**2.3 Geotechnical testing**

Geotechnical testing are generally carried out after the first investigations (prior knowledge of historical building and materials, localisation of heterogeneous areas). The final interpretation of geophysical measurements is only relevant when coupled with geotechnical testing. People interpreting the measurements have to decide to extrapolate - or not – the local tests to the rest of the dike.

Geotechnical testing locally provide physical parameters of the dike body that are required for a good diagnosis. A detailed methodology is given in (Lino et al., 2000)

Penetrometric tests are generally the first geotechnical method used to provide information about the soil density (derived from the measured dynamic resistance (in MPa) with regard to depth) and the layer thickness in the dike body. It consists in hammering a conical tip in soil with some characteristics depending on the penetrometric device. The depth of penetration can easily reach 10 m.

Permeability testing (e.g. the Lefranc test) consists in drilling a borehole, injecting and pumping water in an open-ended cavity, called a lantern, at the bottom of the borehole. It measures the variations of hydraulic head and its flow rate and gives the permeability around the lantern. Some devices evaluate both the soil density and the permeability.

Shear tests with phicometer provide the shear strength and the friction angle of soil. It consists in a probe – metal expansion shells - fitted with horizontal annular teeth inserted into the borehole. The shells move only laterally so that the teeth dig the soil. The method needs a good drilling quality with no lining – not the case in highly heterogeneous soils – and is not suited for soft soils.

A local investigation can be carried out with a mechanical shovel, digging a pit in the dike body or at its toe. It provides the distribution of materials.

Mechanical drilling basically provides the advance speed in borehole, and the location of interface layers. In case of destructive drilling, materials are breaking up and transported to the surface (cuttings) using a circulating fluid or an helicoidal cutting tool (auger). If percussion or rotopercussion conducts drilling (for cohesive and rocky soils), the analysis of cuttings can be difficult, but more information is provided by registered parameters like advance speed, tool pressure, circulation fluid pressure... The auger is applied mostly for loose and poorly cohesive soils and allows to take some material samples for lab-test analysis (water content, Atterberg limit, …). In case of core drilling - non-destructive testing – soil samples are extracted directly from borehole without modifying physical properties of soils. Then the samples can be packed and sent for lab testing. Core drilling is local, more expensive and more time consuming than destructive drilling, but provides very useful information for assessing dike properties.

All these methods require a free access to vehicle in the measuring location (crest and/or toe of the dike).

### **3. The airborne LiDAR as an efficient tool for topographical survey and detection of surface anomalies on dikes**

#### **3.1 Backgound on LiDAR systems**

270 Novel Approaches and Their Applications in Risk Assessment

2007). In that case, ERT provides additional measurements for processing the data. Some temperature probes could also be buried in the dike and the temperature variation could be

Geotechnical testing are generally carried out after the first investigations (prior knowledge of historical building and materials, localisation of heterogeneous areas). The final interpretation of geophysical measurements is only relevant when coupled with geotechnical testing. People interpreting the measurements have to decide to extrapolate - or

Geotechnical testing locally provide physical parameters of the dike body that are required

Penetrometric tests are generally the first geotechnical method used to provide information about the soil density (derived from the measured dynamic resistance (in MPa) with regard to depth) and the layer thickness in the dike body. It consists in hammering a conical tip in soil with some characteristics depending on the penetrometric device. The depth of

Permeability testing (e.g. the Lefranc test) consists in drilling a borehole, injecting and pumping water in an open-ended cavity, called a lantern, at the bottom of the borehole. It measures the variations of hydraulic head and its flow rate and gives the permeability around the lantern. Some devices evaluate both the soil density and the permeability.

