Geotechnical Prospects and Electrical Tomography to Study Slope Instability in the Rif Alboran Sea Shoreline on the Mediterranean By-Road (Northern Morocco)

The traffic along the Mediterranean ring road in northern Morocco is permanently disrupted by ground move - ments. Several sections of the road are not available during the rainy period. The slopes and hillsides that were excavated and remodeled during the construction of the roadway have become unstable and are subject to important landslides. In the present study, focused on the section of road located at kilometer point PK 176+800 between Oued Laou and Jebha City. The aim of this study was to evaluate the degree of instability of the studied slope, in the present and to predict it in the future. In situ and laboratory geotechnical tests as well as the geophysical investigations based on Electrical Tomography were carried out to complement the geomatic analyses performed by the research team. The results obtained testify to the effectiveness of the methodology adopted and confirm the threatening instability of the slope studied, in particularly at landslide area.


INTRODUCTION
Studies and publications dealing with landslide hazards in the Moroccan Rif have often focused on classic landslides that were triggered more or less naturally in the past.In  The development of the Mediterranean (the National 16 or N16) along the northern coast of Morocco began towards the end of the twentieth century and was completed in 2012 (Ministry of Equipment).The earthworks mobilized colossal quantities of cuttings and embankments.The landscape of the inner Rif coast overlooking the Alboran Sea has been transformed.The remodelling of the slopes and the excavation of the hills to follow the road route have caused the creation of several areas of instability, where the naturally stable slopes have become very active and regularly threaten the population and block road traffic.This risk poses a direct threat to people, affects the movement of transport and hinders the sustainable development of the region.
This anthropogenic intervention has given rise to new landslides of significant importance that deserve to be studied and included in the future risk maps of the region.Conducting research studies to map these new landslides, to understand their dynamics and judge their degree of

ECOLOGICAL ENGINEERING & ENVIRONMENTAL TECHNOLOGY
risk, would be of great use by researchers and decision makers, and especially for beneficial to the Moroccan state.In the present study, we focused on the stability of a section of this road, and trying to participate in this reconnaissance campaign of these new landslides.
The studied study area is located in the domain of the inner Rif, at kilometer point PK 176+800 of the Mediterranean road.The choice of this section of road is justified by the importance of the unstable surface of the slope, the consequent morphology of the escarpment and the diversity of ground movements.Also, the availability of recent geotechnical data reinforced the choice.This part of the road is highly degraded, the hydraulic structures are damaged and the majority of the pavement has been carried away by the ground movements.The few attempts to study these new ground movements have been based on the techniques of old investigations based on theory and literature reviews.A scientific diagnostic methodology encompassing geomatics and structural analyses while calling upon geotechnical and geophysical prospection processes as needed is the new research approach followed adopted in this study to understand the dynamics of the active slope and to judge the risk they present for the population and traffic.The aim of this study is to define the causes of instability, the mechanics and geometry of the ground movements in the studied area, and to assess their evolution in order to conclude on the vulnerability of the road to these risks.The results obtained show the effectiveness of the geotechnical surveys used to study landslides locally.They provided precise data on the lithology of the terrain and the lines of weakness.Morphological analyses and topographic surveys allowed the definition of the dynamic regime, the unstable volumes and the extent of the risk.The geophysical survey technique ERT-2D is rarely used in the study of landslides in northern Morocco.In the conducted study, it proved to be very useful to define the geometric aspects, the lithological settings and also to detect the zones of high alteration.
The results obtained from this work show that this slope remains threatening.Several parts in this section are unstable and the main landslide at PK 178+800 is developing into a complex landslide currently moving 2 cm seaward each year.Its risk deserves greater mobilization by the authorities in the region.The present study is an important document for decision-makers to implement a comprehensive road infrastructure strengthening and maintenance programme in this section.

GEOGRAPHICAL CONTEXT OF THE STUDY AREA
The section of road studied (Fig. 1) is located on the National 16 (N16), in the province of Chefchaouen, between Jnan Ennich and Amtar, 22 km from Jebha.It is bounded on the one hand by the Mediterranean Sea (on the north side) and on the south side by an imposing hill that marks the landscape with a very steep escarpment and that follows the road for several kilometres.

