Name = Sirolo Landslide, Adriatic coast, central Italy

Geotechnical Study Area G21
Sirolo landslide, Adriatic coast, central Italy
GEOTECHNICAL STUDY AREA G21
SIROLO LANDSLIDE, ADRIATIC COAST, CENTRAL ITALY
Plate G21 Aerial view of Sirola from the south
1.
INTRODUCTION
The village of Sirolo, called "the pearl of the Adriatic" on account of the beauty of the natural
environment surrounding it, is situated on top of a steep cliff (approximately 125 m above sea
level), a few kilometres south of Ancona and Mount Conero (Figures G21.1 and G21.2). The
area is strongly affected by intense seismic and landsliding activities.
2.
BACKGROUND TO THE INSTABILITY PROBLEM
The cliff is mainly made up of fractured calcareous marls. Three systems of subvertical joints
cross the rock mass, allowing several sections to move apart and create conditions which
promote mass movement.
With the aim of research as well as for public saftey reasons a network of monitoring
equipment was installed in order to reveal, when linked to climatic data, the piezometric
response deep inside the landslide and measure the rate of deformation at several points
(Angeli et al. 1990; Angeli et al. 1991; Angeli and Pontoni 1995; Angeli et al. 1996).
High precision geodetic surveys were carried out (Angeli et al. 1990; Angeli et al. 1991) with the
Geotechnical Study Area G21
Sirolo landslide, Adriatic coast, central Italy
aim of clearly defining stable and unstable areas. This research led to a preliminary hypothesis
of the landslide mechanism and to the introduction of new measures to mitigate the
displacements occurring in the cliff. Continuous adjustments of the monitoring system and the
control works have been made through the period of investigation.
The investigation and works were devised to mitigate the continuous subsidence of the portion
of village directly facing the sea (Figure G21.3, Plates G21a and G21b). The first results of the
monitoring together with some preliminary control works revealed a very large and deep-seated
landslide process, which is controlled by the piezometric fluctuations inside a progressively
opening crack located at the back of the rock mass (Figure G21.4). Further monitoring
installations and new control works have provided the necessary information for the design of
more effective control measures.
As part of the LIFE project the study area contributes towards best practice on landslide
management and focuses on the results obtained following the installation of high strength steel
anchors and especially after the installation of three systems of extremely long sub-horizontal
tubular drains. In the last few years significant leakage from the water pipes and from the
sewerage system has created critical hydraulic conditions, even in the total absence of rainfall.
3.
GEOLOGY AND GEOMORPHOLOGY
The ridge of Sirolo consists of outcrops of the Schlier Formation occurring as strata of
calcareous marls, marly limestones, clayey marls and in some places thin layers with higher
clay contents (Plates G21c and G21d). Due to intense cleavage it is not always possible to
identify the bedding of the material (Plate G21h). The geological structure of the area is
characterized by a SE dipping monocline of 20° to 30° . This monocline is affected by several
systems of faults or joints in a mainly sub-vertical direction. Steep scarps surround the eastern
edge of the monocline.
This creates conditions in which huge blocks (or wedges) of rock are detached along the
above-mentioned system of joints and tend to slide over the bedding planes (Figure G21.5).
These movements do not necessarily follow the maximum slope of the bedding planes, but can
also follow extremely low slope angles. This occurs because of favourable combinations of subvertical joints with the scarp faces.
There are no resources available from large aquifers in the study area, so Mount Conero is the
only source of groundwater, due to its considerable elevation (about 600 m above sea level) in
the vicinity of the village.
Where unfavourable hydraulic conditions occur in addition to the previously described
geometric situation, larger blocks can be set in motion along bedding planes, in directions
almost perpendicular to the dip of the monocline and at very low angles of slope (possibly
favoured by the presence of thin clayey strata). These conditions constitute the most critical
process of instability for the cliff’s overall equilibrium. For this reason they should be accurately
monitored and stopped wherever possible. The remedial works to stabilize the area, described
below, are a successful move towards this goal.
Secondary instability phenomena can be identified in the form of rockfalls caused by toppling.
