Parking-Lot Subgrade Below the Bioretention Rim
Construction of a 120-space parking lot in Grand Rapids requires finished grades to drain to a bioretention basin at the northeast corner. The plan sets the basin rim at 811.20 ft, the lot's low corner at 811.30 ft, and the high corner at 816.10 ft, a 2.0% slope over 240 ft. The contractor has placed subgrade. Your check shots show the high corner at 816.05 ft and the low corner at 811.08 ft; the catch-basin rim is at 811.22 ft. The lot must have a minimum 1.5% slope and positive drainage to the basin. The contractor's 3D machine-control model, built from the design file, is cutting 0.10 ft low compared with your total-station shots. The contractor wants to pave Friday and asks you to 'just certify it.'
1. Compute the as-built slope and determine whether positive drainage to the basin exists. What correction is needed?
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As-built high-low difference equals 816.05 minus 811.08, or 4.97 ft over 240 ft, which is 2.07%. That meets the 1.5% minimum. But positive drainage requires the low corner to be at or above the basin rim. The low corner is 811.08; the rim is 811.22, so the low corner is 0.14 ft below the rim. Water will pond at the low corner and will not reach the basin by gravity. The as-built rim is 0.02 ft above the design rim of 811.20, which slightly worsens the problem. To correct, raise the low corner to at least 811.22, a 0.14-ft fill, or lower the rim and adjust the basin. If the low corner is raised to 811.22, the slope becomes (816.05 minus 811.22) divided by 240, or 2.01%, still above 1.5%. Verify the entire lot's drainage, not just the two corners, and check for local sags.
2. The machine-control model is cutting 0.10 ft low. Diagnose the likely cause and state how to resolve it before paving.
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A consistent 0.10-ft offset points to a systematic datum or model error, not random construction error. Check: (1) the design file's vertical datum and units; (2) the base-station coordinate and antenna height; (3) the geoid model used in the model versus the survey; (4) site calibration; (5) total-station prism constant and instrument height; (6) whether the model surface is the design surface or a stripped subgrade surface. Compare the model to a known benchmark with a level loop. If the model is 0.10 ft low, correct the model and re-cut the subgrade; do not apply a blanket offset to the survey. Resolve the discrepancy in writing with the contractor before paving.
3. The contractor asks you to 'just certify it.' What do you do?
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Do not certify. The lot fails positive drainage because the low corner is below the basin rim. Issue a written nonconformance describing the elevations, the 0.14-ft deficit, and the required correction. Notify the owner and engineer. Re-survey after correction and certify only when the grades meet the plan and the minimum slope. If the contractor proceeds to pave, document that certification was withheld and that the work was not accepted. A seal certifies your professional judgment, not the contractor's schedule.