Yikes. Something you REALLY never want to happen. But what are the causes, and how do we avoid this?
Some of the main causes or reasons for a field or pipes to freeze are the following:
Leaky faucets and fixtures are probably the worst culprits. A small trickle of water coming from a sink or toilet will slowly build ice bridges in the pipes until they are completely frozen solid and burst. They are also severely taxing on your wastewater system... so get them fixed.
High Efficiency furnaces can also be culprits by dripping water into your drains all throughout the day. If you are having problems between the house and your septic tank, be sure to investigate your furnace. A good idea is to have the condensate collect in a bucket or sump with a pump and float, which will send out a large dose to the tank a few times per day, rather than it trickling all day.
I am not a fan anyways of plumbing furnace condensate into the septic system... it is highly acidic, and in my opinion, could cause premature failure to concrete septic tanks and also possibly damage bacteria in the system. My designs always suggest that the plumber routes these extra fixtures in the mechanical room to something like a dry well, separate from the septic system.
Vacancy or limited useage is also an issue. The tank and field depend on regular useage from hot showers, dishwashers, etc for both the warmer water as well as the "food" for the bacteria. Bacteria can keep your tank and field from freezing if they are being fed regularly because they actually produce a very small amount of heat. A common mistake, especially for Canadians, is to pack up and head south for a 2 week vacation on the beach during the coldest months of winter. This puts your septic system at risk of freezing.
If you have to leave for extended periods during cold months, have a house sitter, or have your lines emptied (various ways to do it) as well as your tank pumped. Always leave at least 12" of effluent in the bottom of the tank- this keeps some weight to help counter frost heaves. Many tank manufacturers also require this in order to maintain warranties. You will also want to take care of any pumps or filters that may be damaged by freezing situations.
Improper construction of the piping and field is a large factor. The only thing left in the piping after a dose to the field should be AIR. For systems that have pump tanks and pressure distribution it is important they are properly designed and installed so that the supply line is laid on the proper grade and is properly bedded so the effluent drains back to the pump tank, leaving the pipe empty between uses. Remember also that the drain back into the tank should be through a hole in the supply piping and not back through the pump, which can cause the pump impeller to spin off. Be sure this hole is open and not obstructed. Bows or low spots in the piping that is above frost level can collect effluent and freeze solid.
How can you fix a system that is frozen? The usual solution is to call a steamer truck that will force hot steam or water down the pipe to help thaw it. The danger is that the freezing may have burst, cracked, or damaged parts of your system and a thorough inspection with a sewer camera should be conducted. Steaming a system in not a solution. When weather permits, the proper repairs should be assessed and completed.