Geotechnical Study Area G19 Seychilienne, near , Isère,

GEOTECHNICAL STUDY AREA G19

SEYCHILIENNE, NEAR GRENOBLE, ISÈRE, FRANCE

Plate G19 Séchililienne, near Grenoble, France

1. INTRODUCTION

Between and Seychilienne (close to Grenoble, Isère) within the sector of the "Ruines of Seychilienne", the Alpine valley of the Romanche (Plate G19) is home to a large slope movement which affects the south side of Mont-Sec (at a height of 900m) and threatens the "A" road (route national (RN)) 91 Grenoble to Briançon (Figure G19.1). This instability, which was reactivated during the winter and spring of 1985, showed the existence of a short term slide hazard of a volume of 2-3 million m³ likely to involve the partial destruction of the 'bed' of the Romanche and the severance of RN91. In order to guard against this threat, a diversion for the RN91 and a deviation channel for the Romanche were created in 1986.

Geological and geo-mechanical studies have shown that this instability is developing on the majority of the rocky slope, up to the ridge, and is mobilising a volume of several tens of million m³. This leads to the hypothesis of a major mass slide hazard which could involve the complete destruction of the valley. 2. THE STUDY AREA

2.1 Location

Figure G19.1 shows the site of the Seychilienne Ruines. The south flank of Mont-Sec, affected by a large slope movement, dominates the right bank of the Romanche 20 km upstream of Grenoble.

2.2 Geological and geomorphological context

1 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

The Romanche valley, at the same level as Seychilienne (364m), re-crosses the external section of the Belledonne massif along an east-west line (Figure G19.2). This massif belongs to a Hercynian platform and forms a hillock with a flattened surface (pre-Triassic peneplain of the mountain tops), with coverage towards the south of 2,200 m to the Cross of Chamrousse and about 1,000m in the direction of Laffrey.

The shaping of the valley (Figure G19.3) came about, inter alia, through pressure from the Würm glacier of the Romanche, followed by torrential activity after the glacial retreat. As a result, the average angle of the slopes is steep (35°- 40°) and often shaped in successions of projections and depressions which correspond to glacial erosion on areas of variable hardness.

Lithological and structural outline

The slope comprises mica schists, where white mica predominates, and more compact levels of amphiboles. Threads of quartz can be seen with traces of mineralisation and the zone is bordered to the east by darts of coal-bearing schists, of the Triassic and the Lias. A pronounced sub-vertical metamorphic foliation affects these levels in a mostly meridian direction, i.e. orthogonal to the valley.

This body of discontinuity does not impact upon the stability of the rock mass in this case. From a fracturing perspective the slope is severed by a group of largely east-west fractures, which are clearly identifiable in the morphology, and by undulations to the south of which the most remarkable is visible on the site as an area of quartz and mineralised veins (Pb-Zn) punctuated by former mining works. These undulations are similar to those that are well marked on the Vizille 1:50,000 map, to the east of Seychilienne in the more compact, and therefore more fragile, layers.

Detailed study of the natural deformation at the level of the slope of the "Ruines" of Seychilienne

The groups of fractures and their chronology

Despite the visible confusion, the fracturation of the slope (Figure G19.4) is organised around several easily identifiable groups:

· sub-vertical north-south metamorphic layer; · the north-south and slightly east-west fractures belong to an old system (primary era), reactivated in normal faults (extension phase from the beginning of the second era). This behaviour is particularly marked for the east-west group, engendering fractures associated with a strong angle towards the south, and therefore more or less parallel to the studied slope; · the north-east - south-west and north-west - south east fractures of which a detailed analysis of the sense of displacement makes it seem as though they began (or re- started) in a largely east-west compression (Alpine phase starting at the Tertiary). One will note that the effect of this phase of tightening was to reactivate the opening of the former east-west fractures described above; · the mainly shallow east-west fractures (in the order of 30°), sloping towards the south, which show clear characteristics of vertical traction cracks (subhorizontal behaviour - roughness of surfaces of weak rupture lateral extension). This type of fracture is a well- known consequence in the region of former east-west tightening; · a group of fractures in quite different directions (from north-south to north-east - south- west), but having in common a slope angle of 50 to 70° towards the east or the south- east. It may be a result of the deformation of old subvertical fractures in the former east- west phase of tightening and therefore systematically showing an inverse fault behaviour.

