The approach outlined previously can be extended to cover all the defects experienced on a road, to assess the functionality of a road at a point in time. The recording sheet shown below can be used to assess a ‘functional defect score’ (DS) from a minimum value of 12 to a maximum of 300.
Individual mine sites will need to set their own ‘maintenance intervention thresholds (identified in table as asterisk *)’ at which some road maintenance activity is triggered when exceeded. In the table, typical values are as follows: Road maintenance is recommended if:
■ any single critical functional defect exceeds limits; or • total functional defect score (DS) *140.
■ Road maintenance imminent but still trafficable when *65 DS *139.
■ Road in good condition, no immediate maintenance needs when DS *64. The decision whether or not to maintain the road is not only based on the total DS, but also on critical (identified in table as hash #) individual defect scores too, since these generally adversely effect safety and trafficability. These critical individual defects are generally corrugations, loose material, fixed stoniness, dustiness and wet and dry skid resistance and each has a critical functional defect limit which should also be considered in addition to the overall DS. These values used here are specific to each mine site and it’s operating conditions. Since functionality and road performance in general is influenced by drainage and erosion, it is useful to comment on these two aspects also - poor drainage and/or excessive erosion on the road would normally trigger some maintenance activity in its own right.
The first 5 defects are scored as described in the previous section for rolling resistance, using the visual assessments provided. The scoring for the extra defects considered from a functionality perspective is given in the Figure on the next page.
Another approach is to assess road functionality according to the chart given here. Functional performance acceptability criteria (limits for desirable, undesirable and unacceptable) should be based on your mine’s operating experience - average values for many mines are shown here - but mostly operating in dry and temperate environments. Used on a daily basis, this chart is useful to record how a road deteriorates over time - a road that always returns values in the red sector is probably a good candidate for rehabilitation. If your road segment is always scoring in the yellow and red sectors - even despite frequent maintenance interventions - then it is worth re-evaluating the wearing course functional design and possibly even the structural and/or geometric designs - since the poor performance is not in itself indicative of poor maintenance - rather an underlying design deficiency.
It can also be useful to use this concept in day-to-day road maintenance planning. If roads are evaluated at start of shift, they can be marked with red, yellow or green cones to indicate what segments should enjoy maintenance priority (red). This approach is also useful for truck drivers - it helps them anticipate road (and traffic) conditions and thus operate their trucks accordingly. In either case it is important to retain these records and evaluate how each segment of road changes over time and traffic, to identify those segments of the network that continually under-perform, the reasons for this (using the typical defect to identify the root-cause) and thus the most appropriate remediation strategy.
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