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Engineering Geology 55 (2000) 133147www.elsevier.nl/locate/enggeo

A critical review of landslide monitoring experiences

Maceo-Giovanni Angeli a, Alessandro Pasuto b,*, Sandro Silvano b

aNational Research Council I.R.P.I. Institute, via Madonna Alta, 126-06128 Perugia, Italy

bNational Research Council I.R.P.I. Institute, C.so Stati Uniti, 4-35127 Padova, Italy

Abstract

Over the past few years, the monitoring of natural phenomena has acquired great importance for the scientific

community. It aims to understand the mechanisms of disruptive processes, define adequate prevention measures for

the mitigation of their effects and reduce the loss of human lives and assets. In order to detect the stability conditions

of slopes belonging to different geological and environmental contexts, geotechnical investigations have been carried

out since 1982. The various types of landslides to be investigated, and the diverse socio-economic contexts involved,

have shown the need for constant surveillance, using the most up-to-date technology available. For this purpose,

automatic recording systems connected to different sensors have been installed, (and also serve civil defence purposes).

During this research activity, several problems arose, and several solutions had to be found. In this paper, some of

the main problems concerning the installation and management of monitoring equipment used for the study of three

landslides will be discussed. 2000 Elsevier Science B.V. All rights reserved.

Keywords: Instrumentation; Italy; Landslides; Warning system

1. Introduction This is achieved by means of monitoring kine-

matic, hydrological and climatic parameters in

order to:Hydrogeological disarrangement is one of the$ identify movements before important morpho-most destructive natural events striking civilian

logical changes at the surface have taken place;populations, urban settlements and infrastructures$ define the geometry of the moving mass withworld-wide every year, causing thousands of casu-

precision;alties and serious damage.$

quantify the principal kinematic parametersThe monitoring of natural phenomena has (velocity, acceleration, etc.) and their possibleacquired great importance for the scientific com-correlation with hydrological and climaticmunity since the use of adequate monitoring sys-characteristics;tems is a powerful tool for understanding

$ carry out constant surveillance for events thatkinematic aspects of mass movements and permitsput inhabited areas at risk; andtheir correct analysis and interpretation; in addi-

$ propose reasonable plans to help people intion, it is an essential aid in identifying and check-risk areas.ing alarm situations.Landslides show a great variability not only

from the typological but also from the kinematic* Corresponding author. Tel.: +39-49-829-5800;

and geometrical standpoints. Each landslide isfax: +39-49-829-5827.E-mail address: [email protected] (A.Pasuto) therefore characterised by the way it has devel-

0013-7952/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 1 3 - 7 9 5 2 ( 9 9 ) 0 0 1 2 2 - 2

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134 M.-G. Angeli et al./Engineering Geology 55 (2000) 133147

oped, and this necessarily determines the kind of These events caused the filling of the Tessina

valley with material ranging from 30 to 50 m insensors that should be set up, as well as the number

and location of measurement points, the sampling thickness, seriously endangering the village ofFunes, which is situated on a steep ridge originallyfrequency of parameters, etc. As a consequence,

the standardisation of intervention methods and quite high above the river bed, but now almost

level with the mudflow.of the sensors that should be used is a rather

difficult task. In April 1992, after periods of rather heavy

rainfall and snowmelt, a collapse on the left-handIn order to obtain relevant results, a monitoring

system should be chosen not as a product of slope of the torrent Tessina occurred, involving

approximately one million cubic metres ofvarious technologies but after the preliminary

analysis of a phenomenon by means of adequate material.

The toe of the flow reached the village of Funesconsideration of the data. Similarly, a set of sensors

placed haphazardly inside and around a landslide in just 5 days, moving at an approximate velocity

of 10 m/h (the mass was about 5 m wide and 1 mbody will produce a series of measurements that

are neither easy to interpret nor easy to compare. thick), whereas the main slide, more than 100 mwide, moved at a velocity of about 15 m/day.This may also be due to problems arising in the

sensors installment and functioning and also in Flow movement continued with varying inten-

sity until July 1992, when it almost reached thedata collection.

