Introduction - Geotechnics of Organic Soils and Peat - page 8

Figure 1.10 Organic soil deposit ( Source : http://www.teara.govt.nz/en/soils/7/3/1).
The identification of organic soils is very important because they are much weaker
and more compressible than inorganic (mineral) soils. As such they do not provide
suitable support for most engineering projects. Traditionally, the normal practice is
to avoid such soils, or excavate them or drive a pile though them. However, due
to the scarcity of more suitable land for infrastructure development and agriculture,
avoidance is now no longer an option, as explained later in this topic.
The term peat refers to highly organic soils derived primarily from plant remains.
In other words, peat is unconsolidated superficial deposits with high non-crystalline
colloid (humus) content, constituting the subsurface of wetland systems. It normally
has a dark brown to black colour, a spongy consistency and an organic odour. Plant
fibres are sometimes visible, but in the advanced stages of decomposition they may not
be evident.