Sustainable Ground Improvement Technique for Geo-Disaster Mitigation - Sustainable Practices in Geoenvironmental Engineering - page 442

Agriculture
Infrastructure

Igneous rock

Soil improvement

Soil

Constructed environment

Geodisaster

Carbonation

Rock

Sedimentary rock

Metamorphism
High temperature and pressure

Metamorphic rock

FIGURE 12.1
Weathering of rock, formation of soil, diagenesis, and metamorphism.

The “lesson” to be learned from Figure 12.1 is that the most stable state of the material in
the rock-soil-rock cycle is a solid rock mass. Weathering of rock creates soil, and as we
have discussed in the previous chapters, soil is both a resource material, a signiicant part
of the natural capital of the geoenvironment, and is also the primary constituent of the
surface layer (soil mantle) overlying subsurface rock.
Physical and chemical weathering of rock are the two main natural processes involved
in the disintegration of rock into various sizes of soil particles. Physical weathering pro-
cesses include cyclical heat-cold that shatter rocks because rock components have different
coeficients of expansion. Other physical weathering processes include expansion of water
in rock issures as a result of freezing, and abrasion due to movement of ice and suspended
particles in water. It is useful to note that the weathering processes continue to act on the
disintegrated products (soil particles), altering their sizes and chemical composition.
Chemical weathering processes include hydrolysis, hydration, carbonation, oxidation,
reduction and solution. The processes involve loss of some constituents, addition of pro-
tons, and rearrangement of constituents into new materials—with rates dependent on
temperature, moisture, composition, potential for leaching, and composition of the leach-
ing solution. Protons in the leaching solution can be supplied from CO 2 in the air or from
decaying vegetation or other acids added to the system.
Although the common deinition of diagenesis states that the process sometimes
requires conditions of “high temperature and high pressure,” Worden and Burley (2003)
claim that diagenesis can form sedimentary rocks under conditions of low temperature
and pressure. With this claim, one can readily separate the process of diagenesis from that
of metamorphism. Properties of rock that will be changed when subjected to high temper-
atures and high pressures can be deemed to have undergone the process of metamorphism .