Geotechnical Instabilities in Road Embankments: Analysis of a Landslide in Schistose Road Cut-and-Fill on the Taza-Al Hoceima Expressway, Northern Morocco

Abstract


Introduction
Road infrastructures crossing hilly terrain require the installation of road embankments.Despite their critical role, these embankments pose significant geotechnical stability challenges.Landslides represent a primary consequence of such instability, with the potential to cause considerable disruptions in traffic flow and, under severe circumstances, catastrophic outcomes (Yazidi et al., 2017;Tsoata, 2020;Sissakian et al., 2021;Al-Samarrai, 2022).
A landslide is characterized by the movement of soil or materials along a pre-existing failure surface (Silva and Zuquette, 2013).This phenomenon is influenced by various factors, including mechanical stresses due to road loads (Slimi, 2010), dynamic perturbations such as earthquakes (Jeandet, 2018;Alamanis and Dakoulas, 2019), or meteorological events, like heavy rainfall (Femmam, 2014;Kirat, 2016), the latter being a primary contributor to roadway instability identified in this investigation.Moreover, the influence of geomorphological features (Fressard, 2013, Ezzardi et al., 2015;Al-Dhahi and al., 2023), alongside geotechnical attributes such as geometry, lithology and soil characteristics (Bissaya et al., 2014;Nguyen, 2015;Abidi and al., 2019;Qader, 2020, Akoudad et al., 2024) is critical in dictating landslide susceptibility.Despite progress in understanding these complex mechanisms, pinpointing precise causative factors continues to pose a challenge.Against this backdrop of uncertainty, persisting with research endeavors is essential to refine our capacity for predicting, managing, and mitigating the risks associated with landslides.
This article centers on a particular landslide event that occurred in northern Morocco, precisely at kilometer point 67+800 along the newly constructed expressway connecting Taza with Al Hoceima (Fig. 1).Recent investigations have underscored the elevated landslide susceptibility of this area, covering a probability of 42.47% across the overall area, and rising to 80% in our study sector (Cherifi et al., 2022b).This sector is distinguished by its remarkably complex topography and hydrography.From a geotechnical standpoint, the embankment under scrutiny, extending 11 meters in height, is mainly composed of schists, incorporating both cut and fill configurations.
The primary aim of our study is to deepen the understanding of the factors and mechanisms triggering landslides in embankments.Our specific objectives include: i) lithological, mineralogical and geotechnical characterization of the schists in the study area, and evaluate their influence on the noted instability; ii) exploration of the correlation between the embankment's geographical positioning and its instability risk; iv) examination of the impact of water on instability dynamics; v) decipher the mechanisms triggering the landslide occurrence, and pinpointing the principal contributing factor.This study will highlight the essential requirement for meticulous attention to the geotechnical characteristics of schist, as well as the geomorphological and hydrological specifics of the area, during the road design process.Acknowledging these elements is key to mitigating the potential for instability in road embankments.

Study Area
The study sector is located within the Aknoul commune, part of the Taza Province.It is positioned to the northeast of the city of Taza (Fig. 1).This sector is situated in the southern portion of the External Rif, encompassing the Aknoul nappe (Fig. 2).The nappe extends from Boured to Aknoul and is comprised of allochthonous materials.Stratigraphically, it reveals shales and sandstones at the base, which give way to blackish Cretaceous marls and pélites.This formation is topped by white Eocene-Oligocene marly limestones, culminating in the Numidian sandstone of the Aquitanian (Poujol, 2014).These rock formations are often characterized by their softness (Homonnay, 2019).The region experiences a semi-arid climate with Mediterranean influences, with temperature variations from 3°C to 34°C and annual precipitation levels ranging from 300 to 450 mm.(1997) and Comas et al. (1999).Gharb and Atlantic margin after Flinch (1996)

