Construction Layout: Conflicting Control and a Culvert Pour
You are the field lead on construction layout for a 1,850-ft realignment of a county road. The engineer's plan set gives PI at station 15+82.52, R = 1,200.00 ft, delta = 32 degrees 15 minutes 00 seconds right, and PC at 12+35.60. Control is four 5/8-inch rebar with caps set by another firm; the plan sheet carries a note: 'Hold CP-3; coordinates are NAD83(2011) State Plane, U.S. survey feet.'
Your first setup on CP-3, backsighting CP-4, checks CP-1 within 0.02 ft. A later closed traverse between CP-3 and CP-4 shows a 0.14-ft position difference and a 0.03-ft elevation difference from the published coordinates; the baseline distance is 1,240 ft. In the meantime, the contractor has staked 600 ft of offset line and set forms for a 72-inch culvert crossing at station 14+10, using the published coordinates and the plan's curve data. The culvert has a specified alignment tolerance of plus or minus 0.05 ft. The contractor wants to pour tomorrow.
1. What do you do about the 0.14-ft control discrepancy before the pour?
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0.14 ft is about 1.7 inches. For a culvert with a plus-or-minus 0.05 ft alignment tolerance, that is roughly three times the allowance - enough to put invert elevations, joint alignment, and the tie to the existing channel outside tolerance. The pour should not proceed on the current layout.
Immediate steps:
- Stop layout work in the affected area and notify the contractor and the engineer in writing; keep a copy.
- Verify your own work before blaming the control: instrument level and collimation check, prism constant, occupy CP-3, backsight CP-4, and check CP-1 and a fourth independent point; repeat on a different day or with a different instrument.
- Reobserve CP-3 and CP-4 by an independent method - a closed traverse from CP-1 and CP-2 held fixed, or static GNSS - to determine which point moved or which coordinate is wrong.
- Compare with the engineer's control sheet and any control datasheet; ask the setting firm for observation records and the adjustment.
- Do not unilaterally rescale, translate, or average the control to make the numbers fit. Report the discrepancy with evidence and let the engineer issue corrected control or a decision.
The plan note 'Hold CP-3' tells you which point the designer intends as fixed; it does not authorize ignoring a 0.14-ft disagreement. If the engineer directs you to proceed holding CP-3, get it in writing and note the residual on the staking record. Document everything, because the culvert will be underground and the stakes will be gone.
2. Check the plan curve data and describe how you would lay out the curve.
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Check the geometry before staking anything:
- T = R tan(delta/2) = 1,200.00 x tan(16 deg 07' 30") = 1,200.00 x 0.289104 = 346.92 ft.
- L = R x delta in radians = 1,200.00 x 0.562869 = 675.44 ft.
- LC = 2R sin(delta/2) = 2,400 x 0.277735 = 666.56 ft.
- M = R(1 - cos(delta/2)) = 1,200 x (1 - 0.960657) = 47.21 ft.
- E = R(sec(delta/2) - 1) = 1,200 x 0.040954 = 49.14 ft, which also equals T tan(delta/4).
PC = PI - T = 1,582.52 - 346.92 = 1,235.60, station 12+35.60, matching the plan. PT = PC + L = 1,235.60 + 675.44 = 1,911.04, station 19+11.04. The plan curve data are internally consistent, so a field discrepancy is more likely control than geometry - which supports stopping work rather than recomputing the curve.
Layout: occupy the PC, backsight the PI, and turn deflections. Deflection per foot of arc = (delta/2)/L = 16.125 deg / 675.44 = 0.023873 deg per ft, or 1 minute 25.9 seconds per foot; at 50-ft stations the deflection is 1 deg 11 min 37 sec. Use chord distances computed for the actual arc, not assumed equal to arc length, though at R = 1,200 the difference is negligible. Close by occupying the PT and sighting back to the PI; the measured chord should match the computed LC within tolerance.
3. What do you record and communicate, and what do you do about the stakes already set?
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The file is your defense, and the culvert will be buried.
Record in the field book, with time, party, instrument serial, and weather: the published and observed coordinates for CP-3 and CP-4, the differences, the traverse closure and residuals, the check to CP-1 and the fourth point, the date and method of each observation, and the contractor's stationing and offsets as found. Photograph the forms, the stakes, and the control points. Save the raw data files.
Communicate by memo or email to the engineer, copied to the contractor and, if appropriate, the owner: state the discrepancy, the tolerance it violates, what you have ruled out, and what you recommend. State that you recommend the pour be held until the control is resolved.
Stakes: do not pull the contractor's stakes unilaterally, but mark any stakes you know to be affected - for example, with flagging and a note 'HOLD - CONTROL UNDER REVIEW' - and tell the contractor in writing which stations are suspect. Once the engineer issues corrected control, re-stake from it, label the new stakes, and remove or conspicuously mark the superseded ones so nobody sets forms from the old line. If you are directed to proceed before resolution, obtain that direction in writing and note the deviation on the staking record. The point is to make the decision traceable to the person who made it.