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supply ranges of allowable mechanical properties ofrocks. This set of information is in fact necessary
for stability evaluation, the choice and design of
stabilization operations, or for road coursediversion, as is required in the later stages of an
engineering project devised to improve mountain
road safety.
The aspects and results obtained from ageostructural and geomechanical characterization
procedure applied to the evaluation of potential
instability of rock exposures flanking an endangered
Alpine road (NorthWest Italy) and of a suitablelocation for diversion tunnels are reported in this
paper.
2. ROAD AND SITE DESCRIPTION
The 337 Val Vigezzo national road runs through
the Vigezzo valley for about 30km, connecting
Domodossola -the most important town in this area-to Ponte Ribellasca, where it then crosses the Italian
Swiss border. As this is the only motor-car
connection among the Vigezzo valley villages, andas it crosses an international border, this mountain-
side road must be kept serviceable throughout the
year. Furthermore, thousands of people who livenear the border daily cross the border for work.
The investigation only refers to the last stretch of
the road, which is located in the upper Vigezzovalley. This road stretch, which can be found after
the renowned holiday villages of Santa Maria
Maggiore, Malesco and Re, is about 5km long andconnects the Meis village to the Ponte Ribellasca
border station (Fig. 1). One side of the road leans
against the mountain, while the other faces the
Eastern Melezzo stream. The mountain-side is, to agreat extent, made of steep, often overhanging, rock
faces, while the stream-side slope is shaped for long
tracts by sheer cliffs and gorges. The mountainslope is prevailingly made of metamorphic, often
weathered, jointed rocks. These tectonic units show
verticalized strata and conform to the regionalTonale-Centovalli lineament that shapes the
Melezzo course. The unfavourable geostructural
and geomorphological assett of the Vigezzo Valleygives rise to an unstable behaviour of the mountain
slope, which under adverse meteorological
conditions could cause hydrogeological disasters,
like the floods of 1978 and 1993. These unfortunateevents not only reactivate or increase landslide
activity, but were responsible of a large number of
minor, but very risky, instabilities of different kindsand sizes, due to block/column, boulder and rock
fragment falls onto the road. Examples of this
tottering situation are shown in Fig. 2. It is alsopossible to observe that the Vigezzo railway, which
runs parallel to the national road, but which is
located at a lower level along the mountain side, is
also at risk to the same kind of instability problems.
In order to detect any possible risky conditions and
to obtain technical advice to better manage safety
and serviceability along the upper tract of the 337
road, the national road administration (ANASS.p.A.) commissioned a geostructural-
geomechanical investigation. The in situ
characterization was therefore carried out withgeneral and detailed surveys made through
exploration stations (e.g. ST10sa) located along the
5km long, 1.5 km wide road stretch (Fig. 1).
3. GEOLOGICAL, GEOMORPHOLOGICAL,
AND MACRO-STRUCTURAL SETTINGS
The 337 road passes through a complex geological
area in the Western Alps. Several tectonic units(Sudalpine, Austroalpine and Pennidic nappes),
referred to as the Adriatic and European crustal
sectors, are linked in this part of the Alpine region.In the studied area, these units are divided by two
main tectonic elements that are well know in Alpinegeological literature [6, 7]: a) the Canavese Line
which divides the Subalpine Units (South) from the
Australpine Units (Nord) b) the Centovalli Line thatdivides the Upper Pennidic Units (South) from the
Lower Penninid Units (North). Two main units
outcrop in the Val Vigezzo area (Fig. 1): theigneous and metamorphic rocks of the Ercinic and
Alpine Units and the glacial, fluvial and lacustrine
glacial Quaternary deposits. The first Units outcrop
along the valley sides, are well exposed along theMelezzo River gorge and consist of anfibolithic and
peridotithic igneous rocks mainly exposed on the
southern side of the Melezzo valley. Themetamorphic rocks, which consist of schists and
gneiss with pegmatitic dykes, outcrop along the
northern side. The Quaternary depositsunconformably overlap the metamorphic units, are
exposed along the valley floor and consist of
moraine glacial deposits, fluvial conglomerates and
glacial-lacustrine deposits (about 50 m thick)related to the Quaternary glacial-interglacial
periods.
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Figure 1: Geological-geostructural map along the 337 national road in the upper Vigezzo Valley; the survey stations for stability
evaluation of the road sectors and the proposed diversion tunnels are also shown.