GEOLOGICAL, TECTONICS AND GEOMORPHOLOGICAL ANALYSIS
The Moroccan Rif is a recent mountainous chain of the Betic-Rifan arc, dating from the early Tertiary era, located in northern Morocco on the western coast of the Mediterranean.According to Durand-Delga et al. (1960), the Rif chain is divided into three domains; internal domain, flysch nappes and the external domain.However, the study area belongs to the internal domain (Ghomarides and the Sebtides units), as shown in Figure 1.Subsequently, the Sebtides units consist of metamorphic rocks of kinzigites and gneisses surrounded by peridotite materials.Consequently, these deposits are overlain by the Quaternary materials deposits, affected by metamorphism tectonic (Farah et al. 2021 andMarrone et al. 2021).Therefore, several major contacts, characteristic of the stacking nappes in the internal zones, which crosse-section the Ghomarides and Sebtides units.The most important one is the Araben fault in the peridotites of Beni Bousera massif (Fig. 1), which brings the peridotites with the Filali micaschists to the SE (Gueydan et al. 2015 andMourabit Z et al. 2017), and the Ghomarides to the NW with a vertical thrust.Nevertheless, this contact is often reactivated by small fractured faults accompanied by heavy serpentinisation (Frets et al. 2014).
The stratification of the subsoil is dominated by the Filali micaschist formations (Mourabit et al., 2017).Nevertheless, the Ghomarides units outcrop near to the study area by schist and alternating of sandstone and limestone.These formations are generally impermeable and lack extensive aquifer forms (Gueydan et al., 2014;El Bakili et al., 2020).Moreover, these lithological formations are fractured and have several zones of weakness that induced some typically landslides and debris flow in the area under study.The study of slope instability is delicate due to the gravitational ground movements, which are influenced by many factors such as the geological and hydrogeological structures of the ground as well as the evolution of the mechanical properties of its layers (Bordoni et al. 2015, El Fellah & Mastere 2015).
In fact, the importance of the studied slope instability is defined by its significant geometric characteristics.Actually, the ground moves along the road for a distance of about 1 km.The difference in altitude of this escarpment is 230 m between the highest point and sea level.The roadway is located at a difference in elevation of about 180 m.The average dip is 45°.The surface of the subject area to landslides is approximately 2 Ha.The topographic survey was carried out in September 2020, at a scale of 1/500, illustrating this situation and giving a precise presentation of the relief.
The structural analysis of the discontinuity planes observed on this site allows concluding that the geological formations of this study area are affected by intense fracturation characterized by different families of local faults (fissures and diaclases) with a variable orientation).Although, the deformations observed on the area study are either of a ductile nature exhibited in the anticlinal fold form (Fig. 2B); or in a fractured form associated with the faults (Fig. 2A, 2C).

CLIMATIC ASPECT
The climate of the region is Mediterranean, characterized by a wet and cool winter and a dry and hot summer.Rainfall varies with altitude and exposure of the relief.Average temperatures are generally between 20 and 32°C in summer and 7 and 22°C in winter.The rainfall averages around 500 mm per year in a heavy rainy year.The region is also known for peak daily rainfall of up to 140 mm (Fig. 3).

