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Biogeochemical processes and geotechnical applications: progress, opportunities and challenges

TL;DR: In this article, the authors assess the progress, opportunities, and challenges in this emerging field, which consists of a geochemical reaction regulated by subsurface microbiology, including mineral precipitation, gas generation, biofilm formation and biopolymer generation.
Abstract: Consideration of soil as a living ecosystem offers the potential for innovative and sustainable solutions to geotechnical problems. This is a new paradigm for many in geotechnical engineering. Realising the potential of this paradigm requires a multidisciplinary approach that embraces biology and geochemistry to develop techniques for beneficial ground modification. This paper assesses the progress, opportunities, and challenges in this emerging field. Biomediated geochemical processes, which consist of a geochemical reaction regulated by subsurface microbiology, currently being explored include mineral precipitation, gas generation, biofilm formation and biopolymer generation. For each of these processes, subsurface microbial processes are employed to create an environment conducive to the desired geochemical reactions among the minerals, organic matter, pore fluids, and gases that constitute soil. Geotechnical applications currently being explored include cementation of sands to enhance bearing capacity and liquefaction resistance, sequestration of carbon, soil erosion control, groundwater flow control, and remediation of soil and groundwater impacted by metals and radionuclides. Challenges in biomediated ground modification include upscaling processes from the laboratory to the field, in situ monitoring of reactions, reaction products and properties, developing integrated biogeochemical and geotechnical models, management of treatment by-products, establishing the durability and longevity/reversibility of the process, and education of engineers and researchers.

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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DeJong, J. T. et al. (2013). Ge
´
otechnique 63, No. 4, 287–301 [http://dx.doi.org/10.1680/geot.SIP13.P.017]
287
Biogeochemical processes and geotechnical applications: progress,
opportunities and challenges
J. T. DEJONG
1
, K. SOGA
2
, E . K AVA Z A N J I A N
3
, S. BURNS
4
,L.A.VANPAASSEN
5
, A. AL QABANY
2
,
A. AYDILEK
6
,S.S.BANG
7
, M. BURBANK
8
, L. F. CASLAKE
9
,C.Y.CHEN
10
, X. CHENG
11
,J.CHU
12
,
S. CIURLI
13
,A.ESNAULT-FILET
14
,S. FAURIEL
15
,N.HAMDAN
16
,T.HATA
17
, Y. INAGAKI
18
,
S. JEFFERIS
19
,M. KUO
2
, L. LALOUI
14
, J. LARRAHONDO
20
, D. A. C. MANNING
21
, B. MARTINEZ
22
,
B. M. MONTOYA
23
, D. C. NELSON
24
,A. PALOMINO
25
,P.RENFORTH
26
,J.C.SANTAMARINA
4
,
E. A. SEAGREN
27
, B. TANYU
28
,M. TSESARSKY
29
and T. WEAVER
30
Consideration of soil as a living ecosystem offers the potential for innovative and sustainable solutions
to geotechnical problems. This is a new paradigm for many in geotechnical engineering. Realising the
potential of this paradigm requires a multidisciplinary approach that embraces biology and geochem-
istry to develop techniques for beneficial ground modification. This paper assesses the progress,
opportunities, and challenges in this emerging field. Biomediated geochemical processes, which
consist of a geochemical reaction regulated by subsurface microbiology, currently being explored
include mineral precipitation, gas generation, biofilm formation and biopolymer generation. For each
of these processes, subsurface microbial processes are employed to create an environment conducive
to the desired geochemical reactions among the minerals, organic matter, pore fluids, and gases that
