Etna flank dynamics: A sensitivity analysis by numerical modelling - Volcanic Rock Mechanics - page 142

The bi-dimensional finite difference modelling
(FDM) was performed by Flac 5.0 code (itasca),
the finite element (FeM) by siGMa/W code
(Geoslope international), along the WnW-ese
section, crossing the volcano summit and the Valle
del Bove depression, with a horizontal extent of
61 km and to the depth of 10 km ( Fig. 1 ).

1) volcanic edifice (Ve), 2) subetnean clays (cl),
3) apennine-Maghrebian flysh (Flcl), 4) hyb-
lean plateau (hP), 5) intrusive complex (Dc). The
boundary conditions were imposed by fixing x- and
y-velocities equal to zero respectively at the base
and side boundaries of the model. sea water load
was taken into account, and the hydrogeological
conditions assumed considering first a dry model
and then with Flcl and hP completely saturated
with a static water table ( Fig. 4 ) . The model mesh-
ing has an average resolution of 100 m, and it is
adjusted to fit the topography and the main units.
a first phase of analysis consisted in the initiali-
zation of the stress field in elastic conditions under
the effect of gravity alone.
Then, a second phase consisted in an elastic-
plastic equilibrium under the effect of gravity. in
this phase the lithotechnical units were assigned
a Mohr-coulomb constitutive law and the asso-
ciated strength and deformability properties in
selected value ranges. hoek-Brown properties were
also considered ( Tab. 1 ) .
a sensitivity analysis ( Fig. 5 ) was performed in
order to evaluate the effect of the main assump-
tions concerning: i) topographic complexity;
ii) geometry and asymmetry of the model; iii)
role of the distance of boundary condition from
the area of interest; iv) geometry of the contact
between hP and Flcl ( Fig. 5 ); v) rheology and
constitutive laws attributed to the lithotechnical
units (Mohr-coulomb, hoek-Brown; Table 1 ) ;

3.1 FDM
3.1.1 Conceptual model and sensitivity analysis
The conceptual model was first simplified and
progressively implemented to analyse the effect
of topography, geometry and rheological behav-
iour of the structural units. Five main geologi-
cal and lithotechnical units were considered:

Figure 4. Geological-technical conceptual model. The
vertical black line indicates the location of the inter-
face for the application of magma pressures. at the
sides, boundary conditions (fixed x- or y-velocities) are
imposed. The white dots indicate the average depth at
which equivalent Mohr-coulomb parameters are calcu-
lated as a function of the confining stress.

Table 1. Material properties assigned in the analyses. if multiple values are indicated, sensitivity analyses were
performed.

lithotechnical units

Ve Fl/Flcl cl hP

elastic rock mass properties
Dry unit weight, γ(kn/m 3 )

2500 2600 2300 2700

elastic modulus, e (GPa) 4; 25 6.6; 16 1.9; 10 25

0.3 0.28 0.28 0.28

Poisson ratio, υ

Mohr-coulomb properties calculated at the specified minimum confining stress σ 3
σ 3 (MPa) 17 22 5 -

cohesion, c (MPa) 3.55 5; 2.9; 1; 1 -

Friction angle, φ° 33 29; 30 29 -

Tensile strength, σ t (MPa) 0.035 0.036 0.126 -

0 0 0 -

Dilation angle, δ°

hoek-Brown rock mass properties
Ucs* (MPa) 65 100 50 -

s coefficient 0.0013 0.0067 0.0016 -

mb parameter 2.346 2.0046 0.6300 -

a coefficient 0.511 0.5040 0.5099 -

Disturbance factor D 0 0 0 -

* Uniaxial compressive strength.