RGM RGM - Infoterre

Transcription

RGM RGM - Infoterre

Hydrological and geotechnical investigation of
the La Beline^ slope, Salins-les-Bains, Jura,
France
O. Monge, T. Bogaard
avec la coUaboration de
E. Leroi
July 1997
R 39514
RGM
Hydrological and geotechnical investigation of the 'La Beline' slope, Sallns-les-Balns. Jura,
France - Synthesis of knowledge
Mots clés
:
Keywords:
Reconnaissances, Instrumentation, Synthèse, Glissement de terrain, Salinsles-Bains, Jura, Projet HYCOSI
Investigations, Instrumentation, Synthesis, Landslide, Salins-les-Bains,
Jura, France, HYCOSI project
En bibliographic, ce rapport sera cité de la façon suivante
This report will be referenced as:
:
O. Monge, T. Bogaard avec la collaboration d'E. leroi (1997) - Hydrological and
geotechnical investigation of the 'La BeHne' slope, Salins-les-Bains, Jura, France.
Department of Physical Geography, University of Utrecht. The Netherlands and BRGM
Report R39514, XXX p., 16 fig., 4 tabl.
<ù
liRCíM, 1997, ihis document may not be reiiroJuceJ in whole or in part w iihout the express permission of HKGM.
BRGM report R 39514
Hydmlogical and geotechnical Investigation of the 'La Beline' slope, Sallns-les-Balns, Jura,
Summary
study sites have been selected within tlie HYCOSI (Impact of
HYdrometeorologic Change On Slope Instability) project for field trials on methods and
instruments developed or used during the work. Salins-les-Bains in Jura, France, is the
site in which most efforts have been made in exploration, testing and understanding.
Three
After two years of geophysical, hydrogeological and geotechnical reconnaissance,
instrumentation and modelling, this report presents a summary of the knowledge
obtained about the site and general information useful for the project.
The Salins site has the advantage that it is partially known following the landslide that
occurred during the construction of the Children's Home, and as a result of scientific
work done by C. Laine.
However, the results are only applicable to a limited part of the slope. The initial
investigations carried out for the project consisted in defining the geometry and
characteristics of the formations. The instrumentation phase was then concerned with
the water in the ground whether saturated or not, from ¡ts source (precipitation and other
underground input) to its consequences (displacement and damage).
After
fairly complete
of data and characteristics had
been built up, simulations
were started. Several hydrogeological and geotechnical models were used for the
a
set
simultaneous comparison of tools of increasing complexity, the expected information
gain, and the resulting usage constraints.
BRGM report R 39514
Hydmlogical and geotechnical investigation of the 'La Beline' slope. Sallns-les-Balns. Jum.
Fmnce - Synthesis of knowledge
BRGM report R 39514
Hydmlogical and geotechnical Investigation of the 'La Beline' slope, Sallns-les-Balns, Jum,
Contents
Summary
1.
Introduction
3
7
2. Geology and geomorphology
9
2.1 Geology
2.2 Geomorphology
9
10
3. Investigation methods
13
3.1 Previous investigations
13
3.2 Geophysical investigations
3.3 Instrumentation and metrology
3.4. Additional investigation
13
3.4.1. Hydrology
3.4.2. Geotechnical investigation
4. Investigation results
4.1.
4.2.
4.3.
4.4.
4.5.
4.6.
5.
Geophysical results
Precipitation
Unsaturated zone
Saturated zone
Displacements
Geotechnical parameters
Modelling
17
19
19
19
21
21
25
26
29
30
3
1
33
5.1. Outline
5.2. Hydrological model results
5.3. Stability model results
33
34
35
6. Discussion and Conclusion
41
6.1. Site characterisation
6. 1.1. Geometric characterisation
6.1.2. Hydrogeological characterisation
6.2. Site investigation
6.3. Simulation
41
41
6.3.1. Conceptual models
6.3.2. Calculation program
6.3.3. Modelling
References
BRGM report R 39514
42
44
46
46
47
48
49
Hydrological and geotechnical Investigation of the 'La Beline' slope, Salins-les-Bains, Jura,
List of Figures
Extract from the 1 :50,000-scale Salins-les-Bains geological map
8
Figure 2:
Microgeomorphological map of the 'La Beline' slope
1 1
Figure 3:
Location of sensors on the 'La Beline' slope, Salins-les-Bains, Jura,
Figure
1:
Figure 4:
France
18
Diagram of the instrumentation on the 'La Beline' slope, Salins-lesBains, Jura, France (adapted from Monge and Leroi, 1997)
18
Apparent resistivity map AB = 60 m (Baltassat and Charbonneyre,
Figure 5:
Figure 6:
Figure 7:
Figure 8:
Figure 9:
1995)
20
Bouguer anomaly, profile B, Clucy plateau. Jura, France (Le Jeune
and Besse, 1996)
23
Residual order 3 anomaly, Clucy plateau, Jura, France (Le Jeune
and Besse, 1996)
24
Cumulative precipitation, Salins-les-Bains, Jura, France (Monge
and Leroi, 1997)
26
Soil temperature at the 'La Beline' slope (Monge and Leroi, 1997)
27
Figure 1 0: Volumetric water content at the 'La Beline' slope (Monge
and Leroi, 1997)
27
Average soil water retention curve of remoulded marl at the 'La
Beline' slope (Bogaard, 1996c)
28
Figure 12: Pore pressure in D2, Salins-les-Bains (Monge and Leroi, 1997)
29
Figure 13: Safety factor map at the 'La Beline' slope based on 1-D stability
analysis
36
Figure 14: Safety factor curves for an infinite slope analysis
(C' = 20.5kPa; (p'=26.3°)
37
Figure 15: Safety factor at the 'La Beline' slope for infinite slope analysis
38
Figure 16: Safety factor at the 'La Beline' slope using Talren
39
Figure
1 1
:
List of Tables
Table
1:
Summary of geophysical investigations
Table 2:
Pore pressure cell results (Monge and Leroi, 1997)
Table 3:
Summary of strength parameters determined for samples from
Salins-les-Bains (Bogaard, 1996b)
Table 4:
Summary of stability modelling results
BRGM report R 39514
1.
Introduction
A French-Dutch research consortium consisting of Delft Geotechnics and tfie
Department of Physical Geography of the University of Utrecht from the Netherlands
and JPA consultants, Iris Instruments and BRGM from France, was granted EUenvironment finance for the HYCOSI project which studies the influence of
HYdrometeorologic Changes On Slope Instability. BRGM co-ordinates the HYCOSI
project.
The Salins-les-Bains research site was chosen as a pilot area. The project has invested a
large part of its time and money into the area known as the 'La Beline' slope (see
Figure 1). Salins-les-Bains is a small village in the Jura Department. Geologically it is
situated in the lower part of the Jura mountain chain with regional elevation differences
from 300 m to 800 m above sea level. The 'La Beline' slope lies between the 'La
Furieuse' stream (345 m) and the Clucy limestone plateau (600 m), and extends over
36 hectares. A Children's Home was built at the bottom of the slope, and the
construction work initiated mass movements which attracted a large amoimt of research
attention starting aroimd 1985-1986 with the work of C. Laine. The HYCOSI research
project is a continuation of this work, but at a larger scale.
A large instrumentation scheme was developed in order to study how water flows and
concentrates in this slope. All field instrumentation is controlled and downloaded from
Marseilles by telephone line and modem. Several technical aspects of this operation had
to be overcome. A l:l,000-scale topographic map of the research site was made. New
investigation techniques were tried and efforts were made to determine regional
groundwater flow in the adjacent karst plateau. To obtain the necessary field parameters,
several field surveys were carried out during which additional samples were taken.
This report is a summary of all the work and knowledge of the 'La Beline' research site
in Salins-les-Bains (France) as has been collected by the HYCOSI research group. Its
main function is to summarise existing knowledge and to combine this knowledge into a
synthesis of the mechanisms of rain infiltration, groundwater flow and slope stability.
BRGM report R 39514
Hydrological and geotechnical investigation of the 'La Beline' slope, Saiins-les-Bains. Jura,
Figure 1: Extractfrom the l:50,000-scaie Salins-les-Bains geological map
8
BRGM report R 39514
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jum,
2.
