On ground-mount solar plants the pile is a steel profile driven straight into the ground instead of a concrete footing. How deep it goes sets the cost of the table, the speed of installation and the plant's safety over 25–30 years. There is no single “right depth” — the soil, the loads and on-site testing decide it.
Short answer: on fixed-tilt systems embedment lands between 1.2 and 2.5 m on many sites; soft ground needs more and rocky ground needs pre-drilling. Below is how the depth is set, step by step.
The loads on a pile
Uplift: wind suction on the back of the modules tries to pull the pile out of the ground. On light solar tables this is often the governing load, resisted by skin friction along the pile.
Lateral load and moment: the horizontal wind component creates a moment at the top of the pile, which the soil resists laterally. The longer the pile above ground, the larger the moment.
Compression: the weight of the table and modules plus snow push the pile down; on most soils this governs less than uplift and lateral load.
Step 1 — Geotechnical survey
Boreholes and trial pits across the site establish the soil layers, density (e.g. SPT values), stone and boulder content, groundwater level and the soil's corrosive properties (resistivity, pH, chlorides, sulphates). Distinct soil zones are mapped; each may need its own pile depth. The corrosive properties also drive the coating choice — see our Zn-Al-Mg guide.
Step 2 — Test piles and pull-out tests
At representative points chosen from the survey, test piles of the production section are driven to the planned depths. A hydraulic jack applies load in steps; a load cell measures force and dial gauges measure displacement. A typical programme covers three load cases:
Axial pull-out: the pile is pulled upward; at the design uplift the allowed displacement must not be exceeded.
Lateral load: a horizontal force is applied at the pile head and the head's horizontal displacement is measured.
Compression: vertical loading where the site calls for it.
The designer sets the acceptance criteria and the number of tests; results feed back into the structural calculation to fix pile depth and section for each soil zone. On Y Solar projects the pull-out test results are part of the delivery documents.
Step 3 — Driving, refusal and pre-drilling
Piles are installed with hydraulic pile drivers, often GPS-guided. The main tolerances checked on site are verticality, head height and row alignment; the adjustment slots in the connections are designed to absorb them.
When a pile stops advancing before design depth, that is refusal. The pile should not be forced: it can be driven into a pre-drilled hole, the soil zone can be re-assessed, or rocky areas can switch to ground screws or concrete footings. A thin-walled pile forced into hard ground can damage its tip and scrape off its coating.
Section and depth go together
A stiffer section — a larger C-pile or a sigma pile — deflects less under lateral load; driving deeper raises uplift capacity but adds length and tonnage. The most economical answer comes from optimising section and depth together in a structural calculation calibrated by the test results.
Tracker posts, carrying the dynamic wind loads of a rotating table, are usually heavier and deeper than fixed-tilt piles — see W-beam piles.
Site checklist
Is the geotechnical survey and soil-zone map complete? · Have pull-out and lateral tests been run for every soil zone and fed into the calculation? · Are design depth, pile length and section fixed for each zone? · Is the refusal procedure (pre-drilling, alternative foundation) defined? · Do driving tolerances match the connection slot lengths? · Are the coating repair method and materials on site?