Sewer Grade Conflict: Benchmark or Stake Error?
You are the crew chief staking a 1,200-foot sanitary sewer main at 0.50% grade. The contractor's grade checker tells you the invert at station 8+00 is 0.18 ft higher than the plan elevation. Your total station backsight check was 0.02 ft, and your level loop from the bridge-abutment benchmark closed 0.03 ft over eight setups. The nearest alternate benchmark, a brass disk 900 feet away, differs from the bridge disk by 0.11 ft. The contractor has already laid pipe from 0+00 to 6+50 and is waiting on your decision. You have 45 minutes before the concrete crew arrives. The plan invert at 8+00 is 612.45 ft; your cut sheet shows a rod reading of 4.62 ft on a 5.00-ft benchmark.
1. What is the most likely source of the 0.18-ft discrepancy, and what immediate checks do you perform before restaking?
Read the worked answer
The most likely source is a benchmark or height-of-instrument problem, not the plan grade. A 0.18-ft error over 800 ft is a 0.0225% grade error, far larger than the 0.03-ft loop closure, so random leveling error is unlikely to explain it. The 0.11-ft split between the two benchmarks is a red flag: one may be disturbed, or the 900-ft sight is too long and affected by curvature, refraction, and rod settlement.
Immediate checks:
- Run a closed level loop between both benchmarks with balanced sights under 300 ft.
- Recheck HI on the bridge disk and verify the rod zero and plumb.
- Recompute the cut sheet independently and compare the plan invert.
- Verify the contractor's rod reading and the as-built invert of the laid pipe.
- Check the total station setup and backsight.
Do not restake until the benchmark is resolved.
2. The contractor asks you to simply lower the remaining stakes by 0.18 ft and keep going. How do you respond?
Read the worked answer
Refuse. Lowering the stakes by 0.18 ft assumes the error is in your stakes, but if the bridge benchmark is high, the pipe already laid from 0+00 to 6+50 may be high as well. That would create a grade break and a possible gravity-flow problem. Instead, stop staking, notify the PLS or office, have the contractor shoot an as-built invert on the laid pipe, compare it to plan, and resolve the benchmark. If the error is in the benchmark, a decision must be made about the laid pipe, not a field fix. Document the contractor's request and your refusal in the field notes.
3. What documentation do you provide to the office?
Read the worked answer
Provide the date and time, crew names, instrument and rod serial numbers, benchmark descriptions and ties, loop closure and misclosure, HI, cut sheet computations, as-built invert, contractor communication, photos of the benchmark and pipe, weather, and a signed field note. Also record the alternate benchmark discrepancy and how it was resolved. The office needs enough information to reproduce your decision and to defend the survey if the grade is questioned later.