2

G. Marchi, G. Giacchetti, G. Benedetti, A. Landuzzi – The Scascoli case study

phenomena, which is a long-term process controlled by the dip direction of erodible beds, has been dramatically accelerated by the emplacement of the Scascoli Land-slide.

The rockfalls of 1992, 2002 and 2005 affected the left side of the valley with a complex mechanism of slid-ing and toppling of rock blocks marked off by master joints. All those rockfalls have been prepared by the NW-ward migration of the Savena stream, which led to undermining many rock walls in the left side of the Gorge.

Figure 2. Geological section crossing the cliff of the 2005 rock-fall (left), and the toe of the Scascoli Landslide (right)

On the other hand, stream erosion has also pre-pared the Scascoli Landslide itself, which is now a day responding to stream entrenchment with a continuous, slow motion. Even though at present there is no direct evidence, a relationship might be supposed between high stress driven by deep displacement in the Scascoli Landslide and the large, consecutive rockfalls in front of it. The Large Scascoli Landslide

Near the Savena riverbed, in the right side of the valley, an important road leading from Bologna to Tuscany cuts weathered rocks and debris belonging to the foot of a 20,000,000 m3 landslide (the Large Scascoli Landslide), whose age is supposed to be a thousand years. The body of the Scascoli landslide extends over 0.72 km2, and the toe is 0.9 km long. Rock blocks up to 100 m thick have been involved in its multiple roto-translational movement, for an overall displacement of 200-400 m. Monitoring surveys show that today the whole landslide body is slowly moving.

Rockfall 1992

This rockfall involved seven thousand of m3. The rock blocks fell down from a vertical wall in the left valley side and impacted on the base of slope scree, without reach-ing the road on the right side. This event was the first signal of a more general instability, but it was underes-timated. Rockfall 2002

In October the 15th 2002, part of an overhanging rock cliff 50-80 m high collapsed close to the wall of the 1992 event. The total volume of fallen blocks was estimated as 20,000 m3. The rockfall destroyed a long segment of the road and dammed the Savena riverbed, creating an ephemeral obstruction lake.

Figure 3. Rockfall 2002 The geometry of the fallen rock mass was reconstructed on the base of field data, photographs, pre-fall maps and post-fall topographic surveys. A schematic recon-struction of the failure kinematics is sketched in Figure 4. The movement was probably triggered by progressive breaching at the base of a rock wedge, laterally bounded by an important sub-vertical fracture (ABCD in Figure 4). When the shear strength of the basal rock bridge (segment DE) was overcome, a rotational sliding movement was generated, which undermined the whole overhanging dihedral cliff (DHFB). As a consequence, the rockfall mechanism was mainly initiated with a top-pling. It is interesting to point out that block BCFG fell shortly later than the other rock masses, and crashed on a pile of already collapsed blocks. Indeed, the fall event was more complex than the above-described bi-dimensional sketch, as the rock mass constraints on the landslide evolution were three-dimensional. In particular, the right flank wasn’t con-fined, while along the left flank the tensile strength of rocks has been probably overcome by breaching.

3

Proceedings of the Second World Landslide Forum – 3-7 October 2011, Rome

Figure 4. Rockfall 2002 – Sketch of the most probable instabil-ity mechanism

Figure 5. Rockfall 2002 – The obstruction lake at its highest level

In the hours following the rockfall, a quick interven-tion was planned in order to face hydraulic emergency (Figure 5).

Part of the fallen material was removed (Figure 6), and an outflow channel for the Savena stream was dug. This way, the level of the obstruction lake was lowered.

The outlet was dug in the foot of the opposite slope, in order to keep safe the workers from persistent boulder falling. Part of the removed material was used for the construction of a small bank along the left river side. The rock slope face was partially re-profiled using blasting techniques.

At the same time with the emergency interventions, an important program of geological and geotechnical in-vestigations was planned in order to recognize other ha-zardous situations in the Scascoli Gorge. Most attention

was focused on two huge unstable rock masses, re-spectively known as “Mammellone 1” and “Mammellone 2”.