In this paper, some of the problems involved in outskirts of Lamosano after having overrun the

earlier flows; then, the houses of Funes andthe installation and running of different monitoring

systems employed in the study of three landslides, Lamosano were evacuated.

Up to now, the landslide has developed betweenwith different aims in view (civil defence, research,

remedial measures), will be discussed. the altitudes of 1220 and 625 m a.s.l., with a total

longitudinal extension of almost 3 km and a maxi-

mum width of about 500 m (Fig. 1).

From a geological standpoint, the event has2. Tessina landslideaffected a flysch formation (Middle Eocene), made

up of rhythmic alternations of low-permeabilityIn the autumn of 1960, after a period of intense

precipitation, a landslide, characterised by a source marly-clayey and calcarenite strata with a thickness

of 10001200 m. In some parts, this formation isarea in constant expansion affected by a complex

(rotational slideearth flow) slide movement with covered with Quaternary deposits mainly con-

sisting of vast scree slopes and Wurm morainica slip surface approximately 2030 m deep, was

activated in the province of Belluno (northeast- deposits from the glaciers of the River Piave valley

and from other small local glaciers.ern Italy).

The material from this area, which is intensely From a morphological point of view, it was

possible to distinguish a flat upper accumulationfractured and dismembered, was channelised along

the Tessina valley, where it was progressively area, and a lower accumulation area consisting of

the main flow over two kilometres long, with aremoulded with an increase of water content. Thus,

it underwent increasing fluidification, giving rise steep narrow discharge channel connecting these

two areas.to small earth flows that converged into the main

flow body. In the upper landslide sector, the main morpho-

logical element is a large perimetral scarp, overThe mass movement involved about two million

cubic metres of material, and was responsible for 20 m high, bordering the most active area. The

mobilised material is collected in an almost flatendangering the villages of Funes and Lamosano

(Angeli et al., 1994; Pasuto and Silvano, 1995). area, located at the scarps base, where its primary

characteristics are progressively lost as it turns intoFurther landslides occurred at the site in 1962,

1963, 1973, 1987, 1988, and 1989 after long-term a rather viscous mud, owing to water absorption.

Once transformed, the material flows through arainfall.

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135M.-G. Angeli et al./Engineering Geology 55 (2000) 133147

Fig. 1. Index map, location of the instrumentation and schematic cross-section of the Tessina landslide. Legend: (1) main flow body;

(2) roto-translational slide; (3) scree slope; (4) highly folded and fractured flysch; (5) flysch formation; (6) calcareous and marly-

calcareous formations.

narrow and steep discharge channel where the The lowest landslide sector corresponds to the

main accumulation area, which stretches from thethick sandstone layers making up the flysch forma-

tion crop out; these rocks form a sort of retaining toe of the discharge channel as far as Lamosano,

passing by the hamlet of Funes.structure for the overlying material.

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Several geotechnical tests have been carried out On the upper section of the slide, an automatic

topographic system for measuring surface move-on both the original material cropping out on the

main scarp and the remoulded material making ment was installed. This consists of a motorisedWild TM3000V theodolite with EDM Wildup the main landslide flow. All the materials tested

fall within the field of medium-high plasticity Distomat Di2002. More than 30 targets, made by

retroreflectors (reflecting prisms), are placed eitherinorganic clays. The liquid limit (wl) ranges

between 38.5 and 52.5%, whereas the plasticity within or outside the landslide body.

The telescope body has an integrating focusingindex (PI) is 16.224.7%. Direct shear tests have

provided values of the drained residual friction drive, a CCD camera, an eccentrically placed wide-

angle lens, and a coaxial target light. The targetangle, wr, ranging between 19.9 and 26.1.

By means of field observations and investiga- light generates an invisible infra-red beam of light.

