The advantages of multicriteria analysis for diagnosis of internal erosion phenomena

A. Garandet1 Compagnie Nationale du Rhône, Lyon, France

ABSTRACT

Over the last ten years the Compagnie Nationale du Rhône, part of the energy branch France of Groupe GDF/SUEZ, in partnership with EDG (Européenne de Géophysique), has developed novel techniques to facilitate understanding of thebehaviour of soils and flows. This communication presents the case of a dike under load, subject to internal erosion. Itsbehaviour was determined by a multicriteria analysis based on different in situ investigation methods and a novel method,geophysical tracing. Keywords: dike, geophysical method, water path detection, diagnosis, internal erosion, underground flows.

1 Compagnie Nationale du Rhône - Service DI-CEN, 2 rue André Bonin, 69004 Lyon, France. a.garandet@cnr.tm.fr

1 INTRODUCTION

Despite the development of efficient dimensioning tools and increased knowledge in the domain, the analysis of earthwork structures remains a discipline in which many types of available data are collected in order to evaluate the behaviour and risk of earthworks structures and finally propose actions to correct and reinforce them.

Each type of ground survey technique generally provides only partial data and is not comprehensive. In this paper it is demonstrated that the use of the Water Path Détection technique (WPD), a geophysical method, together with temperature and conductivity measurements leads to a behavioural and dynamic approach. The combination of these criteria validates or invalidates certain hypotheses, refines diagnosis and risk assessment

and determinates the reinforcement strategy to be implemented.

The paper will present a case study, namely the “incident at Chavanay dike”, the diagnosis of which was established following a muticriteria analysis based on review data, classical in situ investigations and, mostly, the global behaviour of the groundwater highlighted by WPD.

2 DESCRIPTION OF THE PROBLEM

The Chavanay dike was built together with the construction of the hydropower development of Péage de Roussillon in 1978. It is about 10 m high.

The considered section is a widened dike that marks an area of transition between a gravel dike and, silt and gravel dike. It is about 300 meters long and some sinkholes have occurred in it. For 25 years of observation, nine sinkholes have

been detected to date. They have mainly damaged the crest of the dike, the upstream slope; and some cases have been also been reported in the downstream slope. Internal erosion is therefore an active phenomenon and reinforcement in the mid-term was decided, taking into account the geometry of the widened dike.

In addition, in this section, the drainage canal had been sealed to prevent any contact between the drainage canal and the well pumping located 30 m from the dike (Petite Gorge well). Drainage of the toe of the dike was made possible by a drain excavated in the downstream gravel crest (figure 1).

2.1 Hydrogeological context

The natural ground is characterised on the surface by a mainly silty-sand covering from 1 to 4 m thick covering a large aquifer of about 25 m. It lies on a marl substratum of variable height forming a "furrow" parallel with the Rhone (probably a paleochannel of the Rhone) towards of the Petite Gorge.

Figure1. Diagram of hydrogeological context

The aquifer is mainly composed of sandy

gravel including sandy levels thick enough to impact the flows of the groundwater. Two sources feeding this groundwater were identified: the slopes of the Pilat mountain, to the west and the reservoir of the Rhône to the east. The pressure exerted by either of these sources fluctuates with the seasons (Figure 1 and [1]).

2.2 Soil and flow parameters

The first geotechnical/geophysical surveys implemented, made it possible to identify the geological structure of the body of the dike and its foundation (cf. figure. 3 and [2], [3]). They showed a lower level of mechanical resistance (using a static penetrometer) within the silty-sand layer of the foundation. This horizon was located on the upper interface.

These investigations provided a set of partial data that could not account for the internal erosion of the dike. Tests using a current meter (water velocity measurements) were performed in two piezometers lowered to the substratum and where significant movements and current speeds were detected. The results led the following investigations towards the use of Water Path Detection2 to identify the flow dynamics in and under the dike.

