Fragmentation during Rock Falls: Two Italian Case Studies of Hard and Soft Rocks

ORI GIN AL PA PER

Fragmentation during Rock Falls: Two Italian CaseStudies of Hard and Soft Rocks

Nicola Nocilla Æ Aldo Evangelista ÆAnna Scotto di Santolo

Received: 28 September 2007 / Accepted: 3 March 2008 / Published online: 13 June 2008

� Springer-Verlag 2008

Abstract In recent years, rock fall phenomena in Italy have received considerable

attention for risk mitigation through in situ observations and experimental data. This

paper reports the study conducted at Camaldoli Hill, in the urban area of Naples,

and at Monte Pellegrino, Palermo, Italy. The rocks involved are volcanic Neapolitan

yellow tuff (NYT) in the former area and dolomitic limestone in the latter. Both

rocks, even though with different strength characteristics, have shown a significant

tendency towards rock fragmentation during run out. This behavior was first

investigated by comparing the volumes of removable blocks on the cliff faces (V0)

and fallen blocks on the slopes (Vf). It was assumed that the ratio Vf/V0 decreases

with the distance (xf) from the detachment area by an empirical law, which depends

on a coefficient a, correlated with the geotechnical properties of the materials

involved in the rock fall. Finally, this law was validated by observation of well-

documented natural rock falls (Palermo) and by in situ full-scale tests (Naples).

From the engineering perspective, consideration of fragmentation processes in rock

fall modeling provides a means for designing low-cost mitigation measures.

Keywords Rock falls � Hard and soft rocks � Fragmentation � In situ full-scale tests

N. Nocilla

Dipartimento di Ingegneria Strutturale e Geotecnica, Universita degli Studi di Palermo,

Viale delle Scienze, 90128 Palermo, Italy

A. Evangelista � A. Scotto di Santolo (&)

Dipartimento di Ingegneria Geotecnica, Universita degli Studi di Napoli Federico II,

Via Claudio 21, 80125 Naples, Italy

e-mail: [email protected]

A. Evangelista

e-mail: [email protected]

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Rock Mech Rock Eng (2009) 42:815–833

DOI 10.1007/s00603-008-0006-0

1 Introduction

In the last few years, systematic investigations of the downhill propagation of single

large blocks or rock falls have been carried out in two locations in Italy, along the

Camaldoli Hill (Soccavo site) in Naples, and on Monte Pellegrino (Acquasanta site)

in Palermo (Fig. 1). Rock falls involve Neapolitan yellow tuff (NYT) in the former

area, and limestone and calcareous dolomite in the latter area. These rocks have

very different fabric and mechanical behaviors (Evangelista et al. 2000).

Survey techniques and analyses based on terrestrial photogrammetry (Nocilla and

Urciuoli 1997; Nocilla et al. 1999), together with the morphology and structural

characteristics of the tuff and dolomitic limestone, were used to define removable

blocks1 in the detachment area (Evangelista et al. 2002, 2004). Measurements were

taken of the characteristics of the rock falls, and of the size, shape, and location of

the fallen blocks along the slope. At Camaldoli Hill, in situ full-scale tests were

carried out (Nocilla et al. 2003). Detailed measurements were taken of the traces of

a significant rock fall that took place on Monte Pellegrino in 1998.

Fig. 1 Location maps of the study areas

1 It is noted that a ‘‘removable block’’ identifies a standalone block on a slope and a block which has

already been dislodged and detached from the cliff face by movements of the order of as a little as 1 dm.

816 N. Nocilla et al.

123

The data show a reduction in block volume during run out. A similar empirical

relationship of volume reduction versus travel distance was proposed for the two

locations. Firstly, this relationship was evaluated by comparing the population

volumes of fallen blocks along the slope with the population of removable blocks on

the rock face. This relationship was then validated by using the results of in situ full-

scale rock fall tests at Camaldoli Hill and the traces of the landslide that occurred at

Monte Pellegrino.

This paper focuses on the phenomenon of rock fragmentation during rock falls by

means of observations and in situ full-scale tests carried out in two Italian sites with

different geological and geotechnical features. The study points out the relevance of

this phenomenon and its impact on the engineering design.

2 Geomorphological Characteristics of the Rock Fall Areas

2.1 Camaldoli Hill, Naples

The Camaldoli Hill (457.5 m a.s.l.) is the highest peak of the Phlegrean Fields and

originates from the collapse of the Ignimbrite caldera (39 ka) (Fig. 1). It is

composed of a succession of different pyroclastic strata deposited during eruptive

phases. Rock falls start at the cliffs of fractured NYT that are present at the peak and

at lower elevations along the slope. There are two subvertical strips of NYT hill

faces which vary in height between 25 and 50 m, from which blocks tend to detach

and propagate along the slope until they reach the urban areas at the toe of the hill.

