Effect of silt interlayer configuration on the cyclic resistance of loose sand in direct simple shear

Liquefaction is a major source of earthquake-induced ground failure, especially in loose, saturated sandy deposits common in Indonesia. Understanding how layered soils behave under cyclic loading is therefore crucial for developing effective strategies to mitigate liquefaction. This study evaluates the influence of silt interlayers on the cyclic resistance of loose sand using Cyclic Direct Simple Shear (CDSS) tests. Three configurations were examined at Dr = 40% under fully saturated conditions: pure sand (S), sand–silt–sand (S–M–S), and silt–sand–silt (M–S–M). The hydro-mechanical response is governed by the drainage-barrier behavior of the silt layers, which modifies pore pressure dissipation during cyclic loading. Experimental results show that the presence of silt interlayers substantially accelerates the generation of excess pore-water pressure ( ru) and reduces the number of cycles to liquefaction (NL). Compared with pure sand, cyclic resistance decreased by about 73% for S–M–S and 82% for M–S–M. The highest ru value (1.096) occurred in the M–S–M configuration, signifying full liquefaction and rapid loss of effective stress. These findings highlight the dominant role of stratigraphic arrangement in liquefaction potential and underscore the importance of identifying thin fine-grained interlayers in hazard assessments for coastal and alluvial deposits.