OVERVIEW OF THE SITUATION
The slope has been visibly modelled by excavation work, creating 4 berms forming stability levels.The two upper levels are not drained and not protected by concrete.These stability platforms are currently highly degraded and have collapsed.The absence of pebble traps to protect the roadway against scree was noted.The crust and vegetation covering the shale bedrock have collapsed, leaving the rock exposed to external degradation (Fig. 4A, 4B).This situation has accelerated the degradation of the surface of the slope and accentuated the instabilities.Thus, the shale and micaschist once covered by the protective crust are now exposed to bad weather and extreme temperatures, not to mention the sea air known for its aggressive salinity.The alteration of the rock has facilitated its repeated degradation in each period of winter or following heavy rainfall.
The ground movements in the studied slope can be divided into two zones (Fig. 6): A large part (Zone 1) where the escarpment is subject to relatively recent and local degradation, which for the moment does not affect the road (Fig. 4).The foot of the slope in this zone is remarkably eroded (Fig. 6).A second surface (Zone 2) can be observed where the berms are clearly curved downwards with more significant degradation and scree.Moreover, this last area is limited by cracks and subsidence, suggesting the formation of a complex landslide with clearly defined boundaries (Fig. 6) and (Fig. 5).The roadway and the shoulders that cross the landslide area are marked by subsidence and intense cracking, the guardrails are detached and damaged (Fig. 5), which explains that the observed landslide is still active and moving towards the sea.The roadway in this landslide area has collapsed and the hydraulic structure is damaged (Fig. 5.D).The vegetation layer still visible in places on the crest is very thin and is gullied by runoff during each rainfall.This further activates the degradation of the slope and the exposure of the shaly substratum.

MATERIALS AND METHODS
The adopted methodology (Fig. 7) is based on a preliminary study including historical and bibliographical research completed by field surveys and a topographic survey at a scale of 1:5,000.After this first phase, the geotechnical reconnaissance tests to be carried out either in situ or in the laboratory were defined.Then, the ERT profile was drawn up, according to the needs of the study but taking into consideration the topographical constraints of the terrain.All the geotechnical and geophysical results were analysed and correlated in order to make correct judgement.

GEOTECHNICAL INVESTIGATIONS
In order to identify the various instability mechanisms in the study area and also to have a clearer view of the subsoil formations, geotechnical investigations are of great use (Abdel-Ilah et al., 2022; Bouafia, 2022).In this context, 4 core drillings are distributed on both sides of the roadway in the landslide area, with a fifth at the level of the slope crest (Fig. 8) and (Table 1).
The drill hole and sampling are made according to the standard EN ISO22475-1.The drill cores obtained were analysed visually and also in the laboratory to define the lithology at each depth.Samples of the various layers were taken to the laboratory for triaxial testing and calculation of the specific weights of the various materials The 5 boreholes were used to carry out Menard pressure meter tests to be able to judge the mechanical performance of the different layers and to detect The tests were carried out using a tri-cellular probe introduced along the borehole; according to the NF P94-110-1 standard.In order to judge the activity and movement of the identified landslide, the core holes SCP1, SCP2 and SCP4, were used to perform inclinometer measurements (Fig. 8) and (Table 1).The inclinometer consists of a mobile waterproof probe 50 cm long, connected to the inextensible cable by a waterproof connector.The contact between the probe and the tube is at least four points.A measurement and data storage system records the position of the low level of the probe guide pin relative to the surface reference mark at the top of the tube.It also measures the inclination of the probe relative to the vertical.The inclinometer used is a GEOKON model GK 604 D, with a digital system.The installation and the follow-up of the results are done according to the NF P94-156 standard.