The slope below, covered by debris, is steep down to an elevation of about

200 m, after which it becomes gentler and evolves into a succession of small

terraces used for agriculture and flower cultivation. The basal plain, where the

inhabited area is located, is flat and densely populated (districts of Soccavo and

Pianura). The wooded areas that once existed here have almost entirely been wiped

out by fires.

As already noted, NYT is present throughout the city of Naples; it is a soft rock,

with varying structure and mechanical characteristics, which consists of a cineritic

matrix with pumaceous and/or scoriaceous and lithic inclusions (Evangelista et al.

2000). The quantity and quality of the inclusions depend on the specific type of tuff

and, even within the same formation, on distance from the volcanic source. The

minerals in the cineritic matrix play an important role in the properties of the rock.

The most common minerals are phillipsite, cabasite, and sanidine.

The mesostructure of the tufaceous mass is characterized by a system of nearly

vertical discontinuities (set 1) that are normal (set 2 and 4) to the slope surface

(Fig. 2). These discontinuities delimit removable rock blocks with parallelepiped or

slab-like shapes. The base of such blocks consists of subhorizontal discontinuities,

dipping between 5� and 38�, which lead to planar sliding. Block size is determined

by the spacing, which ranges from a few decimeters up to 3–5 m in the subvertically

oriented discontinuities.

Besides the blocks that are removable from the cliff face and that are the result of

the unfavorable positioning of the discontinuities, other phenomena associated with

Fragmentation during Rock Falls: Two Italian Case Studies of Hard and Soft Rocks 817

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instability are caused by particularly strong wind erosion at the toe of the cliff or

along very weak rock strata of the hill.

A total of 318 removable blocks, with volumes exceeding 0.5 m3, as shown in

Fig. 3, have been mapped on the cliff. There is a significant number of blocks, with

volumes ranging from 1 to 10 m3, which are generally cubic in shape. Blocks with

volumes exceeding 10 m3 are parallelepiped in shape, with heights greater than

their length or thickness. The geometric features are such that these blocks will

either slide or topple over.

Fig. 2 Pole density contour plot for Camaldoli Hill (Naples)

Fig. 3 Projection onto a vertical plane sub-parallel to the cliff face of removable blocks located in thecliff face below the Eremo dei Camaldoli


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2.2 Monte Pellegrino, Palermo

Monte Pellegrino, which consists of limestone and dolomitic rock, of the Upper

Triassic to the Lower Miocene, is bounded mainly by subvertical or sheer slopes

(Fig. 1). It has a highly complex structure, characterized by folds and faults, reverse

geological sequences, and a dense system of faults. At the toe of the mountain, a

large debris layer has accumulated over coastal quaternary deposits, forming a

35–40� slope near the cliff faces.

On the southern side of the mountain, which overlooks the Acquasanta

neighborhood, Via Bonanno is cut into the debris layer, whose characteristics vary

from area to area. Successful reforestation projects were started in the 1950s here

where the ground consists mainly of gravel and reddish, lime-matrix blocks with the

occasional block exceeding 2 m in diameter. The gravel is generally clumped

together and may even be cemented to varying degrees.

The hill face has an average dip direction of about 85�. There are five

predominant families of discontinuities which can be correlated with tectonic events

represented by N–S, ENE–WSW, and NW–SE oriented fault lines (Fig. 4). Three of

these families are characterized by dips greater than 5�, while the discontinuities of

another two families vary in dip between 30� and 50�.

The rock mass is subdivided into blocks of various shapes and sizes (also

removable) which, in the cortical area of the larger mass, are frequently dislodged

due to displacements of the order of centimetres or even decimetres, or as a result of

rotation around the lower edge.

A total of 554 removable blocks with volumes greater than 0.5 m3 were counted

by terrestrial photogrammetry surveys in the cliff faces above Via Bonanno, the

Fig. 4 Pole density contour plot for Monte Pellegrino (Palermo)


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road providing access to Monte Pellegrino (Nocilla et al. 1999). Figure 5 shows the

blocks identified in the face below the Grattarola Peak.

