Frozen Ground and What It Does to Digging and Setting
Why this matters
Frozen ground makes winter digging slow and hard, and that is the part everyone notices. The part that costs callbacks is what frozen ground does after the crew leaves. A post set with frozen backfill leans by April. A pad set on a frozen base sits level for three months and then tips as the ground under one corner thaws to mud. A trench wall that stood vertical and solid at 8 a.m. sheds a slab of thawed soil at 2 p.m., because the frozen crust that was holding it together warmed.
Frozen soil is not a weak version of ordinary soil. It is a different material that becomes ordinary soil again, sometimes weaker than before, as it thaws, and everything set on it or in it has to survive that change. This card is about recognizing it. How deep frost reaches and what that sets for a footing is owned by 'Frost Depth and What It Sets'; excavation protective systems are owned by 'Trench Safety + Excavation Reference'.
What frozen soil is doing
Soil freezes when the water in its pores turns to ice. The ice cements the grains together, which is why frozen ground digs like soft rock and why a frozen face can stand vertical with no support. Three things follow from that.
- It is strong only while it stays frozen. The strength comes from the ice. Warm it, from the sun, from a heater, from the air on a mild afternoon, from the warmth of fresh concrete, and the strength goes with the ice.
- It can hold more water than the soil had in summer. In frost-susceptible soils, silts most of all and many clays, freezing draws water up from below into layers of ice called ice lenses. That is what causes frost heave: the lenses push the ground and anything frozen into it upward. Clean sands and gravels drain freely and are far less prone to it.
- When it thaws, that extra water has to go somewhere. Thaw often proceeds from the surface down, so the thawed layer sits on top of still-frozen ground that will not let the water drain. For a while the soil is wetter and weaker than it would ever be in summer. This is the soft, pumping ground of early spring.
The practical result is a soil that looks like it will hold anything in January and holds very little in March.
section beside a post hole, cold morning, frost 8 in
spoil pile:
frozen chunks + soft soil
o O o o grade
===============+ +==========================
frozen crust | | frozen crust, 8 in:
8 in, hard, | | digs like rock, stands
stands sheer --+ +-- sheer, lip overhangs
/ \ the soft soil below
thawed soil / \
below: soft, | | sides slough wider
wet, weak | | than the lip; a void
| | behind the crust
| |
+----------+ hole bottom, 42 in:
in thawed soil
The picture is the trap in one section. The hard lip at the top looks like a clean, sound hole. Below it the soft soil has sloughed out wider than the lip, so the crust is a ledge with nothing under it. When it thaws, the ledge falls into the hole.
Digging it
- Breaking the crust. A digging bar, a power auger with a carbide bit, or a breaker for anything larger. Each takes far more force than summer digging, and force is what turns a near miss into a struck line.
- Locating still applies. Every state has a one-call law, and while exemptions differ from state to state, a shop digging post holes or trenches places the locate request first, frozen ground or not, and the marks may need to be placed on snow or re-marked after a snowfall. Inside the tolerance zone around a marked line, which many states set at 18 or 24 inches either side of the marks (check yours), the law requires careful methods such as hand digging. Frozen ground tempts crews to use a bar or a breaker right where that rule bites. Thaw the ground first, or expose the line with careful methods, rather than chiseling blind next to a mark.
- Thawing it. Ground-thaw blankets, heated blankets or simply covering the area overnight with insulation take the frost out without flame. A combustion heater under a tarp or in a tented enclosure produces carbon monoxide that collects where the crew then works: run fuel-burning heaters only in the open, never in an enclosure anyone enters, and use a CO monitor if anyone is working near one.
Setting things in it
The common failures all come from treating frozen material as if it were ordinary soil.
- Frozen backfill. Chunks of frozen spoil tamped back around a post or into a trench do not compact. They bridge on each other and leave voids, and when they thaw the backfill settles and the post leans or the trench line sinks. Backfill with thawed soil, gravel or concrete; never with frozen chunks.
