The Effect of Exopolymers on the Compressibility and Shear Strength of Kaolinite
|Institution:||Louisiana State University|
|Department:||Civil & Environmental Engineering|
|Keywords:||biological soil improvement; biopolymer; EPS; extracellular polymeric substances|
|Full text PDF:||http://etd.lsu.edu/docs/available/etd-01202011-141648/|
Erosion and subsidence threaten coastal infrastructure and natural habitats throughout the coastal United States, and this threat is especially significant along the Louisiana coastline since wetland clays are generally weak and highly compressible. Hydraulic pumping of dredged sediment is a common method for combating damage caused by erosion and subsidence, but the high water content slurry deposited is very compressible with low shear strength. Although conventional soil amendments are effective for reducing compressibility and increasing shear strength, these stabilizers are often caustic or toxic, making them too risky to use. Exopolymers, however, have the potential to improve sediment stability without the environmental risks of typical soil stabilizers. Exopolymers are high molecular weight polysaccharides produced by soil microorganisms. While there have been some studies that correlate soil exopolymer content with improved erosional resistance, there has been no work that measures changes in compressibility and shear strength and relates these changes to Stress History And Normalized Soil Engineering Properties (SHANSEP) models. This dissertation describes methods for using two exopolymer analogues, guar gum and xanthan gum, to change the properties of a pure kaolinite. Changes in the compressibility of biopolymer and kaolinite mixtures were measured using 1D consolidation and triaxial tests. Also, modifications of shear strength were measured using direct shear and triaxial tests. Results from these tests were used to develop SHANSEP and empirical models. The statistically strongest empirical model demonstrated that guar gum produces a 19% mean increase in the inverse of compressibility with a 3% standard deviation at the optimum concentration. It also produces a 9.6% mean increase in undrained shear strength with a 5% standard deviation. Further, cohesive strength meter (CSM) tests showed that guar gum can increase erosional resistance by nine times over the kaolinite on its own. Speculative mechanisms were proposed to aid in interpretation of liquid limit, CSM, 1D consolidation, direct shear, and triaxial test results and to guide future research. This study seeks to advance the understanding needed to develop biological methods of sediment stabilization. It also specifically demonstrates the potential of guar gum amendment for possible use in hydraulically pumped dredged sediment.