Figure 6. Rockfall 2002 – First intervention step Rockfall 2005

The 12th of March 2005, a 27,000 m3 sized wedge fell from the “Mammellone 1” rock cliff (Figure 7).

Figure 7. The “Mammellone 1” rock cliff, before the 2005 event

This rockfall (Figure 8), completely destroyed one hundred meters of road and dammed the Savena ri-verbed a second time, creating an ephemeral lake. The collapsed wedge was 14 m overhanging; the detach-ment niche after the event appeared as a clean dihedral with a height of 60 m and a maximum width of 20 m. The movement developed partly on pre existing master joints, and partly by progressive cracking of the solid rock.

The ephemeral dam forced the Savena stream to shift to the right side of the valley, where it quickly de-leted another section of the road and started undermin-ing the foot of the wide slow-moving Scascoli Landslide.

4

G. Marchi, G. Giacchetti, G. Benedetti, A. Landuzzi – The Scascoli case study

Figure 8. Rockfall 2005

After few weeks, a rotational landslide developed

from the right side of the valley (the toe of the Large Scascoli Landslide), 100 m wide and 80 m high, involv-ing 8500 m3 of soil and weathered rock. The maximum velocity of this movement was 3÷4 m/day, and progres-sively decreased while reaching a new equilibrium con-dition (Figure 9).

Figure 9. Rockfall 2005 and the rotational landslide developed from the right side of the valley highlighted by yellow line (photo by G. Bertolini)

The collapse of “Mammellone 1” had been fore-

seen by means of stability analyses (Figure 10). Taking into account the geomechanical properties and the scale factor, the stability analyses were developed considering the rock mass as a continuous, isotropic medium (finite element method) or as discontinuous medium with pla-nar and wedge sliding (limit equilibrium method). The most dangerous situation (Safety factor = 1.03) was pointed out in the “Mammellone 1” wedge. Because of its large size and overhanging morphology, the rehabili-tation design focused on reshaping it to a stepped slope using blasting. The analysis of the excavation se-quences showed the risk of collapse on the third bench. In order to prevent this, it was planned to excavate down to the second bench, and then knock down the large wedge on the left of the benches. Then reshaping would have been completed by excavation of the benches on the right side. Waiting for the intervention stage, a moni-

toring system was set up for the safety of the traffic. Un-fortunately, the 2005 rockfall took place while setting up of the monitoring system was still in progress.

Figure 10. Slope stability analysis by a distinct and a finite elements method Monitoring systems

Extensive soil investigations and monitoring systems were set up in 2002 and later in 2005, putting in evi-dence the range and the intensity of the instabilities. The investigations included topographic surveys with leveling network points and helicopter laser scan (LIDAR), seis-mic exploration, piezometric measures, geognostic bo-rehole, lab tests on soil and rock. The highest quality da-ta set was given by inclinometric, topographic and ex-tensimeter measures.

The inclinometric pipes placed along the road in the valley, show that the Scascoli Landslide is not exact-ly dormant: it is sliding on a surface placed 10-15 m be-low the riverbed, with an average velocity of 15 mm /year (Figure 2). Those slow displacements explain why there are several large fissures on the old concrete structures along the road.

The topographic leveling network points showed that the bottom of the valley is slowly lifting at a velocity ranging between 0.5 e 2.4 mm/month. This movement is related to the deep sliding of the Large Scascoli Landslide, which is forced to lift by the restrain of the opposite (left) valley side. The velocity fluctuations seem to be related to the stronger rainfalls, which influence the water table level. Also the GPS markers on the body of the Large Scascoli Landslide, indicate the presence of small movements down slope.

Fissure extensimeter had been placed on master joints, which across the cliffs in the left valley side, where the rockfall of 2002 and 2005 appended, and on the high rock slope known as Mammellone 2. On the slope of the 2002 rockfall, the extensimeter, give some trend displacement (sized 0.01 mm/month), which are not clearly interpretable at the present. On contrary, someone of the fissure extensimeter set up on the Mammellone 1 reshaped slope, indicate a clean dis-placement trend (0.3 – 0.5 mm/month), which is related to a large potentially instable rocky mass. At last, some instruments on the Mammellone 2 show a displacement trend of 0.05 mm/month.