HAUL ROAD ENGINEERING – PART 2
Every construction project requires specific care so that the final result is compatible with what you wanted to achieve. In mining, there are many miners that believe the only purpose of haul roads is to connect two points. But is it really?
Haul road design is critical to the longevity of the mine haul road as an asset. The haul road should, in conjunction with the haul trucks using the road, be designed to deliver a specific level of performance and its design, operation, routine, and longer-term maintenance requirements managed accordingly. If not, a critical production asset becomes a significant operating liability.
Roads that make up these sectors are part of a critical and important component of the production process. Roads in poor condition whether due to lack of maintenance or lack of construction quality, result in impacts on productivity, the safety of workers, and equipment life cycle.
A good haul road design is that one with an integrated design approach of sizing, building and maintenance to ensure the safety and quality in the field. The design requires an analysis of several components that meet a set of criteria where the absence or the deficiency of one can harm the others. Key design considerations are discussed below:
How important is Haul Road Geometry?
A good geometric design allows for safety, reduction in fuel consumption and agility in operation.
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The geometry of a haul road must present a horizontal alignment that enables a line of sight distance, which must be greater than the stopping distance to allow the driver see the potential hazards ahead, such as intersections, parked or broken down vehicles, or debris.
Curves of geometric designs must meet criteria in a way that facilitates the transport operation. Curves with a very small radius, require vehicles to slow down and obviously increase the time and costs of operation, although this must be balanced against available space onsite.
The vertical geometry design must provide smooth ramps with a constant grade for better trafficability of mining vehicles. Ramps with greater inclinations make it difficult for the vehicles to reach operating speeds and can result in loss of material during transportation or unnecessarily increasing fuel consumption. Again, this must be balanced against site constraints with the view of optimising the outcome.
How important are Crossfall and Drainage?
Another important component of the designing of haul road is road crossfall and drainage. Drainage elements to consider are surface (tied to crossfall), longitudinal, and subsurface using pipes or culverts. Water results in several defects that may influence mining operations, such as damaging the wearing coarse layer or pavement structure. Constructing and then maintaining the road with a crossfall of 2% to 3% is required to avoid the accumulation of water on the surface, and thus to minimize the damage to surface layers and the percolation through to lower layers.
Adequate longitudinal drainage in the form of v-drains (ditches) to remove/intercept this water is required, and ties into the vertical geometry of the road alignment. Provision to shed this water off through safety bunds or into subsurface pipes must also be considered – as does the need to control erosion at the outlet.
Subsurface drainage is rare in many mines, due to the short-term nature of many roads. However, it should be better considered, particularly where roads cross natural flow paths of concentrated flows. Even in dry environments, water from steep catchments can cause catastrophic damage when not designed for.
How important are Pavement materials?
Pavement design aims to decrease stresses on the subgrade, which the resistance is typically measured by California Bearing Ratio (CBR) test. Low resistance values of the subgrade require higher thicknesses to meet designing criteria and thus can be divided into layers with different resistances. When the resistance criterion is not met, the pavement begins to present permanent displacements and can lead to accumulation of water, delayed operation or collapse of the pavement structure.
Most often we see the competing requirements for production and construction resources limiting what a crew could actually achieve in terms of materials and build quality. Mining is dynamic and we often need to make short-term tactical decisions to maintain production and fleet utilisations. It’s when these short-term ‘fixes’ become a long term that the road problems increase – you can’t ‘undamage’ a haul truck once it’s been run on a poor road for a few shifts. All too often we see sites chasing their tails; poor roads require frequent maintenance grading, often at the expense of the better roads – or surface treatments for dust control – which eventually results in an overall decrease in road conditions across the site.
Haul road pavement must be functional, that is, it requires adequate values of rolling resistance and adhesion. In this way, crushed stones with good quality are recommended for wearing course to ensure good operation performance, safety and to avoid defects such as potholes. Engineering input is required to find this material onsite, source this material from offsite, or if completely unavailable – to make the best of a bad situation.
“A good engineer should be a master of optimisation”
Proper compaction of road embankment and then the pavement itself is another critical factor often missed. This is worse in road maintenance practice. Whilst nearly all mines will have graders running continually, it is extremely rare for rollers to be utilised onsite.
How important is Haul Road Maintenance?
‘Best’ road maintenance practice will vary from site to site, depending on the impact of the deterioration – often assessed in terms of rolling resistance. Using truck speed-rimpull-gradeability data gives us a useful insight into where on a network of roads the impact of deterioration (measured as an increase in rolling resistance) is greatest.
Rutting is a typical example of a structural design issue – not enough or poor quality cover over the weaker sub-grade or in-situ, or for new builds, often poor layerworks compaction too. In this case, simply blading the road will not correct the problem as they’ll reoccur again as the layerworks and sub-grade continue to deform.
The other two defects can be traced back to sheeting (or wearing course) material selection, deterioration, and/or spillage on the road. Even the best sheeting will deteriorate over time, often exhibiting an increase in fine clay fractions, which leads to dustiness when dry and slipperiness when wet. So, the first step if these problems persist would be to check the upper 50mm of your sheeting, how does it compare to the selection criteria?
It’s often only the upper 50mm or so of the sheeting that is regularly bladed and in contact with the truck tyres (and often contaminated with tyre carry-over), sheeting material below that is still in a good condition and, assuming your sheeting was well selected in the first instance, a deep rip and remix will bring the fresh sheeting back to surface and address the problem in the short-term. In the longer term though, eventually, a resheet will be required.
In addition to construction criteria, haul roads require careful operation and maintenance. During hauling of the mined materials, haul roads will fatigue and may present other problems such as corrugation, rutting, and potholes due to loss of in situ materials, displacements/settlement, and weather events. Engineering oversight of maintenance processes must take into account the need to maintain the intent of the original design, for the life of the road.
However, despite all of this, haul roads may still present problems caused by impacts outside of the geometric, drainage or structure design. For example, road generated dust is a problem that can compromise operation and safety.
We can make further improvements to the road during the latter stages of the design, or during operations, if required, but geometric or structural design problems are more difficult to address, being both costly and extremely disruptive – it’s estimated that the cost to repair or rebuild a poor road can be up to 5x the original unit-construction cost. So the value proposition for fit-for-purpose design or improvement lies in the increased safety, durability, trafficability, and productivity of mine haul roads, through the design, build, or rehabilitation to minimize rolling resistance and deterioration, contributing to reducing truck cycle times, improved tire life and fuel efficiency, reduced road maintenance activities, together with enhanced 24x7x365 trafficability and safety.
GRT can assist you to optimise haul road design, construction and maintenance.
Read Part 1: Productivity On Haul Roads Suffers From a Lack of Road Engineering Input
Read Part 3: Materials Optimisation & GRT Products for Haul Road Pavements
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