometer surveys

Deflection surveys using FWD can provide valuable information about the bearing capacity properties of gravel and forest roads constructed on peat. Continuous deflection survey techniques such as TSD or Raptor generally do not work on gravel and forest roads because dust from the road surface creates problems with their laser sensors.

There are several rules of thumb that can be used in FWD deflection bowl analyses on gravel and forest roads constructed on peat. First of all if the 1200 mm deflection is higher than 200 microns, most likely there is peat under the road.

Also the Base Curvature Index (BCI) calculated from the 900 mm – 1200 mm deflections tell how well the road embankment is spreading the load over the peat deposit. If the BCI value is < 60 microns there should not be any major problems, but problems will start to rise if values are higher than 80 microns. If the BCI value is higher than 120 microns deformation problems can always be expected under heavy trucks.

Finally total deflection under the loading plate also provides indicative information of the bearing capacity of gravel roads and forest roads. If the total deflection values are <1000 microns the road should be mainly ok, but if total deflection is >2500 or >3500 microns the road will likely have bearing capacity problems. In really poor gravel roads resting on peat FWD deflections using 50 KN load can be even higher than 5000 microns (5 mm). The E2 values from FWD systems give, in most cases, too pessimistic values of the bearing capacity values on roads on peat. Designing bearing capacity improvements based on these E2 values can give very thick and expensive structures that also trigger new settlements on the road.

5.5. Construction structures

The safest structure when building gravel or forest roads over peat deposit is always soil replacement. But it is also the most expensive one and one that is generally only done where the peat thickness is < 1 m and the peat quality/stability is poor. Designers try to avoid thicker peat deposits but if this has to be done geotechnical design is always recommended.

If roads have to be built over peat the first issue is to evaluate the potential settlement that will take place on the section. This helps in calculating the amount of material needed for the embankment and can be made by using the survey techniques described above. Valuable information for this can be collected from the similar roads built on similar peat deposits close by the area. Sometimes, if material is available close by, a good solution is to build a thick embankment that acts as an overload embankment and this embankment is then easily evened out over a wider base if and when any settlements appear. A high grade line over open peat deposits can be a good solution against drifting snow during the winter, but can also become an environmental issue for not fitting well in the surrounding landscape.

Another important issue in the design is how to separate the materials of the road embankment from the underlying peat, and how to prevent mud from the peat pumping into the road embankment and weakening it. In the old times this was done by building a timber grillage on top of the peat made from trees felled for the roadline, and placing aggregates on top of it. The benefit of timber grillage is its low density and its ability to act as a “floating” layer between the peat and the road structures and thus reduce settlements.

Timber grillages are seldom used currently and have been replaced by installing a geotextile or geogrid between peat and road embankment. This structure, if done properly and using a good quality geotextile or geogrid, has proven to work well in preventing “mud pumping” under heavy truck loads.

5.6. Maintenance and renovation of roads resting on peat

There are not many specific maintenance operations for gravel or forest roads constructed over peat. If the structures are relatively thin then the road can start to widen and lose its shape and in this case the road surface should be brought back to its original shape.

In drainage maintenance there are a few issues that should be kept in mind. If ditches are close to road, deepening them can lead to increasing settlements because road embankment gets heavier (Archimede’s law). Main road culverts through the embankment should be kept open and free flowing, otherwise the water balance on both sides of the embankment can be disturbed causing flooding and damage to the environment.

Where a road over peat has major problems with deformations, pumping or spring thaw weakening, renovation or strengthening structures should be considered. The easiest solution would be to construct thicker structures but this solution can trigger new settlements. Good experiences using steel grids over the last years have been documented in the ROADEX areas where the steel grid has been installed at about 250 mm depth from the road surface. At that depth the forces of the truck load are close to horizontal and thanks to the high tensile strength of the steel grid the wheel load is spread to a much wider area. The steel grid structure has proven to be most efficient at the beginning and end of the peat sections, where the risk for pumping is highest. When installing a steel grid the material under the grid should have sufficiently coarse aggregates in order to ensure good interlocking between the steel grid and the road structure.

Finally if the road embankment settlement continues to be very high, a light weight embankment should be considered, even though they can be quite costly structures. There are not too many gravel or forest road cases reported about these structures in the ROADEX area but woodchips packed inside a geotextile have been tested in some roads in Finland.