Hybrid approach for improving response of rubber-based vibrating foundations

The dynamic response of foundations depends on bed material dynamic properties, which vary with shear strain and confining pressure. Direct evaluation is costly, prompting alternative methods. Thus, the authors integrated free and steady-state vibration tests, in a novel hybrid testing approach, to indirectly assess strain-dependent properties and isolation effect of rubber sheets (0–24 mm thick). Results reveal that increasing rubber thickness reduces stiffness across all Equivalent Dynamic Shear Strain (EDSS) levels (0.001%–0.08%), while damping ratios rise slightly for EDSS <0.02% and drop sharply for EDSS >0.02%. Without rubber, higher deadweight increases stiffness and lowers damping; with a 12 mm rubber sheet, stiffness decreases and damping increases for deadweights >2.1 kN, emphasizing the confining pressure’s influence. The authors developed polynomial equations that give equivalent dynamic properties as functions of EDSS, rubber thickness and deadweight. Validated against independent datapoints, these equations accurately predict machine foundation responses across frequencies without requiring explicit dynamic shear strain, shear modulus or damping measurements. Thus, this innovative indirect hybrid testing approach, utilizing interpolation data — in the tested ranges – for EDSS and other design parameters, offers a simplified, time- and cost-effective method for machine foundation analysis and design.