3 GENERAL DESCRIPTION OF THE GEOPHYSICAL MEASUREMENT METHOD

Analysis of water circulating in the subsoil was considered by brine tracing (NaCl) with the WPD method. This entails both geophysical tracing, in which the spreading of a cloud of salt injected into the soil is tracked by electrodes installed on the surface, and chemical tracing (influence of brine measurable in the piezometers or at another water point). In addition, the measurement is reversible, i.e. the injection of the current at the source of the trace is also used as a source from a target piezometer or a pumping well.

The tracing is compared to a "zero state" that provides an initial image before injecting the brine. This "zero state" partly qualifies the natural flows and those linked to possible pumping. After injecting the brine in the system, the directions of the flows are amplified by the NaCl under the application of an electric current.

2 Water Path Détection ™ (WPD) = Brine + electrical

measurements at a source point + measurements at accessible water points.

In parallel, conductivity (and temperature) measurements are performed at different points to control the chemical effect of the tracing. The measured conductivity permits controlling the speed of transfer, the concentration in dissolved salts and dispersion.

The aim of the interpretation is to monitor the displacement of the brine as accurately as possible in order to obtain information on the circulation of water in the soil. To do this, the initial method employed consists in analysing differences in the variation of potential through time by using modelling results. In practice, although anomalies are usually clear, their interpretation is complex and ambiguities exist.

The method presented in this study consists in obtaining a model of the ground by 3D inversion, by using each recording of electric potential through time. The differences between the models obtained successively must then be visualised, to localise the displacements of the brine.

3.1 General principle, theoretical elements

Processing tests were performed on the results of the numerical model, to study the method's range of interpretation possibilities. During each test, a succession of brine progression states was visualized in the framework of a borehole. We calculated the potentials at the surface when injecting an electric current in the borehole and processed the maps of the potentials obtained by inversion, to acquire a succession of models of the ground. We then calculated the relations between the successive resistivities at the same points of the ground. The isovalues of the surfaces of these resistivity relations were then plotted. This type of simulation was also used to study vertical flows. The simulation included the following successive steps: - State 0, or initial state - State 1: the borehole is filled with brine - State 2: the brine in the borehole invades a

block of ground. Resistivity decreases in the ground.

- State 3: the brine propagates horizontally and invades a second block of the same size.

State 3 is shown in vertical section at the top of Figure 2. The map of the potentials calculated as surfaces can be seen in the middle of the figure. Four numerical 3D models were built. Subjecting these values to an inversion procedure made it possible to provide a model of the ground for each of the four states. The three relations between the successive resistivities R1, R2, R3 were calculated at the centre of each mesh of the models obtained. The 3D view at the bottom of figure 2 shows the surfaces of the iso-values of relations R1, R2 and R3.

The plotted surfaces outline the volumes invaded for the successive states where the resistivity of the ground calculated decreases under the influence of the brine.

3.2 Utilisation at Chavanay

This survey campaign highlighted complex and variable flows from one area to another. It brought to light the following points: - areas of concentrated vertical flows, - superficial flows in the dike taking the

direction of the dike (roughly north-south) in line with the general trend of the seepage accompanying the Rhone, whereas the transfers between the groundwater and the Rhone in the section in the direction of the plain occurred from east to west;

- correlation of the deep preferential flow directions, with areas of vertical flows and areas of sinkholes, see Figure 3 and ([4], [5]).

4 MULTICRITERIA ANALYSIS AND RESULTS

The following table gives criteria classified by theme, which, provide understanding of the dike's behaviour. Only the criteria relevant are showed, hence this partial list:

The results of the multicriteria analysis answered four essential questions for the future treatment of the dike [5]:

Figure 2. Simulation of electric tracing with salt and 3D inversion processing of the potentials calculated by digital modelling.

PREFERENTIALDIRECTION OF

GROUNDWATERIN DEEP LAYER

PREFERENTIALDIRECTION OF

GROUNDWATERIN DEEP LAYER

Figure 3. Synthetic top view of tracing performed.

Figure 4. Hydro-geotechnical behaviour deduced from the multicriteria analysis.

