 | Ice Thickness and Strength |
Every winter it becomes very important to know when the ice is safe to use. Here are some guidelines for determining the safety of freshwater ice. The following table of safe loads is valid ONLY for ice that is clear and sound, with no flowing water underneath. it is not reliable for stationary loads. When in doubt, stay off the ice !
It is highly recommended that you familiarize yourself with theSafety on Floating Ice Sheetsinformation by CRREL.
Loads on Ice
| Required Minimum Ice Thickness in inches | Description of Safe Moving Load |
|---|---|
| 1-3/4 | One person on skies |
| 2 | One person on foot or skates |
| 3 | One snowmobile |
| 3 | A group of people walking single file |
| 7 | A single passenger automobile |
| 8 | A 2-1/2 ton truck |
| 9 | A 3-1/2 ton truck |
| 10 | A 7 to 8 ton truck |
Ice Load Graphs
Graphs have been developed by researchers which include:
What you need to know
Because there can be many variations in thestructure, thickness, temperature and strength of an ice sheet, it isessential to carry out some fairly simple field observations of the icesheet you want to use to support a load. Be cautious! Never go out on anunknown ice sheet alone, and always probe ahead of yourself with a heavyice chisel. Consider wearing a personal flotation device and ropingyourself to an assistant.
The main thing to determine is the ice thickness. This can be done bydrilling holes with an ice auger. Note whether the ice is clear (sometimescalled black ice) or white (due to air bubbles sometimes called snow ice).Measure the thickness of both kinds. Take note of the frequency of cracksand whether they are wet or dry. On rivers, be alert to variations in icethickness that may occur as a result of bends, riffles or shallows,junctions with tributaries, etc.
For both rivers and lakes, warm inflows from springs may create areas ofthinner ice. Also, the ice thickness near shore may be thinner (due to warmgroundwater inflow or the insulating effect of drifted snow) or thicker(due to the candle-dipping effect of variable water levels).
Observe any snow cover as well as variations in its thickness. Obtainthe record of air temperature for the past several days, and continueobserving air temperatures during the period the ice will be used tosupport loads.
Contrary to what many think, a rapid and large air temperature dropcauses an ice sheet to become brittle, and the ice may not be safe to usefor 24 hours.
If the air temperature stays above freezing for 24 hours or more, theice begins to lose strength, and the table no longer represents safeconditions. This becomes the general condition in the spring. Even thoughthe ice may have adequate thickness, the strength is quickly lost thelonger the air temperature is above freezing. In all cases of airtemperature changes, the effects are greatest on bare ice, and are subduedby increasing depths of snow cover. However, no quantitative guidance canbe offered.
Other considerations
An ice sheet must be supported by water.Sometimes, near a riverbank, the water level will drop after the initialice sheet is formed, leaving the ice sheet unsupported near the shore. Thisoccurrence can be detected by hearing a hollow sound when probing with anice chisel. Naturally this is not a safe location for loads on the ice.
Cracks in the ice are either wet or dry. If dry, they do not penetratethe ice sheet and are not a problem. If they are wet, multiply the vehicleclass by 2 to obtain the required minimum ice thickness. Also, drive acrossthe cracks as close to perpendicular as possible, instead of parallel tothem.
On thicker ice with very heavy loads, radial cracks may be observedoriginating from the center of the load. This usually occurs at aboutone-half of the failure load. After the radial cracks develop,circumferential cracks will form and the ice sheet will fail. If radialcracks are seen, the load should be moved immediately. Because ice willcreep, it is only a matter of time before the ice fails. The same processhappens with thinner ice at breakthrough loading, but the process occurs somuch faster that it cannot be relied upon for any warning.