Over the years these rockfalls have created an enormous accumulation of debris extending as
far as the sea (Plate 21f). In the early 1970s the foot of this accumulation was reprofiled and
consolidated with retaining structures in reinforced concrete, built on micropiles. This
reprofiling, which was carried out by the Civil Engineering Department of Ancona, also enabled
the provision of several storerooms from which local fishermen operate. At present this
accumulation is moving very slowly due to creep processes. The movement is completely
independent of the climatic conditions shown to be critical for the main instability process.
4.
GROUND MOVEMENT MONITORING
Since 1990 a large number of boreholes have been drilled (eg. Figure G21.6) and equipped
Geotechnical Study Area G21
Sirolo landslide, Adriatic coast,
central Italy
with instrumentation for manual or automatic reading. An automatic rain gauge was installed
together with a first of a series of pressure transducers for the monitoring of piezometric levels.
Open standpipe piezometers, Casagrande piezometric cells, inclinometer tubes, vertical and
horizontal extensometer bars of glass fibre or extensometer steel wires were installed at
various times in the body of the landslide and at its boundaries.
Subsequently pressure and displacement transducers were connected to the above
instruments and various types of data collectors, both for reading locally and for long distance
transmission, were installed.
5.
EXPERIENCES, SUCESSES AND PROBLEMS WITH THE CURRENT APPROACH
The remedial works carried out in the area consist of a system of pre-stressed steel anchors
and sub-horizontal tubular drains (Figure G21.7). The high strength steel anchors of various
lengths (60-75 m) are intended mainly as a protection against the most severe consequences
of strong earthquakes, rather than a prevention against the prevailingly slow movements of the
landslide mass.
The anchors (at 4 m intervals) have been secured deep in the rock with 20-25m long grouted
anchorage sections and with reinforced concrete beams at the surface, built on micropiles
(Plate G21i. In order to minimize the environmental impact, the beams follow the contour lines
of the slope. However, the subhorizontal tubular drains D1, D2 and D3, arranged radially from
each location, are the principal element of the stabilization measures (Figure G21.7). They were
designed with different lengths (90-150 m) and at different levels to eliminate the hydraulic
thrust at the rear of the landslide body.
The study shows (see Figure G21.8) the relationships between rainfall, piezometric elevation
and strain, considering only a part of the instrumentation installed as representative of the entire
landslide.
In particular piezometer BH1 is typical of the main body of the landslide, whereas piezometers
BH8 and BH11 represent the piezometric behaviour of that part of the relief not affected by
instability phenomena; the extensometers BHH2 indicate the main movement whereas BHH3
represents a small lateral portion of the landslide not yet affected by control works. A1, A2 and
A3 record the three different periods when the anchors were tensioned. D1, D2 and D3 show
when the tubular drains, positioned at three different elevations, began to function.
It is clear that the drains installed at the end of 1994 (D2) produced a significant lowering of the
groundwater table and stopped ground movements. The efficiency of the drainage system D2
was also verified while the works were being carried out. When drilling the last few metres, the
flow in the tubes increased from 0 to 30 litres/minute (approximately).
In the course of a few days tubular drain system D2 (about 100 metres long) completely drained
the reserves of water being held at the rear of the landslide body inside the main tension crack,
causing the groundwater table to fall by about 4 m (Plate G21l). The reduction in water level
occurred up to the elevation of the outlet of the drains.
At the end of 1996 three new tubular drains about 100 m long were installed (system D3), in a
location 5m lower and about 80m north of D2. A check piezometer (BH20) was also installed.
When drilling the last few metres the overall flow in the first two tubes increased from 0 to about
600 litres/min. (Plate G21k). The water level in piezometer BH20 dropped several metres
within 5 hours, thus giving proof of the efficiency of such control works.
