Environmental Engineering Reference
In-Depth Information
unknown ground conditions in the reduced space of
the mountain where the cavern will be located. The
field campaign comprised 15 rotary cored investi-
gation boreholes (vertical and inclined) to a total
of 1.650 m in length and also a complete labora-
tory and in situ program of testing with down-hole
and cross-hole geophysical tests, dilatometer tests,
hydro-fracture stress tests and permeability tests.
Thirteen of the fifteen boreholes were tested
using geophysical optical and acoustic televiewer
probes in order to analyse the discontinuities (3.000
joints were studied). in addition, 2 surface geo-
physical techniques were employed (electrical resis-
tivity and seismic refraction) to obtain geological
subsurface information and depth of weathering.
The field campaign has provided enough data to
have a complete geological model of the Tindaya
Mountain.
Figure 5.
Basalt dyke.
3.2
Geotechnical design assumptions
The cavern and associated structures are located
in relativity massive trachyte body with late stage
steep dipping basalt dykes intruding the trach-
yte. The distribution of the dykes and its thick-
ness, strike and dip extent is somewhat variable,
although a dominant ne structural trend can be
observed.
The dominant trachyte is competent and strong
(Ucs = 40-120 MPa), with a high porosity (9-19%)
and potentially abrasive (cerchar abrasivity index
of 0.6-1.3). Trachyte presents a banding of hydro-
thermal origin, although it was found that this has
a little influence on the mechanical behaviour of
the rock.
The distribution of joints appears relatively uni-
form within the massive trachyte body with four
dominant joint sets. as seen in the borehole logs,
less than 4% of the joints are infilled, such that the
roughness of the joints will govern shear strength.
analysis of the joint shear strength indicates that
representative values are: JRc = 6, Jcs = 60 MPa,
φ
b
= 30°, dn = 7 to 10° and k
n
= k
s
= 200 MPa/m.
The dykes appear to be highly fractured at the
surface and generally also at depth. The width of
basalt dykes and weathering adjacent trachyte is
it is apparent that weathering of Tindaya stone
may occur in a relatively short period of time, not
geological time, and seems to be more susceptible
in areas exposed to moisture.
in situ stresses in the rock mass are higher than
expected with a horizontal to vertical stress ratio of
approximately 2.
Based on the geotechnical tests as well as on
rock mass characterization, Table 1 resumes the
geotechnical assumptions which have been taken
into account for the design.
Figure 6. Model (estudio Guadiana) showing the
dykes crossing the space. The majority will intersect the
entrance tunnel.
Table 1.
Geotechnical parameters.
Trachyte
Basalt*
γ [kn/m
3
]
23
23-27
Ucs [MPa]
62.6 (40-120)
66,7
4.5
-
σ
t
[MPa]
m
i
18
-
e
mass
[GPa]
10-13
5-13
RMR
60-90
-
Q
10-50
2-50
Gsi
65-80
50-80
* Few samples suitable were obtained due to the poor
quality of the rock.
3.3
Hydrogeology
Fuerteventura is a dry island, located within the
sahara Dry Belt area. in Tindaya Mountain,
