Sustainable Ground Improvement Technique for Geo-Disaster Mitigation - Sustainable Practices in Geoenvironmental Engineering - page 444

The lesson learned from the example of rocks is that if the role of diagenesis could be
successfully used as a ground improvement technique, this should provide one with a
robust and durable treatment for soft ground. There is, however, a problem. The natural
rate of process of diagenesis is too slow for practical application. Accordingly, if one wishes
to use diagenesis as a tool, one needs to overcome this problem. The innovative technique
discussed in this chapter demonstrates the eficacy of artiicially induced diagenesis tech-
niques that are both robust and sustainable solutions for ground improvement.

12.3 Carbonate Diagenesis: Carbonate as a Cementing Agent
12.3.1 Definition of Carbonate Diagenesis

It is commonly understood that the strength of sediments is a function of its density—with
density increases due to consolidation and/or compaction—and that the greater the den-
sity, the greater the strength. For example, the values for strength parameters, c (cohesion),
and ϕ (friction angle), for ine-grained soils will increase during consolidation, with the
increases attributed to the decrease in the separation between particles and bonding due
to interparticle action, which has often been confused with the strength increase by bind-
ers between particles, such as carbonates, organic matter, clay, etc.
In geology and lithology, diagenesis is usually deined as carbonate diagenesis since car-
bonate acts as a binder for sedimentary rocks (Morse, 2005). In many cases, sandstone
and shale have been formed with carbonates (Worden and Burley, 2003). Recently, dia-
genesis has been deined as compaction and carbonation processes at low temperature and low
pressure for the formation of sedimentary rock . This is a more advanced deinition because
some obscure factors have been removed. As will be described later, carbonates can be dis-
solved and recrystallized under certain conditions—low temperatures and low pressures,
which strongly suggest that these processes can be included in explaining the process of
diagenesis.

12.3.2 Origin and Fate of Carbonates

12.3.2.1 Sea Bottoms and Ocean Floors

There are several origins in the formation of carbonates. The largest proportion of carbon-
ate formation is attributed to the bodies of coccolith (phytoplankton) and foraminifers
(Fukue et al., 1996). The dead bodies deposited on the sealoor are transformed into lime-
stone or carbonate binders between soil particles, depending on the carbonate content.
The carbonate content of marine sediments varies from less than 1% to more than 80%. In
general, the carbonate content is low near the coast and increases with the distance from
the coast due to the burial effects of materials discharged from the land environment.
Fukue et al. (1996) estimated that from the sedimentation rate of deposits and carbonate
content, the sedimentation rate of the carbonate was approximately 0.95 g/1000 years/cm 2 .
Considering the discharged amount from land, the relationship between total deposits
m tv (g/1000 years/cm 2 ) and carbonate content C (%) i s

tv = 95

m (12.1)

C