Before you do any digging in your yard, you should call for you utility locates. In Colorado, the quick number is 811. Its a simple, FREE and important process. Typically you want to call about 7-10 days before you dig. An operator will ask you for your address and some questions about the type of work you intend to do. If you are contracting the work, your contractor should be responsible for this process. It still does not hurt to make sure he intends to do so. This one phone call will mobilize the major utilities to your residence to mark the utilities with paint and flags. Locates are important for two reasons: 1) to keep you safe! 2) address liability. If you do not call in locates and you damage a line, the cost and responsibility is on you! My house is old. I have encountered major utilities within 8" of the surface. These are not typical depths, but grading over the years has changed the lay of the land. So even if you think you won't be at depths that you would encounter utilities, it does not cost you a thing to make sure this will not be an issue even if you are just planting a shrub!
