bcarwell, Austin soil is highly variable... from Houston Black Clay which is multiple feet deep and grows cotton to shallow calcareous soils that have rocks on the surface and really big rocks just under the surface. What you will be able to do all depends on the soil you are digging in. You say you are intrested in a 3 to 4 inch irrigation line... that's a pretty big line... I hope you have access to an extremely large and cheap source of water, otherwise there is no need to put in such a large line. A smaller line, say 1.5 inches or smaller can be pulled by the subsoiler (assumig you don't hit rocks) at one time for quite a ways when the soil moisture is right (not too wet or too dry). This avoids the need for removing soil from the ditch. As far as a freeze line... if you get the water line 6 inches deep, it won't freeze. There can be other reasons for digging it deeper, but that is clearly sufficient to avoid freezing in Austin. PM me or call to discuss how I have put in water and electric lines in the Jarrell area.