This study investigates the strain-rate-dependent mechanical properties of unsaturated red clay under varying temperatures and matric suction conditions through triaxial shear tests on red clay fill materials from the Sichuan-Tibet Railway region. The tests were conducted with multiple shear strain rates, complemented by advanced microstructural analysis techniques such as mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM), to examine the evolution of pore structure. The results indicate that high matric suction significantly reduces the rate-dependency of strength in red clay fill materials, whereas temperature has a relatively smaller effect. As matric suction increases, the strain-rate parameter decreases across different temperatures, with a diminishing rate effect observed at higher suction levels. Compared to temperature, strain rate has a more pronounced influence on failure time. An increase in strain rate leads to a significant reduction in failure time. At low strain rates, failure time exhibits substantial variability, while at high strain rates, the effects of temperature and matric suction on failure time become less significant. Under high-temperature conditions, the strength of red clay is enhanced, and failure time is delayed. These findings provide critical theoretical support for controlling settlement deformation and predicting failure times of subgrade fill materials under complex climatic conditions, offering valuable insights for engineering applications.

Population growth has resulted in industrialization, massive construction, and significant mining to supply the population's ever-increasing needs. The study deals with waste material utilization for soil properties enhancement, reducing construction costs and benefiting the environment. Bottom ash, one such effluent released from thermal power plants, was used in percentages 0, 20, 50, 70, and 100% by weight. Laboratory tests were conducted, including the sand replacement method, unconfined compression test, and shear test, to study the enhanced mechanical properties of the soil, adding bottom ash to it. Initially, the property of backfill material is enhanced and further it is used behind the wall along with the geofoam placed strategically to significantly reduce lateral stresses exerted on the retaining wall further optimizing the overall structural efficiency. Geofoam of three different densities, 11 kg/m3 (11EPS), 16 kg/m3 (16EPS), and 34 kg/m3 (34EPS), has been tested to understand how the compressive strength and corresponding modulus values change with the unit weight and strain rate. It was observed that with an elevating density of geofoam, unit weight, compressive strength, shear strength, and shear strength parameters increased, whereas water absorption capacity decreased. The results of this study can be used as a reference for the quality control of geofoam. The effective use of geofoam placed behind a stiff retaining wall in reducing lateral stresses brought on by a combination of backfill material and loading conditions was evaluated using a finite element model. The results obtained through the numerical investigations were validated with the differential element method developed. Results obtained through numerical and calculated models were in good accord with a percentage error of less than 20%. The impact of geofoam density, relative thickness, and friction angle of backfill on the efficiency of geofoam in reducing lateral stresses was then investigated using a parametric analysis. Earth pressure reduction obtained for different backfill types and lower density geofoam (11 EPS) of thickness 10 cm was between 23.27 and 62.72% obtained numerically. The highest earth pressure reduction, i.e., 64.17%, was obtained for 11EPS geofoam of thickness 15 cm laid behind the bottom-ash-backfilled retaining wall. Parametric charts prepared with the obtained results can help determine the required thickness of geofoam for any desired earth pressure reduction efficiency.