In short
- A seawall backfill calculator turns wall length, fill depth and width behind the wall into cubic yards and truckloads.
- The math is length x depth x width, divided by 27 cubic feet per cubic yard.
- Loose fill settles, so ordering 10 to 25% extra is the usual planning range.
- A tandem truck carries roughly 10 to 14 cu yd and a tri-axle roughly 15 to 20, but heavy wet fill can be weight-limited.
- The narrow gap between a new wall and an old one is grouted, not filled, so count only the soil behind the wall.
On this page
This seawall backfill calculator is for Cape Coral canal-front owners who want to know how much fill a seawall job might need before they compare bids. Backfill is the soil placed behind the wall after the panels are set. It uses feet and cubic yards only, with no prices, and your contractor’s plan has the final say.
En español: calculadora de relleno detrás de un muro de contención en Cape Coral
| Your numbers | Result |
|---|
Example: 100 ft of wall, a square-edged void 2 ft deep and 3 ft wide, 15% allowance, 14 cu yd trucks.
How the fill number works in four steps
- Size the void
Length along the wall, depth to finished grade, average width back into the yard.
- Convert to yards
Multiply the three, then divide by 27. A wedge is half of a box.
- Add settling
Loose soil packs down. 10 to 25% extra is the usual planning range.
- Divide by the truck
Round up to whole loads and subtract any clean fill already on site.
What do typical voids come to?
| Void | In place | +15% |
|---|---|---|
| 60 ft x 1.5 ft x 2 ft | 6.7 cu yd | 7.7 cu yd |
| 100 ft x 2 ft x 3 ft | 22.2 cu yd | 25.6 cu yd |
| 100 ft x 3 ft x 4 ft, wedge | 22.2 cu yd | 25.6 cu yd |
| 150 ft x 3 ft x 4 ft | 66.7 cu yd | 76.7 cu yd |
These are arithmetic examples, not survey results. Your own measurements decide.
Where does the fill actually go?
Backfill is the soil placed behind the new wall panels and over the tiebacks and deadmen. On a Cape Coral replacement it fills the hole left where old soil washed out, the trench dug for the anchors, and the space between the old and new alignment when the wall is reset. When a new wall goes in front of an old precast wall, the City’s vinyl-wall standard sheets cap that gap at 18 inches and call for it to be grouted, so that gap is not part of the dirt count.
Rock in the fill. The City’s sheet lets rock 6 inches or smaller stay in the backfill and requires larger rock to be removed. Ask your contractor what fill they bring and what they reuse. See our seawall design standards guide for the full list.
Questions to ask your contractor about fill
- How many yards is in the bid, and what is the allowance? Compare it with the number above.
- What material is it? Clean sand, shell rock or reused soil behave differently.
- Who hauls and places it? Ask whether delivery and compaction are in the scope.
- What happens to the yard grade and sprinklers? Settling can leave low spots.
- Is turbidity control included? The city requires screens during backfilling.
Next step: check the rest of the job
Compare scopes with the seawall bid comparison calculator, check rock at the base with the riprap tonnage calculator, and read what the wall must meet in the replacement timeline guide. We are a referral service, not a contractor. We connect visitors with independent local providers through the form below.
Sources
- City of Cape Coral: Engineering Design Standards, Public Works (sheets H-1 to H-3): 18-inch gap, grouted void, rock size in backfill.
- City of Cape Coral: Notice to Industry, Seawalls: permit and design requirements.
- Hello Gravel: how to calculate fill dirt needed: volume formula, 27 cubic feet per yard and the 10 to 25% settling rules of thumb (supplier guidance, not an engineering spec).
- HomeGuide: dump truck sizes: typical tandem and tri-axle capacities.
Frequently asked questions
How do I calculate backfill for a seawall?
Multiply the wall length by the depth of the void and by its average width behind the wall. That gives cubic feet. Divide by 27 for cubic yards, then add an allowance of roughly 10 to 25 percent because loose soil settles when it is compacted or washed in. If the void tapers to nothing at the bottom, a wedge shape is half of a full rectangle.
How many cubic yards of fill fit in a dump truck?
Tandem-axle dump trucks commonly carry about 10 to 14 cubic yards and tri-axles about 15 to 20, but dense wet fill can be limited by legal weight before the bed is full. Ask the supplier for the truck's rated load in tons as well as yards.
How much extra fill should I order?
Fill dirt sellers commonly suggest 10 to 15 percent extra for settling and 20 to 25 percent or more for clay or structural work. These are rules of thumb, not engineering figures. Your contractor knows the soil and the compaction the design calls for.
Is the gap between an old and new seawall filled with dirt?
Not on a typical Cape Coral replacement in front of an old wall. The city's vinyl-wall standard sheets cap the gap at 18 inches from the old panel face to the new cap face and call for the void to be grouted to the mudline. Dirt fill applies to the soil that was lost or removed behind the wall.
Do I need a permit to add fill behind a seawall?
Seawall replacement needs a city permit and a sealed engineer design, and the backfill is part of that job. Check with the City of Cape Coral before bringing in fill on your own, because grade and drainage rules can apply.