Materials and Methods
To achieve a better understanding of the factors and mechanisms responsible for the observed instability, we opted for a combined approach, associating field investigations and laboratory analyses.
During our on-site investigations, we prioritized reconnaissance and assessment of the instability and its related disturbances.Additionally, an analysis of the embankment and its adjacent areas was conducted.Field observations were instrumental in the creation of a block diagram, which effectively illustrated the site morphology, capturing the changes before and after the construction of the road, and emphasized the relative position of the embankment under study.We also carefully sampled both altered and compacted schist, as well as sandstone limestone.These samples, taken from the embankment and nearby outcrops, ensure that the various geological formations in the area are representative.Subsequently, we analyzed the collected samples in the "LABOCONTROL" laboratory to assess their geotechnical properties.The samples of compacted schist and sandstone limestone were assessed using Micro-Deval (MDE) and Los Angeles (LA) tests.These tests are crucial for characterizing the mechanical strength and wear of rock materials.Concurrently, samples of altered schist, were subjected to standard identification tests, including granulometric analysis and Atterberg limits tests, to determine their granulometric distribution and plasticity properties.Additionally, permeability tests were conducted on altered schists utilizing a constant load permeameter equipped with two piezometers, enabling the measurement of the volume of water passing through the sample.On the mineralogical front, altered schists underwent a comprehensive characterization process using X-ray diffraction (XRD) analysis, using a Panalytical X'Pert PRO powder diffractometer equipped with an X'Celerator ultrafast scintillation detector.
Based on the results obtained, we used the Moroccan guide of road earthworks (GMTR, 2011) to classify the samples tested.This approach enabled us to clarify the characterization of the formations in place, anticipate their behavior and suggest guidelines for their possible reuse as embankments.We also incorporated pre-existing data from geotechnical surveys, carried out by the "Provincial Department of Equipment and Water of Taza", into our analysis.This involved a meticulous analysis of the topographical profile of the embankment, as well as the study of data from three boreholes (B1, B2 et B3), each reaching a depth of 15 meters.This information was instrumental in the development of an accurate geotechnical profile for the embankment under study.

History and Description of the Instability
Approximately three years following the completion of the road construction, a significant section of the roadway on the route towards Al Hoceima, spanning around 20 meters in length and 1,80 meters in width, exhibited evident geotechnical instability.Pathologies observed within this section included longitudinal and transversal fissures, some extending over metric scales, as well as subsidence with disparities quantifiable on a centimeter scale.Despite multiple corrective interventions, primarily through the addition of extra layers of asphalt, the recurrence of these pathologies was noted.The most severe incident occurred on the night of April 6, 2022, characterized by heavy rainfall.On-site assessments have demonstrated that this event involved a landslide on the slope adjacent to the above section.This landslide was characterized by a downward movement of materials, a clearly distinguishable detachment niche with a depth of up to two meters, and a curved surface rupture morphology (Fig. 3. a and b).Such features align with the characteristics of rotational landslides.The pronounced inclination of a tree on the slope was a prominent visual indicator of both the direction and magnitude of the landslide (Fig. 3. b).This landslide led to a substantial collapse that impacted almost half of the roadway, resulted in the deterioration of the concrete berm, and compromised the stability of the safety barrier (Fig. 3. b and Fig. 4).Therefore, in order to avoid aggravating the situation, immediate action have been taken, involving the creation of a new temporary concrete barrier to divert run-off away from the affected section (Fig. 4).Furthermore, subsequent field investigations were conducted, including reconnaissance boreholes, lithological and topographical surveys, etc., leading to the initiation of technical studies.Ultimately, a long-term stabilization strategy was decided upon, notably involving the reinforcement of the slope through the construction of a reinforced concrete retaining wall, the reconfiguration of the slope by creating terraces, the use of granular and frictional materials for backfill, and the incorporation of geotextiles.Currently, in February 2024, this approach is in the phase of implementation.

