The team focused on five actively failing slope sections along the corridor, at chainages 16+600, 20+400, 32+300, 34+800 and 35+600, each chosen through field reconnaissance for visible instability markers such as tension cracks, fresh scarps and toe erosion. At each site they excavated test pits between 1.0 and 3.0 meters deep and collected roughly 30 kilograms of disturbed bulk soil, sealed in moisture-tight bags to preserve the in-situ moisture. Disturbed sampling was a necessity rather than a compromise: the coarse, gravelly colluvium that mantles these slopes, in layers typically 2 to 8 meters thick over fractured bedrock, simply cannot yield undisturbed core samples. The shallow investigation depth matches the dominant failure mode, which is shallow translational sliding within the near-surface colluvial mantle rather than deep-seated bedrock movement.

The laboratory program, run to Bureau of Indian Standards codes, produced a classification that is quietly alarming. Wet sieve analysis showed a corridor split between gravel-dominated and sand-dominated materials: the soils at chainages 32+300 and 20+400 are well-graded gravels, while those at 16+600, 34+800 and 35+600 are dominated by sand, with the first two classified as poorly graded, gap-graded sands. Gap grading means the intermediate particle sizes are missing, leaving a metastable skeletal fabric of large, interconnected pores. Water can infiltrate these soils almost unimpeded, and the fine particles may even migrate through the coarse matrix under seepage forces in a process called suffusion, progressively loosening the slope from within. Even the well-graded gravels are highly permeable, with hydraulic conductivities likely exceeding 10^-2 centimeters per second.

The most consequential finding, however, came from the Atterberg limit tests. Although the liquid limits ranged from 17.0 to 23.5 percent, none of the five soils could be rolled into a 3-millimeter thread without crumbling, meaning the plastic limit could not be defined at all. Every sample is non-plastic. That single result reveals the mineralogy: the fines are inert rock flour pulverized from the corridor’s phyllite, quartzite and slate bedrock, not clay minerals capable of providing true chemical cohesion. The strength of these slopes therefore rests entirely on inter-particle friction and on apparent cohesion generated by matric suction, the capillary tension that holds unsaturated grains together. In the dry season this suction is remarkably effective, allowing near-vertical cuts to stand. When the monsoon saturates the pores, it vanishes, and the soil behaves essentially as a cohesionless granular mass on the verge of flowing.

Direct shear tests on specimens remolded to their maximum dry density quantified this precarious strength. Friction angles ranged from 26.1 to 29.1 degrees, and apparent cohesion values from 38.2 to 102.5 kilopascals, measured under normal stresses of 136, 272 and 408 kilopascals corresponding to overburden depths of roughly 7, 14 and 20 meters. The authors are careful to note that this cohesion intercept is apparent rather than true effective cohesion, combining capillary suction with the dilatancy of angular grains during shearing. Because the suction component is destroyed by saturation, they also ran a conservative analysis with cohesion set to zero, under which every section falls well below a factor of safety of one. Compaction testing added further nuance: the well-graded gravel at chainage 32+300 achieved the highest maximum dry density, 2.26 grams per cubic centimeter at just 6.8 percent optimum moisture, while the gap-graded sands needed 11.5 to 12.8 percent water to compact, signaling a strong affinity for water retention that prolongs pore-pressure buildup after rainfall.

The stability calculations used a deliberately transparent, low-cost framework: closed-form infinite slope equations implemented in a spreadsheet, with a representative slip depth of 3.0 meters and pore pressures computed for a water table rising to the slope surface. Under dry conditions, all five sections showed factors of safety above 1.25, apparently stable. Under full saturation, the picture collapsed. The factor of safety dropped by roughly 30 to 36 percent across the critical sections, and at three of the five sites it fell below unity, the mathematical threshold of failure. The worst case was the Mauri Khola debris-flow channel at chainage 20+400, where the factor of safety sank to 0.89, quantitatively confirming the field record of recurrent debris flows there. Chainage 16+600 landed at 1.08, technically stable but far below the 1.3 design standard for permanent highway slopes and vulnerable to any added load.

