geotechnical study area g17 salins-les

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Geotechnical Study Area G17
Salins-les-Bains, Jura, France
GEOTECHNICAL STUDY AREA G17
SALINS-LES-BAINS, JURA, FRANCE
Plate G17 Salins-les Bains, Jura, France
SUMMARY
Under the auspices of the European Programmes HYCOSI, then LIFE, the Béline site at SalinsLes-Bains in Jura was the subject of a particularly in-depth study and instrumentation field
programmes. There were two aims to this monitoring:
·
·
On the one hand to gather the data necessary to feed numerical infiltration models;
On the other hand to test the specific measuring sensors as well as the systems for
acquiring and treating data in a restrictive environment.
In addition, the experiences gained should help to provide an improved methodology to help
manage hazards. The aim of which would be, using a multi-criteria approach (aim, cost,
restrictions etc.), to propose the best solutions for instrumentation and setting up a warning
network.
An identical piece of work has already been undertaken at the Boulc-en-Diois site, the
knowledge acquired from this, show the need for complete autonomy of the system and
therefore the possibility of transmission.
The site at Béline was set up with the aim of obtaining a completely autonomous system that
can be interrogated from afar. The closeness of the children's home in Béline is an important
reason for installing the equipment on the site. It makes it possible to ensure the supply and
repatriation of data via modem in good conditions.
Except for the sensor malfunctions, which are evident since they were installed, and the
lightening incident, the majority of the interruptions in readings result from the connections
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Salins-les-Bains,
Jura, France
(deterioration or ageing). It is therefore of prime importance to move as far away as possible
from wire links, which are the most vulnerable elements at Salins.
1.
INTRODUCTION
Under the auspices the LIFE project (Coastal Change, Climate and Instability) and formerly the
HYCOSI (Hydrometeorological Change On Slope Instability) European Demonstration
Programme, the Salins-les-Bains site in Jura was the subject of a particularly in-depth study
and programme of field instrumentation.
The instrumentation installed was directed by the experience gained by BRGM at the Boulc-enDiois site, which had been the subject of investigations whose main results were written up by
Chassagneux D. and Leroi E. (1995). The distance from the site, the great speed of the
phenomenon and the succession of malfunctions made data acquisition from the site very
difficult. The solution is to turn to transmission by telephone line. This should limit data
losses and allow breakdowns to be detected remotely.
This is why the principal aim at Salins-Les-Bains was to set up a completely autonomous
system, which can be interrogated remotely, whilst responding to the technical requirements of
the HYCOSI and LIFE projects which are:
·
·
On the one hand to gather the data necessary to feed numerical infiltration models;
On the other hand to test the specific measuring sensors as well as the systems for
acquiring and treating data in a restrictive environment.
2.
THE STUDY AREA
2.1
Location
The site at Béline (Plate G17) is located at the southern exit of the town of Salins le Bains in
Jura, opposite and to the east of the outskirts of Bracon, below Belin fort. Figure G17.1 is an
extract of the topographical map no. 3325 for the area at a scale of 1:25,000 by IGN.
2.2
Geology of the slope
From the 1:50,000 geological map of Salins-les-Bains (Figure G17.2), the vegetated slope
where the study site is located extends across medium and upper Lias formations, about 100
metres thick and constituted primarily of marl with several local thin intercalations of more or
less clayey limestone.
In fact, the geotechnical study undertaken in 1977 verified the presence of a marl sub-strata
across the whole site with, locally, a superficial covering of modified old fallen limestone rocks
covered by a clay matrix.
The sub-stratum is represented by blue marl, compact and inflexible at depth, becoming plastic
and much less resistant in the upper part where they are changed.
Locally, the blue marl are covered again by the more or less plastic yellow-ochre clays,
containing the variable sized, modified blocks of fallen limestone. This formation can be up to
several metres thick.
2.3
Morphology of the slope and the study site
The topographical map of the left bank of the Furieuse, upon which the Beline site is located,
shows that there are movements across the whole slope.
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A more detailed analysis of the contours, for studying the hydrological behaviour of the slope,
makes it possible to pinpoint the sloping sub-catchments, from the route of the ridge lines
(Figure G17.3). Areas of water concentration can then be seen (zones A and B) and zones for
which slides will develop, a priori, following the parallel flow lines between them (zones C and
D).
The Béline site is located in zone C half way between the first limestone ridges and the
Furieuse river. More precisely it is situated between the Baud road and the "La Béline" hospital
1
buildings (or Specialised Children's Home (M.E.S. )).
It covers a surface area of 5,000m and extends between the spot heights 370 and 390 on the
French National Grid. The vegetated slope upon which it is situated goes up to a height of 450
to 500m. It is dominated by a very steep, wooded talus with fallen rocks, then by a limestone
cliff which marks the edge of the first Jura plateau upon which Fort Belin is built, at a height of
approximately 580m.
The topographical surface of the Béline site (Figure G17.4) breaks down into three zones:
·
·
·
2.4
a zone forming a gentle shelf in the upper part of the ground (slope at 5º);
a quite pronounced central slope zone (22º) which has been modified considerably
following the stabilization works;
a gentler slope zone (14º) which extends to a carpark delimiting the lower part of the site.
The general context of slope instability at the Salins site
The Salins site is very sensitive to landslides of which there are a number of traces on the
slopes undermined by the Furieuse, the river which deserves its name and whose floods are
aggravated by its karstic feeding regime.
The slides generally develop on the cover of the deterioration of the Charmoutien marl. In 1974
a slide delayed the construction of a building in Le Sicon, at the exit of Nord de Salins and on
2
the right bank of the Furieuse. In 1978, under the auspices of the French ZERMOSS
programme, a map of the Zones Exposed to Ground and Underground Movements was
produced by BRGM (Figure G17.5). The study area was identified as a medium to high risk
area, with the following commentary "The range of the movements and their probability of
occurring remain foreseeable; an in-depth study is necessary prior to all development projects."
3.
HISTORY OF THE SITE
J.P. Asté (1996) undertook a detailed assessment of the work produced on the site up until
1995. The history presented below is a brief overview of it and shows the essential points for
understanding why this site was chosen in developing the research programme launched by
BRGM.
1
2
Maison d'Enfants Spécialisée
Zones Exposées aux Mouvements du Sol et du Sous-sol
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3.1
Operational construction phase (1984-1985)
The Project
At the beginning of 1984, on the Béline site, the company GFTC (Grands Travaux de Franche
Comté), built a hospital for diuretic children (Specialised Children's Home) on behalf of Franche
3
Comté Regional Health Insurance Fund (CRAM ).
The architectural plan comprised two main buildings, A and B, developing largely along the line
level and facing south, with a certain number of annexed buildings (see Figure G17.6).
The preliminary study undertaken by B3G in 1977 provided the geotechnical context and
recommended:
·
·
a slope drainage system,
a type of foundation.
The difficulties encountered whilst undertaking the project
In April 1984, the foundations of building B were built without any major difficulties, as well as
the drainage trenches, and the earthworks necessary for undertaking the foundations of
building A and the construction of the carpark were also started.
In April, during these earthworks, the beginning of a slide appeared on the upper part of the
site, at the edge of the Baud commune road.
In spring 1984, the Veritas office asked BRGM to put four inclinometric tubes in the boreholes
produced under the control of B3G and to make a stability estimate to enable a possible
reinforced structure to be defined (Figure G17.7).
In May, the first inclinometric results showed that the movement was far from being stabilized
and ran to a depth of 4 to 5 metres.
A rapid re-interpretation of the penetrometric data gathered by B3G led to the following
recommendations:
·
·
undertaking a complementary in-depth study before pursuing the earthworks in the
western half of the site;
revision of the earthworks project on the basis of the considerations which will be
explained further below.
