Pipe Installation, Alignment, and Grade
What this domain tests
Pipe Installation, Alignment, and Grade tests whether you can place pipe on line and grade, joint it without damage, and verify invert elevations. Expect grade math, laser and string-line setup, bedding, lowering, restraint, and backfill.
Grade and alignment
- Percent grade equals rise divided by run, times 100. A fall is a negative grade. Convert percent to decimal before multiplying.
- Invert is the inside bottom of the pipe; flow line is the hydraulic bottom. For circular pipe they coincide.
- Cut sheets give station, invert, and grade. Set hubs or offset stakes outside the trench. Use a laser, transit, or total station with a grade rod or target.
- Check the laser against a known elevation; lasers drift and targets get bumped.
Installation sequence
- Prepare bedding; lower pipe with slings, not chains or loader teeth; assemble the joint to the insertion mark; check line and grade before backfill; place and compact initial backfill; then test.
Worked example
Invert at station 0+00 is 104.50 ft; grade is -0.75%. Find the invert at station 2+40. Reasoning: 2+40 equals 240 ft. Drop equals 0.0075 times 240, or 1.80 ft. Invert equals 104.50 minus 1.80, or 102.70 ft. If the laser is set 1.00 ft above the invert, the target at 2+40 should be at elevation 103.70 ft.
Traps
- Measuring grade to the top of pipe instead of the invert.
- Adding a fall instead of subtracting it.
- Using 0.75 instead of 0.0075.
- Trusting a laser without a benchmark check.
- Backfilling before line and grade are checked.
- Exceeding joint deflection to force a curve.