Page 109 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
104 Geotechnical Engineering

Slickensides
In climates with extended wet and dry periods, severely expansive B horizon
clays develop inclined shear planes. These result from lateral stresses that are
high enough to shear the soil after filling of open ground cracks during dry cycles.
Slipping along these planes causes clay particles to be smeared to form slickensides.
Soils that are so severely stressed as to form slickensides are notoriously bad actors.
Slickensides are diagnostic of highly expansive clays.

5.2.4 C Horizon and Saprolite

Unconsolidated soil material underneath the B horizon is called the C horizon.
This may either be sediment such as loess, glacial till or alluvium, or weathered
rock. A C horizon in rock usually is transitional to an underlying saprolite, which
is defined as weathered rock that still retains the rock appearance and structure.
A rock that appears to be sound and falls apart when hit with a shovel most likely
is saprolite, which is from the Greek for ‘‘rotten rock.’’

5.2.5 Caliche and Laterite

In arid and semiarid areas where evaporation exceeds infiltration of water, the
net upward movement can carry with it calcium carbonate that cements the
soil into caliche. Caliche can occur in A, B, or C horizons. It sometimes is used
for surfacing unpaved roads.
Another hard subsoil that occurs in tropical climates is called laterite, which is
soil that is cemented with iron oxides released by weathering. Lateritic layers often
form a mesa-like protective cap on hills, but are differentiated from bedrock
because they cut across bedrock layers.

5.2.6 Subhorizons

Subscripts on A, B, and C horizons further define important characteristics.
Some of the more important subscripts for engineers are as follows. Older soil
surveys may use different subscripts, which will be noted in the survey report.
b buried soil or paleosol (formerly p)
f permanently frozen soil (permafrost)
g gleying, gray color indicating unoxidized soil due to poor drainage conditions
k accumulation of carbonates (formerly ca)
o accumulation of iron and aluminum oxides
p plowed layer
r weathered rock or saprolite, shale, or dense glacial till
ss slickensides, indicating expansive clay
t accumulation of clay
x frangipan, indicating high density and poor seasonal drainage conditions

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