2.3 Consideration of the Geotechnical Properties of the Two Rocks

The mechanical properties of the NYT and the Palermo limestone have been the subject

of numerous studies carried out at the Geotechnical Engineering Department of the

‘‘Federico II’’ University of Naples, and at the Department of Structural and

Geotechnical Engineering of the University of Palermo. These studies show that the

former are low to very low strength rocks, while the latter are medium to medium-high

strength rocks. The two rock materials, classified by uniaxial compression strength

(rc B 10 MPa for the NYT and rc & 100 MPa for the limestone) and the elastic

modulus (E B 10,000 MPa for NYT and E & 100,000 MPa for limestone), fall within

the medium-to-high modulus ratios according to Deere and Miller (1966), Fig. 6.

3 Rock Fall Phenomena

As described above, the tests and observations carried out show a strong tendency

for the rock blocks to break during run out. The reduction in volume which takes

place is shown by comparing the removable blocks that have already broken away

from the mountain face with those present both along the slopes of Camaldoli Hill

and Monte Pellegrino. A comparison of the volumes of removable and fallen blocks

is given below.

3.1 Rock Fall Propagation at the Soccavo Site (Camaldoli Hill, Naples)

Topographic survey techniques and terrestrial photogrammetry were used to detect

and catalogue 120 fallen blocks along the slope (Fig. 7), with volumes equal to or

exceeding 1 dm3; only two blocks had a volume greater than 30 m3.

Fig. 5 Removable blocks identified through terrestrial photogrammetry and shown in projection onto avertical plane sub-parallel to the mountain face below Grattarola peak


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About 30% of the blocks (with volumes ranging from 0.1 to 7 m3) came to rest

on a 40� slope between 300 and 390 m a.s.l., close to the hill face (within 35 m).

About 70% reached elevations below 250 m a.s.l. These blocks can be divided into

the following groups:

• 21% (volumes between 0.2 and 0.8 m3) came to rest between 200 and 240 m

elevation a.s.l., where the slope is about 35�• 26% (maximum volume about 7 m3) came to rest between 160 and 200 m

elevation a.s.l.

• 23% (the remainder, with volumes between 1 dm3 and about 9 m3) remained in

motion until they reached an elevation of about 120 m, propagating to a

maximum distance from the hill face of about 250 m.

The points of rest of the latter group are located in agricultural zones with thickly

wooded areas where the slope has an average dip of 10–15�, with several plain-like

terraces.

Histograms of the removable blocks on the hill face (295 of the 318 identified)

and of the fallen blocks (120) are shown in Fig. 8a, b. The blocks along the slope

tend to be smaller: only about 34% of the entire population had volumes greater

than 2.5 m3. On the hill face, however, removable blocks predominate with

volumes above 2.5 m3, corresponding to about 70% of the sample. Cumulative

frequency diagrams, shown in Fig. 9a, highlight the different volume distribution

on the hill face and along the slope, with smaller volumes being predominant in

the latter.

Fig. 6 Mechanical properties ofthe NYT and of the Palermolimestone


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3.2 Rock Fall Propagation at the Via Bonanno Site (Monte Pellegrino, Palermo)

The points of rest of the fallen blocks not connected to the slope were detected

topographically (Fig. 10). Two main zones were identified:

• Zone 1, below the Volo dell’Aquila Peak and near the S. Maria dei Rotoli

cemetery, with 68 blocks

• Zone 2, below the Grattarola Peak, which embraces the area of the October 1998

landslide, and includes 205 blocks.

Both areas, at the toe of hill, with elevations of 170–190 m and 90–120 m a.s.l.,

respectively, have the following land features:

• slope between 35� and 40� for a distance of about 60 m from the hill face

• slope of about 31�–32� up to 140 m from the hill face

• slope of about 10�–15� beyond (in the vicinity of the inhabited areas).

In Zone 1, below the Volo dell’Aquila Peak, where the rock cliff is 170–190 m in

height, the blocks were distributed along the slope in the following way:

• a few blocks with volumes just below or greater than 1 m3 close to the hill face

• more than 60% of the fallen blocks had rolled to where the hill slope drops to

10–15�• only a few blocks were present in the larger part of the slope, i.e., between the

hill face and its lower section.

Fig. 7 Plan view of the Camaldoli Hill, Soccavo side, with the positions of fallen blocks along the slopeand the location of the field test site


123

In zone 2, below the Grattarola Peak, where the cliffs reach heights of 90–120 m,

the blocks were distributed almost uniformly along the slope for a distance of 120 m

from the face, with only a few blocks reaching greater distances.

