Environmental Engineering Reference
In-Depth Information
Fraction (%)
Specific gravity
Carbonate content (%)
20
40
640
80
100 2.68
2.70
2.72 0.1 0.3 1.0
10 30 100
0
Non
cemented
50
Sand
100
Stone
9 m thick
150
Clay
200
Non
cemented
Silt
250
FIGURE 12.6
Floating sandstone in the diluvial formation, Sakura City, Japan.
diagenesis (Worden and Burley, 2003). A typical example of such a sandstone is the
loating
sandstone layer
in granular sand layers in Sakura City, Chiba prefecture, Japan. The subsur-
face proile shown in Figure 12.6 shows the sandstone layer, thickness of about 10 m, at a
depth of 90 m embedded in the diluvium sand layers (Narita sand formation). Unconined
compressive strengths of about 19 MPa were obtained from samples retrieved from the site
(Fukue et al., 1999). Vertical fracture planes obtained under compression suggested tensile
failure of the samples, meaning that the compressive strength of the sandstone would
likely be higher than 19 MPa. With a carbonate content of 24% in comparison to the less
than 1% carbonate content in the granular sand layers above and below the sandstone, one
can conclude that carbonate contribution to strength is approximately 800 kPa/% (Fukue
et al., 1999).
12.3.5 Calcirudite
Dissolution of limestone (calcite) occurs as follows:
2
+
−
CaCO
+
HO CO
+ ↔+
Ca
2
HCO
(12.7)
3
2
2
3
The reaction shown in the relationship is reversible. Limestone dissolves and recrystal-
lizes as stalactite. Ca
2+
and
HCO
3
−
are transported by groundwater and precipitated in
the soil horizon resulting in the formation of calcirudite—a conglomerate bound with cal-
cite. This reversible reaction (dissolution-precipitation process) is not limited to limestone.
It also occurs with carbonate-composed materials such as shells, coral reefs, carbonates
contained in soils, etc. This dissolution-precipitation process results in a complicated cir-
culation of calcium and carbon dioxide when it occurs in soils. As an interesting piece of
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