A project executive at a national GC asked us a good question last month: if we request precast duct bank in a bid package, what scope items should we be listing to make sure we get the right setup?
Most precast duct bank problems don’t start in the trench. They start in a bid package that described the product but not the performance and by the time anyone notices, the run is set and backfilled.
Here’s what to list, and what each item prevents.
Quick Reference
| Specify this | If it’s missing | What to require |
|---|---|---|
| Cast-in guide pins | Conduit crimped at the joint, found by a mandrel after backfill | Alignment without hands or pry bars in the joint |
| Deviation band | Bends and interferences get priced three different ways | 0–8% elevation and horizontal; everything else cast-in-place |
| Capacity validation | An 8-week quote becomes 6 months mid-project | Confirmed and validated before material release |
| Heat dissipation calcs | Ampacity shortfall discovered after gear is ordered | RHO at submittal phase, test condition named |
| Same-day backfill | Multi-day open trench, access routes blocked | Dig, set, and backfill in one shift |
| Cure policy | Lifting anchors pulling out of green concrete | No shipment before full design strength |
| Piece weight | Installer forced to mobilize a bigger machine | Weights before fabrication, sized to existing equipment |
| Load rating | Rework under a crane path or rail spur | HL-93 minimum; rail or aircraft where conditions require |
The Scope Block
Paste this into your package and edit the bracketed items. Nothing here is manufacturer-specific — any capable precaster should be able to meet all of it.
Precast Duct Bank — Scope Requirements
- Sections shall include cast-in guide pins or equivalent alignment features. Joints shall align without hand guidance, pry bars, or personnel positioned between sections.
- Precast runs shall accommodate 0–8% elevation and horizontal deviation. Corner bends, hard interferences, and transitions outside that range shall be cast-in-place or prefabricated.
- Manufacturer shall confirm and validate production capacity against the project schedule prior to release of materials.
- Heat dissipation calculations shall be provided at the submittal phase, demonstrating thermal resistivity (RHO) meets or exceeds project requirements for [MV / HV] runs.
- Same-day backfill required. Trench shall be opened, set, sealed, and backfilled within one shift.
- No section shall ship prior to reaching full design strength.
- Manufacturer shall provide a lifting plan with each submittal package, including rigging points, piece weights, and lifting hardware safety factor.
- Piece weights shall be provided prior to fabrication and sized to the installing contractor’s existing equipment.
- Manufacturer shall provide allowable subgrade bearing pressure (PSF).
- Load rating: [HL-93 / rail / aircraft] per project conditions.
- Manufacturer shall provide JHA, JSA, installation manual, and SDS documentation.
- On-site technical support required for first installation.
The Five That Get Missed
These are the ones we see cause problems, and they’re almost always missing from the package rather than argued over.
1. Guide pins, because of what happens at cable pull
Specify cast-in alignment features and you’re not just buying a safety mechanism. You’re preventing a defect you can’t see.
Sections muscled into alignment can shift or crimp a conduit at the joint. Nothing about that shows from the surface. It surfaces when the crew runs a mandrel after the entire run is set and backfilled — and then you’re excavating a finished run to find which joint did it.
Write it into the package and the failure never gets buried in the first place.
2. The 0–8% band, because it defines where the scope stops
Precast handles 0–8% elevation and horizontal deviation — roughly 4 feet over 50 feet, with elevation moving about 3 inches per section. That covers routing above or below an existing utility, around bad soil, and past whatever the as-builts got wrong.
Everything outside that band is cast-in-place or prefabricated. State it plainly and every sub bidding the job knows exactly where precast ends and their scope picks up. Leave it out and you get it priced three different ways.
3. Capacity validation, before materials release
Lead time quotes are easy. Sustained capacity against your specific schedule is the thing that slips.
Most precast plants run duct banks across the same form lines as vaults, manholes, and barriers. When demand spikes, duct banks go to the back of the queue — which is how an eight-week quote becomes six months.
Require capacity confirmed and validated before material release. Schedule alignment is cheap to fix in preconstruction and expensive to fix with the trench open.
4. Heat dissipation calcs at submittal, not after
Thermal resistivity drives cable ampacity. If the duct bank’s RHO comes in higher than the electrical design assumed, the fix is more copper or a bigger duct bank — and by then the calcs are done and the gear is ordered.
Ask for heat dissipation calculations at the submittal phase, while the electrical design still has room to move. For reference, our mix runs under 60 RHO oven-dry and we’ll tune below 55 when a spec calls for it.
5. Same-day backfill, stated as mandatory
This is the largest site-logistics win precast offers a GC: trench open and closed in one shift, access routes back by end of day, no multi-day open excavation to manage around.
It’s also rarely a sub’s top priority unless the package says so. If same-day backfill matters to your site logistics, specify it — don’t assume it.
What to Ask For in the Submittal
Anything a manufacturer can’t produce here is worth knowing before award, not after:
- Stamped structural calculations, with turnaround time stated
- Mix design and compressive strength documentation
- Thermal resistivity data with the test condition named
- Dimensional QC on conduit spacing and cover
- Lifting plan and hardware safety factor
- Batch traceability tied to the stamped submittal
- Cure policy in writing
Where Precast Doesn’t Belong
Specifying precast on the wrong run costs more than it saves. It isn’t the right call on:
- Runs under 500 feet, where the crew never reaches production rate
- Congested urban corridors where old utilities cross over each other and decisions get made on the fly
- Live substations and tight sites where an excavator can’t reach the trench
Cast-in-place and fiberglass trench systems exist for those conditions and they’re the right answer there.
Before You Send the Package
Send us your section drawing, conduit count, and depth, and we’ll come back with a piece design, a weight sized to your machine, and a stamped calc in 10 to 14 days. If you want the scope language reviewed before it goes out, we’ll do that too — including for packages we don’t end up bidding.
