Alliant Energy Logo Alliant Energy Geothermal


Call 1-800-723-7635, use our Contact Us form or e-mail .

Home  | About Us  |  Contact Us  |  Sitemap  

Working With It - Installing

Slinky Coils

Loop Length Slinky coils can be installed horizontally at the bottom of a three-foot-wide trench or vertically in a narrow trench. A five- to six-foot trench depth is typical.

Maintain a minimum distance of 12 feet between adjacent trenches and from any foundation walls, and keep the edge of any trench walls at least five feet inside any property line. Trenches do not have to be straight, and 90-degree turns can be used.

Forming the coil

In forming the slinky from a pipe roll, allow the pipe roll to remain in the same circular configuration as manufactured and shipped. A common mistake is to uncoil the pipe as if it were to be placed in a straight line.

Begin with the outside coil of the pipe roll. The individual pipe coils are pulled from the roll to and through the fixture for typing, much the same way a new garden hose would be unrolled.

Ties are applied with the coil in the fixture. They must be strong enough to hold the coil in position during fabrication, transportation and placement. After backfilling, the ties are of no value and can deteriorate with no adverse effect.

Some contractors prefer duct tapes with a sufficient number of wraps. Duct tape secures the pipe over a wider area with less pressure and requires no special tools. The pipe is less likely to crimp and the cost is typically less.

Plastic wire tie wraps with metal catches are one alternative; rebar wire ties are another.

Tie spacing should be about ten inches between each loop. The pipe should overlap and should be tied where the loops intersect at top and bottom. With the extended slinky, the loops are tied where they intersect.

Placing the coil

If the slinky is installed horizontally, it is important that the trench width be slightly greater than the coil height. Alternatively, if the slinky is installed vertically, the trench width must be three to four inches greater than the coil width and deep enough to provide adequate cover over the top of the coil.

The coil is usually placed vertically or edgewise in the narrow trench. The minimum spacing is 15 feet for a compact coil and 10 feet for an extended coil. Spacing can be adjusted based on soil type and moisture content, heat pump run time, and heating- or cooling-dominated climates.


After pressure testing and installation in the trench, the coils are ready for backfilling. The backfilling procedure is critical because heat transfer depends on good contact between the pipe loop and the ground.

Learn more about: Flowable Backfill

One recommendation is to place horizontal loops on top of a two- to four-inch layer of sand or limestone duct, with another two- to four-inch layer on top of the loops.

Silty soils and sands are most easily backfilled using a hydraulic or electric-driven vibratory machine, such as the type used to consolidate concrete. Clay soils must be carefully prepared before returning the material to the trench, since much larger clods occur, particularly if a backhoe is used.

Slinky Installation

There are ways to ensure that the soil is cut into small granular pieces - use a chain trencher with closely spaced teeth a maximum speed, with the trencher travel speed reduce to three or four feet per minute.

If the material is removed in large clumps, a rototiller method of granulating the soil should be used. If the clay is unmanageable, a well-graded sand should be used as the backfill.

Soils containing sharp rocks should not be used as backfill. Rocks can be removed from a narrow trench using a piece of expanded metal over the trench during backfilling. Otherwise, a backfill of well-graded materials should be used.

Use an angle blade set on the grass. Start from the finish end of the trench, since the dirt is deposited toward that end when it is dug out.

The dirt pile should be shoved sideways so that about one-third of the pile on that side falls gradually into the trench. This will minimize bridging and piling of the dirt on top of the slinky that could leave large air pockets around the pipe below.

This can be done only by moving slowly. The rear wheels should be angled away from the trench to prevent walls from caving if the soil is unstable.

If possible, a fire hose attached to a meter from the fire department, then attached to a fire hydrant, works well to knock down bridged soil and sock the backfilled dirt to force all the air pockets to the surface.

Otherwise, haul water to the area and use a 300 psi high-pressure 15-20 gpm pump, or use available water from one or two faucets. Use a six- to eight-foot water lance to knock down any bridging and soak the backfilled soil.

Do not attempt to blade the top one-third of the pile. Make the next one-third backfill pass while continuing flooding to assure all air pockets are removed. The lance can be inserted to the trench bottom every several feet to test for air pockets, which are indicated if the water does not quickly bubble to the surface.

Very little dirt will remain above the surface. Follow-on restoration activities will be minimized. Dirt remaining on the surface along the trench represents the volume of air preventing good heat transfer.

If rock conditions prevent the use of the six-inch narrow trench, a backhoe can be used either for a 12- to 18-inch wide trench of a wider bucket for a three-foot trench with the slinky laying flat on the bottom.

A bedding material such as sand may be required above the slinky of rocks are involved. Keep in mind that forming the coils less than a three-foot diameter may exceed the recommended bending limits.

Shoring is required by OSHA if people work in a trench more than five feet deep. Extended slinky coils are easier to backfill than a compact configuration without bridging problems. However, flooding is still required to remove air pockets.

About Us Legal | Sitemap | Privacy