Biogeochemical processes and geotechnical applications: progress, opportunities and challenges
Summary (5 min read)
INTRODUCTION
- The natural origins of soils, and hence their variability and changes in properties over time, result in engineering mechanics alone being insufficient to address many practical problems.
- Living organisms at other length scales are also present.
Emergence of bio-soils as a subdiscipline
- One of the first explicit discussions of the application of biological processes in geotechnical engineering was presented by Mitchell & Santamarina (2005) , and in parallel it was identified as an important research topic by the US National Research Council (NRC, 2006) for the 21st century.
- The first international workshop on biogeotechnical engineering in 2007 facilitated interdisciplinary discussions and prioritisation of research topics in this emerging field (DeJong et al., 2007) .
- The Second International Workshop on Bio-Soils Engineering and Interactions, funded by the US National Science Foundation, was held in September 2011 at the University of Cambridge.
- This workshop assembled 35 of the leading researchers in the field, and provided an opportunity to assess progress to date, identify the primary challenges and opportunities that lie ahead, and develop strategies for advancing this rapidly developing field.
- This paper presents the outcomes of the workshop, along with a perspective on the possible role of biological processes in geotechnical engineering, including examples of their application and a discussion of salient issues.
POTENTIAL OF BIOLOGY TO MODIFY ENGINEERING PROPERTIES OF SOILS Length scales of biological processes
- The processes by which biology can modify the engineering properties of soil depend on the length scale of organisms, both in absolute dimension and relative to the particle size.
- Multicellular organisms, ranging from plant roots down to insects and invertebrates (e.g. ants, worms), alter soils through both mechanical and biological processes.
- 5 to 3 ìm, and have morphologies that are typically spherical or cylindrical; the latter may be straight (rods), curved , or corkscrew shaped .
- The ability of microbes to regulate processes (depending on the specific process utilised) often stems from the unicells containing the enzyme(s) critical to the geochemical reaction.
- The location of the enzyme, usually within the cell membrane or within the membrane-bound cytoplasm, regulates (through diffusion or active transport) the rate at which the reaction can occur.
Methods of application: processes and products
- Development of a biomediated soil improvement technique requires an application strategy.
- While the former has been the primary strategy used to date in exploring geotechnical applications, the geoenvironmental field is increasingly using biostimulation.
- Biostimulation is generally preferable, owing to the stimulation and growth of native microbes, which are adapted to the subsurface environment, and to the reduction in permission difficulties.
- The primary product is typically the one that is designed to be the desired outcome (e.g. calcite precipitation to bind soil particles together).
- In addition, there are often additional 'by-products' generated by the geochemical process (e.g. ammonium ions).
Potential improvements to engineering properties with biogeochemical processes
- Biomediated geochemical processes have the potential to modify physical properties (density, gradation, porosity, saturation), conduction properties (hydraulic, electrical, thermal), mechanical properties (stiffness, dilation, compressibility, swell/shrink, cohesion, cementation, friction angle, erodibility, and soil-water characteristic curve), and chemical composition (buffering, reactivity, cation exchange capacity) of soils.
- This may be conceptualised by considering how different biogeochemical processes may influence an assemblage of sand grains and/or an aggregation of clay platelets.
- These effects will predictably result in reduced hydraulic conductivity, increased small-strain stiffness, increased large-strain strength, and increased dilative behaviour.
- Biofilm formation, and the production of other extracellular polymeric substances (EPS), are additional biogeochemical processes that can impact on soil behaviour.
- Biogas generation from denitrification or other biogeochemical processes may enable long-term reduction in the degree of saturation of a soil.
RESEARCH ACTIVITY AND APPLICATIONS
- Research activity in biogeotechnical engineering to date has investigated many of the above potential processes, with a significant portion of activity focused on biomediated cementation via calcite precipitation.
- The following examples highlight the extent to which soil properties can be modified or improved by biogeochemical processes.
- These examples are not comprehensive, with additional references provided.
Microbially induced calcite precipitation
- Microbially induced calcite precipitation, or MICP, has been the primary focus of research in biogeotechnical engineering to date.
