Bodengewölbe unter ruhender und nichtruhender Belastung bei ...

Summary 187
10 Summary
In this research work the phenomenon of soil arching in cohesionless soils under static and
cyclic loading and the stabilization of the soil arching by means of geosynthetic
reinforcement (geogrids) has been investigated using experimental, numerical and analytical
methods. The research focuses mainly on the soil improvement system “geosyntheticreinforced
and pile-supported embankment” (GEP).
In addition to the principal topic of the thesis “soil arching under cyclic loading”, some other
special problems related to GEP-systems have also been analysed. These include behaviour of
a GEP-system in the case of triangular pile grids, investigation of the lateral spreading,
calculation of the pull-out length and numerical calculation of the GEP-system under cyclic
loading.
A GEP-system is constructed in areas with soft subsoil. In recent years, it is primarily applied
for highway and railroad embankments in existing or new routes. The basic principle of the
soil improvement system is to relieve the settlement-prone weak soil without disturbing and
altering its natural structure. This is realised by pile-like foundation elements which are
placed in a regular pattern through the soft soil down to a lower load-bearing stratum. Above
the pile heads the reinforcement of one or more layers of geosynthetics (mostly geogrids) is
placed and above this the embankment is built up. The stress relief of the soft soil results from
an arching effect in the reinforced embankment above the pile heads and a membrane effect
of the geosynthetic reinforcement. Part of the load is carried directly by the pile-elements,
another part is first carried by the geosynthetic reinforcement and afterwards transferred to
the pile heads. Finally, the loads are transferred down via the piles into the bearing stratum.
Whereas the system behaviour (arching and membrane effect) under static loading is wellknown,
the bearing behaviour and the settlements expected under cyclic loading is not yet
fully explained and cannot be predicted. Under cyclic loading the arching effect can only be
formed in a very limited extent and part of the load carried directly by the piles can decrease
remarkably, which results in an increase of the load on the soft soil and on the reinforcement.
Due to the reduction of the soil arching, the strains in the geogrid and the surface settlements
increase considerably. The existing design procedures which take soil arching and membrane
effects of the geosynthetics into account are not applicable for such cyclic loading conditions.
Three-dimensional well-instrumented model tests at a scale of 1:3 had been carried out to
investigate the behaviour of soil arching under cyclic loading. The test arrangement M1
consists of a group of four piles placed in a weak soil layer (peat) in a rectangular grid. Above
the pile heads and the soft soil layer a reinforced or unreinforced sand fill of different heights