Purpose
Maintenance is necessary for the preservation of the capital asset, the continued safety of the users and to reduce vehicle operating costs. This is usually attained by retaining the standards of the road as built.
Drainage
It is important that the subgrade strength is retained by having a clear drainage ditch to ensure a water table that is as low as is possible. Otherwise the designed pavement thickness may be compromised and expensive failure and loss of asset ensue.
The main purpose of the ditch is not to transport water, but to intercept springs and depress the water table. Thus in high rainfall areas culverts should be frequent to pass water to discharge on the downhill side with minimum erosion. Silt traps where provided should be cleared regularly.
A well profiled ditch with established grass cover should require very little maintenance. It may be necessary to cut woody growths before they become too substantial. An alternative where environmental constraints allow is the use of herbicides or a combination of both.
When necessary and where the subgrade strength can allow the machine to travel on the subgrade soil, it is usually least expensive to reshape ditches by the use of the motor grader. On weaker soils the work must be done by using excavators which can also re-profile the banks of older roads to encourage the establishment of vegetation.
The reshaping or cleaning of ditches must be done during dry weather to minimise erosion and sedimentation during the soil disturbance. It must be remembered that downstream may be domestic water supply points or fishery interests and local advice should be sought.
Thus good practice aims to achieve appropriate visual and environmental standards, avoid erosion, sediment pollution and to minimise costs by reducing maintenance operations. There is the possibility however that the roadside vegetation can become very interesting botanically and pressure may arise to award almost a nature reserve status.
Pavement thickness
The designed and constructed pavement thickness must be preserved or excessive subgrade rutting will occur. This in turn may lead to deep seated failure requiring a total and expensive reconstruction.
Pavement thickness is lost by the attrition of surface stone. The causes are believed to be:
a. Heavy rain washing away small sized particles and the subsequent release and rolling away of coarser material.b. Wind blowing away small particles in dry conditions and the subsequent release and rolling away of coarser material.
c. Dislodgement and dispersal by vehicle tyres.
This effect can be from all vehicle types. It is worsened by extra traffic, increased speed and by the aggressive tyres of off-road vehicles.
d. Re-grading.
The operation of re-grading can itself lead to some loss of material from the surface. A skilled operator can reduce the loss during the actual re-grading operation, but there seems also to be some consequential loss arising from the previous effects in re-establishing the surface crust. Re-grading should therefore only be done when necessary.
In order to preserve an adequate pavement throughout the life of the road, either additional sacrificial material should be provided initially, or the road periodically re-metalled.
Avoid the use of metalliferous or sulphide rich surfacing materials which may produce a toxic run-off.
Re-metalling costs vary widely with local availability of stone, but usually a crushed stone is required in order to be laid in a thin layer. Typically a 75 mm layer requires some 650 tonnes/km. The rate of attrition is therefore important.
Surface
After grading and roiling, the road surface, even one of good materials, will actually improve under traffic. The surface tightens under the rolling effect of vehicle tyres to form a smooth waterproof crust. Further traffic then begins to cause attrition and increased roughness.
The roughness is quantified in units of millimetre of roughness per kilometre. The millimetre being an integrated amount of vertical movement relative to an inertial datum whilst travelling the kilometre. The method was established by the Overseas Unit of the Transport Research Laboratory of UK. Typical UK values are:
|
bitmac road |
3 000 mm/km |
|
unsurfaced road |
6 000 mm/km (Just regraded) |
|
Onset of pot-holes |
10 000 mm/km |
Tests carried out by the Forestry Commission in Great Britain have shown that typically after 10,000 axles the road surface returns to the same roughness value as a just re-graded and rolled surface. Further traffic increases the roughness in a generally linear relationship, typically at a rate of 1 000 mm/km per 7,000 axles.
The work of the TRL Overseas Unit has shown that an increased surface roughness increases the operating costs of vehicles using the road. On economic grounds, savings in vehicle operating costs might justify efforts to provide an improved surface.
Pot-holes in surfacing materials often seem to be watertight and cause no weakening of the pavement material, thus, the presence of pot-holes is not a particularly damaging condition in terms of the preservation of the asset other than the actual loss of metal. Heavy pot-holing does however lead to a substantial increase in user complaints, more so if the length of journey extends beyond a few kilometres. Eventually the presence of extensive and severe pot-holing becomes, not just an annoyance, but an increased and unacceptable risk to the safety of the user and gives rise to claims for increased vehicle operating costs.