This can be pointed to a retroreflecting target thattions and by analysing the data acquired over the

past few years, it was possible to develop a fairly reflects incident light back to its source, i.e. it acts

as if it was an active luminous target. The CCDreliable landslide evolution model. In practice, the

material involved in the slides affecting the main camera receives the optical beam and projects thevisual field to a video monitor. Image processingscarp or the margin of the upper accumulation

area gives rise to new flows that overlap those allows the theodolite system to identify targets

automatically and measure the angle and distancealready stabilised. Owing to an undrained loading

effect, the latter are partially remobilised over to each target. For target recognition, the CCD

camera is controlled to indicate either the measur-thicknesses of 12 m. This is facilitated during

periods of intense precipitations or snowmelt, ing or the wide-angle field of view to the target.

The theodolite transmits the video signal via thewhen the piezometric rise inside the earth flow

body comes close to the topographic surface. In cameras mains-supply unit to the computer, which

processes the data and calculates the targets dis-these conditions, a sudden overload noticeably

reduces the factor of safety. placements. The monitoring of each benchmark

position is carried out at 13 h intervals duringConsidering this high-risk situation, an alarmand monitoring system has been planned with the emergency periods, whereas in normal times, the

measurement interval is 6 h.main purpose of alerting the population in case of

imminent danger and, secondarily, in order to In order to detect the passage of flow fronts of

a certain thickness that are potentially hazardousacquire useful data for understanding the dynamic

pattern of the phenomenon and for the definition for the hamlets of Funes and Lamosano, two

control units, one consisting of three directionalof possible evolution scenarios.

bars and an ultrasonic echometer, the other of two

directional bars and an echometer, were installed2.1. Instrumentation

on the mudflow some 100 m uphill of the villages.

Three videocameras were also installed toFollowing the 1992 events, a programme of

remedial interventions was started, i.e. building of record and monitor the slide movement in the

most critical areas. Two of these cameras wereretaining walls and installation of a monitoring

and alarm system to guarantee the safety of the connected to two SVHS videorecorders, recording

for 2 s every 3 min, positioned by the hamlets ofpopulation (Fig. 1) (Angeli et al., 1996b).

In particular, two multiple-base wire-extensom- Funes and Lamosano. The third camera, in prox-

imity to the main scarp, sent images directly to aeter units, measuring 280 and 390 m, were installed

in the slide upper section in order to provide a monitor installed at the Lamosano Town Hall,

where personnel with surveillance duties constantlyconstant check of the movement occurring on the

landslide surface. These units consisted of a series checked the evolution of the phenomenon.

Three peripheral stations, fed by photovoltaicof 12 measuring pulleys fitted with an appropriate

scaler system capable of detecting movements as panels and buffer batteries, acquired and pre-

elaborated the data coming from all sensors, check-small as a centimetre.

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Fig. 2. Extensometric devices installed on the Tessina landslide.

2.1.1. Wire extensometers

As regards wire extensometers in the Tessina

landslide, the need to carry out the measurement

of separate, but interrelated, movements and their

arrangement required the planning of a highlyindividual system composed of two anchor points.

One of these was installed in the central part of

the moving zone, and the other in the stable zone,

with the function of keeping cable tension con-

stant. Then, 12 measuring apparatuses (Figs. 2

and 3), consisting of a pulley fitted with an appro-

priate scaler system capable of detecting move-

ments as small as a centimetre, were positioned

along the cable and connected by wire to a peri-

pheral station able to transmit the data via radio

to the central monitoring station at Lamosano.Fig. 3. Layout of the extensometric device. Legend: ( 1) fineThe anchorage points were composed of a cageangular-displacement transducer; (2 ) main angular-displace-

of about 212 m, containing a series of springsment transducer; (3 ) 10-to-1 movement scaler; (4 ) wire; ( 5)pulley. together with a delay mechanism permitting a

cable lengthening of some 10 m, with constant

cable tension (Fig. 4).ing their proper functioning at the same time. DataBearing in mind the high level of risk for theacquisition by the sensors took place every 10 min,

local population and the dimension of the areaand the data were immediately downloaded, viabeing checked, this deformation measurementradio signals, onto a central monitoring station,system was connected to a topographic measure-located inside the Town Hall of Lamosano in order

to define possible danger situations. ment system with automatic surveying to check

