Long-Term Experience with Reinforced Embankements on Soft Subsoil: Mechanical Behavior and Durability
Abstract
Two projects of embankments on soft ground with high-strength basement reinforcemcnt are shortly describcd. Started in 1981 and 1986, respectively they belong to the first one's using preloading plus reinforcement for autobahns and highways in Germany. Site conditions, construction stages, measurement data and evaluation are presentcd, including long-term reinforcement strain measurements. The geotextiles are still under tension. In 1995 high-tenacity polyester woven was exhumed from underneath the first embankment and analyzed for evaluation of durability. Important preliminary results are reported, showing negligible loss of strength in 14 years. In conclusion, findings and recommendations for focal points of design, long-term reinforcemcnt behavior and durability are summarized.
Conclusion
Two projects of embankments on soft subsoil with high strength basement reinforcement have been started in Germany in 1981 ('Rübke') and 1986 ('Grossenmeer') respectively. The Rübke-embankement is a pure test-embankement for the autobahn BAB A 26. The Grossenmeer-embankement is an integral part of the federal highway B 211. In both cases high-modukus polyester woven is used with 200 kN/m an 400 kN/m UTS, respectively, and 10 % ultimate strain. For stability calculations DIN 4084 (Bishop´s method) has been used, modified by reinforcement force. Meansurement programs are performed uo to now, including direct strain meansurements at “Grossenmeer”. Woven from 'Rübke' was exhumed after 14 years under tension below groundwater to durability.
- Stability calculations according to DIN 4084 are correct enough to prevent failure of embankment and/or subsoil, at least when evaluate high tensilc for mobilized to restrain deformations.
- High-moduli for a long period (low creep) are impor tant, because the reinforcement works after consolidation also. In the projects described it is still under tension after 15 and 11 years, respectively.
- The high-strength reinforcement used proved to resist overstressing successfully for a long period.
- Local weaker subsoil zones can easily result in reinforcement overstressing and/or failurc tendncy; it should be kept in mind when selecting safety factors.
- Subsoil squeezing-out is recommended to be checked additionally to DIN 4084-calculations by other methods.
- The analyses of durability and operation conditions inclicate in this case, that (internal) hydrolysis under stress in groundwater is the main reason for the regis tered negligible loss of strength.
- The loss o f strength due to 'aging' (environmental effects) is 1.5% to 2% in 14 years for the high-tenacity polyester woven used.