constitute soil. Geotechnical applications currently being explored include cementation of sands to
enhance bearing capacity and liquefaction resistance, sequestration of carbon, soil erosion control,
groundwater flow control, and remediation of soil and groundwater impacted by metals and radio-
nuclides. Challenges in biomediated ground modification include upscaling processes from the
laboratory to the field, in situ monitoring of reactions, reaction products and properties, developing
integrated biogeochemical and geotechnical models, management of treatment by-products, establish-
ing the durability and longevity/reversibility of the process, and education of engineers and
researchers.
KEYWORDS: chemical properties; environmental engineering; ground improvement; remediation; soil
stabilisation
Manuscript received 2 March 2012; revised manuscript accepted 23
October 2012.
Discussion on this paper closes on 1 August 2013, for further details
see p. ii.
1
Department of Civil and Environmental Engineering, University of
California, Davis, CA, USA.
2
Department of Engineering, University of Cambridge, Cambridge,
UK.
3
School of Sustainable Engineering and the Built Environment,
Arizona State University, Phoenix, AZ, USA.
4
School of Civil and Environmental Engineering, Georgia Institute of
Technology, Atlanta, GA, USA.
5
Department of Geoscience and Engineering, Delft University of
Technology, The Netherlands.
6
Department ofCivil and Environmental Engineering, University of
Maryland, College Park, MD, USA.
7
Department of Chemical and Biological Engineering, South Dakota
School of Mines and Technology, Rapid City, SD, USA.
8
Environmental Biotechnology Institute, University of Idaho,
Moscow, ID, USA.
9
Department of Biology, Lafayette College, Easton, PA, USA.
10
Department of Earth and Environmental Sciences, National Chung
Cheng University, Taiwan.
11
Department of Civil Engineering, Tsinghua University, Beijing,
China.
12
Department of Civil, Construction and Environmental Engineering,
Iowa State University, Ames, IA, USA.
13
Department of Agro-Environmental Science and Technology,
University of Bologna, Bologna, Italy.
14
Research & Development Department, Soletanche Bachy, Rueil
Malmaison, France.
15
Laboratory for Soil Mechanics, Ecole Polytechnique Fe´de´rale de
Lausanne (EPFL), Lausanne, Switzerland.
16
Department of Civil, Environmental and Sustainable Engineering,
Arizona State University, Tempe, AZ, USA.
17
Department of Civil Engineering, Nagano National College of
Technology, Nagano, Japan.
18
Geology and Geotechnical Engineering Research Group, Public
Works Institute, Japan.
19
Environmental Geotechnics Ltd and Department of Engineering
Science, University of Oxford, Banbury, UK.
20
INGETEC S. A., Bogota, Columbia.
21
School of Civil Engineering & Geosciences, Newcastle University,
Newcastle upon Tyne, UK.
22
Geosyntec Consultants, Oakland, CA, USA.
23
Department of Civil, Construction, and Environmental Engineer-
ing, North Carolina State University, Raleigh, NC, USA.
24
Department of Microbiology, University of California, Davis, CA,
USA.
25
Department of Civil and Environmental Engineering, University of
Tennessee, Knoxville, TN, USA.
26
Department of Earth Sciences, University of Oxford, Oxford, UK.
27
Department of Civil and Environmental Engineering, Michigan
Technological University, Houghton, MI, USA.
28
Department of Civil, Environmental, and Infrastructure Engineer-
ing, George Mason University, Fairfax, VA, USA.
29
Department of Structural Engineering and Department of Geolo-
gical and Environmental Sciences, Ben Gurion University of the
Negev, Beer-Sheva, Israel.
30
Office of Research, Nuclear Regulatory Commission, USA.