The strategy adopted for following the works
The concept proposed rested on a certain number of central themes:
·
·
·
to drain the material, to limit the earthworks and to prevent all further deformation,
to reinforce the area that had already slipped,
to try to stabilize the slope as far as possible upslope.
Notably, a suggestion was made to put in place a series of groynes draining across the slope,
installed from west to east in the shelter of the preliminary reinforcement which, in addition to
their drainage function, will improve stability by their mask-weight function and by the friction
that they will bring.
3
Caisse Régionale d'Assurances Maladie
4
Salins-les-Bains,
Jura, France
The drainage groynes will be spaced about a ten metres apart, 3 metres wide and 4 to 6 metres
deep, so that they fit one metre in the ground at Rd > 50 bars under the surface of the most
likely potential slide. Furthermore, the architects looked at transferring building A to the east of
building B.
Implementation of the in-depth study during the phase of works
Implementation
From April to June 1984, the identification and monitoring of the slide surface at depth
remained concentrated around the signs of instability on the surface I1, I2, I3, I4.
A second network was put in place during the month of July to ensure effective control of the
deformations during the phase of works. This network comprised four additional points: I5,I6,
I7 and I8.
The size of the measured deformations lead successively to a progressive deterioration of
certain tubes through which, in the end, the measuring torpedo could no longer pass.
th
A third network was therefore installed on 19 September, I9, in the place of I8 which had
become unusable, and I10 higher under the CD, in the badly drained area mentioned above.
I10 should have provided particularly important information. The rupture surface identified
there was deeper than others (7.50m instead of 4m).
st
A fourth network was installed on 21 November, with multiple aims:
·
·
·
·
to refine the monitoring in the zone between the main collector and runner no. 4, the zone
where the ability to build needs to be preserved: I11 and I12, on either side of I5.
to test the behaviour of the built-up area: I13 and from the zone of transfer: I14.
to characterise the drainage role of the trench installed behind the building already
constructed by using the interstitial pressure cells on either side of this trench: PI15 and
PI16.
to refine the monitoring of the role of water by installing a tube for measuring the content
of neutronic probe. (tube no. 21, between I5 and I6).
Results of the in-depth study
From the beginning of the summer, a striking compatibility was apparent between the rainfall
and the movements. This trend, which was later confirmed was one centimetre of rain
corresponded to one millimetre of movement.
The further detailed study at the beginning of 1988
After the works ended at the beginning of 1986, and the MES was put into operation, the
readings from the inclinometers were taken almost tri-annually until 1988.
In fact many of the inclinometers installed since 1984 had shorn off or been lost. On 15/07/86
only three control inclinometers were still accessible.
In 1988, an additional inclinometer I16 was placed between the curtain pile and the building, at
a depth of 20 metres, and a system of taking the readings remotely was installed on the
surface, following the plan shown in Figure G17.7. The results are brought together on the
diagram on Figure G17.8. The evolution of the vector displacement module is shown on this
document as a function of time (or at least of relative displacement between the heads of the
stake and the rear façade of the building).
5
3.2
Salins-les-Bains,
Jura, France
The scientific phase (1987 - 1989): aims and equipment
The aim of the research given to Christophe Lainé by BRGM in 1987 was part of the
development of the research and works methodologies for preventing hazards due to ground
movement.
Three particular areas of reflection and modelling were pursued:
·
·
·
the simplification of the influence of rain on mudslides on a colluvial slope;
planning the stability of a slope sensitive to vertical re-supply by rain;
the conception of drainage systems.
To meet these objectives, Christophe Lainé worked simultaneously on:
·
·
·
the equipment on the Béline site;
the production of a reduced laboratory model;
the use of a numerical simulation models.
The interest of the Béline site against these objectives
Since 1984, the installation of a network of inclinometers has allowed it to be shown that a
relationship exists between rain and the displacements of the area in movement.
Figure G17.8 shows the correlation between the average cumulated displacements and the
cumulated rainfall for two inclinometers located in the high part of the slide and mid-slope
respectively. The two curves can be likened to a slope line equal to 0.13 and equal to 0.11.
These results show the good correlation that exists, for the Béline site, between rain and
movements in the unstable area. They confirm the importance of water brought by the rain and
which infiltrates into the slope.
Figure G17.8 highlights certain irregularities on the two rain-displacement curves (B3, A6, B6,
C6). They correspond to the movements in the unstable zone for low rainfall, and show the
complexity of the rain-stability relationship of a slope.
All these observations confirm the interest that the Béline site can show in studying the way in
which infiltration from rain behaves, and its influence on the distribution of interstitial pressure in
the slope. In addition, the Béline site was already well known, and it presented the advantages
of a slope that has been studied and altered by man, and where artificially governed
meteorological conditions have been created between the runners by a symmetry of conditions
at the limits on the upper meters.
Experimental site equipment
Taking into account the relative symmetry of the site, and the observations made on the outflow
drained by each runner, the equipment in the study area is concentrated between the first two
drainage runners. (Figure G17.9).
The piezometers
·
The open piezometers
Five open piezometers were installed on the Béline site, all equipped with a pressure
sensor connected to a MADO system. The choice of the depth at which to install the
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·
Salins-les-Bains,
Jura, France
piezometers was guided by the information given by the "rustic" piezometers installed in
1984 in the penetrometric probe holes.
The closed piezometers
Seven closed piezometers (PIEL probes) were installed on the Béline site, all equipped
with a pressure sensor connected to a MADO system.
The tensometers
Two tensometric measuring bases were installed on the slope:
·
·
the first (base T1) is made up of five tensometers installed on a line across the slope
(NGF spot height: 378.40 m).
the second tensometric base (base T2) is made up of four tensometers installed on a line
across the slope (NGF spot height 383.10 m).
The outflow gauges
Two outflow gauges were installed in the collector gathering the water drained by the four
drainage runners.
The outflow gauge, equipped with a rectangular entrance, was located at the level of the
connection between the collector of the spurs and the channelling of the streams. It measures
the outflow drained by the four runners.
The outflow gauge (D2) was installed compared with a drainage runner PD3.
General installation of the equipment
Figure G17.9 shows a "birds eye view" of the study site and the installation of the different
reading sensor. Also see Figures G17.19 and G17.20.
3.3
First results (1987 - 1989)
This decribes of the main results of the instrumentation phase initiated in 1987.
Succinct statistical study of the rainfall readings
Daily and monthly rainfall readings for the study period
Figure G17.10 shows the daily rainfall readings obtained with the help of a rain gauge during
the study period.
Comparison with the decade's rainfall
This statistical study is based on the cumulative monthly values of rainfall. Figure G17.10 also
shows the minimum, maximum and average values of monthly rainfall for the decade (July
1978 to July 1989). The cumulative monthly values for the study period are also shown.
The analysis of the monthly rainfall for the period measured shows a deficit in rainfall for certain
months of the year 1988-89. However, the study of cumulative annual rainfall shows that the
th
study period is not the driest (ranked 5 of the decade). In addition, the anomalies observed on
the Béline site occurred in the context of rainfall which was no less unfavourable than that of the
year 1988-89 (the movements were triggered by the slope ground works). The year 1982-83
appears like an "anomaly" in the decade; it corresponds to a period when a number of large
landslides were observed on the banks of the Furieuse.
Analysis of the piezometric readings
7
The readings taken using the open piezometers
Figure G17.11 shows the fluctuations in the piezometric levels obtained using the open
piezometers. The analysis of the average values of the levels of the piezometers after
stabilization shows large differences from one apparatus to another, which can only be
explained by the presence of the drainage runners. It seems, therefore, that the soil is
extremely heterogeneous.
