Level Loop Misclosure on a Highway Bridge Project
You are running differential levels for a bridge replacement on NM Highway 48. The loop begins on a brass disk set in the bridge abutment (BM 12), runs 1.8 miles through the interchange, and closes on an old railroad spike in a power pole base (BM 27). The loop also includes a painted mark on a curb. Sixteen setups produce a misclosure of +0.14 ft. The project specification allows 0.05√M ft, where M is the length in miles. Three setups near the arroyo have sight lengths of 420 ft, and midday temperatures reach 98°F; the rod is an uncalibrated 25-ft fiberglass rod with a repaired joint. The contractor wants to distribute the misclosure by proportioning it across all 16 setups and proceed with bridge footing layout tomorrow. The design footing elevation tolerance is ±0.05 ft.
1. Does the loop meet the specification, and what is the likely source of the misclosure?
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Allowable misclosure = 0.05√M = 0.05√1.8 = 0.067 ft. The +0.14-ft misclosure is more than twice the allowance, so the loop fails. The pattern is not random: three 420-ft sights at 98°F with a repaired fiberglass rod point to systematic error, most likely rod zero/joint error or refraction rather than a small random scatter. I would check the rod against a steel tape over its full length and at the joint, check the level’s compensator, shorten and balance sights, and re-run the loop. I would also verify BM 12 and BM 27, especially the railroad spike, which is not a stable benchmark.
2. If the loop is re-run and passes, how should it be adjusted?
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If the re-run meets the 0.067-ft allowance, adjust the loop by distributing the misclosure in proportion to the number of setups or to the distance, with the sign reversed. But do not adjust until the blunder or systematic error is removed; proportioning a 0.14-ft error would hide a rod or benchmark problem and could put the footing elevations outside ±0.05 ft. The bridge footings should be set from the adjusted, verified benchmarks, and the adjustment should be documented. If the re-run still fails, use a different benchmark or a more precise method.
3. What should you tell the contractor and record before layout proceeds?
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Before the contractor proceeds, I would issue a written field directive: the loop is rejected, no footing elevations are to be set from it, and re-observation is required. The record should include the raw notes, the rod calibration check, benchmark descriptions, temperatures, sight lengths, and the adjustment computation. The final elevation list should identify the adjusted values and their source. If the schedule forces work, the contractor can use a temporary benchmark only after it is tied to a verified project benchmark, with the risk stated.