Haulroad Design and Maintenance
Major Factors in Haul Road Design
Many factors need to be considered in the design and maintenance of efficient haul roads. Unfortunately there is no single simple rule to follow, however this manual will highlight areas of focus and possible solutions.
“A haul road begins at the load face and ends at the tip head”
Road Geometry
The following criteria needs to be taken into consideration in haul road design:
Equipment size and performance capabilities
a. Safety
b. Topography
c. Mine Life
d. Rolling Resistance
e. Future Plans
Haul Road Design
a. Haul road geometry should provide for the smooth and cost-efficient operation of a haulage truck fleet at the designed operating speed and payload.
b. The road construction should be such that the above criteria is maintained throughout normal mining conditions
Road Width
a. One way - 3 truck widths.
b. Two-way straights - 3.5 truck widths.
c. Two way corners - 4 truck widths.
Grade
a. Maintain smooth grade.
b. Maintain consistent percentage.
Grade
Haul roads should be of constant grade with minimum transitions to ensure optimum performance. The selection of grade should be based on the capabilities of the machine and the expected rolling resistance
Rolling Resistance
For Off-Highway Trucks running radial-ply tyres, assume a minimum RR (rolling resistance) of:
a. Hard, well-maintained road . . . . . . . 1.5%
b. Well-maintained road with flex . . . . . . 3%
c. 25 mm/1” tyre penetration . . . . . . . . .4%
d. 50 mm/2” tyre penetration . . . . . . . . .5%
e. 100 mm/4” tyre penetration . . . . . . . .8%
f. 200 mm/8” tyre penetration . . . . . . .14%
In practice, a 5% increase in rolling resistance can result in up to a 10% decrease in production and a 35% increase in production costs.
Haul Road Alignment:
Geometric elements for operation at normal speeds
Equipment operator sees ahead a distance equal to or greater than the stopping distance.
a. Speed
b. Weight
c. Slope
Design criteria for vertical and horizontal alignment.
Sight distance
Distance from driver’s eye to hazard must equal or exceed required stopping distance.
Vertical. curve crests
Horizontal. curves
Grades
Flatter grades incur greater costs.
Traditional optimum 7%-9%
Reasonable to accept 10% max. sustained
What do you think grades should be?
Horizontal and Vertical Alignment
Design corners and crests that allow excellent visibility at normal travel speeds.
Use worst-case scenarios.
Corners
a. Use maximum practical radii.
b. Employ super elevation for higher speed operations.
c. Use super elevation >10% with caution.
Vertical Curves
To maximise safe working, corners and crests must be designed such that machine operators are capable of seeing and avoiding hazards when travelling at normal operating speeds.
Super elevation of Curves
Negotiating curves can generate high lateral tyre forces. These forces contribute to high tyre wear and ply separation. Super elevating the curve helps eliminate these forces. The amount of superelevation depends on the curve’s radius and the speed at which it is negotiated. The following table is a guide for providing the superelevation necessary to eliminate lateral forces.
Super elevated turns present a danger when slippery. For this reason, curves super elevated over 10% should be used with caution. Unless the proper speed is maintained, matching the elevation of the curve, a vehicle may slide off of the lower edge of the roadway. Super elevated curves should be maintained in good tractive conditions.
Super Elevation Run out
“Correctly designed transitions properly load truck frame, suspension and tyres, as well as prevent spillage.”
Constant Crossfall
To facilitate effective drainage of a haul road, it is necessary to elevate one side of the haul road from the opposite side.
Recommended Crossfall = 1% - 4%
The limiting criteria for maximum crossfall, is as the slope increases, so does the potential for uneven tyre / bearing wear.
One-Way Crossfall
Two-Way Crossfall
Cross-slope
a. Flats
Apply minimum slope to maintain drainage
Use constant crossfall when possible
b. Grades
Minimal cross-slope required
Bench Width
Truck must clear loader under full acceleration.
Minimum width = machine turning radius + width of safety berm.
Drainage and Safety Berms
Drainage system - sized to accommodate maximum rainfall.
Berm size - at least one-half wheel height.
Drainage
Any haul road design is only as good as the drainage capabilities of the design.
The key to maintaining a haul road in the best structural condition, is to limit the exposure of the surface water and to remove any surface water quickly.
If the construction of complex drainage is not feasible or economic, the simplest way to reduce water damage to a haul road is to elevate the haul road above the lay of the land.
Safety Berms (Windrows)
Berms are an effective way of preventing vehicles from straying into dangerous areas. They can be placed at intersections to control traffic and on the edges of steep slide slopes.
A berm should be a minimum ½ wheel height of the largest wheeled machine on site.
Pavement Cross Sections
A well structured and drained haul road is critical to the efficient operation of haul trucks.
The road base geometry is dependent on the capacity of the truck and the materials available.
Not all mines have the same base material properties….. But as long as the CBR (Californian Bearing Ratio) value is know, the depth of the base can easily be determined for a particular load.
The attached table, demonstrates a typical pavement cross section for the range of Cat Mining Trucks.
Summary:
a. Haulage way width
b. Max., sustained grades
c. Rolling Resistances
d. Stopping distance
e. Sight distance
f. Vertical curves
g. Super elevation rate, run out
h. Sub base
i. Surface materials
j. Sharp curves
k. Combined horizontal, vertical alignment
l. Cross slope
m. Typical cross sections