Page 28 - North American Clean Energy May June 2018 Issue

solar power
Northern Exposure
Ground mount solar for anywhere the ground freezes
by Jono Stevens
e sun may be shining brightly, but the ground is frozen—perhaps permanently. Frozen ground presents two challenges for traditional ground mount solar foundations. One is year-round, and involves the need to go deep below the frost line, to avoid problems caused by heaving as the soil freezes and thaws. e other is seasonal, and involves being able to install steel piles or screws in the frozen ground, without damaging them or the installation equipment. e former can make ground mount systems cost-prohibitive; the latter can put solar companies out of business for several months every year.
Both challenges are addressed by the earth anchor foundation system, a recent innovation in the ground mount solar industry. Earth anchors have long been used for a variety of applications, including by electric utilities for securing the guy-wires that tether power poles. e operating characteristics that enable earth anchors to function well - even in the most di cult of soil conditions, from desert hard pan to sand - also make them viable in frozen ground. Before understanding how earth anchors work through freeze/thaw cycles and in permafrost, it is necessary to rst understand some basics about their use.
Earth Anchor Basics
e advantages a orded by earth anchors derive mainly from their shape: bullet-like and typically about 5” long by 1.5” in diameter; penetrating “teeth” at one end; a hole for inserting the drive rod at the other end; and an “eye” in the center for attaching a stainless-steel cable or galvanized rod. Some anchors are more like wedges that are narrower and wider, but either shape gets its rather extraordinary
holding strength from the “inverted cone” of soil above. An anchor set 48” deep, for example, is secured by over 26 cubic feet of contributing soil, for a holding capacity of over 2,000 pounds, under most conditions.
Its relatively narrow pro le and aggressive teeth make the earth anchor substantially easier to install, compared to driven or augered piles or screws. An electric or pneumatic rotary hammer drill, powered by a portable generator or air compressor, and equipped rst with a bit (for drilling a pilot hole) and then with a special drive rod, are the only tools needed to set the earth anchors. After the drive rod is removed, and uplift force is applied to the attached cable or rod, the underground anchor rotates into its nal, horizontal, and locked position.
is installation process a ords another major advantage: e ability to conduct simple, inexpensive, real-time eld load tests to measure the actual (vs. calculated) holding strength of every earth anchor. Such veri cation eliminates the need
for geotechnical reports and related inspections, and virtually guarantees being able to meet the required engineering and design speci cations, easily and cost- e ectively. It is important to note that, should any test fail to reach the speci ed load, the earth anchor can simply be removed and reinstalled at a di erent angle and/or depth. If necessary, a second anchor can also be set, with the load test performed again on the pair.
Below Ground Below Zero
e advantages of earth anchor foundation systems are already proving applicable in frozen soils, making ground mount solar projects viable in some of the worst locations.
Depending on the soil composition,
the basic “warm climate” installation is modi ed by adjusting the depth of the earth anchor and, optionally, using springs to accommodate ground movement, and limit the cyclical forces being applied to the anchors. e easiest case is permafrost where, with no seasonal ground movement, the earth anchors can normally be installed to a depth 36-48", just as they would be at Southern latitudes.
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