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a) b)
Figure 2: a) Toppling rock slabs, covered by nets, on the upper
part of a rock exposure; b) A net torn by a rock block that fellfrom a gallery portal onto the road.
During the late Quaternary era the climatic and
tectonic events produced a change in the riverpattern drainage from E towards W to W towards E,with a quick erosion of the moraine and lacustrine
deposits and the formation of terraced deposits. The
structural setting of the study area is characterizedby the presence of ENE-WSW and E-W North
dipping fault systems. The kinematic indicator on
the fault plane suggests a right transpressive strike-slip movement. NW-SE and NNW-SSE N dipping
fault systems later displaced the strike-slip faults.
4. GEOSTRUCTURAL SURVEY ANDMESOSTRUCTURAL ANALYSIS OF THE
ROCK JOINTS
The metamorphic rocks outcropping along the road
stretch are intersected by a complex joint network.The different joint sets were induced by brittle
deformation mechanisms of tectonic and/or
gravitational origin. Therefore, according to therock mass structure and to the location of the
instability evidence, detailed mesostructural
observations were performed by means of scan-area(Fig. 3) or by scan-line sampling in 24 explorationstations chosen to characterize the 18 sectors of theroad stretch. The steep or overhanging rock slopes
made it very difficult to have access to the rock
outcrops for a close inspection of the natural
jointing, therefore the exploration stations weremostly located along, or in the neighbourhood of,
the road-side (Fig. 1). For the surveyed joint
elements, the following data were collected: jointtype (Fault (F), Joint (J), Schistosity (Sc) Joint,
Tension Crack (TC) or Extensional (E) Joint),attitude, length, spacing, aperture, roughness,
weathering, groundwater, filling and the kinematic
characterization (pitch) for the faults. A furtherdistinction was made for low angle (LA) joint
geometries (dip
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The first set is antipodal with poles in the NE andSW quadrants, and it shows a high azimutal
variation, but is prevailingly made of subvertical
conjugate planes striking N305-330 or N135-155, 70-85 dipping, almost orthogonal to the
schistosity. This pervasive set could give rise to
potential unstable conditions for wedge sliding or
for toppling, depending on the rock slope
orientation.
The poles in quadrant SE define a set of joints that
develops parallel to the schistosity. These
schistosity joints usually show a N225-250 strikeand medium to high 50-75 inclination. They
appear in almost all the surveyed stations and, when
directed parallel or subparallel to the rockexposures, tend to promote toppling.
The central part of the projection diagram shows a
less dense group of poles belonging to the LA joint
set. The LA set is less pervasive but usually morepersistent than the two previously mentioned joint
sets, and it has a SE prevalent dip direction, with an
average dip of 20. Such features, sometimes of alistric shape, being directed down-slope could
induce potential slab or block sliding.
The tectonic structures sampled during the survey
are representative of the field of regional tectonicdeformation. The observed faults in fact show a
general ENE-WSW, plunging NW, trend andtranscurrent kinematics which conform to the major
regional lineaments, such as the Cento Valli fault.These mesostructures can in fact be considered torepresent the deformation zone of such a large
tectonic element.
5. INSTABILITY PHENOMENA AND THEROCK MASS STRUCTURE
A large part of the investigated area is subjected to
landslides that should be considered active. The
erosion activity of the Melezzo stream, favoured by
the high jointing intensity of the metamorphic rockmass and triggered by adverse weather condition
(water, ice-thaw cycles), is the main cause of these
gravitative phenomena that involve both the 337road and the railway below. The landslide evolution
of the rock slopes is, on average, very slow or of the
creeping type (as in the ST1, E2 stations of thePonte Ribellasca sector), however with paroxystic
peaks characterized by rockfall of the unstable
elements when extreme events (like flooding)
occur. Two main kinds of landslides were observedwhich are controlled by joint sets of different
geometrical assets. In the first case, steeply dipping
joints, mostly the Sc antislope joints give rise totoppling phenomena of the slabbing/columnar rock
masses. In the second case, prevailingly the scarp
slope LA joints and sometimes, as in the ST11
sector, medium-high angle joints, give rise to planar
sliding of rock mass portions of different shapes andsizes.
In other circumstances, the different joint sets
combine to define potentially sliding rock wedgesand tetrahedrical or prismatic rock blocks that give
rise to rockfall.
A particular consideration concerning potential
danger should be given to the geomorphologicalevidence of deep-seated gravitative movements.
These deformations, which should be analysed in
greater detail, could be critical for whateverremediation measures are chosen for the present
road layout or for the planned diversions.
