2420
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
3.4
Geosynthetic reinforced column or pile supported
embankment – the use of geogrids
Another way to use geosynthetic material for GI application is
the design of geogrids for the support of embankment, land
levees, yards and structure foundations (slabs and superficial
isolated or continuous footings).
Investigating the use of geosynthetics for reinforcement
under ground mass collapse,
Ponomaryov and Zolotozubov
(2013)
compare the method outlined in British Standard BS
8006 and several design approaches with numerical
calculations. On the basis of experimental elongation results,
they introduce the ratio of actual tensile force to deformation.
Computational assumptions are proposed for the description of
the mechanisms of stress-strain development in the reinforced
ground mass. The authors finally present a comparison between
experimental measurements and the results of seven different
methods used for the calculation of the tensile force in the
geosynthetic, its deflection and the surface settlement.
Mihova and Kolev (2013)
analyze the benefit of a
geosynthetic reinforced pad of crushed stone used for the
foundation of a hall in Sofia over soft saturated soil. Field tests
were performed to estimate the E-moduli before and after
improvement. The authors also conducted Finite Element
analysis to model the consolidation process and to confirm the
design stability under static and seismic conditions.
Dimitrievski et al. (2013)
present a history case of soil
reinforcement with geosynthetics for the construction of a six-
storey structure in Ohrid (Republic of Macedonia). Multi layers
geogrids were designed and the effects of the geostatic,
hydrostatic and dynamic loading conditions were studied with
the help of FEM calculations. The validity of the analysis was
demonstrated with the help of in situ measurements obtained for
a close similar structure.
3.5
Sand compaction piles (SCP’s)
In the sand compaction pile (SCP) method, sand is fed into the
ground through a casing pipe and is compacted by vibration,
dynamic or static compaction to form columns. In practice,
SCP’s are mainly used to prevent liquefaction and reduce
settlement with similar success in sandy and clayey soils. With
the help of laboratory and field tests,
Burlacu et al. (2013)
investigate the potential of columns made of loess-sand-
bentonite mixture for the reinforcement of collapsible loess
deposits in Romania. Indeed, as explained in the paper of
Alupoae et al. (2013)
, these collapsible soils require GI works.
They are characterized by high water sensitivity: when its water
content increases, important deformations in the soil can be
observed. In such a way to illustrate this phenomenon, the
authors present a case study of differential settlement of
buildings founded on loess sensitive to wetting. In spite of the
good realization and control of the foundation, important
differential settlements were measured thereafter as a result of
the defective rainwater recovery system.
3.6
Microbial methods
The use of microbially induced carbonate precipitation (MICP)
to cement cohesionless soils has recently received substantial
attention from geotechnical researchers. The most common
MICP mechanism is hydrolysis of urea. MICP via ureolytic
hydrolysis relies on microbes to generate urease enzyme, which
then serves as a catalyst for the calcium carbonate (CaCO
3
)
precipitation reaction. If it is to date well known that the
mechanical properties of the treated soils are directly correlated
to the amount of (CaCO
3
) precipitation, a gray area still remains
concerning the influence of the original nature of the granular
material on the resulting properties of the treated soil. Within
the framework of a laboratory campaign,
Tsukamoto et al.
(2013)
investigate the influence of the relative density of sand
samples on the MICP. As a result of their study, the MICP tends
to increase as the relative density of the soil decreases.
Nevertheless, considering the results of triaxial tests, maximum
principal stress differences were obtained for the samples with
the highest relative density. In light of these results, this
technique seems to be very promising for the future but due to
the bioplugging (permeability reduction) of the granular
material and to the generation of toxic product (ammonium
salt); soil stabilization using ureolytic MICP remains currently
unusual. According to
Hamdan et al. (2013)
, the use of plant
derived urease to induce the carbonate cementation could be the
solution to avoid these drawbacks.
4 GI WITH GROUTING TYPE ADMIXTURES
4.1
Deep Mixing Method (DMM) and soil stabilization
The deep mixing method (DMM) is nowadays a worldwide
accepted GI technology. In this method, the ground is in situ
mechanically (and possibly hydraulically or pneumatically)
mixed while a binder, based on cement or lime, is injected with
the help of a specially made machine. Numerous reviews and
recent progresses of the DMM are referred in Denies and
Van Lysebetten (2012). In the recent years, the DMM is
undergoing rapid development, particularly with regard to its
range of applicability, cost effectiveness and environmental
advantages, as illustrated by the papers of this paragraph.
In the deep mixing projects, the design can be based on
laboratory mixing tests. Soil-cement samples are then prepared
and tested to study the mechanical properties of the stabilized
soil. But, up to now, many laboratories prepared these samples
without standardized procedure. Actually, molding techniques
have a great influence on the mechanical characteristics of the
stabilized material. According to
Grisolia et al. (2013)
, this
influence is strictly correlated to the workability of the soil-
cement mixture and this latter can be quantified with the
measurement of the torque required to turn an impeller in the
mixture. Five molding techniques have been studied and the
authors propose the abacus illustrated in Fig. 3 to define the
range of applicability of these techniques in function of this
torque.
0 10 20 30 40 50 60 70 80 90 100
mixture's workability, Torque
M
t
(Nm)
Molding technique
Applicable MarginallyApplicable NotApplicable
No Compaction
Tapping
Static
Compaction
50kPa
Dynamic
Compaction
Rodding
65 75
10 15
3 6
Static
Compaction
25kPa
30 40
High
workability,
liquid
Low
workability,
consistent
120 ...
Figure 3. Ranges of applicability of the different molding techniques,
from Grisolia et al. (2013)
The applicability of each molding technique was evaluated by
an “Applicability index”, related to “densest specimens with the
highest strength” and “results repetitiveness”.
Since several decades, DMM has been used for GI works.
But in recent years, this technique has been increasingly used
