Environmental Engineering Reference
In-Depth Information
the anchors operating predominantly in shear and
it was considered that this support system would
carry a high risk of shear failure.
The arch concept was the preferred for the long
term support of the cavern roof. however, it was
necessary to introduce some changes. The number
of anchors would need to be increased to reduce the
individual anchor loads (there were anchors with
loads >200 t). in addition, it was recognized that
horizontal and inclined anchors would be required
to control the potential relaxation of the subverti-
cal joints towards the shaft walls and would be easy
to install between the light shafts.
These conclusions required the development of a
hybrid support concept combining the initial three
concepts. The final idea was to create a reinforced
rock arch within the ground above cavern roof
with curved galleries excavated from the light shafts
and anchors installed around the galleries. longer
ground anchors would also be installed from the gal-
leries to support the rock mass between the arches
and the cavern roof. in addition, rock bolts would be
installed during excavation of the cavern roof to sup-
port the little blocks between the ground anchors.
Figure 9.
sketch of the reinforced rock arch.
Group. This program treats the rock mass as a series
of blocks bounded by joint surfaces.
The three initial support concepts were modelled
by itasca spain. The model included the mountain
slopes and the main underground works (cavern,
light shafts and entrance tunnel) with a simplified
representation of the mapped joints and dykes.
While, in common with similar programs, 3Dec
has a limit on the complexity of the model that can
be run, 8 dykes and 34 master joints were selected
to intersect the cavern and entrance tunnel in the
model. The rock mass between the discontinuities
was modelled as non-deformable blocks.
The strength of the master joints between the
blocks is assumed to have Mohr-coulomb strength
parameters, with zero cohesion and tension. The
shear strength along the discontinuities relies on
friction, dilation and the normal force acting across
the discontinuity.
The excavations and support were wished in place,
apart from a sequential excavation of the cavern roof
area. The excavation was assumed to be in dry on the
basis that the rock mass will be drained by the initial
excavations (pilot gallery in entrance tunnel and roof
galleries) and the recharge of the groundwater table
will be low in the arid climate of the island.
4.4
3DEC model sensitivity and results
The hybrid solution for the support was modelled
in 3Dec and the base case model was investigated
by comparing the displacements and support loads
for the following cases:
- Two sequences of excavation
- Water table: an analysis representing the long
term recovery of the groundwater table close to
the cavern roof level was carried out.
- seismic loading
- Joint orientation: The position of key subverti-
cal joints was varied by 10° in strike and/or dip
- Joint strength: The dilation angle was reduced
from 10º to zero in single degree decrements
- Rock block deformability: as a inal check, the
initial assumption of rigid blocks was changed
to deformable blocks
The base case model was run initially without
support. This model showed that those areas of
the cavern roof adjacent to the light shafts walls
were particularly sensitive to the failure of col-
umns of rock bounded by vertical joint planes and
the shaft walls.
While the hybrid concept offers a solution to
the support of the Tindaya space, the stability
achieved has shown to be sensitive to a variety of
factors, in particular variation in critical master
joint orientations. it is thus considered that the
support will need to be validated or modified using
the joint data collected during the excavation of
the space.
4.3
Hybrid concept
The modelling showed that the rock mass could be
supported with the three initial solutions, however,
the reinforced roof plate would required excep-
tionally high maximum axial loads in the support.
The option with only horizontal anchors was inad-
equate because the orientation of the anchors with
respect to the joints and dykes would result with
