If the diameter of the thread becomes smaller than 3 mm, without any crack formation, it shows that soil is in plastic state and its water content is higher than the plastic limit. We count a stroke as one complete motion of the hand forward and back to the starting position. Normally 80-90 strokes per minute are recommended. We take the reference of a metallic rod for the diameter comparison of the thread. The thread shall be further deformed on each stroke so that its diameter reaches 3 mm taking no more than 2 min. Then the ball is placed on the glass plate and rolled between the palm and the glass plate with little pressure to roll the mass into a thread of uniform diameter throughout its length. We take some amount of this soil mass in hand and form a ball. It is thoroughly mixed with distilled water till it is reached at a consistency at which it can be rolled without sticking to the hands. We take about 30 gm of this sieved soil in an evaporating dish. These particles include fine sand, silts and clays. The particles we get after sieving are all of the size less than 425 micron. 425 micron IS sieve has its mesh openings of 425 micron or 0.425 mm. Because we know Atterberg limits are only significant for the fine grained soils. We can determine the plastic limit using the soil sample prepared for the liquid limit test.įor the determination of plastic limit the soil is air dried and sieved through 425 micron IS sieve to get fine grained soil only. At this water content soil loses its plasticity.īelow the plastic limit soil begins to crumble when rolled into threads of 3 mm diameter. While soil is in the plastic state, if we reduce its water content, soil reaches a point below which it stops behaving as plastic material, that water content is called Plastic limit of soil. As the water content of the soil is reduced the plasticity of the soil is reduced. Plasticity of soil is calculated in terms of its water content.
In plastic state soil can be rolled into threads without breaking them and it can be moulded in any shape. If the soil doesn't have any clay minerals and only contains non clay minerals such as quartz, it would not become plastic whatever may be the fineness of soil or whatever may be the water content. The soil does not become plastic when it is mixed with a non-polarizing liquid, such as kerosene or paraffin oil.Īlso the soil becomes plastic only when it contains clay minerals. The presence of adsorbed water is necessary to impart plasticity characteristics to a soil. Hence these deformations are irreversible which we say they are plastic. When the soil is subjected to deformations, soil particles move to different positions and after the removal of the deformation force, they do not return to their original positions. The clay particles are separated by layers of water which allow them to slip over one another. This phenomenon is known as adsorption with the D of the water and the water so attracted is called adsorbed water. Water molecules, that are dipolar, are attracted towards this negatively charged surface and form a water layer around clay particles. These clay particles carry negative charges on their surface. Plasticity is exhibited by a soil due to the clay minerals present in it. Plasticity in case of soil refers to the property of soil to get deformed without cracking or fracturing under the action of external force and remain deformed after the removal of that force. Plasticity is an important index property of fine grained soils, especially clayey soils. In plastic state soil can be moulded in any shape due to a property called plasticity. Here soil stops being liquid and enters into a state called plastic state of soil. At water content named as liquid limit soil gains infinitesimal shear strength. When water content of this soil drops it starts gaining shear strength and slowly develops resistance to flow.
In liquid state water content of the soil is such that it behaves like liquid and offers no resistance to flow. Depending upon the water content soil may appear in one of these four states.Ĭonsistency limits are water content at which the soil changes from one of these states to another. It is indicated by terms such as soft, medium and hard. Consistency is a term which is used to describe the degree of firmness of soil.
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