Soil Improvement and Foundation Systems with Encased Columns and Reinforced Bearing Layers

Soil improvement and foundation systems with encased columns and reinforced bearing layers have been used for more than 10 years in the foundations of earth structures and dikes. This chapter outlines the different possibilities of combining flexible encased columns or stiff piles with reinforced bearing layers, and describes some interesting projects. Encased columns can also be used as a ground improvement and foundation system in very soft and organic soils, as the radial support of the columns is secured by geosynthetic, which is loaded by ring tensile forces. First, the theoretical background of the bearing system is explained and experiences gained with encased columns in executed road and railway projects and the results of the geotechnical measurements are discussed. After that, the Airbus works 346-acre enlargement into the tidal mud flats of Hamburg’s River Elbe by using more than 70,000 geotextile-encased columns for dike foundations will be shown. This project had certain special boundary conditions, including tidal influence, inaccessible sediment in the riverbed, and very limited construction time. The design and construction of geogrid reinforced bearing layers (i.e., load-transfer mat) over piles or stiff columns are described. The stress relief of the soft soil and, therefore, a large settlement reduction results from an arching effect in the embankment and a membrane effect of the geosynthetic-reinforcement. In 2003 both tracks of the 150-year-old railway between Berlin and Hamburg were improved simultaneously over a length of 10 km, whereby a geogrid-reinforced structure was reconstructed in 6 weeks over grouted stone columns. In another 625-m-long section, the soft organic subsoil (peat and mud) was improved by installing columns in a wet deep soil mixing technique (mixed-in-place method, MIP), on top of which a geogrid-reinforced load-transfer mattress consisting of two geogrids of polyvinyl alcohol (PVA) material was constructed.