All the road sectors were therefore analysed for past
and present evidence of instability trends by lookingat pictures and projections of the joint sets that
caused the specific instabilities. Typical examples
relating the rock mass structure to the instabilitytypes are shown in Fig. 5.
6. MECHANICAL CHARACTERIZATION OFTHE ROCK TYPES AND JOINTS
At the same time as the in situ geostructural survey,
values of the index properties of the rock joints and
of the rock slumps were collected in eachexploration station, along with handy rock blocks
suitable for lab. specimen preparation.
The joint roughness (JRC coefficient, through 10cm
prophilograph) and the joint wall compressivestrength (JCS, through Schmidt hammer index test)
were evaluated from the different joint sets in orderto obtain representative values at the different roadstations. Furthermore, irregular rock slumps,
showing variable weathering degrees, were
collected and tested for strength (Is point loadstrength index, [9]). An evident estimation
variability can be observed in the histograms (Fig.
6a, b) where the prevailing representative values fall
in the 812 classes for the JRC and 2060MPa forthe JCS.
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C=sc
A
B E=LA
A=sc EC
D=LA
ST15sa
Figure 5: Evidences of past rock slope instabilities and of unfavourable joint attitudes surveyed in different stations along the 337
national Val Vigezzo road.
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There is even more scatter for the rock lumpstrength, which was tested through different tests
made orthogonally (n=142) and parallel (n=111)to the schistosity, and through a few ones carried
out on massive rock fragments (nm=9). The
collective histogram of the Is50, obtained by
applying the size correction to the original Isvalues,is reported in Fig. 7, and the following indirect
evaluation of the uniaxial compressive strength canbe derived: cIs=6839MPa, cIs=3713MPa,cIsm=10730MPa.
nsite=24 stations; nlab.=11 specimens
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
4-6 6-8 8-10 10-12 12-14 14-16
JRC
frequency[
-]
s it e l ab .
a)
site, n=24 stations
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
0-20 20-40 40-60 60-80 80-100 100-120
JCS [MPa]
frequency[-]
b)
Figure 6: Histograms of: a) JRC and b) JCS evaluated on rock
joints during in situ exploration (JRC from lab. specimens forshear tests is also shown).
site, n=24 stations
0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
0-1 1-2 2-3 3-4 4-5 >5
Is50[MPa]
frequency[
-]
Figure 7: Histogram of the point load strength index I s50available from the in situ testing.
Although these values reflect the influence thatschistosity and weathering has on the rock strength,
the following average estimation is given
irrespective of the strength anisotropy:
cIs=5724MPa.
For comparison purposes, it should be noted that a
small batch of point load tests (n=11), made on NX
cores recovered during lab. specimenmanufacturing, gave a higher, but highly scattered,
strength estimation: Is50=3.6 2.3 MPa; cIs=86.5 55 MPa. The thus obtained characterization would
suggest a middle rating of 7 for the strength
parameter for use in the RMR and SMR systems.These systems were then used for the rock exposure
qualification.
The in situ collected rock blocks were machined for
lab. specimen preparation, obtaining: 5 cylindricalspecimens for uniaxial compression tests, 12 for
triaxial tests, 8 disks for Brazilian tests, and 11cylindrical specimens containing joints for direct
shear tests. Apart from the mass density (
2.59kg/m3) and wave velocity measurements (high
frequency pulse), the mechanical tests, performed
according to ISRM suggestions [10, 11, 12], allow
the failure behaviour of the rock material to beestimated both according to the Mohr-Coulomb and
the Hoek & Brown [13] empirical failure envelopes.
The parameters evaluated according to the two
criteria are reported in Table 1.
The appraisal of the rock material strength obtained
from the two criteria is somewhat similar for both
the strength from uniaxial compression tests (c781.7MPa) and for the previous estimationsinferred from the mentioned in situ characterizationindexes. When compared to the compression
strength, the indirect tensile strength tb61.7MPaconfirms a good agreement with the 810 c/tratio that can be found in failure envelopes.
Table 1. Mohr-Coulomb and Hoek & Brown failure envelope
parameters of the metamorphic rock (friction angle ,cohesion c, uniaxial strength cM-C; parameters mi,si, uniaxialstrength cH&B).
