By Joseph W. Houk, PG, engineering geologist; and Thomas J. Berglin, PE, cold regions geostructural engineer for Solar FlexRack
Understanding a potential solar project’s ground conditions can influence many design considerations, most importantly what foundation to choose. The most economical foundation design can depend on geographical location, soil type, local building code requirements, groundwater levels, corrosion potential and topography.
Types of foundations
Direct drive foundation posts: Perhaps the most common solar foundation design for both fixed-tilt and tracking projects, direct drive foundation posts include various sized W-section beams, C-channels, hat channels and round pipe.
Helical posts, earth screws: Popular in regions with weak granular soils, helical posts and earth screws rely on the torque axial relationship between the shaft and thread components of the helical/screw and frictional capacity developed within the soil regime or aggregate downhole.
Concrete ballast: Either precast or cast-in-place, concrete ballast is a practical foundation solution on re-purposed brownfield sites, landfills with membrane caps, environmentally remediated/closure sites and also designated Class II wetland sites in some states where minimally invasive foundation designs are required.
A comprehensive geotechnical investigation can be beneficial in understanding how subsurface conditions may impact project design and long-term serviceability. Likewise, full-scale load testing can help validate a foundation design.
Geotechnical investigations may employ test borings, in-situ field testing and/or test pit excavations.
A site investigation consisting of soil borings and laboratory testing will provide, in most cases, a representative cross sectional subsurface profile of the solar array site. The number of borings is usually dependent on site accessibility and size of the project.
Test borings include standard penetration testing (SPT) that provides standardized blow counts or N values. The N values are a measure of the relative density of cohesionless soils (sands) and the relative consistency of cohesive soils (clays), and these values can be reliably correlated to axial and lateral post capacity for a given site.
Test pit excavations are also a valuable tool as an option to soil test borings. Excavation characteristics of the soil can be evaluated, excavation sidewalls will expose soil stratification boundaries, soil penetration resistance readings can be obtained with a hand-held penetrometer instrument, perched seasonal ground water can be observed and representative bulk soil samples for laboratory testing can be collected from the excavation spoils.
Engineering software products can also provide help. The level of accuracy of the software product is a function of the number of specific soil engineering index inputs that are loaded into the program model to simulate subsurface conditions. The model can predict the point of fixity of the structural section being considered in the foundation design as well as a conservative post embedment depth and subsequent overall post length.
The corrosion potential of the soil should also be evaluated to determine if any supplemental corrosion protection is required beyond the standard 3-mil galvanization coating, which is used to ensure the foundation posts will meet or exceed the design service life of the project.
Full-scale load testing performed in concert with a drivability survey is the most accurate methodology to fully validate a foundation design. Three tests posts should be installed in nests. One of the posts should be installed to the target foundation embedment design depth, while one is driven deeper and another is driven shallower for performance comparison.
The static axial capacity of piles typically changes as time elapses after the test post installation, depending on soil/rock properties, pore water pressure and soil structure disturbance induced by installation. A soil rebound period between installation and load testing should be considered and range from three to 30 days.
Load testing loads are derived from the size and type of racking, number of foundation posts per rack and local building requirements for wind loads, snow loads and adfreeze bond stress (frost designs). The test piles are loaded axially and laterally in five-load increments, held for a four-minute duration per increment. The first four increments represent 25%, 50%, 75% and 100% of the design load. The fifth load is a factored design load representing 150% of the design load equivalent to a safety factor of 1.5.
Axial compression testing of test posts is normally performed in extreme cold weather climates where a bond break frost mitigation design has been incorporated into the foundation design, thus transferring the governing loads from axial uplift to compression down. An appropriately sized track excavator serves as the reaction beam testing axially in tension. The bucket is used to test laterally, and the counterweight of the machine is engaged to test axially in compression. A track excavator is ideal for load testing for its speed and mobility accessing difficult terrain, its ability to apply 50,000+ lbs of force and for test pile extraction at the completion of the load test.
The drivability study is performed in concert with the test post nest installation. Drivability indicator posts are driven, usually in a grid pattern to various embedment depths, above and below the target foundation design depth to record drive times and to document subsurface obstructions, boulders and/or bedrock. The drivability survey data is valuable to estimating production drive times and budget purposes to quantify difficult driving or subsurface obstruction areas.
Adhering to ASTM standards in load testing is critical to both repeatability and transparency in design. Many solar racking companies conduct load testing using a range of independent methods, some of which may only be interpreted by the racking company itself.
Good racking manufacturers can help solar contractors choose the best foundation and racking for their project. For example, Solar FlexRack has experience designing solar foundations in climates from the Arctic Watershed area of Northern Ontario to the Hawaiian Islands. With proper help, solar installers can have confidence in the integrity of their solar systems.