Void Filling and Soil Stabilization in Yellowstone County
The mechanism, quoted properly
Much of the valley alluvium around Billings derives from marine shale. NRCS Soil Mechanics Note No. 13 states plainly that “many marine shale formations produce dispersive clays”. In a dispersive clay, sodium dominates the pore water, so individual particles are “repulsed rather than attracted to one another” — the electrochemical bond that normally holds a clay together is not there.
The consequence is the sentence that matters for concrete: individual colloidal particles “go into suspension in practically still water”. Ordinary clay needs real current to erode. This soil does not. Standing water in a trench, a slow seep from a service line, or a sprinkler zone soaking the same edge all summer is enough to carry it away.
That is a different failure from the expansive one. The Bew series — USDA Fine, smectitic, frigid Torrertic Haplustalfs, 50 to 60 percent clay in the argillic horizon, dominantly smectite, with a cation exchange capacity of 50 to 70 meq/100g and slow permeability — swells and shrinks with moisture. It moves the slab and moves it back. Dispersive soil removes the support and does not return it.
The field tests NRCS names
This is a recognised soil condition with an established test suite, not a story invented to sell material. Soil Mechanics Note No. 13 names four approaches:
Crumb test
A soil crumb dropped in still distilled water — a colloidal cloud means dispersion.
Double-hydrometer test
Grain-size distribution measured with and without a dispersing agent, then compared.
Pinhole test
Water forced through a small hole in a compacted sample to see whether the hole enlarges.
Chemical analysis
Sodium as a share of the pore-water cations; above roughly 60 percent is typically dispersive.
How a void is located
Sounding comes first. A slab bearing on soil returns a dull knock; a slab spanning an air gap rings hollow and carries the sound. Walking a slab and marking where the tone changes gives you a rough outline of the cavity before anything is drilled.
The rest is context: which direction the settlement runs, where the water lands, whether the low area follows a utility trench, and how the drill behaves when the first port goes through. Pumping pressure and take-off volume then confirm the size of what you’re filling.
Sometimes the slab is already at the right elevation. In that case the correct job is to fill and support, not to lift — a garage floor that has lost bearing but not grade does not need moving, it needs something under it.
Trenches, leaks and loose backfill
Voids in Yellowstone County recur in the same places. Utility trenches, because the backfill is looser than the ground around it and water follows the easiest path. Service-line leaks, because a slow leak delivers water continuously to a soil that disperses in still water. Foundation backfill, because it was never compacted the way undisturbed ground is.
The Colorado Geological Survey lists the same causes for settlement in semi-arid ground — watering grass and shrubs, failing to repair leaking water lines in utility trenches, impounding water — and describes what happens below as binders that “break, soften, disperse, or dissolve” once water reaches soil that has been dry a long time. Billings averages 14.3 inches of precipitation a year, so the water doing this is usually water we control.
Which is why finding the source is part of the job rather than an upsell. Fill the cavity, lift the driveway, and leave the leaking line or the misaimed head alone, and the soil keeps going into suspension under the new fill.
What it costs in this market
Void filling is normally priced as part of the slab job it belongs to rather than as a separate line — the cavity has to be packed before any lift can hold, so the figures on the driveway, garage, patio and step pages already include it. Montana pricing runs $3 to $9 per square foot for mudjacking and $5 to $20 for foam, with a minimum charge of roughly $300 to $550 per visit; ProMatcher reports $5.01 per square foot for slab leveling in ZIP 59102.
What moves the number on a void job is volume, not area. A shallow cavity under a small pad and a deep washout following a trench can cover the same square footage and take very different amounts of material.
Ranges are published market data, not our quote. Yours depends on lift height, void volume, access and method.
Full cost ranges for this marketQuestions about this work
- What exactly is a void?
- An open cavity between the underside of a slab and the soil that used to support it. The concrete spans the gap until a load or a crack lets it drop, which is why a slab can look fine for years and then fall most of an inch in one wet spring.
- Can you fill a void without lifting the slab?
- Yes, and sometimes that is the right job. If the concrete is still at the correct elevation and has only lost bearing, the cavity gets packed and the slab is supported again — no lift, no change in grade.
- Is dispersive clay a real classification or a sales story?
- It is a recognised soil condition with standard field tests. NRCS Soil Mechanics Note No. 13 names the crumb test, the double-hydrometer test, the pinhole test and chemical analysis of the pore water, where above roughly 60 percent sodium the soil is typically dispersive.