Because the corridor runs close to the Main Boundary Thrust, one of the major faults of the Himalayan collision zone, the team added a pseudo-static seismic analysis using a horizontal seismic coefficient of 0.15, consistent with a regional peak ground acceleration of about 0.30 g. The result is stark: while dry slopes survive design-level shaking, the combination of full saturation and seismic loading drives the factor of safety down to roughly 0.12 to 0.30 when cohesion is conservatively neglected, with the steep cut at chainage 34+800 the most vulnerable. The governing design condition for the corridor, the authors conclude, is the conjunction of monsoon saturation and earthquake shaking, a scenario that is entirely plausible in a region where the two hazards share a season and a tectonic setting.

The study also isolates a distinctly human fingerprint in the failure record. By comparing inferred pre-widening slope angles with the angles measured after the 2015 to 2021 road-widening project, which was undertaken to bring the highway to Asian Highway standards, the analysis suggests that excavation-induced steepening cut the dry factor of safety by about 25 to 45 percent and roughly halved the saturated value. Field observations reinforce the point: retrogressive sliding at chainage 32+300 began where toe excavation removed lateral confinement, tension cracks on the 55-degree cut at 34+800 signal incipient rotational failure despite installed rock netting, and the temporary metal sheet piling at 35+600 is visibly buckling under active earth pressures. This is the pattern researchers have called Nepal’s bulldozer revolution, in which non-engineered road excavation across the hills resets ancient, metastable colluvial deposits into a fresh state of disequilibrium.

The remedial prescriptions follow directly from the mechanics. For slopes governed by pore pressure, notably Mauri Khola and chainage 16+600, deep horizontal drains are identified as the single most effective intervention, because depressing the water table below the slip plane restores effective stress and recovers the frictional strength lost to saturation. For the steep, cohesionless cuts, soil nailing anchored into stable bedrock, combined with high-tensile netting or fiber-reinforced shotcrete, is proposed to confine the loose surface wedge, with stabilizing piles considered where a deeper rotational surface is suspected. Toe scour zones call for flexible mechanically stabilized earth embankments, and the failing sheet piles at 35+600 should be replaced with permanent gabion or gravity structures. Assessment-level modeling suggests drainage plus nailing can lift the critical cases toward the 1.3 target, though the authors stress that detailed design, numerical verification and instrumentation such as piezometers and extensometers must follow before any of these numbers become design values.

Beyond the specific chainages, the study’s broader argument may prove its most exportable result. Deep borehole campaigns and advanced numerical modeling remain financially out of reach for the road agencies of most developing nations, yet regional landslide hazard maps derived from satellites are too coarse to design a retaining wall for a particular curve. By demonstrating that standard index testing, sieve analysis, Atterberg limits, Proctor compaction and direct shear, combined with simple limit equilibrium equations, can correctly reproduce and even predict the observed failure pattern, the authors offer a middle path: a rigorous, inexpensive diagnostic that lets maintenance budgets be aimed at the slopes most likely to collapse next. For a country whose economic arteries run along some of the most fragile terrain on Earth, that framework may matter as much as any retaining wall.

Subject of Research: Geotechnical investigation and slope stability of rainfall-induced landslides along the Mugling-Narayanghat highway corridor in central Nepal

Article Title: Site specific geotechnical investigation and slope stability assessment of the Mugling-Narayanghat highway corridor, central Nepal

Article References: Banjara, S., Poudel, D. J., & Joshi, B. R. (2026). Site specific geotechnical investigation and slope stability assessment of the Mugling-Narayanghat highway corridor, central Nepal. Discover Geoscience, 4(1), Article 305. https://doi.org/10.1007/s44288-026-00678-0

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00678-0

Keywords: slope stability, landslides, geotechnical engineering, Nepal Himalaya, monsoon rainfall, factor of safety, colluvial soils, matric suction, direct shear test, road widening, seismic loading, Mugling-Narayanghat highway

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

Soil shear strength is one of the most fundamental parameters in geotechnical engineering, governing the stability of foundations, embankments, pavements and retaining walls. In developing countries, where locally available soils often display highly variable engineering properties, engineers frequently turn to stabilisers such as lime, cement and fly ash to strengthen weak ground. Yet these conventional binders carry a heavy carbon footprint and can be expensive or difficult to source in remote regions. That has driven a search for low-cost, locally available alternatives derived from biodegradable waste, and eggshell, which is composed of more than 95 percent calcium carbonate, has emerged as one of the most promising candidates.

