Page 110 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
The Soil Profile 105

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 A b horizon is a buried topsoil, and buried topsoils can
be conducive to landslides.

Question: Which other subscripts can have engineering significance?

5.3 FACTORS INFLUENCING A SOIL PROFILE

5.3.1 Climate
Climate, in particular the patterns of temperature and precipitation, exerts
a major influence on weathering. For example, soils in the humid tropics are far
more deeply and severely weathered than those in arctic or desert conditions.

Weathering Stages of a Soil
Climate strongly affects clay mineralogy because of selective leaching and
reconstitution of mineral ingredients. A guide to intensity of weathering is the
weathering stages for the fine-clay size fraction by M. L. Jackson and his
associates at the University of Wisconsin (Table 5.1).

As might be expected, gypsum and its occasional companion, halite or rock
salt, are relatively soluble and leach out first, so a soil containing gypsum is not
highly weathered unless the gypsum came in later as a secondary mineral.

Stage Mineral Formula or common occurrence Table 5.1
1 Gypsum CaSO 4 . 2H 2 O, an evaporite Jackson-Sherman
2 Calcite CaCO 3 , limestone (1953) sequence
3 Hornblende etc. Dark minerals in granite, basalt indicated by minerals
4 Biotite Black mica in granite remaining in the
5 Feldspars Dominant in granite, basalt fine-clay fraction.
6 Quartz SiO 2 abundant in granite Arrows indicate
7 Muscovite K-rich white mica, granite progressive removal
8 Illite, vermiculite, chorite Mica clay mineral, shale of potassium (K) in
9 Expansive clay mineral, soil the series muscovite
!
10 Kaolinite Nonexpansive China clay to smectite, and of
11 Gibbsite Al(OH) 3 , aluminum ore silica (SiO 2 ) in the
Smectite !
12 Hematite Fe 2 O 3 , red iron oxide series smectite to
13 Anatase TiO 2 , soil gibbsite

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