Isotropic collapse load as a function of the macroporosity of volcanic pyroclasts - Volcanic Rock Mechanics - page 105

isotropic collapse load as a function of the macroporosity
of volcanic pyroclasts

a. serrano
E.T.S.I.C.C.P., Universidad Politécnica de Madrid, Madrid, Spain

a. Perucho & M. conde
Laboratorio de Geotecnia, CEDEX, Madrid, Spain

aBsTRacT: Two main types of “macroporosity” in pyroclastic volcanic rocks can be distinguished:
“reticular” and “vacuolar”. The first type is produced when the large pores are located between grain
particles. The second type is produced when large pores are located inside a vitreous rock mass. however,
a mixed type can be defined in most cases, when both kinds of void are present. at other times, the pyro-
clasts do not exhibit any kind of “macroporosity”. an extensive study of how the type of porosity may
affect the strength of the material is being carried out at ceDeX geotechnical laboratory. samples with
different types of “macroporosity” have been tested under isotropic loads. as a result of a theoretical
study, an expression of the isotropic collapse load has been obtained, for any type of “macroporosity”,
and compared with test results.

1 PoRe MacRosTRUcTURe

Two basic pore macrostructures can mainly be dis-
tinguished (ceDeX, 2007; serrano et al, 2007a & b;
santana et al, 2008):
a. reticular structure ( Fig. 1a )
b. vacuolar structure ( Fig. 1b )
The reticular type corresponds to the pore
structure of the rock made up of an aggregate of
particles joined together either by the heat action
produced in their formation, or by some type of
cement. The vacuolar type corresponds to the
pores of a rock which are all vacuolar in type,
formed due to the gas formed inside the viscous
lava as it is expelled from the volcano.
The vacuolar pores generally have no connec-
tion between them while the reticular ones are gen-
erally connected between them.
in general rocks, and particularly macroporous
volcanic rocks contain pores of both types. in this
case they are said to have a mixed pore structure
( Fig. 1c ) , and the total porosity is:

Figure 1. Pore structures: a) Reticular; b) Vacuolar;

c) Mixed.

2 isoTRoPic collaPse loaD

2.1 Rocks with reticular pore structure
Take a ball of macroporous rock with a reticular
pore structure and subject it to an external iso-
tropic pressure, p ( Fig. 2a ) .
The amount of work supplied to the ball by the
external pressure p is:

Te = p ∆ V
∆ V is the total change of volume experimented
by the sample which can be decomposed into two
addends, corresponding to the rock (∆ V i ) and the
reticulated pores (∆ V R ):

n = n R + n V
where n R = reticular porosity and n V = vacuolar
porosity.
More complex cases and structures of pores
may exist where the grains are in turn formed
by micrograin agglomerates which in turn have
microvacuolas.

∆ V = ∆ V i + ∆ V R
The work T i carried out by the internal stresses
of the intact rock is:

T i = σ i ∆ V i