Re-grading should therefore be scheduled according to the condition of the surface and for guidance it is suggested that 80-100 pot-holes per kilometre represents a reasonable level at which to re-grade. In very exceptional circumstances, where high volumes of traffic are carried, it may be necessary to re-grade more than once per year, but this is usually associated with recreational usage where "user satisfaction" is very important.
Without any traffic, an unsurfaced road is likely to become overgrown and the surfacing material becomes contaminated with organic debris from grass, moss and roots. If after some years it is brought back into use, the contaminated surface layer is often lost because it is either deliberately removed to improve traction and for the safety of users, or it simply erodes quickly because it is loose and of low strength. It is possible to avoid the road from becoming overgrown by re-grading on a return period of three to five years depending upon latitude, altitude and climate. The costs of this fall-back re-grading must be balanced by the value of the lost asset from the surface contamination. Only when a long period of non-use is expected and high financial interest rates apply is it considered to be appropriate to temporarily abandon the road.
When in use local factors such as the surfacing material, climate, traffic mix, gradient, traffic speed etc will influence the rate of deterioration of surface condition and although it should be based on actual condition, re-grading after 20,000 vehicles is suggested as a typical interval.
Thus the following re-grading intervals are suggested:
|
Vehicles per year |
Re-grading interval |
|
> 20,000 |
less than one year |
|
12,000: 20,000 |
1 year |
|
8,000: 12,000 |
2 years |
|
6,000: 8,000 |
3 years |
|
5,000: 6,000 |
4 years) If vegetation growth |
|
< 5,000 |
5 years) permits. |
Some surfacing materials are sensitive to disturbance when either too wet or too dry and hence are best worked only at those times of the year when their moisture content is appropriate. Also a grader team will have the capacity to maintain a large quantity of road network necessitating operation over a large territory. It is therefore appropriate for economy to plan ahead to allow the work to be properly scheduled and to minimise movements.
During the re-grading operation it is often advantageous to supply additional metal to replenish sections of heavy attrition loss, such as at comers and on bends.
The running surface will be restored to a camber to shed water to the sides. It is important that the re-grading operation does not leave a raised edge to the road which impedes the escape of surface water.
Winter Conditions
During periods of frost, the moisture content within the body of the pavement increases from water drawn by capillary attraction and as vapour, in extreme cases ice lenses may form.
Whilst frozen this causes no problems, indeed even weak roads, hard frozen, can often be used to advantage. Severe problems however arise when the thaw begins, for then pavements are both unconsolidated and saturated. The change of moisture content taking place mainly within the smaller sized binder material which then loses strength rapidly from even small overall moisture content changes.
Use in that condition by heavy vehicles is likely to cause heavy damage. Drying would be dependent upon the prevailing weather conditions but it may well take a period of several weeks to regain adequate strength. Light rolling in the latter part of the period would help to restore compaction.
Fresh snow often packs down under traffic to ice which binds to the surface layers and its removal is also likely to remove the surface layer of good metal and tight surface finish. It is difficult and expensive to establish a safe running condition on top of surface ice, but it can be done by the generous spreading of coarse grit. It is better to avoid the formation of the surface ice and users should be asked to avoid making journeys if at all possible.
Under no circumstances should salt be applied to an unsurfaced road to remove ice. The rapid thawing it induces renders the surface loose, weak and saturated. If trafficked it will disintegrate, lower levels become affected and major damage can ensue.
It follows from the above that unsurfaced roads are suitable for winter use, but the limitation of non-use in the thaw period must be accepted.
Operations and Mechanical Plant
The grader should be fitted with scarifier tines and perhaps a towing beam to tow the roller if necessary. The use of a loading shovel instead of a tractor adds versatility, to remove abandoned timber produce from ditches or to re-handle metalling material from stockpiles. It also extends the range of duties for a team, allowing car parks and light construction work to be tackled.
With an appreciation of the processes at work related to particular sites and circumstances, the maintenance managers of unsurfaced roads can determine appropriate strategies to provide serviceable roads that continue, in the long term, to be genuinely low cost.
