The second group includes all types of rock that have suffered
physical–chemical alteration due to weathering and/or
hydrotermalism.
For clastic and pyroclastic rocks, the geological criteria that can
be used to set the difference between soft rocks and hard rocks, is
the degree of lithification reached by them. Hoshino [3] studied
sandstone and pelitic rocks in two sedimentary basins in Japan
from samples obtained from deep drilling, up to 3000 m deep.
From the correlation between petrographic characteristics,
petrophysics (porosity) and mechanical properties (uniaxial compressive
strength, module of elasticity, cohesion and propagation
speed of P waves) it was defined three stages of consolidation in
pelitic sediments: dehydration, framework and cementation
(Fig. 2).
In the first stage of lithification, the sediment suffers a process of
compaction with dehydration as a predominant phenomenon. This
stage is characterized by drastic reduction of pore spaces and
mineral transformation (smectite to illite). The rock porosity varies
between 30% and 80% and the UCS between 1 and 30 MPa. The
process occurs at a confining pressure that reaches 30 MPa, at a
temperature of 55–60 C for 1–2 million years (Tables 1 and 2).
The study concludes that the higher the temperatures and