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Alexiew, Dr.-Ing. D.

Ultimate bearing capacity tests on an experimental geogrid-reinforced vertical bridge abutment without stiffening facing


The paper deals with geogrid reinforced soil as solution for bridge abutments. Most important results are presented of a real scale test of a 4.5m high geogrid reinforced vertical soil block loaded directly on top near the edge by a sill beam, high-lightening the low settlements and horizontal displacements measured. In one test, the reinforced embankment was nearly lead to failure, what occurred with a load in the order of 3 times the usual one for this kind of structures.


The tests presented herein on a geogrid-reinforced soil block simulating a real bridge abutment under a sill beam are in no way intended to be a comprehensive scientific analysis. The exercise is much more about testing the behaviour of a system and its reserves (!) in a situation directly related to practice. The use of an already constructed and used for other purposes test object after modificationwas advantageous in terms of time and costs, but it also brought its own restrictions and deficiencies, e.g. that we would have to live with the known insufficient compaction in the upper zone and the possible looser fill zones near the facing resulting from previous tests. The tests described herein are still fairly recent; and so the following remarks are a first, rather incomplete overview, but the most important points are readily recognisable and can be translated into practice.

The tested arrangement should be seen as a "worst case" scenario:

• The sill beam was only 1.0m wide and placed only 1.0m away from the edge

• The front face was vertical

• The facing had no special stiffening elements, being only a geogrid-wrapped-back wall

• The density of the fill in the most sensitive upper zone was only Dpr = 95%, with probably

loosened zones in the front area near the loading beam, some probably as a result of the

previous tests.

The following remarks can be made:

• A contact pressure under the sill beam of up to 650 kN/m2(approx. 3 times the pressure normally experienced) led to no obvious component or system failure. However, because there were signs of serious effects taking place, the situation could be used as a benchmark for the ultimate limit state.

• Acontact pressure of up to 400 kN/m2(approximately twice the usual value) resulted only in completely acceptable deformations.

• The tested system exhibited technically advantageous, ductile behaviour with no discontinuities and seems to have a substantial reserve capacity.

• The overall performance can be considered very good despite the previously found soil density deficiencies.

• The facing consisting of flexible geogrids had no bending stiffness but showed only small local and global deformations (marginal in the relevant load range).

• The settlement behaviour of the sill (indirectly assessed by the modulus of subgrade reaction) was as if it had been sitting on an infinite horizontal plane and not near a vertical slope; the only plausible explanation is the apparently high effectiveness of the incorporated geogrids.

The author would have no reservation using the structure as built and tested (and ideally with better fill compaction) in practice.