Proposal of Axel Ruiken of
Geogrid-Soil Interaction Mechanisms from Plane Strain Model Testing
The aim of this research project is the investigation of geogrid‐soil interaction mechanisms in order to describe the behaviour of geogrid reinforced soil structures. An economic design, however, implies an exact knowledge of the stresses acting within the reinforcement. The current research project is therefore taking the benefits from plane strain testing of geogrid reinforced soil in order to determine the stresses acting in the geogrids. For this purpose a biaxial compression test apparatus is currently being developed with respect to the experience gained from first tests with the box shown in Figure 1. The test box measures (H-W-D) 1m x 1m x 0.45m and has transparent sidewalls. The geogrids can be placed in the box either without any connection to the facing or with one side fixed. The clamps for the fixture are connected to load cells to be able to record the connection loads of the geogrids at the facing. At testing with fixed geogrids the facing can be retracted until it is held completely by the geogrids, i.e. state of equilibrium.
Fig. 1. Plane strain test box for geogrid reinforced soil
Fig. 2. Movement of soil particles from PIV analysis
(Soil reinforced with 2 geogrid layers)
Performance of tests includes homogeneous loading of the geogrid reinforced soil with a compressed air cushion with up to 50 kPa, followed by a controlled horizontal displacement (retraction) of either one facing or both facings simultaneously. The test series include the variation of the number of reinforcement layers (resp. reinforcement spacing), the type of geogrids (resp. geogrid stiffness), the stress level, the type of filling soil and the fixture of the geogrids in order to answer the following questions:
Measurement of forces and pressures is made with standard load cells at the abutments and with high resolution foil sensors (approx. 1 measured value per cm²) between the soil and the facing. Furthermore, redundancy is ensured by measuring the earth pressure also with 20 load cells, each 5 cm high, forming one of the facings. Geogrid strains are being recorded with strain gauges attached to the geogrids. Using the particle image velocimetry (PIV) method to evaluate photographs taken during testing allows to identify shear surfaces (Fig. 2) and areas of interlock.