Experience on the preparation of HPMC viscous fluid for physical modeling in the geocentrifuge

Abstract

Investigating soil response before, during and following large scale, dynamic events like slope failure or impact hammering of monopiles, is challenging. Full scale research into these processes is often conducted in the field, as laboratories don’t offer the required space to conduct these experiments. Apart fromthe monumental costs related to full scale experiments, it is often impossible or impractical to define or portray all boundary conditions, which increases uncertainty. As an alternative to full scale field tests, centrifuge tests on a scaled model are often carried out. When conducting research in the centrifuge, the decrease in geometry is compensated by through the acceleration of the model to N times gravity g. In this way, full scale stress conditions are imposed on the sample. Consequently, the model offers an accurate representation of full scale soil behavior. However, artificial ’gravity’ enhancement impacts a broad range of physical quantities. Scaling laws dictate how physical quantities are affected by conditions in the centrifuge and require careful observation. Yet, the use of scaling laws introduces a discrepancy between the timescale related to dynamic events and diffusive processes. The latter is of particular importance to build-up and dissipation of deviatoric pore fluid pressures. Decreasing the permeability of the soil is generally the best option to eliminate the aforementioned discrepancy. Consequently, instead of water, viscous fluid is used for the centrifuge tests, where the viscosity is increased N times with respect to water. Over the years, various fluids have been developed and utilized in centrifuge experiments. A widely used fluid, consists of aqueous solutions (Hydroxypropyl) Methylcellulose or (HP)MC in short. HPMC molecules form polymeric chains which increase viscosity while largely maintaining the density of the solvent, water. These favorable properties make it a highly sought-after substitute for water in centrifuge experiments. Experience with the fabrication and use of (HP)MC solutions is limited at the centrifuge facility of Delft University of Technology. As part of an initiative to develop in-house knowledge relating to the aforementioned points for physical modeling purposes, this research presents a robust fabrication methodology and maps the viscous properties of HPMC solutions, fabricated usingMethocel® F4M, at various concentrations. Results indicate that advocated preparation methodology enables the fabrication of viscous fluids in the range of 10 to 100 mPa ¢ s of consistent quality. However, overall, the viscosities of the fluids created along the lines of the presented methodology are consistently more viscous than anticipated. Several hypotheses aimed explaining the discrepancy are drafted. However, the nature of the underlying cause remains a topic of debate. Furthermore, it is observed that the HPMC fluids express a substantial degree of shear thinning at high shear rates. The relative decrease in viscosity increases with concentration, causing the viscosities of fluids of different concentration to gradually converge at high shear rates. The latter stresses the importance of quantifying expected shear rates beforehand to prevent behavioral inconsistencies between model and prototype. However, under some circumstances, it is doubtful whether the use of viscous fluids created from Methocel® F4M is suitable to study prototype behavior. In an attempt to facilitate drafting of appropriate recipes for the fabrication of viscous fluid, a general expression is presented to calculate the required concentration, provided the desired viscosity and anticipated shear rate. This generic expression provides adequately describes the experimental data, but requires further tuning in order to fully fulfill its intended purpose. Nonetheless, it provides a valuable indication of the required concentration to obtain a fluid with sought-after properties; thereby shortening the time spent on drafting the ideal fluid recipe.