Local roads news

Road & Transport Research, Jun 2000 by McRobert, J, Giummarra, G

A project recently completed for Transport SA involved the evaluation of the use of wet and dry maintenance practices, from an economic, environmental and social perspective, for the unsealed road network in outback areas of South Australia.

Dry maintenance

Dry maintenance is currently the predominant form of routine maintenance grading carried out on outback roads in SA. The term 'dry maintenance' indicates that no water other than occasional natural rainfall is available to add to the pavement. Typically dry maintenance grading is performed about every 2-3 months, taking advantage whenever possible of wet weather periods. Full 'resheeting' or reconstruction of the pavement surface is required about every 7 years. Dry maintenance comprises the use of a grader only, to smooth the road by respreading the mixed fines and gravel material across the surface.

Wet maintenance

Wet maintenance is a more heavy grading or intensive maintenance process which involves bringing water to the site and then adding it to the surface materials (in addition to any natural rainfall) followed by a means of compaction. Wet maintenance procedures, involve ripping and reforming the pavement, recombining the fines in the pavement layer and then compacting the surface with a roller. This procedure reduces the required number of routine maintenance cycles by providing a tighter more coherent surface. This reduction can be from as many as six or more grading cycles per year to only one or two, i.e every 9 -12 months. Furthermore it has been estimated that major rehabilitation or full resheeting of a wet maintained pavement will be required only every 20-25 years (compared to every 7 years under dry maintenance).

ECONOMIC EVALUATION

In terms of routine maintenance costs, dry maintenance is a substantially lower cost option, at an annual cost of about $1100 per km maintained compared to $3500 per km for wet.

On a full lifecycle costing basis, however, and assuming a 25 year pavement life, there is little difference between the two maintenance alternatives. The increased frequency of major rehabilitation or resheeting works for dry maintenance (between 3 to 4 times) raises the costs in line with wet maintenance. After adding both routine and periodic maintenance costs over a 25 year life cycle (with no discounting) dry maintenance costs are around $150,000/km compared to $128,000/km for wet. With discounting, there is no clear advantage in either maintenance treatment from an economic perspective, while there are some quite significant advantages of wet maintenance for the environment.

ENVIRONMENTAL AND SOCIAL EVALUATION

The road network causes localised environmental damage in outback SA regions, principally due to its interference of the natural drainage system and the exposure of unsurfaced roads. Severe erosion and sedimentation can be associated with the drainage system of the road. Other hydrological impacts from these roads can arise when natural water flows are obstructed during flood events.

Some damage to vegetation communities can take place during road construction and maintenance activities, particularly during materials winning for reconstruction or major rehabilitation. Protective vegetative cover can also be removed where road maintenance equipment operators position their turning circles and at work camp and bore site locations. Additional pressures arise from mining operations and tourists, which all require formed roads or at least access tracks.

In terms of environmental and social impacts, wet maintenance gives increased benefits for most factors. The impact of extra pumping on local aquifers however, in terms of both groundwater quality and quantity, is presently unclear. This remains an unresolved issue and requires further investigation in the event of a substantial shift toward wet maintenance.

Even in the absence of this knowledge it would seem that, on balance, there is strong evidence that wet maintenance is likely to be more beneficial from an environmental and social perspective.

The environmental and social benefits of wet maintenance include:

Reduced new materials usage -4 times less new quarried materials

Reduced sediment transport to waterways

Fewer disturbed sites as borrow pits

Substantially less dust for road users

Improved safety for motorists

Moderately lower fuel usage and greenhouse emissions

CONCLUSIONS

In practice, there will always be a mixture of wet and dry maintenance carried out. Higher class unsealed roads are the most likely to receive full wet maintenance treatment. Logistics may also dictate that even certain sections of these cannot be maintained under a wet maintenance regime or possibly only partially so.

In pavement maintenance and environmental best practice terms however, wet maintenance is the recommended practice where there is a sustainable water supply.

Wet maintenance is expensive in terms of routine maintenance but over the life of the pavement the results are inconclusive.

The study also found there to be a number of existing practices in unsealed road design and maintenance which are environmentally unsustainable. These practices are likely to have wide applicability across many parts of Australia: