
Structural Overview of Earth Anchors For PV Ground Mounted Arrays
May 11, 2017
Author:
Matthew Gilliss, Engineered Power Solutions (EPS)
1405 Spring Street, Suite 204, Paso
Introduction
As the demand for
Typical Earth Anchor Used with the Osprey Ground Mount (Duckbill Anchor Shown as Example)
Conventional Foundations
To fully understand how the earth anchor compares to the other options, an understanding of the design requirements and installation practices of the various foundation types is needed. The purpose of any foundation is to support and anchor the structure above to the ground. A typical concrete slab-on-grade foundation for a building is designed to transfer the vertical loads of the building above to the earth without crumbling, deflecting, or experiencing excess settlement.
The purpose of a foundation for a ground mounted PV racking structure is no different, except that due to the lightweight nature of the racking and the relatively large surface areas of the modules, the foundation also must resist high wind uplift loads. Many
Drilled and cast-in-place concrete piers have been the typical foundation type for small to medium sized projects. The advantages of concrete piers are that minimal equipment is required for installation, and they can be relatively shallow compared to driven steel piles. The disadvantages are that they use concrete, take days to cure, are labor intensive, and require the steel post to be embedded the full depth of the pier (or the use of rebar cages).
Driven steel piles solve many of these issues in that they don’t require concrete or rebar, don’t take time to cure, and can be installed very quickly with a
Both of these options also require hiring a geotechnical engineer to provide the soil design values used to determine the required pier/pile depth. If a geotechnical engineer is not hired, the presumptive soil design values listed in the building code, which are often overly conservative, must be used. Being overly conservative on the required foundation depth by even a foot or two can lead to significant increases in material and labor costs on projects, especially when hundreds, or thousands, of
The use of a ballasted-type foundation system such as concrete ballast blocks or a shallow grade beam (trench dug with a back-hoe and filled with concrete) don’t require deep foundations or geotechnical
Helical piles and ground screws resist uplift loads by engaging the soil around them to resist the applied wind loads and are commonly tested to ensure the most efficient depth is specified for the
Earth Anchors
The earth anchor used on the Osprey units provides a safe and reliable foundation solution with a lower material and labor cost than the typical foundation options. Essentially, earth anchors work the same way as helical piles and ground screws but with much less steel, greater adjustability, and without the need for the specialized installation equipment. Although a few different earth anchor products are on the market, in general they all work similar to a toggle bolt by driving a small (few inches long) steel tip into the ground using a steel rod. The steel tip is driven into the ground with a pneumatic hammer, or by hand with a
The most appealing aspect to an engineer or building
For example, if the engineer determines that the uplift demand on an anchor is 500 lbs., the engineer may require the anchor to be tested to 750 lbs. (factor of safety of 1.5). The installer would then drive the earth anchor, and crank each cable to 1,000 lbs. of tension. If the level of tension is reached without the earth anchor pulling out of the ground, the anchor passes the test regardless of the embedment depth, soil type, etc. If the earth anchor pulls out of the ground, it can be re-driven to a deeper depth and the test repeated. This way, the exact capacity of the anchor is known and proven without the need for an expensive geotechnical report,
Essentially, earth anchors work the same way as helical piles and ground screws but with much less steel and without the need for the specialized installation equipment. Helical piles and ground screws resist uplift loads in the same way and are commonly tested to ensure the most efficient depth is specified for the
Example for Comparison
- Typical 60 Cell Module – Standard Osprey Power Platform Unit
- Racking Tilt:25 degrees
- Design Wind Speed: 100 MPH (3 second gust speed)
- Wind Exposure Cat.: C
- Module Size:65.55” x 39.02”
- Calculated Uplift:-915 lbs. (Back Leg); -280 lbs. (Front Leg)
- Calculated Lateral:480 lbs. (Back Leg); 300 lbs. (Front Leg)
- Soil Conditions:Unknown so Code Presumptive Values Used:
- Lateral Passive:200
psf per foot of depth - Skin Friction:250
psf (1/6 of 1500psf Bearing allowed per code) - Ignore top 1 ft. of soil (typical for topsoil, frost depth, etc.)
- Lateral Passive:200
Requirements Per Foundation Type
Drilled Cast-in-Place Concrete Piers:
- 12” diameter piers
- 6’-0” deep piers for the (2) Back Legs; 5’-0” deep piers for the (2) Front Legs
- Rebar cages required (amount dependent on seismic design category of site)
Driven Steel Piles:
- W6x7 pile assumed (4” wide by 6” deep with a steel weight of 7 lbs. per foot)
- 7’-3” deep piles for the (2) Back Legs; 6’-0” deep piles for the (2) Front Legs
Ballast Blocks (or Grade Beams):
- 800 lbs. of concrete required for Each Back Leg
- 500 lbs. of concrete required for Each Front Leg
- Concrete block shall have enough surface area to resist the lateral load through friction or be embedded into the ground to resist.
- Due to the amount of weight and surface area required at each leg and the space available, it is typical to link the front and back legs together with a single ballast block. Each block would need to be 8 ft. long x 1 ft. wide x 1.5 ft. deep.
Helical Pile or Ground Screw:
- Each helical pile or grounds screw is installed in the range of 5 to 6 ft. (typical).
- Load tests required using a minimum factor of safety of 1.5 and typically higher when only a select number of anchors are tested (per anchor manufacturer).
- Provide a summary of the tested loads to the Engineer of Record for review, approval, and submittal to the governing jurisdiction.
Earth Anchor:
- Each anchor
driven to around 2 ft. deep (average for most Osprey installations) - Load test Back Leg anchors to 1400 lbs. and Front Leg anchors to 500 lbs. (factor of safety of 1.5).
- Provide a summary of the tested loads to the Engineer of Record for review, approval, and submittal to the governing jurisdiction.
Comparisons of Material and Foundation Depths for Each Foundation Option
Comparisons of Material and Foundation Depths for Each Foundation Option
As shown, the earth anchor is a structurally reliable and cost-effective alternative to conventional foundations for
To learn more about our complete range of turnkey solar solutions for you project, schedule a free consultation with the Nuance Energy team today.
Save Time & Money on Your Next Solar Project
Request a QuoteRECENT POSTS
- Why You Should Consider “Auditing” Your Ground-Mount Racking
- How to Continue Installing Ground Mount Solar During the Harsh Winter – Even in Frozen Ground
- How Earth Anchor Technology Simplifies Ground-Mounted Solar for Installers
- The Faster, Smarter, and Cheaper Way to Install Ground-Mounted Solar
- Another Month, Another Cover Story | Solar for Campgrounds
- Nuance Energy’s CEO and Founder Interviewed by Resource World Magazine
- Osprey Honored as Top Product by Solar Power World
- The Osprey: Perfection Through Evolution | Nuance Energy Group
- Helping to Power Puerto Rico
- The Osprey PowerPlatform® Revolutionizing Ground Mount Solar for Previously “Off Limits” Sites