The plastic limit (PL) is the water content where soil transitions between brittle and plastic behaviour. To determine this behaviour boundary, soil balls are rolled into threads of 3 mm diameter and if those 3 mm threads begin to crumble then their water content are plastic limit of soils.
Soil plastic limit could be called as “rolling limit” because technicians have to “roll soil balls into threads” to determine this type of limit.
By doing this, the values of plastic limit vary remarkably and are afftected so much by experience, temper and skill of technicians. Different technicians will produce threads of different diameters or different shapes… Even worse, some threads are hollow or have different moisture content between inside and outside…
In order to increase the reliability of plastic limit test, researchers introduce some devices to conduct the plastic limit test. For instance, a roller is introduced in ASTM or by several manufacturers (Humboldt, ELE, Controls…); an automatic roller manufactured by Wille Geotechnik…
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| Roller for plastic limit determination | Automatic roller manufactured by Wille Geotechnik |
However all devices for plastic limit determination nowadays only facilitate the producing of soil threads in regular size and shape. No breakthroughs in principle of determination of plastic limit exists so far.