The following chart of ice strength was provided by the Forest Resources Association (formerly the American Pulpwood Association), and quoted in Tom Hennessey’s and John Holyoke's past Bangor Daily News articles.
Ice Safety
Estimates |
|
These loads are calculated for clear, hard ice on lakes and ponds. Reduce strengths by 50 percent for slush ice, 15 percent for hard river ice. This table does not apply to parked or standing loads |
|
| Ice thickness |
Permissible moving load |
| 2 inches | One person on foot |
| 3 inches | Group in single file |
| 7-1/2 inches | Passenger car (2 tons) |
| 8 inches | Light truck (2-1/2 tons) |
| 10 inches | Medium truck (3-1/2 tons) |
| 12 inches | Heavy truck (7 to 8 tons) |
| 15 inches | Heavy truck (10 tons) |
| 20 inches | 25 tons |
| 25 inches | 45 tons |
| 30 inches | 70 tons |
| 36 inches | 110 tons |
These are all estimates. The ice you find may not be uniform in thickness, nor condition. Don't take if for granted that the ice is safe because you see someone else out there. Respect Mother Nature!
Excerpt from Bangor Daily News Saturday, December 28, 2002, page D1 (with appropriate web links added):
Authorities differ on how heavy a load ice can carry
One question sports keep asking this time of year is a constant: How much ice is enough ice?
The answer: It depends on who you ask, and what kind of ice you're talking about.
The Maine Department of Inland Fisheries and Wildlife (DIFW) includes a handy ice chart in its ice fishing rule book, but a quick look at three sources will show you that the "estimates" offered on any chart are just that: estimates. For instance, the U.S. Army Corps of Engineers says a single person on skis only needs 13/4 inches of ice, if they're moving, to remain safe. The DIFW chart, which is credited to the American Pulpwood Association, says one person on foot should have 2 inches of solid blue ice underneath them. And according to the Northwest Territories Resources, Wildlife and Economic Development Department, a single person ought to have 3.9 inches of solid ice beneath their feet. Who to believe? It's your life. Isn't worth a bit of extra caution?
A few pieces of information, courtesy of the U.S. Army Corps of Engineers, Cold Regions Research and Engineering Laboratory:
Contrary to what many people think, a rapid and large change in air temperature causes an ice sheet to become brittle, and may cause it to become unsafe for up to 24 hours.
Cracks in the ice surface are either wet or dry. If dry, they do not penetrate the ice sheet and are not a problem. If wet, the ice may be half as strong as charts suggest.
Ice thickness near shore may be either thinner or thicker than the ice on the rest of the body of water.
If the air temperature stays above freezing for 24 hours or more, the ice begins to lose strength, and charts or tables no longer represent safe conditions. Even though the ice may retain adequate thickness, the strength is quickly lost the longer the air temperature is above freezing.
And according to the Northwest Territories Resources, Wildlife and Economic Development Department:
Driving fast over thin ice can create a wave similar to a boat wake, which, under the right conditions can crack the ice ahead of the vehicle.
Driving over a bump or sudden braking increases the effective weight of a vehicle. Drive carefully and slowly.
Frequent and repetitive loading of the ice can drastically weaken it.
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