Analysis of the piezometric readings using the closed piezometers
Figure G17.12 (1 to 7) shows curves of the development of the interstitial pressure measured in
seven points from the study area using the PIEL sensors. The range of the variations in
interstitial pressure is large in every respect from the study area. For the piezometers whose
readings cover a complete cycle (humid period and dry period), the maximum range varies
between 200 mbar (PI1) and 400 mbar (PI4 and PI121).
Contrary to information given by all the open piezometers (with the exception of PZ2), the
readings undertaken with the assistance of the PIEL probes indicate however, significant
fluctuations in the interstitial pressure, over what can be very short periods (PI0).
Increased pressure values can be found in the lower part of the slope, as well as at depth, both
at 10 metres (PI3) and at 5 metres (PI29). Taking into account the installation of the PI29 and
PI3 sensors, and assuming a homogeneous atmosphere, the pressure readings demonstrate
the presence of a climbing vertical hydraulic gradient. In effect, the hydraulic charge in PI3 is
always more than that measured in PI29.
The largest pressure fluctuations are observed in the central part of the study area (PI2, PI4
and PI21) where the piezometers are located in the area influenced by the drainage runners.
Analysis of the tensometric readings
The gross data of interstitial pressure (measured on the tensometers) provides two pieces of
information:
·
·
similar to the readings using the open and closed piezometers, the evolution of the
interstitial pressure as a function of time at different depths. This development, related to
the daily rainfall, made it possible to study the influence of the rain on the variation of the
pressure in the superficial layer.
in addition, in showing the distribution of the pressure on a vertical axis, for each reading
date, it is possible to study the sense of the slides in the hypodermic layer. In order to
obtain this pressure diagram, the hypothesis is put forward that the tensometers of each
base are placed sufficiently close to one another that the values given by each of the
tensometers can be projected over a vertical common axis.
The study of the distribution of interstitial pressure on a vertical axis leads to a plausible
physical explanation for the results of the tensometers. There is a stratification of the
superficial layer into two sub-layers, separated by an area of low permeability. The effect of this
buffer makes it seems like there are two water tables on top of one another, one of which could
be charged up. Figure G17.13 shows a diagram of the behaviour of the buffer area following
the state of saturation of zones 1 and 2.
Interpretation of the pressure readings
Hydrological behaviour of the superficial zone
The study of the evolution of pressure in the first two metres of the ground made it possible to
show the presence of two saturated cross-slope slides, which superimpose themselves
following the stratification of the area. Furthermore, the influence of the rain on the level of
each of these flows was quantified.
8
The existence of an area of lower permeability, at a depth of 1 - 1.5 m has the effect of stopping
the low level infiltration which spread from the surface of the ground. The mudslides, which are
centred on the superficial area, are for this reason mainly oriented in the same way as the
slope.
Study of influence of rain on the repartition of pressure at depth
The analysis of the three characteristic periods of a rain gauge alternation makes it possible to
roughly sketch a schematic diagram of the hydrological behaviour of the different zones.
The mid-slope area: (PI2, PI4, PI21, PZ2), has appeared as the area for which the deep slides
are more influenced by the rain.
A schematic diagram of the section of the ground in the form of several reservoirs each one
corresponding to a layer of different hydrological behaviour (Figure G17.14). The superficial
layer constitutes a reservoir which needs to be divided into sub-reservoirs taking into account
the observations made earlier.
For the schematic diagram of the deep area of ground (between 6 and 11 metres), the reservoir
simulating its behaviour is defined by its inlets and outlets, in the following manner:
A1: the inlets are connected at the surface by a network of discontinuities which crosses the
surface area. The input of the water into the reservoir is rapid (several days after a large rainfall
event) and does not require saturation of the upper ground layers. However, a minimal
quantity of rain is necessary for the water to reach the reservoir (I mini = 50 mm in 2 or 3 days).
D1: in dry periods, the main outlet is located at a depth of 6.70m. The level in the reservoir
never rises above this value. The D1 outlet is connected to the drainage system.
D2: in dry periods, the level in the reservoir is located below D!; this outlet becomes ineffective.
The drainage of the slope can be represented diagrammatically by a D2 outlet at a depth
greater than 10m.
To understand the influence of the phenomena observed in the superficial area on the
behaviour of the reservoir R3, it is essential to monitor the interstitial pressures at an
intermediate level (between 2 and 6 metres).
There is an open piezometer at the base of the slope (PZ29) and a closed piezometer (PI29)
installed at a depth of 5.5m.
Unfortunately, the behaviour of this section of the study area is very different from that of the
mid-slope area. The information provided by these piezometers cannot be compared with the
readings of PZ2, PI4, PI2 and PI21.
The area at the base of the slope: (PI3, PZ3, PI29, PZ29) behaves in a way that is more difficult
to interpret. The piezometers installed at 5.5m note the variations of pressure and the
piezometric level in phase, but without relation to the rainfall: the pressure increases up until
the end of the month of June 1989.
The comparison with the PI29 and PI3 readings, show very different pressure developments
according to depth.
One can attempt to explain the behaviour of this section of the slope by drawing a schematic
diagram of the section of the ground in the form of three reservoirs (Figure G17.15).
The R1 reservoir shows the superficial layer of the ground.
The R2 reservoir corresponds to the layer of ground between 2m and 8m. It is fed by the base,
by R3, and empties by draining slowly into the slope.
9
The R3 reservoir schematically shows the deep zone constituted of sound marl. The water
content of this reservoir is loaded and the supply comes from upstream.
The zone located at the top of the slope (PI, PZ, PI).
There is a lack of trustworthy data to roughly sketch a schematic diagram.
Conclusion
The analysis of the development of the interstitial pressure over the period of one year, and in
different points of the study site, showed the complexity of the resulting phenomena. The main
results obtained relate to the superficial and the deep (10m) area of the Béline site. They
confirm the importance that must be attributed to the behaviour of the first metres of ground,
which are where the large slides directly linked to rainfall events are located. The influence of
the groundwater levels, which appear in wet periods, on the distribution of pressure at depth,
was identified but not clearly defined. In effect, due to the complexity of the hydrology of the
site, a unique slide scenario could not be demonstrated for the study area. The qualitative
study of the phenomena relating to infiltration from rainfall into a colluvial slope made it possible
to highlight three main phenomena corresponding to the successive steps for infiltration in the
ground of the volume of water brought by the rain:
·
·
·
progressive increase in the degree of saturation in the form of an infiltration front;
appearance of a transversal subsurface slide which feeds a saturated infiltration front
which progresses vertically towards the deep groundwater;
resealing of this infiltration front with the capillary fringe which shows up as a recharge of
the deep groundwater level.
The visualisation of these phenomena on a site with equipment which is characteristic of the
slopes subject to slow slides which are reactivated by large rainfalls was only partially realised.
In effect, the saturation stage of the hypodermic layer (between the ground surface and about 3
metres depth) and the apparition of a subsurface transversal slide were clearly identified.
Variations in interstitial pressure at depth were also observed, which were well correlated with
the rainfall conditions.
The recharge of the deep groundwater level occurs simultaneously at different points in the
slope, the concept of the water tower (resupply from upstream) is not applicable in the case of
the studied slope. However, due to the complexity of the hydraulic behaviour of the site at La
Béline and the lack of readings in the zone located at a depth of between 3 and 8 metres, it was
not possible to prove that the re-supply of the deep groundwater level happened vertically in a
way that was described above. The progression stage of the saturated infiltration front, fed by
high groundwater, was not demonstrated. This is why a slide simulation model was used,
which made it possible to quantify the influence that high groundwater could have on interstitial
pressure at depth.
