Geosynthetics October/November 2022 - 30

Drainage geocomposites and PVC geomembranes for highway tunnel lining
excavation). Usually, a good estimate of
the load can be made by considering
the theoretical-empirical formulations
based on vast practical experience and
on a deep knowledge of the theoretical
principles that rule the soil mechanics.
The most used (and universally recognized)
classification of the soils and of
the consequent load conditions in tunnel
excavations is Terzaghi's classification
(Terzaghi, 1946).
Based on the geological survey, calcuThe
behavior
of a draining
geonet is strongly
influenced by the
normal pressure
applied over it.
lation of the earth pressure acting on the
vault was accomplished by considering a
stratified and jointed rock. The load, in
this case, can be conventionally expressed
as the equivalent load of a fraction of the
total height of the covering soil over the
tunnel. It strictly depends on the dimensions
of the cavity (B and Ht) and on the
intrinsic characteristics of the material
(internal friction angle, cohesion, unit
weight, water content).
Calculation of the
compressive strength
and flow rate
The tunnel has been excavated in flysch.
The geotechnical parameters of
this material are very low, particularly
in terms of internal friction angle. This
fact can heavily reduce the capacity of
the tunnel vault to generate the arching
effect, typical of stratified sedimentary
rocks; consequently, the earth pressure
over the tunnel can increase in a considerable
way, reaching values of the load
Rock Mass Category (RMR system)
3 fair rock
4 poor rock
5 very poor rock
K [-]
0.350
0.725
1.100
TABLE 2 Pressure on the vault and at the base of the tunnel
30
Geosynthetics | October November 2022
ϕ [º]
33
23
13
σv [kPa]
159.891
331.203
502.515
σh [kPa]
103.983
229.576
439.959
height comparable to those typical for
blocky and seamy rocks. The empirical
formulation used to estimate the vertical
and horizontal stress level to consider for
the dimensioning of the drainage system
of the tunnel has the following shape:
σv =γk (B+Ht)
vertical pressure over the vault
σh = γk Ht (B+Ht) tan2(45° -ϕ/2)
horizontal pressure at the base of the
tunnel where:
γ = unit weight of the rock mass
[kN/m³]
ϕ = friction angle of the rock mass [°]
B = width of the tunnel [m]
Ht = height of the tunnel [m]
k = coefficient for the calculation
of the equivalent load height [-]
The values for the coefficient K and
for the friction angle of the rock depend
on the jointing level of the rock mass.
They can be estimated with reference to
the Geomechanics Classification or the
Rock Mass Rating (RMR) system, published
by Bieniawski (1976 and 1989).
The rock present in the project was classified
into three categories. The values
used for the definition of the stresses are
shown in Table 2.
The joint friction angle and the K
coefficient have been determined after
Bieniawski (1989). Under the specified
load, each section of the geonet should
guarantee the discharge of the water coming
from the rock vault and from the
upper part of the geonet. The vault has
been divided into 12 sectors, six on each
side. Five sectors, on the curve part of
the tunnel, have length equal to 6.1 feet
(1.87 m); the bottom part, vertical, has a
length that is higher (8.3 feet [2.53 m])
(Figure 5).
The maximum flow measured,
as shown in Table 1, is equal to 11.25
m³/h/m (3.13E-03 m³/sec/m).
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