INTRODUCTION
Biology in the evolution of geotechnical engineering
The field of geotechnical engineering has advanced steadily
throughout history; the durability of several ancient geotech-
nical systems (e.g. Egyptian dams and canals, Greek strip
and raft foundations, and Roman bridges along the Appian
Way) testifies to a working knowledge of geotechnics by
their creators. However, formal initiation of the discipline
may be attributed to Coulomb’s definitive work on earth
pressures in the 1770s. Numerous advances in mechanics
and water flow followed Coulomb’s work, including Darcy’s
law (Darcy, 1857), Boussinesq stress distribution (1871),
Rankine earth pressure theory (1875), Mohr’s circle of strain
(1885), Reynolds volumetric behaviour (1885), and more.
Karl Terzaghi’s work, from the 1920s onwards (e.g. Terzaghi,
1924), revolutionised the discipline by developing the princi-
ple of effective stress and analyses for the bearing capacity
of foundations and consolidation of soils. Later work by
Gibson on analytical techniques (see the first issue of
Ge
´
otechnique), by Taylor (1948) on dilation and interlock-
ing, by Roscoe et al. (1958) and Schofield & Wroth (1968)
on plasticity and critical state, and by many others since
then and through to the present day, have continued the
development of mechanics concepts and analysis methods
for geotechnical systems.
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. The geologic origin of a soil, its depositional
mode and environment, thixotropic processes, and other
post-depositional phenomena must often be considered.
Early on (e.g. Terzaghi, 1955), the importance of considering
these formational and time-dependent processes was recog-
nised and addressed through the field of geology. Work by
Mitchell (e.g. Mitchell, 1975) and others in the second half
of the 20th century recognised the critically important role
of chemistry in the behaviour of fine-grained soils, and how
macro-scale performance depends directly on micro- (or
nano-) scale conditions. For example, some ground improve-
ment methods specifically target chemical changes in the
clay fabric to stabilise soils for construction (e.g. pozzolanic
changes).
Harnessing of biological processes in soils promises to be
the next transformative practice in geotechnical engineering.
For many years, the influence of plant roots on slope
stability has been recognised and exploited (e.g. Gray &
Sotir, 1996). There is now the opportunity to exploit many
chemical processes that are mediated by biology. Although
ignored for centuries with respect to geotechnical behaviour,
microbes are ubiquitous in soils at surprisingly high concen-
trations, almost regardless of saturation, mineralogy, pH, and
other environmental factors. Near the ground surface, more
than 10
12
microbes exist per kilogram of soil (Mitchell &
Santamarina, 2005). At depths typical in geotechnical sys-
tems (e.g. 2 to 30 m), the microbial population decreases to
about 10
11
to 10
6
microorganisms per kilogram respectively
(Whitman et al., 1998) (for context, about 10
14
bacteria exist
in the typical human body; Berg, 1996). Living organisms at
other length scales are also present. For example, worms at
larger length (cm) scales recompact soil, create preferential
drainage conditions, and otherwise impact on soil character-
istics, and spores at smaller length scales (, 1 ìm) can be
transported into smaller pore spaces.
A permanent biological presence (microbes have been
active geotechnical engineers for 3+ billion years, much
longer than the 0
.
0002 billion years for humans; Kohnhau-
ser, 2007) in soil provides opportunities for geotechnical
engineering to consider soil as a living ecosystem rather
than as an inert construction material. Biological activity in
soil can potentially explain observations in some case
histories that have troubled experts (Mitchell & Santamar-
ina, 2005) and provides the opportunity to manipulate soil
using natural or stimulated processes (as expanded upon
herein).
Emergence of bio-soils as a subdiscipline
One of the first explicit discussions of the application of
biological processes in geotechnical engineering was pre-
sented 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 engineer-
ing in 2007 facilitated interdisciplinary discussions and
prioritisation of research topics in this emerging field
(DeJong et al., 2007). Research advanced quickly, with a
bio-geo-civil-engineering conference in 2008, and several
dedicated sessions at national and international conferences,
additional papers assessing the potential of the field (Ivanov
& Chu, 2008; Kavazanjian & Karatas, 2008; Ivanov, 2010;
Seagren & Aydilek, 2010; DeJong et al., 2011; Hata et al.,
2011), and more than 100 technical conference and journal
papers dedicated to this topic since. Research programmes
on biogeotechnical engineering are currently active in more
than 15 countries around the world.
The Second International Workshop on Bio-Soils Engin-
eering 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 engineer-
ing properties of soil depend on the length scale of organ-
isms, 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. For ex-
ample, ants are effective at soil grading, densification, and
creating preferential flow paths (macropores); they also adapt
and optimise their efforts considering capillarity forces at
particle contacts (Espinoza & Santamarina, 2010). Similarly,
mucous excretion from worms can strengthen soil along
tunnelling paths, and help bind (geotechnically strong) faecal
pellets to such an extent that the cone penetration test (CPT)
measures the strength increase (Kuo, 2011).
Unicellular microbial organisms in soil consist primarily
of bacteria and archaea (see Woese et al., 1990, for defini-
tions of terms), which typically range in diameter from 0
.
5
to 3 ìm, and have morphologies that are typically spherical
(coccus) or cylindrical; the latter may be straight (rods),
curved (vibrio), or corkscrew shaped (spirilla). These are
present in soil either through entrapment during deposition
(the typical mode in fine-grained sediments offshore; Reba-
ta-Landa & Santamarina, 2006) or through migration
through pore space via hydraulic flow transport or self-
motility. Geometric compatibility between bacteria and ar-
chaea and the pore space (pore throats to be specific)
288 DEJONG ET AL.