The research team, led by MD. Moin Akon, set out to answer a question that most previous studies had sidestepped: does eggshell powder behave the same way in fundamentally different soils? They collected two contrasting materials from locations across Bangladesh. The first was a clay of intermediate plasticity, classified as CI under the Unified Soil Classification System, with a liquid limit of 46.72 percent and a plasticity index of 22.09, sourced from Matidali in the Bogra district. The second was a poorly graded sand, classified as SP, taken from Gabtoli in Dhaka. Both samples were extracted from depths of one to one and a half metres to minimise the influence of organic matter and surface desiccation.

The eggshells themselves were collected from households and restaurants in Dhaka, washed to remove the inner membranes, oven-dried for 24 hours at 105 degrees Celsius, crushed and sieved through a 1.18 millimetre mesh. The powder was then blended into each soil at three dosage levels, 8.5, 12 and 15 percent by dry weight of soil, a range chosen after preliminary mixing trials showed that lower contents produced negligible changes while higher contents caused segregation during specimen preparation. All treated specimens were sealed and cured for 28 days at 25 degrees Celsius, matching the standard benchmark used in comparable stabilisation studies.

The testing programme followed established ASTM standards. For the sand, direct shear tests under normal stresses of roughly 31 to 93 kilopascals, corresponding to shallow depths typical of road subgrades and earthwork fills, revealed a striking rise-and-fall pattern. The untreated Gabtoli sand exhibited an internal friction angle of 29 degrees, a value the authors attribute to the low confining stresses, the sub-rounded shape of its particles and its poor gradation, which together limit interlocking. When 8.5 percent eggshell powder was added, the friction angle climbed to a peak of 38 degrees, an improvement of about 31 percent. Maximum shear stress rose from 52 to 84 kilonewtons per square metre, an increase of roughly 62 percent. A similar value was recorded at 12 percent, but at 15 percent the friction angle fell back to 31 degrees, signalling that the benefit had been lost.

The clay told the opposite story. The untreated Matidali clay had an average undrained shear strength of 68.11 kilonewtons per square metre, corresponding to a stiff consistency. Adding just 8.5 percent eggshell powder caused that figure to collapse to 22.57 kilonewtons per square metre, a reduction of approximately 67 percent. At 12 percent the strength remained low at 24.99 kilonewtons per square metre, and even at 15 percent, where a partial recovery to 39.30 kilonewtons per square metre was observed, the treated clay was still about 42 percent weaker than the untreated soil. In other words, at no dosage did the eggshell powder restore, let alone improve, the clay’s original strength.

The authors propose that these contrasting outcomes stem from physical rather than chemical interactions, and they are careful to frame their explanations as working hypotheses, since no scanning electron microscopy, X-ray diffraction or energy-dispersive spectroscopy was performed. Eggshell powder is essentially calcium carbonate and, unlike lime or cement, it does not readily release reactive calcium hydroxide unless it is calcined or chemically activated. Without that pozzolanic reactivity, its effect on soil depends almost entirely on particle-scale physics. In sand, the finer eggshell particles appear to fill the voids between the larger grains, increasing the density of particle contacts and improving load transfer, while their irregular, angular morphology may raise sliding resistance at contact points in a sand whose own grains are sub-rounded. Beyond a critical fines content, however, the added particles begin to separate the sand grains rather than pack between them, disrupting the direct grain-to-grain contacts that carry load and eroding the strength gains.

In clay, the same physical additions appear to work destructively. Clay derives its undrained strength from electrochemical bonding between platelets and from the delicate fabric they form. Introducing coarser, non-cohesive calcium carbonate particles may break up that bonded fabric, while the increased specific surface area of the mixture can raise water demand and local pore water pressures under undrained loading, reducing effective stress. Crucially, without alkaline activation, the eggshell powder remains largely inert and cannot form the cementitious bonds that lime and cement create. The result is a filler that disrupts rather than reinforces the clay matrix, a conclusion consistent with earlier work showing that plain, non-activated eggshell powder offers limited or no benefit in high-plasticity clays and that benefits in expansive clays diminish beyond a few percent dosage.