Mohr-Coulomb Hoek & Brown
()
c(MPa)
cM-C(MPa)
r2
58.7
13.6
97.10.78
mi(-)
si(-)
cH&B(MPa)
r2
26.7
1
94.50.58
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As far as the rock material deformability isconcerned, the representative values of Youngs
modulus Ei17.41.7GPa and of the Poisson ratio i0.250.07 agree with the corresponding, althoughhighly scattered, dynamic deformability valuesderived from the wave velocity analysis: Edyn
20.410.4GPa, dyn0.210.09.
The joint shear strength fundamentally controls therock block stability and a significant effort was
made for the mechanical characterization, through
direct shear testing, using the Hoek shear box [14].
The roughness evaluation of the jointed specimens,before shear testing was performed, gave the
histogram in Fig. 6a which compares well with that
obtained in situ, namely, the higher JRC frequency
falls in the 812 classes in both cases. Shear testswere performed, applying a constant normal stressto the joint surfaces in the 0.5-5.0MPa interval.
The results of the experimental data set, for both the
peak and the residual conditions, are reported in
Fig. 8a,b along with two least squares regressionmodels of a linear type (C) and a non linear type
(Bartons (B) [15]) that represent the shear strength
of the metamorphic rock joints at the lab. scale.
These interpolations allow one to infer thefollowing representative values for the shear
strength parameters of the two models for the peak
condition: jp 41.7, cjp 0.29MPa, JRCp 8.2, JCSp
59.3MPa, b 34.3, and for the residual condition:jr30.0, cjr0.37MPa, JRCr 12.6, JCSr30.0MPa, r23.0. The range of the frictional strength parameter
is: 26
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suitable to provide a consistent judgement: RMRand above all SMR and modified SMR (SMRm) of
the overall rock mass quality against potential
instability of the rock slopes, RMR and Q of therock condition for the diversion tunnels. The GSI
[17, 18] was also used as a reliable index and for
comparison with the previously mentioned
qualification tools.
The RMR geomechanics classification [19, 20, 21]
and the Q system [22, 23], along with their quality
numbers (RMR89, Q - environmental condition not
included) are well known and widely applied(mostly for tunnels), while the SMR [24, 25] and
the SMRm [26] introduce adjustements to the basic
RMR, taking into consideration four Fi factorsrelevant to rock slope stability. The expression of
the Slope Mass Rating SMR is:
SMR=RMRbasic-(F1xF2xF3)+F4 (1)
where: RMRbasic is the RMR value computedwithout considering the discontinuity orientationrating; F1 is associated to the parallelism between
the slope and the discontinuity strike direction; F2is
related to the discontinuity dip inducing planefailure; F3concerns the slope angle compared to the
discontinuity dip angle; F4 is related to the slope
excavation method.
The difference between SMR and SMRm is in the
way a potential wedge instability is considered.
According to Romana, the two joints that make up awedge must be separately evaluated for SMR, and
the lowest SMR is assumed. The modified SMR is
instead evaluated using the plunge and trend of theintersection line of the wedge. Finally, a stability
assessment is given by assigning stability classes (I:
fully stable to V: fully unstable) to the (0100 SMRrange) and support measures are assigned for
remediation purposes in each class (Table 2).
The classification schemes are then applied toqualify all the 18 road sectors, that are labelled
according to the codes of the pertinent explorationstations. The different characterization indexes in a
given road sector are evaluated with reference toeach joint set (or to the couple that make up the
wedge when SMRm is considered) and a
representative quality judgement can be assigned tothe road sector by averaging these index values.
When the average RMR and the SMR evaluations
are compared it is possible to observe that the RMRjudgement is one class lower than the SMR (Fig. 9).
According to Romana, the lowering of the RMR,because of the unfavourable joint orientation
(weight up to 60), could induce this rather
pessimistic judgement of rock mass quality whenreferring to rock slope stability problems.
When the stability judgement of rock slopes is
related to remediation operations, the worst
potential failure mechanism must be consideredgiving the lowest SMR or SMRmvalue. The results
of this qualification are shown in Table 3.
Both SMR and SMRmsuggest that at least 65% of
the road could be classified as unstable (U) or evenfully unstable (F.U.), while the rest can be
considered partially stable (P.S.).
However a potential wedge type failure, which is
better identified by the SMRm, qualifies some roadsectors as being (F.U.) and not (U.). Referring to the
map (Fig. 1), it can be seen that the difficult sectors
are prevailingly located in the last part of the road,from the Olgia tunnel to the border station at PonteRibellasca. Previous instability phenomena and
deformation evidence of rock slopes support the
impression of a potentially dangerous road layoutand confirm the severe stability judgement reached
by the rock mass classifications. In other words,
huge remediation measures should besystematically provided along the road (e.g.
anchors, reinforced concrete toe walls, deepdrainage) to improve the stability of the rock slopes.
