Analytical and numerical analyses of a real scaled geogrid reinforced bridge abutment loading test
The paper deals with the study of a geogrid-reinforced soil (GRS) solution for bridge abutments. In the first part of the paper a full scale test is briefly presented summarizing the important parameters and results required for further theoretical analyses. A 4.5 m high geogrid reinforced vertical soil wall was directly loaded, near the top front edge, using a reinforced concrete block and hydraulic jacks, simulating the bridge sill beam. Loading and unloading cycles were performed, where the load was increased up to 3 times the normal load for this kind of structure. Settlements and horizontal facing deformations were measured during the test.
In the second (and main) part of this paper analytical and numerical analyses of the full scale test are presented.
The analytical procedures include methods commonly used worldwide e.g. Bishop, Janbu, Block Sliding etc. The numerical analyses were made using FEM with the commercially available Plaxis V8.6 program.
The analytical analyses focus on the ultimate limit state and the numerical analyses on both, serviceability and ultimate limit state. The comparisons made between analytical and numerical procedures on the one hand, and test behaviour on the other hand, will assist in gaining a better understanding of the systems behaviour and application, and for better guidance in relation to the appropriate design procedures and assumptions for heavily loaded geogrid-reinforced bridge abutments, both in regards of ultimate and serviceability limit state.
This paper presents first the most important test results being published in detail elsewhere (Alexiew 2007) of a real scale loading test of a geogrid reinforced vertical soil wall used as bridge abutment. The test results demonstrate the high capability of geogrid reinforced soil walls and the versatility as bridge abutment since the bearing capacity and also the deformations fulfil the stringent requirements.
The obtained test results are now analysed herein first with different analytical limit equilibrium methods. The calculations are made with the non-factored (say: "characteristic") parameters of the soil and the load. The tensile strength of the geogrid is reduced only due to creep and installation damage: no additional partial factor of safety is applied. The circular failure plane methods of Bishop and of Krey and the polygonal failure plane methods of Janbu and block-sliding are applied. Very good agreements are found between the analytical methods and test results, only Janbu seems to be a bit conservative.
In a second step numerical FEM analyses were performed in order to simulate the registered load-deformation behaviour of the test wall for the full load range from 50 kN/m2 up to 650 kN/m2 (when the test wall approached failure) under the loading sill beam. The most important foundings from the FEM analyses are:
• It was confirmed that for the system under discussion the load-settlement behaviour of the sill beam on top of the reinforced wall is similar to that of an even unreinforced infinite half-space (with the same soil parameters) demonstrating the efficiency of the reinforcement used.
• It was not possible to simulate the sill settlement and the horizontal displacements of the wall facing in a precise way for the lower loads. For them the numerical simulation overestimates both sill settlements and facing bulging. Possible explanations and comments are given.
• An artificial increase of the tensile stiffness (modulus) of the geogrids in the FEM-simulation results in a better simulation of wall behaviour, especially for the lower load range.
• The points of maximum tensile force in the geogrids at the maximum load of 650 kN/m2 from the FEManalysis correspond very well to the critical Bishop-circle from the analytical analyses.
• The wraparound facing from the geogrids Fortrac 80/30-35M without any other stiffening elements seems to provide sufficient support/confinement redirecting the main stresses downwards probably due to the grids tensile stiffness.• Despite the problems faced, FEM seems to be an acceptable tool for analysis and modelling of the general tendencies in the behaviour of the prototype test wall. A main problem seems to be the appropriate simulation of the early geogrid mobilisation.