Solar Carport Foundation Design: UK Ground Conditions

Four UK foundation types

(1) Pad-and-screw foundations — concrete pad plus mechanical screw piles; suitable for stable cohesive soils; fast install (no curing). (2) Driven piles — steel H-piles or screw piles driven to refusal; suitable for soft cohesive or made ground. (3) Continuous concrete pad — reinforced concrete strip foundations; suitable for poor ground or high-load sites. (4) Ballasted foundations — concrete ballast blocks at column bases, no excavation; suitable where digging is prohibited (contaminated land, asbestos).

Ground investigation requirements

Every commercial solar carport project requires a Phase 1 (desk study) and Phase 2 (intrusive — typically 4-8 cable percussion boreholes plus dynamic probe tests) ground investigation. Cost: £4k-£12k depending on site size. Output: ground profile, water table depth, contamination assessment, foundation type recommendation.

Cost differential by foundation type

Pad-and-screw: £180-£280 per column position. Driven piles: £350-£550 per column position. Continuous pad: £400-£700 per linear metre of structure. Ballast: £450-£750 per column position. A 100-column carport (200 bays) foundation cost ranges from £18k (best ground, pad-and-screw) to £75k (made ground, driven piles).

Made ground and brownfield sites

Many UK commercial car parks sit on made ground (concrete, brick, demolished structures). Made ground requires either driven piles to natural ground (typical 4-9m depth) or contaminated land working procedures. Made ground typically adds £8k-£18k per 200-bay carport vs natural ground.

Underground services interference

UK commercial car parks often have multiple sub-surface services: water, electric, telecoms, drainage, fuel lines, fibre. Before foundations: full PAS 128 utility survey (~£3k-£8k) plus CAT and Genny scanning at each foundation point. Where services fall within a foundation footprint, column positions are typically offset by 0.5-1.5m to avoid them.

Contaminated land considerations

Sites with historic industrial use may have contaminated sub-surface. Ground investigation includes contamination testing; if found, options are: (1) remediation (expensive); (2) ballasted foundations that avoid disturbance; (3) deep piles to bypass contaminated zone. Contaminated land typically adds £15k-£40k to a 200-bay foundation budget.

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