Stability and durability of polymers and clay-polymer composites in geotechnical environments

• Applicants: Dr.-Ing. Wiebke Baille; Professor Dr.-Ing. Marc Wichern; Professor Dr.-Ing. Torsten Wichtmann
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 279823280

Polymers are used in numerous geotechnical and geoenvironmental applications to replace bentonite or improve its performance, e.g. as support fluids, in geosynthetic clay liners, as lubrification agent and soil conditioner in mechanized tunnelling. However, polymers are subjected to degradation during their service life time. In soils, degradation of the polymer is caused by mechanical and chemical stresses or due to biological activities. Degradation alters the relevant polymer properties, such as molar mass, charge distribution and composition of the polymer. Subsequently, also the relevant engineering properties permeability, strength, and volumetric behaviour of the polymer are found to alter. Even the release of toxic degradation products has been reported in literature. This leads to significant uncertainties with respect to the stability and durability of polymers within geotechnical applications. A reduced stability, thus increased mobility, and an insufficient durability can imply possible pollution of the soil.The current research proposal focuses on the alteration of polymers and polymer enhanced soils caused by relevant hydraulic, mechanical and biological impacts in geotechnical applications. Both synthetic polymer and biopolymer are considered. The project is divided in the main parts: (1) development of techniques for the characterization of polymers adopted to soil mechanics laboratory testing, (2) experimental investigation on degradation of polymers and polymer-modified soils subjected to frictional flow, mechanical stresses and biological activity, (3) assessment of the mobility of polymers and their degradation products using numerical modelling.In the first part, by an extensive experimental program, correlations between the molar mass and the viscosity are established. The charge properties will be related to a change in viscosity by the known change in salt concentration. A simple outflow test is calibrated against rheometer test results for determining viscosity using Hagen-Poiseuille equation and is planned to be used for quick characterisation of the various polymers. In the second part, the investigation of degradation induced by frictional flow of anionic polymer solutions, by mechanical shear stresses acting on dry (brittle) or wet (ductile) soil modified with cationic polymers, or by biologically active microorganisms in cationic and anionic polymer-soil mixtures will be performed using adequate experiments. Degradation will be quantified based on measured plasticity properties, permeability, viscosity, peak strength, stress-strain behaviour and undrained shear strength, and swell index prior and after the respective degradation impact. The third part of the research project covers a numerical study (with PLAXIS 3D PlaxFlow) on the mobility of polymers and degradation products using viscosity as the main parameter. For this, an earlier developed numerical approach will be modified with respect to polymer solutions.

• DFG Programme: Research Grants