Lithology
Following fieldwork, lithological analysis reveals a notable predominance of schist formations in the studied area.At the surface, these geological formations appear altered, but they become compact and affected by deep-seated fracturing.Boreholes B1, B2, and B3, substantiate these observations.Positioned along a linear axis, these boreholes conducted at various levels: on the roadway, on the slope, and at its base.The precision of the results obtained from these boreholes has facilitated a thorough analysis of the lithology of the slope in question.
The examination of the borehole logs (Fig. 5) identifies a schist backfill stratum positioned immediately beneath the road infrastructure, with the total thickness of these two layers totaling 2.00 meters.It is crucial to acknowledge that this backfill originates from locally procured compact schist fragments, allocated to the GMTR category R34.Subsequent examination of the logs uncovers layer of altered schist, with thicknesses ranging from 5.50 to 10 meters.Embedded within this layer is a distinct passage of sandstone limestone, singularly detected in borehole B1.A more profound exploration via the boreholes exposes compact schist formations that extend to the deepest levels explored (15 meters).An essential observation to underscore is the detection of water flow within the altered schist layers, signifying pronounced hydrogeological dynamics

Mineralogy
X-ray diffraction (XRD) analysis conducted on four schist samples; Fig. 6 uncovers a diverse mineralogical composition.The spectra predominantly show notable quartz (Qz) presence, evidenced by pronounced peaks, mainly in the 2-theta ranges between 20° and 30°.Calcite (C) also prominently features across all samples, as suggested by significant peaks.While dolomite (D) and ankerite (A) are less prevalent, they are identifiable through their distinct peaks.The detection of manganese oxide (Mo) is also noted by minor peaks.The presence of manganese oxides and ankerite is indicative of chemical interaction with water (Ahmat et al., 2022).Moreover, the detection of calcite and dolomite, minerals renowned for their susceptibility to dissolution (Eppner, 2016), highlights the vulnerability of these schists to structural compromise.These findings emphasize the significant role of water-mineral interactions in the deterioration of the embankment's integrity.

Permeability
The lithology of the sector, ranging from altered schist at the top to compact schist at depth, has a significant influence on permeability.Laboratory permeability tests conducted on samples of altered schist demonstrated high permeability, exhibiting an average permeability coefficient in the order of Kp=10 -3 m/s.As for the compact schists, although the means available do not allow their direct evaluation, their fracturing makes them permeable, but their dense, low-porosity structure indicates a significantly low permeability, particularly in comparison with the altered schists.These results suggest that the altered schist layers are the preferred zones for water circulation in the sector.

Physical and mechanical properties
The compacted schists revealed significant disparities in terms of resilience and hardness, underlining their heterogeneity.Indeed, the values observed for Micro-Deval (MDE) coefficients range from 67 to 100 %, while Los Angeles coefficients vary from 33 to 57 % (Table 2).According to the Moroccan guide of road earthworks (GMTR), this range of values classifies them in the R34 category, attributed to the fragmentable argillaceous rock family.This classification is attributable to their intrinsic fragility, inducing major modifications in their geotechnical properties, including granulometric alterations and increased plasticity, manifested by the liberation of fine particles.Consequently, the evolutionary nature of these schists indicates a marked tendency towards instability.As a result, and in line with GMTR recommendations, it is imperative to carefully examine the applicability of these rocks, particularly in the construction of embankments, a central aspect of our case study.In comparison, sandstone limestone revealed MDE coefficients between 22 and 30 %, and Los Angeles coefficients oscillating between 16 and 20 % (Table 2).These parameters qualify it for category R21 (GMTR, 2011), illustrating its high degree of hardness.This increased hardness justifies its unconditional use as a backfill material, in line with GMTR recommendations.These points to the widespread applicability of sand-lime in geotechnical engineering scenarios, offering a viable and resilient solution for various road construction applications.As for altered schists, test results (Table 3) show a percentage of particles under 0.08mm ranging from 35% to 40%, and those under 2mm from 57% to 80%.The maximum particle size observed in all samples does not exceed 50mm.Such findings delineate a soil texture profile that is primarily finegrained, yet demonstrates notable variability in soil composition.Atterberg limit assays refine the determination of the soil's plasticity characteristics, exhibiting liquid limit (LL) values from 28% to 38%, and plasticity indices (PI) spanning 13 to 22%.In reference to the GMTR classification, these schists are identified as fine soils, categorized precisely as class A2.While the GMTR indicates that soils within this category are generally suitable for use as backfill material, evidenced by the positioning of the samples relative to line A on the PI-LL discriminant diagram (Fig. 7), warrants careful consideration.This is especially pertinent in scenarios where engineering requirements demand enhanced stability and the soil is prone to saturation, because the plastic nature of the soil can markedly affect its volume change behavior.