2 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

Impact on the mechanism of the current deformation of the slope

The list above shows that there is not one general type of fracture across the site, which is largely parallel to the slope, and that could easily lead to the basal discontinuity for a simple slope model.

The east-west fractures, stimulated in the last resort by tension, were open at that time and therefore very sensitive to gravitational actions linked to the presence of the slope. As a result they cut off the mass movement in vertical sections separated by large holes (as a consequence of the post-glacial evolution of the slope).

The mass studied is clearly delimited by the two combined fractures belonging to the north-east - south-west and north-west - south-east group, which is enough to show their importance vis à vis the problem.

2.3 Climate and hydrology

Rainfall

Annual rainfall varies between 900 and 1,100 mm. These figures are relatively moderate taking into account the average altitude. There is no particular trend in the recent fluctuations in the rainfall, but there was a period of three years showing much lower levels ending in 1985.

Snowfall has only been measured at the site since 1992, when a specific meteorological station was installed.

The catchment

The topographical catchment of the slope of the Ruines is in the order of 2.2 km² whilst the surface of the unstable zone is 0.72 km² and its catchment in the order of 1 km². No information is available on the effective hydro-geological basin likely to be feeding the unstable zone.

The infiltration conditions

On the whole, the mica-schists of Belledonne are impermeable rocks. As a result, given the steepness of the slopes, infiltration levels should be negligible. All the same, in the unstable zone and close to the surface, the overall hydraulic conductivity of the massif is high due to the disruption which results in the discontinuities opening up.

The extrusions

The only surface extrusions found were either in the unstable sector or close to the catchment. As the Romanche's alluvium is about 100 metres thick, it is likely that the water circulating within the slope flows into the groundwater of the Romanche below the level of the mass of fallen rocks at the foot of the slope.

2.4 The Responsible Authorities

The Commune of Saint-Barthélémy-de-Séchilliene is responsible, as in the cases of Roquevaire and Criel sur Mer, for the provision of safety for people and information of its citizens (Study Areas G16 and G20).

The DDE 38, is in charge of the upkeep of the road, rail and waterways network, and should ensure continuity and safety within it. In order to do so it calls on the Centre for the Technical Study of Facilities in Lyons (CETE1) and to the IRIGM.

1 Centre d'Etude Technique de l'Equipement de Lyon

3 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

3. THE THREAT OF SLIDES TO THE SEYCHILIENNE RUINES - STUDY OF THE PHENOMENA

3.1 Background

Historic events

Historic research teaches us about both the slope of Ruines and about the section of the Romanche valley situated just below.

The slope of the Séchilienne Ruines

The instability of the slope is verified by multiple witnessed events, since the 17th century at least. The place names confirm the very bad agro-forestry-pastoral conditions in this sector in former times.

Precise information about slides is rare. In 1726, a rock of about 5 metres in size fell into the bed of the Romanche at an area called La Fausette under the Ruines of Séchilienne. In 1762, three block of about 15 cubic metres detached themselves from the mountain of Rivoirbands and completely obstructed the road. The same scenario happened again in 1794. The road was damaged in several places. In 1833, a series of enormous rockfalls occurred along the Ruines, which intercepted the road each time. The last big recorded fall happened during the night of the 23rd to 24th February 1906.

In the 19th century, the old road which crossed the area was regularly destroyed by rock falls and slides. Difficulties in its upkeep and the decline in land use meant that it was soon abandoned in favour of other communication routes.

This slope was also subject to mining activity. The extraction of zinc dates from the middle of the 19th century ending in 1914. A certain number of tunnels and shafts still cross the area. No plan of these works has been discovered to this day, despite rumours of it which even detail the existence of haulage tunnels between the concession of the Ruines and that of Pierrerousse located on the other side of the mountain.

The Romanche valley

The information stems on the one hand from the consecutive works on the route and maintenance of the RN91 and, on the other hand, from the containment trenching works of the Romanche.

Between the beginning of the 17th Century and the middle of the 18th Century, the small road between Grenoble and Briançon was built at the foot of the slope of the Ruines where there are continual rockfalls. At the same time, the first containment works eased the concerns of the inhabitants of the Île de Falcon. From the beginning of the 19th Century and despite major works, the Romanche regained ground on the right bank. The floods of 1856 and 1859 destroyed almost all of the RN91across this entire sector. The route was re-established at the foot of the slope using large excavation works between the Rivoirands and the Ruines. The first continual containment projects for the Romanche were launched on this occasion.

