Soil category and determination of other geotechnical parameters

The last component of the processing phase is the one that allows the determination of the soil category and geotechnical parameters using the results obtained during the computation.

Here is shown the average of the velocity Vs in the first thirty meters, weighed on the thickness of the layers defined in the model. From this data and using some additional definitions, is determined the category to which the examined soil belongs.

Finally, are also calculated:

• Shear deformation modulus;

• Edometric modulus;

• Volumetric compressibility modulus;

• Young's modulus;

• NSPT ,Qc.

How to read these moduli

The four moduli are expressed in MPa and are derived directly from the velocities and the density of the layer, according to the formulas above: G0 = rho x Vs^2, Ed = rho x Vp^2, M0 = rho x (Vp^2 - 4/3 Vs^2), Ey = 2 rho Vs^2 (1 + v). With density in kg/m3 and velocities in m/s the result is in Pascal, and it is converted to MPa.

These are dynamic, very-small-strain moduli: not comparable with geotechnical tables

A seismic test strains the ground by about 10^-6, that is in the range where soil is stiffer than it will ever be under a structure. Hence G0 is the initial shear modulus (G_max) and Ey the corresponding dynamic Young's modulus.

The values in handbooks and geotechnical tables are operational moduli instead, measured or estimated at working strains (10^-3-10^-2): they are typically 5 to 20 times smaller. Finding 150 MPa here and 15 MPa in a table for the same soil is not a unit error: they are two different quantities.

To go from seismic values to design values a stiffness reduction factor is applied (G/G₀ degradation curves), chosen according to the expected strain level and the soil type.

Symbols

Ed is the oedometric (constrained) modulus derived from the compressional wave velocity; M0 is the bulk modulus. The Poisson's ratio v used in Ey is the one computed from the Vp/Vs ratio of the layer.

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