Morphostructural evolution and related kinematics of rockfalls in Campania (southern Italy): A case study

ENGINEERING GEOLOGY ELSEVIER

Engineering Geology 36 (1994) 197-210

Morphostructural evolution and related kinematics of rockfalls in Campania (southern Italy): A case study Paolo Budetta and Antonio Santo Department of Engineering, Applied Geology Institute, Naples University "'FedericoH", 80125 Naples, Italy (Received May 27, 1993; revised version accepted October 21, 1993)

Abstract

On the southern slope of the Sorrentine peninsula there are many subtriangular depressions in which the main towns of the area are located; the depressions are linked to block-rotation during strike-slip fault activity and are often marked by high subvertical cliffs and related rockfall phenomena with consequent damage to the underlying towns. On the basis of back analyses carried out using Hoek's method, an in-depth study of some of the rockfalls enabled us to define for one of these towns the most suitable shock restitution coefficients of the detrltical pyroclastic deposits present on the terraces situated at the foot of the cliffs. On the basis of the main kinematic parameters of actual rockfalls and approximately 6600 simulated rockfalls, a planning of the high-level rockfall risk valley bottoms was drawn up. The size of the high-level rockfall risk sites depends above all on the presence, number and size of the terracings where citrus fruit trees are cultivated. The calculation method suggested by Paronuzzi was also adopted in order to make a detailed evaluation of the role the terraces play in modifying the trajectory height, the translation velocity and the endpoint of the boulders. Although there are methodological and statistical differences between the two methods, results from both studies showed sufficient correlation, in particular concerning the trajectories' endpoints.

1. Introduction

The town of Atrani* (Fig. 1) is situated near the m o u t h of the narrow valley b o t t o m of the T. Dragone gorge, and is dominated on both the east and west by steep rock cliffs from which boulders, at times huge ones, often fall. Historical chronicles dating back to the 15th century (in particular 1401, 1588, 1764 and 1780; see Camera, 1881) include references to rockfall *According to historical sources, the name "Atrani" derives from the Latin "Atrum" (neuter noun of the adjective "Ater"), meaning a dark, sombre place. oo13-7952/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0013-7952(93)E0064-T

phenomena and the consequent damage to the town. The damage was sometimes particularly serious and generally occurred during earthquakes or heavy rainfalls. More recently, at the beginning o f this century, Atrani was placed on the list of towns declared unstable and was entitled to reinforcing work to be carried out, as provided for by law no. 445/1908 (Ministero LL. PP., 1963). In the light of this law, various incomplete reinforcements were carried out on more than one occasion, but, more often than not, they were not based on proper design criteria. As the rockfalls persisted, particularly after the

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197-210

198

o

,~o

I

300 m I

Fig. 1. Locationof the studyarea, earthquake in the Campania region on 23 November 1980, and more recently in 1986 and 1991, it was considered important to thoroughly study the geostructural features of the rock faces and the rockfall kinematics (especially for several phenomena which were well reconstructed in detail), with the aim of locating not just the rock failures and the trajectories traced by the boulders along the slopes, but also the high-level rockfall risk to valley bottoms. This study enabled the morphostructural features of the Atrani area to be defined and, by extension, further contributed to the knowledge of the southern side of the Sorrentine peninsula's geostructural evolution. This study is part of an on-going research project carried out over the last few years by the Applied Geology Institute of the Faculty of Engineering at Naples University, and is working towards a definition of the landslide risk in the rock slopes of the Sorrentine Peninsula (Budetta and De Riso, 1988; Budetta et al., 1991; Budetta and Calcaterra, 1991 ).

2. Morphostructural outline The area studied lies on the southern edge of the Sorrentine Peninsula which is a horst transver-