Recent events (since 1937, the date of the first available aerial photography mission)

From the five series of IGN aerial photographs available at the Bron CETE (1937, 1948, 1956, 1982, 1987) a net change in the activity of the site can be seen following the winter of 1984-85.

· On the 1937 plate, the zone of the East corridor at that time (which limits the lower East frontal zone to the east) is marked by a labyrinth of ravines. · Between 1937 and 1948, development of this sector can be seen on the upper section. · On the 1982 plate, a strong development of the East branch of the East corridor can be seen, which was well individualised from then on.

4 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

· In 1987, the entire zone of the 1937 ravines is active and disrupted, which signifies a net acceleration in the degradation of the sector.

This change in activity on the site coincides with block falls observed on the RN91, which attracted the attention of this section in 1980. During site visits, a development was noticed between 1980 and 1984, followed by a clear worsening of the situation after the winter of 1984- 85.

3.2 Analysis of movements

Regular monitoring of different measurement systems installed on the site enabled a first approach to the phenomenon through an analysis of more than 16 months of measurements.

This confirmed the hypothesis of a deformation of the entire active zone (Figure G19.5), increasing over time, with fractures opening at the eastern end through active dislocation.

All the measurements show that the movements and deformations follow an average direction of north - north-west - south - south-east, normal to the general direction of the major fractures and north-east - west - south-west, emphasised by very marked, characteristic depressions in the morphology of the slope. The comparable range of movement and deformations measured on the entire unstable area and on the rear fractures far from the façade is the result of an instability mechanism of the whole slope affecting part of it.

The displacement vectors are at a slight incline (Figures G19.6 an G19.7) (20 to 25° on average on the upper section and 5 to 10° on the lower section). These inclines are very much less than the slope angle and indicate the existence of a large horizontal deformation. This is the result of a large expansion of the mass in the direction of movement, linked to pronounced subsidence of the depressions corresponding to major fractures.

The range of the movements measured in the median part of the slope, which is lower than for other points, seems to indicate the presence of more resistant areas controlling the current development of the instability. Situated in the general line of the movement, the presence of these hard points leads to a concentration of pressure in this area and a risk of violent rupture and the detachment of a large, total or partial mass slide of the moving zone.

The existence of a clear cyclical component in the opening of the fractures in the frontal zone (speeding up in winter and spring, slowing down in summer and autumn), is the result of the influence of hydraulic control from the mechanism for change. This is confirmed by the study of the correlation between the speeds of deformation and the cumulated contributions of water from the massif (data from the meteorological station of Mont-Sec corrected by evapotranspiration and thawing, see Figure G19.8). This study demonstrated an integrated response for the lower frontal zone corresponding to the medium to long term effects, showing the complex hydrological behaviour of the site.

3.3 Hypotheses on the rupture mechanisms

The analysis of the massif's fracture systems reinforces a hypothesis that a compression mechanism existed already. This is particularly evident from the important development of fractures orientated north - north-east - south - south-west linked to the general direction of the deformations and collapses, which cut obliquely across the slope, the size of cumulated displacements, and the orientation and size of movements,

The homogeneous nature of the whole area and the distribution of the displacements and deformations are a result of instability in the mass affecting a fairly large area.

Consideration of structural data about the massif and the characterisation of former and current movements lead to the consideration of a complex internal rupture mechanism of the mass involving gravitational compression and expansion. The existence of different massif

5 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

fracturation systems, and, in particular, sub-vertical or steep slope fractures forms a likely element in the development of a mechanism of this type.

The observation of the influence of climatic factors confirms the existence of a relatively deep rupture mechanism affecting a large volume of rocks. This probably results in the influence of the hydraulic conditions of the massif, of which no evidence of water can be seen on the slope.

4. THE SOCIO-ECONOMIC IMPACT OF SLIDES

4.1 Local importance of the RN 91

The RN 91 runs as far as Briançon via the Lautaret pass and thus provides access to the Montgenèvre pass i.e. to Italy, but is also of the utmost importance for economic and tourist reasons for the département of Isère.

It also serves:

· the valley of Eau d'Olle (i.e. Allemont), the Grand'Maison barrage and the Glandon pass); · the town of Bourg d'; · access to the important ski resorts of Alpes-d'Huez and and to the Vénéon valley, i.e. to the mountainous centre of Bérarde.