Shear tests with phicometer provide the shear strength and the friction angle of soil. It consists in a probe – metal expansion shells - fitted with horizontal annular teeth inserted into the borehole. The shells move only laterally so that the teeth dig the soil. The method needs a good drilling quality with no lining – not the case in highly heterogeneous soils –

A local investigation can be carried out with a mechanical shovel, digging a pit in the dike

Mechanical drilling basically provides the advance speed in borehole, and the location of interface layers. In case of destructive drilling, materials are breaking up and transported to the surface (cuttings) using a circulating fluid or an helicoidal cutting tool (auger). If percussion or rotopercussion conducts drilling (for cohesive and rocky soils), the analysis of cuttings can be difficult, but more information is provided by registered parameters like advance speed, tool pressure, circulation fluid pressure... The auger is applied mostly for loose and poorly cohesive soils and allows to take some material samples for lab-test analysis (water content, Atterberg limit, …). In case of core drilling - non-destructive testing – soil samples are extracted directly from borehole without modifying physical properties of soils. Then the samples can be packed and sent for lab testing. Core drilling is local, more expensive and more time consuming than destructive drilling, but provides very useful

All these methods require a free access to vehicle in the measuring location (crest and/or toe

correlated to the presence of water in the dike body (Radzicki & Bonelli, 2010).

for a good diagnosis. A detailed methodology is given in (Lino et al., 2000)

**2.3 Geotechnical testing** 

not – the local tests to the rest of the dike.

penetration can easily reach 10 m.

and is not suited for soft soils.

information for assessing dike properties.

of the dike).

body or at its toe. It provides the distribution of materials.

Airborne laser scanning (also called ALS) or LiDAR (Light Detection And Ranging) is an active remote sensing technique that provides georeferenced distance measurements between an airborne platform and the surface. It measures the time-of flight of a short laser pulse once reflected on the Earth surface. Strips of several kilometres, with a high overlapping ratio, provide the surveyed topography. The attitude of the airborne platform is acquired by both a GPS and an inertial measurement system. Distance measurements are then transformed into georeferenced 3D points. A detailed description of the processing chain can be found in (Mallet & Bretar, 2008) and (Shan & Toth, 2009).

The height accuracy (resp. horizontal accuracy), at the top end process, is less than 0.05 m (resp. about 0.40 m ) or less depending on the flying conditions as well as on the surveyed topography.

Moreover, such active systems, called multiple echo LiDAR, allow detecting several return signals for a single laser shot. It is particularly relevant in case of vegetation areas since a single LiDAR pulse allows acquiring not only the canopy, but also points inside the vegetation layer and on the ground underneath.

In recent years this technique has been applied over natural landscapes to extract terrain elevation (Kraus & Pfeifer, 1998; Bretar & Chehata, 2010) or to classify land cover (Antonarakis et al., 2008; Yoon et al., 2008, Bretar et al, 2009).

In the particular case of dike monitoring, we need a high flexibility in the flight planning in terms of altitude (100-300 m) and heading, and also a high accuracy because dikes are civil engineering structures with a relative low height (less than 7 m) and with a lot of small surface singularities. As a result, it is advised to use a corridor mapping system like FLI-MAP (Fast Laser Imaging and Mapping Airborne Platform) developed by Fugro-Geoid (Gomes Pereira & Wicherson, 1999).

Embedded in an helicopter, FLI-MAP can provide†, over a 105 m wide corridor at a fly height of 150 m, a point density of 80 pts/m2, with an absolute height accuracy (Z) of 0.03 m. The Pulse Frequency Rate (PRF) of the latest version can reach 250 kHz with a field of view of 60 ° in the cross track direction. The survey is done following three scan plans in the flight direction (vertical for 50% of the points, front 7 ° and rear 7 ° for 2 x 25%), which reduce the effects of shadows.

The trajectory of the helicopter is recorded by two dual frequency GPS and an inertial measurement unit. A digital camera in nadiral position, synchronised with the LiDAR system, records the surveyed landscape and is used both to build a mosaic of georeferenced images and to colorize in real time the 3D point cloud so that a user should have a better understanding of the scene (Fig. 5). The system also includes two frontal and oblique cameras (photo and video). These data are particularly popular for dike managers who use

 † Example based on a recent application of the FLI-MAP technique on the Loire levees near Orléans, in the context of the FloodProBE European research project.

