Construction of a military helipad at an altitude of 2400 m involves 88000 m³ (loose) of excavation area in soft soil. This task needs to be completed in 200 working hours. The company entrusted two dozers, each with an output of 220 m³/h under job conditions for task execution. It also holds wheel loaders and 22 m³ dump trucks. One loader can load in trucks, about 120 m³ of excavated soil per hour. The dump truck cycle time for disposal of excavated materials is 40 minutes. This includes 8 minutes of loading time by a loader team that consists of 2 loaders. In this case, the total number of dumpers required to complete the task on time is :
- A6
- B12Correct
- C18
- D24
Solution
Concept
Matching loader and hauler fleets
Formula
Trucks per loader team = (cycle time / loading time) + 1
Step-by-step solution
1. Required output = 88 000 / 200 = 440 m³/h (equal to the two dozers: 2 × 220). 2. One loader team (2 loaders) loads 240 m³/h, so loader teams needed = 440 / 240 = 1.83 → 2 teams. 3. Trucks per loader team = 40 / 8 + 1 = 6. 4. Total dumpers = 2 × 6 = 12. (Direct check: 440 / (22 × 60/40) = 13.3 trucks, which is close to 12.)
Explanation
A loader team keeps its trucks busy only if there are enough trucks to cover the cycle time while one truck is being loaded. The number of trucks per team is cycle time divided by loading time, plus one for the truck being loaded.
Why the other options are wrong
6 trucks serve a single loader team, which can load only 240 m³/h, not the 440 m³/h required. 18 and 24 would need three or four loader teams.
Shortcut
Trucks per team = Tc/Tl + 1 = 6; teams = 440/240 → 2.
Common mistake
Forgetting the loader limit and using only the truck capacity.
Correct answer: B) 12