It is important to observe that between June and August 1995, in total absence of rainfall, an
important piezometric rise took place, just before new rainfall events induced the largest
piezometric peak ever recorded (see piezometer BH1 in Figure G21.9, and also Plate G21j). A
similar situation was also recorded between August and October 1992. The former rise led to a
general increase in the piezometric base level until the drainage (D2) installed at the end of
1994 started to work, leading to the lowest base level ever recorded. The 1992 piezometric rise
Geotechnical Study Area G21
Sirolo landslide, Adriatic coast,
central Italy
was interpreted as the impact of changes in the obstruction of natural drainage paths induced
by the progress of landslide movements.
The comparative analysis of these two events (1992 and 1995) highlighted that significant and
increasing leakage from the mains water supply pipes might have occurred. Since this situation
could have posed a hazard for the stability of the landslide, the local authorities were warned in
good time about the danger caused by leakage water filling up the crack located at the back of
the landslide mass or, in other words, by the possibility that higher than expected piezometric
peaks might follow rainfall events. In spite of this, during the critical situation of August 1995
the drainage proved to be adequate to enable rapid reduction of the dangerous piezometric
peak (Figure G21.9).
In June 1996, after long investigations, significant leakage from the waterpipes was detected
and repaired.
In October 1997, at the start of the LIFE project, a microbiological analysis of the water
discharge from the tubular drains showed the presence of a large coli bacillus concentration.
This demonstrated the pollution of the groundwater from the sewerage system with water filling
the sub-vertical landslide crack; a continuing critical situation. At the beginning of the summer
of 1998 (Figure G21.10), again in total absence of rainfall, an important piezometric rise took
place. Since this situation could have been a hazard in terms of stability of the landslide, the
th
local authorities were warned again in good time about the danger. As a result on the 28
September 1998 new significant leakage from the main water pipes was detected and repaired.
The monitoring data showed that a decrease in drainage discharge took place after the repair
work but in many areas in the landslide still showed high piezometric elevations. During this
period significant landslide movement took place, as shown in Figure G21.10.
The search for new points of leakage and a check of the integrity of the water supply and
sewerage system is now in progress, following the suggestion to use the newest techniques for
detection such as telecameras and a receiver of induced sound signals.
The situation described above appears a paradox to scientists devoted to mitigating landslide
evolution, but sadly it is a widely occuring problem in many historical villages, where their
infrastructure is now obsolete due to very rapid development superimposed on to an ancient
urban plan.
Despite this the long-term success of the drainage systems can be summarised by the
significant and permanent drop of the average water level present in the main crack at the back
of the landslide body.
This effect of the remedial drainage works is of great significance especially if related to the
highest rainfall events which now induce piezometric peaks less dangerous than before (as an
example see in Figure G21.9, the critical situation of August 1995, when two days of heavy
precipitation provided a total rainfall exceeding 300 mm, which is 1/3 of the average annual
total). Indeed, because the piezometric changes which follow heavy rainfall begin from a lower
base level, they lead to a significant and constant reduction of the water pressure at the rear of
the landslide body, resulting in the drastic mitigation of movements.
6.
LESSONS LEARNT
The approach adopted to landslide management in Sirolo bears some similarity to the Ventnor
Undercliff (Study Areas G1 to G8). Following a geomorphological investigation a range of
engineering stabilization measures were commissioned, including coastal protection, drainage,
slope support and ground anchoring. At Sirolo the drainage works have been particularly
successful due to the extent to which changes in groundwater levels, identified using
monitoring, could be associated with leakage from main water supply and sewerage pipes, and
the appropriate co-ordinated action be taken by the local authorities, utilities and the
geotechnical monitors to address the problem and reduce risks. Apart from the unfavourable
geological conditions at Sirolo the risks arising from seismic activity have necessitated an
integrated approach to landslide management.
Geotechnical Study Area G21
Sirolo landslide, Adriatic coast,
central Italy
The hydrological conditions have been examined in greater detail in Sirolo than in the Isle of
Wight Undercliff, where this issue needs addressing. However at both locations coast
protection at the toe of the landslide was regarded as a priority. In Sirolo building damage
occurred over time and as in Ventnor a number of properties were built directly against the
landslide scarp faces resulting in structural damage when ground movement took place. The
accompanying best practice guide produced by this LIFE project (‘Managing ground instability
in urban area -A guide to best practice’) recommends that buildings should be separated from
such cliff faces.
