Environmental Engineering Reference
In-Depth Information
uncertainties and risks involved for the conditions
encountered. The choice of this overdesign Fac-
tor, required for anchorage calculation, obeys
to the great variability observed in respect of the
geomechanical properties of the rock mass, being
thus the upper value of the recommended interval
(F s ≥ 1.3 to 1.5) according to the spanish “Founda-
tion Design Guide for Road Works” (Ministerio de
Fomento. Dirección General de carreteras 2003).
achievement of the necessary structure reaction
against the ground pressure requires a sufficient
wall displacement, as a consequence of employing
passive bolts. its calculation has allowed to check
the compatibility of this displacement with road
operation.
Two different analysis methodologies has been
applied to check the rock slope stability:
- When the rock mass presents deined discon-
tinuity surfaces (according to stereographic
projection of joint sets), determining the pos-
sibility of cinematic instabilities, analyses using
equilibrium limit methods applied to planar or
wedge rock slides have been performed. To cal-
culate the shear strength of rough rock joints,
the aforementioned Barton-choubey criterion
was utilized. Rock wedges are studied with the
hoek-Bray method.
- if the rock has not presented deined discontinu-
ity surfaces, as in the event of extremely fractured
rock masses with joints of all directions, failures
have been assumed on cylindrical and planar
slip surfaces, being analysed with equilibrium
limit methods assuming the mass rock perform-
ance similar to a dense coarse-gravel soil, with
a Mohr-coulomb failure criterion. in that case,
stability is studied both in drained and und-
rained conditions.
in every former situation, the slope Factor of
safety against all the actions is checked. a mini-
mum overdesign Factor of 1.5 has been required
for the stability. if the calculated F s results lesser,
a force must be applied against the sliding slope
until this factor is equal or over. Precisely, this force
coincides with the minimum reactive force that the
retaining structural system must guarantee to sta-
bilize the slope with the specified safety, keeping in
consideration the reaction dip with respect to the
horizontal.
it should be remembered that methods of total
equilibrium do not consider the plasticization of
the slices and consequently, species in the prox-
imity of the re-enforcement interventions, local-
ized breaks could realistically occur even when
the retaining structure is able to offer a reaction
sufficient to stabilize the slope: this since the
ground may not be able to transmit the exerted
pressure of the slope to the intervention due to
poor geotechnical characteristics or reduced thick-
ness in the upstream zone of the retaining struc-
ture. in the interaction between slope and retaining
structure the analysis includes not only the sliding
surface that without a retaining structure presents
a minimum safety factor, but also includes all the
potentially unstable sliding surfaces. The safety
factor value is, in fact, a relative quantity while that
of the retaining structure reaction is absolute, and
it is for this reason that the force necessary to stabi-
lize a slope along a sliding surface with a minimum
safety factor may not be sufficient for another
surface with an initially greater safety factor but
with a unit weight, mass, and then force very much
superior in play.
in addition, the rock slope nailing underneath
the concrete retaining wall is analysed. its safety,
with respect to overall and local rock wedge and
planar instabilities, has been also checked. here,
loads on the top of this slope are bigger because
of the wall weigh concentrated load. The applied
analysis methodologies and failure criteria are the
same. likewise, the stability of the rock slopes
where retaining walls are unnecessary was verified,
designing the necessary passive anchor system to
assure the specified Factor of safety ( Fig. 4 ) .
With the results of the required stabilizing reac-
tion forces, whose maximum values are presented
in Table 2, the anchorages were designed.
For the reason of simplifying the installation,
taking into consideration the complicated access
to site and working conditions, a typology of
passive steel bar anchor was the ultimate choice.
The system is constituted, as aforesaid, by passive
corrugated steel B500s GeWi ® bars of the same
diameter, only varying their length depending
upon each road cross section.
To form a mechanical and chemical reinforce-
ment, the space between the bolt and the borehole
Table 2. Total required stabilizing pressure on the
anchors system to assure a specified overdesign Factor
(F s = 1.5) against planar and wedge sliding.
Factor
of
safety
F s
Required
Reaction
(kn/m 2 )
height
(m)
slope
station
slope behind
the retaining
wall
5 +000
7.17
64.00
1.50
slope beneath the
retaining wall
2.50
54.40
1.50
4 +940
slope with anchors
(without retain-
ing wall)
8.00
32.50
1.50
4 +960
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