A concrete slab is only a few inches thick, and it depends on the ground under it. We strip the soft material, shape the subgrade, dig the thickened edges and place and compact the gravel base so your concrete crew can pour on something solid.
Slab Excavation in Boise
Most of the cracked garage floors and sunken shop slabs we see around the valley didn't fail because of the concrete. They failed because of what was under it. Sod left in place, fill dumped and never compacted, a base that was too thin, or water that had nowhere to go.
We prep slab-on-grade foundations in Boise, Nampa, Kuna, Star, Parma and Caldwell. That includes single-story homes, attached and detached garages, shops, ADUs and the floors inside pole barns. We don't pour. We hand the concrete crew a pad that's ready.
What Goes Under a Slab
From the bottom up, a typical slab sits on three things. Each layer has its own job, and a problem in any one of them shows up in the concrete sooner or later. We build the first two and leave the third to the crew that pours.
- Subgrade. The native soil, or compacted fill, shaped flat and to the right height.
- Gravel base. A layer of crushed gravel, compacted, that spreads the load and lets water drain out from under the slab.
- Vapor barrier. Under heated living space, a plastic sheet usually goes on top of the base. The builder or concrete crew typically handles that.
The plans or the building department set the base thickness. A shop floor that will hold a tractor or an RV usually calls for more than a patio does. The type of gravel matters too. Crushed rock with sharp edges locks together when it's compacted, while rounded pea gravel or river rock keeps rolling and never gets firm. We place the base in lifts, compact each one and check the top against grade, so the slab comes out the thickness on the plans across the whole pad.
Stripping Organics and Finding Good Ground
The first step is getting rid of anything that will rot or compress. That means sod, roots, topsoil and any old fill full of debris. On a lot that used to be a pasture or a garden, the organic layer can be deeper than people expect.
Once we're down to firm soil, we shape the subgrade. If a spot pumps or feels soft when we drive over it, we dig it out and replace it with compacted gravel. Wet clay is the usual culprit. You can't compact clay that's soaked, so after a spring rain the right move is often to wait a few days.
Thickened Edges and Interior Footings
Most slabs that carry walls have a thickened edge around the outside. On a monolithic slab, the edge is dug deeper and wider so the concrete forms a footing and floor in one pour. Interior bearing walls or posts may need their own thickened strips or pads.
We dig those to the depth and width on the plans. For many buildings in Boise, that means getting the bottom of the footing to the 24-inch frost depth, but the plans make the call. If your shop or garage needs a full stem-wall foundation instead, see foundation excavation.
Why Slabs Crack When the Base Is Wrong
Some hairline cracking is normal. Concrete shrinks as it cures, and the concrete crew's control joints are there to guide it. The bad cracks, the ones that open up, drop on one side or let the slab rock, usually come from below.
- Loose fill settles under the weight and leaves the slab hanging over a void.
- Organics rot and leave soft spots.
- Clay swells when irrigation or rain soaks it, then shrinks when it dries out.
- Thin or uneven base puts more stress on some parts of the slab than others.
- Water under the slab from bad grading or downspouts softens the subgrade.
Good soil compaction in thin layers, at the right moisture, is the fix for most of these. Grading is the other half. Ground around the slab should fall away from it, and downspouts should drain away from the edges so water doesn't work its way underneath. When the engineer or building department wants compaction testing, a third-party geotech firm does it, and we leave each lift ready for them to check.
Shops, Pole Barns, Garages and ADUs
Pole barns and post-frame shops
On acreage around Star, Kuna and Middleton, many shops are post-frame. The posts go in first, often with augering, and the floor gets prepped inside the building later. We set the pad grade so the slab ends up where the doors and skirt boards expect it.
ADUs and slab homes
A slab with living space on top usually has plumbing, sewer and sometimes electrical running under it. Those lines go in before the gravel base, so we line up with your plumber and leave the trenches ready for inspection before we place the base.
Garages
A garage slab has to meet the driveway at the right height and slope toward the door. We check that before the base goes down, not after. If the driveway is new too, see driveway excavation.
Before We Start a Slab Job
A slab job goes faster when a few things are settled up front. We'll want the plans showing the slab size, the finished floor height and the thickened edge details. If plumbing or electrical runs under the slab, we need to know when those trades are coming. The Digline locates have to be called in, and a dump truck needs a way to get close enough to drop gravel. For a detached shop or ADU in a backyard, that access is often the part that takes the most planning.
Why Hire Ultimate Excavation LLC
We're an Idaho registered contractor (RCE-65836) based in Caldwell, with a 5.0 rating on Google. One reviewer called us hard workers who do a great job for a fair price. On a slab, the work you can't see once the concrete is poured is the part we take care of. Call (208) 697-2429 or request a free estimate.
