Plasticity Index
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Plasticity Index

Also known as Atterberg limits, is simply the sticking nature of soil material. It is usually used to determine the amount of clay in the soil there are mainly two procedures under geotechnical engineering materials for obtaining the atterberg limit namely; casangrande and penetrometer methods The following atterberg test procedure is based on Bs 1377


a)      Obtain a sample of air dried soil and sieve it on a 0.425mm sieve then take approximately 250g of the sieved material

b)      put the sample on a clean glass surface and mix with distilled water

c)       put some small amount of the mixed sample in the cup and trim off the excess material

d)      adjust the cone until the tip touches the sample and note the dial gauge reading

e)      release the cone and record the reading after five seconds

f)       press the dial gauge swiftly and record the new dial gauge reading

g)      the difference between the two readings gives us the penetration

h)      remove some of the soil from the cup and add some more, do this at least 3 times

i)        liquid limit test-take two samples at the 20mm penetration for the determination of the moisture content, this is the liquid limit(liquid limit of soils is the moisture content that produces a penetration of twenty millimeters after five seconds)

j)        Also put some for rolling to produce threads of approximately 3 mm thick, take two samples and determine their moisture content. This is our plastic limit

k)      remix the sample and repeat the process from  b) to h) at least 4 times

l)        using the vertical scale for the cone penetration and the horizontal scale for the moisture content, draw a best fit -line graph

m)    the moisture content corresponding to a penetration of twenty millimeters is the liquid limit

Plasticity index results

Plasticity index is the difference between liquid limit and plastic limit= LL – PL


a)      Repeat steps a) and b) above

b)      Take a portion of the wet sample and level it on the casagrade apparatus, the material should be parallel to the cup

c)       Take the cutting groove and cut along the center of the soil

d)      Rotate the handle at a revolution of about two per second until the soil comes into contact at the base of the groove

e)      Separate some portion to be used later for moisture content determination, weigh and place in the oven

f)       Repeat the procedures b) to f) at least five times without adding more soil or changing the sample

g)      Draw a graph to report the results with the penetration on the vertical scale and moisture content on the horizontal scale. Take note that this is the opposite of the penetrometer method.

h)      Join the scattered points using a best fit line

i)        Liquid limit-reported as the moisture content that corresponds to twenty-five blows


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