The transfer time calculated for a slope configuration similar to the site at La Béline, led to the
conclusion that if there was a vertical progression of the water from the superficial area of the
ground, it would not be in the form of an infiltration front affecting the whole of the unsaturated
area, but through the intermediary of a network of fissures connected to the ground surface. In
addition, the lower section of the study area shows a hydraulic behaviour that is completely
different from the rest of the slope. It is therefore necessary to have a complementary system
to be able to put forward a diagram of the slides on the La Béline site.
10
4.
INSTRUMENTATION OF THE SITE UNDER THE AUSPICES OF THE RESEARCH
PROJECTS
Under the auspices of the European Programme HYCOSI then the LIFE project, the SalinsLes-Bains site was the subject of a particularly in-depth site and equipment study. The principle
of the works undertaken, conforming to the aims of the HYCOSI programme, is to better
comprehend the way in which water circulates and is concentrated in the slopes.
4.1
The interest of the site
Under the auspices of the HYCOSI project, the BRGM report, Chassagneux D., Leroi E. (1995)
outlined the state of knowledge acquired about equipment on the Boulc-En-Diois site (26). The
main problem concerned the remoteness of the site and above all the lack of transmission
which prevented all remote diagnosis of the site. In order to push the research further, it was
therefore necessary to develop transmission techniques. The aim of the site at La Béline was
therefore to develop a data collection system that could be completely remotely interrogated.
The observations made in section 5, about the good correlation that exists, for the La Béline
site, between rain and movements in the unstable areas show the importance of water brought
by rain and which infiltrates into the slope. The interest that the La Béline site could provide for
studying the way in which rain infiltration occurs, and its influence on the distribution of
interstitial pressure on the slope is evident. In addition, the general conditions are a good way
of testing the material for monitoring and surveying the water in the soil in order to obtain
warning systems.
4.2
Complementary studies
Geophysics
This field investigation was undertaken by BRGM in two parts:
·
Localised field investigation. Two types of investigation were undertaken:
- geo-electric methods;
- seismic refraction;
·
Ongoing geo-electric readings. The reports by Baltassat and Charbonneyre (1995) and
from Mathieu and Miehé (1996) brought together the results.
Geo-electric methods
The geo-electric readings along the slope profile (electric trail) were undertaken using two four
point systems of different lengths.
·
·
Seismic refraction
Upstream of the communal road, nine seismic bases of 100m in length were developed
using explosives as a source of the seismic waves. Downstream, due to the proximity of
the hospital, other houses and public highways, it was not possible to use explosives.
Two bases of 60m and six bases of 120m were developed using a hammer.
Spontaneous polarisation
Five profiles measuring the differences in potential were produced in relation to the
contour lines one metre apart. Using the electostatic rod, six lines of current were
followed. The complete results are presented in report 95.1023 (Cazin, 1995).
Laboratory boreholes and trials
The laboratory boreholes and trials shown in Figure G17.16 result from an analysis of the
geophysical results and include the contributions of the Spontaneous Polarisation field study.
11
The summary of the investigations is provided in the document no. 95.269/177 (Demartinecourt
1995). The boreholes put in place and the equipment installed are as follows:
·
·
·
·
·
·
core sample borehole C1 at 22m equipped with a pressure cell at 19.5m,
destructive borehole D1 at 12.5m equipped with a pressure cell at 4.5m,
destructive borehole D2 at 11m equipped with a pressure cell at 7.5m,
destructive borehole D3 at 30m equipped with an inclinometer,
destructive borehole D3 at 11m equipped with a pressure cell at 9m,
destructive borehole D4 at 13m equipped with a pressure cell at 6m.
In the laboratory, the following trials were undertaken:
·
·
·
·
4.3
17 water content,
4 Attenberg limits,
2 densities,
2 triaxial consolidated drainage trials (CD).
Instrumentation
A series of photographs of the site follow this description.
Generalities
The principle of the works undertaken (Figure G17.17) is, on the one hand, to better understand
the way in which water circulates and concentrates within the slopes and, on the other, to test
the instrumentation systems in an operational way. To do this, sensors are used that are
spaced across the site, with the aim of characterising the following over the course of time:
rainfall, the saturated environment and the non-saturated areas. In addition, the deep ground
movements (landslides, flows) are followed on a monthly basis by an operator using an
inclinometer. Table 1 summarises the equipment installed.
Location
Clucy
La Béline
Sensors
1 bucket rain gauge
1 bucket rain gauge
La Béline
5 interstitial pressure
cells (TELEMAC)
6 HUMLIOG stations
(IRIS Instrument)
2 panels of 24
conductivity
electrodes
Inclinometer
La Béline
La Béline
Site
Collection system
Madotel
OSIRIS central
exchange
OSIRIS central
exchange
OSIRIS central
exchange
SYSCAL
Use
Marseille via modem 1
Operator
Operator
Table 1 List of the equipment at the Salins-les-Bains site
Conductivity electrodes (SYSCAL)
The SYSCAL R1 Plus is an automatic resistivity gauge specially developed by IRIS Instruments
for prospecting electricity in surface areas. It also makes it possible to study both the variations
in resistivity as a function of the depth (electric boreholes) and the variations in resistivity along
a profile (electric profile or trail). IRIS Instruments also developed a so-called "intelligent" multielectrode guiding system. The electrodes managed in this way cover a large surface by getting
rid of the movement of the four traditional electrodes.
12
The SYSCAL R1 Plus brings together in the same container: the supply source, the transmitter,
making it possible to reach 400V of tension, and the receiver. It generates the current,
measures the tension between the potential electrodes and shows the value of the apparent
resistivity. It may work with a variety of electric systems such as the Schlumberger and Wenner
profiles and boreholes, the gradient and the dipole.
From the resistivity gauge and using the RCM (Remote Controller Module) command module, it
is possible to manage the electrode network. Once the electrodes are installed and connected,
the resistivity gauge automatically switches on the emitting and receiving electrodes following
the predefined sequences. It was interesting to associate these functionalities with the
repeated readings looking to continuously follow the electric resistivity.
SYSCAL is used to detect the water circulation in the ground and to follow over time the
variations in the water conditions of the soil. The permanent installation of a SYSCAL system
on a ground movement site for detailed investigations creates a certain number of constraints,
notably:
·
·
·
·
on the safety of people vis-à-vis the human environment,
the condition of the electrodes,
the supply and automation of the SYSCAL system,
remote access.
Taking account of the fact that the SYSCAL system is a generator of current, the maximum
tensions injected in the ground to measure the apparent resistivity was limited to 48 V. By
contrast, in order to prevent all risks of accidental short-circuiting by a person (an electrode in
each hand when injecting), two steps were taken: to bury all the cables from the electrodes and
to reject an injection electrode of the measuring system in infinity by separating their respective
cable routes.
Under the auspices of the equipment at the Boulc-en-Dois site (Baltassat and Mathieu, 1995),
a first version of SYSCAL was developed to allow data to be remotely collected and stored
automatically. The experience showed that for sites where conditions are very aggressive,
equipment was likely to degrade, this did not carry enough of a guarantee for an in-depth
operational study. A more advanced conditioning of the electrodes in a pipe system, which
existed at that time, should have made it possible to overcome most of the problems
encountered, but not allow the same flexibility of use as SYSCAL, notably for the changes in
the geometric configurations of the profiles. With regard to Salins-les-Bains (Mathieu and
Miehé, 1996) the system evolved further to only allow electrodes and cables on the site and
bring together the most sensitive components in a protected area. The electrodes which could
be addressed were totally rethought in this way and all electronic equipment was brought
together within "mutinode" boxes functioning on the same principle as the multiplexer boxes
used with the OSIRIS data collection system.