The average annual traffic flow is about 8,000 vehicles / day but, during busy periods, notably during the winter school holidays, this number can exceed 20,000 vehicles / day.

There is no diversion for this route that can cope with this level of traffic, the only useable 'B' roads are narrow and tortuous and can only be used for short periods when absolutely necessary.

4.2 Sociological impact

The DDE 38 ordered by the CETE, completed technical studies of socio-economic research in order to better determine the problems.

Faced with a phenomenon, of which the seriousness is not always well understood, and above all because of the slowness of the compulsory purchase and victim compensation procedures, public opinion is very negative and some campaign groups have been launched.

5. RECONNAISSANCE, THOROUGH EXAMINATION AND IMPLEMENTATION

5.1 Towards protection of users in the short to medium term

Succession of events since the hazard was taken into account

In 1980, following the block falls, a first visit to the slope undertaken at that time, resulted in a statement that the stability of the most active façade was precarious and this resulted in road protection measures being recommended. These measures consisted of a reshaping of the foot of the slope, with the installation of a landing pit and a barrier of mobile material about 3 m high along the edge of the road.

Further visits took place in 1984 which showed that that the instability had continued to develop and that notable changes could be seen compared to what was observed in 1980.

During the harsh winter of 1985, the phenomenon accelerated suddenly and many block falls occurred, with considerable impact on the protection zone. During the month of March the slope investigations had been so worrying that they led to the immediate installation of permanent monitoring, with the possibility of interrupting the flow of traffic using traffic lights. They also provided the opportunity of generally raising awareness of the size of the problem:

6 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

the masses in question were such that they could have not only caused the RN 91 to be completely cut off, but also to cause a dam across the valley which would be likely to impede the normal flow of the Romanche and to flood the hamlet of Ile-Falcon situated within the Commune of Saint-Barthélémy-de-Séchilienne, immediately downstream of the dangerous area. Moreover, a collapse phenomenon could have repercussions as far as Grenoble.

Some studies were commissioned as soon as possible with two main aims:

· to define short term protection works for the RN 91; · to define a hazard in the medium or long term and the steps to take to protect against it.

The short-term protection works project was developed quickly enabling the works to be carried out in June and July 1985 (creation of a landing pit along the edge of the road with a wall of prefabricated concrete blocks and a detector net).

In terms of the hazard in the medium to long term, it rapidly appeared necessary to plan to divert the road to the left bank to get it as far away from the toe of the slope as possible and to develop a new river bed for the Romanche in order to ensure that it continued to flow in the event that the current bed became damaged.

Taking into account the detection of a lateral extension of the hazard and/or the money required for further studies, and of the development in the movement of the area being monitored, it had been necessary to pursue permanent monitoring and to install a diversion with two temporary ends.

This diversion has been in service since July 1986. It comprises two temporary works of clearing the Romanche, a new route of 1,100m protected by a 20 m high mound of earth and the new riverbed of the Romanche, which will only be used if needed. The total amount for the works carried out in 1985 and 1986 was about 26 million francs.

Emergency measures

The large growth in hazards linked to block falls, which have been frequent on the RN 91 since the end of the winter of 1985, led, in parallel to the study of this phenomenon and the clarification of more general solutions, to the implementation of emergency protection measures comprising: · the installation of continuous monitoring of the slope using permanent surveillance (functioning 24 hours a day) installed on the opposite bank of the Romanche, allowing traffic to be interrupted on the RN 91 to the right of the zone exposed if a fall were to happen; · the creation of a continuous structure at the base of the slope, along the RN 91, to protect against the block falls, in the form of a protection pit completed downslope by a raised structure or a concrete block wall, depending on the zones, with a net on top to detect the rock falls.

The dimensions of the emergency protection works were the subject of a computerised simulation study of the characteristic trajectories to be taken into account in designing the structure. The analysis of the spread conditions of the fallen rocks of numerous block falls on the RN 91 was undertaken using numerical models, called "trajectory envelopes", developed by the Ponts and Chaussées Laboratories. Fine-tuning the calculation models benefited from numerous observation data relating to the rock falls of the winter of 1985.

The spread study was supported by a photogrammetric survey at 1:1,000 in the area concerned, providing a detailed reconstruction of the morphology of the slope, which was used to support the analysis of the general line of spread and the main characteristics of the probable trajectories. These elements enabled the installation and sizing of temporary emergency protection works to be defined, which were undertaken along the length of the RN 91 during the spring of 1985.