sally oriented towards the Apennine chain, separating two tectonic depressions, i.e., the Campania Plain to the north and the Gulf of Salerno to the south. The structure is characterized by a monoclinal inclined towards the NW which on the southern side is faulted by major NE-SW-oriented faults and is affected by numerous transverse faults (NW-SE-oriented), many of which are strike-slip faults (Fig. 2). The Sorrentine peninsula is made up of thick Mesozoic dolomitic limestone sequences on which Miocene flysch has been conserved in small structural depressions; lesser quantities of Quaternary clastic deposits together with pyroclastics have also been conserved. The present-day orographic setting of the Peninsula is the result of a series of phases of tectonic uplift and phases of erosion (Cinque, 1986). A detailed examination reveals that the first uplift occurred between the end of the Miocene and the beginning of the Pliocene, followed by a long erosive phase leading to the formation of a large paleosurface which was successively dissected into numerous horst and graben structures during a new tectonic phase of the Lower Pleistocene; remnants can be seen in isolated relic slabs on the peaks of the Peninsula (Mount Erasmo, Mount Cerasuolo and Mount Comune) and in some depressions such as the plains of Agerola or Ravello. In the Middle Pleistocene a new tectonic uplift caused the rejuvenation of morphologic features and resulted in the present height of the relief. The combined action of linear erosion and karst erosion which was most active during the last Glacial Wt~rm (when the sea level was approximately 130 m lower than at present) markedly modelled the relief. Recently, new views on the southern Apennine chain and a number of geostructural papers (Patacca and Scandone, 1991; Capotorti and Tozzi, 1991) have highlighted the presence of strike-slip faults in this area which are probably linked to the last compressive phases of the chain. Our re-interpretation of the structural data collected by aerial photogrammetries in Civita et al. (1975) and Budetta et al. (1991), and new geological data acquired during this study have enabled

P. Budetta, A. Santo/Engineering Geology36 (1994.) 197-210

authors' opinion be attributed to block rotation movements brought about by strike-slip faults, as already observed in other geological contexts by Nur et al. (1988). Thus, for the area of investigation, the angle at the top of the coastal subtriangular depression (approximately 30°), in which Atrani is situated, was obtained on the basis of the dip direction measurements of the joints present on both sides of the gorge and enabled a synthetic reconstruction

us to highlight the presence of numerous strikeslip faults in relation to which coastal subtriangular depressions are often noted exclusively on the southern slope of the Peninsula; these are marked by secondary tectonic lines along which nearvertical cliffs are often found (Fig. 2). These triangular depressions generally have vertical angles varying between 20 ° and 45 ° and contain the main coastal towns situated between Vietri sul Mare to the NE and Nerano to the SW. They can to the

N~'~NaPleIs~

1 \

199

~ ~,~ra z" "

i2

Nerano

"

J,/

31

'i

Fig. 2. Structural sketch of the Sorrentine Peninsu|a, I: Main morphostructural features; 2: main strike-slip faults; 3 coastal subtriangular depressions and their vertical angles.

I

.~s.l. A

B

t

-

NE

C

Fig. 3. Block diagram referring to the morphostructural evolution of the Atrani area during the Plio-Pleistocene. A. NW-SE oriented faults (a) and strike-slip faults (b) with their relative cataclastic zones; initial sketch of the subtriangular depression linked to block rotation. B. Uplift of the area; formation of the Dragone Paleo stream and, consequently, the beginning of an active phase of karst erosion. C. Present-day situation with U-shaped slopes imposed on the cataclastic zones and development of karst interbedding caves.

200

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197-210

of the main morphostructural phases which, in our opinion, have been affecting the area since the Pleistocene (Fig. 3).

3. Geostructural framework and slope failure patterns The town (Figs. 4 and 5) is dominated to the east and west by two near-vertical and/or overhanging rock cliffs composed of Liassic dolomitic limestones and calcirudites, sometimes with centimetric marly-clayey interbeddings (Iannace, 1991 ). The east front is approximately 320 m long and 100 m high and presents a regular progression with a bearing of strike towards the N W - S E and basically coincides with a fault plane whose dip direction is 210°-220 °. The west front is oriented with a bearing of

strike of N 20°W and a dip direction of approximately 70 °. It too coincides with a fault plane. However, the rock face is much more articulated due to the presence of frequent subhorizontal surfaces and secondary transversal cliffs between 20 and 50 m height which interrupt the vertical and horizontal continuity. The area linking the cliff base and the valley bottom has, especially to the west, many very steep slopes (mean angle of dip around 45°). The slopes are made up of pyroclastic talus deposits; often, these have been terraced using traditional dry gravity walls to enable the local citrus fruit trees to be cultivated (Figs. 6 and 7). The geological survey of the rock masses investigated on both cliffs was carried out on the basis of 470 different data on joint orientations gathered from several sites (Fig. 4). The joint orientations of each cliff, according to

Fig. 4. Morphostructural sketch of the study area. 1: Main tectonic lines; 2. valley morphologies imposed on the cataclastic zones; 3: interbedding caves; 4: landslides; 5: measurement stations; 6: bedding plane orientations; 7: A-B is position of geological crosssection of Fig. 7.

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197-210

201

Fig. 5. Detail of the east cliff.