However, external reinforcement or reexcavation
operations might not be possible in some zones
where the rock slopes are particularly high or areunaccessible.
In these cases, in at least 25% (F.U. sectors) of the
entire road stretch, the difficulties due to the
location, design, and construction of suitableremediation operations -when possible- and the
related costs make the choice of diversion tunnels
the only feasible alternative.
As far as a variant of the present road course isconcerned, 4 diversion tunnels (G1, G2, G3, G4)
were planned (Fig. 1) according to a preliminary
design. An engineering assessment of each tunnelwas made based on classification systems, using the
geostructural-geomechanical information collected
in the exploration stations located near the tunnel
portals.
Different judgements were therefore formed, for a
given tunnel portal (e.g. I1G1, I2G1 - the two
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portals of tunnel G1), according to the stations usedfor the characterisation.
These evaluations only refer to the portals and to
the tunnel tracts close to the portals, therefore
outcrop information could also be used to qualifythe underground excavation.
Table 2: SMR class ratings and suggested support measures
for rock slope stability remediation [25].
Class SMR Support category & type
IaFully Stable
91-100 S1a: None
IbFully Stable
81-90 S1b: None. Scaling
IIaStable
71-80 S2a: (None. Toe ditch or fence)Spot bolting
IIbStable
61-70 S2b: Toe ditch or fence. NetsSpot or systematic bolting
IIIaPartially Stable
51-60 S3a: Toe ditch and/or netsSpot or systematic boltingSpot shotcrete
IIIbPartially Stable
41-50 S3b: (Toe ditch and/or Nets)Systematic bolting. Anchors
Systematic shotcreteToe wall and/or dental concrete
IVaUnstable
31-40 S4a: AnchorsSystematic shotcreteToe wall and/or dental concrete
(Reexcavation) Drainage
IVbUnstable
21-30 S4b: Systematic reinforced shotcreteToe wall and/or concreteReexcavation. Deep drainage
VaFully Unstable 11-20S5a: Gravity or anchored wall
VbFully Unstable
0-10 Np Not possible
(i) very often several different support methods are used in
the same slope(ii)Less usual support measures are in brackets
0
10
20
30
40
50
60
0 10 20 30 40 50
RMR
SMR
60
Figure 9: Comparison of the RMR and SMR ratings for the 18road sectors.
Table 3: SMR and SMRm classes and ratings, stability
condition and support category evaluated at each road sector.
Sector
#
Class Rating
SMR/SMRm
Stability
SMR/SMRm
Support
SMR/SMRm
ST1 III 54 / III 54 P.S. / P.S. S3a/S3a
ST2 IV 38 / IV 38 U. / U. S4a/S4a
STE2 IV 31 / V 16 U. / F.U. S4a/S5a
ST3 IV 30 / IV 30 U. / U. S4a/S4a
SP IV 35 / IV 35 U. / U. S4a/S4a
ST6 III 42 / IV 34 P.S. / U. S3b/S4a
ST7 IV 23 / V 15 U. / F.U. S4b/S5a
ST10 V 19 / V 2 F.U./F.U. S5a/Np
ST11 V 0 / V 0 F.U./F.U. Np/Np
ST12 IV 39 / IV 39 U. / U. S4a/S4a
ST13 III 43 / III 43 P.S. / P.S. S3b/S3b
ST14 III 46 / III 35 P.S. / U. S3b/S4a
ST15 IV 34 / V 13 U. / F.U. S4a/S5a
ST17 III 45 / IV 36 P.S. / U. S3b/S4a
ST18 III 41 / III 41 P.S. / P.S. S3b/S3b
ST19 IIIa 52 / IIIa 52 P.S. / P.S. S3a/S3a
ST20 IIIb 41 / IIIb 41 P.S. / P.S. S3b/S3b
ST21 IIIb 43 / IIIb 43 P.S. / P.S. S3b/S3b
Two slopes would need to be excavated to prepare a
tunnel entrance: one excavation to make the portal
(e.g. I1G1p), the other to make the sideway slope(e.g. I1G1s), as the considered tunnel tracts (e.g.
I1G1t) are near the portal zones. The classification
results suggest that difficult conditions should beexpected during portal excavation as the stability
judgement prevailingly pointed out (U) or even(F.U.) conditions (Table 4).
