feature stories | managing geotechnical risks: landslide at athmenia managing geotechnical risks landslide at athmenia dam by Nassim Abi Fadel, GHCE The Athmenia Dam and Reservoir project, completed in 2007, is part of the Beni Haroun hydro-scheme, a water transfer project supplying water to six eastern Algerian cities. The reservoir is dammed by a 630-m main dike K1, and K2 a 900-m long saddle dike. Work on the project started in late 2002, and the 32 Mm3 reservoir was filled in August 2008. The reservoir’s water is regulated through an intake tower and a diversion gallery crossing K1. It is able to supply a maximum flow rate of 60 m3/s for drinking water and 2.5 m3/s for irrigation. The dams’ fill material - marly and silty clays - was extracted from the site’s immediate vicinity. It was during excavation work for K1 that a slide occurred on its left abutment. The operational main road at the top of the left bank, right above the rupture, made it impossible to excavate the slide and attain a smoother slope. Still, any solution required some light machinery to clasp onto the slope of the slide and perform initial stabilization work. The solution To get around the constraint imposed by the slide location, a retaining screen made of vertical micropiles was positioned to intercept the sliding surface and support the earth on the northwestern side of the excavation, then active anchors were added to ensure the lateral stability of the screen. The micropile screen The structural vertical screen catered for allowable efforts and displacements and anchor provision kept the maximum shear load in the screen at an acceptable value. Calculation resulted in a screen of micropiles 25 cm in diameter - reinforced with metallic rings distanced 50 cm with a cement grout of a 25-MPa compressive strength. The anchors Four rows of vertical active anchors - of 44-ton capacity and 2 m apart stabilized the retaining screen. Horizontally, the anchors were spaced 1.5 m apart in the two upper rows and 1 m apart in the lower rows. The solution designed was implemented with zerodelay and in the safest way possible to the works and the civilian traffic on the Athmenia-Mila Road above the slide location. In July 2003, the extremity of the left bank of K1, a naturally steep limestone zone, ruptured during excavation works for the dam’s foundation. Any solution had to be completed before October - the start of Algeria’s rainy season. The soil investigation Eight continuous coring vertical borehole ground explorations in the vicinity of the unstable zone resulted in an average core recovery of 75% and uncovered a water table at the base of the limestone formations. The instability was in the second soft clay layer between the limestone layer and the compact claystone. The rupture mechanism The rupture was a planar sliding at the interface between the superficial clay/limestone and the deep claystone. The presence of water at the base of the limestone layer coupled with the steep slope of the limestone bed, triggered the slide during excavation. 42