Reliability of SPT-Based Bearing Capacity Estimation Compared with Direct Shear Tests in Rwandan Soils along the Bugesera Nyanza Gisagara 110 kV Transmission Line Corridor

This paper presents a geotechnical case study of the Bugesera–Nyanza–Gisagara 110 kV transmission line corridor in Rwanda. Seven boreholes (BH1–BH7) were drilled to 12 m depth, with Standard Penetration Tests (SPT) performed at 1 m intervals and laboratory direct shear tests conducted at 2 m and 4 m depth. Two questions are addressed: (1) how reliably does SPT estimate allowable bearing capacity for transmission tower foundations in geologically variable Rwandan soils; and (2) how well do SPT-derived effective friction angles (φ′) agree with direct shear test results? Allowable serviceability limit state (SLS) bearing capacities ranged from 89 kPa at BH3 (soft, organic-rich soils) to 400 kPa at BH1, BH5, and BH7 (competent sands and silts). SPT-derived φ′ agreed with direct shear results to within 0.5–6.0° at non-organic boreholes (mean Δφ′ = 4.5° at 2 m depth), but diverged critically by 14.74° at BH3 at 2 m, where soft peat caused SPT to overestimate friction angle by nearly 100%. Key findings include: the unsuitability of SPT alone in organic soils; the risk of interpolating geotechnical conditions between investigation points; and the practical adequacy of SPT — corrected per Skempton (1986) and interpreted per Eurocode 7 — for bearing capacity assessment in non-organic soils. This study represents one of the first published comparisons of SPT and direct shear data in Rwandan soils, providing actionable guidance for practitioners working on infrastructure corridors in East and Central Africa.