Cyclic behavior of clayey lumps intruded quaternary alluvial sand in the Indo-Gangetic Plain: an experimental assessment

Abstract

The dynamic behaviour of soils under cyclic loading is a key concern in geotechnical engineering, particularly in seismic-prone regions. The Middle Ganga Plain is characterised by unique geomorphological features, containing collapsible cohesive alluvial lumps that naturally form at the interface between cohesive and cohesionless soils. The proportion of these lumps in natural sand varies between 10% and 20%, depending on location, and significantly impacts the mechanical properties of soils. However, the behaviour of these lumps under cyclic loading remains poorly understood. This study investigates the effects of varying lump content on the dynamic properties of lump-sand mixtures, with a focus on shear modulus and damping ratio. Strain-controlled cyclic triaxial tests were performed on mixtures with different percentages of cohesive lump-sand mixture, and grain size distribution was analysed before and after testing to assess the lumps’ disintegration. The study proposes a novel empirical modification in Dobry’s excess pore water pressure generation model for quaternary alluvial sand intruded with cohesive lumps. The results reveal that with the increase in the lumps content, significant degradation of shear modulus is observed, indicating reduced soil stiffness and cyclic resistance. The damping ratio increased during the initial cycles for soils with more than 10% lump content, influenced by the particle size distribution and cyclic loading. Additionally, higher cohesive lumps content leads to an increased void ratio, altering the intergranular matrix and promoting structural rearrangements under cyclic loading. This research underscores the critical impact of cohesive lump content on the dynamic behaviour of soils, highlighting its implications for seismic site response, liquefaction potential, and foundation performance. The findings reveal that neglecting the effects of cohesive inclusions may lead to an underestimation of seismic demand and post-cyclic settlements. By quantifying these effects, the research offers valuable insights for the design and evaluation of foundation settlement, embankments, and other geotechnical structures in alluvial seismic regions such as the Middle Ganga Plain.