- Bacillus pasteurii (American Type Culture Collection 6453), which was recently reclassified as Sporosarcina pasteurii (ATCC 11859), an alkalophilic bacterium with a highly active urease enzyme (Ferris et al., 1996) , has been used in laboratory studies where bioaugmentation has been performed to produce calcite precipitation (Mortensen et al., 2011) .
- Microscopy techniques show that the calcite structure varies with treatment formulation (Al Qabany et al., 2012) , cementation occurs preferentially at particle contacts (Chou et al., 2008; Martinez & DeJong, 2009) , calcite precipitation occurs directly on or around individual microbes and their aggregates, and cementation breakage during shearing occurs within the calcite crystals (DeJong et al., 2011) .
- MICP has also been shown to increase cone tip resistance (Burbank et al., 2012b) . (f) (g) (e) (b) (c) (d) Modelling of MICP requires coupling of biological, chemical, hydrological, and mechanical processes.
Biofilm formation
- Biofilms form when microorganisms adhere to a surface and excrete EPS as part of their metabolism.
- This 'slimy' EPS enhances further attachment of more microorganisms and other particles, thereby forming a biofilm that can affect the physical properties of soils (Fig. 2 (a); Banagan et al., 2010) .
- Close to the surface in riverine and marine environments, biofilms play an important role in trapping and stabilising sediments, and increasing the resistance to erosion (Stal, 2010) .
- In the subsurface, it has been shown already that the growth of biofilms can reduce hydraulic conductivity (Slichter, 1905) , a process referred to as bioclogging.
- Talsma & van der Lelij (1976) observed that water losses from rice fields were limited, owing to bacterial clogging.
Biopolymers and EPS
- Both in situ and ex situ applications of biopolymers for soil improvement have been explored.
- Biopolymers mixed with soils have been shown to reduce hydraulic conductivity and increase shear strength (Kavazanjian et al., 2009; Nugent et al., 2010) .
- The observed reduction in hydraulic conductivity is a function of soil grading and the applied hydraulic gradient (Jefferis, personal communication, 2012) .
- Biopolymers are used in biodegradable drilling muds owing to their propensity for bioplugging (Hamed & Belhadri, 2009) .
- Furthermore, there are many case histories of clogging of filters in dams, landfills and water treatment plants caused by the growth of biofilms (Cullimore, 1990; Ivanov & Chu, 2008) .
Mechanical processing by marine worms
- Many deep ocean clays are subject to thousands of cycles of biological activity that transform virgin material into processed material.
- Burrowing invertebrates , through the process of bioturbation, digest sediment that has fallen through the water column to the seabed.
- They are one example of a biological agent that has been active for millennia.
- These pellets' collective presence (in some cases 20-60% of total sediment by dry mass; Kuo, 2011) can be measured as a crust-like feature during in situ strength testing with conventional tools, including ball and T-bar fullflow penetrometers (see Fig. 4 (b), following Kuo & Bolton, 2011) .
- Because of their proximity to the seabed, faecal pellets are of significant engineering interest in the design of offshore pipelines and shallow foundations.
Shallow carbon fixation through plant roots
- It may be possible to design a carbon sequestration function in soils through exploiting and extending natural processes of pedogenic carbonate function.
- It has recently been shown that this process also occurs in urban soils, as a consequence of reaction between root exudates and calcium derived from the dissolution or weathering of cement-based construction materials (Manning, 2008; Renforth et al., 2009 Renforth et al., , 2011;; Washbourne et al., 2012) .
- Plants exude 10-30% of carbon captured from the atmosphere by photosynthesis through their roots and associated mycorrhizal fungal associations (Kuzyakov & Domanski, 2000; Taylor et al., 2009) .
- Root tissue compounds are released into the soil as exudates (Jones et al., 2009) , which are complex materials composed of polysaccharides, proteins, phospholipids, cells that detach from roots, and other compounds.
FIELD APPLICATIONS Completed/ongoing field trials
- To date, only a few field trials have been performed in which microbes have actively been used to either increase the strength and stiffness of soils by microbially induced carbonate precipitation or reduce the hydraulic conductivity through biofilm formation, although such processes will have been occurring naturally for millennia.
- Contractor Visser & Smit Hanab applied a MICP treatment for gravel stabilisation to enable horizontal directional drilling (HDD) for a gas pipeline in the Netherlands in 2010 (Fig. 6 ; from Van Paassen, 2011) .