dictates mobility (Mitchell & Santamarina, 2005; DeJong et
al., 2010; Phadnis & Santamarina, 2012) and survivability
(Rebata-Landa & Santamarina, 2006).
Unicellular activity, in general, does not affect soil proper-
ties directly. Rather, it is how biological activity locally
exploits geochemical processes, which in turn affect soil
properties. Microbe activity creates ‘microniche’ conditions
surrounding individual cells that critically alter when, where,
and at what rate geochemical processes occur. A given
geochemical process can often occur in the absence of
biological activity, and indeed, for it to occur as a microbial
process, it must be viable in the absence of biological
activity, although the rate may be exceedingly slow (i.e.
bioprocesses are often regarded as biocatalysis). However,
doing so may result in widely distributed reactions, resulting,
for example, in precipitation in the pore fluid that is subse-
quently transported outside the targeted treatment zone.
The ability of microbes to regulate processes (depending
on the specific process utilised) often stems from the uni-
cells 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, regu-
lates (through diffusion or active transport) the rate at which
the reaction can occur. Increased enzymatic activity within a
given cell or an increased number of cells both increase the
bulk reaction rate. Although not widely explored to date, it
may also be possible to use free enzymes (a non-biological
approach) to improve soil properties in applications where
the treatment time is relatively brief (e.g. dust suppression;
Bang et al., 2011).
Methods of application: processes and products
Development of a biomediated soil improvement tech-
nique requires an application strategy. The two primary
strategies bioaugmentation (where the required microbes
are injected into the soil) and biostimulation (where natural
microbes are stimulated) build on bioremediation tech-
niques developed over the last 30+ years. While the former
has been the primary strategy used to date in exploring
geotechnical applications, the geoenvironmental field is in-
creasingly using biostimulation. Bioaugmentation is gener-
ally considered less favourable than biostimulation, owing to
the introduction of exogenous (non-native) microbes, in
some cases the permissions required, the increased costs, the
practical difficulty of uniform application in the subsurface
due to filtration of microbes (akin to filter design intended
to protect clay cores of dams), and the potential for die-off
or dormancy if the environment is not favourable for their
proliferation. 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. However, many challenges exist in
applying biostimulation, including obtaining uniform treat-
ment across a site, and accommodating the increased time
associated with stimulation and growth. A compromise be-
tween these two approaches may be bioaugmentation at a
low concentration followed by stimulation in situ, or ‘micro-
dosing’ (Martinez, 2012).
Geochemical processes regulated through biostimulation
or bioaugmentation often result in multiple products. 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). The generation, transport and fate of these
by-products must be addressed, often as a waste, although in
some cases they may be repurposed for other applications
(e.g. ammonium for fertilisation of plants).
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, compres-
sibility, 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 assem-
blage of sand grains and/or an aggregation of clay platelets.
Biomineralisation processes that precipitate inorganic
solids (including microbially induced calcite precipitation, or
MICP) can clearly have a mechanical effect: for example,
reduction in pore space, brittle cementation at particle con-
tacts, increased fines in the pore space, and increased
stiffness. These effects will predictably result in reduced
hydraulic conductivity, increased small-strain stiffness, in-
creased large-strain strength, and increased dilative behav-
iour.
Biofilm formation, and the production of other extracellu-
lar polymeric substances (EPS), are additional biogeochem-
ical processes that can impact on soil behaviour. These
processes generate organic solids that occupy a portion of
the pore space with a soft, ductile, elastomeric-like material
that reduces pore size, reduces rearrangement of particles
during soil deformation, and increases ductility. These
changes can reduce hydraulic conductivity, and perhaps
reduce rapid strain-softening during undrained shearing.
However, property changes due to biofilm and EPS produc-
tion may be lost, and thus be applicable only for short-term
ground modification, as these living systems must be con-
tinuously nourished or their engineering performance may
become unreliable.
Biogas generation from denitrification or other biogeo-
chemical processes may enable long-term reduction in the
degree of saturation of a soil. Reduction in the degree of
saturation increases pore space compressibility, and may
thereby reduce excess pore pressure build-up during cyclic
loading, mitigating earthquake-induced liquefaction potential
in some soils.
Other processes that have been identified, but are currently
less developed, include algal and fungal growth for near-
surface soil stabilisation, bacteria and worms for methane
oxidation, biopolymers for drilling applications, organic slur-
ries for hydraulic control, and silicate precipitation.
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 exam-
ples 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 en-
gineering to date. In MICP, the creation of calcium carbo-
nate (calcite) occurs as a consequence of microbial
metabolic activity (Stocks-Fischer et al., 1999; Ramakrish-
nan et al., 2001). Calcite precipitation may be achieved by
many different processes (DeJong et al., 2010), including
urea hydrolysis (Benini et al., 1999; Ciurli et al., 1999);
BIOGEOCHEMICAL PROCESSES AND GEOTECHNICAL APPLICATIONS
289

