Determination of dynamic properties of soil - Method of test
1992 Edition

IS 5249 recommends several in-situ test approaches for determining dynamic soil properties such as: Standard Penetration Test (SPT) for empirical correlations of dynamic modulus and damping; Field Vane Shear Test to assess shear strength relevant to dynamic analyses; Crosshole and Downhole Seismic Tests for measuring shear wave velocity (Vs), a critical parameter for dynamic modulus calculations; Spectral Analysis of Surface Waves (SASW) as a non-invasive Vs profiling method; and Resonant Column Tests (either in field or laboratory) for assessing small-strain shear modulus and damping characteristics. The shear wave velocity obtained is related to maximum shear modulus by the formula G_max = ρ × Vs², where ρ is soil density.

According to IS 5249, the dynamic shear modulus of soil varies with both the strain amplitude and the confining pressure. As shear strain increases, the dynamic shear modulus decreases due to nonlinear soil behavior, approaching the maximum shear modulus at very low strains. Meanwhile, effective confining pressure positively influences the maximum shear modulus, with G_max typically increasing proportionally to the square root of confining pressure. This reflects that higher confining stresses densify the soil fabric, enhancing shear stiffness. Thus, dynamic soil stiffness is modeled as a function dependent on both strain magnitude and effective confining stress.

IS 5249 specifies that the vibrating block used in block vibration tests must be sufficiently rigid and massive—generally about 300 mm by 300 mm in dimensions—with an operating frequency range between 10 and 50 Hz, and controlled amplitude to avoid excessive soil disturbance. For cyclic plate load tests, the standard calls for circular steel plates ranging from 300 mm to 750 mm in diameter, with loading systems capable of applying cyclic loads at frequencies around 1 to 5 Hz. Load measurement devices such as load cells should have an accuracy within ±1%, while displacement measurement instruments (like dial gauges or LVDTs) should detect movements as small as 0.01 mm. Additionally, data acquisition systems and stable reaction frames are essential to ensure precise and reliable test results.

IS 5249 outlines two principal methods for damping coefficient determination: the Free Vibration Decay method and the Forced Vibration method. The Free Vibration Decay approach involves exciting the structure and observing the natural decrease in vibration amplitude over time, calculating the damping ratio using logarithmic decrement based on the ratio of successive peak amplitudes. The Forced Vibration method applies harmonic loads at varying frequencies to identify resonance peaks and bandwidth, from which damping is inferred. Accuracy relies on using sensitive instruments such as accelerometers, minimizing external noise, and averaging multiple test results to enhance reliability.

IS 5249 provides comprehensive guidance for designing machine foundations by integrating dynamic soil-structure interaction principles. It emphasizes the utilization of dynamic soil parameters including dynamic modulus of elasticity, damping ratio, and Poisson’s ratio—derived from field or laboratory dynamic tests. The code advises ensuring the foundation’s natural frequency is distinct from the excitation frequency of the machinery to prevent resonance. Soil stiffness values (soil spring constants) are calculated based on dynamic soil properties for vertical, horizontal, and rocking directions and incorporated into vibration analyses. Consideration of soil damping allows for accurate prediction of vibration amplitudes. Ultimately, foundation mass, size, and embedment are designed to achieve acceptable dynamic response and structural stability.