In order to quantify the design load for buildings due to an earthquake the Eurocode 8 (EC8) and the DIN4149 require the classification of the ground. The dominating or average shear wave velocity in depths down to 30 m is most often used for this classification. Beyond that the standards request certain design features as flexible foundation beams based on the class of the subsoil which are affecting the costs of a building. A seismic survey can often reduce the costs because otherwise necessary safe assumptions concerning the classification are avoided. Surface wave measurements such as MASW are especially suited to determine shear wave velocity profiles at these shallow depths.
MASW measurements were made in the planning phase of a warehouse to classify the building site and to determine the necessity of flexible foundation beams. The result in the figure to the left shows the calculated shear wave velocity profile, where the velocity depth profile is assigned to the middle of the geophone array. The average shear wave velocities determined at depths of 3 to 20 m is the so called dominating shear wave velocity. This velocity is used to classify the foundation site according to DIN 4149. For this example the site was assigned as subsoil class B with dominating shear wave velocities between 350 and 800 m/s. For class B foundation sites, situated in low risk earthquake zones, the use of flexible foundation beams is not required.
Calculated shear wave velocity
Elastic soil parameters at small strains are crucial for the design of dynamically loaded foundations and structures. They govern the dynamic soil-structure interaction at the interface between ground and foundation. At a larger scale they are used to predict the amplification characteristics of a site related to earthquake induced accelerations leading to site specific response spectra.
Seismic velocities are directly linked to the elastic soil parameters. Borehole tests as seismic crosshole and downhole tests can provide these velocities in a straight forward fashion.
A seismic crosshole and downhole survey was conducted in order to estimate accumulated settlements of a dam caused by cyclic changes of the water level due to the operation of a near ship lift. The results show a vertical profile of the seismic P- and S-wave velocities, which enable the calculation of material properties, such as Young's modulus, shear modulus, bulk modulus and Poisson's ratio with depth.
Downhole survey at a ship lift
1D seismic velocity profiles