Muller, were represented on stereographic nets (equatorial stereographic nets, see Figs. 8a and b) by identifying the main sets which were later represented on equatorial stereographic nets (lower hemisphere, see Figs. 9a and b). The joints examined belonged, according to typology, to bedding planes, faults (at times with obvious strike-slip lines) and joints most of which can be attributed to sets and a smaller number which was distributed randomly. Comparing Figs. 8 and 9, the following conclusions can be drawn: (a) The bedding plane (S) is slightly inclined and the stable direction of dip is towards the SE. It demonstrates a relative rotation of approximately 30-35 °, as is clear from the dip direction angles going from the east cliff (~---130 °) to the west cliff (Qt= 166°); (b) The systems K2 and K5 discovered on the east cliff demonstrate very similar angles to those of the west cliff systems of K4, K3 and K2

(approximately 210 ° and 240 ° , respectively); these systems on both cliffs demonstrate bearing of strikes (N30°W and N60°W, respectively) with angles of approximately 30 ° between the systems. Concerning the single cliffs, the following can be noted (Table 1): (c) The mean planes of the joint sets K2, K3 and K5 on the east cliff (Fig. 9a) have downslope steeper wedges with angles ranging between 75 ° and 80°; the bedding planes are oriented oblique to the cliff and have a mean spacing of 0.80-1.00 m. They contribute towards isolating the aforementioned wedges to the top and the bottom. Because of the strength of this structural feature the entire cliff shows frequent indentations in the form of dihedral angles coinciding with old rockfall crowns. (d) On the west cliff (0t= 70 °, 13was not unequivocably definable), the joint set intersections K1-K3 and K 1 - K 4 (Fig. 9b) demonstrate wedges with a lower rock failure probability, due to their

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197-210

202

Fig. 6. Detail of the west cliff; detail of the terraces at the foot; the arrows indicate an open joint.

Torre delloZirro

,•

SW

NE

'

,

¥ sea 19vel

1

2

~

4

Fig. 7. Geological cross-section (for location see Fig. 4). 1: detritical pyroclastic deposits; 2: alluvial deposits; 3: dolomitic limestones; 4: faults.

203

P. Budetta, A. Santo~Engineering Geology36 (1994) 197-210 [A]

N

[AI

W

E

W

E

S

S

[B]

N

N

[B]

N

s

-2%

2-4%

4-6%

6-8%

-10%

Fig. 8. Isodense percentagesof the points of the joints relating to the studied cliffs. Equatorial stereographic nets (lower hemisphere). A: East cliff; B: West cliff. low-to-average downslope. Besides they demonstrate the dip direction angles of the lines of intersection (trends) which diverge to angles of approximately 70 ° compared to the dip direction of the cliff itself (Table 1). The joint set intersection K 1 - K 5 is excluded because it gives steeply sloping wedges; therefore, we can find here a higher rock failure probability. Moreover we can see here numerous secondary cliffs, which are almost orthogonal to the main cliff and often coincide with the main faults and joints, belonging to a K5 system (Figs. 4 and 9b). On these secundary cliffs there are several surfaces of various inclinations coinciding with bedding planes of the S system. This means that it is kinematically impossible to stop plain sliding failures. The morphostructural outline of the two cliffs

Fig. 9. Cyclographicprojections of the average representative surface of the joint sets indicated in Table 1. Equatorial stereographic nets (lower hemisphere). S: pole of the average surface of bedding planes; K~: pole of the averagejoint surface. A: East cliff; B: West cliff.

is completed by the presence of several huge interbedding caves which developed preferentially in relation to the formation of major joint sets and cataclastic zones. In particular, the geostructural survey of the west cliff allowed the identification of what caused the formation of a huge deep cave ("Grotta di Masaniello", see Fig. 10), i.e., the systems K2, K3 and S with which several major low-angle " r a n d o m " joints, inclined towards the cliff, are associated.

4. Rockfall back analyses The first phenomenon of which the dynamics and trajectories were actually reconstructed in detail occurred in 1991 on the west cliff overhang-

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197-210

204

Table 1 Summary of the joint sets of the two cliffs and the possible intersections which favour wedge failures Joint set

Dip direction

Dip

~(°)

13(°)

Wedge failures Joint set intersection

Line of intersection Trend (°)

Plunge (°)