Geology and geomorphology
2.1 GEOLOGY
The Salin region (see Figure 1) is underlain by a group of Mesozoic beds oriented
approximately NE-SW. The northwestward overthrust of the allochthon, favoured by
the décollement at the level of the evaporitic Triassic, occurred during the Pontian (MioPliocene) phase.
On the scale of the entire site, this structure appears as an enclosed syncline bound by
two faults. The terrain between La Furieuse stream that flows along the bottom of the
site to the Clucy plateau belongs to the Lias, mainly marly, overlain by Dogger
limestone.
The thickness of the Lias is 100 to 200 m; and in particular there is a massive limestone
bed about 10 m thick exposed in the La Furieuse stream bed, followed by marl and
marly to sandy limestone of the Middle Lias, and finally grey-blue, greasy and schistose
marl of the Upper Lias. The Dogger forms the limestone substratum of the Clucy
plateau and is visible particularly in cliffs such as the 'Roche Pourrie' and Fort Belin at
the edge of the site.
This Bajocian to Callovian limestone is the site of karstic streams feeding springs
commonly located at the contact between the limestone and the underlying marl.
Although tracing experiments have shown that most subsurface flows on the plateau
lead towards the Gouaille waterfall, a 0.5 km^ hydrological basin exists between Fort
Belin and the village of Clucy which could have a different outlet, particularly towards
the site (Hemel et al., 1997).
Exploratory boreholes drilled on the marl slope (Demartinecourt, 1995) provided
information for a more precise description of the superficial lithologies. This layer has
been weathered and remoulded by solifluction phenomena, and may comprise three
main layers and a few local variations, before reaching fresh marl:
1
-
2 3
-
4 -
Small limestone gravel imbedded in grey clay with a thickness of between
4.0 and 5.1 m;
Abundant limestone gravel in a yellow matrix of clay, silt and sand found in
all the destructive boreholes;
Silty, grey marl of average density with some gravel content observed only
in the cored borehole;
Stiff, dark blue and grey marl, at depths of 17 and 20.2 m (in D3 and CI
respectively).
BRGM report R 39514
Hydrological and geotechnical Investigation of the 'La Beline' slope, Salins^es-Balns, Jum,
2.2 GEOMORPHOLOGY
In summer 1996 a microgeomorphological map (also called a geomorphological map)
was drawn by the University of Utrecht. Its principle goal was to make an unbiased
(uninterpreted) map of the morphology of the slope. A secondary goal was to map
different movement areas in detail and to incorporate hydrological information. It was
also attempted to combine field measurements, for example saturated permeability, with
geomorphological units.
The l:l,000-scale topographic map was used as a basis for the mapping. Because of the
amount of detail on this map, no additional geodetic equipment was used to locate all
phenomena. The legend was kept simple with only four gradient classes, namely 0-10,
1 0-20, 20-40 and > 40 degrees, with concavity and convexity symbols. The map also
shows local depressions (valley-shaped or closed), wet areas and the limits of the niche
and the deposition area defined by clear slumps.
The geomorphological map is shown in Figure 2. It shows a very complex, steep terrain.
No global structure can be distinguished in the first impression. One landslide area that
is still active is shown in the northeastern part of the map. A second area that appears
liable to movement is the westem part close to the forest undemeath Fort Belin. Other
striking features are a central valley and several closed depressions.
In general the slope can be characterised as having abundant topographic irregularities,
such as bulges, niches and scarps without any clear, large-scale structure. The
interpretation is highly complicated by the different former and current land-use
activities of the dozens of owners. For example, it is obvious that some of the land was
terraced at one time, while the slope on an adjacent parcel is smooth. More recently,
activities result in an obvious difference between parcels grazed by cows and those
covered by grass and fmit trees.
Considering the above, the observed features were interpreted as local and small scale.
The outstanding questions are whether or not they are still active and under what
conditions they have been active.
10
BRGM report R 39514
LEGEND
I
Oí
H
Built up area
<^
Source
^
S w a m p y area
LjJ
Dry valley
O
Closed depression
-A
Stone-wall
Pi
I
Small surface discontinuity < 1 m
Convex slope
Concave slope
Niche, slope > 40
step > 1 m
ft
Î
i
100m
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jura,
12
BRGM report R 39514
Hydrological and geotechnical Investigation of the 'La Beline' slope, Salins-les-Bains, Jum,
3. Investigation methods
3.1 PREVIOUS INVESTIGATIONS
A number of investigations had been carried out (Asté, 1995) on the Beline site and for
the constmction of the Beline Specialised Children's Home, before the HYCOSI project
was started:
preliminary geotechnical study by the B3G design office in 1 977;
complementary study by the Franche-Comté BRGM branch in 1983;
intervention by the Lyon BRGM branch following damage observed during
constmcfion in 1984;
scienUfic study for a thesis between 1987 and 1989 (Laine, 1989).
Preliminary investigations included electrical sounding, excavations with power
shovels, piezometers, penetrometers and laboratory tests. Inclinometer tubes were
subsequently installed to monitor the damage. Later, the scientific approach adopted at
the site introduced equipment more suitable for monitoring water, including a raingauge (1), tensimeters (9 in 2 bases), both open (5) and closed (7 PIEL probes)
piezometers and flow meters (2).
All the investigations were applicable to
a
limited area on the downstream part of the
slope.
3.2 GEOPHYSICAL INVESTIGATIONS
Different geophysical methods were used at the site (see Table
1)
with the aim of:
gaining a better understanding of the stmcture of the slope through geoelectrical
measurements and seismic refraction,
clearly defining an assumed additional karstic supply through a microgravimetric
study,
applying specialised techniques such as Self Polarisation (SP) and Proton Magnetic
Resonance (PMR) in the context of ground movements.
Geoelectrical resistivity sur\'ey has been carried out using two different mechanisms,
namely electrical sounding and profiling. Electrical sounding determines the vertical
resistivity distribution beneath the measurement point. By uniformly increasing the
BRGM report R 39514
13
Hydrological and geotechnical Investigation of the 'La Beline' slope, Salins-les-Bains, Jura,
.
distance between electrodes, the vertical sequence of the different beds with contrasted
resistivities can be demonstrated. Electrical profiling consists in carrying out electrical
measurements along profiles to determine the horizontal distribution of resistivities at a
globally constant depth for a given line length AB. The Geoelectrical survey was carried
out in May 1995. Fifteen electrical sounding tests with a depth (<200 m) equal to line
length AB were made together with 16 electrical profiling tests using two 4-pole devices
representing about 350 measurement points.
Investigation by seismic refraction can be used to determine the depth of the compact
substrate and the stmctures that affect it. This substrate constitutes a priori the main
refractor studied. Measured seismic velocities give an indication of the compactness and
type of ground present. The seismic refraction campaign took place in July 1995. Nine,
100-m long seismic bases were measured above the road passing through the site using
explosives as the seismic wave source. A drop hammer was used as the seismic wave
source below the site. Two, 60-m bases and six, 120-m bases were measured. The
geophone spacing was 5 m, reduced to 2.5 m close to shot points. Between three and
five shootings were made for each base, depending on site constraints.
In hydrogeology, self polarisation is based on the principle that water circulation in the
ground generates an electric ciurent that induces an electric potential at the ground
surface; SP surveys attempt to measure this potential. The first work was done in
September 1995 to determine the instmmentation layout for the site. Five potential
measurement profiles were recorded using contours at a 1-m interval. Six current lines
were monitored using an electrostatic rod. Measurements were subsequently taken along
two of these lines. Furthermore, repeated measurements were made on four horizontal
profiles and one current line in June and July 1996 to evaluate the variability of
measurements with time (Gemndo, 1996).
Microgravimetric prospecting was carried out in September 1996 to determine if there
were any cavities, karsts or loose areas on the Clucy limestone plateau above the 'La
Beline' slope. Four test profiles separated by about 100 m and with a measurement
interval of 5 m were preferred to a regular grid on a smaller area.
NUMIS, developed by BRGM and IRIS instmments, is a Proton Magnetic Resonance
tool designed for direct detection of groundwater down to a depth of 100 m. Hydrogen
atoms of water molecules are excited by alternating current pulses at a natural frequency
transmitted into a loop laid on the ground. The magnetic field they produce in retum is
measured and analysed for various excitation pulses. Measurements can be interpreted
to estimate the water content and the mean pore size of each layer at depth. Two
locations were investigated in January 1997, one on the slope and the other at the same
location as the Microgravimetric prospecting.
