How deep should I bury pipes so they don't freeze ?

I want to run a flow and return to my garage as I want to install a radiator in there but the garage is 8 meters from my house. I don't want them to freeze in winter so how deep should I bury them or is there any insulated pipe I can buy so they don't need to be buried so deep ?
The worst musical instrument ever invented. Bury them at least 6 feet under,
600mm. at least.
Just get builders insulation and wrap it around your pipes. The pipes will need to be at least a half-metre deep irrespective if they have insulation or not (British Standards).
I would not bury them, I would cut a duck on the surface & put the pipes in the duck with insulation, this way if the pipes block or freeze you can get at them without having to dig up the pipes.
A good plumpers shop would tell you of the correct type of insulation and thickness.
There is a hot water pvc pipe but go with copper if you can afford it. Use a rubber foil outer wraping and cover with a roll of insulation before replacing the dirt. If you are going to run hot water through a radiator then the water will be warm enough not to freeze even on the return. If you turn off the system then you should have the lines buried at least 15" or more if you live in the deep freeze zones. Inquire as to the freeze line in your area. After replacing the ditch dirt cover the run with a good wide and thick mulch bed to create a blanket effect. The idea is for the water temperature not to drop but a few degrees on it's way to the garage. Be sure the pump is adequate to send the water through at a fast rate. Keep the radiator bled!
depending on where you live of course determines the frost line, but you should be safe if you buried them 4 ft
You would first need to find out from someone where your frost line is for your area. The depth at which freezing stops, some time this can be fairly deep. If it were me I would use some heat tape (hardware store) at an exposed part of it. Its tape you can wrap around and plug in to keep the pipe warm enough not to freeze.
phone a local contractor and they will tell you what the legal code is in your area
It all depends on where you live and how cold it gets in winter. In Arabia, I've seen water supplied by firehouse laid on top of the ground. Needless to say, they didn't have to worry about freezing. In the far northeast of the USA, winter temperatures get down to -40 F or C (same thing at the temperature), and pipes must be buried 4 feet or 1.25 meters deep to protect them from frost. To get an idea of what you need to do, talk to the regular salespeople at the professional plumbing supply dealer in your area. They should be able to tell you what the standard depth is for burying water pipe where you live.
Mains water pipes have to be at least 750mm below ground to stop them freezing, but they are not normally insulated. As your pipes will be hot, you will need to insulate them so you might be able to lay them at a shallower depth. The suggestion to use a duct (not a duck, they just won't stay put and make a lot of noise when you try to stuff them into the dirt) to carry both the pipes and the insulation is a good one. You can also get something ready made for the purpose that has an outer sleeve, two pipes within it, and the remaining void full of insulation. Solves everything at once. Oh yes, bear in mind that copper pipes can corrode in direct contact with the ground and plastic pipes are prone to vermin damage.

Sure you still want to do it?
The frost line is 36 inches that is the same depth for concrete footings in foundations. In Boston MA there is a service called digsafe that will come out and check where you are going to dig to check for gas and electric before you dig. It's a free service you should call before you start digging.

Happy Digging
It's all dependant upon where you live and soil conditions. Here in Toronto, you need to go to 5 feet to be below the frost line. Where you live may very well be different. I would insulate those pipes, with the premade foam wraps, avaliable at a major hardware store. It'll improve your effeciency at the garage end, minimizing your heat loss to the ground. Every little bit helps.
Four foot depth is pretty much a standard in any part of the US where the temps. get really cold in the winter months.
check your local building codes to find the frost line in your area this will let you know the depth you need.
You did not say where you live. In UK I would suggest the use of 22m.m. plastic pipe covered in Armaflex rubber insulation, and the pipes run inside old or new downpipes or both inside 4inch plastic ducting buried 9inches below ground. (Plastic ducting protects and insulates, also against vermin attact, i,e,rats.) In the USA it would be threequarter pipe and good foam insulation and also much deeper for the north