Figure 11a shows the volume histograms for 548 out of the 554 removable

blocks identified on the hill face, while Fig. 11b shows the histogram of the 302

fallen blocks. Figure 11a does not include six boulders with volumes greater than

50 m3, four of which had volumes of about 110, 137, 183, and 300 m3 . It may be

observed that blocks with volumes less than 2.5 m3 predominate on the hill face

(57%) and along the slope (78%). Also at this site, the blocks remaining on the hill

face outnumber those along the slope (fallen blocks).

Fig. 8 Volume histograms of removable blocks on the Camaldoli Hill face, Soccavo side (a) and ofblocks fallen on the slope (b)


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By comparing Figs. 8, 9, and 11 the following observations can be made:

• The population of removable rocks on the hill face is always more numerous

than the population of fallen rock; the difference between the two is larger for

the Camaldoli site.

• The number of fallen blocks with volumes greater than 5 m3 is larger at the

Monte Pellegrino site.

Overall, block fragmentation at the Camaldoli site is greater than that on Monte

Pellegrino, which is consistent with the mechanical properties of the rock materials

involved.

4 Volume Reduction Analysis

The volume distribution of the removable blocks depends essentially on the

mesostructure of the rock mass, a characteristic that can be considered as having

remained virtually unchanged in recent millennia. For this reason, the present

population of rock blocks on the hill face may be assumed to be the same as the

population of fallen blocks on the slope. As already noted, considering the different

Fig. 9 Cumulative frequency diagrams of fallen blocks on the slope and removable blocks on the hillface: a Camaldoli Hill; b Monte Pellegrino


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volume composition on the hill face and along the slopes, it follows that the sizes of

removable blocks on the hill face are larger than those estimated at the point of rest.

It may be suggested that the volume reduction is due to impact processes and to

erosion during run out (rolling and/or sliding phases).

The result of these phenomena intensifies with increasing distance between hill

face and point of rest. It can be assumed that the volume (Vf) at the point of rest

depends on the initial volume (V0) on the hill face, and that the Vf/V0 ratio is an

inverse function of the distance from the hill face (xf). The following empirical

reduction law can be suggested:

Vf ¼V0

ð1þ xfaÞ; ð1Þ

where Vf is the volume at a certain point of rest at a distance xf from the hill face,

V0 is the initial volume on the hill face, xf is the distance from the hill face to the

point of rest, and a is a reduction coefficient [L-1] correlated to the mechanical

properties of the materials involved in the rock fall.

Fragmentation intensity increases as a increases. The first approach for

evaluating the a coefficient was obtained as follows:

• Different values were assumed for the coefficient a, and hypothetical initial

volumes V0 were evaluated starting from Vf and xf.

Fig. 10 Plan view of Monte Pellegrino, Acquasanta side, with locations of fallen blocks on the slope anddelineation of the 1998 rock fall zone


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• The cumulative frequency of these volumes was then calculated and compared

to the removable blocks on the hill face to find the best fit.

In Fig. 9 (a cumulative frequency diagram of the fallen blocks along the slope),

in situ removable blocks and those calculated by means of (1) are shown for the two

sites. The calculated V0 values reach a best fit with in situ values for a coefficient aequal to 0.015 m-1 for the calcareous dolomite, and a = 0.02 m-1 for the NYT.

Equation 1 was then validated by observation of well-documented natural rock

falls at Monte Pellegrino (Palermo) and by the results of in situ full-scale tests

carried out along the Camaldoli slope (Naples).

Fig. 11 Volume histograms of removable blocks identified on the hill face of Monte Pellegrino, aboveVia Bonanno and the Acquasanta neighborhood (a), and of fallen blocks present along the slope (b)


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5 Full-Scale Testing at Camaldoli Hill (Naples)

The test site was set up along the eastern border of Camaldoli hill (Fig. 6). Distance-

measuring posts and four video cameras synchronized with a longitudinal time code

(LTC) system were placed on the slope. Three of these cameras were positioned

according to the maximum dip of the slope for lateral viewing. The fourth camera

was located further down in order to provide a frontal view of the slope.

Along the slope, which dips 40� at the test site, a pyroclastic debris layer is

present, as well as thick and uniform vegetation reaching a height of about

80–100 cm. There is a copse of beech trees at about 300 m from the hill face.

When blocks were already detached from the hill face, the dislodgement process

was carried out manually by using a lever wedged into a discontinuity (removable

block) or by isolating rock portions with a precut process carried out by placing

expansive mortar in neighboring vertical fissures. The initial volume of the blocks

ranged from 1 to 12 m3. The trajectories down the slope, recorded by the cameras,

were analyzed to determine block velocities, heights of bounce, and run-out

distances. For each major impact of the block, the translational and rotational

velocities before and after the impact, and consequently the restitution coefficients,

were evaluated.