14
BRGM report R 39514
s
Ï
3
Geophysical Investigations
Cd
en
Geoelectrical methods
Date
M a y 95
Contents
15 electrical soundings
13 electrical profilings A B = 60 m M N = 10 m
References
§
Baltassat & Charbonneyre
(1995)
Q.
1
(about 260 points)
3 electrical profilings A B = 120 m M N = 20 m (about 90 points)
Seismic refraction
July 95
9 x 100-m bases (explosive)
2 x 6 0 - m bases (drop hammer)
Baltassat & Charbonneyre
(1995)
6 x I20-m bases (drop hammer)
Self polarisation
September 95
5 SP profiles (equipotential)
2 SP profiles (current lines)
6 profiles with electrostatic rod
Self polarisation
June/July 96
Cazin(I995)
CO"
Charbonneyre (1997)
4 SP profiles (equipotential)
Gerundo(1996)
1 SP profile (current line)
Charbonneyre (1997)
Le Jeune & Besse (1996)
Legchenko & Beauce (1997)
Microgravimetry
September 96
4 measurements at 5 - m interval
Proton Magnetic Resonance
January 97
2 PMPv surveys
Table 1: Summary of geophysical investigations
3
o o
5 . CD
a
3"
I
I
Cfl
16
BRGM report R 39514
Hydrological and geotechnical investigation of the 'La Beline' slope, Sallns-les-Balns, Jum,
3.3 INSTRUMENTATION AND METROLOGY
A large amount of instmmentation has been used on the site since January 1996 in order
to monitor variations with time of the main significant parameters for the study of the
'La Beline' slope (see Figure 3). Measured magnitudes concern the hydraulic trigger,
groundwater and signs of movements (Leroi and Monge, 1996).
Two rain-gauges were installed close to the Beline's children's home and at Clucy on
the plateau to measure precipitation and evaluate its variation with altitude.
Eighteen soil moisture sensors (IRIS Instruments Humilogs) record the temperature and
water content by volume, to determine the state of the unsaturated zone. They are laid
out at three depths (between 0 and 1 .5m) along six measurement profiles.
Five pore pressure cells (TELEMAC type CLl) were distributed across the site in the
saturated zone, using destmctive or cored boreholes. Two of these cells are located
directly beneath the water content measurement points.
The groundwater is indirectly and globally monitored through time by two electrical
panels, permanently located in the downstream part of the site.
A 30-m
deep inclinometer has been installed at the site in borehole D3 to locally
monitor deep-seated displacements.
Although all measurement points are connected to a remote interrogation acquisition
system (see Figure 4), an operator takes monthly measurements using the inclinometer.
The measurement sampling interval is specific to each acquisition station - 10 minutes
for Madotel, 3 hours for Osiris and when ordered for Syscal.
BRGM report R 39514
17
Hydrological and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jura,
SA
Humilog a 0,4 m
0.6 m
1,3 m
Humilog a 0,4 m
0.8 m
1,0 m
Cellule 0/5 bars
IHUM320
HUM255
Cellule 0/5 bars { 7.5 m )
4,5 m
IUM155
Humilog a 0,4 m
0.8 m
1.3 m
'HUM22I
HUM085
«HUM
PLUVIOMETRE
Hurmlog a 0,35 m
0.45 m
0,80 m
.04
Cellule 0'5 bars ( 6,0 m )
Humilog a 0.5 m
0.75 m
1,3 m
Cellule 0/2 bars ( 19,5 m ]
Humilog a 0,4 m
0,8 m
1,1 m
L
Figure 3: Location of sensors on the 'La Beline' slope, Salins-tes-Bains, Jura,
France
Pluviometer
Madotel
Modem 1
Clucy
4 8 Conductivity
electrodes
I
1 Pluviometer
18 Soil moisture sensors
5 Pore pressure cells
Syscal
Osiris
Modem 3
Modem 2
Salins-les-Bains
Telephone network
Marseilles
Figure 4: Diagram of the instrumentation on the 'La Beline' slope, Salins-les-Bains,
Jura, France (adaptedfrom Monge andLeroi, 1997).
18
BRGM report R 39514
3.4. ADDITIONAL INVESTIGATION
Two types of material characteristics had to be determined, namely hydrological for
calculating groundwater flow, and geotechnical for stability modelling. The
hydrological investigation consists of m situ saturated permeability tests and laboratory
retention curve analysis (including porosity and bulk density). Tlie geotechnical
parameters are the cohesion and angle of internal friction. These were determined in the
University of Utrecht laboratory (Bogaard, 1996b) and by Hydrogeo (Demartinecourt,
1995) using direct shear and triaxial tests (CD) respectively.
3.4.1. Hydrology
For characterisation of the unsaturated zone, a soil retention curve was determined
which relates soil-water content to pressure-head tension. A soil-water retention curve
also reveals porosity (water content at saturation) and dry bulk density (stove dried
sample). The soil-water retention curve was determined during the drying stage. The
retention curve can be used to calculate imsaturated permeability as a function of soilwater content (Van Genuchten, 1980) if the saturated permeability is known. A total of
26 X 100 cm-^ samples were taken. For the groundwater flow calculations, 64 in situ
saturated permeability tests were determined using inverted auger hole tests (Kessler
and Oosterbaan, 1974).
3.4.2. Geotechnical investigation
From the cored borehole SCI, triaxial samples were taken in the first layer of remoulded
marl at 1.4 - 2.7 m and in the unweathered fresh marl at 21.3 - 21.6 m (Demartinecourt,
1995). Three triaxial tests were performed with 50, 150 and 400 kPa axial load. In
summer 1996, superficial direct shear samples were taken to be tested at the University
of Utrecht (Bogaard, 1996b). Twelve subsamples were taken, consolidated and saturated
before testing. The axial load ranged from 50-250 kPa and the deformation rate was 2
mm/dav.
BRGM report R 39514
19
Hydrological and geotechnical investigation of the 'La Beline'slope, Salins-ies-Bains, Jura,
6j>
<*
¿
SALINS-LËS-BAINS
(Jura)
"Les Prés Moureaux"
/
/
/
CARTE DE RES1STIV1TE APPARENTE
AB = 60 m
K//VXJ
Résistivilés comprises entre 25 ei 30 ofim.m
^ ^ ^ ^ flésisiivilés supérieures à 3 0 o h m . m
40"
Cou'De d'isorésisiiviié en o h m m
0
20
40 m
"ífi
Figure 5: Apparent resistivity map A B = 60 m
(Baltassat and Charbonneyre, 1995)
20
BRGM report R 39514
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Satlns-les-Bains, Jum,
4. Investigation results
4.1. GEOPHYSICAL RESULTS
part of the geoelectrical resistivity survey carried out by Baltassat
and Charbonneyre (1995), the apparent resistivity maps created using the electrical
profiling method show low resistivity variations with values between 8 and 45 ohm.m;
these values are conformable with those expected for marl and clay (see Figure 5) .
As
The electrical sounding data were analysed to create a rough model involving:
-
a series
of layers, between
0 and 12-25 m depth, with variable thickness and
resistivity
(12 to 95 ohm.m) assigned to remoulded formations;
- an underlying layer with lower and more constant resistivity (5 to 15 ohm.m)
corresponding to fresh marl.
However, the resistivity contrast being low, the interface between fresh and remoulded
marl cannot be identified at all points. In particular, the presence of a deep conductive
layer is not obvious from electrical sounding No.5 (SE5).
refraction prospecting carried out by Baltassat and
Charbonneyre (1995) and the analysis of the data assuming layers with constant velocity
increasing with depth, three units were defined over the entire studied area based on
their seismic velocity (Vp):
Following the seismic
-
a
poorly characterised surface layer with a wide range of velocities (200 to 800 m/s),
- an intermediate layer with velocities varying between 800 and 1400 m/s,
-
the main refractor with velocities varying between 2000 and 3000 m/s.
Velocities between 200 and 1400 m/s are obtained for very loose, uncompacted
(v < 400 m/s) to medium compacted (v > 000 m/s) ground, which can be attributed to
displaced ground or marl weathered in situ.
