Foundation defects are mainly caused by the introduction of water into the pavement and subgrade, either by inadequate surface drainage or by seepage from the surrounding environment. They are generally expensive to fix but need to be rectified, otherwise ongoing problems will occur. Foundation defects include widespread subgrade failure and more localised rutting, and soft spots.
The use of lower tyre pressure in heavy vehicles has been shown to reduce the occurrence of defects in fully compacted pavements. The lower tyre pressure creates a larger footprint for the tyre, leading to less impact on the road. The use of low-pressure technology has increased but its use is dictated by the load carried and the speed of the vehicle. On forest roads, loaded trucks equipped with central tyre inflation should reduce tyre pressure.
9.6.1 Rutting, soft spot, and subgrade failure
Movement of material within the pavement causes localised soft spots. Generally, these are found in the wheel paths, but they can also occur elsewhere across the pavement surface. Soft spots are often found in material containing a high proportion of fines, which have not been well compacted. Another cause is the entry of water into the pavement from either the surface, or water seeping up via capillary action into the subgrade. The best way to help overcome the formation of soft spots is to dig it up and replace with correctly graded, compacted, or stabilised material. Correct compaction at optimum moisture content (OMC) of well-graded aggregate material will reduce the risk of soft spot formation. Maintaining superelevation and crossfall will ensure adequate drainage. Good roadside drainage will prevent water from ponding, and reduce the chance of water entering the pavement from the surrounding environment. In some cases, subsoil drainage may be required.
Rutting is longitudinal deformations in the wheel paths, caused by traffic. The traffic movement compacts the material directly under the wheel, creating rutting. Rutting in dry conditions occurs in non-cohesive materials such as sands or gravels, which have a low fines content. In wet conditions, rutting usually occurs in materials that are sensitive to water or have high water content.
Rutting is due to failure of the subgrade, base course or surface material. Excessive quantities of water entering the pavement or subgrade, along with excessive fines, poor compaction of pavement and inadequate pavement depth for the wheel loading contributes to rutting. Surface rutting will reduce through maintaining or rehabilitating the crossfall and compacting the running surface. As with corrugations and potholes, ruts can be removed by cutting the surface to just below the rut, and reshaping and compacting the pavement. For subgrade failure, additional material is required to increase the pavement depth and, therefore, distribute the applied load. The risk of rutting is reduced if the correct pavement depth is applied at construction for the strength of the subgrade. Also, the correct material gradation will allow effective compaction and water fastness. Maintaining crossfall and optimum compaction of pavement material will also reduce the risk of rutting. Ensuring that adequate pavement depth is maintained will better distribute the load onto the weak subgrade.
Subgrade failure is usually identified by large, soft or depressed areas within the roadway. They primarily occur in the wheel paths, but can be found across the entire road pavement. The same reasons for more widespread subgrade failure apply to those described above for rutting. The only solution to subgrade failure is to remove all soft and poor-quality material. The depth of extraction should continue until a solid base is found. Backfilling with good quality aggregate will be required. It must be compacted to form a stable base, and subsurface drains may also be needed. In some situations, other options are needed, such as in poor weather conditions combined with operational requirements that demand a rapid fix. These include using geosynthetics, log corduroy and subsurface drains. Geosynthetics and corduroy stop the mixing of pavement and subgrade materials that can cause foundation defects, and they also help to spread the applied load evenly over the soft subgrade. In situations where the road is low lying, and where water entering the pavement is a potential problem, subsurface drains in the subgrade and pavement will help to drain the affected area, reducing risk of occurrence. Subgrade failure can often be prevented. Logging on non-compacted new or ‘green’ roads may lead to subgrade failure. These often expensive and inconvenient failures can be prevented through having an ahead roading position or by constructing roads so that are robust enough for ‘green’ use. Also, the addition of aggregate material is a preventative measure, since this increases the pavement depth and the distribution of the applied loads onto the subgrade.
Shaping the subgrade material will also encourage water runoff before and during construction, and in the event of any water entering the pavement. In situations where a stable base cannot be achieved, stabilisation with lime, cement or other chemical product may be required.