A below-grade ice house gets cold from the ground but needs three things settled before digging: the floor must sit above the water table, the walls must be supported, and the drain must reach daylight downhill. On a hillside falling 10 percent, a drain from a pit floor 2.7 m down, laid at 0.5 percent, daylights only 28 m away, so site the pit where such a drain is possible or the pit will fill with water.
Part of Ice Houses and Cold Storage
Below-grade design covers ice houses that are dug fully or partly into the ground, where the earth supplies a cold surround and the engineering problems are groundwater, wall stability and drainage.
The Ground Helps Only If It Is Dry And Drained
Below ground the temperature barely changes through the year. At depth the ground sits near the local average annual air temperature. The ice-house parent article describes earth at depth as staying near 10-15 degrees C in most climates, and that figure is the warm end of the possibilities: in a climate with freezing winters the ground may be cooler, and in a mild climate it will be at the top of that range. Whatever the figure, the ground is warmer than the ice, so a pit still needs insulation. See Insulation Methods. What the ground provides is stability, not freezing.
The biggest risks are water and collapse. A floor below the water table fills. A wall in loose soil caves in. A drain with no outfall at a lower level holds the meltwater against the ice.
What You Need
Three checks come before any digging: groundwater, wall stability and drain level.
- A soil and groundwater check — dig a test hole to the planned depth and leave it. If water seeps in, the water table is above your floor. See Soil Drainage Capacity.
- A level and a slope measurement — to check the drain can daylight.
- Wall lining — dry-stacked stone, brick or timber, per Ice House Construction.
- Shoring — planks and props for the pit while digging.
Method: Check The Site Before You Dig
Step 1: Dig the test hole. Dig to the planned depth, 2-3 m. Cover it and leave it a day or after rain. Water in the hole means the site is wrong.
Step 2: Check the soil’s stability. Firm clay or compacted gravel stands for a while. Loose sand or wet silt will not. The pit will need a lining and shoring either way.
Step 3: Check the drain can daylight. Work out where the drain meets the ground surface downhill. See the example below.
Step 4: Dig in stages and support the walls. Shore the pit as you go. A pit more than a metre or so deep with unsupported walls can collapse without warning.
Step 5: Build the lining and floor as you reach depth. Do not leave the walls bare overnight in wet weather.
Worked Example: How Far Away The Drain Daylights
Inputs (assumed): the pit top is at ground level (height 0); the drain at the pit centre is 2.7 m below it (2.5 m pit plus the gravel layer); the hill falls 10 percent (0.10 m per metre); the drain pipe falls at 0.5 percent (0.005), the minimum gradient used for drainage ditches in the parent flood article.
Let x be the distance in metres downhill from the pit.
- Ground height at x: -0.10 x.
- Pipe height at x: -2.7 - 0.005 x.
- Daylight when they are equal: -0.10 x = -2.7 - 0.005 x, so 2.7 = 0.095 x, and x = 2.7 / 0.095 = 28.4 m.
So the drain pipe runs about 28 m before it reaches the surface. On a flatter hill, with a fall of 3 percent, x = 2.7 / (0.03 - 0.005) = 108 m, which is usually impractical. On flat ground the drain never daylights and the pit needs a soakaway or a sump. This is why the parent article places ice houses on well-drained slopes: the slope is the drain.
Common Mistakes
- Digging before testing groundwater — the pit fills in the first wet spell.
- A flat site — the drain has nowhere to go.
- Leaving walls unsupported — the pit caves in.
- Assuming the ground is freezing-cold — the parent article gives 10-15 degrees C, which is well above ice. The pit still needs insulation.
- Draining toward a well or house — the meltwater should go downhill of both.
Safety
Pit walls collapse and the pit is a confined space
Soil weighs on the order of 1.5 to 2 tonnes per cubic metre, so a cave-in on someone in a pit can crush the chest and bury them in seconds. Protective systems are required in many places for trenches deeper than about 1.5 m, and loose soils need support at lesser depths. Shore the walls, never work alone and keep spoil back from the edge. Once built and sealed, a pit with decaying plant insulation can become oxygen-deficient; follow the ventilation and tether rules in Ice House Construction, and never use a flame to test the air.
What’s Next
- Learn next: Meltwater Drainage — the drain itself
- Combine with: Earth Berming — half-buried structures
- Combine with: Underground and Earth-Sheltered Housing — ground temperature at depth