GEOPHYSICAL SURVEY
The geophisical and geotechnical tests provided a lot of information on the nature of the materials that constitute the body of the escarpment studied.They also allowed understanding its structural, lithological and geological context.However, these results remain relatively punctual and cannot be applied to the whole slope.The geological and geomorphological study allowed exploiting the geotechnical results and making a complete interpretation of the way in which the various formations are set up and also their dynamic behaviour.The use of geophysical prospection means allowed refining the data obtained and to understand the subsoil structures.
The expected objective of the geophysical surveys was to confirm the continuity and consolidation of the shaly substratum in the subsoil, to have a clear idea of the thickness of the weathered surface layer, to check whether there are points of intense weathering and finally to try to understand the disposition of the fractures and cracks observed on the surface.The geophysical survey method used is Electrical Resistivity Tomography (ERT-2D).It is based on the measurement of apparent electrical resistivity of the subsurface along a set of straight cables connected to a defined number of electrodes, for a large number of positions and spacing of the electrical current injection electrodes and those for the calculation of the potential difference.This geophysical prospecting technique allows for a geological and geometric description of the subsurface based on the resistivity contrast of the formations (Günther & Rücker 2012;Zhou 2018;Sassioui et al. 2022).
Geoelectrical data acquisition was carried out with the MAE "X612EM+" resistivity meter.In order to have a compromise between spatial resolution and depth of investigation, the authors opted for the combined Wegener -Schlumberger device, the distance between the electrodes was fixed at 10 m.The results were evaluated using the zoneres2D software, with an RMS of 6.3% after the fifth iteration.The profile was implanted to the right of the roadway towards Oued Laou following the ENE-WSW orientation, parallel to the topography and perpendicular to the direction of the slide.It also passes next to the core sample SCP1 (Fig. 8).The length of the profile is 220 m; the number of electrodes used is 23.
The regularised least-squares inversion method was used (Zhdanov & Portniaguine, 1999) which is a version of least-squares inversion using a smoothing factor to give a block-smooth distribution model with constant resistivity.Yet, the equation in matrix form for this inversion is as follows: where: A -a matrix of partial derivatives of apparent resistivity (the Jacobian); C -a smoothing operator; W -a matrix of relative errors of measurements; m -a vector of the model parameters; μ -a regularization parameter; Δf -a vector of deviations between calculated and measured values, R is a tuning factor.
This inversion method is better adapted to the case where the distribution of the resistivity of the soil is very contrasted (case of well-defined limits between geological elements or layers), it enables to give a model which clearly shows the limits of the various layers or contacts which corresponds perfectly to the investigated case, since shale formations crossed by faults or diaclases are explored.

RESULTS AND DISCUSSION
The crust that covers the hill is thin (50-100 cm).This layer is still visible at the crest.It is made up of breccia and shale debris embedded in a brownish clay matrix (Fig. 2A), (Fig. 11) and (Fig. 12).During the construction of the Mediterranean bypass, the slope underwent excavation and demolition work to make way for the road along the planned road layout.
However, to stabilize the new morphology of the slope, 4 berms were built.Only a part of these stability levels is protected by concrete and just one undersized hydraulic structure has been observed in this section, which is made of a DN 1000 concrete nozzle.Currently, this pipe is blocked at the foot of the slope, on the other side; a great part of it is broken and carried away by the landslides (Fig. 5D).Thus, the dominant formation of altered micaschist, folded and fractured along unfavourable cleavage planes (Fig. 2A), have facilitated the infiltration of water and the acceleration of the alteration of the rock.

SYNTHESIS OF GEOTECHNICAL DATA
The analysis of the core samples from SCP1 and SCP2 (Fig. 9) and (Fig. 12), shows that at the level of the roadway, the shaly substratum is reached after about 5 meters of depth.Therefore, a mixture of shale debris and the backfill material forming the pavement body forms of the surface layer.Yet, the greyish shale in the subsoil is heavily fractured and contains whitish calcic seams.In the SCP3 borehole, which is located on the roadway next to the hydraulic structure, it can be seen that the material extracted from the cores consists of backfill material to a depth of around 9 m (Fig. 12).The shale can be identified after this depth, but it is much crushed and is embedded in a greenish or greyish silty-marl matrix.Borehole SCP4, located in the landslide right-of-way, shows a disordered stratification, confirming the structural disorder in this area due to the landslide and the dynamic activity of the slope at this location.At a depth of between 12 m and 15 m, a layer of silty marl, about 2 m thick, can be seen (Fig. 10) (Fig. 12).This layer certainly represents the landslide line and its thickness.The SCP5 borehole (Fig. 11) (Fig. 12), located at the crest of the hill, confirms the small thickness of the crust, not exceeding one meter in thickness.Below this, alternating fractured schistose layers separated by greenish or greyish marl and clayey cleavage joints can be seen, sometimes crossed by white calcic seams.After a depth of about 20 meters, the shale is relatively hard with a decrease in the fracture rate with depth.
Analysis of the results of the Menard pressure-meter tests (according to NF P 94-110-1), carried out in the five boreholes shows that the shaly substratum is consolidated to over-consolidated, but there are zones of altered and fragile weakness at the upper level of borehole SCP4 (Fig. 13), reflecting the development of progressive failure phenomena affecting the slope in the slide zone.Consequently, the deeper layers are much healthier, owing to the deep shale layer which is relatively unaltered and reconstituted by surface movements.In fact, to be able to judge the activity of the observed landslide, inclinometer tests were carried out in boreholes SCP1, SCP2 and SCP4.Although, the tests were carried out in accordance with the NF P 94-156 standard.The measurements were taken from 17/12/2021 to 06/06/2022.The analysis of the results shows a displacement of the whole in the direction of the sea.This displacement is greater at the level of the inclinometer of borehole SCP4, it is of the order of 10 mm for a period of 6 months, and the slip line is located at a depth of about 10 m (Fig. 14).The deformation is of the order of 1.02° (Fig. 15).

