Caracterización de las propiedades dinámicas no lineales del suelo para la respuesta sísmica en Islamabad, Pakistán

This study presents a detailed experimental characterization of the small-strain dynamic behavior of fine-grained soils in the Islamabad capital territory, Pakistan, an area of moderate to high seismic hazard with limited site-specific geotechnical data. A comprehensive series of resonant column tests was conducted on reconstituted silty clay and clayey silt samples collected from 15 boreholes across the region. Tests were performed under confining pressures of 50, 100, and 150 kPa to evaluate shear wave velocity, maximum shear modulus (Gmax), the modulus reduction (G/Gmax) with shear strain, and strain-dependent damping behavior. Results reveal that Islamabad soils are characteristically stiff at small strains, with Gₘₐₓ ranging from approximately 40–80 MPa and increasing systematically with confining pressure, while typical Vs values ranged from 160–260 m/s. The normalize shear modulus (G/Gmax) curves showed moderate nonlinearity, with G/Gmax reducing to 0.3–0.5 at maximum strain levels (~0.1–0.2%). Reference strain (γref) values obtained from curve fitting fell between ~0.02–0.08%, further quantifying the onset of modulus reduction. Damping ratios increased from ~0.5% at microstrains to a maximum of 8–12% at higher strains, with minor variation across confining pressures. When compared with standard empirical models, the measured curves exhibited stiffer behavior and lower damping at equivalent strains, highlighting the importance of site-specific calibration. These findings provide critical input for improving seismic site response analyses in Islamabad, directly informing local microzonation studies and contributing to the refinement of Pakistan’s seismic design guidelines.