7 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

5.2 The Reconnaissance Techniques - The in-depth investigation

Two distinct types of reconnaissance were used on the site, on the surface and at depth:

· geological mapping (1:1,000 for the lower frontal east zone, 1:5,000 for the entire slope on the right side of the Romanche, structural surveys of the outcrops and the tunnels (those on the 585 m hill spread over 240 m in a N-S direction), photo-interpretation. · digging of a 240 m reconnaissance tunnel in 1984, orientated N-S, on the 710 m hill, and corresponding structural survey.

The cost of works (installation, track, security, digging, bending) was 2.5 million francs.

For this project, it would have been preferable to have had a series of five vertical boreholes from the surface, the cost of which would have been equivalent but very difficult to use.

In-depth investigation and monitoring operation in the active area

The aims of the in-depth investigation :

There were two distinct aims of this operation:

· the analysis of movements and the identification of mechanisms to improve the understanding of the phenomenon. · setting up a monitoring infrastructure for the site giving rise to the management of safety aspects, within the framework of protecting the zones exposed to rock falls. In addition to site instrumentation, this infrastructure comprises the implementation of a pre-warning system defined in a safety plan, in case of a detected worsening judged as dangerous by the experts responsible.

The operation of the thorough investigation :

The geodesic network

From the end of the winter of 1985, a first in-depth investigation operation was set up, allowing the movements of the façade of the unstable zone to be determined and monitored using an in-depth geodesic examination.

The monitoring network comprises:

· a reference framework made up of seven support points using concrete pillars installed on the slope of the left bank of the Romanche, as well as on the right bank along the RN 91 (Figure G19.9). · a collection of ten targets with reflective prisms installed on the active zone and fixed on the characteristic rock flows.

The measurements work on the basis of triangulation and electo-optic measurements of the distance of the reference points, from the observation pillars whose stability is periodically checked by the control measures of the support network.

This network currently comprises 35 measuring points and has been broadened to make use of optical and satellite geodesic methods (GPS). The precision of the determination of the points allows the displacement vector components to be determined with an uncertainty of less than 1 cm.

Cost of establishing the initial network: 500,000 francs.

8 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

Cost of the complementary equipment: 300,000 francs. Cost of a complete geodesic campaign: 150,000 francs.

The tele-extensometer network

After the month of June 1995, a network of extensometers was installed to cover the main fractures. This system was completed as the observations progressed. It currently comprises 26 sensors installed on the main fractures of the site and relayed by radio beacons to the site terminal of Thiébauds (Figure G19.10). Permanent monitoring is undertaken by the Lyons Operation Centre (installed at the Bron CETE). This terminal interrogates the beacons every hour during normal times and every quarter of an hour in times of crisis (the parameters of the measurement process can be modified from the Lyons Operation Centre) but it also operates the technical control of the network and controls the measurements, which are compared to the projected values set on the previous three days. If an anomaly occurs or the threshold is exceeded, the terminal sends a warning to the constraint staff (voiced telephone messages, pager).

Cost of equipment and installation: 2 million francs. Cost of maintenance and daily monitoring: 300,000 to 400,000 francs / year.

About fifty topometric reference points

These are installed in the main fractures and allow the control and recalibration of the remotely monitored extensometers. The measuring equipment consists of a set of (portable) Invar lines, regularly calibrated at the LPC in Lyons, and a graduated dynamo- meter (set to a standard line tension). The length of the measurement bases can reach 50 metres, the accuracy is between 0.1 and 0.5 mm depending on the length of the base.

Cost of a base: 500 to 15,000 francs exclusive of tax depending on the type (bracket etc.). Cost of the complete case of measuring equipment: 50,000 francs exclusive of tax.

5.3 Study of mass rock slide hazards

The analysis of geological, structural and geo-mechanical data, the study of the characteristics of the deformations and the movements brought to light by the thorough investigation, and the consideration of the hypotheses relating to the rupture mechanisms, all confirm the existence of a major mass rock slide hazard affecting a large volume of the currently destabilised area, in the order of 2 million to 3.5 million m³. Consideration of this major hazard in the research calls for global protection solutions at the bottom of the valley.

