Our research in bio-soils is examining the interaction between biological processes and the engineering properties of soil with two objectives:
In our research we have linked with microbiologists Dr. Klaus Nusslein (UMass Amherst) and Dr. Doug Nelson (UC Davis). We have also received funding from the National Science Foundation and have linked with select industry companies to advance this technology towards full-scale field implementation.
Our current investigations focus on creating and controlling calcite precipitation within granular soils (e.g. sands) through a bio-mediated process in which ureolysis by aerobic microbes raises the pH in a supersaturated solution, forcing precipitation of calcite.
Current methods to improve the engineering properties of sands are diverse with respect to methodology, treatment uniformity, cost, environmental impact, and site accessibility requirements among others. All of these methods have benefits and drawbacks, and there continues to be a need to explore new possibilities of soil improvement, particularly as suitable land for development becomes more scarce.
The utilization of aerobic and anaerobic bacterial processes provides a unique opportunity to harness natural biological processes for engineering purposes. In this study bacterial (microbial) mineral precipitation of calcium carbonate (calcite) was performed. The creation of calcium carbonate (calcite) cement occurs as a consequence of bacterial metabolic activity which raises the pH of the proximal environment. The local pH rise may be achieved by the production of ammonia and carbon dioxide resulting from the enzymatic hydrolysis of urea. Bacillus pasteurii, a common soil bacteria, uses urea as an energy source and produces ammonia which increases pH, in turn causing Ca2+ and CO32- to precipitate as CaCO3. Microbiologically-induced calcite precipitation occurs according to the reactions:
A microscale investigation was performed to directly observe the characteristics and degree of bonding between particles as well as the compositional nature of the cementing agents. Results from scanning electron microscopy and X-Ray compositional mapping are presented below. The individual particle images show calcite cement (in a lighter shade of grey) on the particle surface and at particle contacts. It is apparent in the 2-D images that many of the particles are connected via cementation even though the actual particle-particle contacts are few.
The presence of calcite on the silicate grains was confirmed by X-Ray compositional mapping of the same biologically cemented specimen used for SEM images. The whiteness in the image is proportional to the abundance of the element under consideration (denoted above each image).
Cementation of the microbially induced calcite precipitation (MICP) specimens occurs shortly after the initial biological treatment. A plot of shear wave velocity, Vs, (measured with bender elements) versus time for one triaxial specimens during treatment is shown below. As evident the shear wave velocity increases from about 200 m/s to a value of 540 m/s. Generally it is feasible to improve soil from a NEHRP site classification of E/F (problematic soil requiring site specific analysis) to B/C (soft rock).
Following treatment the triaxial specimens were subjected to undrained monotonic shearing. Throughout shearing the shear wave velocity was measured, providing insight into the progressive degradation of cementation. The results of both an untreated specimen (for reference) and a bio-treated specimen are shown below.
The untreated specimen exhibited conventional monotonic hardening with strain as presented in q/p’consol versus global axial strain (εa) where q = σ1-σ3 and p’ = 1/3(σ1’ +2σ3’) (Figure 2a). The effect of microbial induced cementation on undrained shear response was readily evident. The microbially treated specimen provided a stiff initial response, approaching a peak q/p’consol ratio of 3.8. The shear wave velocity decreases significantly within the first 0.7% axial strain, after which it approaches a value similar to the untreated specimen. As evidenced, the bio-mediated improvement technique significantly altered the mechanical behavior of sands, in this case effectively preventing monotonic undrained collapse during shear.
Optimization of the bio-mediated treatment technique, in terms of both the chemical-biological reaction network and the geotechnical parameters is underway. As an example, efficiency of the calcite precipitation depends on the calcium concentration provided in the tretment solutions. As shown below, the rate at which the soil stiffness increases (a secondary indicator of the magnitude of precipitation that has occurred) depends on the calcium concentration. Similarly, the range of soils which can be successfully treated is also likely limited to soils with primarily sand-sized particle content.
A series of tests have been undertaken to realize the benefits observed at the laboratory element scale at the field scale. Using 1-D tube, 2-D planar, and axisymmetric bench scale models, the parameters required to obtain uniform treatment throughout the treatment zone are being identified.
In addition, the potential extent of benefits in geotechnical system performance is being explored in 1-g models. An example of this is a simple spread footing foundation. The images to the left show the footing load and the extent of the treatment zone after excavation, while the figure on the right shown an 80% decrease in foundation settlement when the treatment technique is implemented.
Reference for Original Work:
DeJong, J.T., Fritzges, M.B., and Nüsslein, K. “Microbial Induced Cementation to Control Sand Response to Undrained Shear” ASCE Journal of Geotechnical and Geoenvironmental Engineering, 2006, Vol. 132, No. 11, pp. 1381-1392. [Link]