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What are Shear Wave Velocity Measurements?
It is well known that the soil conditions beneath a structure have an impact on the propagation of ground motions from the bedrock to the ground surface. The soil conditions may amplify certain spectral accelerations and attenuate spectral accelerations at other periods. Ultimately, the spectral accelerations are used to estimate the earthquake induced forces that are imparted to a structure. In addition, the shear stresses imparted to the soil by the earthquake are affected by the soil properties and have a direct effect on the liquefaction potential of the soils. The shear wave velocity (or shear modulus) of the soils is the soil property that has the greatest effect on the determination of an appropriate response spectra and estimation of shear stresses. SASW Test:
The SASW method is based on the dispersive characteristics of surface waves. It is a non-intrusive test making field measurements of surface wave velocities and determine the Vs profile through an inversion process. Vertical velocity transducers with a natural frequency of 1Hz are used as receivers and a falling weight is used as the source. |
Many methods have been adopted for evaluating shear velocity (Vs), some of them are:
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Seismic Cone tests:
The seismic cone (SCPT) consists of a conventional piezocone behind which a seismic module equipped with a miniaturised triaxial geophone is mounted. The shear wave is generated from the surface using a sledge hammer striking on a metal beam pressed against the soil by one of the stabilising actuators of the truck. Every meter in depth, a shear wave is generated from the surface and recorded by the triaxial geophone. Out of these measurements the total travel time and interval travel time is calculated and, since the distance to the geophone or between two recordings is known, the shear wave velocity can be calculated. |
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Crosshole method:
Crosshole (ASTM D4428) Vs measurements are typically performed between boreholes, one with a mechanically-activated source and at least two others with receivers. Crosshole testing allows for the generation and measurement of both vertically (SV) and horizontally (SH) polarized shear waves. Source and receivers can be lowered to different depths in the boreholes for additional measurements or the Vs can be measured on inclined paths to generate a tomographic shear wave velocity image. Downhole Method:
The downhole (ASTM D7400) method is performed in one borehole. A source at the surface generates shear waves which propagate downwards to receivers in the borehole. In general, the down-hole method is less expensive than crosshole because only one borehole is required. Downhole tests can also be used to measure both SV and SH waves. |
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