DEVELOPMENT OF A SOIL PROFILE

5.2.1 Weathering

Weathering almost inevitably involves chemical changes that contribute to expansion and disintegration. For example, exfoliation or flaking-off of the surface of a granite boulder, and its eventual disintegration into sand, are the result of hydration reactions that cause a volume expansion of the grains of feldspar.

Chemical weathering converts rock minerals into new minerals, in particular a distinct class of minerals called clay minerals. Water plays a key role in the conversions, as OH
ions are essential building blocks in clay mineral crystal structures. As indicated in the preceding chapter, clays can be concentrated by erosion and deposition as sediments, but with rare exceptions their ultimate genesis is by weathering.

Because weathering is most intense at the ground surface, there is a transition from highly weathered soil to unconsolidated ‘‘parent material’’ that may be either a weathered rock or a sediment. Different layers in the weathered zone constitute the soil profile. The layers are called ‘‘horizons’’ and are designated by capital letters, the most common from the ground surface down being A, B, and C. Various subscripts are used to designate specific properties. For example a subscript A1

designates a dark-colored topsoil that develops under grassy vegetation whereas A2signifies a light gray or white layer that develops under forest. Some examples of soil profiles are shown in Fig. 5.1, where the Tunbridge series shows a strong A2

horizon. The zone of maximum development of the B horizon is called the B2.

5.2.2 The A Horizon

The A horizon is the common ‘‘topsoil.’’ (The scientific name is ‘‘epipedon,’’ which means over-soil.) A horizons often are rich in humus and organic plant residues and have a loose, loamy texture preferred by gardeners. An A horizon usually is darker in color than the underlying soil except under forest conditions, where intense leaching by humic acids derived from forest litter creates an ash-gray or white layer. This is particularly true under conifer forest, where Russian soil scientists who pioneered the classification of soils named these ‘‘podzols,’’ which means ash. A white or ashy layer therefore is diagnostic of development under forested conditions and may be designated an E (for eluviated) horizon.

An example is shown at the right in Fig. 5.1.

The thickness of the A horizon typically ranges from a few centimeters (inches) to about 0.6 m (2 ft). Because of its high content of organic matter the A horizon

Figure 5.1

Three examples of soil profiles. In all cases, A horizons should be removed prior to use in engineering. The Houston Black is an expansive clay with a B horizon that extends to a depth of about 2.6 m (9 ft) and causes extensive engineering problems. A somewhat shiny appearance is caused by slickensides or smearing of clay along shear surfaces. Tunbridge is officially designated the Vermont state soil. R denotes rock. (From a calendar publication of the Soil Science Society of America, 2002.)

usually exhibits undesirable engineering characteristics such as high compress-ibility and rebound, low shear strength, resistance to compaction, and variable plasticity. In construction operations the A horizon generally is removed and saved for topping-out and seeding, sodding, or planting for erosion control or aesthetic purposes.

The A horizon also is the site for intensive weathering to form clay minerals, but these tend to be carried downward by percolating water to be deposited in the next lower layer, the B horizon. This process is called eluviation.

In engineering the A horizon is the layer to avoid. A horizon soil that is left on a slope and buried under fill can cause a landslide, no matter how well the fill soil has been compacted. Acceptable construction practice involves removing all vegetation and A horizon material, and cutting horizontal steps prior to placing soil fill on a hillside. Exploration borings must penetrate all of the way through fill material to ensure that there is no buried topsoil, trash, or organic material that may cause future problems. Any construction on such a site also must bear an additional cost for removal of the poor material. Structures that are built on top of weak or compressible layers may suffer severe damage and require expensive remedial measures.

5.2.3 B Horizon

In humid or subhumid climates, clay minerals are concentrated in the B horizon by eluviation. Most clay-rich B horizons form a valuable reservoir for water for plants during periods of drought, and clay minerals act as temporary holding sites for fertilizers.

Two expansive clay B horizons are shown in Fig. 5.1, the highly expansive Houston and the moderately expansive Holdrege soil series. The Holdrege also has a transitional BC horizon.

As shown in Fig. 5.2, because soil horizons follow contours of hills, they are transected by horizontal floors and foundations, resulting in differential uplift or settlement. Expansive clay B horizons often are removed and replaced with nonexpansive soil. This can be the same clay stabilized by mixing with a few percent hydrated lime. Expansive B horizon clay can readily push a basement wall off its foundation, as shown at the bottom of Fig. 5.3. A failure to recognize and deal with expansive clay is an invitation for future problems and lawsuits.

Blocky Structure

Soil containing expansive clay typically develops vertical shrinkage cracks during periods of dry weather. Such cracks can extend a meter or more deep, extending to a nonexpansive soil layer or to a depth below which the moisture content remains stable.

Moisture cycling of a B horizon containing expansive clay creates a ‘‘subangular blocky’’ structure with the individual blocks or peds measuring a centimeter or so across. Clay coatings on the outside of the individual peds are planes of weakness that maintain the blocky structure. A blocky structure therefore is diagnostic of expansive clay.

Figure 5.2 Typical damages from an expansive clay B horizon.

This crack pattern indicates

shrinkage.

Figure 5.3

Basement wall and sewer pipe being pushed in by wet-dry cycling of an expansive clay B horizon. This may happen years after

construction, as cycling is required to compress loose soil backfill placed against the outside of the wall. Also a severe problem with expansive shale C horizons as in the Denver area.

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

Y accumulation of gypsum

Z accumulation of soluble salts, for example in dried lakebeds

Many of these substripts have important implications in engineering and in geology. For example, an Abhorizon is a buried topsoil, and buried topsoils can be conducive to landslides.

Question: Which other subscripts can have engineering significance?