Analysis of Environmental Influence
The block diagram presented in Fig. 8, illustrates the location of the slid embankment, and provides an intricate representation of the site's morphology.With altitudes over than of 1300m, combined with steep slopes, the resulting landscape has produced a dense hydrographic network and hydraulic dynamics.The configuration of the relief proves decisive in the trajectory and intensity of water flow.In accordance with the fundamental laws of gravity, water follows the slopes and depressions of the land, ultimately converging towards the river known as Tizi Ouadrene (Fig. 8).
Furthermore, the predominance of schist in the sector accentuates water infiltration, particularly in the altered schist horizon witch its increased permeability.The position of the embankment in the lower areas of the mountainside, combined with its orientation towards the river, aligns with the guidelines of the hydrological scheme in the sector.Faced with this situation, and in order to minimize the adverse effects of water on the stability of the road and its appurtenances, a road drainage system has been installed (Fig. 9).This mechanism comprises a side ditch (Fig. 9.a and c).Its role is to canalize runoff from the roadway, shoulders and adjacent slopes.Its function is enhanced by its connection to a culvert (Fig. 9.c), essential for redirecting water towards the river.To counter the risk of erosion, especially where the embankment contains backfill on the downstream side of the road, a concrete berm has been built at the junction of the shoulder and the top of the embankment (Fig. 9.b).It serves to channel water from the road, preventing its dispersion and directing it towards the adjacent Wadi.However, while this drainage system ensures the collection and evacuation of surface runoff, the issue of groundwater persists.

Discussion
The roadway's construction involved substantial earth-moving activities across a span of about 40 meters in width.The process included reshaping the upstream side into a slope by excavating altered schist to achieve a 91° incline.In contrast, the downstream side, which is the focal point of our study due to noted instability, utilized a cut and fill approach to balance the topography.By synthesizing topographic data, borehole logs, field observations, and geotechnical tests, we have derived a comprehensive geotechnical profile (Fig. 10).This profile uncovers crucial embankment characteristics and identifies the potential break line.The outlining of this rotational line is basically derived from the indications of the detachment niche observed in situ.Examining this profile highlights the embankment's complex geometry, marked by a steep incline of approximately 75% and an elevation of 11 meters.Prior research (Yazidi et al., 2017;Peridikou, 2019;Çellek, 2020), has confirmed that such geometric attributes increase the risk of landslides, by enhancing the gravitational forces exerted.Furthermore, the particular disposition and orientation of the underlying compacted schist stratum created a conducive pathway for the displacement of the overlaying altered schists.
The altered schists, exhibiting a plastic consistency and significant fines content (Table 3), are classified as category A2 according to the GMTR (2001).Such classification underscores their sensitivity to water, a property further corroborated by mineralogical analyses (Fig. 6) revealing the presence of highly soluble minerals like calcite and dolomite (Eppner, 2016).The detection of manganese oxide and ankerite within these formations points to active chemical weathering processes, a clear indicator of significant geochemical dynamics within the studied sector (Ahmat et al., 2022).
The initial manifestations of instability were directly attributable to the suboptimal performance of backfill materials, which led to settlement and fissuring on road surfaces, facilitating water ingress to the embankment core.These materials, sourced from local excavations, categorized under fragmentable argillaceous rocks R 34 .Rocks within this group evolve, releasing fine, plastic, and water-reactive particles, alongside a decrease in mechanical strength.This aligns with Cherifi's 2022 study on our sector's schists.Temperature fluctuations worsen this evolutionary process, causing expansion and contraction of the materials, that weaken their structural integrity.Excavation and road traffic also increase stress on these materials.The GMTR (2001) stipulates several criteria for the use of such evolving materials.These include a rigorous compaction, post-extraction fragmentation, and stabilization with lime (particularly under conditions of increased moisture).In addition, preliminary evaluations are required to identify appropriate extraction and compaction techniques, ascertain the particle size distribution, and devise designs prioritizing impermeability.Adhering to these recommendations can enhance the performance of backfilled schists.
As previously discussed, schists as backfill or present in their natural state, are inherently sensitive to water.Furthermore, the pronounced permeability of altered schists facilitates water flow, which predisposes them to instability risks.This susceptibility is further exacerbated by the destabilizing effects of hydraulic pressures and erosion (Sun, 2020).Despite the drainage system's capacity to manage surface runoff, it falls short in preventing subsurface water infiltration, leading to pronounced percolation.This situation was significantly worsened by the heavy rainfall event on April 6, 2022, identified as a major trigger for the subsequent landslide.Several authors (Kim et al. 2004;Xue et al. 2008;Fort, 2011), have underscored the substantial rainfall as a critical triggering factor.
Aware of the significant impact of hydrology on slope stability, this article proposes an enhancement of the initial drainage system to align with the hydrological, morphological and lithological characteristics of the studied site.The initiative advocates for the strategic installation of a deep drainage trench aimed at managing groundwater, as detailed in Fig. 11.The trench structure is equipped with an impermeable geomembrane acting as a barrier against lateral water infiltration towards the embankment, complemented by filling with water-resistant draining material, that guide the water to a perforated pipe, thus directing the flow towards a suitable outlet.The addition of a geotextile is recommended to maintain the functionality of the drainage system by preventing clogging by fines.This approach constitutes a systematic geotechnical strategy addressing the challenges posed by adverse hydrological dynamics.

