The research, led by Arpita Biswas of the Indian Institute of Technology Kharagpur together with Sankar Kumar Nath of the University of Burdwan, drew on an extraordinary geotechnical and geophysical database of approximately 6,000 investigation sites across the basin. These included Standard Penetration Test boreholes, downhole seismic surveys, multichannel analysis of surface waves, ambient noise measurements and resistivity surveys, supplemented by laboratory tests on soil samples for grain size, moisture content, plasticity and density. The sheer scale of the dataset allowed the team to move beyond the city-level case studies — focused previously on Kolkata, Dhaka and Chittagong — that had dominated liquefaction research in the region.

The geological setting could hardly be more favourable for liquefaction. The basin contains the world’s deepest Neogene-to-recent sedimentary accumulation, exceeding 18 kilometres in thickness, and about 77 percent of the study area is blanketed in Quaternary alluvium — loose silt, sand, clay and gravel deposited by the Ganges, Brahmaputra and Meghna rivers. Statistical analysis of the borehole data revealed that the upper 15 metres of basin sediment is very loose to medium dense, with corrected penetration values averaging between 6 and 35 depending on depth, and a mean fines content of just 36.8 percent, indicating predominantly coarse-grained soils. Grain-size curves for sites in Dhaka, Rajshahi, Kalyani and Bhubaneswar showed that most soil layers fall within the internationally recognised boundaries of ‘most liquefiable’ and ‘potentially liquefiable’ soils.

Groundwater compounds the danger. Post-monsoon water tables across the basin range from just 0.27 to 4.71 metres below the surface, and shallow groundwater dramatically lowers the effective confining stress on sediments, creating ideal conditions for pore-pressure buildup during shaking. Historical precedent is sobering: the 1885 Bengal earthquake of magnitude 6.8, the 1897 Shillong earthquake of magnitude 8.1, the 1918 Srimangal earthquake of magnitude 7.6 and the 1934 Bihar-Nepal earthquake of magnitude 8.1 all triggered documented liquefaction, leaving ground fissures, sand boils, lateral spreading and soil slumping in their wake. Even moderate events such as the 2003 Kolabunia earthquake of magnitude 5.6 produced ground rupture along riverbanks in Rangamati.

Methodologically, the study is a tour de force of integrated hazard modelling. Because instrumental recordings are absent for the great historical earthquakes, the team used the EXSIM finite-fault stochastic simulation technique, which discretises a fault plane into numerous subsources and sums their contributions with realistic rupture propagation and travel-time delays. The resulting bedrock motions were then fed into two-dimensional nonlinear finite element site response analyses in PLAXIS 2D, capturing soil behaviour that simpler one-dimensional equivalent-linear methods miss. The simulated ground motions were validated against Modified Mercalli intensities derived from historical isoseismal maps, yielding a root-mean-square error of 0.91 — a respectable agreement for regional-scale work.

The probabilistic component is equally ambitious. Combining a comprehensive probabilistic seismic hazard model with fourteen ground-motion prediction equations, the researchers computed bedrock peak ground acceleration for a 475-year return period — the conventional design-basis earthquake — ranging from 0.08 to 0.58 g. Multiplying these values by spatially interpolated site amplification factors, which ranged from 1.30 to as much as 4.60 in the soft coastal deposits of the Sundarbans, produced surface-consistent hazard values reaching a remarkable 1.40 g near Sylhet in the northeast, close to the Dauki fault and the Indo-Burman subduction zone. Cities such as Mymensingh and Sylhet fell into the ‘severe’ category, while Dhaka, Kolkata and Khulna were rated ‘moderate’ and Bhubaneswar and Dhanbad ‘low’.

With these ground motions in hand, the team evaluated liquefaction using the stress-based framework of Idriss and Boulanger, computing factors of safety for every soil layer, then aggregating them into the Liquefaction Potential Index, the Liquefaction Risk Index, the probability of liquefaction and the probability of ground failure. The results are stark: under the probabilistic scenario, roughly 42 percent of the basin’s area falls into the ‘severe’ liquefaction potential class, with a further 35 percent rated ‘high’. Dhaka, Sylhet, Mymensingh, Chittagong, Barisal, Khulna and Rangpur all exceed the severe threshold of LPI greater than 15, while Rajshahi and Kolkata sit in the high class. The risk index map places nearly half the region — about 48 percent — in the ‘extremely high’ risk zone. At representative sites including Dhaka, Chittagong, Haldia and Kalna, probabilities of liquefaction exceeded 65 percent between depths of 5 and 20 metres, and cities from Dhaka to Barasat showed probabilities of ground failure above 0.9 — ‘extremely high to absolutely certain’.