Generally an ‘‘explosion’’ accompanied the initial impact of each block, which

was characterized by a rapidly expanding dust cloud from which blocks with fan-

like trajectories initiated after a few tenths of a second. The fragments traveled

distances less than those predicted by the numerical analyses (Richards 1988;

Labiouse 2004; Pfeiffer and Bowen 1989); almost all of the rock fragments were

stopped by the beech copse (Nocilla et al. 2003).

Video recordings of the rock falls showed that the impact down the slope gave

rise to the subdivision of each block into fragments and the formation of fan-like

trajectories. The fragments fan out with a width of about 30–35% of the travel

distance. This width is 2–20% greater than the values reported in the literature

(Azzoni and De Freitas 1995).

Imaging techniques made it possible to determine the approximate size of the

blocks prior to and immediately after the impact. For the dislodged blocks, Fig. 12

shows the ratio of the largest volume fragment (V) to the initial volume (V0) versus

the travel distance (x) from the starting point. The same figure highlights the rapid

volume reduction, equal to about 80% of the initial volume, during the first 50 m

travelled. The data were interpolated by using the following relationship:

V

V0

¼ 1

1þ xað Þ ; ð2Þ

which is the same as in Eq. 12, assuming different a values.

The values of a provide a satisfactory interpolation with the experimental data,

ranging from 0.02 to 0.06 m-1 with a mean value of 0.04 m-1. There was only one

2 Equation 1 can be used to determine the initial theoretical volume starting from the final volume of the

blocks along the slope. From Eq. 2 the block progressive volume reduction under propagation can be

derived.


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test, trial number 16, in which the reduction coefficient a was about 0.008 m-1. For

this test, the final volume Vf was found to be 40% of the initial volume V0, which

was the smallest volume reduction process in all the tests performed along the

Camaldoli Hill.

The landslides initiated in the experimental field were simulated with a three-

dimensional numerical code (Scioldo 2000). The numerical analysis conducted,

which simulated the beech copse as damping elements (E = 200 kJ), showed that

masses with constant volume equal to the initial value of V0 would have continued

their path and reached the rockfall protection barrier at the bottom of the hill. At any

rate, the insertion of dissipative elements proves incapable of impeding the masses

when considering the initial volume in the simulations. Good agreement between

the observed and calculated arrest points is obtained only by assuming partially

reduced mass volumes according to Eq. 2.

6 The Monte Pellegrino Landslide (Palermo)

The significant landslide that took place at Monte Pellegrino in October 1998 was

also analyzed. A boulder of 64 m3 broke away from the mountain side at an

elevation of 228 m a.s.l. (Fig. 5). The outline and initial volume V0 of the in-face

blocks were estimated using photographic measurements taken in 1997. The boulder

had already broken down into numerous blocks within the face, the largest of which

had a volume of 38 m3 (Fig. 13).

During the slide, the rock broke into 43 fragments with volumes ranging from 0.2

to 23 m3 (Fig. 14), and numerous smaller fragments with volumes less than 0.2 m3.

Most of the fragments (around 60%) came to rest close to the hill face at a distance

of about 40 m; 24% came to rest near Via Bonanno at about 120 m from the hill

face; nearly 8% reached the slope between two of the road hairpin curves. One

fragment (number 41), with a final volume Vf of 23 m3, and thus a subfragment of

Fig. 12 Volume reduction versus travel distance


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the highest initial volume (V0 = 38 m3), reached the edge of an old abandoned

quarry at a distance of about 200 m from the toe of the hill face.

This block followed a straight trajectory, shearing tree trunks and branches in its

path, and left a well-defined circular wake, 1–2 dm deep, and a slight bulge at its

perimeter. Figure 13 shows the trajectory which was inferred from the outline of the

impact craters, from the heights of broken tree trunks or snapped/scraped branches

sheared by the flying boulder, from trees contiguous to the trajectory, and from

traces of sliding in the area immediately before the final point of rest.

The boulder mostly flew through the air, bouncing and hitting the slope. The

separation of fragments from the initial rock volume (V0 = 38 m3) probably

occurred near the site of first impact at the toe of the hill face, which is covered by a

debris layer consisting primarily of rock blocks. Numerous rock fragments with

signs of recent fractures were identified within a 25–30 m radius.