SYNTHESIS OF GEOPHYSICAL DATA
The ERT-2D model obtained (Fig. 16) confirms the results of SCP1 and SCP2.It indicates that the shale massif that forms the basement structure is homogeneous and forms the entire bedrock of the slope.Also, no voids or groundwater were detected.After a depth of about 10 m, the resistivity in the profile varies between 300 and 800 ohm-m, these The bedrock has a variable compactness due to its intense fracturing, which explains this variation in resistivity.The profile also shows very wet and weathered surface areas corresponding to the much degraded zones, especially at the beginning of the profile, corresponding to the limits of the landslide.

CONCLUSIONS
The escarpment studied presents several risks of landslides, which can be differentiated according to two categories: A periodic danger linked to landslides that disrupt road traffic during each rainfall or winter period, due to the debris and blocks of rock that slide down the slope, invading the road in the absence of an abutment or pebble trap.Another eminent risk would come from the volume of crushed soil in the landslide right-of-way which is currently in early equilibrium and could rupture at  any time.This landslide is significant because of the volume of material estimated at 3800 m 3 with an average density calculated in the laboratory of 2.66 tonnes/m 3 , i.e. a total mass exceeding 10,000 tonnes.The slope of the slide is dangerously close to the angle of friction calculated in the triaxial test.Finally, this escarpment is sawn by local faults and fractures which favour the infiltration of water at depth.This further weakens the shaly layers substratum and creates new zones of weakness.The section of the road at KP 178+800 is in a threatening state of instability, particularly at the landslide level.The intervention of the competent authorities to address this problem must take into consideration these instabilities in their entirety.The solutions and protective structures to be implemented must take into consideration all the instabilities listed above.In this study, the results obtained by geotechnical and geophysical methods clearly showed the performance of them in slope instability study, mainly for the coastal slopes in northern Morocco, which are not economically developed yet.Hence, these results can be used by different decision makers for the development, in order to avoid unnecessary material damages in the future.

Figure 1 .
Figure 1.Geological map of the study area

Figure 2 .
Figure 2. (a) Lithological presentation at the top of the slope, (b) example of an anticlinical fold used on site, (c) fracture marking the limit of the slide

Figure 3 .Figure 4 .
Figure 3. Rainfall and maximum temperatures measured in the counting stations of Afghan, Koudiat Kourirene and Jebha from 2009 to 2020

Figure 5 .Figure 6 .
Figure 5. (a) Cracks and subsidence in the road sides, (b) limit and direction of the slide, (c) crack in the wall of a small house, (d) collapse of the roadway and damaged structures

Figure 7 .
Figure 7. Process and methodology

Figure 8 .
Figure 8. Boreholes and ERT profile location

Figure 9 .Figure 10 .
Figure 9.The first two core drilling plates SCP 1 & SCP 2: A SCP1 borehole from 0 to 6 m; B SCP1 borehole from 6 to 14 m; C SCP2 borehole from 0 to 9 m; D SCP2 borehole from 9 to 15 m

Table 1 .
List of boreholes and destination