The dike along the Rhone river, close to the town of Aramon in the Provence, was affected by a zone of active leakage through the dike body taking out fine materials over years. The leakage impact did not only bring fine soil material to the dike surface but left decompressed areas in the dike body leading to unsafe dike conditions.
The actual rehabilitation of the dike extended over a length of 570 m along the right bank of the river Rhone close to the village of Vallabrègues. The main target of the rehabilitation was to create an impermeable barrier through the dike axis into less permeable ground stopping the existing leaks at the dike foot and achieving equalization of the piezometric levels along the concerned dike area.
The achievement of these objectives will ensure the durability of the dike with regard to the phenomenon of internal erosion and improve overall dam safety.
The Mixed-in Place (MIP) method was selected to create the impermeable barrier. The design specified a 550 mm wide continuous wall providing embedment into low permeable ground layers to achieve a permeability > 10–7 m/s. The geology can be described as superficial fill within the dyke body above sand and gravel layers with few limestone blocks, followed by weathered limestone above hard limestone.
The wall depth extended up to 20 m below dam crest in the southern zone. Along the active leakage zone and at the connection area the wall depth increased towards 23 m to embed into the weather limestone. To reach the specified challenging depth and for embedment into the weathered limestone a modified drilling rig BG 40 was utilized for the works. The quality control during cut-off wall works execution included material testing, verticality monitoring during MIP installation, upstream and downstream piezometer readings, on-site leak observation methods along the dike foot and final verification of the water stream along the axis of the installed MIP wall employing ground temperature sounding (specialist subcontractor for CNR).
As an immediate observation after completion of the cut-off wall the leakage at the dike foot at the affected zone disappeared, thus showing the positive impact of the installed wall.
Employer/Client: Compagnie Nationale du Rhone (CNR)
Scope of Works:12,500 m² Mixed-in-Place Cut-off Wall; thickness 55 cm; depth up to 23 m
Equipment: BG 40
Geology: superficial fill, gravel, sand, limestone blocks, weathered limestone
Construction period: 2018 - 2019