FOUNDATION AND SUBSTRUCTURE DESIGN
What a solar foundation actually has to do
A solar foundation is not carrying a heavy building. The permanent load of a module and its supporting frame is small. What the foundation has to do is hold the structure down and stop it rotating when wind acts on a large, light, inclined surface, and to keep doing so for a twenty-five year design life in ground that varies across the site.
That inverts the usual design logic. Most foundations are sized by bearing capacity. Solar foundations are usually sized by pull-out resistance and by overturning, which are governed by different soil parameters and demonstrated by different tests.
01 · THE GOVERNING CASE
Uplift, not weight
Wind actions are derived under BS EN 1991-1-4 and its UK National Annexe, then combined under BS EN 1990 so the governing case is named rather than assumed. For most solar structures the critical combination is minimum permanent action acting together with maximum wind uplift, because the self weight available to resist that uplift is small.
The consequence is that a foundation which is comfortable in bearing can still be inadequate. Pull-out capacity, not bearing pressure, is usually what decides whether a pile or a ground screw is long enough.
Perimeter and leading-row positions carry the highest uplift. A single foundation design applied uniformly across an array field will be conservative in the middle of it and potentially inadequate at the edge, which is the opposite of where the margin is wanted.
02 · THE GROUND
The ground investigation is the design input
Geotechnical design follows BS EN 1997 and its UK National Annexe. The input is a ground investigation. Where one has not been carried out, the honest position is that the foundation cannot be finalised, only bounded, and we will say so rather than proceed on a guess.
Ground varies across a field. Made ground, variable fill, shallow rock, a high water table and aggressive soil chemistry each change the answer, and each of them can be present on part of a site and absent from the rest. Interpreting the investigation, rather than reading a single figure off it, is the work.
Where a design depends on pull-out capacity, that capacity should be demonstrated by on-site test rather than taken from a manufacturer table. A tabulated figure is a starting assumption. A site test is evidence, and it is the difference between a design that is defensible and one that is merely plausible.
03 · SELECTION
Four solutions, one selection criterion
Driven piles, ground screws, concrete pads and ballasted trays are all legitimate. The selection criterion is not installation speed or unit cost. It is whether the solution develops the required resistance in the ground that is actually present, and whether it stays durable for the design life.
Driven steel sections
Suit competent granular ground and install quickly, but drivability has to be confirmed and a corrosion allowance set against soil aggressivity.
Ground screws
Develop pull-out through the helix and the surrounding soil. Sensitive to obstructions and to variable fill, so consistency across a site cannot be assumed.
Concrete pads
Resist uplift by mass, and are often the answer where the ground will not accept a driven solution. The cost is material, excavation and programme.
Ballasted trays
Avoid ground penetration altogether, which suits contaminated land, capped sites and hard standing. The ballast requirement is set by the wind case and can become the dominant cost.
04 · PLANT AND ANCILLARY STRUCTURES
Inverters, transformers and battery storage
A solar scheme is not only the array. Inverter stations, transformer plinths, DNO substation bases, switchgear housings and battery storage bases all need substructure design, and they are the elements most often left until late in the programme.
These are conventional structural problems: bearing capacity, settlement, plinth and holding-down design, and in the case of battery storage a base suited to a heavy container with a defined support arrangement and defined access for replacement. They are ordinary work for our engineers, and they are best designed alongside the array rather than after it, because the ground investigation that serves one serves the other.
05 · WHAT WE PROVIDE
Deliverables
- Foundation type selection with the governing combination named under BS EN 1990
- Pull-out, bearing, sliding and overturning checks to BS EN 1997 and its UK National Annexe
- Pile, ground screw, pad and ballast sizing against site-specific actions
- Substructure design for inverter stations, transformer plinths, switchgear and battery storage bases
- Interpretation of ground investigation data, or a stated scope for the investigation required
- Specification for on-site pull-out testing where the design capacity depends on it
- Calculations and a stated conclusion, reviewed and signed by a qualified structural engineer
- Output written to be relied upon by a funder, an insurer or a certification body, not only by the party who commissioned it
Quotations to a 24-hour target, reports to a 48-hour target from confirmed scope. £5m professional indemnity cover in force.
FREQUENTLY ASKED
Foundation and substructure questions
What governs the design of a solar foundation?
Usually uplift rather than bearing. The permanent load of a solar structure is small, so the critical combination under BS EN 1990 is normally minimum permanent action with maximum wind uplift derived to BS EN 1991-1-4 and its UK National Annexe. That means pull-out resistance and overturning decide the foundation, and a design that is comfortable in bearing can still fail the governing case.
Do you need a ground investigation before foundations can be designed?
For a final design, yes. Geotechnical design under BS EN 1997 takes ground parameters as its input, and without them a foundation can only be bounded rather than confirmed. Where no investigation exists we will state the scope of investigation required, and can issue a preliminary assessment on clearly stated assumptions so that procurement is not held up while it is arranged.
Which foundation type is best for ground-mounted solar?
There is no single answer, because the selection is decided by the ground rather than by the array. Driven piles suit competent granular ground. Ground screws develop pull-out through the helix and are sensitive to obstructions and fill. Concrete pads resist uplift by mass where a driven solution will not work. Ballasted trays avoid ground penetration altogether on contaminated, capped or hard-standing sites. Durability and corrosion allowance form part of the selection, not a later adjustment.
Can you design the bases for inverters, transformers and battery storage?
Yes. Inverter stations, transformer plinths, DNO substation bases, switchgear housings and battery storage bases are conventional substructure design and long-standing work for our engineers. They are best designed alongside the array, because the same ground investigation informs both and the same engineer holds both sets of assumptions.
Is foundation design something you do outside solar?
It is core work for our engineers across commercial and industrial projects and has been for many years. The discipline transfers directly: a pile resisting uplift is a pile resisting uplift. What is specific to solar is the action applied to it, and that is derived from the array rather than assumed.
THE WIDER STRUCTURAL SCOPE
One engineering team, from the module to the ground
The foundation is the last element in a load path that starts at the module. We design and check that path end to end, whether the structure above it is a roof, a table or a canopy, so the assumptions at the top and the capacity at the bottom are set by the same engineers.
Solar Structural Engineering Ground-Mount Structures Solar Carports
Ground Interrogated.
Foundation Proven.
Foundation and substructure design for solar PV across the UK, signed by a qualified structural engineer.