9.4 Equipment Selection and Production Rates

Equipment Selection and Production Rates

Why This Topic Matters for the PE Construction Exam

Estimating project duration and cost heavily relies on understanding equipment productivity. A construction engineer must select the right size and type of equipment for the job and calculate how much material it can move per hour. The PE exam tests your ability to calculate cycle times, apply efficiency factors, account for soil swelling/shrinkage, and balance fleets (e.g., matching the number of trucks to an excavator) so that neither equipment sits idle.

Soil State and Volume Changes

Earthwork calculations must account for the physical state of the soil, as volume changes significantly when soil is excavated or compacted.

• Bank Cubic Yards (BCY): Material in its natural, undisturbed state. Most earthwork quantities and payment terms are based on BCY.
• Loose Cubic Yards (LCY): Material after it has been excavated. Excavation introduces voids, causing the soil to expand. This expansion is called swell. LCY = BCY * (1 + Swell Factor)
• Compacted Cubic Yards (CCY): Material after it has been placed in a fill and mechanically compacted. Compaction removes voids, often making the soil denser than its natural state. This reduction from BCY is called shrinkage. CCY = BCY * (1 - Shrinkage Factor)

Note: Equipment capacities (like a truck bed) are usually limited by LCY or weight limits, not BCY.

Cycle Time Analysis

The productivity of any piece of cyclic equipment (excavators, scrapers, haul trucks, loaders) is determined by its cycle time.

Total Cycle Time = Fixed Time + Variable Time

• Fixed Time: The time spent loading, dumping, maneuvering, and accelerating/decelerating. It is generally independent of the haul distance.
• Variable Time: The time spent traveling loaded (haul) and traveling empty (return). It is directly related to the haul distance and the speed of the machine. Variable Time = (Haul Distance / Haul Speed) + (Return Distance / Return Speed)

Travel speed is affected by the power of the equipment and the resistances it must overcome:

• Rolling Resistance: Resistance from the tires sinking into the ground. Hard paved roads have low rolling resistance; soft mud has high resistance.
• Grade Resistance/Assistance: Resistance from traveling up a hill (adds resistance) or traveling down a hill (assists movement).

Production Calculations

Once the cycle time is known, the maximum theoretical production can be calculated:

Cycles per Hour = 60 minutes / Total Cycle Time (in minutes)

Theoretical Production = Cycles per Hour * Volume per Cycle

To find the Actual Production, you must apply efficiency and capacity factors:

1. Fill Factor (Bucket Factor): Buckets are rarely filled to 100% of their struck capacity due to voids and the nature of the material. A fill factor adjusts the heaped capacity to a realistic LCY volume.
2. Job Efficiency Factor: Accounts for operator skill, minor breakdowns, delays, and breaks. Often represented as "working minutes per hour" (e.g., a 50-min hour = 50/60 = 83% efficiency).

Actual Production = Theoretical Production * Fill Factor * Job Efficiency

Fleet Balancing

In operations involving loading units (excavators) and hauling units (trucks), production is limited by whichever fleet is slower.

To perfectly balance a fleet, the number of trucks required ($N$) is the ratio of the truck cycle time to the loader loading time (which is the time it takes the excavator to fill one truck).

$N$ = Truck Cycle Time / Loading Time per Truck

• If $N$ is a fraction (e.g., 4.3), using 4 trucks will leave the excavator waiting (trucks govern production). Using 5 trucks will leave trucks waiting in line (excavator governs production). Economic analysis dictates which configuration is preferred.

Worked Example: Fleet Productivity Calculations

Scenario: A hydraulic excavator is loading articulated haul trucks with earth.

• Excavator bucket capacity: 3 LCY per cycle.
• Excavator cycle time: 0.5 minutes.
• Bucket fill factor: 0.90.
• Truck capacity: 15 LCY.
• Truck haul time (loaded): 8.0 minutes.
• Truck return time (empty): 5.0 minutes.
• Truck maneuver/dump time: 1.5 minutes.
• Job efficiency: 50 min/hour.

Question 1: What is the Loading Time per Truck?

• Actual volume per excavator cycle = 3 LCY * 0.90 = 2.7 LCY.
• Number of excavator cycles to fill one truck = 15 LCY / 2.7 LCY = 5.55 cycles. (Must round up to full cycles, so 6 cycles).
• Loading Time per Truck = 6 cycles * 0.5 min/cycle = 3.0 minutes.

Question 2: What is the Truck Cycle Time?

• Truck Cycle Time = Loading Time + Haul Time + Dump/Maneuver Time + Return Time
• Truck Cycle Time = 3.0 + 8.0 + 1.5 + 5.0 = 17.5 minutes.

Question 3: How many trucks are required to balance the fleet?

• $N$ = Truck Cycle Time / Loading Time = 17.5 / 3.0 = 5.83 trucks.
• Conclusion: To ensure the excavator never waits, 6 trucks are required. If 5 trucks are used, production is limited by the trucks.

Question 4: What is the estimated hourly production (in LCY/hr) if 6 trucks are used?

• If 6 trucks are used, the excavator determines the maximum production (it never stops).
• Excavator Cycles per 60-min hour = 60 / 0.5 = 120 cycles/hr.
• Theoretical Production = 120 cycles/hr * 2.7 LCY/cycle = 324 LCY/hr.
• Actual Production (applying efficiency) = 324 * (50/60) = 270 LCY/hr.