Stiffness and Dilatancy Improvements in Uncemented Sands Treated through MICP
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 141, Issue 11
Abstract
Laboratory testing shows that microbially induced carbonate precipitation (MICP) through microbial denitrification can improve the mechanical properties of a sand without inducing significant interparticle cementation. Consolidated isotropically undrained triaxial compression testing of Ottawa 20–30 sand treated with denitrifying microorganisms shows that, even at low carbonate contents and with no observed cementation, soil treated through MICP exhibits significantly improved stiffness and dilatant behavior. These improvements are also evident when the treated soil is dried, reconstituted, and retested, indicating that the stiffness and dilatant properties of the soil can be improved by MICP in the absence of interparticle cementation, particularly at low strains. However, these improvements may be reduced or eliminated when the soil is reconstituted and tested multiple times. These results indicate that small amounts of MICP can induce significant improvement in treated soils, potentially leading to savings in time and money if this technology is applied in the field.
Results of consolidated isotropically undrained triaxial compression (CIUC) tests presented in this abstract suggest that, in addition to increasing shear strength through cementation of soil particles, microbially induced carbonate precipitation (MICP) may also increase the stiffness and dilatant properties of the sand by increasing particle roughness. However, repeated shearing may reduce or eliminate these increases. MICP occurs when microorganisms alter the geochemistry in soil pore water containing divalent cations (e.g., calcium) by increasing the alkalinity and pH. This, in turn, drives the precipitation of carbonate species (e.g., calcite). MICP is generally assumed to improve soil behavior through carbonate cementation of soil particles. Several different MICP processes are being explored by investigators worldwide, including ureolysis (Whiffin et al. 2007) and denitrification (van Paassen et al. 2010).
MICP was induced through denitrification in columns of Ottawa 20–30 silica sand. The columns were formed at a relative density of 40% and inoculated with a mixed culture of bacteria. After inoculation, the columns were treated 13 (CUIC-1) and nine (CUIC-2) times at 2-week intervals with a pore fluid consisting of calcium nitrate, calcium acetate, calcium chloride, and magnesium sulfate. Ion chromatography and mass balance indicated that the carbonate content of the treated sand columns was 0.7 and 0.5% by weight, respectively.
Following treatment, the columns were drained, flushed with deionized water to remove residual salts, and subjected to CIUC testing. Following testing, the specimens were washed with acetone to dissolve microbial biomass and heated to approximately 300°C to burn off residual biomass. The sand was then reconstituted once (CUIC-1) or twice (CUIC-2) to a relative density of 45% and subjected to CIUC testing. An untreated specimen of Ottawa 20-30 sand at a relative density of 45% was also subjected to CIUC testing. All CIUC test specimens were back-pressure saturated () at a confining stress of 100 kPa and tested at 0.5% strain per min to a maximum strain of 15%.
The CIUC test results (Fig. 1) show a substantial increase in the stiffness and dilatancy following MICP-treatment compared with the untreated soil. Similarly, the first reconstituted MICP-treated specimens show significant increases in stiffness and dilatant behavior at small strains. However, following the second reconstitution, CIUC-2 showed little improvement compared with the untreated specimen.
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Implications
The CIUC test results presented in this abstract suggest that very little calcite precipitation is necessary to induce significant increases in the stiffness and dilatant behavior of Ottawa 20–30 sand. They also show that the stiffness and dilatancy of the MICP-treated sand is significantly improved at small strains with no interparticle cementation. These findings could lead to savings in time and money if MICP is applied for soil improvement in the field. It is hypothesized that the observed improvement in stiffness and dilatancy of the reconstituted treated sand is caused by particle roughening as a result of calcium carbonate coating the particles. Degradation of improvement with subsequent reconstitution is presumably attributable to the removal of the calcium carbonate coating from the soil particles by abrasion during triaxial testing and sample reconstitution. Testing is currently under way to determine if these findings hold true for cyclic loading.
References
van Paassen, L. A., Daza, C. M., Staal, M., Sorokin, D. Y., van der Zon, W., and van Loosdrecht, M. C. M. (2010). “Potential soil reinforcement by biological denitrification.” Ecol. Eng., 36(2), 168–175.
Whiffin, V. S., van Paassen, L. A., and Harkes, M. P. (2007). “Microbial carbonate precipitation as a soil improvement technique.” Geomicrobiol. J., 24(5), 417–423.
Information & Authors
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Published In
Journal of Geotechnical and Geoenvironmental Engineering
Volume 141 • Issue 11 • November 2015
Copyright
© 2015 American Society of Civil Engineers.
History
Received: Jun 5, 2015
Accepted: Jul 20, 2015
Published online: Aug 17, 2015
Published in print: Nov 1, 2015
Discussion open until: Jan 17, 2016
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