Soil Tests Reveal Hidden Danger Beneath Rwanda’s Power Line Corridor

Beneath the rolling hills of southern Rwanda, along a 120-kilometre corridor earmarked for a new 110 kV electricity transmission line, engineers have uncovered a cautionary tale about one of geotechnical engineering’s most trusted tools. The Standard Penetration Test, or SPT, has been the workhorse of site investigation for decades, prized for its simplicity and low cost. But a new study published in Discover Geoscience shows that in the organic swamp soils of Bugesera District, this venerable test can overestimate soil strength by nearly one hundred percent, a discrepancy large enough to have produced dangerously under-designed tower foundations had laboratory testing not intervened.

The research, conducted by Jeremie Iradukunda of Rincent BTP Rwanda, is the first published quantitative comparison of SPT-derived strength parameters against direct shear test measurements from Rwandan soils. Seven boreholes were drilled to twelve metres at angle tower positions spread across three geologically distinct districts: Bugesera, Nyanza, and Gisagara. At each borehole, penetration tests were performed at one-metre intervals, while undisturbed samples retrieved at two and four metres were subjected to consolidated-drained direct shear testing in the laboratory. The resulting dataset of fourteen paired observations allowed the author to test a deceptively simple hypothesis: that SPT-based estimates work well in mineral soils but break down catastrophically where organic matter dominates.

The empirical correlations at the heart of the study were never designed for African terrain. Formulas developed by Skempton, Stroud, and Hatanaka and Uchida, which convert corrected blow counts into friction angles and bearing capacities, were calibrated on temperate European clays, North American sands, and Japanese alluvial deposits. Rwanda’s soils tell a different geological story. The corridor crosses lateritic red soils formed by intense chemical weathering, volcanic-derived profiles rich in allophane and halloysite, and valley-bottom swamps where peat and organic silt have accumulated over millennia. Each of these materials behaves in ways the original calibration datasets never captured, from cementation that inflates blow counts to metastable fabrics that collapse during penetration.

The results split cleanly into two regimes. Across twelve non-organic paired observations, SPT-derived friction angles agreed with direct shear values within a mean absolute discrepancy of 3.6 degrees, with a mean signed bias of just 0.66 degrees and 95 percent limits of agreement spanning minus 7.3 to plus 8.7 degrees. SPT overestimated the friction angle in seven of those twelve cases and underestimated it in five, a scatter pattern consistent with normal empirical uncertainty and comfortably absorbed by the safety factors of two to three routinely applied in foundation design. At competent sites such as BH4 and BH7, agreement was excellent, with discrepancies below one degree, demonstrating that in uniform mineral soils the SPT remains a reliable screening tool.

The organic sites told an entirely different story. At borehole BH3, which sits in a swampy topographic low underlain by blackish peat and organic silt, the direct shear test measured an effective friction angle of 16.26 degrees at two metres depth. The SPT correlation, applied to a corrected blow count of 11.4, predicted 31.00 degrees, an overestimate of 14.74 degrees, or nearly one hundred percent. The mechanism is physical rather than statistical. In peat and organic clay, the split-spoon sampler advances with little resistance not because the soil is strong but because the saturated, highly compressible matrix deforms and flows around it. The blow count reflects viscous remoulding of waterlogged organic matter rather than particle-to-particle friction, so any correlation calibrated on mineral soils becomes physically inapplicable.

A second, independent failure at borehole BH5 confirmed the pattern was no fluke. There, a peat layer extends from 2.25 to 10.5 metres, and at four metres depth the SPT recorded an anomalously high corrected blow count of 40.4, most likely because the sampler struck a localised sand or gravel lens embedded within the organic sequence. The correlation accordingly predicted a friction angle of about 40 degrees, while the direct shear test on a more representative specimen returned 31.09 degrees, a divergence of 9.31 degrees. Crucially, at BH3 the discrepancy reversed at four metres, where the profile transitions out of the organic layer and the two methods agreed within 2.83 degrees, showing that the failure tracks soil type rather than any systematic flaw in the test itself.

The spatial variability documented along the corridor is equally striking. Allowable serviceability bearing capacities ranged from 89 kilopascals at the organic site BH3 to 400 kilopascals at competent sites BH1, BH5, and BH7, a fivefold contrast within a single corridor and a near-fivefold change across roughly eighteen kilometres within Bugesera District alone. Borehole BH4 offered its own lesson: increasing foundation embedment by just one metre, from three to four metres, more than doubled the allowable capacity from 115 to 271 kilopascals. In terrain this heterogeneous, the study argues, interpolating conditions between investigation points is unsafe, and site-specific testing at every critical tower location is essential rather than optional.

The author is careful about the limits of the evidence. Fourteen paired observations from seven boreholes cannot support formal population-level statistical inference, and the study deliberately stops short of recalibrating the SPT correlations for Rwanda, which would be statistically indefensible on such a small dataset. Confounding factors at BH3, including borehole disturbance, energy losses in the rod string, and water level maintenance, were examined and found unable to explain the magnitude of the divergence. Seasonal groundwater adds further caution: BH3 was drilled during the dry season, when the water table stood about one metre deep, whereas wet-season saturation would lower effective stresses further, making the dry-season strength estimates potentially optimistic for the most critical loading period.

The practical recommendations emerging from the work are straightforward and inexpensive. Wherever borehole logs reveal blackish, high-moisture, or fibrous material, or where blow counts appear anomalously low relative to surrounding depths, SPT-based strength estimates should be flagged as unreliable and validated by direct shear or vane testing before entering design calculations. Future investigations in similar terrain should pair the SPT with cone penetration testing for continuous profiling, add oedometer consolidation tests in compressible zones to address settlement, and install piezometers to track seasonal groundwater at sensitive locations. At BH3, the study notes, the foundation problem is ultimately governed by deformation rather than strength, since even a foundation safe against shear failure would suffer long-term creep settlement incompatible with the tolerances of a high-voltage tower.

Beyond its immediate engineering value, the study makes a case for building a national geotechnical evidence base in Rwanda, which currently has no published database of SPT results correlated with laboratory data. The soil environments documented here, lateritic residual profiles, volcanic-derived materials, and organic valley deposits, recur across Uganda, Tanzania, Burundi, and the eastern Democratic Republic of Congo, where infrastructure is accelerating but published data remain scarce. The qualitative conclusion travels well beyond one corridor: in organic soils, the Standard Penetration Test must always be treated with suspicion until independently validated, and the systematic publication of paired field and laboratory datasets is how regional correlations will eventually be built.

Subject of Research: Reliability of SPT-based bearing capacity and friction angle estimation compared with direct shear testing in Rwandan tropical and organic soils

Article Title: 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

Article References: Iradukunda, J. (2026). 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. Discover Geoscience, 4(1), Article 313. https://doi.org/10.1007/s44288-026-00675-3

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00675-3

Keywords: Standard Penetration Test, bearing capacity, direct shear test, friction angle, organic soils, peat, Rwanda, geotechnical engineering, transmission line, lateritic soils, Eurocode 7, site investigation