Table 1. Analyzed criteria

Themes Criteria Detection indicators and resources

Geomorpho-logical context

Presence of a paleochannel alongside the

Rhone

Plans from records and bathymetric

survey of the Rhone

Roof of substratum: 30m below the dike

Records of the development scheme

and soundings Nature and size of

the aquifer Hydrogeological

studies Behaviour of the

groundwater Piezometers installed

in the site

Geological/ Hydrogeo-

logical context Flows:

Rhone→groundwater and groundwater

→dike

Geophysical tracing and temperature measurements

Nature of the materials: sand-silt Coring

Quality of materials: weak area at the sand-

silt/gravel interface

Penetrometric soundings

Presence of sinkholes in the

dike

Localisation and number (9 sinkholes

in 25 years)

Dike body

Infiltration of water from the upstream

slope

Thermometric surveys, analysis of

piezometry

- Identification of malfunction: active

internal erosion was observed, more specifically erosion at the contact between silty-sand foundation and the gravel of the aquifer.

- Degree of degradation of the dike: this was difficult to quantify, but a weaker layer of silty sand 2 meters thick was identified, and sporadic decompress areas were found in the dike body.

- Origin of the phenomenon: high groundwater circulation speed and vertical flows.

- Geographical extent of the phenomenon: it appeared to be focused in 2 deep flow paths.

5 CONCLUSIONS

5.1 Innovative part of the method

Geophysical tracing permits dynamic acquisition of knowledge of the phenomena involved. It is a new way of approaching diagnosis of earth structures.

Indeed, analysis of dike incidents often involved a number of surveys and tests that give an image of the dike at particular moment, whereas this method underlines the importance of visualising the phenomenon dynamically in the case of internal erosion (a phenomenon itself governed by flow dynamics).

The large number of tests performed, (without awareness of these dynamics), failed to detect the extent of the behaviour of the plain groundwater and its impact on the dike at Chavanay. Geophysical tracing used in combination with other measurements and data (temperature, conductivity, state of materials and historical observations) showed that the overriding factor involved in the malfunctioning of this dike was its location in a specific and complex hydrogeological context, impacting on the foundation materials used in the dike.

5.2 Advantage for reinforcement design

The reinforcement of a dike under permanent load cannot be performed without having analysed the whole problem. This means that the entire structure must be considered; not only the manmade dike itself, but also the foundations and the geographical, geological and geomorphological context specific to each site.

Global understanding of the phenomenon (identification of the mode of malfunctioning, the state of degradation of the dike, the origin of the phenomenon and its perimeter) lead to finding an adapted solution that: - responds to the reinforcement of the

incident area in the perimeter defined, - validates the implementation of the

reinforcement, - ensures that the reinforcement does not

harm the areas bordering the area treated (displacement of the problem upstream of the dike, an indirect effect caused by plain groundwater).

Depending on the results of the analysis, reinforcement can include strengthening the ground, drainage works and sealing works.

5.3 The next steps and development of the method

The analysis method has been implemented on other incident areas of the dikes managed by the Compagnie Nationale du Rhône, with the aim to validating and improving the methodology (3 studies in progress). These latter case studies also rely on temperature, which can be a good criterion of seepage.

ACKNOWLEDGEMENT

I would like to thank Pierre Frappin, from Européeenne de Géophysique, who helped realizing this paper and I am grateful for the support provided by Laurence Duchesne.

REFERENCES

[1] Service Régional d’Aménagement des eaux Rhône-Alpes, Nappe de plaine de Chavanay - Puits de la petite Gorge - Etude de la pollution par le fer et le manganèse 3-HYD-1986 BB /RMB -Avril 1986.

[2] Rapport de sondages - Campagnes de sondages au pénétromètre statique, forages carottés, mise en place de piézomètres et essais d’eau - SONDALP, CS 2897 –Avril 2008.

[3] Sondages pénétrométriques, sondages carottés, essais Lefranc, essais de laboratoire vers le PK48 (SIC Infra – AMAP’Sols, Décembre 2006.

[4] Européenne de géophysique (EDG), Diagnostic de digue par traçage géophysique -affaire n°08.11.451/42 –ERP n° 00010541, Février 2009.

[5] Compagnie Nationale du Rhône (CNR), Direction de l’ingénièrie, Aménagement de Péage de Roussillon – incident de digue de Chavanay – Diagnostic, Novembre 2010.