According to Huang (2003) the placement of a drainage layer directly under the asphalt or concrete pavement surface is preferable, because the water in the pavement, either percolating through cracks or entering from the sides, is quickly allowed to move to a lower level from where it can easily be drained. No pore pressures can develop because of the high permeability and rapid dissipation properties of the OGDL eliminating any chance of pumping occurring. Furthermore, it eliminates the final, negative, effects of water or frost. A properly designed and constructed permeable granular base layer may have a similar structural performance as a conventional base. However, OGDLs have a number of disadvantages:
• the deficiency of fines in the drainage layer may cause stability problems. They are difficult to compact into a stable foundation on which higher pavement layers can be constructed. Perhaps even more problematic is the trafficking of the OGDL by the construction plant that will lay the next pavement layer;
• the water in the sub-base cannot drain readily into the drainage layer;
• if the outlet becomes blocked the drainage layer becomes a reservoir for pore water that can develop pressure pulses under passing traffic thereby causing erosion and loss of bearing capacity. This can be a particular problem under jointed pavements where ingress of water along joints may be rather large once joint sealants have failed.
A recent study in Finland showed that more open-graded sub-base is a satisfactory means of reducing the moisture content of a granular base course, but that such a sub-base should not be used beneath an open-graded base course due to stability issues.
Typically, open-graded unbound granular materials are used as permeable layers, however the use of cement or asphalt treated permeable bases can add some extra strength and stability to the drainage layer (and, hence, the pavement) if needed. The resulting material is some kind of buried “no-fines” concrete or porous asphalt. Normal OGDLs are relatively expensive to source, due to the wastage of the fine fraction, and expensive to compact due to the stability issue. Treatment only adds to this.
Table 13.2 Example of gradation of unbound granular permeable bases in Spain
Sieve size (mm) 25 20 8 4 2 1 0.500 0.250 0.063
Passing (% by mass) 100 65-100 30-58 14-37 0-15 0-10 0-6 0-4 0-2
Although authors differ over the precise value, pavement layers can be considered as permeable when their coefficient of permeability exceeds approximately 10-5 m/s. Therefore, many of the conventional dense-graded unbound granular layers cannot be considered as permeable. Table 13.2, shows a typical grading of permeable granular layers used in Spain, where material passing the 1 mm size sieve is limited to 10% (by mass). Since sometimes segregation problems have been detected when pavers are used to lay the layer down, a uniformity coefficient, Cu, less than 4 is required. In addition, in order to get enough stability during the construction of the layer above, the permeable base must mostly be made of crushed aggregate.
Illinois experimented with the use of thin treated OGDLs (7.5-15 cm thick) beneath both concrete and asphalt surfaced pavements during the late 1980’s and early 1990’s (Winkelman, 2004). Four projects were constructed to monitor the effectiveness of the drainage layer and the performance of the pavement. Five additional projects were constructed based on the early performance of the monitored projects. However, continued monitoring of the initial projects, and additional projects, indicated two of the pavements were quickly deteriorating. Surface pavement distress, severe lane to shoulder differential settlement, and high pavement deflections for these two projects indicated a failure of the pavement structure.
The immediate response was to stop using OGDLs, although the non-failing pavements built over OGDLs were not removed. Six of the projects were monitored through from construction until 2003 to ascertain the longer term performance of these pavements and to see whether the OGDL had been beneficial. Some contained cement-treated OGDLs, some asphalt-treated OGDLs.
It was concluded that:
i) The use of an OGDL is more expensive than the use of a standard stabilised base material or lime modified soil;
ii) Some limited benefit due to drainage may be achieved, particularly early during the life of the pavement. However, the longer term performance of the monitored pavements was not much better than that typical for other pavements in the area built without OGDLs;
iii) The intrusion of fines from the subgrade and the aggregate separation layer into the OGDL resulted in settlement, faulting, and eventually premature failure of the pavement, therefore, the use of a geotextile fabric or dense graded aggregate filter under the OGDL to prevent the intrusion of subgrade fines is recommended;
iv) The benefits of using cement-treated OGDLs over asphalt-treated OGDLs, or vice versa, could not clearly be determined;
v) For continuously reinforced concrete pavements, the limited benefits of using an OGDL do not outweigh the increased costs, construction difficulties and maintenance requirements;
vi) Segregation is problematic during construction and careful use of plant is needed to minimize it; and
vii) Careful consideration should be given to subgrade soil analysis, topography and surface drainage, and pavement type. OGDLs are not suitable for all situations.
Most of the pavements were quite heavily trafficked and it may be that different findings would have been obtained on more lightly trafficked pavements.