1
The main refractor is assigned to indurated ground in situ. Measured values are
conformable with values for fresh marl (Vp between 2000 and 2500 m/s) or weathered
limestone (Vp between 2750 and 3000 m/s). This refractor is well defined from bases
BRGM report R 39514
21
where an explosive source was used, and slightly less well with a drop hammer source.
Its depth oscillates between 3 and 9 m over the entire site, and deepens (over 11 m) in
the eastem half of base E8.
Isobaths
of the upper boundary of the main refractor show similarities with the high
resistivities measured by electrical profiling. However, the interface identified by
geoelectrical methods is systematically deeper. Sounding observations tend to agree
better with the geoelectrical interface.
The self polarisation surveys (Cazin, 1995; Gemndo, 1996) measured relatively low
potential differences of less than 10 mV (June and July 1996) or 15 mV (September
1995). However, the profiles mn demonstrate the heterogeneity of flows at the scale of
the slope.
The microgravimetric study (Le Jeune and Besse, 1996) along test profiles on the
Clucy plateau demonstrated minor anomalies with very large amplitudes (150 to 200
microgals, see Figure 6) oriented along an NNE-SSW axis (see Figure 7). They are
superposed on the ridge that can be seen on the relief map. These anomalies may
correspond to cavities, karsts or decompressed zones. It was impossible to specify the
depth or extension of the objects that created the anomaly, but the assumption of a site
with karstic water supply is well founded.
Proton Magnetic Resonance (Legchenko and Beauce, 1997) depth invesfigafion and
the value
of the measured signal depend on the device used.
For the 'La Beline' slope, which is about 15° on average and within which water
circulates superficially in the upper part of the marl, the Numis application test
demonstrated two limitations. Firstly, the site geometry and the depth to be investigated
made it necessary to reduce the size of the antenna placed on the ground. Furthermore,
even if the presence of water aggravates the situation at a landslide site, the quantities of
water present are very much lower tlian the quantities encountered an aquifer context.
Consequently, the equipment must be adapted to the low water contents encountered in
unsaturated layers.
The Numis tool was used on the Clucy plateau to attempt to associate a water
circulation depth with minor anomalies demonstrated by microgravimetry. The results
did not identify any such circulation, although it was impossible to determine whether
this was because the volume was too low to be detected by the instrument used, or if
circulation occurred at a depth of more than 100 m, which is the maximum depth at
which the equipment is effective, and also the approximate thickness of the limestone
overburden at this point.
22
BRGM report R 39514
Hydrological and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains. Jura,
Initial measurements were made using the electrical panels when they were installed as
part of the instrumentation for the Salins-les-Bains site (Mathieu and Miehé, 1996),
followed by isolated measurements during 1996, when a number of improvements were
made to the system (Monge and Leroi, 1997). Finally, a daily measurement campaign
was launched in February and March 1997.
The electrical panels showed abnormal resistivities one year after their installation.
These results were explained by a discontinuity in the wiring, probably due to
deterioration (accidental breaks, rodents, ageing, etc.).
The few measurements made in 1996 were stable, even compared with the electrical
sounding data (SE 6 and S E 7, Baltassat and Charbonneyre, 1995). This observation
does not show any practical usefulness in monitoring the variation of resistivity with
time in clayey ground with low resistivity contrasts.
800
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100
200
DISTANCES EN M E T R E S
300
FIG.5
Figure 6: Bouguer anomaly, profile B, Clucy plateau^ Jura, France
(Le Jeune and Besse, ¡996)
BRGM report R 39514
23
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28
Hydrological and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jum,
4.2. PRECIPITATION
At Clucy, rainfall monitoring started in mid-Febmary
1996 and continued until
September, when the measurements were distorted due to the rain-gauge being blocked
by leaves. Subsequently a measurement failure was caused by discharged internal and
external batteries.
The Beline rain-gauge was put into service at the end of January 1996, and has since
been in operation, except for two periods when the rain-gauge was clogged (July 1996
and between mid-August and 13 October 1996).
In order to get around these measurement gaps, data acquired on the site were compared
1 133 mm of rain fell during this
with the local Météo-France station at Arbois, where
particularly wet year.
The total accumulated precipitation makes it possible to include time, without focusing
on the periods for which measurements were unreliable. The two sites can be
realistically compared by artificially starting the accumulated amount for the Clucy and
Beline rain-gauges at the value reached on the same day at the Arbois Météo-France
station (Figure 8).
The shapes of cumulative precipitation curves for the various sites are very similar. This
confirms that each rainy episode starts at the same time, without any time lag or
discrimination. However, as at the Arbois station, precipitation is higher at the Clucy
site than at the Beline site. This observation is not constant throughout the year,
probably due to a measurement problem (dirt).
The slope of the curve can be used to evaluate the total rainfall during specific periods.
Precipitation was uniform during the month of May, and of a stormy nature in July.
During 1996, the average monthly input was 95 mm. The wettest months were July,
August and October (about 115 mm/month). May (143 mm/month) and particularly
November (255 mm/month).
BRGM report R 39514
25
Hydroiogical and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jura,
Time
Clucy
pluviometer
La Beline
pluviometer
Meteo-France
Arbois
Figure 8: Cumulative precipitation, Salins-ics-Bains, Jura, France
(Monge and Leroi, 1997)
4.3. UNSATURATEDZONE
Water contents by volume measured by Humilogs provide a certain amount of
information (Monge and Leroi, 1997). A stratification of physical magnitudes is clearly
identified by a surface layer less than 1.5m thick (see Figure 9). Seasonal and sometimes
daily variations are observed, and are superposed on the fast fluctuations following a
rainy episode. O n the other hand, the reduction in the water content is similar to drying
curves and can involve a several different kinetics.
26
BRGM report R 39514
Hydrological and geotechnicai investigation of the 'La Beline' slope, Saiins-les-Bains, Jura,
Figure 9: Soil temperature at the 'La Beline' slope (Monge and Leroi, 1997).
The m a x i m u m values of water content are of the order of 40 to 55%, compatible with
k n o w n porosity values for this type of material (Bogaard, 1996c). These values are
reached (see Figure 10) when the surface soil is saturated following a rainy episode. It is
impossible to refer to a perched water table since this would suggest a certain durability,
it is more like an infiltration front. T o prove its presence, it would be necessary to be able
to monitor it and display its progress over a greater soil thickness.
1
45.0
-
j- -Y
__ __ _
.— _ _ _
\
V
1
1
1i
^
1
-H1
—
—
Hy P
1
1
Figure 10: Volumetric water content at the 'La Beline' slope
(Monge and Leroi, 1997).
BRGM
report R 39514
27
Hydmlogical and geotechnical investigation of the 'La Beline' slope. Salins-les-Bains, Jum.
Conceming the characterisation of the unsaturated zone, 26 soil water retention curves
were determined in the laboratory and 64 saturated permeability tests were executed.
The former were used to produce the bulk density, porosity, and average soil water
retention curves. The latter were used to produce bulk saturated permeability values of
the superficial layer. Together the results can be used to calculate the unsaturated
permeability as a function of water content of the soil according to the Van Genuchten
equations (Van Genuchten, 1980).
The bulk density varied between 1 and 1.8 g/cm^ with an average of 1.5 g/cm^ and the
porosity varied between 36 and 58%, with an average porosity of 43%. No relationship
with depth was found. The average retention curve is given in Figure 1 1 .
Salins-les-Bains
Average retention curve
Van Genuchten parameters
5
alpha = 0 044
0 -i
n=
1
1
4.0 -i
¡0 20
1.0
0
1(1
0 20 n ."0 o-Jii (I y.) 0(0
N'oltiinoliic
ni(»istiiio cdiid'iil
Figure 11: Average soil water retention curve of remoulded marl at the 'La Beline'
slope (Bogaard, 1996c)
Using a non-linear least square curve fitting the Van Genuchten parameters (see
Bogaard, 1996c for more detail), values of alpha=0.044 and n=l.l were determined with
a residual soil moisture of 5%. This gives the following Genuchten-Mualem equation
K(Q)=K..*Q"'[1-(1-Q"f""f
for the unsaturated permeability
as
function of soil moisture:
©^
(Qs-Qr)
where
28
BRGM report R 39514
Hydrological and geotechnical investigation of the 'La Beline' slope, Salins-ies-Bains, Jura,
The saturated permeability was determined in-situ using the inverted-auger hole method
(Kessler and Oosterbaan, 1974). The inverted-auger hole test is an infiltration test in the
unsaturated zone. Afterfillingthe hole with water, the rate at which the water level
drops is measured. The tests were executed at 40 c m below surface.