denitrification (Karatas et al., 2008; Van Paassen et al.,
2010a; Hamdan et al., 2011b); sulphate reduction, inducing
dolomite precipitation (Warthmann et al., 2000); and iron
reduction, inducing ankerite and other mixed mineral pre-
cipitation (Roden et al., 2002; Weaver et al., 2011). Enzy-
matic hydrolysis of urea by microbes is the most energy
efficient of these processes (DeJong et al., 2010), and urease
activity is found in a wide range of microorganisms and
plants (Bachmeier et al., 2002). Bacillus pasteurii (American
Type Culture Collection 6453), which was recently reclassi-
fied 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 bioaug-
mentation has been performed to produce calcite precipita-
tion (Mortensen et al., 2011). More recently, biostimulation
of native microbes has reportedly been successful (Burbank
et al., 2011, 2012a), and the influence of competing microbe
species/processes has been explored (Gat et al., 2011).
Limited studies have also explored precipitation of other
minerals (e.g. Chu et al., 2011; Weaver et al., 2011).
Research has provided insights into MICP from micro-
metre- to metre-length scales (Fig. 1). Microscopy tech-
niques show that the calcite structure varies with treatment
formulation (Al Qabany et al., 2012), cementation occurs
preferentially at particle contacts (Chou et al., 2008; Marti-
nez & 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). Laboratory-scale ele-
ment tests have shown substantial (. 10
3
3) increases in
strength (DeJong et al., 2006; Whiffin et al., 2007; Van
Paassen et al., 2009; Chu et al., 2011, 2013; Al Qabany &
Soga, 2013), decreases in hydraulic conductivity greater than
two orders of magnitude (Al Qabany, 2011; Rusu et al.,
2011; Martinez et al., 2013), increases in small-strain stiff-
ness by three orders of magnitude (DeJong et al., 2006; Van
Paassen et al., 2010b; Van Paassen, 2011; Esnault-Filet et
al., 2012), and an increase in dilative tendencies (Chou et
al., 2011; Mortensen & DeJong, 2011; Tagliaferri et al.,
2011). Even after cementation degrades owing to shearing,
the reduction in pore space (or increase in solids density)
due to the precipitated calcite alters the behaviour of the
material. Re-establishment of properties following MICP
degradation from shearing can be rapid and efficient (Mon-
toya, 2012). Geophysical methods (shear wave velocity in
particular) are effective for real-time monitoring of the
cementation process, as the precipitated calcite stiffens parti-
cleparticle contacts (Al Qabany et al., 2011; Montoya et
al., 2012; Weil et al., 2012).
One-dimensional column, two-dimensional flow and three-
dimensional model tests have enabled enquiry into treatment
uniformity, formulation optimisation, and self-equilibrating
ability, as well as demonstration of conceptual ideas about
property improvement due to MICP (DeJong et al., 2009;
Martinez & DeJong, 2009; Van Paassen, 2009; Van Paassen
et al., 2009; Inagaki et al., 2011a; Tobler et al., 2012;
Martinez et al., 2013) (Fig. 1). One-dimensional column
experiments have shown pulsed flow injection and flow
reversal to increase uniformity, lowering the molar ratio of
urea to calcium to reduce by-product formation, and geophy-
sical seismic measurement to spatially monitor improvement
(Martinez, 2012). Two-dimensional models of field treatment
patterns have explored the efficacy of bioaugmentation com-
pared with biostimulation, and linkages between microbial
distribution, ureolysis activity, shear wave velocity and total
precipitated calcite (Al Qabany, 2011; Martinez, 2012). Scale
model tests have demonstrated the effectiveness of MICP in
reducing wind- and water-induced erosion (Bang et al.,
2011), improving resistance to liquefaction (Inagaki et al.,
2011b; Montoya et al., 2013), creating impermeable crusts
(f) (g)(e)(b) (c) (d)
350
300
250
200
150
100
50
0
0 3 6 9 12 15
Axial strain, : %ε
z
Deviatoric stress,: kPaq
(h)
Experimental untreated
Experimental treated
Numerical untreated
Numerical treated
(i) (j) (k) (l) (m)
km
μm
mm
Length scale
cm
dm
m
(a)
cm
Fig. 1. Overview of upscaling of MICP: (a) urease enzyme structure housed within microbes (Benini et al., 1999); (b) Sporosarcina
pasteuri microbe (image supplied by DeJong); (c) bacterial impression within precipitated calcite (Martinez & DeJong, 2009);
(d) structure of precipitated calcite (Day et al., 2003); (e) MICP-cemented sand grains (Chou et al., 2008); (f) CT scan of MICP-cemented
sand (image supplied by DeJong); (g) MICP-cemented triaxial specimen (Mortensen et al., 2011); (h) modelling of MICP triaxial
compression test (Fauriel, 2012); (i) 1D column tests (Martinez, 2012); (j) radial flow treatments (Al Qabany, 2011); (k) MICP
impermeable skin for retention basin (Stabnikov et al., 2011); (l) MICP treatment of 100 m
3
sand (Van Paassen et al., 2010b; Esnault-Filet
et al., 2012); (m) field trial
290 DEJONG ET AL.