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams 273

completeness of these visual clues (Clement & Mériaux, 2007). Geophysical survey or geotechnical testing will then characterize possible extension of surface singularities in the

Fig. 6. 0.02 m resolution digital image acquired during the LiDAR survey. It shows a pump

Fig. 7. Laser profile of a bank under the vegetation (source Cemagref-FUGRO)

interest for people in charge of operating field measurements.

Finally, high resolution topographic data contribute to build specific geomechanical model of the dike that, after incorporating data provided by geophysical and geotechnical surveys, are integrated in the calculations of the structure stability. The quality of the geomechanical model also depends on the accurate location of in situ geophysical and geotechnical surveys so that one should interpret the results with relevance. In this regard, a decimeter resolution DTM acquired with a LiDAR system or derived topographic plans at 1: 100 is of high

line through the base of a dike (source Cemagref -FUGRO)

dike body or in the foundation.

the images for later processing and for marketing/communication actions towards the public or financial sponsors.

## **3.2 Surveying a dike with LiDAR data**

A LiDAR system is able to acquire data on a dike structure of up to 80 km per day, which makes the use of this technique valuable in case of emergency situations (after a major flood, for example). Provided that it exceeds a length of up to 60 - 80 km (corresponding to a day of helicopter), the costs are competitive with regard to conventional field topographic techniques (in the order 2000 euros / km) and provide additional valuable products like precious information on dike slopes and crest or their near environment (river banks, etc.). The high-resolution digital images allow to measure with accuracy visible objects. Figure 6 shows the identification of a pump line through the base of a dike.

Fig. 5. Colored 3D point cloud over a dike (source FloodProBE - FUGRO)

Moreover, in case of vegetation, LiDAR data makes possible to study invisible structures from images. Fig. 7 illustrates the way the erosion of riverbanks under vegetation can be quantitatively analysed with laser profiles.

The field visit (Fig. 8) confirmed this erosion process. The possibility of studying the vegetation is also of high importance: the development of woody vegetation near or onto the dike is a major risk factor (Mériaux et al., 2006).

Surface singularities are often signs of disorder or suspected disorder in the dike itself: for example a subsidence or a sinkhole on a ridge may result from internal erosion or karst collapse. Such singularities, once pre-identified on the images are, of course, to be confirmed by field visits, but the contribution of high resolution LiDAR data is to improve the

the images for later processing and for marketing/communication actions towards the

A LiDAR system is able to acquire data on a dike structure of up to 80 km per day, which makes the use of this technique valuable in case of emergency situations (after a major flood, for example). Provided that it exceeds a length of up to 60 - 80 km (corresponding to a day of helicopter), the costs are competitive with regard to conventional field topographic techniques (in the order 2000 euros / km) and provide additional valuable products like precious information on dike slopes and crest or their near environment (river banks, etc.). The high-resolution digital images allow to measure with accuracy visible objects. Figure 6

shows the identification of a pump line through the base of a dike.

Fig. 5. Colored 3D point cloud over a dike (source FloodProBE - FUGRO)

quantitatively analysed with laser profiles.

dike is a major risk factor (Mériaux et al., 2006).

Moreover, in case of vegetation, LiDAR data makes possible to study invisible structures from images. Fig. 7 illustrates the way the erosion of riverbanks under vegetation can be

The field visit (Fig. 8) confirmed this erosion process. The possibility of studying the vegetation is also of high importance: the development of woody vegetation near or onto the

Surface singularities are often signs of disorder or suspected disorder in the dike itself: for example a subsidence or a sinkhole on a ridge may result from internal erosion or karst collapse. Such singularities, once pre-identified on the images are, of course, to be confirmed by field visits, but the contribution of high resolution LiDAR data is to improve the

public or financial sponsors.

**3.2 Surveying a dike with LiDAR data** 

completeness of these visual clues (Clement & Mériaux, 2007). Geophysical survey or geotechnical testing will then characterize possible extension of surface singularities in the dike body or in the foundation.