The 64 saturated permeability tests had a geometric mean of 0.17 m / d , a median of
0.10 m / d and a range of 0.014 - 27.16 m / d . Bogaard (1996c) analysed the measurements
and found no relationship with surface characteristics, such as slope angle or
morphology. The existence of matrix flow and preferential flow was observed.
4.4. SATURATED ZONE
In the saturated zone, the five cells installed have satisfactorily followed the variations of
pore pressures since the end of January 1996 (Monge and Leroi, 1997). However, the
pressure measured by the cell in Cl has been dropping continuously since it was
installed. Consequently, its value is not yet usable. The reverse phenomenon occurred at
D l , which has only been stable since July 1996.
In general these sensors react to precipitation with a low amplitude (see Figure 12) and
within a few days.
0.130
-,
i
i
i.
0.110
- -
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A
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i
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TA
A.
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r
r
h
L
\
A
f
V
/«
!i
i
i
i
i
i
Figure 12: Pore pressure in D2, Salins-ies-Bains (Monge and Leroi, 1997).
Measured pore pressures can be converted into heads (see Table 2), but with some
reserves about the validity of this measurement method. In particular the water level in
Cl after sampling w a s observed at a depth of about 5 m , although instrument
measurements, which must be treated with caution, indicate a value exceeding 14 m .
BRGM report R 39514
29
Hydmlogical and geotechnical Investigation of the 'La Beline' slope. Salins-les-Bains, Jura,
CI after sampling was observed at a depth of about 5m, although instmment
measurements, which must be treated wdth caution, indicate a value exceeding 14 m.
This once more demonstrates that measuring a piézométrie level in a clay slope is a
difficult exercise, and that the pore pressure is only an isolated, damped and lagged
value along a current line.
Boreiiole
Ground
Surface
Sensor
depth
Pore pressures (kPa)
Equivalent
piézométrie
depth
Elevation
(m)
(m)
average
minimum
maximum
amplitude
(m)
CI
404.9
19.5
540*
493
585
92*
about 14
Dl
389.4
4.5
106**
118**
25**
3.4
D2
430.9
7.5
105
83
121
38
6.4
D3b
450.9
9.0
486
478
514
36
4.1
D4
419.3
6.0
75
59
122
63
5.2
93**
non-significant values (continuous decrease since emplacement)
measurements used since 01/07/96
Table 2: Pore pressure cell results (Monge and Leroi, 1997)
4.5. DISPLACEMENTS
Quadrilaterals distributed on the slope have not yet been used to quantify superficial
displacements for which the manifestations are sometimes visible between two
interventions on the site.
At depth, only one point is monitored over the entire site, with the implied limits on its
representativity. After a year of measurements, there is a clear defomiation of an
inclinometer tube between the surface and a depth of about 6.5 m. Tliis movement took
place in two phases; initiation in July 1996 and a much more significant amount in
November 1996. The order of magnitude of the maximum deformation at a depth of
about 3 m is
and 8 mm respectively for the first and second phase. It would be
1
reasonable to relate these displacements to the high precipitation described at the
beeinninc of Julv and then in October and November 1996.
30
BRGM report R 39514
Hydmlogical and geotechnical Investigation of the 'La Beline' slope. Sallns-les-Balns, Jura,
4.6. GEOTECHNICAL PARAMETERS
The triaxial and direct shear tests have given the following results (see Bogaard, 1996b
for more detail). The direct shear test revealed a cohesion of 20.5 kPa and an internal
friction angle of 26.3°. The triaxial tests on samples between 1.4 m and 2.7 m gave a
cohesion of 17 kPa and an internal friction angle of 27°. The triaxial tests carried out on
unweathered marl at a depth of 21 m show no cohesion and an intemal friction angle of
21°
Depth (m)
Number of
Cohesion (kPa)
Intemal
friction angle
tests
Direct shear
0.40
12
20.5
26.3°
Triaxial
1.4-2.7
3
17
27°
Triaxial
21.5
3
0
21°
Table 3: Summary of strength parameters determinedfor samples from
Salins-les-Bains (Bogaard, 1996b)
As shown in Table 3, the strength parameters for weathered marl are almost identical for
the direct shear and triaxial tests. The results for unweathered marl show a clear
difference.
BRGM report R 39514
31
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jum,
32
BRGM report R 39514
Hydmlogical and geotechnical Investigation of the 'La Beline' slope, Salins-tes-Balns, Jum,
5. Modelling
5.1. OUTLINE
The initial concept of the Beline site has been refined after almost two years of
investigations, instmmentation and studies.
Geologically, the two part system composed of Lias marl and Dogger limestone is
superficially affected by erosion and weathering processes. This results in the breaking
up of cliffs such as the 'Roche Pourrie' and remoulding of surface marl. Piedmont rock
falls are also a potential source of material adding broken limestone (Dogger) to the
weathered marl, thus modifying the characteristics, particularly hydrogeological.
Water circulation appears to take place temporarily and preferentially within the
remoulded marl, in which two main layers are distinguished, a top layer (around 5 m
thick) with relatively high permeability and a relatively permeable intermediate layer
(although very irregular) overlying an almost impermeable lower limit, the blue marl.
The assumption of a karstic supply originating from somewhere in the Clucy plateau
remains plausible after a microgravimetric study (Le Jeune and Besse, 1996) and a
tracing campaign (Hemel etal., 1997), although this has not been proven. To confirm
this assumption, it would be necessary to idenfify any springs, which are probably
hidden by rock falls at the contact between the marl and limestone.
Occasional traces of old stone-lined farm ditches to increase resistance against soil
erosion are visible at the site both at the surface and at shallow depth. These "paved"
lines, although partially dismantled, now form a network that artificially redistributes
water within the site and plays a role in superficial instability. In places, they are the
source of small springs.
A morphological study of the site and displacement data shows much evidence of
limited damage (not necessarily recent). It would appear that the slope is subject to
creep and slides of limited amplitude (less than 10 m depth). No signs of largeamplitude movements were detected.
The 'La Beline' slope was modelled hydrogeologically and geotechnically based on the
this global approach.
BRGM report R 39514
33
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jura,
5.2. HYDROLOGICAL MODEL RESULTS
The groundwater model for the 'La Beline' slope is described in detail by Bogaard
and Monge (1997). The present report only contains a short summary of the general
results, without discussing specific model aspects.
Two different types of model were used. Firstly, linear reservoir models (EPL and
GARDENIA) and secondly, fully physical deterministic groundwater flow models
(Marthe and Modflow). The former have the advantage that the number of parameters is
limited and the disadvantage that they do not describe the physical process of
groundwater flow. The latter need a large number of subsurface parameters, which are
generally not available.
The linear reservoir approach is only possible in its most basic form. A model such as
Gardenia (using a series of linear reservoirs) needs several years of input data, and so is
incapable of accurately modelling fluctuations in groundwater level. Physical
deterministic models for describing groundwater flow have not been developed for
undulating regions. In principle, it is possible to use them in this type of area, but
several mathematical problems were encountered. In its current state, the Marthe model
tumed out to be mathematically unstable when modelling the steady-state groundwater
level. The Modflow model worked well in the two- and three-dimensional models that
were made.
The combined model results showed two things. First of all, a time lag of about 7 days
exists between a rain event and the response of the pore pressure device. Both the clayey
material and the type of pore pressure device installed are responsible for this lag.
Secondly, it was shown that recharging was highly dependent on time. In autumn, the
fraction of precipitation that becomes recharge is small whereas in spring, this fraction
is double that of autumn. The exact amounts are very difficult to determine and depend
on the chosen saturated permeability values which are not known in sufficient detail.
Generally it was found that it was impossible to model groundwater level fluctuations in
detail because the calibration period of one year was too short. For the physical
deterministic models, detailed informaUon of the subsurface (especially the first 5-10 m)
is also very important. It is almost impossible to collect sufficient data at this depth. The
average groundwater level and the average fluctuation could be modelled reasonably
with EPL and Modflow, although the timing of the model results was always earlier
than the measured data.