East rope S K1 K2 K3 K4 K5

130 025 210 163 006 240

16 64 80 75 72 85

K2-K3 K2-K5 K3-K5

161 173 169

75 78 75

166 052 238 206 208 146

15 85 74 84 54 85

K1-K5 K1-K3 K1-K4

92 130 139

83 68 28

West Hope S K1 K2 K3 K4 K5

ing the town. A 1.4 m 3, 3.65 tonne elongated polyhedrical-shaped boulder, fell from a height of 100 m above sea level (data gathered after the boulder became detached, fell and stopped, including presumable partial crushing phenomena). The rock mass followed an initial free fall trajectory with secondary impacting on small morphological roughnesses of the cliff, it rebounded past two citrus fruit terraces and stopped on the edge of a third terrace approximately 45 m above sea level. During its 50-m fall, the boulder partially destroyed a metal structure which was built there. The slope geometry survey (carried out on the basis of on-site measurement and thorough photographic documentation) of several impact marks which were still visible on the tilled land of the terraces and the endpoint, enabled us to reconstruct in detail the trajectory of the boulder. The crown was still visible on the cliff whose mean angle of dip is approximately 80 °, and is defined by the intersection of at least two joints which lie in such a way that a wedge was formed with the line of intersection sloping at an angle of at least 60o-65 ° . The presence of certain morphological roughnesses on the cliff and in particular a small fiat surface 80 m above sea level must have influ-

enced the first phase of the free fall trajectory by halting the boulder along a parabolic trajectory after the first impact; the total horizontal distance covered from the detachment point to the endpoint can thus be estimated at approximately 20 m. The second rockfall studied, which occurred in 1986, was from the edge of the cliff situated approximately 80 m north of the "Torre dello Zirro" (Fig. 10). This fall was much more difficult to reconstruct, because there were no direct witnesses (the zone is particularly inaccessible) and because of the time elapsed after the event. Nevertheless, evidence gathered in the vicinity of the block shows that the rock-slide induced block fell from a height of approximately 20 m and shattered at the foot of the cliff while hitting a large morphological surface which coincided with a bedding plane. Because this surface was approximately 15 m wide it was covered by talus deposits with different medium trunk trees (cypress and duster pine) whose trunks showed visible signs of impact marks (Fig. 11 ). The cliff where the rock fall occurred is almost vertical and has overhangs especially at the bottom due to karst erosion. The boulder experienced a free-fall and reached an estimated speed of approx-

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197-210

205

Fig. 10. Detail o f the west cliff; cultivated terraces and several karst interbedding planes can be seen (the "Grotta di Masaniello" is in the centre of the photo). The arrow indicates one o f the landslides described in the text.

Fig. 11, Rockslide-induced blocks from the west cliff during the landslide referred to in Fig. lO.

imately 20 m/s upon the main impact on the talus deposit. After the impact, the crushing phenomena released several huge fragments which were in turn

stopped by trees and the immediate surroundings of the point of impact. Isolated smaller rockfalls, i.e., rarely greater than 0.5 m 3, have been noted on the east cliff above the town ("Torre Civita"); the endpoints of the rocks in all of these cases can be observed on the first terrace at the bottom of the cliff. Several analyses have been carried out on the two main rockfalls described using Hoek's rockfall program (Hock, 1987) in order to reconstruct the trajectories of the boulders and estimate the most reliable kinematic parameters of the blocks governing the evolution of the phenomena (Fig. 12). The program is written in Basica for an IBM-PC or compatible computer. The main input data are: - - coordinates for the beginning of each cell segment of the slope; - - initial velocity components for falling rock;

206

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197 210 100 m s.I.

N = 200

50 number

80

12 16 distance (m)

60

20

4

2

40 0

10

20

30

40 m

Fig. 12. Back analysis of one of the west cliff landslides using Hoek's method. N: number of trajectories examined; dp: detrital pyroclastic deposits; dl: dolomitic limestones.

- - friction angle on plane 1 (for plane sliding); - - mean and standard deviation values for both e. and et shock restitution coefficientes, for each cell; and - - the number of runs required for each analysis. The output consists of a graphical representation as follows: - - a plot of the rockfall trajectories for each set of "Montecarlo" simulations; - - a histogram showing number of blocks with respect to the ultimate locations. As is known in this program, which incorporates a subroutine for "Montecarlo" probabilistic block propagation analysis, the kinematic parameters of blocks which have the greatest influence on the results are the normal shock (e.) and tangential (et) restitution coefficients. These coefficients, if partially inelastic shock criteria are assumed, enable to calculate the normal and tangential

rebound velocities, starting from initial impact velocities to the slope surface. There were no existing experimental calculations of the sites in question for the e. and et coefficients relating to the dolomitic limestone sections of the slope. Reference was made to the values deduced from special experiments carried out in old quarries at the "Italcementi" works at Castellammare di Stabia (northern slope of the Sorrentine Peninsula). The experiments consisted of handmade throws and free fall tests by fragmentation of rock using explosives, of dolomitic limestone boulders on rocky surfaces and on talus deposit of the landslide fans of 1986 (Urciuoli, 1988; Budetta and De Riso, 1988). Rock impact values for en and et were obtained (0.20 and 0.53, respectively), and although they were on average lower than those generally found in literature (Richards, 1988) they tallied with the