A particular success of the LIFE work at Sirolo has been the monitoring of instability and
groundwater movements as set out in Task two of the project. Interpretation of monitoring
results by the National Research Council and its consultants is making a positive contribution to
landslide management in an area where an understanding of the hydrological regime is vital;
the method of presenting results is also clear and readily understandable.
During the course of the LIFE project further stabilization works have been undertaken in
vulnerable locations in response to the results from the monitoring programme; further new
works and maintenance will be required. The authorities at Sirolo are undertaking a works
programme following thorough ground investigation and monitoring. The approach is similar to
that of the Isle of Wight for both locations are doing as much as possible to reduce the impact
of landsliding an their communities with limited financial resources.
7.
REFERENCES
Nb. Please also see Reference list at the end of Study Area G22
(Grottammare).
Angeli M-G., Gasparetto P. and Pontoni F. 1996. Long term monitoring
and remedial measures in a coastal landslide (Italy). Proceedings of
VII ISL, 17-21 June 1996, Trondheim, Vol. 3, 1497-1502.
Angeli M-G. and Pontoni F. 1995. Il monitoraggio e gli interventi di
stabilizzazione in una falesia di roccia fratturata. Atti del XIX
Convegno Nazionale di Geotecnica, Pavia, Italy, Vol. 3, 1-6 (in
Italian).
Angeli M-G., Barbarella M. and Pontoni F. 1991. Instability of a sea cliff:
Sirolo landslide, Italy. Proceedings of VI ISL, 10th-14th February
1992, Christchurch, Vol. 2, 1093-1100.
Angeli M-G., Barbarella M., Dramis F., Garzonio C. A. and Pontoni F.
1990. A monitoring project for the definition of the geostructural
model of Sirolo landslide (Italy). Proceedings of VI ICFL, 12th
September 1990, Milano, 175-186.
Milan
Venice
Ancona
Sirolo
Perugia
Grottammare
Rome
Naples
0
Figure G21.1
Location map of Sirolo and Grottammare landslides.
200 km
Figure G21.2 Block diagram of the slope.
Figure G21.3 Historical map of Sirolo (1850); the buildings in dark have been destroyed by successive
landsliding.
Figure G21.4 Geomorphological map, Sirolo.
Figure G21.5 Landslide model, Sirolo.
Figure G21.6 Map of the levelling surveys, Sirolo.
Plate G21.7 Map of the monitoring system and control works, Sirolo.
Figure G21.8 Long term recordings of rainfall, piezometric elavation and displacement, (1997-2000).
Figure G21.9 Critical event of August 1995.
Geotechnical Study Area G21
Sirolo landslide,
Adriatic coast, central
Italy
Plate G21a/b Aerial
views of Sirolo
village showing the
edge of the town
square cut by
landsliding
Geotechnical Study Area G21
Sirolo landslide,
Adriatic coast, central
Italy
Plate G21c Aerial view from the sea (east)
Plate G21d View of Sirolo cliffs from the sea
Geotechnical Study Area G21
Sirolo landslide,
Adriatic coast, central
Italy
Plate G21e Aerial photo (USAF, 1955)
Plate G21f Landslide toe, with a large debris accumulation (1920)
Geotechnical Study Area G21
Sirolo landslide,
Adriatic coast, central
Italy
Plate G21g Landslide occurred near the “Urbani Cave”
Plate G21h Open joints in the rock (20-25cm)
Geotechnical Study Area G21
Sirolo landslide,
Adriatic coast, central
Italy
Plate G21i Drilling operations of the tubular drains (L=120-150 m)
Plate 21j “D2” drainage system at work during the critical event of August 1995
Geotechnical Study Area G21
Sirolo landslide,
Adriatic coast, central
Italy
Plate G21k Maximum discharge of “D3” drainage system (36m3 /hour)
immediately after the drilling
Geotechnical Study Area G21
Sirolo landslide,
Adriatic coast, central
Italy
Plate G21l “D2” drainage discharge some days after the drilling