The paired experimental design is what gives the study its force. Most published research on eggshell stabilisation has examined a single soil type in isolation, often treating the powder as a potential lime substitute for expansive clays, which makes it difficult to judge how widely positive results can be generalised. By testing a clay and a sand side by side under identical curing conditions, mixing protocols and dosage levels, the Bangladeshi team produced a directly comparable dataset demonstrating that the response to eggshell powder is governed by soil type. The authors also compared their findings with earlier studies, noting that improvements reported in soft Egyptian clay at 5 to 15 percent dosage, or in expansive clay at lower dosages, may reflect the weaker baseline of those materials, where the compaction and void-filling effect of the powder outweighs fabric disruption, whereas the stiffer intermediate-plasticity clay used here was more vulnerable to being disturbed.

The practical implications are pointed. For Bangladesh and similar deltaic regions dominated by fine-grained alluvial deposits and poorly graded river sands, eggshell powder shows genuine promise as a low-cost, locally sourced additive for improving sandy subgrades and fills, particularly for lightly loaded roads and earthworks, where a friction angle gain of this magnitude could meaningfully increase shear capacity. But the study issues a clear warning for fine-grained ground: used alone and without chemical activation, eggshell powder can actively degrade the performance of clay subgrades and should not be deployed as a standalone stabiliser for cohesive fills or embankments. The authors acknowledge limitations, including only two replicate specimens per dosage in the compression tests, a single 28-day curing period, and the absence of microstructural analysis, and they recommend consolidated triaxial testing, consolidation tests, pavement-focused California bearing ratio tests and evaluation of alkali-activated formulations as next steps. For now, the message is one of conditional optimism: the egg on your breakfast plate may indeed help build roads, but only where the ground beneath them is made of sand.

Subject of Research: Use of eggshell powder as a waste-derived additive to modify the shear strength of cohesive and cohesionless soils

Article Title: Eggshell as soil shear strength amplifier for cohesive and cohesionless soils

Article References: Akon, M. M., Shovon, K. M. M. I., Chowdhury, A. A., Ahmed, S., & Turja, A. S. (2026). Eggshell as soil shear strength amplifier for cohesive and cohesionless soils. Discover Soil, 3(1), Article 100. https://doi.org/10.1007/s44378-026-00251-8

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00251-8

Keywords: eggshell powder, soil stabilisation, shear strength, cohesive soil, cohesionless soil, geotechnical engineering, calcium carbonate, direct shear test, unconfined compression test, Bangladesh, sustainable construction, biodegradable waste

The research team, led by MD. Moin Akon of the University of Asia Pacific in Dhaka, set out to answer a question that has lingered in geotechnical literature: how does the same fly ash perform when blended into two fundamentally different soil types under identical laboratory conditions? Most previous studies had tackled either clay or sand in isolation. The researchers collected a clay of intermediate plasticity from Matidali in Bogra and a poorly graded sand from Gabtoli in Dhaka, both from depths of one to one and a half meters to avoid organic-rich surface material. The sand, classified as SP under the Unified Soil Classification System, was uniformly graded with a coefficient of uniformity of just 1.67, while the clay carried a liquid limit of 47 percent and a plasticity index of 22.37 percent, marking it as moderately plastic and vulnerable to moisture-driven volume change.

Characterizing the fly ash itself proved crucial to interpreting everything that followed. Drawing on published X-ray fluorescence analyses of Barapukuria ash, the team confirmed that the combined silicon dioxide, aluminum oxide, and iron oxide content reached 92.9 percent, while calcium oxide sat at a mere 0.56 percent. Those figures comfortably satisfy the ASTM C618 criteria for Class F fly ash, the low-calcium variety that lacks self-cementing ability and needs an external calcium source to drive pozzolanic reactions. In practical terms, this ash would not behave like a glue on its own. Any strength gains would have to come from physical rearrangement of particles or from limited chemical reactions drawing calcium from the surrounding soil, a distinction that shaped the entire experimental design.

The experimental program was methodical. Oven-dried soil and fly ash were blended at proportions of 0, 8.5, 12, and 15 percent by dry weight, moistened to the optimum moisture content determined by Standard Proctor compaction testing, and compacted into specimens. Treated samples were then sealed in plastic wrap and cured for 28 days at a controlled temperature of 25 plus or minus 2 degrees Celsius, giving any potential pozzolanic chemistry time to develop. Sandy specimens went into a direct shear apparatus under normal stresses of roughly 31, 62, and 93 kilopascals to extract the internal friction angle, while clay cylinders were crushed in unconfined compression tests to determine undrained shear strength, calculated as half the peak axial stress. At least two specimens were tested for every condition, with mean values used throughout.

