Sand and silty-sand soil stabilization using bacterial enzyme induced carbonate precipitation (BEICP)

The concept of bio-geotechnics represents an innovative, new technical merger between three traditional disciplines: geotechnical, material, and environmental engineering. As originally conceived decades ago, biogeotechnology mechanism uses live micro-organisms to improve and stabilize soils, by which their suitability for construction realizes engineering, environmental, and economical benefits. More recently, though, this concept has been broadened to include a suite of possible strategies, including: 1) using whole-cell microorganisms to secure ‘Microbial Induced CaCO3 Precipitation’ (MICP), 2) using cell-free, free-‘Enzyme Induced CaCO3 Precipitation’ (EICP), and 3) using ‘Microbial Induced Desaturation and Precipitation’ (MIDP). Although none of these biogeotechnical methods have yet reached a pragmatic level of commercial application, promising results have been achieved within laboratory, and in limited instances of large-scale and field-scale evaluation. This dissertation documents the outcomes achieved during an investigation of a novel modification of the latter ‘EICP’ method which could be similarly employed to secure bio-mediated soil improvement. In this case, however, the operative catalytic enzyme (i.e., urease) was extracted from a bacterial source and then used in its free-enzyme form to secure a so-called ‘Bacterial Enzyme Induced Carbonate Precipitation’ (BEICP). A sonication method was applied to lyse living cells of S. pasteurii to obtain the desired urease solution. The urease activity rate of this bacterial extracted enzyme was higher, at an approximate 2X magnification, even though the volume of the sonicated solution had only been reduced one-fourth as compared to that of the original bacterial solution. Furthermore, extending beyond this benefit realized with producing an even higher rate of enzymatic activity, the performance results obtained when using BEICP soil processing demonstrated several additional performance-based benefits. This dissertation consequently documents the engineering properties achieved with BEICP-treated sand processing, as well as comparing these findings against that of traditional MICP treatment. These lab-level research results offer positive evidence for two possible benefits with the BEICP method: 1) mechanical stabilization of sands, and even including that of loose sandy soil materials, and 2) an ability to retain post-treatment permeability of the bio-cemented sands (i.e., as compared to MICP’s typically higher reduction in treated soil permeability). The advantage of BEICP’s free-enzyme processing approach stems from its nano-sized (water-soluble) catalyst dimension, where these nano-enzymes are far more easily able to penetrate the small pore space of a silty sand matrix. In turn, this BEICP method was successfully applicable to the solidification of silty-sand soil. The measurement of unconfined compression strength of BEICP-treated samples ranged from 0.4 to 1.1 MPa, and from 0.23 to 0.84 MPa with silt-sand mixtures at silt levels of 10 and 20 %, respectively. These results accordingly validated the biological treatment process BEICP as a prospectively applicable means of successfully solidifying natural sand and silty-sand soil systems. As previous mentioned, BEICP treated is a new bio-based method, and this dissertation’s accompanying research has further evaluated a variety of processing factors which might impact the resultant engineering properties of bio-cemented sand. Notably, a series of test-tube experiments was conducted to investigate the effects between the bacterial cell and urease in the chemical conversion ratio. The results showed that the precipitation ratio reduced when the concentration of chemical agents increased. These experiments also characterized the urease activity of biological sources and chemical concentration for sand column tests. Two types of sand, including both coarse- and fine-grained sands, were examined in order to evaluate how these size factors impacts product strength and permeability with BIECP treatment. These findings correlated with previous studies on MICP and EICP, where the size of particle and the CaCO3 content played a vital contributing factor relative to both strength increase and permeability reduction. However, more engineering factors, such as injection flow, temperature, chemical concentration, etc., needs to be studied in order to optimize the BEICP-treatment process. Another significant aspect of BEICP-treated soil is that of the durability of the biocemented soil under the freeze-thaw cycling. Sandy soil and silty-sand soils which were originally packed in a loose condition were treated with BEICP processing as well as with commercial Portland cement and fly ash additions. The strength reduction following freeze-thaw cycling was examined on treated samples. This investigation revealed that the BEICP-treated samples retained higher strengths than that in Portland and fly ash cemented samples after freeze-thaw cycling. This approach suggests that this method may have beneficial use when applied to stabilize sub-grade and sub-base materials underlying pavement layers within cold regions. This research effort subsequently started with the development of a sonication technique to lyse viable S. pasteurii bacteria cells in order to release their intracellular urease materials. A particular advantage of using this new method is that it produces distinctly higher levels of urease activity. The extracted enzyme was then used to treat a group of test columns bearing different percentages of coarse- and fine-grained soils by weight. The engineering properties of BEICP-treated soil were evaluated via a series of lab tests. Another clear advantage for BEICP processing is that this method can form calcium-bearing crystals as bridges between fine (silt) and coarse (sand) soil grains, which then increases the overall strength of our silty-sand columns, while at the same time not unduly decreasing matrix permeability