Due to parasite phenomena, SYSCAL can only be fed by batteries and not directly from the
mains. However, using such energy and wanting to get rid of handling and battery charging, a
system was specially installed by IRIS instruments. It consists of a battery charger connected
to the mains and activated in non-peak hours using a timer.
Once the autonomy of the system has been ensured, remote interrogation is possible in a
sustainable manner. However, in the first instance, a telephone call allowing data to be
collected or stored should last until the end of the operation at risk of interrupting them. And
yet some of the data collection configurations last more than one hour. In order to use
telecommunications less, a box with a relay system was developed for keeping SYSCAL in
working order at the end of a telephone call, for a length of time that can be regulated by a
computer programme.
With these various developments, SYSCAL became autonomous at that time allowing
continuous monitoring of the site.
13
Data transfer
Sensors and data acquisition systems
In order to show the variability of the rainfall on the site, it was decided to install two rain
gauges. If the first is located in the lower part of the unstable slope, close to the children's
home, the second was installed in the Clucy commune on the limestone plateau. Comprising a
unique sensor, the rain gauge is relayed to a MADOTEL. The frequency of the sampling is
purposely very high to finely characterise the rainfall signal. This configuration (Table 2) is
autonomous as it is supplied by an internal buffer battery and recharged by an external battery.
In addition, the modem link allows it to be guided and data to be collected remotely from a site
further afield (BRGM in Marseille).
Close to the Béline children's home, a control cupboard shelters in particular an OSIRIS data
acquisition exchange. With the aid of three multiplexers, 24 sensors occupying 42 lines can be
guided. Data collection across all of the sensors lasts about 25 minutes, the frequency of the
sampling is fixed to three hours for all the lines. Data collection is maintained using a modem
link from Marseilles, using the software Concept developed by IRIS instruments.
·
·
·
The rain gauge
The rain gauge installed on the Béline site is strictly the same as that at Clucy. In order to
calibrate the sensor, it is necessary to know that the gross value measured is a
cumulative counter. The tension, expressed in mV is deduced from the gross value by a
factor of 1.90735E-2. Then, knowing that the absence of bucket rotation is also 3mV and
that each rotation represents an increment of 1mV, and it is necessary to convert the
gross rainfall value.
The humilogs
The humilogs are the capacity sensors measuring the volumetric water content. Each of
the 18 humilogs installed on the site occupies two lines from the OSIRIS data exchange.
In effect, as explained in detail in annex 3, the volumetric water content reading using a
ground dielectric permittivity requires a temperature reading to put in a correction. The
gross value is translated into a current reading across all the lines by a factor of
4.768376E-5. The intensity "I" obtained in this way is taken into account in the calibration
parameter A1 to obtain the apparent permittivity and the temperature.
The interstitial pressure cells
Five interstitial pressure cells of the type TELEMAC CL1 (0-2 or 0-5 bars) are connected
to the OSIRIS central data exchange. It is a question of reading a vibrating thread in a
maintained way. The gross value is directly the frequency of vibration of the cell thread,
expressed in one hundredths of Hertz.
The pressure, expressed in bars, is calculated using the equation (1) using the
extensometric coefficient K, obtained during the calibration in the laboratory, and the
initial frequency N0, in Hertz, measured on site using the installation of the cell.
P = K· (N² - N0²) (1)
On the OSIRIS central exchange, IRIS Instruments and TELEMAC propose the
relationship (2) to obtain the relative extension (in _m/m). In order to directly have the
pressures and taking into account the units (hundredths of Hertz), modified parameters
following the equation (3) are introduced.
-5
2
2
_L/L = 625.10 K_(N -N0 ) (2)
-5
-4
P= _L/L_ 1/625.10 .10 (3)
Data storage
The management and treatment of the data collected requires that it be rigorously stored and
treated. A certain number of programmes facilitate the management of data. The name of the
files whose role is explained elsewhere follows strict conventions. The letters AA, MM, JJ
14
Salins-les-Bains,
Jura, France
which can be used in the descriptions representing respectively the two numbers of the year,
the month and the day, of when a version of a file or a directory was saved.
The data collected in Clucy by the Madotel only concerns rainfall. Under "Mado2fr", the
interface software with the Madotel, the downloading of data creates gross value files
(extension .BRU) then transforms into text format (extension .MAD). A programme then allows
it to be transferred into Excel format and the rainfall to be displayed.
The choice of a sampling step reduced to 10 minutes allows an appreciable level of precision,
but ends up in storing a large amount (52,560) of data each year, a very large proportion of
which is useless (no rainfall). The role of the programme "Ote0.xlm" is to suppress all the
periods of time without rain and to only save the necessary data.
It is then sufficient to insert this data, which is pre-treated, into that already collected, as and
when it is collected. A calculation sheet shows the data relating to each rain gauge
independently or to two rain gauges together.
For the Béline site:
The Concept software developed by IRIS Instruments allows the management of the OSIRIS
central data collection exchange. Each time data is saved a file is created, for which the
nomenclature used is "\OAAMMJJ\". Concept allows the selective creation of a text file. In
selecting a path or a group of paths of identical size rainfall, humilog or even pressure sensor
data are isolated. In this way a programme allows the transfer to an EXCEL format.
5.
ASSESSMENT AND COMMENTS ON DATA COLLECTION
5.1
The results of the system
Meteorology
Having tried and tested the homogeneity of the rainfall recorded on the site with that of the
French Meteorological Office in Arbois, the latter serves as a reference and can be favourably
substituted for the instrument readings. In addition to the daily readings, the potential evapotranspiration has been provided by the French Meteorological Office since July 1995.
Given the homogeneity of the readings recorded at Clucy with those of the French
Meteorological Office in Arbois, this sensor was not set up again and has since been
dismantled.
The rain gauge at Béline was started at the end of February 1996. However, this sensor
regularly diverts to the rain gauge in Clucy and the French Meteorological Office in Arbois. This
way of measuring is due to the periodic clogging up of the rain gauge receptacle.
Electric panels of the Béline slope
In February and March 1997, following the developments described elsewhere which made the
system practical and autonomous (Monge and Leroi, 1997), daily measurements were taken
from the electric panels installed under the auspices of the Salins-les-Bains site system
(Mathieu and Miehé, 1996).
One year after their installation, the electric panels showed abnormally high resistivity. These
results can be explained due to discontinuities in the cabling due to their deterioration
(accidental severance, rodents, ageing etc.). After a site inspection, tests and the reparation of
this device, although this argument is undeniable for the furthest electrode, it has not yet been
proved for the electrodes of the electronic panels.
15
Salins-les-Bains,
Jura, France
The operational interest of monitoring the resistivity in the clayey ground showing weak
contrasts of resistivity over time remains therefore to be demonstrated. To do this it is
necessary to look for a much longer life from the cables, in particular by protecting them more
effectively, or even to follow an experimental application under the auspices of an irrigated site
that is examined on a daily basis.
Non-saturated zone
State of affairs
In order to characterise the non-saturated area, there is a network of 18 water content gauges.
Spread across 6 reading stations, these HUMILOG gauges enable the site to be covered as a
whole, and for the effects of depth to be taken into account locally. Each of the gauges has two
lines: temperature and water content.
At the end of 1996 a certain number of gauges no longer worked, showing a reading that was
either wrong or dubious. New breakdowns (several gauges failing on the same date) occurred
in 1997. Table 2 shows the balance.
The 18 HUMILOG gauges can be classified in three different states; those that work, those out
of service and those which give a signal that is not always trustworthy. Table 2, classifies the
gauges using this nomenclature at the beginning and end of 1997. The number of gauges out
of order for 1998 is of course the sum of those out of order in 1996 and the breakdowns which
occurred in 1997.