The permanent safety of the tunnel entrances shouldbe assured by profiling or by systematic
reinforcement of the excavated slopes and buildingtracts of artificial tunnels to protect the entrances
from unforeseable rock falls from the high rock
spurs.
A preliminary estimation of the tunnel rock load isgiven in Table 5 according to different empirical
relations based on classification ratings [22, 27, 28].
The rock engineering qualification that was made in
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the different road sectors could also be used tosupply mechanical parameters of the rock mass
when the characterization indexes are applied to the
corresponding intact rock parameters.
The following representative values of rock massstrength and deformability could be suggested for
design computations:
=cm 6.92.7MPa, =bm 4.91.2 (-),=s 0.0060.005 (-), =mE 127GPa.
Table 4: SMR and SMRm classes and ratings, stability
condition and support category evaluated at each tunnel portal.
Portal
#
Class Rating
SMR/SMRm
Stability
SMR/SMRm
Support
SMR/SMRm
I1G1p-St1 III 44/ III 44 P.S./P.S. S3b/S3b
I1G1p-St2 III 46/ III 46 P.S./P.S. S3b/S3b
I1G1p-StE2 III 47/ III 42 P.S./P.S. S3b/S3b
I1G1p-St3 III 60/ III 55 P.S./P.S. S3a/S3a
I1G1s1-St1 IV 35/ IV 35 U./U. S4a/S4a
I1G1s1-St2 III 42/ III 42 P.S./P.S. S3b/S3b
I1G1s1-StE2 IV 31/ IV 31 U./U. S4a/S4a
I1G1s1-St3 III 59/ III 59 P.S./P.S. S3a/S3a
I2G1p-St7 IV 29/ IV 29 U./U. S4b/S4b
I2G1s1-St7 IV 25/ IV 23 U./U. S4b/S4b
I1G2p-St10 IV 40/ V 10 U./F.U. S4a/Np
I1G2s1-St10 V 19/ V 3 F.U./F.U. S5a/Np
I2G2p-St13 III 54/ III 54 P.S./P.S. S3a/S3a
I2G2p-St14 IV 24/ IV 24 U./U. S4b/S4b
I2G2s1-St13 III 47/ III 47 P.S./P.S. S3b/S3b
I2G2s1-St14 IV 39/ IV 39 U./U. S4a/S4a
I1G3p-ST15 IV 28/ IV 28 U./U. S4b/S4b
I1G3s1-ST15 IV 27/ V 20 U./F.U. S4b/S5a
I2G3p-St17 III 53/ III 53 P.S./P.S. S3a/S3a
I2G3s1-St17 III 54/ III 56 P.S./P.S. S3a/S3a
I1G4p-ST21 III 43/ III 43 P.S./P.S. S3b/S3b
I1G4s1-St21 IV 38/ IV 38 U./U. S4a/S4a
Table 5: Evaluation, through rock mass classifications, of the
tunnel rock-load (vertical PVi, horizontal PHi) according to: 1Bartons [22], 2 Unal [27], 3 Goel & Jethwa [28] empirical
relations.
Tunnel
#
Pv1
[MPa]
Ph1
[MPa]
Pv2
[MPa]
Pv3
[MPa]
I1G1t-ST1 0,141 0,104 0,214 0,116
I1G1t-ST2 0,103 0,076 0,217 0,120
I1G1t-ST3 0,086 0,063 0,163 0,078
I1G1t-STE2 0,141 0,104 0,204 0,106
I2G1t-ST7 0,111 0,081 0,192 0,095
I1G2t-ST10 0,120 0,089 0,220 0,124
I2G2t-ST13 0,071 0,052 0,138 0,067
I2G2t-ST14 0,061 0,045 0,167 0,079
I1G3t-ST15 0,071 0,052 0,163 0,078
I2G3t-ST17 0,072 0,053 0,157 0,074
I1G4t-ST21 0,080 0,059 0,176 0,085
8. EVALUATION OF THE POTENTIAL
INSTABILITY MECHANISM THROUGH THE
BLOCK THEORY AND KINEMATIC TESTS
Besides the qualification obtained using theclassification schemes, an evaluation can be
performed of the potential instability of the rock
slopes, along the different road sectors or at theentrance of diversion tunnels using the Block
Theory and kinematic tests.
