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.

PurposeThe purpose of this paper is to evaluate the influence of a mixture of nutrient solution, bacteria and biofilm on the consolidation, unconfined compression and desiccation characteristics of two soils that could be used in waste containment applications.Design/methodology/approachExperimental work was conducted to investigate the influence of biofilm on the desiccation, strength and consolidation characteristics of two barrier soils. The soils were evaluated with water alone and with a biofilm solution composed of nutrients, bacteria and exopolymeric substances (EPS). These solutions were mixed with a locally available clay (“red bull tallow” (RBT)) as well as a mix of 65 percent sand and 35 percent bentonite (65‐35 Mix).FindingsReductions in strength and increases in ductility are observed with biofilm amendment for two soil types. The shear strength was reduced from 413 to 313 kPa and from 198 to 179 kPa for RBT and 65‐35 Mix, respectively. Desiccation tests reveal an increase in moisture retention for early time increments in amended specimens, while both increases and decreases are noted after extended drying. Increases in the rate of consolidation and modest decreases in the compression and swell index were observed. In particular, the consolidation coefficient was increased from 0.036 to 0.064 cm2/min and from 0.060 to 0.093 cm2/min for RBT and 65‐35 Mix, respectively.Practical implicationsThese results are useful in establishing the broader impacts of using biofilm as an additive to increase the performance (e.g. reduce hydraulic conductivity and increase resistance to crack formation) of barrier materials in waste containment applications. Moreover, the data provide insight into the geotechnical implications of biofilm‐producing methanotrophic activity that occurs naturally in the covers of municipal solid waste landfills.Originality/valueVery little research has been published on the influence of biofilm on the behavior of barrier materials in general, and on geotechnical properties in particular. This paper is unique in making the connection between methanotrophic activity, soil modification and barrier material performance.

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