arma-05-739_geostructural and geomechanical characterization of rock exposures for an endangered...

8/12/2019 ARMA-05-739_Geostructural and Geomechanical Characterization of Rock Exposures for an Endangered Alpine Ro

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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-St14 IV 24/ IV 24 U./U. S4b/S4b

I2G2s1-St13 III 47/ III 47 P.S./P.S. S3b/S3b


I1G3p-ST15 IV 28/ IV 28 U./U. S4b/S4b

I1G3s1-ST15 IV 27/ V 20 U./F.U. S4b/S5a


I2G3s1-St17 III 54/ III 56 P.S./P.S. S3a/S3a

I1G4p-ST21 III 43/ III 43 P.S./P.S. S3b/S3b


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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