The new federal highway (Autobahn) A26 in Germany with high geosynthetic reinforced embankments on soft soils
In 2001 the construction of the Federal Highway (Autobahn) A 26 in north of Germany was started. In this area the underground is of very low bearing capacity as it consists of soft layers (clay, peat) with a thickness of up to 13 m. Because of financial, technical and ecological reasons a soft soil replacement was even not under discussion. The pre-loading procedure (consolidation method) was chosen for this construction. Short- and long-term stability is ensured by the use of high-strength woven geosynthetics for reinforcement at the embankment base. The vertical and horizontal deformations and the stress in the soil, caused by the preloading procedure, have to be controlled by special measurements. Additionally, the strain behaviour of the high-strength geosynthetic is measured below the higher parts of embankments.
Among others, a very sensitive issue was the construction of an embankment ramp with a height of up to 16 meters situated at a distance of only 20 m from an important railroad under traffic. Concept, measurement program (controlling by evaluation and feed-back of the construction process (DIN 1054)) and important findings regarding the geosynthetic's behaviour - especially the different behaviour of two geosynthetic layers installed at different height in the embankment base - are shortly presented.
The experience gained during the construction of the 11 km long first section of the Highway A26 on very soft saturated marsh soils including a very high ramp adjacent to a railroad results in following more important conclusions:
1. One time more, it is confirmed that construction of high embankments on soft soils using strong basal geosynthetic reinforcement is a successful technique.
2. It seems to be economically more efficient to use higher pre-loads and stronger basal reinforcement instead of EPS.
3. The solution with two "weaker" basal geosynthetic layers is less efficient and even to some extend risky in comparison with one single "stronger" layer, because in the two-layer solution the bottom layer is "overstressed" and the upper layer "understressed" (not sufficiently mobilized) at all construction stages.
4. The use of appropriate measurement devices and program including the strains in the Geosynthetics allow to control the construction process in an optimal way and to gain useful information about the systems behavior to be used for next similar projects.