The sandy soil told a strikingly non-linear story. Untreated sand, whose shear resistance comes almost entirely from grain-on-grain interlocking, produced a friction angle of 29 degrees. At 8.5 percent fly ash, essentially nothing changed; the failure envelope remained virtually identical, suggesting the modest ash dose did little to alter how load transferred between grains. But at 12 percent, the friction angle jumped to 37 degrees, an improvement of roughly 27.6 percent. The researchers attribute this to three complementary physical mechanisms: the fine ash particles fill the voids between sand grains, densifying the packing and multiplying the contact points that carry load; the irregular, angular morphology of ash particles roughens the contact interfaces, raising sliding resistance; and at the optimum dose the balance between these effects peaks. Push the dose to 15 percent, however, and the friction angle slips back to 36 degrees, still a healthy 24.1 percent gain but a clear sign that excess fines begin to separate sand grains rather than nestle between them, replacing efficient grain-to-grain contacts with weaker sand-to-ash interactions.

The clay responded through a different route and delivered an even larger headline number. Untreated clay averaged an undrained shear strength of 79.63 kilonewtons per square meter, corresponding to a firm to firm-to-stiff consistency. Adding 8.5 percent fly ash lifted that to 97.60 kilonewtons per square meter, a 22.6 percent gain, and at 12 percent the strength climbed to 106.36 kilonewtons per square meter, a 33.6 percent improvement with remarkably tight agreement between duplicate specimens, the coefficient of variation sitting at just 1.5 percent. In a low-calcium Class F system, the authors argue, this improvement most likely reflects a combination of reduced plasticity, improved compaction behavior, and limited early-stage pozzolanic activity, in which reactive silica and alumina from the ash react with whatever calcium hydroxide is available to form modest quantities of cementitious calcium silicate hydrate and calcium aluminate hydrate.

The 15 percent clay results, however, exposed a messy complication that anyone who has worked with fine powders will recognize. One specimen recorded a strength of 67.03 kilonewtons per square meter, actually 15.8 percent below the untreated baseline, while its twin reached 121.45 kilonewtons per square meter, a 52.5 percent improvement. The coefficient of variation ballooned to roughly 39 percent, compared with 13 percent at 8.5 percent ash and 1.5 percent at the optimum dose. The team attributes this scatter to the difficulty of manually dispersing a large volume of fine ash uniformly through a clay matrix: localized ash-rich and ash-deficient zones create wildly different reaction environments within nominally identical specimens. The average value at 15 percent, 94.24 kilonewtons per square meter, was used for trend analysis but with an explicit caveat that it masks substantial within-treatment variability.

To turn these observations into something a designer could use, the researchers fitted quadratic regression models to the strength data, a natural choice given the parabolic rise-peak-decline pattern in both datasets. For sand, the friction angle was modeled as 29 plus 0.62 times the fly ash percentage minus 0.022 times its square; for clay, cohesion was modeled as 79.6 plus 3.45 times the dosage minus 0.12 times its square. Differentiating each equation and setting the derivative to zero yields theoretical optimum dosages of about 14.1 percent for sand and 14.4 percent for clay, hinting that the true mathematical peak may lie slightly above the experimentally tested 12 percent, in the 13 to 14 percent window. The authors are candid about the limits of these models: each was fitted to only four data points, the bare minimum for a second-order polynomial, and they should be read as empirical trend-fitting tools for these specific soils rather than validated design equations.

The engineering and environmental implications travel together. A 27 to 34 percent gain in shear strength parameters translates into meaningful increases in bearing capacity and reductions in deformation for subgrades and embankments, exactly the infrastructure elements where weak deltaic soils cause the most trouble in Bangladesh and similar regions. Meanwhile, every ton of ash diverted into stabilization is a ton kept out of an ash pond, attacking a waste disposal problem and a weak-soil problem with a single intervention. The optimum range of roughly 10 to 15 percent also aligns with earlier findings by Prabakar and colleagues and by Sengul and colleagues for comparable soils, lending the result broader credibility. The authors are equally clear about what remains unknown: no scanning electron microscopy or X-ray diffraction was performed, so the proposed mechanisms are inferred from macroscopic behavior and literature rather than directly observed at the particle scale, and field-scale validation with wetting-drying cycles and long-term loading has yet to be carried out.

