Conduit Crossing Under an Arterial with a Sewer Conflict
A 4-in PVC conduit run must cross a 62-ft-wide four-lane arterial. The plan shows 24 in of cover under the pavement, but the local agency's standard requires 36 in under arterial roadways. A 12-in sanitary sewer crosses the alignment at about 6 ft depth and at a 60-degree skew; at the planned grade the conduit would pass about 18 in above the sewer. Groundwater is encountered at 5 ft. The contractor proposes a one-night open cut across all four lanes with lane closures, dewatering, and same-night patch. The run continues about 300 ft to the next pull box, and the crew plans to use 90-degree bends at each end to drop into the box.
1. Evaluate the proposed 24-in cover against the agency's 36-in requirement and the sewer crossing. What are the conflicts?
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The 24-in cover fails the agency's 36-in requirement for arterial roadways, so the profile must be revised. Going deeper may reduce the 18-in separation from the sewer or create a crossing conflict; verify the agency's minimum vertical separation and whether the sewer is above or below the conduit. Pothole the sewer to confirm its actual elevation and alignment before finalizing the profile. The revised profile may require a crossing detail, a casing, or a local reroute. Resolve the conflict through a plan revision or RFI before trenching, and do not let the crew install to the plan grade while the cover and separation issues are unresolved.
2. Should the crossing be open-cut or bored? Explain the factors.
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Open-cutting all four lanes in one night is high risk: groundwater at 5 ft, required cover plus bedding means excavation deeper than 5 ft, dewatering is needed, pavement restoration is extensive, and traffic exposure is significant. Boring, jacking, or directional drilling may be preferable if the soil permits and the pits fit. It avoids full-depth pavement removal and reduces traffic exposure, but it requires accurate grade control, clearance from the sewer, and contingency for subsidence or frac-out. Compare cost, schedule, restoration, and utility risk, and obtain approval for the selected method before mobilizing.
3. What pull box and bending requirements apply to the 300-ft run?
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Check the agency's maximum spacing between pull points and the bending limit. A common rule is no more than 360 degrees of total bend between pull points, with pull-box spacing often limited to 150 ft, although some agencies allow up to 300 ft for straight runs. A 300-ft straight run may be acceptable, but 90-degree bends at both ends plus any offset can exceed the bend limit and make pulling difficult or damage the conduit. Add a pull box or use long-radius sweep bends, and size the box for the number and size of conduits. Confirm the requirements with the agency's standard before installation.