Construction: Methods, Scheduling, and Cost
Construction questions test whether you can move between drawings, quantities, schedules, and money without losing track of units. Core concepts: earthwork volume by average end area or prismoidal formula; shrinkage and swell factors (bank, loose, compacted states); production and cycle time for excavators, trucks, and compactors; critical path method (CPM) with forward/backward pass, total float, and free float; earned value (PV, EV, AC, CPI, SPI); and quantity takeoff from plan dimensions.
Traps
- Confusing bank (in situ) with loose volume - always convert with the swell factor before matching to truck capacity.
- Reporting total float when the question asks for free float (free float = early start of successor minus early finish of the activity).
- Forgetting that a change in one activity's duration only affects the project if it is on the critical path.
- Mixing cost and schedule indices: CPI = EV/AC, SPI = EV/PV.
Worked example
An excavator with a 1.5-cy bucket, 45-second cycle, 50-min effective hour, and 0.85 bucket fill produces: cycles/hr = 50/0.75 ≈ 66.7; loose volume = 66.7 × 1.5 × 0.85 ≈ 85 cy/hr loose. If bank density gives a 1.25 swell factor, bank production = 85/1.25 = 68 bank cy/hr. Rule of thumb: convert to bank measure before comparing to a bid quantity, since payment is usually by bank volume.