for catchment facilities (Stabnikov et al., 2011; Chu et al.,
2012), healing/stabilising cracks in concrete and masonry
(Ramachandran et al., 2001; Bang et al., 2010; Yang et al.,
2011), treating waste (Chu et al., 2009), immobilising heavy
metals (Fujita et al., 2004, 2008, 2010; Hamdan et al.,
2011a; Li et al., 2011), and performing shallow carbon
sequestration (Manning, 2008; Renforth et al., 2009, 2011;
Washbourne et al., 2012). MICP has also been shown to
increase cone tip resistance (Burbank et al., 2012b).
Modelling of MICP requires coupling of biological, chem-
ical, hydrological, and mechanical processes. Modelling ef-
forts have generally focused either on prediction of biogeo-
chemical processes (Barkouki et al., 2011; Fauriel & Laloui,
2011b, 2012; Laloui & Fauriel, 2011; Martinez et al., 2011;
Van Wijngaarden et al., 2011, 2012; Martinez, 2012) and
calcite distribution, or on prediction of the mechanical be-
haviour of biocemented soils (Fauriel & Laloui, 2011a;
Fauriel, 2012). Models to date have captured and predicted
biogeochemical processes, provided first-order predictions of
precipitated calcite distributions, and captured the mechanic-
al behaviour of MICP-treated sand (Fig. 1(h)).
As discussed later in the section on field applications, two
field trials using MICP have been performed to date.
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 environ-
ments, 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. Much
of the research on bioclogging is focused on preventing and/
or removing the clogging material (Howsam, 1990), for
example by flushing with formaldehyde, in order to restore or
maintain the functionality of wells or drains (Baveye et al.,
1998). However, some researchers have found that biofilm
formation in soil could also be advantageous (e.g. Mitchell et
al., 2009). For example, Talsma & van der Lelij (1976)
observed that water losses from rice fields were limited,
owing to bacterial clogging. Attempts have been made to use
bioclogging to decrease hydraulic conductivity in situ beneath
and within dams and levees, to reduce infiltration from ponds,
to reduce leakage at landfills, to plug high hydraulic conduc-
tivity layers surrounding oil bearing layers, and to control
groundwater migration with subsurface barriers (Fig. 2(b);
Seki et al., 1998; James et al., 2000; Lambert et al., 2010).
Biogas generation
Biological activity in the subsurface is frequently accom-
panied by the development of discrete gas bubbles in other-
wise saturated environments. A variety of gases can be
produced by microbial processes (e.g. carbon dioxide, hydro-
gen, methane and nitrogen), with both the organism and the
oxidative/reductive environment of the pore fluid influencing
the ultimate products of the reaction. For example, aerobic
microbes (obligate or facultative) use oxygen as the terminal
electron acceptor during the process of microbial respiration.
Typically, an organic molecule is used as the carbon and
energy source, and the products resulting from the reaction
include water and carbon dioxide. By contrast, anaerobic
respiration by methanogenic archaea occurs in the absence
of oxygen, and results in the production of methane and
often carbon dioxide (i.e. part of the carbon is oxidised to
carbon dioxide and part is reduced to methane). The process
uses part of the carbon, as opposed to oxygen, as the
terminal electron acceptor. Respiratory denitrification occurs
through the reduction of nitrate, producing nitrogen and
carbon dioxide gas as the end products of the reaction in
environments that have high ratios of nitrate to carbon.
Numerous laboratory studies have demonstrated the feasibil-
ity of producing microbially generated discrete gas bubbles
at the bench scale, with a review of the processes given in
Rebata-Landa & Santamarina (2012), and shaking-table tests
provided by He et al. (2013). In practical terms, the pre-
sence of gas bubbles within an otherwise saturated soil
results in a decrease in the measured P-wave velocity; this
decrease is transient in coarse-grained soils, but not in fine-
grained soils that can trap the generated gas bubbles (Fig.
3(a); Rebata-Landa & Santamarina, 2012). Even small re-
ductions in the level of saturation of a soil are known to
significantly reduce a soil’s susceptibility to liquefaction
(Fig. 3(b); e.g. Sherif et al., 1977; Chaney, 1978; Yoshimi et
al., 1989; Ishihara et al., 2001; Pietruszczk et al., 2003).
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
(a)
0
50
100
150
200
250
300
350
400
450
0 102030
Discharge flow: l/h
Time: days
(b)
Drain 2
Drain 1
Fig. 2. Example results of biofilm treatment for permeability
reduction; (a) laser confocal fluorescence microscopy image of
biofilm-coated sand grains; (b) field data showing reduction in
seepage flow in a dam following biofilm hydraulic conductivity
reduction treatment (Blauw et al., 2009; used by permission)
BIOGEOCHEMICAL PROCESSES AND GEOTECHNICAL APPLICATIONS 291