Gauges
Working in 1996
Unreliable readings in 1996
Our of order in 1996
Unreliable reading in 1997
Interruption in 1997
Working in 1998
Table 2
Temperature line
13
3
2
5
5
6
Water content line
11
3
4
4
6
4
Summary of the water content gauge network
From the table a number of particularly important simultaneous breakdowns can be seen, due
in part to an accidental severance of the cables. In addition, the number of unreliable readings
increases a little, leading to a change in the gauge or a progressive deterioration of the wires
(insulation). However, in this category some gauges seem to be irreparably out of service and
others seem to work intermittently. All these things explain the low number of gauges still
working in 1998 after functioning for two years:
·
·
6/18 i.e. 33% for the temperature line;
4/18 i.e. 22% for the water content line.
Aside from the malfunctions of the gauges, evident practically since they were installed, the
majority of the problems seem to come from the connections. It is therefore of prime
importance to get rid of wire links, the most vulnerable part of the water content gauge system.
Results
Each gauge measures the temperature of the ground and the volumic water content. The
results are the most interesting when water contents are compared between 1996 and 1997.
In general terms, the ground temperature plays a buffer role:
·
the annual range of temperatures is much higher the nearer you get to the surface;
16
·
Salins-les-Bains,
Jura, France
significant daily oscillations are evident on the gauges that are closest to the surface.
To the right of a reading station, the volumic water content data shows the different behaviour
of the surface levels compared with a similar response. In the same way, both the range of the
variation in water content and the saturation or desaturation kinetics can be also be different.
Periods of saturation are suspected when the variation in water content is sudden and goes
back to a limit level.
It is interesting to consider the water content column for one sensor. The similarity between
years of the variations (range and kinetic) shows the reproducibility of the readings, confirms u
posteriori their validity and demonstrates the proven seasonal behaviour.
In this way, in both 1996 and 1997 there was a very marked reduction in water content during
the summer period for the following sensors:
station no. 3; H155 at 0.45m,
station no. 6; H225 at 0.5m,
station no. 6; H225 at 0.75m.
Although it is not possible to compare across the two years, this behaviour is also discernible in
the three captors at station no. 5.
During the summer, rainfall is no longer effective as it is counterbalanced by a strong potential
evapo-transpiration. This summer de-saturation and autumn re-saturation appears later
however in 1997 (station no.6; H225 at 0.5m). The potential evapo-transpiration which was
very high in June and July 1996, and in August 1997 explains in part this gap during desaturation. The earlier re-saturation in 1996 is due to the autumn rain which came earlier in
1997.
Saturated zone
Appraisal
In the saturated area, the five cells installed at the end of January 1996 satisfactorily followed
the developments in interstitial pressure up until the 7 August 1997, between 1800 and 2100.
On this date, three cells stopped working at the same time. The section of a cable network,
first thought to be the cause, was invalidated by the cable continuity test using the ohm-meter.
The malfunctions did not however involve the interstitial pressure cell connector, which is
particularly resistant due to the fact that the cables are reinforced. The simultaneous nature of
the breakdown of 3 of the 5 cells would rather seem to be explained by damage caused by
lightening (demagnetisation or deterioration of the electro-magnets).
The French
Meteorological Service confirmed that there was lightening activity in the commune of Salins on
th
the 7 August 1997.
In these conditions, the loss of pressure cells is less of a problem due to the effective lightening
conductor protection of the data collection exchange.
Results
The 1997 data from the interstitial pressure cells at Salins-les-Bains has been brought together
and also viewed within each cell during the two years of in-depth investigation:
·
·
·
·
·
interstitial pressure at Salins-les-Bains in 1997,
interstitial pressure in C1. in 1996 and 1997,
interstitial pressure in D1. in 1996 and 1997,
interstitial pressure in D2. in 1996 and 1997,
interstitial pressure in D3bis. in 1996 and 1997,
17
·
Salins-les-Bains,
Jura, France
interstitial pressure in D4. in 1996 and 1997.
The variations in interstitial pressure are fairly similar in 1997 to those in 1996. The orders of
scale of the pressure are identical:
·
·
about 10kPa for D1, D2 and D4,
around 50kPa for C1 and D3bis.
The range of pressure measured is low, these are not sensitive to hydro-meteorological
variations, due a priori to a dampening of hydraulic prompting. In this capacity, the pressure
cells make it impossible to summarise the hydrodynamic evolution of a marl slope in a
transitory system.
During the two years, the pressure measured by the cell used in C1 fell continuously from the
time it was installed. The value is therefore unusable.
The comparison of 1996 and 1997 of cell D1, confirms the validity of a stabilized level reached
since July 1996, after 8 months of balancing (for 6 months of which there are readings). This
assessment goes further in the sense of huge inertia in the system which makes up the marl
environment and the filter for the cells, explaining in part the low sensitivity of the cells to rainfall
events.
In D2, the data for 1996 and 1997 are different in terms of detail, but similar in terms of their
range (about 4kPa) and their seasonal behaviour.
The cells located in D3bis are very similar, with little variation. As for cell D1, you can say that
from then on that the balancing of the cell, from November 1995 to March 1996, shows a
certain inertia. By contrast, the high interstitial pressures evident in August 1996 is certainly
very much less, and delayed due to the hydraulic promoting.
In D4, it is interesting to see that over the course of the two years you find a same summer
phase during which the signal is either more unstable or more sensitive to environmental
variations. However, the values measured in 1997 are about 2 kPa higher that in 1996.
5.2
Appraisal of the system in 1999
The last report (BRGM Report, R39995, March 1998), outlined the measures taken in 1997.
These measures included:
·
·
·
·
·
2 bucket rain gauges;
2 panels with 24 conductivity electrodes;
6 vertical profiles of 3 water content gauges;
5 interstitial pressure cells;
1 inclinometer.
Since then there have been three operations on the site. The aim of these operations was to
gather information on the state of the system after several years of operation.
The first operation (at the beginning of 1998) consisted of repairing the resistivity panel system,
which was giving abnormally high resistivity readings. The inspection and the tests after repair
showed the discontinuities in the remote electrode cabling, as well as the deterioration in the
cabling of the electrode panels. Undeniably, the importance of the wire network of such a
system is very fragile in the long term.
The aim of the second operation (in August 1998) was to re-establish modem contact with the
data collection exchange and to diagnose the state of the gauges in place. Only 4 water
content gauges were still working, all of these were therefore withdrawn. Their diagnosis made
18
it possible to attribute the malfunctions to water activity which had penetrated the gauges
through the cables which had become porous over time. In addition, the withdrawal of the
gauges was very difficult, it showed the importance of considering the way in which they are to
be repaired (PVC tube and traction cable to the surface) right from when they are installed.
The last operation (in September 1999) should have made it possible to re-establish the
modem link. On site, the assessed malfunction of the central data exchange OSIRIS, made it
necessary for this to be repaired. We were able to assess at that time, that the central data
exchange was late in taking readings. The type of TELEMAC probes installed are sensors with
a vibrating thread, it is necessary to stimulate these sensors after a certain time using an
electric pulse (using in particular a measuring system on site, the PC6 of Telemac). And yet
when these readings no longer sent a sufficient electric impulse the OSIRIS central data
exchange regularly re-sent the data and never got a response. However, after the lithium
battery had been withdrawn and the late stopping of the readings, the contact could not even be
re-established, the probable cause is the malfunction of the internal modem at the central data
exchange.
Nowadays, the five interstitial pressure cells, the rain gauges and the inclinometer are left on
the site as well as the data collection material consisting of two multiplexers, a lightening
conductor system, a tension converter all contained within a wooden box.