Fig. 6. 0.02 m resolution digital image acquired during the LiDAR survey. It shows a pump line through the base of a dike (source Cemagref -FUGRO)

Fig. 7. Laser profile of a bank under the vegetation (source Cemagref-FUGRO)

Finally, high resolution topographic data contribute to build specific geomechanical model of the dike that, after incorporating data provided by geophysical and geotechnical surveys, are integrated in the calculations of the structure stability. The quality of the geomechanical model also depends on the accurate location of in situ geophysical and geotechnical surveys so that one should interpret the results with relevance. In this regard, a decimeter resolution DTM acquired with a LiDAR system or derived topographic plans at 1: 100 is of high interest for people in charge of operating field measurements.

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams 275

on the dike crest, slope, or toe. Whereas the geo-electrical behaviour of dikes evolves in 3D, recorded and processed data are based on a two dimensional measurements and

interpretation - 2D inversion software like Res2dinv® (Loke & Barker, 1996).

Fig. 9. Major influences of internal erosion processes on selected parameters

The principle of an inversion process is to find a model that best explains measurements obtained on the field plus other constraints. Consequently, in the case of a 2D inversion realized on a 3D medium the 2D inversion process inevitably leads to 3D artefacts. They are mainly due to the topographic effects, the siltation of the reservoir, the water reservoir

However, external information collected from preliminary studies Fig. 1 can help to reduce these effects as presented in Table 1. Indeed, the location or depth of the anomaly can be given by a visual inspection or a morphodynamic study of the river to focus ERT acquisitions. The knowledge of the geomaterial of the dike and/or the foundation can be supply by a geological study to constrain the inversion. Then, the depth of the foundation or the thickness of a repaired breach can be available after historical investigations and can also constrain the inversion. Finally, the topography of the dike can be available (e.g. from

Here, we aim to illustrate pitfalls and misinterpretations of 2D-ERT inversion, before

presenting methodological improvements without acquiring full 3D data.

(Johansson, 1997).

effect, and the clay core effect.

LiDAR data or in situ measurements).

Fig. 8. Bank erosion. The red arrow represents the 3.5 m elevation gap seen on Fig. 7 (source Cemagref)

#### **4. Investigation and monitoring of dikes and dams with Electrical Resistivity Tomography**

#### **4.1 Quest for complementarity**

Internal erosion processes and overtopping phenomenon represent more than 90% of dike failure (Foster et al., 2000; Fell & Fry, 2007). This section focuses on internal erosion processes which are more complex, and above all, should be detected by geophysical methods before the rupture of the earthwork.

Among them, the **DC-Electrical Resistivity Tomography (ERT)** is of particular interest (Johansson, 1997) for dike monitoring in heading condition. This technique is considered highly sensitive to the induced physical phenomenon such as changes in clay or water content, temperature and porosity. Fig. 9 presents the main interactions regarding the effects of internal erosion on electrical resistivity.

The main purpose of ERT campaigns is an insight of the subsurface via 1D, 2D, or 3D representations of the spatial and/or temporal variations of the electrical resistivity. One of the advantages of the method is its double resolution capacity:


This double resolution capacity can be exploited in two ways:


For cost effectiveness purposes, in the case of dike survey, ERT is usually applied in a "classical" way (2D): a set of equidistant electrodes is aligned along the longitudinal direction

Fig. 8. Bank erosion. The red arrow represents the 3.5 m elevation gap seen on Fig. 7

**4. Investigation and monitoring of dikes and dams with Electrical Resistivity** 

Internal erosion processes and overtopping phenomenon represent more than 90% of dike failure (Foster et al., 2000; Fell & Fry, 2007). This section focuses on internal erosion processes which are more complex, and above all, should be detected by geophysical

Among them, the **DC-Electrical Resistivity Tomography (ERT)** is of particular interest (Johansson, 1997) for dike monitoring in heading condition. This technique is considered highly sensitive to the induced physical phenomenon such as changes in clay or water content, temperature and porosity. Fig. 9 presents the main interactions regarding the effects

The main purpose of ERT campaigns is an insight of the subsurface via 1D, 2D, or 3D representations of the spatial and/or temporal variations of the electrical resistivity. One of

Instant survey for imaging the apparent resistivity distribution of the observed

Temporal monitoring to follow the evolution of the electrical resistivity of the

For cost effectiveness purposes, in the case of dike survey, ERT is usually applied in a "classical" way (2D): a set of equidistant electrodes is aligned along the longitudinal direction

(source Cemagref)

**Tomography** 

medium,

earthwork.

