Micro- and macroscopic interaction behaviour of geogrid-reinforced soils
- Mikro- und makroskopisches Interaktionsverhalten von geogitterbewehrten Erdkörpern
Derksen, Jan; Ziegler, Martin (Thesis advisor); Fuentes Gutierrez, Raul (Thesis advisor); Grabe, Jürgen (Thesis advisor)
Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023
The complex interaction between geogrids and adjacent soils is key to the performance of geosynthetic-reinforced soil (GRS) structures and needs to be understood from the micro- and macroscopic perspective to enable safe and economical designs. For this purpose, the micro- and macroscopic interaction behaviour of geogrid-reinforced soils was investigated in this thesis by combining experimental results with numerical modelling. Interaction experiments with transparent soil were performed to investigate the modes of interaction and the microscopic load transfer. A new experimental setup was presented to provide a more realistic insight into indirectly activated geosynthetics by the movement of the surrounding soil. The transparent soil technique was used to record the processes at the interface unobstructed and continuously by digital image analysis. The microscopic interaction performance was evaluated regarding deflections of geogrid transverse members, locally mobilised tensile strains and loads, and interface shear stresses resulting from the relative displacements between geogrid and adjacent soil. The effects of geogrid aperture size, tensile stiffness, geogrid type and reinforcement configurations, properties of the fill, confining stress, and load plate length and arrangement on the development of interaction zones and interface performance were investigated. Three distinct modes of interaction were identified for indirectly activated geogrids: pushout, pullout and interlocking. In the pushout zone, loads from the soil were transferred to the geogrid. The direction of load transfer was reversed in the pullout zone, where the geosynthetic tensile force was anchored to the fill. In the interlocking zone, equilibrium was established, where neither relative movement nor load transfer occurred between geogrid and adjacent soil. Additionally, small-scale and large-scale 1g experiments were conducted under plane strain conditions to analyse the macroscopic structural behaviour of GRS walls regarding bearing capacity failure in the foundation soil. The small-scale tests provided qualitative and fundamental insights into the development of shear surfaces during failure based on image analysis. However, the small-scale tests suffered from scale effects because the mechanical similarity was not completely fulfilled. Therefore, large-scale experiments were additionally performed to quantify the ultimate load-bearing capacity and to obtain data on soil stresses and internal soil movements, wall deformations and reinforcement strains. The experimental results revealed that the bearing capacity failure was initially triggered at the rear end of the reinforced zone. However, the effective reinforcement length relevant to the bearing capacity of the subsoil was progressively reduced during failure by shear bands intersecting the bottom geogrid layer. The results confirmed the quasi-monolithic behaviour of the reinforced zone. A multi-body failure mechanism formed below the base of the wall, consisting of an active and passive wedge connected by a transition zone. In numerical modelling, appropriate interface formulations are required to predict the deformations and stresses for GRS structures. Therefore, the capability of modelling geogrid-soil interaction with different interface approaches in finite element analyses was investigated. For this purpose, the two main approaches with zero-thickness (ZTE) and finite-thickness (FTE) interface elements were analysed in combination with conventional (MC, HS, HSS) and user-defined (UDM) constitutive models. For the UDM, a hyperbolic approach was implemented in the finite element software Plaxis to model the non-linear and stress-dependent shear stiffness of geogrid-soil interfaces. The interface behaviour was initially studied under direct shear movement in isolation. This pre-analysis revealed that the behaviour of the default ZTE interface was characterised by a linear-elastic, ideal-plastic stress-strain relationship, even for constitutive models covering non-linear shear behaviour. In contrast, the FTE interfaces assigned with HS, HSS and UDM constitutive models accurately represented the experimentally observed hyperbolic stress-dependent shear behaviour because the non-linear degradation of shear stiffness was enabled. Finally, the performance of the interface approaches was examined in two boundary value problems using the interaction experiments with transparent soil and the large-scale experiment of the GRS wall.
- Chair of Geotechnical Engineering and Institute of Geomechanics and Underground Technology