The way in which the readings will be monitored has not yet been decided. There were already
a lot of lessons about which monitoring methods to set up and about the survival of the
equipment. In particular, it enabled information to be gathered which would be useful for
producing information sheets to help decisions, and information sheets about the problems of
instrumentation and the solutions used.
5.3
The general problems linked to the site and to ground movement
The closeness of residential areas and human presence (particularly children), required us to
adapt the in-depth study for the sensitive areas. It made it necessary to ensure transparency in
bringing together the data collection systems and in burying the cables. However, this was an
advantage in terms of bringing together the data collection system in one safe place and being
supplied by a line from the French electricity company.
The phenomena relating to the infiltration of rain in the marl slopes are very complex. Added to
this are human factors, numerous paved ways were able to be established (old drainage
systems installed for irrigation purposes). They disrupt the natural flow.
As for the Boulc en Diois site, the remoteness is a limiting factor. Its impact is greatly reduced
by the use of a data collection system and transmission.
The equipment problems
·
The open piezometers
- They are not reusable and are difficult to use (ground-sensor contact).
- Their sensitivity is too low compared to the permeability of the ground. It is difficult to
increase the sensitivity, as the sensors used to follow the fluctuations in water level in the
piezometer have a minimum diameter.
- The solution found is to use closed piezometers with PIEL probe pressure sensors.
These probes are reusable, but the difficulties in using them, though reduced, still remain
in terms of ground-sensor contact.
·
The closed piezometers (PIEL probes)
The readings show:
- artefacts corresponding to the technical problems experienced for the most
part during the period when it froze;
- that the malfunction of one of the sensors caused large fluctuations in the
readings, for
the sensors connected to the same collection system.
19
Salins-les-Bains,
Jura, France
·
The interstitial pressure cells (TELEMAc probe)
- Three out of five of the cells broke down at the same time, the most probable
hypothesis is that they were struck by lightening (demagnetisation or deterioration of the
electro-magnets).
- When they were installed, the bronze filter was tightened too quickly leading to a
lowering in pressure during the first months due to the dissipation of the high pressure
caused.
- However, before they broke down, the range in pressure reading was low, as they were
not sensitive to hydro-meteorological variations, due a priori to the absorption of the
hydraulic prompts. As such, the pressure cells do not make it possible to draw together
the hydrodynamic evolution of a transitory marl slope system. There is a large inertia in
the system which consists of the marl environment and the cell filters, explaining in part
the low sensitivity of the cells to rainfall events.
- The daily fluctuations (in the order of some kPa) may be explained by the fact that
certain cells were not quite waterproof, and the influence of atmospheric pressure.
- The Telemac probes are responsible for the problem of contact with the OSIRIS central
data exchange. In effect, after a certain amount of time, it is necessary to re-stimulate
the vibrating wire in order to get readings, as when the OSIRIS central data exchange
demands data it cannot send sufficient energy to stimulate the vibrating wire. When it
does not get a reading, it relaunches the request for data and quickly ends up late in
taking a reading. Constantly, in interrogating the sensors the telephone link is then no
longer possible.
·
The bucket rain gauges
- Two rain gauges were installed. One of them no longer gave any readings as both the
internal and external batteries had run down. The other gave readings attributable to the
regular clogging up of the receptacle. Furthermore, the periods during which the
receptacle was clogged up are particularly visible from September 1997, on both the
precipitation columns and the monthly values or on the cumulative precipitation.
- The two rain gauges were withdrawn, without prejudice to the on-site readings, as the
values collected validated those of the closest French Meteorological Office station.
The water content sensors (HUMILOG)
- After two years of operation, the number of sensors still in service were: 6/18 for the
temperature line; 4/18 for the water content line.
- Added to this are the readings from the signals which are not always reliable.
- A large number of simultaneous breakdowns are noted, attributable to the cable being
severed by accident. Except for the sensor malfunctions, after they were installed (5/18
for the temperature line and 7/18 for the water content line), the majority of the problems
stem from the connections and the oxidation of the sensors. The oxidation of the inside
of the sensors can be explained in two ways: 1) water entering the connections; 2) water
entering via the cables.
- It is therefore of prime importance to do away with wire links as they are the most
vulnerable element. In addition, the sensors in the ground do not really produce
according to a profile type.
Solutions used:
- decrease in the length of the cable (Léaz site),
- use of a cable coated with polyethylene or polyurethane insulation,
- development of the HUMITIB (see the Roquevaire site, Study Area G20).
·
·
The intelligent electrodes
Several problems are linked to a certain number of constraints:
- on the safety of people (proximity to residential areas),
- on the condition of the electrodes,
- the supply and autonomy of Syscal (due to parasite problems, Syscal cannot
be directly supplied by the mains),
- remote interrogation.
Solutions used:
- more thorough conditioning of the electrodes as a pipe system,
20
Salins-les-Bains,
Jura, France
- bringing the most sensitive components together on a protected site (leaving only the
cables and the electrodes on site), the electrodes, which could be addressed were
rethought and all of these electronic components have been brought together within the
"multi-node" boxes, developed from a system for recharging the batteries from the mains,
downloaded in off-peak hours.
- in terms of remote interrogation, the Syscal system is linked to a modem. However,
certain configurations for taking readings take more than an hour. A box consisting of a
relay was developed to maintain the Syscal system in working order at the end of the call
for a regulatable period of time using a timer.
Despite all these improvements, one year after they were installed, the electric panels
show abnormally high resistivity levels. These results can be explained as a result of
discontinuity in the cabling, due to damage (accidental severance, rodents, ageing etc.).
·
·
Mercury tensometer
- The hypodermic areas of the ground are saturated for a long period of the year; the
tensometers therefore give positive interstitial pressure values, hence a gap in readings.
- Installation is delicate, it is necessary to ensure good contact between the porous cell
and the earth.
- The temperature influences the reading due to the dilution of the liquids, water and
mercury. The order of size of the variations can be very large: up to 20mbar difference
between readings has been noted on the Béline site. It has proved impossible to
establish a relationship which would allow the readings of the mercury pressure gauge
to be corrected. To minimise this source of error, the readings are all taken at the
beginning of the day (uncertainty in the order of 10mbar).
- Even though the Humilog water content gauge does not make it possible to do as fine a
study on the hypodermic area as that done using mercury tensometers, it still answers
the principal problems. It can be interrogated remotely, a temperature correction is
made, the installation is less exact as the cell reads the water content in an area of
approximately 30cm around the sensor.
Debitmeters (2 pressure sensors)
- The calibration is very difficult and was managed case by case.
Collecting the readings
From the beginning several sensors gave incoherent readings, often due to bad
implementation. This problem is recurrent. It is absolutely necessary for all site assessments
to plan if possible a system for recovering the sensors, which enables them to be diagnosed.
The choice of installing these sensors is of prime importance, and is made after a fairly
thorough investigation of the site. The discovery of old drainage networks and of a source
after the installation of the humilogs and the mercury tensometers meant that data collection
could not be optimised as some sensors were permanently saturated.
This site made it possible to bring the data collection systems together in a sheltered place,
thanks to the proximity of the residential areas. In this way, in contrast to Léaz, this has not
suffered too much, but it required the lengths of the cables to be multiplied. The importance of
the cabling and its relative vulnerability were a very detrimental element.
6.
CONCLUSIONS
Except for the sensor malfunctions, evident practically since they were installed, or the
lightening incident, the majority of the interruptions in readings seem to have come from the
connections, due either to accidental causes or ageing. It is therefore of prime importance to
do away with wire links, which are the most vulnerable elements in the system network, as soon
as possible.
21
Salins-les-Bains,
Jura, France
The development of a system that can be interrogated entirely remotely showed other
weaknesses however in the network for taking readings such as problems with connections,
and also the interrogation procedures and the installation of equipment.