Crucially, the modelled hazard aligns with reality. The predicted severity zones show an increasing trend in the number of documented liquefaction occurrences, providing qualitative validation of the approach. The spatial pattern also makes geological sense: the northeast around Sylhet and Mymensingh combines high seismic loading from nearby active faults with young floodplain deposits and shallow groundwater, while the southwest, underlain by older, stiffer sediments near the Indian Shield, fares considerably better. Notably, the study shows that strong shaking alone does not guarantee severe liquefaction — the Chittagong Fold Belt’s uplifted, folded sediments partially offset high seismic demand, whereas the unconsolidated Holocene deltaic plains of Khulna and Barisal remain highly susceptible despite more moderate loading.

Perhaps the most practically valuable contribution is the back-analysis of target penetration resistance. By reversing the standard liquefaction triggering equations, the researchers calculated the improved SPT-N values — denoted N-improved — needed at each site to guarantee no liquefaction under the probabilistic scenario, assuming a target factor of safety of 1.5. Mapped at depth intervals from 5 to 20 metres, these values range from as low as 1 in the stable west to 50 at 20-metre depth in the most demanding northeastern locations. Such maps give engineers, contractors and planners a first-order estimate of the densification or reinforcement required at specific sites, bridging the persistent gap between regional hazard assessment and ground-level engineering decisions.

The authors are candid about limitations: the empirical triggering relationships were calibrated on global case histories rather than Bengal Basin-specific data, uncertainties were not formally propagated through the entire framework, and the maps represent regional trends rather than site-specific design criteria. Ground improvement techniques — vibro-compaction, stone columns, gravel drains, grouting, dewatering and reinforcement — each have their place depending on geology, land use and economics, and the study suggests densification suits the young deltaic south while targeted foundation measures may be more realistic in congested megacities like Dhaka. Still, as a unified, basin-scale portrait of liquefaction threat across one of the world’s most populous regions, the work offers an essential foundation for risk-informed urban planning — and a sobering reminder that beneath the streets of Dhaka and the fields of Sylhet, the ground itself may be the region’s most insidious seismic hazard.

Subject of Research: Assessment of earthquake-induced soil liquefaction potential and mitigation targets across the Bengal Basin using probabilistic seismic hazard analysis and historical earthquake scenarios

Article Title: Probabilistic seismic hazard and historical scenario-based assessment of liquefaction potential in the Bengal Basin for liquefaction mitigation strategies

Article References: Biswas, A., & Nath, S. K. (2026). Probabilistic seismic hazard and historical scenario-based assessment of liquefaction potential in the Bengal Basin for liquefaction mitigation strategies. Discover Geoscience, 4(1), Article 293. https://doi.org/10.1007/s44288-026-00672-6

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00672-6

Keywords: Bengal Basin, soil liquefaction, seismic hazard, liquefaction potential index, probabilistic seismic hazard analysis, ground motion simulation, site amplification, Standard Penetration Test, ground improvement, earthquake engineering, Ganges-Brahmaputra delta, geotechnical investigation

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

Why does shear wave velocity matter so much? When an earthquake strikes, the ground does not shake uniformly everywhere. The speed at which shear waves, the transverse waves that produce most of the destructive shaking in buildings, propagate through the upper layers of soil fundamentally controls how strongly the ground amplifies seismic motion. Soft, loose sediments can slow these waves dramatically and trap their energy, turning a moderate distant earthquake into a locally violent shaking event. Hard, dense soils and rock, by contrast, transmit the waves quickly and tend to amplify them less. This is why modern seismic design codes around the world, from ASCE 7-16 in the United States to Eurocode 8 in Europe, classify building sites primarily by the average shear wave velocity in the top thirty meters, a parameter known as VS30. Getting this number right is one of the first and most consequential steps in designing an earthquake-resistant structure.

The gold-standard way to measure shear wave velocity is geophysical: techniques such as the Multichannel Analysis of Surface Waves, or MASW, record seismic waves at the ground surface and invert their dispersion to recover a velocity profile with depth. In the one-dimensional MASW variant used in this study, a seismic source generates Rayleigh surface waves, an array of geophones records them, and the analysis exploits the fact that different frequency components sample different depths. The method is non-invasive, relatively fast and reliable, and its good practice has been codified in international guidelines such as the InterPACIFIC project recommendations. But in many urban investigations, particularly in developing regions, budgets and schedules do not always allow dedicated geophysical surveys at every site of interest. Boreholes with penetration testing, however, are drilled routinely for ordinary building projects.

That is where the Standard Penetration Test enters the story. The SPT, standardized as ASTM D1586, is one of the most widely performed in-situ geotechnical tests in the world. A split-barrel sampler is driven into the soil at the bottom of a borehole by a standardized hammer, and engineers count the number of blows required to advance the sampler a specified distance, yielding the N value. This humble number encodes a great deal about soil density and consistency: loose sands and soft clays yield low blow counts, while dense gravels and stiff clays demand many more. Because SPT data already exist in thousands of archived borehole logs, an empirical correlation linking N values to shear wave velocity would allow engineers to retrofit velocity estimates onto decades of existing geotechnical records, effectively converting old borehole data into modern seismic site classifications without drilling a single new hole.

