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 urgency of this work is rooted in global urbanization trends. According to United Nations projections cited in the study, roughly 45 percent of the world’s population currently lives in urban areas, a figure expected to climb to about 60 percent by 2050. As cities expand vertically and laterally into topographically complex terrain, engineers must build on geologically difficult ground where slope failure threatens not only infrastructure but also environmental goals. Landslides degrade land, strip away vegetated cover, disrupt drainage systems, and expose urban communities to escalating geohazard risk, directly undermining the United Nations Sustainable Development Goals on sustainable cities and life on land.

The study’s setting is the Almaty Basin in Kazakhstan, a region flanked by the Zaili Alatau Mountains and blanketed in thick Quaternary sedimentary deposits. These soils, formed from the weathering of deeply buried Paleozoic bedrock, consist of heterogeneous mixtures of pebbles, sands, sandy loams, and clay-rich loams. Laboratory testing on soil sampled from Almaty at a depth of four meters classified the material as low-plasticity silty clay under the Unified Soil Classification System, with an effective friction angle of just 19 degrees and moderate cohesion of 33 kilonewtons per square meter. These weak shear parameters mean that failure surfaces tend to develop as shallow mechanisms within the weathered soil layer rather than in the intact rock below, particularly when rising pore water pressure accompanies rainfall infiltration.

To confront these conditions, the researchers turned to the Geobarrier System, or GBS, an integrated slope protection technology that evolved from the capillary barrier concept. A capillary barrier exploits the hydraulic contrast between an overlying fine-grained soil layer and an underlying coarse-grained layer: water infiltrating the fine layer is retained and diverted laterally by capillary forces rather than percolating downward into the slope. While effective, conventional capillary barriers are limited to slope inclinations of up to 45 degrees. The Geobarrier System overcomes this by adding retaining elements, drainage provisions, and geosynthetic reinforcement, allowing stable designs on slopes as steep as 70 degrees. Its facing is built from geobags filled with an Approved Soil Mixture that supports vegetation, turning the structure into a green, nature-based solution rather than a bare concrete wall.

In standard GBS practice, biaxial geogrids are connected to the geobags and extend into the soil mass behind the structure, mobilizing tensile resistance through frictional interaction with the compacted backfill. The customary minimum embedment length is 0.7 times the height of the structure, a figure consistent with guidance from the Federal Highway Administration and AASHTO for geosynthetic reinforced soil systems. Shortening the geogrid saves material and cost, but it risks insufficient anchorage: if the reinforcement does not extend beyond the potential failure surface, tensile stresses cannot be transferred into the stable region of the slope, and deformation escalates. The central question of the new study was whether steel J-pin anchors attached to the free ends of shortened geogrids could restore the lost stability.

Answering that question required a numerical model capable of simulating three distinct interactions simultaneously: soil against geogrid, geogrid against J-pin, and soil against J-pin. The team selected the general-purpose finite element software ABAQUS over the geotechnical package PLAXIS because ABAQUS offers the flexibility to model structure-to-structure contacts directly, something PLAXIS handles only through predefined interface elements. The researchers used ABAQUS/Explicit with central difference time integration to handle the severe geometric nonlinearity and complex contact conditions, while carefully monitoring the ratio of kinetic to internal energy, which stayed below 0.8 percent throughout the analyses, well under the 5 percent limit accepted for quasi-static behavior. Factor of safety values were computed with the strength reduction method, progressively scaling down the soil’s cohesion and friction angle until failure occurred, automated through a Python-based scripting framework.

The model represented a 12-meter-high, 25-meter-wide block of Almaty soil supporting a 4-meter-high GBS with eight reinforcement layers, a 70-degree face inclination, and a 10 kilonewton per square meter surcharge on the crest. The geogrid properties matched those of the Miragrid GX-80/30 product, providing 80 kilonewtons per meter of longitudinal strength, while the J-pins were modeled as 12-millimeter-diameter S275 steel rods, 0.5 meters long, with bilinear elastoplastic behavior. Before adding pins, the team validated their ABAQUS setup against PLAXIS 2D for the baseline 0.7H configuration and shortened variants of 0.6H, 0.5H, and 0.4H. Deformation differences between the two platforms ranged from 0.3 to 6.63 percent and factor of safety differences from 0.97 to 3.2 percent, comfortably within the 10 percent tolerance considered acceptable when comparing different computational platforms.

The validation runs alone revealed a stark pattern. Cutting the geogrid from 0.7H to 0.6H raised maximum deformation only modestly, from 6.98 to 7.37 millimeters, but shortening to 0.5H pushed deformation up by roughly 52 percent to 10.58 millimeters, and 0.4H produced a 140 percent increase to 16.74 millimeters. The factor of safety fell in parallel, from 1.34 at 0.7H to 1.24, 1.15, and finally 1.09 at 0.4H. When J-pins were then added one layer at a time, starting from the lowest geogrid, the results split cleanly. For the 0.6H model, deformation decreased steadily up to five pins before a slight rebound, and the factor of safety peaked at the same five-pin configuration. The 0.5H model improved up to six pins. The 0.4H model, however, showed only marginal gains, with its factor of safety hovering near unity regardless of how many pins were attached.

Force distribution data explained why. With eight pins installed, the J-pins carried 17.2 percent of the total reinforcement force in the 0.6H model and 19.3 percent in the 0.5H model, evidence of effective composite load transfer between geogrid and anchors. Individual pin contributions were non-uniform: middle layers, which experience the greatest driving forces, mobilized the most tensile resistance, while the topmost and bottommost pins contributed the least. In the 0.4H model, by contrast, the pins carried only 13.3 percent of the force and their contributions were evenly spread, a signature of a geometry-controlled failure mechanism in which the shortened geogrid simply cannot mobilize meaningful tensile resistance beyond the slip surface. The conclusion is unambiguous: anchorage density can compensate for moderate geogrid shortening, but below roughly 0.5H no number of pins can rescue the design in weak colluvial soil.

The team put their findings to the test in a pilot-scale field construction on a 3.5-meter slope, using the 0.5H design with five J-pins attached to the lowest five geogrid layers. Recycled concrete aggregates served as both the fine and coarse capillary barrier layers, underscoring the sustainability credentials of the approach, and young plants were established in vegetation pockets on the structure’s face. Numerical analysis of the as-built design predicted a maximum deformation of 6.5 millimeters and a factor of safety of 1.424, exceeding the long-term stability minimum required by British Standard BS 6031. One month after planting, the vegetation had grown into a substantial green cover, and the structure has shown no significant cracking or movement since completion. The authors caution that their model assumes idealized pin anchorage without progressive soil yielding around the shaft, so field-scale monitoring remains important, but the demonstrated threshold of 0.5H with five pins offers designers a practical, material-efficient rule for deploying geobarrier systems on the landslide-prone clay slopes found not only in Almaty but across comparable mountainous terrain worldwide.

Subject of Research: Hybrid geogrid and J-pin reinforced geobarrier systems for landslide prevention on clay-rich colluvial slopes

Article Title: Landslide preventive approach incorporating hybrid geogrid-J-pin reinforced geobarrier system

Article References: Landslide preventive approach incorporating hybrid geogrid-J-pin reinforced geobarrier system. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: landslide prevention, geobarrier system, geogrid reinforcement, J-pin anchorage, slope stability, capillary barrier, finite element analysis, Almaty Kazakhstan, colluvial soils, strength reduction method, geosynthetics, nature-based solution