We left this type of soil to the last principally because it does not conform to easy identification by grain size.  In this category we have peat's and bog soils whose distinguishing feature is their high level of organic content.  The mineral portion is usually clay or clay and silts but sands are often present.  Poor drainage and lower soil horizons are essential for such formations.  Because of their high organic content and fine grain size, these soils are highly plastic.  This represents a special problem for compaction, and will be treated separately in the analysis of compaction problems.

To sum up, soils are compounds containing mineral and organic elements.  The grain size is the factor which determines what type of mineral soil we are dealing with.  Two other important factors are climate, which determines the degree and kind of leaching action that takes place, and the relative age of the soil.  The following table shows the relative sizes of soil particles as expressed and used in this text.

Soil Composition by Grain Size

Type	Diameter Limits	Screen Size
Gravel	76.2     to   2.0       mm	3/4'      to   No 10
Coarse Sand	2.0       to   0.42     mm	No 10   to  No 40
Fine Sand	0.42     to   0.075   mm	No40    to  No 200
Silt	0.075   to   0.005   mm	No 200 to  No 300
Clay	0.005   to   0.002   mm	No 300
Colloidal Clay	0.002   to   0.001   mm

Detailed within this manual is a unified soil classification system which details the major divisions group, symbols, typical names and laboratory classification criteria of soil types commonly found.  This can serve as a useful guide. 
 

Other Method of Identification
As well as the classification system previously discussed there are other subjective methods which can be used to give an indication as to the quality of the material. This can be an examination of:

    a) environment
    b) profile
    c) texture
    d) hand plasticity tests

Again this information is not compulsory but is useful in the field to identify the soil type.  It is especially useful when maximum information needs to be obtained at a minimum time and cost prior to a specific engineering testing method that has been chosen.  More detail on this is given in Appendix 1.

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