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Fig. 4. Device used to keep the wire in constant tension.

and verify the movements of some benchmarks snow, during which topographic measurement

cannot be carried out.and all the extensometric measurement equipment.

This linkage between the two systems has demon- However, the intensity of the movement under

study has created notable difficulties for the man-strated the advantage of continuous surveillance

of the evolution of the source area, even in adverse agement of the entire measuring apparatus.Maintaining the steel wire in constant tension byclimatic conditions, e.g. intense rainfall, fog and

Fig. 5. Example of correlation between rainfall and displacement recorded by an extensometric device during the 1993 critical phase.

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means of the apparatus shown in Fig. 4 was practi-

cable up to a stretch limit of some tens of metres,

beyond which complete resetting of the extensome-ter devices was necessary.

Moreover, considering that each extensometric

apparatus placed within the body of the slide was

connected to a data logger with an electric cable

for data transmission, intense movements have

often broken the cable, thus necessitating the

reconnection of the cables, sometimes under

difficult conditions.

Other problems arose from the tilting of meas-

uring equipment in the course of slide movements,

leading to misalignment between it and the wire.

Also, in these cases, the original equipment posi-tion needed to be reset to ensure its correct

functioning.

Despite all this, the system has allowed contin-

ual checking of the movements and the forecast

of several critical situations that have led to the

collapse of substantial sections of the slope

(Fig. 5).

2.1.2. Directional bar

This is a simple instrument consisting of a bar

that, when suspended from a cable runningthrough a channel, records the passing of

mud/debris flows and avalanches (Fig. 6). TheFig. 6. Directional bar suspended from a cable running throughworking principle is based upon the tractional andthe Tessina Valley.

rising effect of the moving mass on the bars.

Groups of two and three bars were installed

upstream of Lamosano and Funes, respectively,

working as alarm systems (Fig. 1). They were meter was added to each group of bars, which

constantly measured the altitude of the flow sur-calibrated so as to set offan alarm signal by the

closure of a mercury switch only if they were tilted face, to confirm and back-up the alarm signal from

the directional bars.more than 20 from the vertical position for more

than 20 s. This threshold was introduced as a way Thanks to the simplicity of the apparatus con-

struction, no particular operational problemof reducing false alarms usually due to the effects

of frequent gusts of wind for which this valley is occurred. In addition, the possibility of false alarm

was further reduced due to checks by means ofrenowned.

The entire system was connected to specifically developed software, set up to record

only the passage of mud flow deeper than 34 mmeasuring/checking stations, which sent the vari-

ous alarm signals to an operative centre. and with a velocity of at least several metres per

minute, such as the present situation of danger forIn fact, the installation of several directional

bars, placed at reasonable intervals from one the inhabited areas.

Notwithstanding this, false alarms took placeanother, allowed registration of both the mud flow

and its velocity so that the varying risk levels of on two occasions, due, on one occasion, to excep-

tionally intense and persistent winds that kept athe area could be detected. An ultrasonic echo-

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Fig. 7. Giau Pass landslide: location of boreholes and evolution scheme of the slope (reference to cross-section CC).

bar tilted for more than 20 s, and, on another dAmpezzo) has been monitored since the early

1980s.occasion, to the passage of a fallen tree carried by

the flow, whose branches reached and displaced a On 25 April 1988, the landslide suddenly col-

lapsed, and its evolution was automaticallybar. However, in both cases, the remote control

of data produced by the echometer permitted recorded (Fig. 7).

