Collection System Design, Plans, and Specifications
Core concepts
Know gravity sewer design basics: slope, Manning's equation, minimum pipe size (commonly 8 inch for public gravity mains), cover, and alignment. Slopes must produce self-cleansing velocity at design flow while limiting scour. Manholes are placed at changes in grade, size, alignment, and at maximum spacing (commonly 300-500 ft depending on jurisdiction). Drop manholes handle steep incoming grades.
Reading plans and specs
Understand plan-and-profile sheets: stationing, invert elevations, rim elevations, slope, and pipe class. Specifications define material, bedding, testing, and compaction. Know the difference between design drawings and as-builts; as-builts reflect field changes. Standard details govern manhole construction, bedding, and trench backfill.
Design traps
Questions test whether you can compute slope from inverts: slope = (upstream invert - downstream invert) / length. Watch units (feet per foot, not percent unless asked). Do not confuse invert with rim. Minimum cover protects pipe from loads and frost. Also know that flatter slopes require larger pipes to carry the same flow.
Rule of thumb
Slope (ft/ft) = invert drop / pipe length. Example: 0.60 ft drop over 300 ft = 0.002 ft/ft, or 0.2%. Check this against minimum velocity before accepting.
Worked example
Upstream invert 100.50, downstream 99.90, length 300 ft: drop = 0.60 ft; slope = 0.60/300 = 0.002 ft/ft (0.2%). Verify velocity meets the ~2 ft/s minimum.