Conclusions
Our comprehensive analysis of geotechnical instability in road embankments, with a focus on a landslide event along the Taza-Al Hoceima expressway in Northern Morocco, has determined that intense rainfall serves as a primary trigger for such occurrences.Secondly, the inadequate performance of schist backfills, combined with the embankment's complex litho-geometric structure, has been pinpointed as a crucial determinant of this instability.Laboratory-based geotechnical and mineralogical analyses have verified the vulnerability of schists, attributed to their reaction to water and natural evolution.Concurrently, a notable failure in the drainage system's ability to handle subsurface water infiltration has been observed.This shortcoming is exacerbated by the schists' permeability, alongside the locale's hydrological and geomorphological aspects, culminating in pronounced water percolation.Ultimately, the strategic implementation of a drainage trench offers a promising solution to these issues.This study underscores the crucial role of the synergistic interplay among geotechnical, mineralogical, hydrological, and morphological factors in the instability of embankments.It stresses the importance to take into account the geotechnical characteristics of schists, and underscores the vital need to design road drainage systems that are specifically tailored to the local environmental conditions.Implementing these recommendations is anticipated to significantly improve the reliability and durability of road infrastructures.Moreover, the development of a detailed geotechnical profile of the studied embankment, has paved the way for new research perspectives.This entails using the limit equilibrium method in inverse analysis to determine the shear strength parameters of altered schist.Such a method is crucial for advancing towards the modeling and optimization of reinforcement strategies, such as the construction of retaining walls.

Fig. 3 .
Fig. 3. Field photos from two different angles depicting landslides.(a): highlights the detachment niche, along with damage to the berm and safety barrier.(b): highlights the inclination and the falling of the tree, showcasing the slope failure.White dashes: initial location of the tree prior to sliding.Red dashes: Final position of the tree after sliding.

Fig. 4 .
Fig. 4. Overview of the roadway post-landslide, illustrating the collapse that affected nearly half of the roadway and the installation of a concrete containment system

Fig. 7 .
Fig. 7. IP-LL discriminant diagram showing the positioning of altered schist samples (S1 and S2) relative to line A

Fig. 8 .
Fig. 8. Block diagram of the site morphology.A: Condition prior to road construction.B: Condition following road construction, with emphasis on the relative position of the landslide

Fig. 10 .
Fig. 10.Geotechnical profile of the embankment in question (conditions before sliding)

Fig. 11 .
Fig. 11.Geotechnical solutions to mitigate the impacts of negative hydrological dynamics

Table 2 .
Results of geotechnical tests for rock formations (Compact schists and sandstone limestone)

Table 3 .
Results of geotechnical tests on soil formations (Altered schist)