Soil Treatment with a Thixotropic Fluid: An Autoadaptive Design for Liquefaction Prevention

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Liquefaction of loose cohesionless soils, associated with the effective stress loss produced by rapid repeated loading, is an important cause of damage to civil infrastructures during earthquakes. While many factors affect the liquefaction resistance of soils, both laboratory studies and field observations support the conclusion that the presence of plastic fines reduces the liquefaction susceptibility of a site. This concept is the premise for the work proposed here which is an in-depth investigation of the use of thixotropic fluids, specifically bentonite-based suspensions, for treatment of sands for liquefaction susceptibility mitigation. This idea, which originated from an NSF sponsored workshop focused on the use of autoadaptive media in civil engineering, is supported by the results of a short term project funded by NSF through the Small Grant for Exploratory Research Program. This study demonstrated that the presence of 5% bentonite (by dry mass of sand) dramatically increased the liquefaction resistance of a loose sand. It also highlighted the difficulties of delivering the bentonite into the sand pores due to the rheological properties of bentonite suspensions. The proposed research builds on the results of this preliminary study to a) provide a more in depth investigation of the behavior of sand-bentonite systems; and b) to tackle the challenge of delivering the bentonite to the pore space in a time-release fashion. The work will be performed by an interdisciplinary team from Purdue University's School of Civil Engineering and Department of Chemistry. The specific objectives of the first component of the research are not only to expand the range in testing conditions (CSR and frequency of load applied during cyclic loading, confining stress level, bentonite percent, etc.) investigated, but also to address the more fundamental question of the micromechanism(s) responsible for the increased liquefaction resistance of sands in presence of bentonite. In addition, work will also be performed to assess the self healing nature of the bentonite treatment under repeated cyclic events, which potentially makes the sand-bentonite system a truly autoadaptive material. For delivering the bentonite to the pore space of sands, the research will explore a novel treatment based on coating the bentonite particles with Chitosan, a widely available and inexpensive biodegradable cationic carbohydrate. Chitosan will be used to inhibit swelling of the bentonite thus facilitating permeation of the suspension into the porous medium. The compound will then degrade due to the action of soil bacteria restoring the swelling properties of the bentonite. One broader impact of this research is to develop an innovative and cost-effective solution for a problem that has great significance to society. The novel approach for treating bentonite by means of a biodegradable compound has the potential of being relevant for a number of other applications. An interactive web page will be created to host a collection of images for use in teaching the complexity of the micro/nanostructure of clays. The web page will be complemented by a manual that will describe its use for education in junior high through university level courses. Work will continue on the development of modules for use in K-12 education with the mathematics teacher with whom initial work has already been conducted thanks to a RET (Research Experience for Teachers) grant from NSF.