2613
Influence of relative density on microbial carbonate precipitation and mechanical
properties of sand
L'influence que la densité relative du sol donne dans précipitation du carbonate microbienne et
propriétés de la mécanique
Tsukamoto M.
Kawasaki Geological Engineering Co., Ltd. (Formerly Public Works Research Institute), Minato-ku, Tokyo, Japan
Inagaki, T. Sasaki, Y.
Public Works Research Institute, Tsukuba, Ibaraki, Japan
Oda K.
Osaka University, Suita, Osaka, Japan
ABSTRACT: There exists a ground improvement technology that uses calcium carbonate precipitated from carbon dioxide generated
by microbial metabolism and calcium sources in the pores of soil. It is known that the mechanical properties of the improved grounds
correlate with the amount of calcium carbonate precipitation, but it is unclear how soil density influences calcium carbonate
precipitation and the mechanical properties of the improved soil. Toyoura sand specimens of three relative densities are used to
precipitate calcium carbonate through microbial metabolism. The injection-improved test and the triaxial test (consolidated-drained
condition) are conducted to investigate calcium carbonate precipitation and the mechanical properties of the soil. The results show
clearly that more calcium carbonate precipitation occurs in soil with lower relative density, but that in soil with higher relative
density, the mechanical properties strengthen as calcium carbonate precipitation increases.
RÉSUMÉ : Dans la nature, il existe des micro-organismes qui capturent le dioxyde de carbone et les ions calcium présents dans la
terre pour ensuite rejeter du carbonate de calcium. Ces dernières années, en s’appuyant sur le fonctionnement de ces micro
-
organismes, une technique renforçant la résistance du sol a été d
éveloppée. Dans le cadre du développement d’une technologie qui
renforce le sol à l'aide de métabolisme microbien, la présente étude a mis en évidence expérimentalement que l’influence de la densité
relative du sol exerce un effet bénéfique. Tout d’abord,
à travers une série de tests de cisaillement, le comportement au cisaillement du
sol renforcé par le métabolisme de ces micro-
organismes a été mis en avant. Ensuite, il s’est avéré que plus la densité relative du sol
était petite, plus la quantité de carbonate de calcium déposée était importante. Cependant, nous avons aussi compris que plus la
densité relative du sol était grande, plus les effets renforçant la résistance du sol étaient visibles.
KEYWORDS: micro-organism, ureolysis, soil improvement, mechanical properties, triaxial test, calcium carbonate
1 INTRODUCTION
To ensure the efficient maintenance of civil engineering
structures, which is an issue, ground improvement technologies
can be applied to reinforce existing structures. Ground
improvement technology that uses calcium carbonate
precipitated from carbon dioxide generated by microbial
metabolism and calcium sources in the pores of soil is expected
to be applicable to ground directly under existing structures
because the viscosity of the injected grout is low (Wiffen et al.
2007). It is known that the mechanical properties of grounds
improved through the use of this method correlate with the
amount of calcium carbonate precipitation (Inagaki et al. 2011),
but it is unclear how the relative density of the soil influences
the precipitation and the mechanical properties of the improved
soil.
In this study, we used
Sporosarcina pasteurii
(ATCC11859)
to stimulate microbial metabolism via the following reactions.
Our aim was to investigate the relationship between the
soil’s
mechanical properties and calcium carbonate precipitation in
three types of Toyoura sands that were compacted by microbial
metabolism.
(Ureolysis)
CO(NH
2
)
2
+ 3H
2
O
→
2NH
4
+
+ 2OH
-
+ CO
2
(1)
(Calcium carbonate precipitation)
CO
2
+ H
2
O
→
HCO
3
-
+ H
+
(2)
HCO
3
-
+ Ca
+
+ OH
-
→
CaCO
3
+ H
2
O
(3)
In the tests, microbial broth and a nutrient mixture were
injected into the Toyoura sand specimens with three relative
densities to improve compaction. In addition, we carried out a
triaxial test (consolidated-drained [CD] condition) on the
improved specimens. Then, we examined the influence of the
relative density of the soil on calcium carbonate precipitation
and the
soil’s
mechanical properties.
2 TEST METHODS
2.1
Method for production of specimens
A half-split mold made of PVC, 15 cm in height and 5 cm in
diameter, was used to create the specimens (Fig. 1 and Photo 1).
Silicone grease was applied on the internal surface of the mold
to prevent the generation of water paths along the wall. Toyoura
sands having the physical properties listed in Table 1 were used
to create the specimens, and the molds were filled with sand
using the air-drop method. The specimens had a relative density
of Dr = 15% (
d
= 1.372 g/cm
3
), Dr = 60% (
d
= 1.504 g/cm
3
),
and Dr = 85% (
d
= 1.589 g/cm
3
). The specimens were checked
for weight and density, fitted with a collar on top, and saturated
with distilled water supplied from the bottom. The surface of
the specimen of Dr = 15% sank significantly during the
hydraulic filling. This settlement was measured using vernier
calipers, and it was confirmed that the relative density after
hydraulic filling was about Dr = 30%.