In conclusion, some promising results were obtained in modelling fluctuations in the
groundwater level. However, it is obvious that we are far from a fully physical
deterministic model of groundwater fluctuations on slopes. These data are necessary for
slope stability calculations and thus for waming systems.
34
BRGM report R 39514
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Sallns-les-Balns, Jum,
5.3. STABILITY MODEL RESULTS
Depending on the degree of knowledge conceming geometric and mechanical
conditions, variably sophisticated geotechnical models can be used to analyse the
stability of a slope (Boeijen and Teunissen, 1997):
- one-dimensional failure mechanism;
- two- or three-dimensional methods for displacement or failure, based on assumptions
about this line or surface.
Tlie one-dimensional failure mechanism involves the natural slope (P) given by the
digital elevation model and the intemal friction angle (9'), but ignores the presence of
water and other material properties (equation 5.1).
Safety factor=
tan((p)
r r
Equation 5.1
Two-dimensional failure take account of hydrological and mechanical conditions. The
geometry selected for hydrological modelling is then considered, assuming a failure
plane parallel to the ground surface. The safety factor is then expressed as a fimction of
the slope and the proportion of saturated soil as follows (equation 5.2):
Safety factor =
Y
z
I
^^
j-^r
y z
ir-^
sin(p)-cos(p)
Equafion 5.2
Circular failure is considered using Bishop's method, assuming a geometry in which the
real profile passes through boreholes CI, D3 and D3b. This calculation was done using
the Talren program.
Finally, if there is sufficient knowledge about the site formation and hydrology, the
desire to take account of local features at the scale of the slope could jusUfy the use of a
three-dimensional model such as 3DSTAB or 3D.PENT. This type of calculation has
been carried out on the 'La Beline' slope, and is presented in a special report (Asté,
1997).
One-dimensional failure (see Figure 13) and plane failure have been dealt with using
parameters (see Figure 14) and cartographically (see Figure 15), based on certain
assumptions. Using the data available to us, these various methods produce variable
results (see Table 4), which can be used to determine whether or not there is any local or
global slope instability as a function of the assumptions made.
BRGM report R 39514
35
Hydrological and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jura.
The stability analysis carried out using the Talren program gives a minimum safety
factor varying from about 0.95 to 1.85 (see Figure 16), depending on the mechanical
properties used. These results provide bounds for the problem and emphasise the
importance of a complete investigation survey to evaluate local variations.
221300.00
221200.00
20.00
221100.00
10.00
221000.00
5.00
2.00
220900.00
1.50
220800.00
1.25
220700.00
1.00
220600.00
0.00
220500.oa
869900.00
870100.00
870300.00
870500.00
Figure 13: Safety factor map at the 'La Beline ' slope based on 1-D stability analysis
36
BRGM report R 39514
Hydrological and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jura,
os
Saturated soil ratio
-
- Ftf Ln ISm
Fafl(#« Sm
NMxjim slop» 15*
-faun
Nflutri slope IS*
5m
Ndurti slop* 20*
- Fdhx» ISm
Kdtxd slop* 20*
Figure 14: Safety factor cun'csfor an infinite slope analysis (¤'= 20.5 kPa; ç'= 26.3°j
Method
Assumptions
Safety factor
-D failure
Equation 5. 1
^ = 26.3°
(Lias marl formations)
1.85if/?=15°
1.35ifp = 20°
2-D plane failure
Equation 5.2
C' = 20.5kPa; ^'=26.3°
Map Fig. 13
Nomogram Fig. 14
1
/=17.5kN/m^
failure at 5 m or 1 5 m
C' = 2ü.5kPa; ^'=26.3°
2-D plane failure
Equation 5.2
Map Fig.
15
X=17.5kN/m^
failure at 15 m; h, /z = 75%
2-D circular failure
Bishop (TALREN)
Bishop (TALREN)
Bishop (3D.PENT)
C'=20.5kPa; (p'=26.3°
r=n. 5 kN i ir?
C' = ükPa; í^ = 21°
1.85
0.95
/=I7.5kN/m^
of properties:
PGl to PG4
4 sets
4 failure surfaces:
GLIA to GLID
Table 4: Summary of stability modelling results
BRGM report R 39514
37
Hydrologicaf and geotechnical investigation of the 'La Beline'slope, Satins-les-Bains, Jura,
In order to compensate for the variability of mechanical properties, four sets of
parameters were considered in the three-dimensional modelling of the stability of the
'La Beline1 slope, together with four ellipsoid shaped sliding surfaces (Äste, 1997). The
calculated safety factors vary from 0.85 to 2.00 depending on the assumptions. The
lowest safety factor values correspond to the landslide that occurred during construction
of the children's h o m e . The case of an old, large amplitude slide was also observed, and
an attempt was m a d e to reconstitute the topography prior to its occurrence.
221300.00
221200.00
20.00
221100.00
10.00
221000.00
5.00
220900.00
2.00
1.50
220800.00
1.25
220700.00
1.00
220600.00
0.00
220500.00
869900.00
870100.00
870300.00
870500.00
Figure 15: Safety factor map at the 'La Beline' slope for infinite slope analysis
38
BRGM report R 39514
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Hydrological and geotechnical Investigation of the 'La Beline' slope, Salins-tes-Balns, Jum,
All methods used give consistent results, although they are obviously closely dependent
on the assumptions conceming the mechanical properties and hydraulic conditions.
Although this observation is valid for all methods, there is no doubt that the more
sophisticated methods take better account of singular points.
40
BRGM report R 39514
Hydmlogical and geotechnical Investigation of the 'La Beline' slope, Salins-tes-Balns, Jum,
6. Discussion and Conclusion
The procedure proposed for the HYCOSI project has been applied to the Salins-lesBains site in the French Jura. It can thus be used to evaluate methods and means used
during each of the successive steps in order to determine the role of water, in space and
time, with respect to slope instabilities, namely:
- characterisation;
- investigation;
- simulation.
The tasks carried out during these steps can be broken down and analysed as a function
of objectives and achievements, the means used and their efficiency, and proposed
adaptations or improvements.
6.1. SITE CHARACTERISATION
In our case, site characterisation means recognising, at any time, the 3-D geometry of
formations, their characteristics and hydrogeological state.
6. 1.1. Geometric characterisation
A number of disciplines are concerned in geometrically and qualitatively evaluating the
ground that exists on the 'La Beline' slope.
The geoIog>' was described by referring to the explanatory notes for the Salins-les Bains
sheet, together with site observations extending beyond the study area. This was then
clarified by investigation boreholes. This sequence of techniques, in increasing order of
costs and results, provides global information, then superficial information and finally
specific information. For specific information, the concept may or may not be close to
reality, depending on the complexity of the site and the amount of investigation. Since
there were several objectives in putting down boreholes at Salins-les-Bains, including
lithological observations (cored boreholes), metrology (pore pressure cells and
inclinometry) and taking samples, not all surveys were extended as far as the fresh marl
substratum, which was only encountered on two occasions (CI and D3). Systematically
taking boreholes down to the fresh marl would have provided additional information,
but would also have cost more. The ratio between these two variables is specific to each
site. This is why it is sometimes preferential to use a combination of investigation
boreholes and instrumentation techniques.
BRGM report R 39514
41
Geophysical methods, such as electrical exploration and seismic refraction, can
compensate for this deficit when they are fully operative. They are intended to clarify
and extend geological knowledge, which in turn is used for calibration. The
geoelectrical methods
sounding and profiling
applied to the marl at the site give
globally significant results but without much contrast, and are therefore difficult to
interpret and use in detail. This is simply a fact inherent to the method which is not
really suitable for this type of material. Seismic refraction is more promising and gives
global guidelines with clear evidence of a refractor. By retro-analysis and referring to
geological observations, the refractor found does not represent a lithological boundary.
The interpretation of dromochomes with a seismic wave velocity increasing along a
gradient in each layer would appear to be more correct than the assumption of layers
with constant velocities. The specific equipment used during the present study could not
handle this type of treatment.
In order to geotechnically characterise known formations, borehole parameters were
recorded and samples were taken for laboratory tests. Not many deep samples were
available since they were taken from the only cored borehole. On the other hand,
samples near the surface are better distributed over the entire site, but they only
characterise the superficial layer down to a depth of 2 m. A better sample distribution
would also represent an extra cost similar to the cost of exploration boreholes.
