Environmental Engineering Reference
In-Depth Information
- The loss of resistance in the matrix rock of the
mass or in the material in the discontinuities due
to the fracturing or the progressive alteration of
the material.
Throughout this process it must be taken into
account, apart from the overall structural stability
of the group, the problem presented by the exist-
ence of the open joints in the rock mass where
resistance is fundamentally mobilised in the con-
tacts between the irregularities of both sides. These
circumstances give rise to the possibility that two
bending (traction) areas exist in some parts of the
layers and very high compression stress in the con-
tacts of the rugosities. another factor is alteration
due to the attack capacity presented by the exist-
ence of the network of cracks and by the large spe-
cific surface area of the mobilised ledges.
This situation, which can be assumed to be gen-
eral for the entire subterranean complex, is partic-
ularly critical in the entrance areas of the Jameos,
where on a joint basis there are four unfavourable
characteristics which act complementarily and are
the following:
- The larger dimension of the cross sections of the
cavern. This means a greater “equivalent” span
between supports than in the rest of the cavern.
- Reduced thickness of the solid rock mass in the
keystone of the corresponding section.
- an almost lat arrangement of the section roof
due to the loss of the “shoulders” area. it is
indeed this shape which is the clearest symptom
of how critical the current situation is. it must be
remembered that it has developed until becom-
ing a situation in strict equilibrium.
- a greater ease of alteration due to the proximity
of the free surface area.
Vano autoestable-espesor de roca. Hipótesis voladizo
6, m
5, m
4, m
3, m
2, m
1, m
, m
, m
1, m
2, m
3, m
4, m
5, m
6, m
7, m
8, m
Longitud de vano autoestable (FS=1) (m)
Tracción máxima 11, T/m2
15, T/m2
Vano autoestable-espesor de roca. Hipótesis bi-apoyada
6, m
5, m
4, m
3, m
2, m
1, m
, m
, m
2, m
4, m
6, m
8, m10, m12, m14, m16, m
Longitud de vano autoestable (FS=1) (m)
Tracción máxima 11, T/m2
15, T/m2
resistance to simple compression to these materials
ranging between 11 and 15 MPa. This hypothesis
is conservative if taking into account the results of
all the tests; however this conservatism has to be
used when there is a relatively limited amount of
available information. With regard to the specific
weight a value of 21.7 kn/m
3
is proposed, an aver-
age value for all the tests.
There is no information on resistance to simple
traction. it is necessary to assess this based on nor-
mally accepted reasonable criteria and which usually
allow assessing it as a tenth part of the resistance to
compression i.e. between 1.1 and 1.5 MPa.
The cantilevers and openings observed were,
in general, close to those calculated. This would
imply a strict equilibrium of the structure of the
solid mass which would require a reinforcement to
allow separating the reinforced situation from the
current one with regard to the safety factor. This
reinforcement can be focused from the point of
view of modifying the working hypotheses of the
solid mass using anchoring modifying the thickness
of competent rock, thus increasing, if possible, its
average resistance to traction (a possible approach
would be to incorporate an artificial stratum over
the surface) or a combination of both.
3.2.2
Simplified calculations carried out
to assess the condition of the Jameos
The first attempt to be made was that correspond-
ing to a minimum cantilevering and opening which
would be self-supporting and for which a simpli-
fication was used treating them as a built-in-free
and bi-supported beam (maximum tractions in the
centre of the opening) subjected to its own weight.
These hypotheses do not take into account the
three-dimensional nature of the problem, nor the
fact that the numerous sub vertical fractures can
make blocks become detached. Whilst aware of the
degree of conservatism and simplification of the
problem that has been assumed the objective was
however essentially to have a reference framework
of how the group functions.
For the purposes of the geomechanical char-
acterisation of the upper unit and roof it can
be considered as being the one only, allocating a
3.3
Second-level problems isolated instability
by average-sized blocks
These correspond to instability produced due
to faults of lesser importance than previously
