Recent advances in the geotechnics of organic soils and peat - Geotechnics of Organic Soils and Peat - page 212

Prasad et al. (2012) observed that the field behaviour of an interior column when
a large number of columns are simultaneously loaded can be simulated by the single
column behaviour using a unit cell concept. Similar behaviour of a group of columns
is also reported by Dhouib and Blondeau (2005) and Maurya et al . (2005).
7.11 NEW DEEP MIXING METHODS (DMM) FOR STABILIZATION
WITH NEW CHEMICAL BINDERS
DMM, a deep in situ soil stabilization technique using cement and/or lime as a stabiliz-
ing agent, was developed in Japan and the Nordic countries independently in the 1970s.
Numerous research efforts have been made in these areas investigating the properties
of treated soil, behaviour of DMM improved ground under static and dynamic con-
ditions, design methods and execution techniques. In the past three to four decades,
traditional mechanical mixing has been improved to meet changing needs. New types
of technology have also been developed in the last 10 years - e.g. the high-pressure
injection mixing method and a method that combines mechanical mixing and high
pressure injection mixing technologies (Kitazume and Terashi, 2013).
In this DMM, soil mixing is usually carried out using mixing augers through which
a grout is introduced and mixed with the soil, resulting in stabilized soil. The most
important factor in such an application is to ensure that the injection and mixing
process of the additives with the soil are thorough and effective so that homogeneous
well-mixed soil additives in monolithic columns are produced (Al-Tabbaa et al ., 1999).
Miura et al . (1998), Shen (1998) and Shen et al . (2003a,b) indicated that there exists
an influence zone of property changes in soil surrounding the injection columns, rang-
ing from the edge of the column to a distance of about 2-3 times the columns' radius.
Within this influence zone, the moisture content decreased while the pH values and
the concentration of cations increased. In this condition, the shear strength of the soil
decreased during the installation, which is very helpful for doing injection in the soil,
and it was restored after a short curing period. Furthermore, there is a consolidation
process involved in the surrounding soil after installation of the injection column. This
is due to the dissipation of excess pore-water pressure developed during the installa-
tion, which contributes to the strength increase in the surrounding soil (Randolph et al .,
1979; Asaoka et al ., 1994; Shen, 1998). Four major factors were identified by Shen
et al . (2003a) that caused property changes in the surrounding soils during and after
installation of injection columns: (1) soil fracturing, (2) disturbance, (3) thixotropy and
consolidation and (4) cementation effects as a result of the diffusion of the chemical
binder.
Since injection columns are constructed by mixing in situ soft soils with chem-
ical admixtures using rotating blades, there exist two types of forces acting on the
surrounding soil. The first is created by an expanding action caused by the injection
pressure of the chemical binder. The second is caused by a shearing action resulting
from the blade rotation. These dual effects can generate excess pore-water pressure and
disturb the surrounding soil, with the end result being that a plastic zone is formed
around the column (Shen et al ., 2003a).
There are three binder injection methods: (1) injecting all the binder while pen-
etrating with the mixing blades, (2) injecting all the binder while withdrawing the