A study of the simulation of a mass rock slide of a volume of 2 million m³ with an extension of up to 3.5 million m³ was carried out to analyse the impact of such a slide and the consequences arising from it (obstruction of the bed of the Romanche, interception of communication lines, destruction of buildings etc.). This study was carried out using numerical models developed by the LPC allowing consideration of the mass which plays an essential role in the propagation mechanisms.

The analysis of the results of the simulation calculations show that the development of a mass slide involving a volume of 2 million m³ results in a partial filling up of the bottom of the valley, covering the RN 91 and obstructing the bed of the Romanche, which would be pushed back to its left bank. The existence of a natural channel remaining at the foot of the slope of the left bank, showed by the study, enabled the planning of the re-establishment of links through this passage upstream and downstream of the valley: through the creation of a reserve channel for the Romanche and the diversion of the RN91.

6. CONCLUSIONS

9 Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

From a technical point of view, the study of the phenomenon is very satisfactory on the whole. Nowadays, the continuation of the reconnaissance tunnels regularly provides us with new information about the geology and brings us closer to understanding the phenomena.

However, the problem of the impact on the citizens remains less well managed. Compulsory purchase procedures have not been applied.

10 Figure G19.1 Map showing the site of Séychilienne Ruines. The south flank of Mont-Sec, affected by a large slope movement, dominates the right bank of the Romanche 20 km upstream of Grenoble. Figure G19.2 Detailed geological map of the most active frontal zone. The volume is estimated at 7 million m 3. Figure G19.3 Geomorphological map of the slope of the Séychilienne Ruines. The unstable zone, marked with dotted line, is notable from the large EW to NE-SW depressions corresponding to large tectonic fractures which control the deformation of the slope. SE 1200 NW Mont-sec Collapse crevasses Crevasses d'effondrement

1000

800

zone de developpement de la rupture 600 Rupture development The Romanche zone 200 m La Romanche 400 Altitude (m) Scale (m)

Figure G19.4 Schematic cross-section of the slope of the Seychilienne Ruines. The large, almost vertical, tectonic fractures, penetrate many hundreds of metres. Figure G19.5 Thorough geodesic examination of the south slope of Mont-Sec. The measured planimetric movements of Feb. '88 to Dec. '95 confirm the existence of an active movement affecting the whole upper section of the massif. They are relatively homogeneous with the exception of the most active frontal area (to the east) where they reach 20cm / year. Scale of displacements Echelle des déplacements

200 mm

Figure G19.6 Cross-section of the displacement vectors measured by geodesy. The low inclination of the displacement vectors is one the arguments in favour of a complex internal rupture mechanism of the massif with expansion. The general orientation N150 of the displacement vectors is not that of the largest slope but rather perpendicular to the direction of the main fractures. SLOW PERIODS / PERIODES LENTES Opening (in mm) / Ouverture (en

FAST PERIODS / PERIODES RAPIDES

Drainage time (in years) / Temps écoulé (en années)

Figure G19.7 Extensometric measurements of the crevasse opening at the head of the most unstable frontal zone, from 1985 to 1996. The seasonal variation of rapid periods (30 to 65 mm / month) and slow periods (10 to 25mm / month) indicate hydraulic control of the deformation.

Water contributions cumulated over 45 and 10 days / Apports hydriques cumulés sur 45 et 10 jours Average daily speed / Vitesse moyenne journalière

water contributions / apports hydriques Speed (mm/v) / Vitesse (mm/j)

speed A22 / Vitesse A22 Contributions (mm of water) / Apports (mm d'eau) Contributions (mm of water) /

Time (in months) / Temps (en mois)

Figure G19.8 Correlation between the noted average daily speeds and the water contributions cumulated over 45 and 10 days. The cumulated water function using two delay periods gives the best correlation coefficients with displacement speeds of 45 and 10 days. This purely numerical observation pinpoints how the phenomena is controlled by hydrology; the component over 45 days represents the charging of the massif by indirect feeding whilst that of the 10 days shows the infiltration of rainwater into the unstable massif. Figure G19.9 Installation of the geodesic support framework. The measurements of the 35 points of the network, undertaken annually from 1988 to 1996, have been repeated more frequently since 1996 due to the installation of an automatic theodolite. The precision by which these points are determined means that the uncertainty of the displacement vector components is less than 1 cm. Geotechnical Study Area G19 Seychilienne, near Grenoble, Isère, France

Plate G19a Séychillienne Landslide, near Grenoble

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