Footings, Piers and Frost Depth

Two Depths Competing for One Hole

A footing bottom has to satisfy two demands that have nothing to do with each other. One is structural: it must sit on material stiff enough to carry the load without settling. The other is climatic: it must sit below the depth at which the ground freezes, or be protected so that freezing cannot reach it. Neither number is derived from the other. They come from different documents, produced by different parties, and they are almost never the same.

Whichever depth is deeper governs. That single sentence settles most of the arguments that happen at the edge of a trench. A crew that digs to the frost depth on the drawing and stops has done half a check; if competent soil starts a foot lower, the footing is still wrong, and the inspector who passes it is only confirming the frost number. Run both checks, on every footing, and write both elevations on the layout sheet before a machine moves.

Which one wins changes site to site, and sometimes footing to footing on the same site. A southern lot with three feet of undocumented fill over native clay has a nominal frost requirement and a real bearing problem. A northern lot with dense glacial till at grade has excellent bearing close to the surface and still needs the footing far below it. Interior footings inside a heated, enclosed building answer only to bearing; the garage thickening ten feet away may answer to frost.

Finding the Bearing Depth

Bearing depth gets found rather than specified. Topsoil, root mat, organic silt and old fill all have to come off before anything is judged, and the elevation where competent material begins is a field determination confirmed against the geotechnical report — not a line copied off a section detail. Test pits dug with an excavator bucket before layout are the cheapest information available on any foundation job.

Look for the change: colour, moisture, and the way the bucket teeth load up. Where a report exists, it will reference standard penetration testing to ASTM D1586 Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils, and classification to ASTM D2487 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). On small residential work a hand penetrometer and a probe rod, used at every hole rather than at one, catch most of what goes wrong.

Undocumented fill is the recurring villain. Backfill from a demolished foundation, a filled swale, a buried stump, a utility trench crossing under a corner — all of it looks like dirt and none of it bears. Fill either comes out to native material or gets re-engineered as compacted structural fill with a test record. Splitting the difference by digging a bit deeper produces a footing bearing partly on native soil and partly on trash, and differential settlement follows that boundary.

Finding the Frost Depth

Frost depth is not measured on site; it is looked up. Model codes leave the design frost depth blank and require the local jurisdiction to fill it in, so the governing number comes from the building department with authority over the parcel — not from the last county you worked in, not from a supplier's chart, and not from what the neighbouring subdivision did twenty years ago. Get it in writing with the permit.

Heated and unheated construction are treated differently, and the difference bites on attachments. A conditioned building keeps its own perimeter warm; a detached garage, an open porch, a deck, a stair landing or a freestanding canopy does not. Crews routinely carry the house footing depth out to the porch piers and are surprised when only the porch moves.

Digging is not the only way to satisfy the frost side. ASCE/SEI 32 Design and Construction of Frost-Protected Shallow Foundations sets out the alternative: rigid insulation placed horizontally and vertically to push the freezing isotherm away from the footing, allowing a much shallower foundation. It carries conditions — building heating status, insulation type and thickness, ground cover, and specific treatment at corners, which lose heat in two directions. Insulation used has to be a product qualified under ASTM C578 Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation, with a documented compressive resistance for the load above it.

When Bearing Wins

Competent soil deeper than the frost line leaves two routes. Deepen the footing until it reaches bearing material, or over-excavate the poor material and replace it with compacted structural fill so the footing can stay high. Which is cheaper depends on how much deeper, how wide the excavation has to open, whether shoring becomes necessary, and whether groundwater shows up.

Over-excavation has geometry that gets skipped. The replacement zone extends laterally beyond the footing edge — the geotechnical engineer specifies the spread, commonly on a slope related to the depth of replacement — because a footing on a narrow plug of good fill inside soft ground punches through the sides. Fill goes in controlled lifts, compacted to a percentage of a laboratory maximum density established under ASTM D1557 Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort, with field verification by nuclear gauge under ASTM D6938 or by sand cone. Fill placed without that record is undocumented fill again, only newer.

Deepening changes the numbers you size to. Allowable bearing pressure generally improves with embedment and confinement, so the pad that needed a certain plan area at the shallow elevation may need less at the deeper one — while the stem or pier above it gains volume. Trench safety enters at the same moment: past the trigger depth, sloping, benching or a shield is required, and that requirement, not the concrete, usually sets the cost of going deeper.

When Frost Wins

The opposite case — good bearing near the surface, frost depth far below it — describes most deck, porch, carport, sign and pole-structure work. Load is small, required depth is large, and a conventional spread footing formed at the bottom of a deep hole becomes a large volume of concrete under a single post. A drilled or augered pier is the efficient answer: the shaft carries the load down through the frost zone and the bearing is developed at the bottom.

