Road pavement layers on Algerian terrains: from the Tell to the Sahara

Below everything is the subgrade (plate-forme), the natural or filled ground after earthworks, whose bearing capacity is classified from the soil tests. Where the ground is weak, a capping layer (couche de forme) of selected material raises it to the class the design needs. Then come the pavement layers proper: a sub-base (couche de fondation) and a base (couche de base), which spread the wheel loads, and the surfacing (couche de roulement), which carries the traffic, sheds water and gives grip. Between the base and the surfacing a tack coat or a binder course may be added, and the whole is drained by shoulders and ditches, without which no stack lasts.

Algerian road design relies in large part on the catalogue of structures for new pavements issued by the public works technical control body (CTTP), together with soil and material tests by the national laboratories. The catalogue method classifies the traffic (by heavy vehicles per day and the design period), the bearing capacity of the subgrade, and the climatic zone of the site, then proposes structures for each combination and type of network. Engineers also use the CBR method and the French catalogue as references. The point for a contractor or an owner is that a structure is not improvised on site: the layers, the materials and their thicknesses come from the study, and the contractor’s skill lies in producing and compacting them to the specified density and evenness.

Unbound layers use graded crushed aggregate (grave non traitée, GNT) with a controlled curve such as 0/31.5, compacted at the right moisture content; its quality depends on the hardness of the rock (Los Angeles and Micro-Deval tests) and the cleanliness of the fines. In the southern and Saharan zones, gypsum-limestone crusts known as tuf are a traditional first-choice material for sub-base and base, with their own design rules because they behave differently from standard GNT: they gain cohesion when compacted dry but can degrade with water and fatigue, which is why their use is specified with care.

Bituminous layers are produced in a hot-mix plant from crushed aggregates, crushed sand and road bitumen; the 40/50 penetration grade is the one most commonly used in Algeria, with 35/50 also met in specifications, both suited to a hot climate. Laying follows the paver, the compactors and the temperature windows of the mix, and stops when the air is too cold or the wind cools the mat too fast.

On the coastal Tell, rain, clay soils and high water tables put the emphasis on drainage, capping layers and the protection of the subgrade; cuttings in marl and the stability of embankments are the engineering problems. In the Atlas ranges, the alignment itself is the challenge: rock cuts, high embankments, retaining structures and steep gradients, with a short paving season at altitude.

On the High Plateaus, at around a thousand metres, winters bring frost and occasional snow, so the design guards against frost penetration and the saturation of fine soils in spring, and the programme places the bituminous works in the warm months. Aggregate demand is high because the alignments are long, which makes a nearby quarry the deciding factor in the cost of the unbound layers. In the Sahara, the problems invert: heat, which limits the hours for asphalt and concrete and raises the choice of bitumen grade; wind-blown sand, which encroaches on the road and abrades the surface; the scarcity of water for compaction; and the use of local tuf and crushed material where good rock is far away. Flash floods in wadis dictate the hydraulic structures and the height of the embankments.

Whatever the terrain, the layers only perform if compacted to the specified density at the right moisture content, which is why vibratory rollers, water tankers and a laboratory on site are as important as the paver. In the interior of Algeria the water for compaction is itself a logistical question, answered by tankering or by a well on the alignment. The haulage of aggregates from the quarry to the site, with trucks and tipper semi-trailers, and the supply of bitumen in insulated tankers complete the picture: a road project in these regions is a logistics project as much as a construction one.

SARL ETAHG builds roads with its own complete fleet, from earthworks and grading to compaction, bitumen spraying and asphalt paving, in direct contract or as a subcontractor, and supplies the crushed fine aggregates from its own quarry and crushing plant at Oued Sdeur near Aïn El Ibel in the wilaya of Djelfa. Established in 1997, it has completed road projects in many regions of Algeria. See road construction, fine aggregate production and equipment rental.