Frequently Asked Questions
Why are driven H-piles used for solar farms?
Driven steel H-piles (W6×9, W8×31, or C-section) dominate ground-mount PV because they can be installed without excavation or concrete, achieving cycle times of < 2 min per pile. They offer high axial capacity in sand and excellent pullout resistance — critical under wind uplift loads from PV trackers.
How does EN 1997-1 DA2 differ from DA1 for pile design?
DA2 applies resistance factors directly to calculated pile resistances (γ_b, γ_s, γ_t) and uses unfactored ground parameters. DA1 uses two combinations: Comb1 factors loads; Comb2 factors ground parameters (γ_φ=1.25, γ_c=1.25). For driven piles γ_b=γ_s=γ_t=1.0 in both. DA3 factors both loads and ground strength — strictest approach.
What tension reduction factor should I use for uplift?
EN 1997-1 §7.5 does not prescribe a single factor; practice ranges from 0.70 to 0.90. For driven H-piles in sand, use 0.80 (conservative β-method under reversed loading). In cohesive soils the α-factor drops from 0.50 to 0.35 for tension (Tomlinson). Always verify with pile load tests on site.
When does the Broms method apply for laterally loaded piles?
Broms (1964) applies to short rigid piles where L/D < 20 (cohesionless) or L/D < 15 (cohesive). PV driven piles with 8ft (2.44m) embedment and 6in equiv dia have L/D ≈ 16 — borderline short. For longer piles use p-y curves (API RP 2GEO or LPILE). Broms gives a quick conservative estimate suitable for preliminary design.
What is the Converse-Labarre group efficiency formula?
η = 1 − (θ°/90) · [(n−1)m + (m−1)n] / (n·m), where θ = arctan(D/s), n = rows, m = cols, s = spacing. PV foundations typically use 1×2 or 2×2 groups at s ≥ 5D — giving η ≥ 0.95 and negligible group effect. Closer spacing in weak soils requires full group analysis.
Do solar farm piles need a full geotechnical investigation?
Yes. EN 1997-1 §3 requires geotechnical category assignment. Utility-scale solar (>1 MW) typically falls in GC2. A minimum investigation includes SPT or CPT at 100–200m grid spacing across the site. Pile design based on SPT correlation or CPT (Bustamante-Gianeselli) without load tests requires a model factor ξ applied to the characteristic resistance.