Citations
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TL;DR: In this study, different metabolic activities leading to calcium carbonate precipitation, their native environment, and potential applications and challenges are reviewed.
Abstract: Calcium carbonate represents a large portion of carbon reservoir and is used commercially for a variety of applications. Microbial carbonate precipitation (MCP), a by-product of microbial activities, plays an important metal coprecipitation and cementation role in natural systems. This natural process occurring in various geological settings can be mimicked and used for a number of biotechnology such as metal remediation, carbon sequestration, enhanced oil recovery and construction restoration. In this study, different metabolic activities leading to calcium carbonate precipitation, their native environment, and potential applications and challenges are reviewed.

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Abstract: The global magnitude (Pg) of soil organic carbon (SOC) is 677 to 0.3-m, 993 to 0.5-m, and 1,505 to 1-m depth. Thus, ~55% of SOC to 1-m lies below 0.3-m depth. Soils of agroecosystems are depleted of their SOC stock and have a low use efficiency of inputs of agronomic yield. This review is a collation and synthesis of articles published in peer-reviewed journals. The rates of SOC sequestration are scaled up to the global level by linear extrapolation. Soil C sink capacity depends on depth, clay content and mineralogy, plant available water holding capacity, nutrient reserves, landscape position, and the antecedent SOC stock. Estimates of the historic depletion of SOC in world soils, 115-154 (average of 135) Pg C and equivalent to the technical potential or the maximum soil C sink capacity, need to be improved. A positive soil C budget is created by increasing the input of biomass-C to exceed the SOC losses by erosion and mineralization. The global hotspots of SOC sequestration, soils which are farther from C saturation, include eroded, degraded, desertified, and depleted soils. Ecosystems where SOC sequestration is feasible include 4,900 Mha of agricultural land including 332 Mha equipped for irrigation, 400 Mha of urban lands, and ~2,000 Mha of degraded lands. The rate of SOC sequestration (Mg C ha-1 year-1 ) is 0.25-1.0 in croplands, 0.10-0.175 in pastures, 0.5-1.0 in permanent crops and urban lands, 0.3-0.7 in salt-affected and chemically degraded soils, 0.2-0.5 in physically degraded and prone to water erosion, and 0.05-0.2 for those susceptible to wind erosion. Global technical potential of SOC sequestration is 1.45-3.44 Pg C/year (2.45 Pg C/year).

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TL;DR: A review of the use of MICP for soil improvement can be found in this article, where the authors discuss the treatment process including the primary components involved and major affecting factors, as well as the potential advantages and limitations.
Abstract: Biocementation is a recently developed new branch in geotechnical engineering that deals with the application of microbiological activity to improve the engineering properties of soils. One of the most commonly adopted processes to achieve soil biocementation is through microbially induced calcite precipitation (MICP). This technique utilizes the metabolic pathways of bacteria to form calcite (CaCO3) that binds the soil particles together, leading to increased soil strength and stiffness. This paper presents a review of the use of MICP for soil improvement and discusses the treatment process including the primary components involved and major affecting factors. Envisioned applications, potential advantages and limitations of MICP for soil improvement are also presented and discussed. Finally, the primary challenges that lay ahead for the future research (i.e. treatment optimization, upscaling for in situ implementation and self-healing of biotreated soils) are briefly discussed.

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Journal ArticleDOI
TL;DR: In this article, the evolution of the shear strength and stiffness of sand subjected to undrained and drained shearing is evaluated using triaxial tests using MICP treated sands with cementation levels.
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Journal ArticleDOI
TL;DR: In this paper, a species of Bacillus megaterium was used to trigger calcite precipitation, and the results showed that the improvement in the engineering properties of the MICP-treated residual soils is comparable to those of treated fine sands.
Abstract: Studies of soil improvement by microbial-induced calcite precipitation (MICP) have focused primarily on fine sand. This paper explores the viability of the MICP technique for improving the engineering properties of a typical tropical residual soil. A species of Bacillus, B. megaterium, was used to trigger calcite precipitation. Four variables were considered in this study: the concentration of B. megaterium, the concentration of the cementation reagent, the treatment duration, and the flow pressure of the cementation reagent. The results show that the improvement in the engineering properties of the MICP-treated residual soils is comparable to those of treated fine sands. The preferable treatment conditions for the soil studied are B. megaterium concentration of 1×108 cfu/mL, cementation reagent concentration of 0.5 M, flow pressure of 1.1 bar of the cementation reagent, and treatment duration of 48 h. Using this combination of parameters, the obtained shear strength increase and hydraulic conduct...

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References
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TL;DR: It is proposed that a formal system of organisms be established in which above the level of kingdom there exists a new taxon called a "domain." Life on this planet would be seen as comprising three domains, the Bacteria, the Archaea, and the Eucarya, each containing two or more kingdoms.
Abstract: Molecular structures and sequences are generally more revealing of evolutionary relationships than are classical phenotypes (particularly so among microorganisms). Consequently, the basis for the definition of taxa has progressively shifted from the organismal to the cellular to the molecular level. Molecular comparisons show that life on this planet divides into three primary groupings, commonly known as the eubacteria, the archaebacteria, and the eukaryotes. The three are very dissimilar, the differences that separate them being of a more profound nature than the differences that separate typical kingdoms, such as animals and plants. Unfortunately, neither of the conventionally accepted views of the natural relationships among living systems--i.e., the five-kingdom taxonomy or the eukaryote-prokaryote dichotomy--reflects this primary tripartite division of the living world. To remedy this situation we propose that a formal system of organisms be established in which above the level of kingdom there exists a new taxon called a "domain." Life on this planet would then be seen as comprising three domains, the Bacteria, the Archaea, and the Eucarya, each containing two or more kingdoms. (The Eucarya, for example, contain Animalia, Plantae, Fungi, and a number of others yet to be defined). Although taxonomic structure within the Bacteria and Eucarya is not treated herein, Archaea is formally subdivided into the two kingdoms Euryarchaeota (encompassing the methanogens and their phenotypically diverse relatives) and Crenarchaeota (comprising the relatively tight clustering of extremely thermophilic archaebacteria, whose general phenotype appears to resemble most the ancestral phenotype of the Archaea.

