Pipe Network Clash Nobody Modeled
A storm sewer runs 620 ft from a headwall at station 0+00 to an outfall, with five structures. The pipe is 18-inch RCP at a 0.5% slope. A sanitary sewer runs parallel and crosses beneath the storm line near station 4+20. The storm profile shows the pipe invert at 298.40 ft at station 4+20. The sanitary line's invert at the crossing is 297.10 ft, and its pipe is 12-inch. The survey crew set the storm structures from the plan, but the as-built survey shows structure 3 (at station 4+20) is 0.6 ft lower than designed. The design used a pipe network with rules setting minimum cover at 3.0 ft. The corridor's finished grade at station 4+20 is 305.20 ft.
1. Compute the vertical clearance between the storm and sanitary lines at the crossing and determine whether there is a clash, showing your arithmetic.
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Storm pipe: 18-inch (1.5 ft) RCP, invert at 298.40 ft. Crown = 298.40 + 1.5 = 299.90 ft.
Sanitary pipe: 12-inch (1.0 ft), invert at 297.10 ft. Crown = 297.10 + 1.0 = 298.10 ft.
Clearance between storm invert and sanitary crown = 298.40 − 298.10 = 0.30 ft. That is 3.6 inches between the outside of the sanitary pipe and the invert of the storm pipe - effectively a clash for any reasonable bedding and pipe wall thickness.
Now apply the as-built: structure 3 is 0.6 ft lower than designed, so the storm invert at 4+20 becomes 298.40 − 0.6 = 297.80 ft. That is below the sanitary crown at 298.10 ft. The storm pipe now physically intersects the sanitary line. The as-built condition turns a marginal 0.30-ft clearance into a 0.30-ft interference.
2. The design rules set 3.0 ft minimum cover, yet the clash was not flagged. Explain how the rule set and the design workflow allowed this to pass.
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The 3.0-ft minimum cover rule checks the storm pipe against the finished grade surface, not against other utilities. At station 4+20, grade is 305.20 ft and the storm crown is 299.90 ft, giving 5.3 ft of cover - comfortably passing the rule. The rule set was doing exactly what it was configured to do; it simply had no knowledge of the sanitary line.
Second, the sanitary line is a separate network or an external reference. Unless the two networks are in the same model and a clash check is run between them, nothing compares their elevations. The storm profile shows only the storm line.
Third, the crossing was likely designed by matching the storm invert to the profile and assuming the sanitary line was deep enough. The 0.30-ft clearance suggests someone checked it roughly but did not account for pipe wall thickness, bedding, or the as-built tolerance.
The lesson: minimum-cover rules are a grade check, not a utility-conflict check. A separate cross-network clearance check is required.
3. Given the as-built 0.6-ft drop, what are the remediation options, and which would you recommend and why?
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Options:
- Lower the sanitary line locally to restore clearance. Requires re-grading a reach of sanitary sewer, which is expensive and may conflict with upstream grades.
- Raise the storm line by re-setting structure 3 and adjusting adjacent structures. The as-built is 0.6 ft low; raising it back toward design restores the 0.30-ft clearance, which is still marginal.
- Reroute the storm line around the crossing with a short horizontal offset or a crossing at a different station where the sanitary line is deeper.
- Replace the storm pipe with a shallower structure - a flatter pipe or a box section - to gain clearance.
- Accept and protect: encase the crossing in concrete and document the reduced clearance, if the geotechnical and structural engineers approve.
Recommend option 2 combined with option 5: raise structure 3 back toward design and encase the crossing. Raising restores the intended 0.30 ft, and encasement protects against future settlement. Option 1 is the most robust but the most disruptive; option 3 is only worth it if the sanitary line is deep enough elsewhere. The deciding factor is the sanitary line's profile upstream and downstream - if it has slack, option 1 becomes viable.