Recently natural hazards like earthquakes, landslides, subsidence, glacier bursts and flash floods have severely impacted Himalayan cities, including Joshimath, Uttarakhand. In January 2023, significant ground cracks were observed, leading to the evacuation of nearly 800 buildings. This study investigates the underlying causes of ground failure and building damage through various geotechnical and geophysical tests at 12 sites in Joshimath, including plate load test, dynamic cone penetration test, field direct shear test, multi-channel analysis of surface waves and horizontal-to-vertical spectral ratio. Soil samples are analyzed for natural moisture content and grain size distribution. There is large heterogeneity in the test results which are highly variable. The field tests indicate the soil fabric of Joshimath is a complex mixture of boulders, gravels and soil. Internal erosion in such soils causes the instability of the whole fabric and results in the readjustment of the boulders resulting in subsidence. Internal erosion, driven by subsurface drainage from rainwater, ice melting and wastewater discharge, destabilizes the soil matrix and causes subsidence. It has also been observed that even at greater depths, no clear uniform strata is present and similar heterogeneous strata extend. Lower shear strength and bearing capacity are observed at several sites, potentially contributing to building damage. The study emphasizes that individual test results alone may not adequately capture site conditions. Instead, a combination of multiple test results is essential for a comprehensive assessment. Based on the test results, a vulnerability map of the area is presented.

This study aims to develop a finite element model (FEM) of a plate load test (PLT) to evaluate the performance of a geogrid-reinforced two-layer soil system. The PLT with a reinforced base over a soft subgrade is replicated with a two-dimensional (2D) axisymmetric FEM with a size of a 4-m radius and a 4.4-m depth after a rigorous testing for appropriate dimensions. The critical responses of the performance of the two-layer soil system such as the vertical surface deformation and compressive strains and stresses at the top of the subgrade are estimated numerically from a commercially available FEM program. The developed model is verified by benchmarking with a published data from literature, and the results are found to be in good agreement. The PLT deflection results are used to estimate the modulus of the subgrade reaction for an unbound base over the soft subgrade. The numerical results show that the performance of the geogrid-reinforced two-layer soil system is enhanced with an increase in the modulus of the subgrade reaction by a factor of 1.3 with a relative deformation ratio of up to 34% due to the impact of the degradation of the modulus ratio and the angle of stress distribution, effects of cyclic loading and the influence of the granular base layer's thickness. Regarding the improvement in the bearing capacity, the results show that it increases by a factor of around 1.06, and hence is considered nominal and not effective to use the soft geogrid reinforcement.

Soil liquefaction is an important problem that threatens the safety of buildings during earthquakes due to its destructive effects caused by a serious decrease in soil strength. In addition to the existing soil improvement methods, a reasonable approach to tackle this problem could be a superabsorbent polymer called sodium polyacrylate (SPA), which has a 125 ml/gr water absorption capacity as well as an extraordinary swelling pressure, used as grouting inside the liquefiable soil. This mitigates the effects associated with liquefaction -induced damage by changing the phase of excess water into a gel formation within the ground without changing the soil skeleton. In this study, the use of SPA is evaluated against post -liquefaction problems of saturated liquefiable soil, such as pore pressure generation, surface settlement and bearing capacity both in shaking table tests in a laboratory and full-scale in -situ tests thanks to its viscous gel structure and swelling potential. Firstly, an optimum SPA column spacing was determined to be four times the drilling diameter according to the shaking table tests in terms of pore water pressure generation and surface settlement. After this, an original chemical grouting device was designed, produced, and mounted on a drilling machine with two different mechanisms that provides multiple injections in a dry state and multiple boring at the same time. Finally, the results of the full-size liquefaction tests within an 8 m3 test pit demonstrate that generation of excess pore pressure is dissipated at up to 150%, settlements are reduced, and liquefaction resistance is increased due to the maximum 0.4 pore water pressure ratio value, after improvement through single, double, and triple SPA column application.

The availability of quality materials for the construction of pavements has been a problem in some regions. This scarcity enforced geosynthetic materials as one of the quests for su1stainability in the pavement industry. This study attempted to stabilize marginal (tunnel muck, reclaimed asphalt pavement material) and industrial waste (zinc slag) materials by employing high density polyethylene geocell to appraise the effectiveness of geosynthetic reinforcement in enhancing the characteristics of pavement materials and mitigating their rutting. The cyclic plate load tests are performed on marginal and industrial waste material with geocellreinforced and unreinforced base layers over black cotton soil as the subgrade. The tests were conducted following the trapezoidal loading pattern with a 0.77-Hz frequency. The geocellreinforced pavement performance was evaluated for resilient deformation, accumulated permanent deformation, rut depth reduction, traffic benefit ratio, and base layer thickness reduction. The mechanistic empirical pavement design guide (MEPDG) permanent deformation performance (PDP) model framework was used to predict the accumulated permanent deformation in unbound granular layers and to differentiate the rutting performance in geocellreinforced and unreinforced sections. This study determined the material constants of the MEPDG PDP model for marginal and industrial waste materials used in the pavement for quantifying the reduction in base layer thickness.