Consolidation and Strength Characteristics of Biofilm Amended Barrier Soils

Abstract

Experimental work was conducted to investigate the influence of biofilm on the consolidation and strength characteristics of two barrier soils. Biofilm has potential as a low-cost additive for soil stabilization, and it may be formed naturally in landfills throughout the developing world. The EPS-producing bacterium Beijerinckia indica was used to prepare solutions of varying concentration of exopolymeric substances (EPS). These solutions were then used as the molding moisture for compacted specimens of locally available clay ("red bull tallow," RBT) as well as a mix of 65% sand and 35% bentonite (65:35 mix). As compared to tap water, the influence of the nutrient solution or biofilm on RBT is to increase the compression index (C c ), although this trend is variable for increasing EPS concentration. While the effect of biofilm on the 65:35 mix is less uniform, the largest increase in C c was observed for the highest level of biofilm amendment (EPS-5, 300 mg/L). Amendment with biofilm results in both increases and decreases in the rate of consolidation (c v ). The c v values ranged from 0.4 to 13.6 m 2 /year and from 0.2 to 19.3 m 2 /year for RBT and 65:35 mix, respectively. In general, EPS has a decreasing effect on observed strength. For example, the peak unconfined compressive strengths for unmodified RBT and 65:35 mix were found to be 667.0 and 395.3 kPa, respectively. Many of these values decreased with increasing biofilm amendment, and for the highest level of amendment, the observed peak strengths were 159.1 and 98.8 kPa. To the extent that naturally-occurring methanotrophic activity in landfill cover systems results in biofilm production, the results suggest potential concerns with cover stability.

Hydromechanical Effects of Micro‐Organisms on Fine‐Grained Sediments During Early Burial

“…The observation that sediments become more compressible (or less stiff) with added micro‐organisms is consistent with some results from Daniels et al. (2009), who showed that C c values increase with increasing amounts of biofilm for a lean clay (with sand‐size grains present) but not for a sand–bentonite mixture (65% sand; 35% bentonite). While their results are inconclusive, our results clarify and affirm that micro‐organisms increase C c .…”

“…This decrease in porosity with progressive burial results in a decrease in mudstone permeability, where porosity and permeability have a log linear relationship (e.g., Neuzil, 1994, 2019; Reece et al., 2012; Schneider et al., 2011). However, the effects of micro‐organisms on the compression behavior of fine‐grained sediments are poorly constrained (Daniels et al., 2009) and their effects on the permeability of fine‐grained sediments are unexplored.…”