From a geological viewpoint, the layers crop-immediate rectification.

Maintenance and checking of the bars function- ping out in the area surrounding the landslide

belong to the Werfen Formation (Scythian). Thising caused considerable difficulties because they

were hanging from a cable some hundred metres formation is composed of a well-bedded, reddish,

marly limestone, alternated with marls and sand-in length, positioned transversally over a valley.

Retrieval and repositioning of the instrumentation stones. These beds dip upstream with an average

slope angle of about 30. The overlying morainerequired delicate and laborious manoeuvres owing

to the inaccessibility of the slope. deposits, which form the landslide body, are com-

posed of a fine silty-sandy matrix, including gravel,

pebbles and blocks of dolomitic limestone from

the uphill dolomite peaks. Sandy and clay lenses3. Giau Pass landslide

are also present.

Since the first investigation, the landslide hasA landslide of some 500 000 m3 made up of

morainic material and located in the Dolomites appeared to be a complex slide (Skempton and

Hutchinson, 1969), characterised by a large graben(near the Giau Pass, about 30 km from Cortina

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area in the upper part of the slope and by a

translation of the main mass occurring on an

almost sub-horizontal slip surface. Late in 1981,the main scarp, located at the back of the graben,

had already reached a greater extent: 6 m in height

and 250300 m in length. Further enlargements of

the main scarp occurred in subsequent years, until

the complete slope collapsed, during which the

scarp reached a height of 20 m, and the landslide

body underwent a translative displacement of the

same intensity, before stopping against the oppo-

site slope.

The maximum thickness of the landslide body

was about 50 m, whereas its maximum length was

150200 m. The slip surface was identified as beinglocated at the contact surface between the moraine

deposits and the Werfen marl bedrock, at least as

regards its rectilinear extent. Its curvilinear extent

is completely contained within the moraine depos-

its (Fig. 7).

Hydrogeological and geotechnical investiga-

tions have been carried out since 1982.

Laboratory tests were carried out on the finer

matrix of soil samples collected either from bore-

holes or directly along some parts of the slip

surface, which was formed after the 1988 collapse.The matrix of most of the soil samples (mainly

constituted by coarse-grained moraine materials)

taken from the slip surface was classified as a clay

of intermediate plasticity (with a PI of 15.3 Fig. 8. Correlation between rainfall, groundwater level andinclinometric displacements over 7 years of recording. The land-17.4%). Residual shear strength tests carried outslide collapsed on 24 April 1988.on the finer matrix material (passing through the

B40 sieve) gave wr values ranging between 15

and 17.

tions and inclinometric displacements are quite

evident.

The automatic instrumentation installed in the3.1. Instrumentation and data obtained

landslide body, in particular the electric pressure

transducer and a steel wire extensometerFollowing the first investigations, the slope was

gradually equipped with standard and automatic (Corominas et al., 1999), with a sample interval

of 2 h, allowed a detailed recording of two criticalinstrumentation: three Casagrande piezometric

cells, two electric pressure transducers, two deep- events concerning stability (Angeli et al., 1990a).

Movements of a few centimetres were measuredseated steel wire extensometers, four inclinometric

tubes, a rainfall gauge, a snow gauge and an air in April 1987, along with piezometric variations

of about 1.5 m, whilst in the same month of thethermometer. A summary of standard hydrological

and kinematic data collected over 7 years on the following year, the slope collapsed, with a displace-

ment of more than 20 m but with a rise in piezo-slope examined is shown in Fig. 8. The correlations

between precipitation depth, piezometric eleva- metric level of only 0.5 m (Fig. 9).

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Fig. 9. Comparison of data recorded during two critical hydrological events in 1987 and 1988.

In both cases, the data established that the reprocessing of piezometric data was therefore

taken into account. Considering the solution ofbeginning of movement happened before the

recorded piezometric peak and in particular that the differential equations of water flow (Hvorslev,

1951) inside the piezometric apparatuses, a recal-it coincided with the flexure point of the piezomet-

ric rise curve, rather than with its maximum value. culation of piezometric data was obtained.