**4.1 Quest for complementarity** 

methods before the rupture of the earthwork.

of internal erosion on electrical resistivity.

the advantages of the method is its double resolution capacity:

This double resolution capacity can be exploited in two ways:

High resolution imaging for selected short stretch

Low resolution imaging for high outputs with fast zoning techniques,

on the dike crest, slope, or toe. Whereas the geo-electrical behaviour of dikes evolves in 3D, recorded and processed data are based on a two dimensional measurements and interpretation - 2D inversion software like Res2dinv® (Loke & Barker, 1996).

Fig. 9. Major influences of internal erosion processes on selected parameters (Johansson, 1997).

The principle of an inversion process is to find a model that best explains measurements obtained on the field plus other constraints. Consequently, in the case of a 2D inversion realized on a 3D medium the 2D inversion process inevitably leads to 3D artefacts. They are mainly due to the topographic effects, the siltation of the reservoir, the water reservoir effect, and the clay core effect.

However, external information collected from preliminary studies Fig. 1 can help to reduce these effects as presented in Table 1. Indeed, the location or depth of the anomaly can be given by a visual inspection or a morphodynamic study of the river to focus ERT acquisitions. The knowledge of the geomaterial of the dike and/or the foundation can be supply by a geological study to constrain the inversion. Then, the depth of the foundation or the thickness of a repaired breach can be available after historical investigations and can also constrain the inversion. Finally, the topography of the dike can be available (e.g. from LiDAR data or in situ measurements).

Here, we aim to illustrate pitfalls and misinterpretations of 2D-ERT inversion, before presenting methodological improvements without acquiring full 3D data.

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams 277

Therefore, it is necessary to develop inversion methods capable of taking this non-linearity into account. To limit the financial cost of the acquisition and the computational cost of inversion, new inversion codes specifically dedicated to the dike and dam context have been developed (Fargier et al., 2011). The code InGEOTH-2D+ proposes a 2D inversion that integrates part of the full 3D geo-electrical behaviour of a dam (topography and water reservoir are included). The purpose of this code is twofold. The first purpose is to provide new discretization capabilities to better state the problem. The second purpose is to allow

To test the relevance of the presented techniques a measurement campaign has been carried out at the crest of a dam. An historical research, a topographic survey, a geological study,

A dense Wenner-Schlumberger protocol was used because of its spatial resolution and robustness. Fig. 10 a) shows one electrical resistivity section obtained after inversion of the raw data without any external information. Fig. 10 b) represents the same section after normalization of the water reservoir effect and the topography. Fig. 10 c) shows the final result of the inversion obtained with InGEOHT - 2D+. Fig. 10 d) illustrates the inverse model used for the inversion shown in Fig 10 c). For all three results, and after four iterations, the

Fig. 10. Results of the the inversion process obtained a) without any correction procedure (Res2dinv®), b) with normalization of water reservoir effect and topography effect (Res2dinv®), c) with the InGEOHT - 2D+ inversion code. d) presents a view of the measurement campaign and The 2D+ inversion model used to inverse the result.

A first interpretation of the inverted section in Fig. 10 a) indicates that the medium is quite regular in the longitudinal direction and composed by two layers. The upper layer whose wall varies between 9 m and 12 m has a resistivity oscillating between 500 .m and 2500 .m. The resistivity of the lower layer decreases to 40 .m. In Fig. 10 b) the electrical resistivity of the water reservoir was integrated in the inversion process (81 .m). The effect

the inclusion of any explicit prior information that the geophysicist provides.

and a visual inspection were realized before the geo-electrical survey.

convergence data criterion is less than 1%.

**4.5 Results** 


Table 1. External information gathered form preliminary studies.