What emerges is a compelling, if carefully hedged, case for turning industrial waste into ground improvement. The Barapukuria results suggest that low-calcium Class F fly ash works primarily as a physical strength modifier, excelling in cohesionless soils through void filling and interlocking while offering more modest chemical benefits to clays under ambient curing. Future work, the researchers recommend, should test the unexplored 13 to 14 percent dosage range, adopt a minimum of three specimens per condition, add microstructural characterization, and extend curing durations before the approach moves from the laboratory to the highway. If those steps confirm what the shear boxes and compression machines have already shown, one of South Asia’s most stubborn waste streams may find itself quietly holding up the region’s roads.

Subject of Research: Fly ash stabilization of cohesive and cohesionless soils for enhanced shear strength

Article Title: Shear strength enhancement and optimum dosage prediction of fly ash-stabilized cohesive and cohesionless soils

Article References: Akon, M. M., Shovon, K. M. M. I., Chowdhury, A. A., & Ahmed, S. (2026). Shear strength enhancement and optimum dosage prediction of fly ash-stabilized cohesive and cohesionless soils. Discover Soil, 3(1), Article 128. https://doi.org/10.1007/s44378-026-00288-9

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00288-9

Keywords: fly ash, soil stabilization, shear strength, geotechnical engineering, Class F fly ash, direct shear test, unconfined compression, quadratic regression, sustainable construction, Bangladesh, coal combustion by-products, subgrade improvement

A team led by Chaojie Wang and Chengchao Guo set out to answer a deceptively simple question: how strong is the interface between polymer-stabilized soil and concrete, and how quickly does that strength fade when the interface is submerged? The researchers used a two-component permeable polymer grout, mixing a polyether polyol-based component with an isocyanate-rich component in equal masses. The liquid penetrates soil pores under controlled grouting pressure, then cures into a solid that binds soil grains together and glues them to concrete surfaces. This chemistry, based on urethane and urea linkages, gives the material its high bonding strength and low viscosity, but those same chemical bonds are vulnerable to hydrolysis, the slow reaction in which water molecules break chemical links apart.

The experimental program was meticulous. Soil excavated from a construction site in Zhengzhou, Henan Province, was air-dried, sieved, and re-wetted to controlled moisture contents of 6 and 11 percent. Concrete cubes of three strength grades, C30, C40, and C50, were cast, cured for 28 days, and then precision-cut with a CNC machine into half-cubes whose cut faces formed the test interfaces. The researchers machined grooves into some of these faces to create four levels of surface roughness, quantified using the joint roughness coefficient scale developed by Barton and Choubey, ranging from perfectly smooth to a JRC of 19.5. Soil was compacted in layers onto the concrete surfaces inside a custom mold, and a purpose-built constant-pressure grouting rig, monitored by a digital pressure sensor sampling at 1 hertz, injected the polymer until penetration was complete.

Once cured, the specimens faced their ordeal. After seven days of natural hardening, they were submerged in water for 7, 21, or 56 days, then sheared along the polymer-stabilized-soil-to-concrete interface in a servo-controlled direct shear machine under normal stresses of 200, 400, and 600 kilopascals. The resulting shear stress-displacement curves told a consistent story across every combination of concrete grade, soil density, grouting pressure, and roughness. Each curve rose elastically, peaked, softened, and settled to a residual plateau, the classic signature of a quasi-brittle interface that fails suddenly rather than stretching plastically.

The numbers reveal a striking three-stage decay pattern. The most dramatic loss of strength came early: within the first seven days of immersion, peak shear strength dropped by just over 20 percent on average, and the decline was steepest of all in that opening week. Between 7 and 21 days the erosion of strength continued but slowed, and between 21 and 56 days the curves flattened into a plateau. After 56 days underwater, the peak shear strength had fallen by roughly 30 percent overall, with a worst case of 35.15 percent. Crucially, the interface never collapsed entirely. The residual bonding capacity stabilized at approximately 60 to 65 percent of the original dry strength, suggesting that the polymer possesses an intrinsic water resistance that prevents complete bond loss.

