Frequently asked questions¶

The G0, Ed, M0 and Ey moduli look far too high compared with geotechnical tables. Is it a unit error?¶

No. The values are expressed in MPa and they are correct; what differs from the tables is the quantity they represent.

First check: the numbers add up¶

The four moduli are derived directly from the density and the velocities of the layer, so the check can be done by hand in a minute, starting from a row of the Other geotechnical parameters table.

Example, first layer of a real case: density 1400 kg/m³, Vs 198.71 m/s, Vp 371.75 m/s, Poisson's ratio 0.30.

Modulus	Formula	Computation	Value
G0	ρ · Vs²	1400 × 198.71² = 55.28 × 10⁶ Pa	55.28 MPa
Ed	ρ · Vp²	1400 × 371.75² = 193.48 × 10⁶ Pa	193.48 MPa
M0	ρ · (Vp² − 4/3 · Vs²)	1400 × (138,198 − 52,648)	119.77 MPa
Ey	2 ρ Vs² (1 + ν)	2 × 1400 × 39,486 × 1.30	143.73 MPa

With density in kg/m³ and velocities in m/s the result is in Pascal; the program converts it to MPa by dividing by 10⁶. These are exactly the values printed in the table and in the report.

The check that rules out a unit error

If those values were in kg/cm², the G0 of the example would be 564, not 55: 1 MPa is about 10.2 kg/cm². The numbers themselves are therefore enough to rule out a unit mix-up.

Why they are higher than the tables: they are very-small-strain moduli¶

A seismic test strains the ground by about 10^-6, that is in the range where soil is far stiffer than it will ever be under a structure. What Easy MASW computes is therefore the initial shear modulus G0 (also written G_max in the literature) and the corresponding dynamic Young's modulus.

The values in handbooks and geotechnical databases are operational moduli instead, measured or estimated at working strains (10^-3–10^-2). For the same soil they are typically 5 to 20 times smaller.

Finding 150 MPa here and 15 MPa in a table for the same soil is therefore not a sign of an error: they are two different quantities, and the first is not to be used in place of the second.

How to use them¶

Directly, with no reduction, in small-strain dynamic analyses: site seismic response, vibration studies, dynamic soil-structure interaction.
Reduced, in static geotechnical checks: a stiffness reduction factor is applied (G/G₀ degradation curves), chosen according to the expected strain level and the soil type.

The symbols in the table

Ed is the oedometric (constrained) modulus, derived from the compressional wave velocity; M0 is the bulk modulus. The Poisson's ratio used for Ey is the one computed from the Vp/Vs ratio of the layer, not an imposed value.