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

The research, published in the journal Discover Soil, was carried out by Donald Okechukwu Nnebedum of the Department of Geology at the University of Nigeria, Nsukka. The target was an edifice approaching completion in the Nsukka urban area, where fractured foundation soils had been suggested as a primary cause of structural distress. The site sits within the Campano-Maastrichtian succession of the Anambra Basin and is underlain by the Maastrichtian Ajali Formation, a unit composed of medium to coarse-grained sandstones and conglomerates, including angular to subangular quartz-arenites. Fresh samples of this formation are typically whitish with iron staining, but weather to a brownish colour at the surface, a sign of its high iron content. Outcrop evidence suggests the formation is between 25 and 50 metres thick, with fluvio-deltaic and fluvial channel depositional features such as trough and planar crossbedding, mud-draped foresets, and Ophiomorpha trace fossils.

Crucially, the region has a documented history of tectonic deformation. Post-Santonian events have left two fracture trends, trending northwest-southeast and northeast-southwest, in the Campano-Maastrichtian formations around nearby Enugu. These pre-existing weaknesses matter because even though cohesionless coarse-grained sandstones have lower bearing pressure limits of about 440 kilopascals, and medium-grained sandstone soils about 240 kilopascals, fractures can slash that strength dramatically, posing an extreme threat to any structure founded directly above them. Classical bearing capacity analyses, such as Prandtl’s and Terzaghi’s methods, assume a homogeneous soil mass and may simply miss fracture-induced heterogeneity altogether, which is precisely where geophysics earns its keep.

The study deployed two techniques side by side on identical survey plans. Electrical resistivity imaging, or ERI, is the established workhorse for mapping near-surface inconsistencies, and its effectiveness in fracture detection is well documented. Its companion, induced polarization imaging, or IPI, measures a subtler phenomenon. When current is injected into the ground and then abruptly switched off, the measured voltage does not vanish instantly. A residual polarization voltage, known as overvoltage, decays over time as electrical charges stored in the subsurface gradually dissipate. The magnitude of this effect, expressed as chargeability in milliseconds, reflects how well the ground acts like a capacitor. In non-mineralized soils, this response arises from electrolytic or membrane polarization, in which positively charged ions concentrate at electrolyte boundary layers and extend along the pore walls of clastic rocks.

That physics is exactly what makes the method sensitive to fractures. Open fractures enhance porosity and create conduits for fluid movement, and the groundwater filling them behaves as a weak electrolyte rich in dissolved minerals. Fracture zones therefore act like capacitors, accumulating charge that decays slowly once the current is cut off, producing anomalously high chargeability. Fractures that juxtapose two separate blocks or bring chemically dissimilar fluids together can amplify the effect further, and even clay-filled fractures generate a polarization response when current is interrupted. According to Archie’s law, which governs electrolytic conduction in clastic rocks, increasing porosity also drives down bulk resistivity, so fractured zones should appear as abrupt low-resistivity features in resistivity images. The two techniques thus provide independent, complementary fingerprints of the same geological defect.

Four ERI surveys and four IPI surveys were executed along parallel lines crossing the foundation zone, using the same Wenner alpha electrode configuration for both. A base electrode spacing of 10 metres was expanded through separation factors of one, two, and three, reaching a maximum spacing of 30 metres and a median depth of investigation of about 15.57 metres, adequate for non-high-rise foundation studies. Data were acquired with an ABEM SAS 1000 Terrameter configured for simultaneous resistivity and time-domain induced polarization measurement, offering resistivity accuracy of about one percent and chargeability precision of 0.1 milliseconds. Stainless steel electrodes kept contact resistance below 5 kilo-ohms, current injection ranged from 50 to 150 milliamperes, and four to eight stacking cycles per reading boosted the signal-to-noise ratio, with reciprocal measurements on ten percent of data points confirming repeatability errors below three percent. Current and potential cables were routed separately to suppress electromagnetic coupling, a refinement previously shown to improve time-domain IP data quality in multi-electrode systems.

The raw apparent resistivity and apparent chargeability data were organized in block model format and inverted with a least-squares approach using RES2DINV software, a format noted to deliver good solutions for fault modelling. Reliability was assessed carefully. A depth of investigation index, computed by comparing inversions initialized with reference half-spaces of 100 and 1000 ohm-metres, indicated values below 0.2 and therefore well-resolved model parameters to depths of 12 to 14 metres, comfortably enclosing the interpreted fracture zones at 5 to 12 metres depth. Interpreted fracture positions carry lateral uncertainty of roughly 1.5 metres and depth uncertainty of about 1 metre, while absolute resistivity and chargeability values vary by roughly 12 to 18 percent and 9 to 15 percent respectively. Importantly, the spatial coincidence of low-resistivity and high-chargeability anomalies remained stable across perturbation tests.

The results were emphatic. On survey line one, the inverted resistivity section revealed an inclined zone of low resistivity, below 600 ohm-metres, at offset distances of roughly 70 to 90 metres, and the corresponding chargeability image showed an anomalously high zone of 7.77 to 8.98 milliseconds against a background of just 0.5 to 3.0 milliseconds at the same position. Survey line three, on the opposite side of the site, mirrored the pattern, with a low-resistivity anomaly near 203 ohm-metres or less between 75 and 105 metres and high chargeability of 11.6 to 24.8 milliseconds between 80 and 105 metres. Together, the paired lines outlined a fracture trending north-northwest to south-southeast, dubbed the Alpha fracture. Survey lines two and four mapped a second defect, the Beta fracture, trending northeast-southwest, with line two showing resistivity anomalies from 107 to 695 ohm-metres and chargeability from 200 to 395 milliseconds, and line four showing resistivity of 169 to 388 ohm-metres and chargeability between 195 and 296 milliseconds, plus a smaller zone exceeding 603 milliseconds. Signal-to-background ratios for all IPI anomalies exceeded the accepted threshold of 2.0, confirming that the signatures were genuine subsurface features rather than noise artifacts.

The decisive test came from the ground itself. A post-survey site visit revealed pronounced fissures on the foundation floor at the same spatial locations the imaging had flagged, with one set trending north-south, consistent with the Alpha fracture, and another set trending east-west, matching the Beta fracture. These surface expressions tally with the two post-Santonian fault trends previously identified in the Campano-Maastrichtian formations of the Anambra Basin. The study’s multi-evidence framework, combining co-located anomalies, stable inversion convergence, cross-line replication, and surface correspondence, demonstrates that conventional two-dimensional time-domain induced polarization imaging, a cost-effective and easily deployed technique, can stand shoulder to shoulder with electrical resistivity imaging in mapping fracture-induced heterogeneity beneath foundations. The author notes that future work should incorporate targeted borehole data to link geophysical responses directly to specific fracture properties, but for engineers worried about what lurks beneath their next project, the message is clear: the ground has an electrical memory, and it will tell you where it is broken.

Subject of Research: Use of 2D induced polarization imaging to detect fractures in foundation soils at a failing building site in Nsukka, Nigeria

Article Title: Efficacy of 2D induced polarization in detecting fractures in foundation soils

Article References: Nnebedum, D. O. (2026). Efficacy of 2D induced polarization in detecting fractures in foundation soils. Discover Soil, 3(1), Article 108. https://doi.org/10.1007/s44378-026-00268-z

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00268-z

Keywords: induced polarization, electrical resistivity imaging, foundation soils, fractures, geophysics, Nsukka, Anambra Basin, Ajali Formation, chargeability, Wenner array, site investigation, building failure