Haul Road Rolling Resistance Production Shortfall Case Study

Open-pit truck productivity can fail for a reason that is visible on the road but hidden in the production schedule. After rain, a haul road may remain passable and safe at reduced speed, yet still impose enough rolling resistance, rutting, queueing, and speed loss to starve the crusher.

This case study follows a mine that misses daily ore-feed targets even though the shovel is available, the crusher is mechanically healthy, and the truck fleet size appears adequate in the monthly plan. The root cause is a wet and rutted loaded-haul segment that increases truck cycle time and fuel use. The case is hypothetical and intended for engineering education.

The engineering question is:

Should the mine add trucks to recover production, or should it first repair the haul road condition that inflated the cycle time?

The answer is that the fleet must be judged by measured cycle time and effective truck availability. In this case, adding trucks treats the symptom; restoring road condition removes the constraint.

Case Context

The mine feeds a primary crusher from an open-pit ore bench. Dispatch records show that ore trucks spend longer on the loaded uphill segment after a storm event. The road is not closed, but rutting and poor drainage have increased rolling resistance.

Item	Value
Daily ore feed target	55{,}000\ \text{t/day}
Scheduled operating time	20\ \text{h/day}
Truck fleet	16 trucks
Rated payload per trip	180\ \text{t/trip}
Planned average cycle time	45\ \text{min/trip}
Measured cycle time after rain	53\ \text{min/trip}
Mechanical availability	0.88
Operating utilization during scheduled time	0.85
Loaded mass on wet segment	340{,}000\ \text{kg}
Wet loaded-haul segment length	3.2\ \text{km}
Planned rolling resistance coefficient	0.025
Measured wet-road rolling resistance coefficient	0.055
Net ore contribution margin	8.00\ \text{USD/t}

The production symptom is a crusher feed shortfall. The operational symptom is slower loaded travel, more queuing near the shovel, more bunching near the dump pocket, higher fuel burn, and more road callouts from operators.

Planned Fleet Check

The effective number of working trucks is:

where N is the fleet count, A is mechanical availability, and U is operating utilization during scheduled time.

With:

the effective fleet is:

The planned trips per day are:

where:

and:

Therefore:

Planned ore movement is:

The apparent planning margin is:

or about 4.4\%. The monthly plan therefore looked feasible, but only if the 45\ \text{min} cycle remained valid.

Measured Production Shortfall

After the storm, the measured average cycle time is:

Using the same availability, utilization, payload, and operating time:

Daily ore movement becomes:

The daily shortfall is:

The crusher feed rate implied by the target is:

The wet-road feed rate is:

So the crusher is not the primary constraint. The haulage system is delivering about:

less ore than required.

Rolling Resistance Evidence

Rolling resistance force can be screened as:

where C_{rr} is the rolling resistance coefficient, m is loaded vehicle mass, and g is gravitational acceleration.

The incremental coefficient caused by the wet road is:

The extra tractive energy on the 3.2\ \text{km} loaded segment is:

Using:

gives:

or:

This is a lower-bound screen because it does not include additional acceleration loss, gear selection, waiting on grades, tire deformation heating, or traffic interference.

Fuel and Emissions Penalty

Assume diesel fuel energy content is:

and the effective drivetrain efficiency for this duty is:

The extra diesel per loaded trip is:

At the measured wet-road production rate:

the extra fuel is:

If the mine tracks a diesel emission factor of 2.68\ \text{kg CO}_2/\text{L}, the avoidable emissions are approximately:

The production loss is the main economic issue, but the fuel and emissions penalty confirms that the wet road is a real energy loss, not only a dispatch accounting artifact.

Required Cycle Time

The cycle time required to meet target with the existing fleet is:

where P is truck payload.

Substitute:

and:

Then:

The mine does not need a perfect road to meet target, but it does need the average cycle time to return from 53\ \text{min} to about 47\ \text{min} or better, with enough margin for normal variability.

Adding Trucks Does Not Remove the Cause

If the wet-road cycle time remains 53\ \text{min}, the effective truck count required for the target is:

With the same availability and utilization:

The mine would need about 19 trucks, or three additional trucks, just to compensate for the wet road. That option adds traffic density, queueing, tire exposure, fuel burn, maintenance demand, and operator hours while leaving the road defect in place.

The engineering review therefore rejects “add trucks” as the first response. A temporary truck addition may be justified for a short recovery campaign, but it is not the primary corrective action.

Corrective Road Package

The selected corrective package includes:

1. regrade the loaded uphill segment to remove rutting and restore crossfall;
2. clean side drains and culverts so rainfall is diverted before it reaches the running surface;
3. add wearing-course material at the soft section identified by dispatch speed loss;
4. assign a grader inspection after each rain threshold event;
5. add dispatch alarms when loaded-cycle p50 or p90 exceeds the control limit;
6. verify tire temperature and truck speed on the corrected segment before returning to normal release.

After the corrective work, the measured average cycle time is expected to return to:

Expected daily movement becomes:

The corrected margin is:

or about 2.2\%. This is a narrow but usable first recovery margin if short-interval control continues. It is not enough to ignore future rain, shovel delays, crusher downtime, or maintenance events.

Economic Screen

The daily ore shortfall value is:

where c_m is net contribution margin.

Using:

gives:

The road package costs:

Cost item	Value
One-time regrade, drainage, and wearing-course work	120{,}000\ \text{USD}
Additional wet-season road maintenance	8000\ \text{USD/day}

The net daily recovered value is approximately:

The simple payback is:

This is not a full mine economic model. It excludes metal price uncertainty, grade variation, processing recovery, maintenance backlog, tire damage, contractor constraints, and safety risk. It is enough to show that road restoration is not merely housekeeping; it is a production control action.

Release and Validation

The corrected road condition should not be accepted from a single good hour. The production system needs evidence that cycle time, crusher feed, and road condition remain stable under representative operation.

Acceptance evidence should include:

1. dispatch cycle-time distribution before and after the road repair, separated into loaded travel, empty travel, loading, dumping, and queueing;
2. p50 and p90 loaded uphill segment speed for the corrected road section;
3. measured rolling-resistance proxy from fuel burn, speed, grade, payload, and truck telemetry;
4. crusher feed rate and starvation minutes during normal shovel assignment;
5. road inspection records after rainfall thresholds;
6. tire temperature, truck event logs, and operator reports for the wet segment;
7. dewatering and drainage inspection confirming that water is not being routed onto the running surface;
8. production reconciliation between dispatched ore tonnes, surveyed movement, and crusher belt scale;
9. maintenance plan showing grader and water-management resources assigned before the next rain event;
10. stop or reroute criteria if loaded-cycle p90 exceeds the limit for two consecutive dispatch intervals.

Release criteria should include:

with:

and:

for representative ore movement without increased safety events, tire alarms, manual dispatch overrides, or unplanned road closures.

Engineering Lessons

The first lesson is that truck fleet capacity is not a static number. It depends on cycle time, payload discipline, availability, utilization, queueing, grade, road condition, weather, and traffic rules.

The second lesson is that a road can be operationally open but production-limiting. Rolling resistance converts directly into cycle time, fuel, heat, congestion, and crusher feed loss.

The third lesson is that adding equipment can mask a process defect. If road condition is the cause, more trucks may increase queueing and cost while leaving the production system fragile.

Good mine planning therefore treats haul roads as production infrastructure. The schedule, dispatch data, road maintenance plan, dewatering controls, crusher feed record, and economic screen must agree before the mine calls the shortfall solved.