The fundamental input data considered were:This behaviour was attributed to the large

volume that piezometric apparatuses used (open diameter and length of the filter, diameter of the

piezometer and permeability of the ground.standpipe piezometers). The electric pressure trans-

ducers were placed inside a 80 mm diameter pipe, The application of a computer routine has thus

permitted recalculation of the new curves relativeand this fact meant that a significant time-lag

occurred between the levels of the GWT inside to the piezometric level variations. These new

piezometric elevation values show an oppositeand outside the tube.

trend to those recorded (Fig. 10).

The April 1988 situation (measured peak value:3.1.1. Piezometers

In order to correct the observed time-lag and a 27.82 m; calculated value: 19.7 m) was

assessed as being much more severe than thepossible cut-off of the maximum peaks, a

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quence, there has been a significant reduction in

the number of buildings in the old part of the

village.The stratigraphic sequence of the cliff is com-

posed of thick or massive layers of marls and

calcareous marls. In some places, thin layers of

clay are present.

Structurally, the area is characterised by a

monocline dipping SE from 20 to 30, which is

affected by several systems of faults and joints,

generally subvertical. Steep escarpments partly sur-

round the monocline above.

Under these conditions, large blocks of rock,

set free by the systems of joints and by the presence

of the above escarpments, tend to slide on the

bedding planes (Figs. 11 and 12).

Since 1990, a number of horizontal and vertical

boreholes have been drilled and equipped with

geotechnical instrumentation. In particular, BHH2

and BHH3 boreholes were drilled horizontally, at

a slightly different elevation, in order to install

several fibre-glass extensometric bars of different

lengths (20, 40, 60, 80 m). The direction of the

boreholes and the different lengths of the extenso-

metric bars were established in order to check theFig. 10. Measured and calculated piezometric head during two extent to which the differential movements in thecritical events (1987 and 1988).

jointed rock mass could take place. They reached

a stable zone well below the inhabited area.

This type of extensometer is generally used inprevious situation occurring in 1987 (measured civil engineering for monitoring retaining struc-peak value: 26.94 m; calculated value: 22.3 m) tures such as bulkheads, large supporting walls or(Fig. 9). The new calculated piezometric curves,

inner tunnels for controlling the stability of rockshowing peaks preceding movement and much

walls. It has been successfully used for the firsthigher than the values measured, fully explained

time for monitoring a landslide using considerablythe occurrence and the collapse mechanism.

long bars.

In the case of Sirolo, rectangular-section

(35 mm) bars were installed and anchored atdifferent depths in order to identify the precise4. Sirolo landslidelocation of the slip surface. The bars are protected

by rigid sheaths, with their upper extremity pro-The village of Sirolo, located on top of a steeptruding from the ground surface, on which amarly cliffalong the Adriatic sea, several kilome-manual centesimal comparator is placed. Thetres south of the harbour of Ancona (Centralmanual comparator may be substituted by a linearItaly), is widely affected by instability phenomenadisplacement transducer, which, when connected(rock falls, toppling of rock pillars, slides) (Angelito a suitable logger, allows continuous movementet al., 1990b, 1991, 1996a; Angeli and Pontoni,recording.1995). The area is highly tectonised and subject to

recurrent high-intensity earthquakes. As a conse- Concomitant with the installation of extensome-

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Fig. 11. Map of the Sirolo landslide area showing the location of the equipped boreholes and the control works.

tric bars, a rain gauge and several electric transduc- Each head of the anchor is connected to the

others by steel-reinforced beams, running horizon-ers of pressure and displacement were connected

to automatic data loggers. High-precision geodetic tally along different contour lines in the slope.

Moreover, horizontal tubular drains weresurveys were also carried out.