Although the rock slope side of the road could be
subject to large or complex landslides, the simpleidentification of different kinds of block types that
the rock mass structure could release from the rock
faces is nevertheless of paramount importance for
the assessment of the local stability and the safetyof the road. The Block Theory, for the potential
sliding or falling modes of different block types
[29], or the graphical test, for the potential topplingof rock slabs [30], were applied to each road sector
and to the excavated slopes of the tunnel portals.
In order to apply these analyses at a given location,
the complex structure of the metamorphic rockmass was simply schematized by assuming design
joints with constant attitudes equal to the average of
the joint set, while the friction angle j along thejoints was assumed to be 25, according to the
previously mentioned characterization.
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The PTWorkshop program [31] was then applied toproduce the stereographic drawings that are typical
of the Block theory where the different block types
are labelled according to the half-space sequencesthat make up the removable joint pyramid codes
(JPrem). The potential instability modes, the safety
factors (SF) and key blocks (KB) were also given
by the program, along with a 3D view of the block
type. The value of the friction angle, Jlim, which isrequired for the limit equilibrium condition, wasalso computed. The results of the analyses are
summarized in Table 6 for the road sectors and
Table 7 for the tunnel portals.
The tables show that the rock mass structure has anunfavourable interaction both with the rock slopes
of the present road layout and with the excavations
that will be needed to excavate the portals of thediversion tunnels.
Table 6: Evaluation, at each road sector, of the potentialsliding (one or two planes) or toppling modes and safety factor
SF of rock blocks of a given removable JP rem and of the
friction lfor limit equilibrium.
Sector JPrem. mode S.F. l[] KB Toppl.ST1 11000 A/E 3.18 8.3 N
ST2 01000 C/E 10.00 2.7 N
STE2 0001 B/C 0.55 40.1 Y
ST3 11100 E 1.28 20 N A=Sc, Y
SP 1110 D 1.00 25 Y
ST6 11001 C 0.69 34 Y
10001 B/C 0.75 31.7 Y
ST7 11001 C/D 0.66 35.1 Y E=Sc, Y
10001 B/D 0.99 25.1 Y
ST10 10011 A/B 0.77 31.2 Y C=Sc, Y
ST11 0111 A 0.27 60 Y D=Sc, Y
0011 A/B 0.58 38.9 Y
ST12 101 B 0.84 29 Y
ST13 111101 E 0.17 70 Y A=Sc, Y
110101 C/E 0.60 37.9 Y
ST14 101 A/B 2.94 9 NST15 10011 B/C 0.28 59.3 Y
10010 C/E 0.32 55.3 Y
10000 C/D 0.93 26.5 Y
ST17 110110 C/F 0.35 52.9 Y
110010 D/F 13.9 1.9 N
ST18 1010 B 0.66 35 Y
1010 B/D 0.70 33.5 Y
ST19 - - - - - -
ST20 1101 C 1.74 15 Y
ST21 0101 A 1.15 22 Y
In compliance with the classification evaluations, ahigh potential for rock slope instabilities was
confirmed, as approximately 90% of the analyses
carried out for the present road and over 65% ofthose made for the future portal zones show a
tendency towards sliding and/or toppling of the rock
blocks. However, when compared to the present
widespread dangerous road conditions, the rock
slopes that need to be excavated for the portals ofthe diversion tunnels have well-defined locations
and are of limited extent. This would allow thedesign of reinforcement, protection operations and
monitoring to be made that could prevent
unforeseable instabilities from ocuurring in thesemore easily controlled zones.
Table 7: Evaluation, at each tunnel portal, of the potentialsliding (one or two planes) or toppling modes and safety factor
SF of rock blocks of a given removable JP rem and of the
friction lfor limit equilibrium.
