Combined effects of blasting and geological structure on rock mass stability—a case study from the Marrakech–Agadir highway, Morocco

The Marrakech–Agadir highway crosses mountainous areas of the Western High Atlas of Morocco with a high risk of slope instability. The use of explosives as an excavation method, especially at kilometric point 33 on the Imintanout–Argana section, has triggered major ruptures. The regional geological setting, together with the lithological and structural characteristics of the rock mass, represent the major factors influencing this operation where the degree of disturbance is estimated as 0.82. The studied slope is located along the trajectory of a submeridian fault corridor remobilised because of blasting during excavation works. This slope reveals instabilities associated with tectonic planes (F₁ and F₂) and/or bedding (S₀) and the presence of argillites and siltstones that coincide with bedding. These unconsolidated layers, inclined in the direction of excavation, act as slide planes. Structurally, the Pk33 slope can be subdivided into three zones depending on the displacement type being recorded and may be a fortiori related to the geomechanical properties of the substrate. The rheological characteristics of zones B and C, highly fractured and with low competence, respectively, explain their relatively different displacement patterns from that of zone A. The maximum instantaneous explosive load used for offloading the upper part of the sliding mass has been estimated in order to increase the safety factor for the instable slope.