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197-210

reasonable mechanical resistance properties obtained from the rock mass analyzed. According to the Deere-Miller Classification these come under class C (50 500) (Urciuoli, 1988). The back analyses carried out on the area of Atrani, based on the kinematic parameters thus defined, enabled us to obtain values of 0.10 and 0.20, both for the en and for et coefficients, concerning the remolded pyroclastic deposits from the terraces situated at the base of the cliffs. These values can be considered sufficiently reliable, especially if they are compared to those obtained experimentally by Urciuoli (1988); the latter studied the impacts on detritus of the fans present at the foot of rock cliffs at the old Italcementi quarries, and obtained a practically zero value for en and 0.24 for et. It is worth noting that, although from a granulometric point of view, there are considerable differences between the pyroclastic deposits on the terraces and the landslide debris of the fans, however, the degree of compactness of the former more than justifies a higher normal shock restitution coefficient compared to the latter which to all effects and purposes can be considered "loose".

5. Rockfall hazard planning The rock slope failures and the parameters deduced from back analyses can be considered sufficiently reliable and, therefore, enable us to confidently predict future kinematics for potential rockfall phenomena. It was considered worthwhile to draw up a plan of the entire valley bottom in order to identify high-level rockfall risk areas, to evaluate the most suitable means of intervention, and to properly assist any town in planning decisions in the future. Therefore, fourteen topographic cross-sections of the entire east slope and nineteen cross-sections of the west slope were reconstructed in considerable detail (based on fieldwork on site and a wealth of photographic documentation); these cross-sections were located in what were considered high-level rockfall risk zones due to the effect of unfavourable morphostructural situations on

207

the cliff and the location of the nearest constructions at the base of the cliffs. Using Hoek's "Rockfall" program, the maximum potential endpoint was statistically identified for each section by hypothesizing an initial plane sliding along the first straight line cell of the slope, i.e., mostly free fall, a main impact at the base of the cliffs, followed by secondary rebounding and rolling until reaching its endpoint. In the block trajectory simulations (approximately 6600 in all) there were no considerations for impacts against the dry gravity walls of the terraces. Because they were carried out with dolomitic limestone quarry faces, they would cause rebounds with fiat trajectories thus enabling the boulders to come to a halt at a much lower level. These eventualities, however, appear statistically less probable given the particular position of the walls in relation to the hypothetical trajectories. The coordinates of the boulders' maximum potential endpoint obtained during various simulations have been shown in a topographic map in order to outline the areas of high-level rockfall risk (Fig. 13). The average size of an area at risk at the foot of the east slope is approximately 20 m with a maximum of approximately 35 m. Because there are a few private dwellings and a public building (church) of particular architectural importance in this area, the extent of the risk is considerably high. The risk area at the foot of the west slope is slightly larger due to the greater height of the cliffs and the presence of numerous morphological roughnesses on the rock cliff, which could lead to larger rebounds and flatter trajectories. Because the urban centre is, however, further away from the foot of the afore-mentioned vertical slope, the greatest risk is limited to that part of the town situated immediately below the highest peak at 200 m above sea level in the "Torre dello Zirro" area. In most of the simulations with terraced zones at the foot of the slopes, it was noted that the first two to three terraces came under greater risk compared to lower terraces, which had a much lower probability of boulders coming to a halt there. In general, the size of the rockfall risk sites at

208

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197-210

Fig. 13. Atrani Area. 1: Topographic cross-sections; 2: sites with a high risk of rock fall.

the foot of both slopes strictly depends on a variety of mainly topological factors such as the detachment point from the cliff, the presence of morphological roughnesses on the cliff, and especially the presence, number and size of cultivated terraces at the foot of the cliffs. The presence of the latter is no doubt the most influencing factor when reducing the size of the areas at risk, because of both the thick tree growth and the fact that the ground has been tilled, thus making it "softer". To further highlight the role played by terraces in modifying trajectory heights, translation velocities and endpoints, it was considered worthwhile to use P. Paronuzzi's rockfall calculation program (Paronuzzi, 1987, 1989) to verify the results obtained. Unlike Hoek's program, this program gives the impact and rebound velocities of the individual trajectory points. The program is written in HP Basic and runs on PC HP-86B. The main input data consist of the following: - - coordinates of the slope profile;

