An increasingly popular approach, especially in residential systems, is a "slinky" coil.
A slinky is a coil of plastic tubing spread out and overlapped in a trench and buried. It concentrates the heat transfer surface into small volume, requiring less land area and shorter trenching - a big plus for homeowners.
A compact slinky will reduce trench length by about two-thirds compared to two pipes and four- and six-foot depths. An extended slinky will reduce trench length by about one-third. Specific design lengths will vary with the climate, soil and the heat pump's run fraction.
Slinky coils are installed horizontally at the bottom of a three-foot-wide trench.
Learn more about: Designing a Slinky Coil System