Point Loads From Jacks, Stands and Dumpster Wheels

You can read a failed slab from the underside. We broke out a shop apron last fall and flipped the pieces over on the pile, and pressed into the crushed base were four dish-shaped depressions, each about the width of a dumpster wheel and a good inch deep. The concrete had not been crushed anywhere. The dirt had been pushed down, over and over, until the slab was spanning air.

I run compaction equipment, and my work is hauled off before the first ready-mix truck arrives. Everything I hear about a slab afterward is a report card on dirt I put down months earlier. That slab mostly prints out what is underneath it.

A point load is a bearing problem first

A loaded 20-yard roll-off weighs 12,000 to 20,000 pounds and sits on four small steel wheels or a pair of rails. A jack stand holding 3,000 pounds of car corner has a base pad around 3 by 3 inches carrying all of it.

Those numbers push people to ask whether the concrete is strong enough in compression. It almost always is. Four thousand psi concrete is not being crushed by a dumpster wheel. What the load does is bend the slab, and a slab on grade acts like a stiff plate floating on a bed of springs. Whether it cracks depends far more on how stiff those springs are than on the last 500 psi of mix strength. The slab side of that is covered in slab thickness and psi for heavy vehicle traffic. My end is the springs.

The springs have a number, and it gets measured

Ground stiffness under a slab is the modulus of subgrade reaction, written k, in pounds per square inch per inch of deflection. A plate load test measures it. Typical values off the report sheets I see:

50 to 100 pci for soft clay, wet silt, or fill nobody compacted.
100 to 200 pci for a firm natural sandy or silty subgrade.
200 to 400 pci for four to six inches of well-compacted crushed base over that same subgrade.

Moving a site from 100 to 300 pci does more under a dumpster wheel than adding an inch of concrete, at a fraction of the cost. The FHWA pavement engineering material shows how support value feeds thickness design.

Support goes missing in predictable places

Edges and corners. A slab edge has roughly half the support of the interior, with no concrete on the far side to share the bending. Dumpsters land at the apron edge and jack stands go near the garage door, which is the worst real estate on the slab.
Utility trenches. Backfill is often dumped in one or two big lifts and wet down with a hose. Placed that way it settles two to four percent of its depth, so a four-foot trench can drop over an inch, and the crack runs straight along the line.
Pumping. Water gets under the slab, a wheel rolls over, and the water squirts out carrying fines with it. Tan staining fanning out of a joint after rain is base material coming up, and it means a void is growing.
Old roots. A stump ground out and buried leaves a cavity that shows up two to five years later.

What I do before there is any concrete

Proof roll first. A loaded tandem dump or water truck gets driven slowly over the prepared subgrade while somebody walks alongside watching the ground, not the truck. Deflection over about half an inch, or any spot that pumps water to the surface, gets marked in paint. That hour finds problems that cost thousands later.

Soft areas get undercut, dug out 12 to 24 inches deep and replaced with crushed stone. Everything goes back in 6 to 8 inch loose lifts, each compacted before the next lands on it, moisture within about 2 percent of optimum. Too dry and the material will not knit; too wet and the roller just makes waves. Target is 95 percent of modified Proctor residential, 98 percent under a truck or container pad, verified with a nuclear density gauge or sand cone rather than by how the ground sounds under a boot.

Where a dumpster or a lift will live, we thicken the section and widen the base under it. A 10 by 10 thickened pad adds roughly $300 to $900. Undercut runs $12 to $25 per cubic yard to dig and haul, plus $35 to $65 per ton for imported stone.

Frequently asked questions

Will a roll-off dumpster crack a residential driveway? It can, and the wheels are the reason rather than the total weight. Two layers of 3/4 inch plywood or a piece of 1/2 inch steel plate under each wheel spreads a 4,000 pound load from a few square inches to a few square feet. Keep the container off the slab edge and off any visible crack, and avoid parking it in one spot through weeks of wet weather.

Do jack stands crack a garage floor? A 4 inch floor over compacted base handles normal car work without complaint. Failures show up on floors poured over loose fill, or at the outside edge of the slab near the door. A 12 by 12 plywood pad under the stand foot drops the bearing pressure by better than a factor of fifteen.

Does adding rebar prevent point load damage? Steel does not stop a crack forming. It holds the pieces together and keeps both sides at the same elevation, which matters enormously, because a tight crack is cosmetic and a crack with a lip is a trip hazard and a water path. Support first, steel second. Cabinetry and benches bridge point loads the same way — a custom millwork shop spreads a heavy top across a frame rather than trusting two legs on a slab edge.

Pavement guidance: American Concrete Pavement Association.