Geotechnical Properties

Some physical properties of geologic materials are important in geotechnical engineering, but are not useful for geophysical interpretation. These properties are the dry strength, dilatancy plasticity, and toughness of the material. All four properties may be easily quantitatively categorized in the field by handling the materials.

Dry Strength

Dry strength is how strong the material is when it is dry. The dry strength is categorized in the field by the engineer modelling a small ball, adding water if necessary until the material has the consistency of putty, then applying finger pressure. If the ball crumbles with the pressure of handling, it has no dry strength. Categories progress through low, medium, and high, with very high dry strength indicating a material that cannot be broken with the pressure applied by squeezing the sample between a thumb and a hard surface.


Dilatancy is how the volume of a cohesion less soil will expand under loading or shear deformation. The dilatancy of a sample can be qualitatively categorized in the field using basic tools. The field technician will need to mould the specimen into a small ball, adding water until it is soft, then smoothing the surface with a blade or spatula. Then, he or she will shake the ball horizontally, striking the side of one hand against the other, and observing how quickly water appears on the surface of the ball. Then the engineer squeezes the balled sample by closing his or her hand, or by pinching the material, and observing if and how quickly the water disappears back into the ball. The dilatancy is then categorized as none, slow, or rapid depending on how quickly the water appears and disappears when the ball is manipulated.


Plasticity is how far a material may be deformed under constant stress, without cracking or dilatancy. The plastic limit can be determined by rolling the material into a thread, then folding and re-rolling the material until the thread crumbles. After reaching the plastic limit (when the thread crumbles), the field technician kneads the material into a lump, continuing to knead until the material crumbles. The plasticity is qualitatively categorized by how the material behaves with more or less water than the plastic limit, particularly how long it must be rolled to form a thread, then how a lump of the material behaves as it dries out.


Toughness is the ratio of the plasticity index to the flow index. It is tested concurrently to the plasticity test. Qualitatively, it is categorized into low, medium, and high by how much pressure is required to rolling the material into a thread and kneading it into a lump during the plasticity test, and the stiffness of the thread and lump.

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