- MICP was monitored using electrical resistivity, groundwater sampling and physical sampling for calcite content measurements, with varying degrees of success.
- These trials have employed injection of dissolved molasses and urea in the target treatment zone (calcium available in groundwater), with contemporaneous withdrawal of groundwater from a well several metres away from the injection point.
- At the Rifle site the well-to-well cycle is closed by reinjection of withdrawn water.
Normally consolidated
- In Austria, nutrient solutions were injected through a screen of injection wells in the crest of a 'leaking' dike along the Danube river in Greifenstein (Blauw et al., 2009; Lambert et al., 2010) .
- During the second treatment phase (July-August), a significant reduction in discharge rate was observed (Fig. 2(b) ).
- Whether it is the biofilm itself that clogs the pores, or some trapped particles in the biofilm, or perhaps the biogeochemical conversions that stimulate attachment, detachment or precipitation of particles that can reduce the hydraulic conductivity still needs to be resolved.
- Geochemical measurements indicated that microbial growth and a significant increase in microbial diversity were observed.
Challenges for field implementation
- The process of upscaling to the field, following experimental and modelling research at the element and bench scales, raises the following challenges that must be considered and addressed.
- The treatment scheme selected depends largely on whether the nutrients and/or microbes can be delivered relatively uniformly across the treatment zone through injection; this uniformity is directly a function of solution viscosity and density as well as microbe size relative to soil pore throat size and of course, critically, the soil uniformity.
- Central to any improvement method -biogeochemical or conventional -is monitoring during treatment to verify that the required distribution and magnitude of improvement are realised, and, after treatment, to verify that the improvement level remains adequate throughout the service life.
- With respect to MICP, soil improvement with calcite production requires less carbon than cement stabilisation, but additional analysis is required to study the energy required for manufacturing of the urea and calcium chloride, for injection of the improvement media into the ground, and for treatment of by-products.
Feasibility for different applications
- Realistically, biogeochemical-based soil improvement technologies will never replace all conventional ground improvement techniques.
- Considering the general attributes of biogeochemical processes, the challenges for implementation in the field, and society's needs, the applications with the highest likelihood of success will, in general, require simple implementation, provide a unique answer to a problem, have competitive costs, and have a potential for rapid take-up by industry and society.
- The applications that seem most feasible in the near term include erosion control, environmental remediation, dust control, improvement of rural roads, surface carbon dioxide sequestration, repair of sandstone structures, and solidification of fly ash.
- All of these applications still require further development, but they all represent problems for which current solutions are insufficient.
- If capital costs are merely competitive with current industry methods, the triedand-true established methods in industry that have decades of experience will often be preferred.
CLOSURE: RESEARCH AND DEVELOPMENT NEEDS
- The rapid development of biomediated soil improvement methods over the last decade has generated exciting advances in geotechnology, from the micro scale up through successful field-scale application.
- The focus on MICP is simultaneously encouraging, as this focus has resulted in a successful field-scale trial within a decade of its initial development in the laboratory, and unsatisfying, as there are probably so many other biogeochemical processes that have yet to be identified and/or be subject to intensive research.
- Monitoring techniques to verify treatment success, and to monitor durability and performance over the project's service life, have been identified as an important consideration.
- While appropriate monitoring techniques will vary, depending on the biogeochemical pro-cess selected, geophysical methods have a high potential for indirectly mapping the effect that a treatment process may have on engineering soil properties.
- Activities designed to raise awareness may be needed, as well as industry training, as field-scale applications of biogeotechnology become increasingly common.
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Citations
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...Unicellular microbial organisms in soil consist primarily of bacteria and archaea (see Woese et al., 1990, for definitions of terms), which typically range in diameter from 0.5 to 3 m, and have morphologies that are typically spherical (coccus) or cylindrical; the latter may be straight (rods),…...
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...At depths typical in geotechnical systems (e.g. 2 to 30 m), the microbial population decreases to about 1011 to 106 microorganisms per kilogram respectively (Whitman et al., 1998) (for context, about 1014 bacteria exist in the typical human body; Berg, 1996)....
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