The available data include the initial volume V0 of the blocks involved in the

slide as well as the total volume V0tot, the initial elevation, the point of rest of the

main fragments, and their corresponding volumes (see the table in Fig. 14).

Fig. 13 Monte Pellegrino: trajectory of the V0 = 38 m3 fragment from the 1998 rock fall event


123

Starting from fragment number 41, with a volume of 23 m3, an estimate was

made of the different volumes of the block during its trajectory by assuming:

Vi ¼ Vi�1 � ti; ð3Þ

where ti is the volume of the fragment found along the trajectory, Vi-1 is the

volume of the block before impact i, and Vi is the residual volume after the impact.

Figure 15 shows the Vi values normalized with respect to V0tot = 64 m3, versus

the distance of the point of rest from the hill face. The data may be interpolated

Fig. 14 Monte Pellegrino: location of the fragment points of rest from the 1998 rock fall event


123

using Eq. 1 by assuming a = 0.01 m-1. The volume of the same fragment number

41 has been normalized with respect to its probable initial volume V0 = 38 m3.

Equation 1 yields an a value equal to 0.0035 m-1.

In the first case (V0tot = 64 m3), a volume reduction process occurs which is

slightly smaller than expected by comparing the rock populations on the slope with

those on the hill face (a = 0.015 m-1, Fig. 8b). The application of the value

a = 0.0035 m-1 to volumes Vf of blocks along the slope obtained in the second

case (V0 = 38 m3) would yield a theoretical population of removable blocks on the

hill face which is only significant for the larger volumes, for example, those greater

than 10 m3.

7 Conclusions

The analysis of the populations of blocks on Camaldoli Hill in Naples, and on

Monte Pellegrino in Palermo shows a progressive reduction in size during rock fall,

from detachment to the final point of rest. The phenomenon appears to be well

described by the reduction law given in Eq. 1. The final volume is found to depend

on travel distance and on the coefficient a, which can be correlated with the

mechanical characteristics of the rock mass and of the hill slopes.

Full-scale rock fall tests carried out on the NYT (rc B 10 MPa) at Camaldoli

Hill, and at a landslide which occurred in 1998 in the Monte Pellegrino limestone

(rc * 100 MPa), were analyzed. In both cases the rock systematically shows a high

level of boulder fragmentation right from the initial impacts with the hill face and

talus debris layers, in addition to a fan-like trajectory of particle dispersion along the

slope. The fragments, whose volumes become smaller as travel distance increases

and which reach sizes that are only 10–20% of the initial volume in the case of

NYT, are generally distributed over a broad front.

Fig. 15 Variations in the volume of the cluster of blocks versus travel distance during the 1998rockslide. The curve where a = 0.0035 m–1 refers to the initial volume V0 = 38 m3. The curve wherea = 0.01 m-1 refers to the total fallen volume V0

tot = 64 m3


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The data from the Camaldoli test site (NYT) are in agreement with Eq. 1, but

highlight a non-negligible variation in the parameter a, between 0.06 and 0.008 m-1.

The back-analysis of the Monte Pellegrino landslide (limestone) confirms the

validity of Eq. 1 and indicates that the fragmentation processes, with values of aranging from 0.01 to 0.0035 (depending on the assumed initial volume), are generally

much less intense than those recorded for the NYT.

The experimental observations lead to the conclusion that the forecast of rock fall

phenomena can be described more realistically by considering the reduction in

volume of single blocks versus their travel distance. It is suggested that the

reduction law to be adopted, when field tests are not feasible, may be that obtained

by comparing the volumes of the fallen blocks present along the slopes with those

detached but still lying on the hill face. The range of a values to be considered is

given in Fig. 16, where higher-intensity fragmentation processes were not taken into

account (a[ 0.02 for NYT and a[ 0.01 for limestone).

It is noted that passive protection barriers may be designed more rationally and

economically by considering the fragmentation of the blocks that occurs during

impacting and subsequent sliding and rolling. In other words, based on our

experience, in design procedures, smaller volumes can be considered than the

volumes of removable blocks lying on the hill face. Given our current level of

knowledge in this respect, the choice of the a reduction coefficient to be used needs

to be based on field tests and cautious evaluations.

Acknowledgment This research was partly funded by MURST (Ministry for Universities and

Research), Italy, within PRIN 2005 2005085322_004 (National Research Project, 2005).

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Fig. 16 Ranges for the reduction coefficient a for the NYT from Camaldoli hill and for the calcareousdolomite from Monte Pellegrino


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