What are the key design criteria for haul road grades in relation to truck type and payload capacity?

The design of haul roads requires balancing the mechanical limits of the truck fleet with the physical demands of the payload. The “grade” (the slope of the road) is arguably the most critical factor, as it dictates cycle times, fuel costs, and component longevity. The following sections detail the key design criteria for haul road grades.

Optimal vs. Maximum grade

While modern trucks can technically climb grades as steep as 18 – 20%, doing so is rarely economical. The optimal grade for most large-scale surface mines is 8% to 10% [1]. A common regulatory and safety limit is a gradient of 1:10 (10%), though some operations push to 1:12 for smoother performance. Steep “break-out” grades of up to 12–15% may be used for very short distances (less than 50m) to minimize the footprint of a pit exit, but these significantly increase tire spinning and engine stress.

Truck performance & retarding curves

Engine and braking capabilities vary by truck type (e.g., mechanical vs. electric drive). Designers must consult the manufacturer’s Performance and Retarding Curves. The grade must allow the truck to maintain a steady speed without “hunting” between gears. If a grade is too steep for a specific payload, the truck may drop to 1st gear, leading to engine overheating and excessive fuel burn.

For loaded trucks descending, the grade is limited by the truck’s retarding capacity. The braking system must be able to dissipate the heat generated by the payload’s gravitational energy. If the grade is too steep, the truck must travel at a dangerously slow crawl to avoid “brake fade.”

Rolling resistance (the “Hidden” grade)

Designers use Total Resistance to determine if a truck can handle a slope [1]. Total resistance is the sum of the physical grade and the rolling resistance caused by the road surface.

Total resistance = Grade (%) + Rolling resistance (%)

• Hard/maintained roads: ~2% rolling resistance.
• Soft/muddy roads: Up to 10% rolling resistance.
• Impact: If a road has a 10% grade but is poorly maintained (adding 5% rolling resistance), the truck effectively feels a 15% incline. This can stall a fully loaded truck or cause the tires to slip and “slice” on the rock.

Weight distribution and tire life

Payload capacity is directly tied to how weight shifts on an incline. The static distribution is usually 33% front / 67% rear on flat ground [2]. As the truck tilts back, more weight shifts to the rear dual tires [3]. On a 12% grade, the rear tires can bear significantly more than their rated capacity, leading to internal heat buildup (TKPH-Tons Kilometers Per Hour limits) and premature tire failure. If the grade is too steep and the road surface is wet, the “coefficient of traction” may be lower than the required rimpull, causing the wheels to spin and destroying the expensive tire treads.

Vertical curves & sight distance

The transition between different grades (the vertical curve) must be gradual. At the top of a ramp (crest), the curve must be flat enough so the driver can see an obstacle in time to stop. For very large trucks (e.g., CAT 797 or Komatsu 930E), a sharp “dip” (sag) at the bottom of a grade can cause the truck’s bumper or frame to strike the ground due to the compression of the suspension under payload.

Summary table: grade design impact

Reference

[1]        A. Shakenov, “OPTIMAL SLOPES OF MINE HAUL ROADS,” Jul. 2022.

[2]        “ZKG international.” Accessed: Jan. 15, 2026. [Online]. Available: https://www.zkg.de/en/artikel/zkg_Road_Maintenance_Safety-2273698.html

[3]        pcm_admin, “Optimised haul roads key to efficiency and longevity,” Quarry. Accessed: Jan. 15, 2026. [Online]. Available: https://www.quarrymagazine.com/optimised-haul-roads-key-to-efficiency-and-longevity/