The motorway A29 between Amiens and Saint-Quentin, 63 km long was opened to traffic in June 2001. It spreads over rich agricultural lands of the north of France (plateaux of Santerre, of Vermandois) crossed by the valley of Somme. Geologically, the chalky substratum (senonian chalk, superior Cretaceous) outcrops locally in certain dry, small valleys with a silty cover the thickness of which is very variable. The surface formations are mainly made up of silts of plateaux (quaternary), silty and chalky colluviums, sands from Thanetian and clays from Sparnacian constituting some pilot hillocks. At the design stage, the project was optimized by developing to the maximum materials of the site (treatment of the silts and chalks) for re-employment in capping layer, upper part of earthwork (PST), technical blocks close to the structures. Several scenarios of earthmoving were studied in order to have a perfect control of risks of this type of building site (geotechnical, meteorology, yield coefficient of materials...) leading thus to a balance. This balance is obtained for each scenario, by adaptation of the project (modification of the longitudinal profile, extra width of certain cuts), and excluding recourse to stock-pile and borrow-pit which would have been of a higher cost and badly accepted taking into account the high quality of the arable lands. During the building site, optimization was continued and the longitudinal profile was modified along 10 km long to take account of a material yield coefficient of the silts, different from the basic assumption. The PST, made up either of silts (90 % of the layout) or of chalk, was systematically treated. Along half of linear, a 0,35 m-thick capping layer in silts treated in a stabilisation plant, by hot lime and cement, made it possible to obtain a formation PF4. On this formation, the structure of roadway is made of an asphaltic concrete for wearing course (BB) (7 cm) and of a road base asphalt (GB) (10 cm). Along other half of linear, 40 cm-thick sand treated in a stabilisation plant constitute the capping layer/road base, which is surmounted by a 2 cm-thick anti-cracks layer made of sand-asphalt and 8 cm-thick BB. These layers are the road structure of the motorway. The maximum exploitation of on-site materials for the capping layers or capping layer/road base not only made it possible to minimize the external material contribution to construct the roadways but also to avoid any recourse to borrow-pit or stock-pile for a 63 km long motorway and more than 4,5 million m 3 moved soils. For the covering abstract see ITRD E135448.
Abstract