Portal JPrem. mode S.F. l[] KB Toppl.I1G1p-ST1 01001 A/D 1.11 22.8 N
I1G1p-ST2 01001 C/D 3.72 7.1 N
I1G1p-STE2 0001 B/C 0.55 40.1 Y
I1G1p-ST3 11100 E 1.28 20 N
I1G1s1-ST1 10000 B/E 8.84 3 N A. Y
I1G1s1-ST2 01100 E 1.35 19 N
I1G1s1-STE2 1011 A/B 5.06 5.3 N
I1G1s1-ST3 00011 A/B 1.09 23.1 N
I2G1p-ST7 10101 B/C 2.53 10.4 N A. Y
I2G1s1-ST7 10011 B 0.67 35 Y
00011 A/B 0.76 31.5 Y
10001 B/D 0.99 25.1 Y
I1G2p-ST10 10100 A/D 0.79 30.3 Y
I1G2s1-ST10 10011 A/B 0.77 31.2 Y C=Sc. Y
I2G2p-ST13 001110 F 0.81 30 Y C. Y
I2G2p-ST14 001 B 0.75 32 Y
I2G2s1-ST13 111101 E 0.17 70 Y A=Sc. Y
110101 C/E 0.60 37.9 Y
I2G2s1-ST14 101 A/B 2.94 9 N C=Sc. Y
I1G3p-ST15 10110 E 0.27 60 Y A=Sc. Y
10100 D 0.91 27 YI1G3s1-ST15 11011 C 0.22 65 Y
10011 B/C 0.28 59.3 Y
01011 A/C 0.64 35.8 Y
I2G3p-ST17 110011 D 0.50 4.3 Y
010111 A/C 0.59 38.3 Y
010011 A/D 9.23 2.9 N
I2G3s1-ST17 110110 C/F 0.35 52.9 Y
110010 D/F 13.99 1.9 N
I1G4p-ST21 0111 A/C 1.29 19.8 N
I1G4s1-ST21 0111 A/C 1.29 19.8 N
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9. CONCLUSIONS
A rock engineering assessment of the presentconditions of the rock exposures along a stretch of
the endangered 337-Val Vigezzo national road in
North-West Italy has been performed with thepurpose of identifying the potential instability
problems and evaluating the suitability of
permanent remediation by means of diversiontunnels. The assessment was carried out by means
of in situ geological-structural exploration,
laboratory mechanical characterization of the rock
and joints, rating of the rock slopes through rockmass classification systems and identification,
above all through the Block Theory, of the potential
instability modes of the rock blocks. The results thatwere made available from this qualification can be
summarized as follows.
The metamorphic rocks have a complex fracture
network of tectonic (joints and faults) orgravitational (extension joints) origin that, apart
from large variabilities of fracture attitudes, can be
described by three main joint sets.
The mesostructural analysis made at the studystations located in the different road sectors allows
the rock mass quality to be evaluated according to
classification systems and the stability conditions ofthe rock slopes that flank the road to be rated.
The results of mechanical laboratory testing of the
metamorphic rocks and joints, along with theindexes based on the rock mass structure and joint
condition, allow a reasonable appraisal of the
strength and deformation parameters of the rockmass and of the distinct joint features to be made,
which could be used for the design of remediation
operations.
Most of the road sectors show evidence of previousinstability and the potential for further evolutions.
The instability phenomena can be classified as a
tendency to planar or wedge failure of the rock
blocks resting on low angle or, sometimes, medium-high angle joints striking subparallel to the slope, or
to the toppling of slabs or columns prevailingly
made up of schistosity joints. The SMR and themodified SMR consistently defined a poor rock
mass quality and classified 12 out of 18 road sectors
unstable. This unstable tendency evolves to fullyunstable in 5 sectors, that are prevailingly located
between the Olgia gallery and the border station at
Ponte Ribellasca.
The analysis of the potential of rock block failure insliding modes, made using the Block Theory, and of
toppling, systematically confirmed the unstable
trend that was obtained through the classifications.
The possible danger of an unstable evolution of therock-face side of the road was confirmed by the
traces of previous instability phenomena and an
active deformation trend underlined by the evidenceof open or dislocated joints.
Huge remediation measures would need to be
systematically provided along the road (e.g.
anchors, reinforced concrete toe walls, deepdrainage) to improve the stability of the rock faces.
However, external reinforcement or reexcavation
operations would not be possible in zones where the
rock slopes are exceeding high or unaccessible.
The difficulties of a precise localization, design and
setting up of effective remediation measures and the
related costs would suggest a variation of thepresent road layout by means of 4 diversion tunnelsat least in the sectors classified as being fully
unstable. This choice should restrict the important
protection and stabilization operations on the rockslopes to the tunnel portal zones.
From the more general point of view of land
protection, it is important to consider the diffusionand variety of landlside processes that have
occurred in the upper Vigezzo valley. The landslide
movements due to valley erosion are prevailinglyslow, but, however, with the possible primer of
sudden rock mass failure (rock fall, translational or
complex movement), above all when extremeevents, like flooding, occur. Evidence also exists of
deep-seated gravitational movements (e.g. natural
trenches, ridge splitting, counterdipping slopes, orslope toe heaving) that could extend to a large part
of the mountain side. These processes, due to the
sizes involved and potential implications, should be
carefully considered when planning
countermeasures for the safety and protection of themountain side.
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