Djedra Dam is located in north-eastern Algeria, in the vicinity of the city of Souk Ahras and the border with Tunisia, at the confluence of rivers Hammam and Akiba. Rockfill dam with upstream concrete screen, with height of approx. 65m and length of approx. 450m at dam crest is executed as per the solutions from the Detailed Design by company Zarubezhvodstroy (Russia) and Construction Documentation of EP Hidroinzenjering, which, since the end of 2012, has been actively monitoring construction of this dam, carrying out continuous modifications of the initial design as a result of unfavourable geological structure, inconsistencies of the received documentation – structural and technical part thereof with geological documents, complex structures and technological and administrative limitations of our Algerian partners in implementation of this project.

Geological structure of the terrain, in the narrow area of investigation of the site of prospective dam, is characterized by a complex composition in terms of structure and texture. The youngest geological formations in the locality of dam site section of Djedra, are of Eocenic origin. The oldest geological formation consists of Triassic formations with gypsum as the predominant formation.

Triassic formations had an active role in creation of a geologically complex structure of the terrain and they contributed to creation of steep relief with atypical topography with subsequent raising of already formed terrain in the conditions when at great depths, anhydrites come in contact with water and turn to gypsum. In this process, the volume of this formation increases, sometimes by as much as 30%, older geological formations expand, rise and reach geologically younger formations which are hypsometerically higher. This was also the last phase of formation of the geological structure of the terrain.

Frequent changes in lithological members with unclear position of spreading of geological structures in all directions, under conditions of diapir, occur as a rule rather than exception in this locality.

Apart from very complex geological structure of the terrain in the zone of diversion structures, contemporary geodynamic processes are developed in the form of two active landslides. Design bureau prepared for preparation of technical solutions of prospective Djedra dam, taking into account similar experiences well known from Mosul dam (continuous eluviation of gypsum in the conditions of use of the facility), but uncoordinated structural and technical documentation from Detailed Design phase with geological structure of the terrain brought the designer of the project back to the beginning. Designer was forced to offer complex technical solutions of terrain remediation under conditions of construction of the project, which have not been seen in Energoprojekt for decades.

When it comes to structures, the structures within the diversion posed a serious problem:

1. Shaft spillway with water intake tower. Design provides for flood water discharge from the reservoir of Djedra dam through shaft spillway. Apart from its complex geometry, this structure had the following problems as well:

Unlike the standard engineering practice where the tendency to bury such structures at least in one third of their height, this shaft spillway is fully excavated, which together with defined high-risk seismic zone offers very dynamic influences transferred to the ground.

Nothing less of a problem is joining 2 fully different structures, a massive shaft spillway and water intake tower into a single structure with shared wall and foundations. This atypical merger created

a unique structure which lost axial symmetry, its centre of gravity and geometric centre shifted from the axis of the shaft and torsion was introduced as a significant load which would not exist if the structures were separated.

The problem of foundations of this high structure of 30,000 m3 of concrete added up to the all of the previously mentioned ones – marl of inadequate load-bearing capacity for this type of structure. The stability of the structure, permissible pressures in the ground and withstanding the impacts of the entire structure were secured with foundations, as well as replacement of the soil with lean concrete, while the piles are arranged in 2 concentric rings.

2. Diversion tunnel. Excavation for the structure which has a role to divert the river during construction and ensure dry conditions for construction of the dam, while during the period of its use ensures discharge of flood water from shaft spillway through stilling basin to the river bed downstream from the dam, posed a great challenge in terms of shoring system which would enable safe operation. The solution to this problem was found in the excavation with pipe roofing system.

3. Outlet structure of diversion tunnel. The conditions in the downstream tunnel were additionally aggravated at the point of outlet structure. Apart from bad properties of the shale which forms the base of the slope, there were also 2 registered landslides with depth down to 10m, which even before commencement of the works, in the natural condition of the terrain, included almost the entire slope to the riverbed. The situation in the terrain was additionally aggravated by the contractor who formed an embankment and constructed a building at the top of the landslide.

Considering the geological conditions of the terrain, the possibility of execution of the excavation from the surface of the natural terrain was discarded in its entirety. Instead, after removal of surface layers of the terrain, we provided for vertical excavation with strong shoring – by forming walls of piles that formed a box with 15m sides and enabled descent to the required level – the level of the bottom of the tunnel. Channel constructed on piles was connected to this box, and then the stilling basin laying at the toe of the landslide.

Unstable and moving body of the landslide was removed within the preliminary works and series of piles were formed, which with protection of the slopes with gunite and anchors enabled for a part of the excavation be carried out from the surface of the terrain forming an area for vertical excavation as per the adopted solution, with steel shoring of piles which were sequentially placed as the excavation progressed and dismantled аs the internal concrete structure progressed taking over the load from the shoring and giving the structure its final outline.

In order to enable safe excavation of the spillway in very bad material, wing walls constructed on piles were formed. During these 7 years of active construction, most of the time was dedicated to the stabilization of the terrain and enabling execution of the works. Since the river was officially diverted approximately 10 months ago, that allowed for quicker advancement of works and opening of a wider front of the works and the accent was slowly shifted to realisation of the dam, as well as to anti-filtration measures to reduce the speed of gypsum dissolution which could result in collapse of the ground in the zone of the dam and jeopardize of the stability of the dam and watertightness of the reservoir.

Taking into account the progress of works, as well as the potential problems with watertightness of the terrain due to unfavourable geological structure (primarily the presence of gypsum in the foundations of the dam), additional attention will be paid to the program of observation, both through the use of modern auscultation equipment and through forming of geodetic systems with regular observation during construction and later during the use of the structure.