Sustainable Ground Improvement Technique for Geo-Disaster Mitigation - Sustainable Practices in Geoenvironmental Engineering - page 446

12.3.3 Formation of Sedimentary Rock due to Carbonate Diagenesis

Examples of diagenesis resulting from precipitation of carbonate are sandstones cemented
with carbonate (Morad, 1998; Moore, 2001). Fukue et al. (1999) found that the strength of
marine sediments was strongly related to their carbonate content—with the dead bodies
of marine animals such as foraminifer and phytoplankton (coccolith) as the source of the
carbonates. The relationship between strength and carbonate content of marine sediments
in Sagami Bay (Fukue et al., 1999) is shown in Figure 12.2. Although the proportions of clay
silt and sand below a depth of 0.5 m remain relatively constant, the vane shear strength
shows a decrease in strength with depth after a depth of approximately 1.2 m—indicating
the fallacy of assuming a strength relationship between sediment strength and depth of
sediment. The results clearly show, however, that the vane shear strength of sediments is
strongly inluenced by carbonate content.
Figure 12.3 shows that the trend lines for the proiles of unconined compressive strength
and carbonate content for the sediments obtained from Tokyo Bay (the off-shore Haneda
Airport at a soil depth ranging from 42 to 44.5 m) are remarkably similar—indicating
that a quantitative relationship can be established between strength and carbonate con-
tent. Using the slopes of the trend lines (“itting” lines), a correlation between unconined
compressive strength and carbonate content can be obtained as shown in Figure 12.4. The
vertical axis in Figure 12.4 represents the slope of unconined compressive strength, i.e.,
Δ q u / Δ D , and the horizontal axis represents the slope of carbonate content Δ C/ Δ D, where q u
is the unconined compressive strength, C is the carbonate content and D is the sediment
depth. Using data from the sediments from off-shore Urayasu (Chiba Prefecture, at a soil
depth ranging from 42 to 43 m) and those from Figure 12.3, the results show that Δ q u / Δ C ,
i.e., increasing rate of strength, is approximately 64 kPa/%.
Surface sediments obtained from Tokyo and Osaka Bays were also examined to obtain
relationships between strength and carbonate content, with the results showing a similar

Fraction (%) Void ratio Carbonate

content (%)

Vane shear
strength (kPa)

020406080 1.5 2.5 2 4 10 20

0

Sand

1

Silt

2

Clay

3

FIGURE 12.2
Proiles of sediments from the Sagami Bay at water depth of 1020 m.