Correlations of this kind have been published for many cities and soil types worldwide, from Lucknow and Kolkata in India to Korean alluvial deposits and Norwegian clays, but the literature carries a consistent warning: these relationships are strongly region-specific. Soil genesis, mineralogy, stress history and cementation all influence both penetration resistance and shear stiffness, and a formula calibrated in one basin can be badly biased when exported to another. For Cochabamba, a city of more than a million people sitting in a seismically active intermontane valley of the Bolivian Andes, no locally calibrated relationship existed for its fine-grained soils. The Bolivian seismic design norm and the national geotechnical study standard both demand site characterization, yet practitioners lacked a validated tool tailored to local ground conditions. The new study set out to close that gap.

The researchers assembled their dataset by pairing two independent measurement streams at coincident locations across three representative zones of the city. On the geophysical side, they performed one-dimensional MASW surveys to extract shear wave velocity profiles. On the geotechnical side, they collected SPT records from boreholes in the same areas. The overlap produced 164 data points, each tying a measured velocity to a penetration resistance at a comparable depth. With this dataset in hand, they fitted power-law models of the form VS equals a coefficient multiplied by N raised to an exponent, the classic functional form used in essentially all such correlations, reflecting the nonlinear stiffening of soil as density increases. Crucially, they evaluated the models twice: once using the raw field blow count and once using the energy-corrected value N60, which normalizes the measured blow count to a standard hammer energy of sixty percent of the theoretical maximum, following the correction framework established in the liquefaction literature since the 1980s.

The verdict on energy correction was clear. The model built on N60 outperformed the one based on uncorrected field values, and the team’s recommended relationship is VS equals 129.59 times N60 raised to the power of 0.2836, with velocities in meters per second. The statistical indicators are respectable for an empirical soil correlation: a coefficient of determination of 0.6713, a correlation coefficient of 0.8209 and a root-mean-square error of 30.81 meters per second. In practical terms, the equation predicts that a soft soil with a corrected blow count of 5 would have a shear wave velocity of roughly 210 meters per second, while a stiff soil with N60 of 30 would reach around 320 meters per second, differences large enough to shift a site from one seismic soil class to another. The exponent of about 0.28 sits within the range reported in international studies, suggesting the Cochabamba soils behave in a broadly familiar way, while the local coefficient captures the specific character of the valley’s fine-grained deposits.

The implications for Cochabamba itself are significant. The city’s valley, whose geology and hydrogeology have been documented in German-Bolivian cooperative studies, is filled with alluvial and lacustrine fine-grained sediments that can amplify earthquake shaking, and the region has a documented seismic hazard that motivates the national design code. With the new correlation, engineers reviewing borehole logs for a school, hospital or apartment block can now estimate the shear wave velocity profile and hence the site class directly from the SPT data that every geotechnical campaign already produces. That means faster, cheaper preliminary seismic screening across the urban fabric, and a way to populate site-class maps from the enormous archive of existing penetration test records. The authors position the model as a reliable estimation tool that contributes to the local seismic characterization of the Cochabamba valley, complementing rather than replacing direct geophysical measurement where critical structures demand it.

The study also carries a broader lesson for earthquake engineering in data-scarce regions. The workflow demonstrated here, fusing routine geotechnical borehole data with targeted geophysical surveys and validating the result with transparent statistics, is replicable in cities across the Andes and beyond that face similar combinations of seismic exposure, soft soils and limited measurement budgets. Comparable efforts in Peru, Ecuador and India have shown how local correlations sharpen regional hazard assessments, and the Cochabamba equation now joins that growing library. The authors, who conducted the fieldwork and analysis at Universidad Mayor de San Simón with support from the university’s geotechnical laboratory, note that the underlying data will be made available on request, an important step for independent verification and future meta-analyses.

For a city where a single well-placed hospital or bridge must stand for decades, the difference between assuming a generic imported correlation and using one calibrated on 164 local measurements could translate into real margin of safety. The equation itself is compact enough to fit on a single line of a design report, but behind it lies an unglamorous and essential truth of earthquake engineering: the ground beneath a building is as much a part of the structure as its columns and beams, and the more precisely that ground is characterized, the better the odds when the next earthquake arrives. Cochabamba’s engineers now have one more tool, rooted in their own valley’s soils, to tip those odds in their favor.

Subject of Research: Empirical correlation between shear wave velocity and SPT blow counts for seismic site characterization of fine-grained soils in Cochabamba, Bolivia

Article Title: Empirical correlation between shear wave velocity (VS) and SPT results using 1D MASW in fine-grained soils of Cochabamba, Bolivia

Article References: Copa, P. A. D. C., Padilla, A. F. C., & Rocha, J. H. A. (2026). Empirical correlation between shear wave velocity (VS) and SPT results using 1D MASW in fine-grained soils of Cochabamba, Bolivia. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02663-x

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02663-x

Keywords: shear wave velocity, Standard Penetration Test, MASW, seismic site characterization, Cochabamba, Bolivia, fine-grained soils, geotechnical engineering, earthquake engineering, N60 correction, power-law correlation, VS30