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 team focused on tension-type prestressed anchors, the workhorse of early slope reinforcement in China. Unlike compression anchors, whose grout stays squeezed and therefore rarely cracks, tension anchors place their grout in a state that invites cracking. Those cracks become highways for groundwater, oxygen, chloride, and sulfate ions to reach the steel tendon, which is itself held under high stress. The result is a particularly vicious form of deterioration: stress corrosion that can transform a ductile steel strand into a brittle one, quietly stripping away the load-carrying capacity that engineers counted on at design time.

To capture this decay mathematically, the researchers built on an electrochemical corrosion rate model originally developed by Hong, adapting it to anchor cables. Their equation links the corrosion rate to ambient temperature, relative humidity, the thickness of the grout cover wrapping the tendon, and the compressive strength of the grout itself. The physics is intuitive once unpacked: warmer conditions accelerate the electrochemical reactions through Arrhenius-type kinetics, moisture drives the cathodic half of the corrosion cell, and a thicker, stronger grout cover slows the ingress of aggressive species. Because fully enclosed anchors corrode far more slowly than those exposed to wet-dry cycling, the team applied a correction factor, effectively assuming the grout remains intact until corrosion products expand and crack it from within.

From this corrosion rate, the researchers derived time-varying expressions for three competing failure modes. The first is debonding at the mortar-rock interface, where the entire grout column pulls out of the borehole. The second is slippage at the tendon-grout interface, where corrosion products and chemical degradation of the cement matrix erode the bond that transfers load from steel to rock. The third is tensile yielding of the free segment, as the effective cross-section of the steel shrinks and its yield strength drops. Crucially, the anchorage system behaves as a series, or weakest-link, system: whichever of the three resistances is smallest at any given moment governs the anchor’s capacity, and failure of that single link means failure of the whole.

The study’s most striking mechanistic finding is that this weakest link does not stay put. In the early decades of service, the mortar-rock interface typically controls the anchoring force. But as corrosion progresses, expanding rust products crack the grout, allowing water, oxygen, chloride, and sulfate ions to flood the tendon surface. The tendon-grout bond then collapses far faster than the other resistances, and control of the anchoring force shifts to that degrading interface. In the case study, a twelve-meter anchor’s tendon-grout bond resistance plummeted from 2,714 kilonewtons to just 291 kilonewtons over a century, while the free segment’s yield resistance fell more gently from 1,316 to 1,033 kilonewtons. This mechanism transition is what produces the alarming acceleration of anchoring-force decay in the later service life.

To translate anchor degradation into slope-scale risk, the team embedded their time-dependent anchor capacities into a pseudo-static limit equilibrium framework based on the horizontal slice method and the Mohr-Coulomb strength criterion. Seismic demand was not applied uniformly: following the Chinese seismic design code for hydraulic structures, the horizontal acceleration coefficient was distributed trapezoidally along the slope height, rising from the toe to the crest to reflect the well-documented topographic amplification of ground motion. A genetic algorithm searched for the critical slip surface, and the First-Order Reliability Method, implemented through the JC algorithm with equivalent normalization of non-normal variables, computed failure probabilities at each service age. Peak ground acceleration was treated as a Gumbel-distributed extreme-value random variable, consistent with standard probabilistic seismic hazard practice.

The case study examined a 28-meter rock slope cut into homogeneous slate at 47 degrees, reinforced with seven rows of high-strength strands and analyzed under four seismic intensity levels ranging from 0.1 g to 0.3 g. The results trace a clear three-act drama. For the first forty years, failure probability stays essentially flat, governed mainly by seismic parameters rather than corrosion. Then, between roughly fifty and seventy years, the curves bend upward: under 0.1 g shaking, failure probability jumps from 8.3 percent to 17.5 percent, a 110 percent increase, while under 0.3 g it climbs from about 50.7 to 59.1 percent. In the final decades, the system enters what the authors call a high-risk state, with failure probability exceeding 38.5 percent at 0.1 g and reaching 67.8 percent at 0.3 g by year one hundred.

Perhaps the most counterintuitive insight concerns how seismic intensity and corrosion interact. The absolute gap in failure probability between the 0.3 g and 0.1 g scenarios over a century is about 29.3 percentage points, but the relative difference shrinks dramatically, from 7.7-fold to 1.8-fold. In other words, long-term corrosion accumulation erodes system reliability most severely in low-seismicity environments, where a corroded anchor system has little residual margin to absorb even modest shaking. The authors also benchmarked their results against Chinese code target reliability indices, finding that under a 0.2 g scenario the failure probability surpasses 40 percent after about 85 years, a reliability index below 0.25 that would be unacceptable for most structures, while early-life indices above 1.0 are only marginally adequate even for low-consequence applications.

The sensitivity analysis delivers a rare piece of genuinely actionable engineering guidance. Increasing the grout cover thickness over the tendon from 20 millimeters to 50 millimeters cuts the hundred-year failure probability by 42 percent, raising the reliability index from roughly 0.31 to 0.82. Thin covers show accelerated failure probability growth beginning around thirty years, whereas thicker covers delay that inflection to around fifty years. Conversely, every 10-degree-Celsius rise in average temperature or 20 percent rise in relative humidity boosts the failure probability growth rate by 15 to 25 percent, a sobering prospect for coastal and monsoon-climate regions and for a warming world. Among all parameters examined, the environmental corrosion coefficient exerts the strongest influence on reliability, followed by humidity and temperature, confirming that durability, not seismic demand alone, dominates long-term performance.

The authors are candid about the limits of their framework. They assume uniform rather than pitting corrosion, treat corrosion across the anchor array as statistically independent rather than spatially correlated, and model fully saturated conditions without the strengthening effect of matric suction in unsaturated soil, all choices that make their baseline estimates conservative. Even so, the practical implications are hard to ignore. The fifty-year inflection point offers a quantitative target for maintenance scheduling, pointing to interventions such as secondary grouting or anchor supplementation well before reliability collapses. For major projects with design lives beyond half a century, the study argues, durability design must incorporate environmental acceleration factors and hygrothermal coupling, and slopes in hot, humid regions with inadequate grout cover may fall below acceptable reliability thresholds well within their intended service life. What emerges is a clear message for infrastructure owners: the anchors holding back the world’s slopes are aging on a schedule that static, time-invariant safety checks were never designed to see.

Subject of Research: Time-variant reliability analysis of corroded prestressed anchor-reinforced slopes under seismic loading

Article Title: Time-variant reliability of anchored slopes considering coupled corrosion and seismic effects

Article References: Wang, H., Meng, X., Yang, X., Jia, J., Tu, B., Zhang, L., Pan, X., & Han, Q. (2026). Time-variant reliability of anchored slopes considering coupled corrosion and seismic effects. Discover Geoscience, 4(1), Article 339. https://doi.org/10.1007/s44288-026-00700-5

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00700-5

Keywords: anchored slopes, prestressed anchors, corrosion, seismic loading, time-variant reliability, slope stability, geotechnical engineering, failure probability, grout cover thickness, limit equilibrium method, FORM, infrastructure durability