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"Biogeochemical processes and geotec..." refers background in this paper

  • ...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),…...

    [...]

Journal ArticleDOI
TL;DR: The number of prokaryotes and the total amount of their cellular carbon on earth are estimated to be 4-6 x 10(30) cells and 350-550 Pg of C (1 Pg = 10(15) g), respectively, which is 60-100% of the estimated total carbon in plants.
Abstract: The number of prokaryotes and the total amount of their cellular carbon on earth are estimated to be 4-6 3 10 30 cells and 350-550 Pg of C (1 Pg 5 10 15 g), respectively. Thus, the total amount of prokaryotic carbon is 60-100% of the estimated total carbon in plants, and inclusion of prokaryotic carbon in global models will almost double estimates of the amount of carbon stored in living organisms. In addition, the earth's prokaryotes contain 85-130 Pg of N and 9-14 Pg of P, or about 10-fold more of these nutrients than do plants, and represent the largest pool of these nutrients in living organisms. Most of the earth's prokaryotes occur in the open ocean, in soil, and in oceanic and terrestrial subsurfaces, where the numbers of cells are 1.2 3 10 29 , 2.6 3 10 29 , 3.5 3 10 30 , and 0.25-2.5 3 10 30 , respectively. The numbers of het- erotrophic prokaryotes in the upper 200 m of the open ocean, the ocean below 200 m, and soil are consistent with average turnover times of 6-25 days, 0.8 yr, and 2.5 yr, respectively. Although subject to a great deal of uncertainty, the estimate for the average turnover time of prokaryotes in the subsurface is on the order of 1-2 3 10 3 yr. The cellular production rate for all prokaryotes on earth is estimated at 1.7 3 10 30 cellsyyr and is highest in the open ocean. The large population size and rapid growth of prokaryotes provides an enormous capacity for genetic diversity. Although invisible to the naked eye, prokaryotes are an essential component of the earth's biota. They catalyze unique and indispensable transformations in the biogeochemical cy- cles of the biosphere, produce important components of the earth's atmosphere, and represent a large portion of life's genetic diversity. Although the abundance of prokaryotes has been estimated indirectly (1, 2), the actual number of pro- karyotes and the total amount of their cellular carbon on earth have never been directly assessed. Presumably, prokaryotes' very ubiquity has discouraged investigators, because an esti- mation of the number of prokaryotes would seem to require endless cataloging of numerous habitats. To estimate the number and total carbon of prokaryotes on earth, several representative habitats were first examined. This analysis indicated that most of the prokaryotes reside in three large habitats: seawater, soil, and the sedimentysoil subsur- face. Although many other habitats contain dense populations, their numerical contribution to the total number of pro- karyotes is small. Thus, evaluating the total number and total carbon of prokaryotes on earth becomes a solvable problem. Aquatic Environments. Numerous estimates of cell density, volume, and carbon indicate that prokaryotes are ubiquitous in marine and fresh water (e.g., 3-5). Although a large range of cellular densities has been reported (10 4 -10 7 cellsyml), the

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    [...]

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01 Jan 1976
TL;DR: In this paper, the authors developed an understanding of the factors determining and controlling the engineering properties of soil, the factors controlling their magnitude, and the influences of environment and time, and developed a two-part book which contains the following chapters: Part 1 - the nature of soils; bonding, crystal structure and surface characteristics; soil mineralogy; soil formation and soil deposits; determination of soil composition; soil water; clay-water-electrolyte system; soil fabric and its measurement; Part 2 - soil behavior; soil composition and engineering properties; effective, intergranular
Abstract: The book is intended to develop an understanding of the factors determining and controlling the engineering properties of soil, the factors controlling their magnitude, and the influences of environment and time. The two-part book contains the following chapters: Part 1 - the nature of soils; bonding, crystal structure and surface characteristics; soil mineralogy; soil formation and soil deposits; determination of soil composition; soil water; clay-water-electrolyte system; soil fabric and its measurement; Part 2 - soil behavior; soil composition and engineering properties; effective, intergranular and total stress; soil structure and its stability; fabric, structure and property relationships, volume change behavior; strength and deformation behavior; and, conduction phenomena. /TRRL/

3,283 citations

Book
01 Jan 1968

2,552 citations


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  • ...…by Gibson on analytical techniques (see the first issue of Géotechnique), by Taylor (1948) on dilation and interlocking, by Roscoe et al. (1958) and Schofield & Wroth (1968) on plasticity and critical state, and by many others since then and through to the present day, have continued the…...

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Book
01 Jan 1948

2,107 citations