- - slope lithology (debris or rock); - - impact criterion carried out on the basis of shock restitution coefficient (e); - - type of movement (sliding, free fall or air rebound, rolling); sliding and rolling dynamic friction angles; and movement condition: detachment point, initial rebound angle and initial velocity (for free fall). The output consists of graphical and numerical output as follows: - - geometric features of the slope profile; - - plot of rockfall trajectory; - - impact and rebound velocities and angles; and - - endpoint. As is known, Paronuzzi's method assumes a single shock restitution coefficient value (e) for various substances, and the simulation results are considerably conditioned by the initial rebound angle (So) of the various segments of the trajectories. In a probabilistic procedure both parameters, e and So,mUSt be considered as independent stochastic variables which have continuous values -

-

-

-

P. Budetta, A. Santo~Engineering Geology36 (1994) 197-210

within a specific range; the probability density function (PDF) for these values, in agreement with Chowdhury (1987) must be considered a "beta distribution" (Paronuzzi, 1989). The proposed method and its statistical type formulation was thus applied to two sections (Fig. 14), which were held to be sufficiently representative of the most frequent topographical and morphological conditions of the site of Atrani. A total of 100 simulations were carried out in the hypothesis of motion caused by free fall, rebounding and rolling. The following can be deduced from a comparison of the results obtained for both sections: (a) the presence of terraces causes higher rebound trajectories, they, however, rapidly reduce the average translation velocity along the slope; (b) whereas all boulders reach the toe of a nonterraced slope and their endpoint depends on their average translation velocity, about 64% of trajectories on terraced slopes come to a halt on the first two terraces; 31% reach the third and fourth terraces and only 5% of trajectories end up at the toe of the slope. Although the two methods of trajectory analysis

s.n~.~

14~I0~I\}~\

N = 50

A

209

present methodological and statistical differences, they do provide results which are sufficiently comparable, especially when identifying the endpoints of the trajectories in relation to the terraces closest to the base of the cliffs.

6. Conclusions

In the study area, the data collected highlighted the close dependence of rockfall phenomenon on present-day morphological features brought about by complex tectonic phases which must still be considered susceptible to further changes. With regard to the above, it can be seen that the block rotation phenomena linked to the strikeslip fault activity we identified for the first time in the Atrani area (in analogy to the findings of Nur et al. (1988) although in very different geological contexts), must still be confirmed in other areas of the Sorrentine Peninsula for which detailed geostructural studies are not available. However, as far as landslide phenomena are concerned, the study of known rockfalls has enabled us to gather a first set of shock restitution

smt.~

1 ~I~ 4I\'°I

S

N = 50

',,,

1124\ 100

100

t '130 r n / s

0

~

=

0

i

20

,

.

K

-

,

40

m 60

0

20

40

m 60

Fig. 14. Graphic display of the probabilistic analysis of the trajectories using Paronuzzi's method. (A) For a normal slope; (B) for a terraced slope. Representative cross-sections of the morphology and topography of the slopes of the Atrani area are shown. N: number of trajectories examined; 1: envelope to maximum height of rock fall trajectories; 2: trajectory for boulder with maximum reach; 3: envelope to maximum velocities; 4: frequency distribution of blocks coming to rest in specific zones; 5: slope geometry.

210

P. Budetta, A. Santo~Engineering Geology 36 (1994) 197 210

coefficient values for pyroclastic deposits, which are very common on many slopes of the Sorrentine Peninsula, and to propose a planning for the highlevel rockfall risk valley bottoms. This will provide the possibility of intervening at the toe of the slopes and install, for example, protective rockfall barriers of the correct size and also to help in guiding future decisions in town planning. Finally, even though this kinematic study of rockfalls has revealed the important role of terraced surfaces in lowering the amount of potential rockfall risk sites, it could in the future be greatly improved if we had more exact reconstructions of the rock cliffs' geometry obtained from terrestrial photogrametric surveys.

Acknowledgements The authors would particularly like to thank Professors E. Hoek and P. Paronuzzi, who provided copies of their respective calculation programs. This work was carried out with financial contributions from MURST 6 0 % , 1991, cat. 18/cap.02/fund 16/93 (under Prof. R. de Riso).

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