- Setting on frozen subgrade. A pad, a slab, a footing or a precast block placed on frozen ground is resting on ice. When the subgrade thaws, it softens and settles, and it rarely settles evenly, because the sun-side thaws first. Never pour concrete on frozen ground, and never set a pad on a frozen base; see related: Concrete for the Small Pour in Cold and Hot Weather.
- Frost jacking. Even a post set correctly below frost depth can be lifted if frozen soil near the surface grips its sides and heaves. A concrete collar that flares out at the top, a mushroom shape from overfilling the hole at grade, gives the frost something to grab. A smooth-sided footing with a slightly wider bottom than top resists it.
- Shallow work above the frost line. A pad, a post or a pier that does not reach below the local frost depth moves with the ground every winter. Whether it needs to reach that depth is set by the structure and local code; see related: 'Frost Depth and What It Sets'.
Worked example: two gate posts in January
A handyman crew is setting two 4x4 gate posts for a new yard gate in January. The local building department gives a design frost depth of 36 inches for footings, and the crew plans each hole 10 inches across and 42 inches deep: 36 inches to frost depth plus 6 inches for a gravel base under the post. The ground has been frozen for two weeks. A probe at the first hole finds the frost line about 8 inches down, so the top 8 inches of the 42, a little under a fifth of the hole depth, is frozen crust and the rest is thawed, wet clay-silt.
The locate. Marks were placed the previous week and a snow has fallen since. One mark, a buried low-voltage line to a yard light, runs within 2 feet of the second hole. The crew asks for a re-mark before digging rather than guessing where the paint was. The light line turns out to be 14 inches from the hole edge, inside the state's tolerance zone, so that hole is opened by hand: the crust is thawed under an insulated blanket left down overnight, and the line is exposed by hand before any auger work.
Digging. The auger breaks through the crust on the first hole and drops into soft soil. At 42 inches the bottom is firm thawed clay. The spoil pile is about a fifth frozen chunks and four-fifths soft, wet soil. When the lead looks down the hole with a light, the soft soil below the crust has sloughed outward, leaving the crust as a lip a couple of inches wider than the hole wall below it on one side.
Setting. Three calls, each driven by the frozen material:
- The lip comes off. The crew knocks the overhanging crust into the hole and cleans it out before setting the post. Otherwise it thaws in March and drops into the footing collar, leaving a void at the top.
- No frozen spoil goes back. The post goes on 6 inches of compacted gravel, and the hole is filled with concrete to just below grade, sloped to shed water, with no flare at the top. The frozen chunks are hauled off or spread to thaw elsewhere.
- The concrete is protected. The top 8 inches of the hole wall is frozen, concrete is not placed against frozen soil, and fresh concrete must not freeze before it gains early strength. The crew thaws the crust under blankets before the pour, then covers each post's footing with insulated blankets for the period the cold-weather concrete guidance calls for, which runs to days, not one night.
What it would have looked like otherwise. Post set, frozen spoil tamped back around it, the crust lip left in place. In March the lip and the frozen chunks thaw, the backfill settles, and the gate post with a gate swinging on it leans until the latch no longer meets.
Trenches in frozen ground
A trench cut through a frozen crust looks unusually safe. The walls stand sheer and the crust bridges over soft soil below, exactly as in the post-hole section, but over the whole length of the trench. As the day warms, or as the crew works in the trench, the crust thaws and the face loses the ice that was holding it.
Thawing is a change in the trench's condition. Under 29 CFR 1926.651, a competent person inspects an excavation daily and after any occurrence that increases the hazard, and a thawing face is one. The frozen face does not justify skipping a protective system the soil would otherwise need, and the trench card covers that decision; this card's point is that a frozen wall at 8 a.m. tells you nothing about the same wall at 2 p.m.
References
- See related: Frost Depth and What It Sets
- See related: Trench Safety + Excavation Reference
- 29 CFR 1926.651, Specific Excavation Requirements (inspections by a competent person)
- Your state's one-call law and its tolerance zone definition
- See related: Concrete for the Small Pour in Cold and Hot Weather; How to Protect a Small Concrete Pour From a Freeze