Observation of all the data collected has permit- drilled at different locations along the slope

(Figs. 11 and 12). Sometimes, they reached ated a first hypothesis on the main landslide mecha-

nism. In particular, a huge block-type slide length of 150 m. Often, they allowed the rapid

outlet of large quantities of water coming fromoccurring on a subhorizontal plane was identified.

Fluctuation of the groundwater table above a fixed the opened cracks existing at the back of the

landslide body.threshold was identified as being responsible for

periodic landslide reactivations. A sufficient number of critical hydrological situ-

ations recorded before and after the remedialRemedial measures consisted of a series of very

long pre-stressed steel anchors and subhorizontal measures have revealed a significant reduction in

the piezometric peaks in the landslide body,tubular drains.

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Fig. 12. Schematic cross-section of the Sirolo landslide body: bedding planes dip 20 to 30 toward the observer.

accompanied by a cessation of movement 5. Final remarks(Fig. 13).

A particularly innovative feature of this experi- From the three cases briefly illustrated, the wide

spectrum of situations in which even simple moni-ence was the use of extensometric bars made of

fibre-glass, which proved especially useful for the toring systems can be employed advantageously is

evident. Usually, the best possible results can onlydetection of very small movements. This equip-

ment is, in fact, able to detect movements of the be obtained if the position of the sensors is mean-

ingful, and this can result only from a previousorder of hundredths of a millimetre and is thus

particularly useful for rock-slide applications. The in-depth understanding of the phenomena,

obtained through collection and analysis of all theeffectiveness of these apparatuses was also con-

firmed both by the long activity period (about previous documentation. Geological conditions

must also be accurately investigated.10 years) and by the accuracy of the measurements

obtained, thus guaranteeing the continuity and Another very important aspect is the choice of

parameters and frequency of sample data collec-reliability of the observations, which would not

have been possible with the equipment normally tion. It is indeed important to optimise data collec-

tion so that large amounts of poorly significantused (inclinometers, wire extensometers).

Precision is required in its installation, and the data can be discarded.

In the case of alarm systems, it is very importantmeasurements themselves should be taken by

expert personnel. At present, checking is carried to avoid false alarms, to choose appropriately the

critical thresholds for the various parameters takenout manually. The choice of a sufficiently accurate

automatic data logger for continuous data collec- into consideration, and possibly to use alternative

measures if such thresholds are exceeded.tion that also permits manual checking at the same

time is currently being examined. Finally, it should be pointed out that the instru-

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Fig. 13. Relationships between monthly rainfall, piezometric rise and landslide movement; piezometer BH1 is representative of the

landslide body, whereas piezometers BH8 and BH11 represent the piezometric behaviour of the stable area; BHH2 and BHH3 indicate

horizontal extensometric bars; A1, A2 and A3 refer to the three systems of anchors installed, and D1, D2 and D3 refer to the three

sets of tubular drains.

mentation used in the study of landslides is a Acknowledgementspowerful tool of investigation, but in no case

This paper is part of the CEC Environmentshould it be the aim of research. In this case, theProgramme Project NEWTECH (ENV-main objective (i.e. the knowledge, study, observa-CT96-0248) New Technologies for Landslidetion and management of landslides) would be

lost. Hazard Assessment and Management in Europe.

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Angeli, M.G., Gasparetto, P., Pontoni, F., 1996a. Long termReferencesmonitoring and remedial measures in a coastal landslide

(Italy). In: Senneset, K. (Ed.), Landslides. Proc. 7th ISL

Angeli, M.G., Gasparetto, P., Pasuto, A., Silvano, S., 1990a. Trondheim, Norway. Balkema, Rotterdam, pp. 14971502.Analisi di un caso di frana in materiali morenici (Val Fioren- Angeli, M.G., Pasuto, A., Silvano, S., 1996b. Landslide hazardtina, Belluno). Mem. Sci. Geol. 42, 129154. in high mountain areas: some case histories (Italy). J. Geol.

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