Seismic piezocone tests (SCPTU) give shear modulus at small (micro) strains. The equipment (Figure 1) consists of a standard electric cone in which a small electronic device mounted at the top of the cone which is capable of detecting the arrival of shear waves. This device can consist of an accelerometer or a geophone.
Figure 1. Seismic test
The test is carried out as shown in Figure 1. Shear or S waves are generated at the soil surface by striking a steel plate with a hammer. The steel plate should be securely fixed on the soil surface by a weight or other method. The hammer is electrically connected to a triggering circuit in the data acquisition system.
The seismic test is normally carried out during cone penetration pauses to add new rod lengths. As the cone is at depth zi, the strike plate is hit by the hammer and the triggering circuit starts the data acquisition. A signal versus time is obtained at each test depth zi and corresponds to the arrival time ti of the shear wave. The shear wave velocity Vs between two successive depths zi and zi+1 is given by:
The shear modulus Gmax is then computed according to the following equation:
where γ is the soil unit weight and g is the acceleration of gravity.
Small strain Gmax values can be corrected to the macro-strain domain, which corresponds to most geotechnical engineering applications by the use of a single laboratory test or theoretical degradation curves.
It is recommended that more than one hammer blow is given at each test depth. The number of blows varies between two to five at each depth. SPAS 2009 enables a check in the repeatability of signals giving the user the option to include in the analysis only the signals with good repeatability.