Normal stress turned out to amplify the damage. Under identical immersion times, specimens sheared at higher normal stress lost a larger fraction of their strength than those tested at lower stress, because water lubricates the sliding surfaces more effectively when they are pressed together harder. The same pattern appeared in the residual strength, which is governed almost entirely by sliding friction once the bond has failed. After 56 days of immersion, residual shear strength had decayed by about 35 percent, exceeding the peak-strength decay, with a maximum of 38.44 percent. Fitting the data to the Mohr-Coulomb framework showed why: the interfacial friction angle, a measure of how much the rough surfaces resist sliding, proved more water-sensitive than cohesion, the chemical-glue component of strength. After 56 days, the friction angle at peak strength had decayed by up to 36.57 percent, and at residual strength by as much as 40.67 percent, while cohesion losses peaked near 26 percent.

Microscopy explained the mechanism. Scanning electron microscope images of the stabilized soil showed that water dissolved protruding polymer films, loosened surface soil particles, and smoothed the once-rough interface, eliminating the mechanical interlocking that had helped resist shear. ImageJ analysis of the micrographs revealed that apparent porosity climbed from about 25 percent in unimmersed samples to just under 27 percent after 56 days, with most of the growth occurring early. Mercury intrusion porosimetry confirmed the trend: the porosity of grouted soil rose from 21.49 percent before immersion to 26.49 percent after 56 days, while average pore size grew by 43 percent. Yet even the degraded soil remained far denser than the ungrouted silt, whose porosity of nearly 50 percent dwarfed anything measured after treatment. The water damage concentrated in pores between 7 and 200 micrometers, carving the fine through-cracks and voids that directly weaken the bond.

The failure surfaces themselves shifted character with soaking. In dry conditions, shear failure typically tore through the interior of the stabilized substrate, leaving soil smeared across the concrete, a mode the authors call type A. With longer immersion, less soil adhered to the concrete after failure, and the fracture plane migrated to the polymer-soil interface itself, the cleaner type B mode. Quantitatively, the proportion of soil clinging to the concrete surface fell steadily with immersion time and stabilized, while higher normal stress pushed failures back toward the substrate. The team also established a practical quality-control clue: specimens with total porosity above roughly 23 to 24 percent were far more likely to fail at the interface, and porosity correlated linearly with strength decay regardless of normal stress.

To turn these observations into a design tool, the researchers derived a three-stage damage constitutive model that captures the full bond-slip curve: a smooth rising branch, an S-shaped softening segment after the peak, and a frictional residual plateau described by the Mohr-Coulomb law. Fitted to the experimental data with the Levenberg-Marquardt algorithm, the model achieved coefficients of determination between 0.9275 and 0.9944 across all immersion times and stress levels, and it correctly reproduced the post-immersion shift toward greater ductility, in which the interface deforms further before reaching its diminished peak.

The authors are candid about the limits of their work. The five test combinations came from a preceding orthogonal design rather than a full factorial matrix, vertical displacement during shearing was not measured, and the hypothesis that strength stabilizes because accessible hydrolysable bonds are consumed early, leaving hydrophobic-protected links intact, rests on microscopy and literature rather than direct spectroscopy. Still, the practical message is clear. For trenchless tunnel repairs in water-rich ground, the finding that roughly 60 to 65 percent of interfacial strength survives 56 days of immersion offers a conservative starting point for design, while the porosity threshold gives inspectors a measurable warning sign. Water attacks the glue quickly, but it does not dissolve it completely, and knowing exactly where the decay stops may be the most valuable number this study delivers.

Subject of Research: Shear strength degradation and damage modeling of the polymer-stabilized soil to concrete interface under water immersion

Article Title: Research on the shear strength and damage constitutive model of interface between polymer stabilized soil and concrete under water immersion

Article References: Wang, C., Ding, L., Diao, Y., Zhang, X., Guo, C., Wang, F., & Du, X. (2026). Research on the shear strength and damage constitutive model of interface between polymer stabilized soil and concrete under water immersion. Case Studies in Construction Materials, 25, Article e06528. https://doi.org/10.1016/j.cscm.2026.e06528

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06528

Keywords: permeable polymer grouting, tunnel reinforcement, soil-concrete interface, direct shear test, water immersion, shear strength decay, Mohr-Coulomb parameters, damage constitutive model, mercury intrusion porosimetry, scanning electron microscopy, hydrolysis, geotechnical engineering