Physical and Numerical Analyses of a Geogrid-Reinforced Soil System for Bridge Abutments
The paper deals with a study of a geogrid-reinforced soil (GRS) solution for bridge abutments, which was tested in a real full scale test at the LGA Nuremberg and also analyzed with analytical and numerical methods.
In the first part of the paper the full scale test will be presented. A 4.5 m high vertical GRS wall was directly loaded by a 1 m wide reinforced concrete beam and hydraulic jacks, which were located near the top front edge. The concrete beam, simulating the bridge sill beam, was subjected to several loading and unloading cycles by the hydraulic jacks, where the load was increased up to three times the normal load for this kind of structure. Settlements and horizontal facing deformations were measured during the test.
The paper presents analytical analyses of the full scale test. The analytical design procedures include methods commonly used worldwide e.g. Bishop, Krey, Janbu and Block Sliding. The focus of the analytical analyses is on the ultimate limit state.
Furthermore numerical analyses of the full scale test will be presented. The numerical analyses were made using FEM with the commercially available Plaxis V9.2 program.
Different material models for the soil, such as Mohr-Coloumb, Hardening Soil and Hardening Soil with Small Strain Stiffness, have been used. The focus of the numerical analyses is on both the ultimate and serviceability limit state. A comparison of the analyses results between the different material models and with the measurements from the full scale loading test will be presented.
The comparisons made between analytical and numerical procedures on the one hand, and the behaviour tested 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.
The paper presents the most important test results of a real scale loading test of a geogrid reinforced vertical soil wall used as bridge abutment, which are published in detail in Alexiew,
2007. 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 meet the stringent requirements.
Different analyses of the obtained test results using analytical as well as numerical methods are presented in this paper. Good agreements are found between the results of the real scale test and the analytical and numerical analyses. All analyses confirm the high bearing capacity of the reinforced soil structure under the very concentrated and heavy loads as well as the compliance with the stringent requirements regarding the deformation of a bridge abutment.
It is found that the exact simulation of the step-wise construction of a geogrid reinforced soil structure is not easy and some limitations exist.
The most important findings from the FEM analyses are:
• The influence of the soil model (MC, HS and HSsmall) on the results of the numerical calculation in the case studied is smaller than expected. Therefore all further calculations are made with the HS model.
• It is 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.
• In the case studied it was not possible to simulate the sill beam 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 beam settlements and facing bulging. The numerical model is reacting softer then the real structure.
• An artificial increase of the tensile stiffness (modulus) of the geogrids in the FEMsimulation results in a better simulation of wall behaviour, especially for the lower load range, but an inferior agreement was found for the higher loads.
• The points of maximum tensile force in the geogrids at the maximum load of 650 kN/m2 from the FEM-analysis correspond very well to the critical Bishop-circle from the analytical analyses.
• The activated geogrid forces in the numerical simulation correspond well with the tensile forces used in the analytical calculation.
• 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, especially in cases of very high beam loads. For lower loads the FEM results seem to be on the safe side from the point of view of load-deformation behaviour.