6.1.2. Hydrogeological characterisation
A hydrogeological inventory also
for site characterisation. This is
done by distinguishing the hydrodynamical environment, water input to the site, and the
various water conditions within the slope.
needs to be made
In order to determine the hydrodynamical characteristics, permeability tests were carried
out in situ and in the laboratory conceming the saturated or unsaturated state. These
measurements were distributed around the site but even so are incomplete since they are
superficial with respect to the corresponding inherent limits. Permeabilities of deep
layers (more than 2 m) were estimated but not measured. Furthermore, this type of test
concerns very small sample sizes and therefore, not all components such as
macroporosity or bulk permeabilities are considered. Additional resources derived
through drilling (even rudimentary such as a motorised auger) would be useful to
improve the distribution of samples taken for geotechnical and hydrogeological
purposes. Special tests must be made to eliminate scale effects. For example, a trench or
a borehole could be drained or filled by gravity (permeability tests) or under forced
conditions (test pumping). Their feasibility needs to be considered by examining the
damage, cost, combined usefulness (geological observation, instrumentation) efficiency
depending on the type of material and consequences with respect to stability.
The water supply to a slope normally consists of three components - meteoric water,
surface water and groundwater. With a fairly small topographic catch basin and the lack
42
BRGM report R 39514
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Salins-les-Bains, Jum.
of any watercourse, even intermittent, surface input is negligible on the 'La Beline'
slope. The meteoric component is evaluated by placing two rain-gauges in the lower
part of the site and on the Clucy plateau dominating the slope. A comparison between
the results validates the data acquired in each and régionalises them. However, MétéoFrance data are used as a reference due to their low cost and abundance, and particularly
because the data sets are directly comparable with each other in the case of Salins-lesBains. Finally, groundwater supply is a reasonable hypothesis since there is a limestone
plateau above the site. A tracing experiment was performed at the scale of the plateau in
an attempt to identify this component, and eliminated some areas which have a different
outlet. However, no conclusion could be drawn about a relation between Clucy and Fort
Belin, particularly because the microgravimetry campaign carried out at this location
showed a strong minor anomaly that could be caused by zones that could act as vector
for karstic circulations. A Proton Magnetic Resonance profile was made to improve
knowledge, but did not demonstrate any significant water circulation. In conclusion,
outlet traces still have to be foimd at the site; these are probably at the marl-limestone
boundary, but concealed under fallen rocks at the bottom of the cliff. It is particularly
important to solve the problem conceming the groundwater supply to the slope, since
the effective proportion of precipitation is precisely known. As far as possible,
consistency should be maintained between the orders of magnitude of the precision for
each supply component.
Three geophysical methods were used to determine the variation of water conditions
within the slope. Electrical panels placed below the site make use of geoelectrical
methods. Consequently, the same comments about the relevance of the absolute values
obtained can be made as for the survey as a whole. Most work carried out as part of the
HYCOSI project concentrated on developing the system, and it now remains to be
demonstrated whether or not it is relevant to use this as a means of continuously
monitoring the moisture condition of the soil.
The configuration of the Proton Magnetic Resonance tool tested on a marl slope within
the project proved to be unsuitable for this type of topography and lithology. A new
acquisition system that also better satisfies other criteria (for example environmental)
should be developed experimentally.
Finally, self polarisation profiles provide qualitative information that deserves further
investigation, repeated testing with time, and combination with qualitative water
measurements.
Pore pressure cells, which are preferred to open piezometers, were installed at the site in
order to indirectly measure water levels. This type of sensor, which is very robust, is
placed in a sealed chamber and measures a local pressure. Spatially, it can help to
individualise groundwater flow, however, this can cause representativity problems in
environments that are too heterogeneous for the number of available sensors. In the long
term, this value is only valid as a stationary value, or in very pemieable environments.
Fluctuations are damped and there is a lag before the piezomctric surface varies in the
case of a clay slope. The results from pressure cells must be analysed with caution,
particularly depending on their position with respect to the failure surface. Due to the
BRGM report R 39514
43
Hydmlogical and geotechnical investigation of the 'La Beline' slope. Sallns-les-Balns, Jum,
damping and lag, a deeper cell could underestimate the state of instability of a slope and
an imminent failure.
Humidity sensors (Humilog) are designed principally for taking measurements of water
content by volume in unsaturated zones, and were found to be useful in confirming
assumed periods of environment saturation and stratification of reactions of superficial
layers. Their layout is currently very superficial, and it would be worthwhile installing
more of these sensors in depth and on the site. Hopefully, it would be possible to
indirectly demonstrate and monitor a vertical infiltration front and lateral flows. The
depths of saturated soils is an indicator reducing developed pore pressures. In practice,
this does not cause any particular additional problems, however the reliability of this
sensor tested during the study needs to be improved somewhat. Extreme care is
necessairy in placing it and the difficulty inherent to this type of instrument, namely how
to place it in a borehole and then take representative measurements in an undisturbed
environment, has to be solved. One solution, although expensive, is lateral penetration
from a borehole. In any case, this technique remains destructive.
6.2. SITE INVESTIGATION
Site investigation involves monitoring £ind quantifying changes with time of a range of
representative or relevant parameters. An acquisition system or manual intervention is
necessary in order to obtain a measurement taken with sensors.
The physical magnitudes that are sought at a site with ground movement such as Salinsles-Bains concern water and signs of movements (strain gauges and inclinometers).
Through its meteoric component, water forms part of the system input data and through
its groundwater component, it translates the state of the environment. The objective of
placinginstruments on this type of slope has been achieved for rain, water contents and
pore pressures. These magnitudes can be monitored automatically with a sampling
interval of less than 3 hours using two rain-gauges, soil moisture sensors and pore
pressure cells. Soil resistivity was also measured, remotely but based on a specific
command rather than in a predefined acquisition mode. This procedure is more flexible,
but also more constraining.
On the other hand, signs of movements perceptible using an inclinometer and
quadrilaterals were measured manually and not automatically. At Salins-les-Bains, this
decision was made partly because of the acquisition interval which is pointlessly high
considering variations in the monitored magnitudes, and partly because of the cost of
automatic equipment (clinochain and strain gauges), considering the frequency (monthly
or several times monthly) of measurements made at an easily accessible site.
Furthermore, if several strain gauges were necessary for each quadrilateral, there would
be problems of volume occupied considering the land used, without any guarantee that
they would be effective in the long tcmi.
44
BRGM report R 39514
There are no particular technical difficulties with the sensors for monitoring water and
movements in an accessible and relatively unagressive site such as Salins-les-Bains. A
compromise has to be made between the required acquisition frequency and the cost of
sensors, compared with the work that would be required by an operator on a site with a
given accessibility. However, the representativity of pore pressure measurements made
in a marl slope is uncertain, and measurements of water content by volume in
sufficiently deep boreholes may be used as well or in preference. Similarly, it is wise to
choose and to be able to adapt the acquisition mode so that measurements can be made
at regular intervals, whenever an alarm threshold is exceeded or at any other time.
Another objective of site monitoring is to evaluate the robustness of sensors. Although
pore pressure cells and rain-gauges behaved well, since all sensors were in working
condition after 16 months of activity, a third of the soil moisture sensors became
defective when they were first installed or later. Defects included doubtful
measurements or characterised break downs, caused partly by imperfect installation
(soil/sensor contact), and also unidentified causes. Furthermore, the resistivity panels
installed were unusable after twelve months on site. Like other sensors in isolated cases,
this problem is caused by the cable. Cables connecting the acquisition system to sensors
are particularly vulnerable to extemal aggression (accidental cuts, damage by animals,
ground movements), both for buried cables such as syscal, and locally visible cables. In
order to improve the life of sensors, HF type links should be developed and used to
replace long distance cable connections as soon as possible. Apart from reducing the
vulnerability of the instrumentation, this would provide the advantage of lowering
installation costs and a subsequent increased flexibility of the installation during
operation. A more efficient method of placing soil moisture sensors will have to be
found. An optimum installation procedure may be considered making use of a kit for
drilling a preliminary hole and then forcing the sensor to its required depth under good
conditions, or a set of sensors could be installed on the same vertical line. A readerrecorder (for example Hydris, IRIS Instmments) could be used to make an immediate
check if it works, which would avoid the situation of having sensors that don't work
immediately after their installation.