The choice of remotely controlling the system required communication to be prolonged and the
system to be protected. The protection of the system was effected by burying the cables and
bringing the data collection equipment together in a safe place. However, this generates
lengths of cables which make the system more fragile. Future options should be planned in the
development of autonomous stations in terms of power supply (batteries and solar panels) so
that they are as close as possible to the sensors.
Another important element in the case of transmission is the formalisation of data interrogation
and safety procedures. In effect, during each communication the user should fill in a certain
number of sheets for which the formats and names should be made the same.
7.
BIBLIOGRAPHY
Baltassat J.M., Mathieu F. 1995. Installation et test d'un système de mesures à électrodes
intelligentes sur le glissement de Boulc-en-Diois (Drôme). Note BRGM, 95 DGA 001.
Bogaard T.A. 1996c. Determination of hydrological parameters of remoulded marls, Salins-lesBains, France. HYCOSI technical report no.4.
Chassagneux D. and Leroi E. 1995. HYCOSI - Impact of Hydrometeorologic Changes on
Slope Instability. Synthèse des connaissances acquises sur le site de Boulc-en-Diois.
Rap. BRGM R38777.
Leroi E. and Monge O. 1996. HYCOSI - Impact of Hydrometeorological Changes on Slope
Instability. Reconnaissances et instrumentation du site de Salins-les-Bains (39): compterendu d'intervention. Rap. BRGM, R38890.
Mathieu F. and Miehé J.M. 1996. Installation et test d'un système de mesures à électrodes
intelligentes sur le glissement de terrain de Salins-les-Bains (Jura). Rap. BRGM R39995.
22
Monge O. and Leroi E. 1997. HYCOSI - Impact of Hydrometeorologic Changes on Slope
e
Instability. Instrumentation du site de Salins-des-Bains, rapport d'avancement 3 année.
Rap. BRGM, R39172.
23
Figure G17.1 Extract of the topographical map no. 3325 at a scale of 1:25,000 by IGN.
Extrait de la carte topographique no.3325 à 1/25.000ème de L'IGN.
Figure G17.2 Extract of the 1:50,000 geological map, enlarged to 1:25,000.
Extrait de la carte géologique 1/50 000, agrandie à 1/25 000.
Figure G17.3 Sketch of the hydrological map of the slope.
Esquisse de carte hydrologique du versant.
ZONE AFFECTED BY
THE 1994 SLIDE
ud
e
ed
Ba
ut
Ro
ZONE WHERE MEASURING EQUIPMENT
HAS BEEN INSTALLED
Building
Parking
Figure G17.4
Topographical cross-section of the study area.
Schema topographique de la zone d'etude.
Figure G17.5 Extract from the ZERMOS map.
Extrait de la carte ZERMOS.
TRVAUX DE STABILISATION REALISES EN 1984
SLOPE STABILISATION WORKS UNDERTAKEN IN 1984
Eperons drainants
et fronttants
BAT
B
Coupe transversale
sous batiment B
New installation of building A
Figure G17.6
General cross-section of the project.
Plan général schématique du projet.
Key:
1977 B3G Field Programme:
Electric boreholes
Penetrometric boreholes
Core sample borehole
Piezometers
Metrology thesis Ch. Lainé
Borehole probes
Tensometers
Piezometers
Debitmeters
Piles of the Youth Workers Accommodation plot
Figure G17.7 Plan of the installation of the piles and the detailed study tools.
Plan d'implantation des pieux et des outils d'auscultation.
a SALINS (13 en 1984)
DEPLACEMENTS CUMULES (en mm)
CUMULATIVE DISPLACEMENTS
100
80
60
40
20
0
0
100
200
300
400
500
PLUIES CUMULEES (en mm)
600
700
600
700
a SALINS (16 en 1984)
DEPLACEMENTS CUMULES (en mm)
CUMULATIVE DISPLACEMENTS
70
60
50
40
30
20
10
0
0
100
200
300
400
PLUIES CUMULEES (en mm)
Figure G17.8
Correlation between rain and movement.
Correlations Pluie-Mouvements.
500
PI :
PZ :
T:
PD :
Tube d'acce's à la sonde de pression interstitielle
Piézométre ouvert
B
Tensiométre
Palin drainant
P10
Pluvlographe
PZ1 P11
T21
T25
A3
A2
A3
T11 T15
P14
P12
PZ2
A2
PZ2r
PZ2g
PI29
P13
PZ3
PZ4
PD1
B
Figure G17.9
PD2
PD3
Debimetre D2
PD4
Collacleur commun
Bird's eye view of the study site.
Vue cavaliere du site d'etude.
Debimetre D1
Figure G17.10 Cumulative monthly rainfall at Salins during the study period and monthly rainfall
for the decade.
Pluviométrie mensuelle cumulée à Salins pendant la période d'étude et pluviométrie
mensuelle sur la décennie.
Figure G17.11 Fluctuations in the ground water levels of the open piezometers.
Fluctuations es niveaux piézométriques sur les piézomètres ouverts.
Figure G17.12 1 to 7 - Development of the interstitial pressure readings on the PIEL.
Evolution des pressions interstitielles obtenues sur les Piel.
Figure G17.13 Behaviour of the buffer zone following the state of saturation.
Comportement de la zone tampon suivant l'état de saturation.
A2
?
D1
6mr0
R3
D2
10m
Figure G17.14
Reservoir simulation.
Simulation-reservoir.
R1
=2m
R2
=8m
Figure G17.15
R3
Reservoir simulation.
Simulation-reservoir.
Figure G17.16 Installation of the boreholes.
Implantation des sondages.
HYCOSI
INSTRUMENTATION
MARSEILLE
Mo
SALINS-LES-BAINS
.............................................
CLUCY
EDF220
EDF220
Mo
Mo
Mo
BE
Mt
CB
CB
12 V
BI
O
S
BE
BI
M
MP16
12 V
1 pluviometre
MN16
MP16
MN16
MP16
1 pluviometre
5 pression
18 Humilogs
MN16
48 Electrodes
Legende :
S : Syscal O : Osiris M : Madotel Mo : Modem
MP16 : Multiplexeur
MN16 : Multinode
BE : Batterie externe
B1 : Batterie interne
Mt : Minuteur
CB : Chargeur de batterie
EDF220 : Courant alternative 220
RCM : Remote contoller module
Key :
S : Syscal
O : Osiris M : Madotel
Mo : Modem
MP16 : Multiplexer
MN16 : Multinode
BE : External battery
BI : Internal battery
Mt : Timer
CB : Battery charger
EDF220 : Alternative current 220 V
RCM : Remote controller module
Figure G17.17
Diagram of the principle of the equipment at Salins-les-Bains.
Schema de principe de l'instrumentation de Salins les Bains.
Rainfall
Figure G17.18 Clucy acquisition chain.
Chaîne d'acquisition de Clucy.
Figure G17.19 Plan General Implantation des sondages et de l'instrumentation.
General map showing installation of sensors and equipment.
Figure G17.20 Plan General Implantation des sondages et de l'instrumentation.
General map showing installation of sensors and equipment.
Figure G17.21
Evolution des d formations mesur es par inclinom trie en fonction du temps et de la pluviom trie.
(a) Camouflage des cables
Plate G17a
(b) Implantation du sondage D1 et d'une cellule
TELEMAC
(c) Creusement d'un chemin de cable
Plate G17b a) Station de mesure no.3 (rupture de pente á Lintérieur du perimeter
matérialisé
b) Implantation du sondage carotté C1 et station de mesure no.6(en avant des pneus)
25

geotechnical study area g17 salins-les

Transcription

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