If the freeze is near the surface, pouring hot water over the specific patch of ground can help transfer heat downward, though this is inefficient in extreme cold. A more effective variation involves using a portable steam generator (a "steamer") often utilized by utility companies. Steam is injected into the valve box or manhole, where it permeates the soil and surrounds the pipe with moist heat. Steam is far more effective than hot air or water for thawing because it carries latent heat energy, releasing massive amounts of thermal energy upon contact with the cold pipe.
You cannot use an open flame (like a blowtorch) on underground pipes—the ground will trap heat and melt the pipe. Instead, use gentle, penetrating heat. how to thaw a frozen pipe underground
First, ensure the problem is ice, not a leak. If the freeze is near the surface, pouring
Discovering that an underground pipe has frozen is a homeowner’s nightmare. Unlike exposed pipes in a crawlspace, you can’t simply aim a hairdryer at an underground pipe. Worse, if you rush the process, you can crack the pipe or cause a catastrophic flood when the ice plug melts. Steam is far more effective than hot air