Civil Engineering Reference
In-Depth Information
but not to engineers, except to save and replace as landscaping after construction is
complete.
4.3 Geological processes
Soils and rocks close to the surface of the Earth are exposed to the atmosphere and are
weathered, eroded, transported and deposited, while deep burial converts soils back
to rocks. The general cycle of soils and rocks is illustrated in Fig. 4.2.
Collisions between drifting continental plates raise mountain chains like the Andes,
the Rockies and the Himalayas. Rain, snow and sunshine weather rocks and soils; this
may consist simply of mechanical breakdown of a rock mass into chunks of material
that have the same composition as the parent rock or chemical alteration to new clay
minerals. Water, ice and wind transport these weathered rock fragments and, at the
same time, degrade, polish and sort them into different sizes. When these transporting
agents slow down, the soil particles are deposited and as further material is deposited
above they become compressed.
The cycle illustrated in Fig. 4.2 is, of course, highly simplified and there are many
additional influences and processes. For example, rocks weathered
in situ
form residual
soils while tectonic activity recycles moltenmaterial frombelow the crust to the surface,
sometimes causing rocks and soils to metamorphose in the process.
4.4 Stratigraphy and the age of soils and rocks
A borehole drilled down into the crust will pass through strata, or layers, of soils
and rocks which generally become older with depth. Stratigraphy is the study of the
sequence of strata that represent geological history. At a particular location there
will have been periods of volcanic activity, mountain building and erosion and so a
single borehole will not reveal the complete sequence of Earth's history. A break in
the stratigraphic column in a borehole record is known as an unconformity; often the
materials at an unconformity have been eroded before deposition of new material.
Figure 4.2
Simplified representation of the cycling of rocks and soils.
