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The principle compound behavior of geogrid reinforced soil is being investigated at RWTH Aachen University. For this purpose, a laboratory apparatus has been developed that allows biaxial compression testing of geogrid reinforced soil. To study the compound stress-strain behavior and the complex interaction mechanisms between soil and the geogrids in a non-scaled ratio, the apparatus admits large scale testing of specimens with dimensions of 800 x 820 x 460 mm³ (H x W x D). Furthermore, it has a transparent side wall to determine the reinforcing effect to the kinematic behavior during compression. The tests are being carried out under plane strain conditions with constant confining pressures applied via vacuum. In this paper, the results of first series of biaxial compression tests are presented. Generally, a significant increase in strength has been observed for all reinforced samples due to the confining effect of the geogrids. With an initial test series an appropriate arrangement of the geogrids in the specimens, in terms of the ultimate bearing capacity, the kinematic behavior, as well as the stress distribution at the specimen top surface, has been determined. Subsequent parameters have then been studied with specimens reinforced with two geogrid layers placed at 1/4th and 3/4th of the height. As an important parameter, the tensile stiffness of the geogrids has been varied in a common range between J0-2% = 530 and 1650 kN/m. It is shown that an increase of the geogrid tensile stiffness leads to both, a higher stiffness and a higher bearing capacity of the reinforced soil.