Things You'll Need
Rigid copper pipe
Tar base weatherproofing
If your old pipe is galvanized use a copper to metal fitting to connect the two.
Always wear safety glasses. If the underground water line looks too worn, replace the entire line or the same problem will probably occur again.
How to Repair a Broken Underground Water Line. If you suspect that your water pipes are leaking because your water bill has gone through the roof, there's a damp or muddy spot on the lawn and the rest of the yard is bone dry or you actually can see water spraying up through the ground, you can take several steps to repair the damage. It isn't incredibly easy to repair a broken underground water line, but it is doable.
Locate the spot. This task is easy if you have "Old Faithful" exploding in your front yard. If your first hint was merely a high water bill, then you have a little bit of a task. In dry weather you can always locate 2 things--the first is the septic tank and the second is a broken underground waterline. Look where the grass is greener. In wet and stormy weather, check the ground for spots with puddles or softer soil.
Turn off the main water shut off once you find the leak. If you're in the city you need someone from the city waterworks to do this. The main valve is in the yard and they have a special tool. If you're in the country without access to city water, then you need to turn off or unplug the pump.
Dig a trench along the area that shows signs of water leaking. Go down deep enough until you uncover the pipe. Clear the pipe several feet in front of the leak and behind it. Dig down 1 foot under the pipe. When you make a repair to the underground water line you need to expose more than just the spot that leaks.
Check the line for wear, besides the area of the leak. Frequently roots can cause damage to pipelines in one spot but do no harm in others. Evaluate the possibility of replacing the entire pipe from the street to the house.
Cut out the spot that leaks; make it at least 6 inches on each side of the leak. Get rigid copper pipe for the section. If you have to purchase the pipe in longer lengths, such as 5-foot lengths, you may as well use it all. Cut the exact measurement within 1/16 to 1/8 inch shorter. You need a little wiggle room. Clean off the weatherproofing on the old pipe. Sand all ends, including the ends of the pipe to be added and the existing pipe.
Sand the inside of 2 slip couplings that fit the pipe with a specially made wire brush for this procedure. Slide the couplings onto the new pipe toward the center of the pipe. Fit the new underground water line pipe into place and slide the slip coupling so that one half of each coupling is on the old pipe and the other is on the new.
Heat the coupling in the center from underneath with a propane torch and as you do put a piece of lead-free solder wire from the spool, on top of the edge of the coupling that connects to the old pipe. When you see the solder drip from the bottom of that joint, put the solder atop the other edge of the coupling on the new pipe side. Do this on both couplings. Weatherproof with a tar base insulator on the new section.