Sustainable Ground Improvement Technique for Geo-Disaster Mitigation - Sustainable Practices in Geoenvironmental Engineering - page 448

trend to those shown in Figures 12.3 and 12.4 (Fukue et al., 1999)—but with lower values
because the surface sediments were looser and much younger in age in comparison to the
deeper sediments shown in Figures 12.3 and 12.4.
12.3.4 Formations of Carbonate Nodules and Sandstones
Carbonate nodules can be found in the mines of bentonite clay, as shown in Figure 12.5a.
The nodules consist of calcite (calcium carbonate) and soil particles. They are very hard
but are easily dissolved with strong acid. Carbonate nodules can also be found in old
and newly formed sediments as shown, for example, in the carbonate nodules found in
the alluvial sediments shown in Figure 12.5b. Although the mechanisms responsible for
formation of the nodules are not fully understood, it is likely that the natural processes of
nodule formation involved some element of microbial activity.
Another natural process of carbonate precipitation is the formation of sandstone, with
a likely process involving the dissolution of shells and the recrystallization of carbonates
binding sand particles (Worden and Burley, 2003; Morad, 1998; Moore, 2001). These natu-
ral processes that produce cementation of soil particles with carbonate can be deined as
(a)
(b)
FIGURE 12.5
(a) Carbonate nodules found in bentonite mine. (b) Carbonate nodules found in alluvial sediments near a river
mouth.