Furthermore, measures that were made correctly may be distorted, as in the case of the
pore pressure cell in borehole SCI, but also apparently with the Beline rain-gauge. A
perceptible drift is observed after comparison with the Clucy and the Arbois MétéoFrance station rain-gauges, probably due to gradual or deliberate dirt accumulation in its
receptacle. The only way of discerning and avoiding this measurement problem is to
perform regular maintenance or by using a larger number of measurement points, and
particularly a reliable reference. This would not be possible on a site with a more
difficult access or with very heterogeneous precipitation conditions (summer storms,
influence of mountains, etc.).
Monitoring the variation of parameters with time ideally requires a standalone,
permanent acquisition system with a remote access. Its autonomous operation depends
largely on the power supply to the system. At the Beline site at Salins-les-Bains, the
BRGM report R 39514
45
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Sallns-les-Balns, Jum,
Osiris and Syscal systems were plugged in to the mains power supply, which meant that
modifications had to be made to Syscal, which normally mns on batteries and can be
affected by the background noise on the 220 V power supply. The Madotel controlling
the Clucy rain-gauge is powered by an extemal battery that recharges the intemal buffer
battery. This system worked for 10 months until the first cold spell of the winter, which
probably caused a severe discharge in the extemal battery. Therefore, this solution
requires more maintenance. Alternative power supplies making use of "renewable"
energies were not tested within this project, and it would be necessary to make sure that
the time necessary to recharge the batteries is compatible with a sufficiently frequent
acquisition interval.
Remote management of acquisition systems through a telecommunication network and
modems did not cause any technical problems. This is a relatively reliable operation
conceming sensor calibration, management of the system and importing data.
Modifications to instmments were necessary to avoid the need to use the telephone
network continuously when making measurements using the electrical panels.
The acquisition system in the Beline children's home was placed inside an electrical
cabinet enclosed in a padlocked wood shed to protect its integrity. Lightning protection
was also provided by fuses.
Once collected, measurement data must be imported, processed, stored and then used. A
well-defined procedure is applied to facilitate this sequence of operations, in particular
using macrocommand modules created to carry out repetitive tasks. However, the
procedure was not completely automated. This would be a desirable objective, for
example using the concept control software for the Osiris system.
6.3. SIMULATION
The main objective of a site study after having characterised and observed it, is to
understand it, which means being able to conceptualise and then reproduce or simulate
what is happening. Several complementary models, or with increasing complexity, may
be developed. All must be compared with the reality of observations to evaluate their
accuracy.
6.3.1. Conceptual models
A model cannot be created unless the site has been sufficiently characterised and its
dimensions and parameters are known. Its boundaries and permanent or temporary
conditions also have to be described. Finally, the main physical magnitudes involved in
a model must be described.
46
BRGM report R 39514
Hydrological and geotechnical Investigation of the 'La Beline' slope. Salins-les-Bains, Jum,
In the case of Salins-les-Bains, the perceptions of the phenomena involved were refined
firstly by the prior knowledge of the site and then in the light of new investigations. The
first result was an implicit model based on the expertise, globally describing the site in
more detail. This outline was then rationalised when the description of a facies is
improved by a permeability measurement or a layer thickness. Finally the concept
becomes clearer when a larger number of measurements are made or more detailed
geometry is taken into account. The objective is to know the necessary and sufficient
information density for the complexity of the model being used, which must satisfy
expressed requirements. For example, initially there is no need to use 3-D models if the
geological layers are of constant thickness and parallel to the surface. A judgement can
be made regardless of the information that is available, and this judgement will be more
reliable if exhaustive and detailed information is available. An equilibrium between the
reliability of the results, the cost of investigation and response times has to be found.
At Salins-les-Bains, apart from a simple qualitative description, the precision of the
model is limited by a lack of hydrogeological and geotechnical characteristics for deep
facies and uncertainty about some boundaries such as the top of fresh marl, or the extent
of the saturated zone. The model could be improved by increasing the amount of
conventional exploration and carrying out specific investigations using better adapted
methods.
6.3.2. Calculation program
After considering the qualitative approach, quantitative modelling very quickly involves
equations and even computer programs to more explicitly predict the state of a
magnitude at a given time. The purpose of the HYCOSI project is thus to test and
develop tools and methods (quantitative and/or qualitative) so that the hydrogeology and
stability can eventually be simulated in three dimensions. The first step should be to
explain the global approach, followed by the use of partial operational models that are
improved in steps. With this in mind, a number of calculation programs were selected
following bibliographic studies (Van Esch, 1995) and the development of conceptual
models (Leroi, 1995).
The simplest hydrogeological models used are global. They consist in a series of
sequences of reservoirs in series or in parallel. Their usefulness is limited since the best
they can do is to simulate the variation with time of a local magnitude considered to be
representative. Furthermore, simulation parameters do not always have any physical
correspondence.
The stability calculation methods used provide partial solutions with increasing
precision depending on the assumptions made.
The use of more or less detailed deterministic models, possibly designed for other
applications, were tested using more and more precisely formulated assumptions. The
transposition of hydrogeological models developed and used in the context of aquifers
with low gradients and fairly flat topography creates new problems when working on
BRGM report R 39514
47
Hydrological and geotechnical investigation of the 'La Beline' slope, Sallns-les-Balns. Jum,
the subject
of slope instabilities. Thus, finite difference models such
as
Modflow or
Marthe inevitably introduce "steps" when modelling a slope topography or a geological
layer with a non-negligible dip. Depending on their amplitude, the analyst and the
calculation program manage these step configurations to a varying degree. Therefore a
compromise has to be found between the size of the mesh and the size of the computer
model. Working on finite element calculation programs is undoubtedly one method to
be improved.
The mathematical convergence of a model may depend on assumptions about boundary
conditions. In this case, a search should be made for a numerically correct solution that
does not conceal the physical reality of the phenomena, and vice versa.
6.3.3. Modelling
The objective of modelling is to reproduce the reality as it is perceived, partly through
instmmented measurements. Deterministic hydrogeological models introduce many
constraints and are difficult to use. Furthermore, the results obtained are inconsistent.
Marthe is still incomplete in that problems may arise in finding a mathematical solution
or in the physical validity of the results; results obtained from Modflow are more
encouraging.
It is also much easier to maintain control over hydrogeological modelling through
continuous instmmented measurements, than modelling of instabilities which predicts a
state that cannot easily be observed.
Deterministic modelling is a long and difficult exercise in which a large number of
parameters have to be determined. Therefore in principle it is only justified for very
precise purposes.
Simple models, sometimes with a few difficulties or restrictions, frequently give rapid
results but with a usefulness limited to what can be expected from this type of tool.
Global monitoring also requires prior knowledge of the site or a learning phase.
48
BRGM report R 39514
Hydrological and geotechnical Investigation of the 'La Beline' slope, Salins-les-Bains, Jum,
References
AstéJ.-P. (1995) - HYCOSI- Impact of Hydrometeorologic Changes on Slope
Instability. Synthèse des travaux 2ème année (1995) : historique de la
connaissance sur les sites de Boulc-en-Diois (26) et Salins-les-Bains (39)
(Summary of work done in the 2" year - ccmipilation of knowledge about the
Bould-en Diois (26) and Salins-les-Bains (39) sites). JPA Consultants report
10395.
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BRGM report R 39514
49
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' dossier n° 95.269/177.
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spontaneo. Rapporto finale 'sull'attivita'di ricerca svolta presso il BRGM/DR,
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de Salins-les-Bains (Jura) (Microgravity study in test profiles on the Salins-les
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Leroi E. (1995)- HYCOSI- Impact of HYdrometeorologic Changes On Slope
Instability : rapport d'avancement 1ère année (1994) (First year progress report 1994). BRGM report R 38389.
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Slope Instability. Reconnaissance et instmmentation du site de Salins-les-Bains
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50
BRGM report R 39514
Hydmlogical and geotechnical investigation of the 'La Beline' slope, Satins-les-Balns, Jum.
Monge O.and Leroi E. (1997) - HYCOSI - Impact of Hydrometeorologic Changes on
Slope Instability. Instmmentation du site de Salins-les-Bains (39) : rapport
d'avancement 3ème année (Instmmentation of the Salins-les-Bains site (Jura) 3^*^ year progress report). BRGM report R39172.
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BRGM report R 39514
51

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