Geoscience Reference
In-Depth Information
induce carbonate precipitation (Mitchell and Santamarina, 2005). Also, in fine sands or
coarser materials bioclogging could occur when bacteria are adsorbed or strained by the
solid grains, which could result in limited treatment distance for ground reinforcement
purposes (Van Paassen
et al
., 2010).
There is an important problem with biogrouting: the limited dispersion of bacteria
injected into soils. Bacteria often stick fast to solid surfaces. In the absence of a strong
hydrogeological gradient the organisms remain localized at the origin of injection,
resulting in the fouling of wells and inadequate dispersion of bacteria. The direct
transportation of bacteria from injection wells to other zones would be advantageous
to augmentation approaches used for
in situ
remediation (De Flaun and Condee, 1997).
It causes a heterogeneous diffusion of CaCO
3
away from the injection points.
The lack of CaCO
3
close to the injection points could be the result of a higher flow
velocity, causing more bacterial flush out and hence lower activity and less CaCO
3
.
Another explanation for the lack of CaCO
3
around the injection points considers the
kinetics of CaCO
3
precipitation and transport of crystals. Initially the crystals are
still small or not even present if the solution is not yet sufficiently oversaturated that
nucleation has taken place, which is likely in quartz sand (Lioliou
et al
., 2007), that
they are still easily transported through the pores. Once the flow velocity drops or
crystals become bigger, they are more easily trapped in the narrow pores (Van Paassen
et al
., 2010).
The control and predictability of the
in situ
distribution of bacterial activity and
reagents and the resulting distribution of CaCO
3
and related engineering properties
in the subsurface are not yet sufficient, and form the greatest challenge for further
optimization, especially if biogrouting is applied in an open system (Van Paassen
et al
.,
2009). Further research should demonstrate what mechanisms are responsible for the
observed heterogeneity in the deposition of carbonate and the consequent geotechnical
parameters and what the implications of this heterogeneity are for the designed purpose
(Van Paassen
et al
., 2010).
7.7 ELECTRO-BIOGROUTING IN ORGANIC SOILS AND PEAT
Electro-biogrouting is a new method to stabilize the organic soils. There are noticeable
qualities in organic soils that make a suitable environment for utilization of electro-
biogrouting techniques, i.e. (1) the saturated mass is a good environment, (2) the high
charge and high specific surface area of the humus increase the presence of cations for
water momentum in an electroosmotic phenomenon, (3) the surface charge can cause
electroosmosis to occur, and (4) the decomposition processes affect the electroosmotic
behaviour. The bacteria are rod-shaped with many negative charges on their surface,
and they can move across the soil under an electric gradient. The urease is negatively
charged and can diffuse with electric potential. Indeed, the urease produced can be
mixed with ammonia and transported in organic soils under an electric gradient. The
calcium chloride solution is then added in an electric injection process. This method
can induce carbonate precipitation (CaCO
3
). It can operate in fine organic clay and
peat. However, the surface charge of bacteria in the electrokinetic environment, the
effect of pH on the bacteria, and the transport rates of ammonia, urease and calcium
chloride in organic soils are all challenges that need to be studied.