Straight shaft or belled is a soil question before it is a capacity question. Belling adds bearing area at the base without widening the entire shaft, but it needs soil that will stand open long enough to under-ream — cohesive material above the water table. Running sand, saturated silt or a caving gravel seam rules it out unless the hole is cased, and casing changes the pour sequence entirely.

Order concrete against measured holes, not nominal diameter. Augered shafts run oversize where the auger wobbles at the collar, where a soft layer sloughs, and everywhere the flight has been withdrawn and re-entered. Twenty piers that each take a bit more than the drawing says adds up to a short load and a cold joint at the worst possible elevation. Cover the holes if they sit overnight, and pour the same day you drill wherever the soil will not stand.

The Shaft That Climbs Out

Reaching below frost protects the bearing surface from heave underneath it. It does nothing about the sides. Freezing soil bonds to the shaft and lifts it — adfreeze uplift — and a light structure has no dead load to argue with. Decks, sunrooms, small porches and freestanding stairs are the usual casualties, because they weigh almost nothing relative to the grip a frozen silt develops around a rough concrete column.

Countermeasures work by breaking the grip or by anchoring against it. A smooth, slick form face and a shaft that does not widen toward the top reduce what the ice can hold. A bell, or a pad wider than the shaft, gives the frozen soil something to fail against — provided vertical reinforcement is continuous from the bell up into the shaft, because that load path is pure tension and unreinforced concrete has none to offer. Backfilling the annulus with clean granular material rather than the silt that came out of the hole removes the frost-susceptible soil from the one place it does the most damage.

Failure reads as movement that is seasonal and cumulative. A deck lifts each winter and comes back down not quite all the way; by the third or fourth season the ledger flashing has torn, the guard posts lean, and the door onto the deck no longer latches. Piers that heave differentially rack the frame, and the repair is almost always full replacement of the affected piers, working under a structure that now has to be shored.

Water Runs the Frost Problem

Heave needs three things at once: freezing temperatures, a frost-susceptible soil, and water available to feed growing ice lenses. Remove any one and the mechanism stops. Temperature is not yours to change, which leaves soil and water — both of which are.

Silt does the damage. It has enough capillary rise to pull water toward a freezing front and enough permeability to keep supplying it, which is exactly the combination that grows lenses. Clean gravels and coarse sands barely heave at all. Classification and plasticity testing under ASTM D2487 and ASTM D4318 Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils tell you which one you are backfilling with, and the answer should change what goes back in the hole.

Drainage has to be set relative to the footing, not the wall. A perimeter drain installed above the bottom of the footing leaves the bearing surface sitting in the water it was meant to remove; the pipe belongs at or below that elevation, on a continuous fall, wrapped in a graded filter or fabric matched to the surrounding soil, and discharging somewhere real. Roof water dumped at the corner of a building will defeat any of this — downspout extensions and positive grade are part of the foundation, not the landscaping.

The Bottom of the Hole Is Perishable

Sequence protects the thing you just spent money establishing. Excavate to a few inches above final elevation, then cut the last lift when the crew is ready to form and place. A bearing surface that has been open through a rainstorm, walked on by a plumbing crew, or frozen overnight is no longer the surface the report described, and it has to be recut to sound material before concrete goes anywhere near it.

Never place on frozen ground, on ponded water, or on softened material that a boot heel sinks into. Where a delay is unavoidable, a thin mud slab seals the bottom and gives the steel a clean working platform. Water that collects has to be pumped from a sump outside the footprint rather than skimmed across the bearing surface, which is how a good bottom turns into a slurry.

Tolerances and cover are the quiet items. ACI 117 Specification for Tolerances for Concrete Construction and Materials covers footing plan dimensions and thickness; ACI 336.1 Specification for the Construction of Drilled Piers covers shaft work; and ACI 318 Building Code Requirements for Structural Concrete sets concrete cover, which for concrete cast against and permanently exposed to earth is substantially greater than for formed faces. Reinforcement resting on dirt has effectively no cover on that face, and chairs or dobies cost nothing next to the corrosion that follows.

Hold points belong on the schedule, not in someone's memory. Many jurisdictions require an open-hole or footing inspection before reinforcement is placed and again before concrete; pouring through a missed inspection can mean exposing the footing or coring it. Photograph every hole at final elevation with a tape in frame, note who verified the bearing material, and file the compaction reports with the same set. That file settles the argument two years later when something moves.