The plasticity index (IP)
- The plasticity index (IP) of a soil is the difference between the liquid and plastic limits
Liquid limit test
Test procedure
- Select, prepare a mature specimen (over 24 hours)
- Check apparatus
- Mix and place sample into cup
- Measure cone penetration
- Measure moisture content
- Repeat twice adding more water
- Each time calculate a plot graph
The cone penetration is the difference between R1 & R2
- Check the apparatus. The cone should fall freely when the release button is held in.
- Place the sample directly below the cone, adjusting the cone so that the tip just touches the soil. Record the dial gauge reading (R1)
- Allow the cone to fall for 5 seconds by pressing the button. Record the dial gauge reading (R2)
Take the initial gauge reading
Take the final gauge reading
- Plot the moisture content results against the cone penetration results and draw the best-fit line between them.
- The liquid limit (Wl) is the moisture content corresponding to a cone penetration of 20mm
A few tips for good results
- On a glass plate, mixing 500g of specimen, thoroughly with distilled water into a thick homogenous paste, removing any course particles. The mixing time should be at least 10 minutes
- Press the soil paste in a metal cup, so as to avoid trapping air.
Simple dry sieving
BS13177 : part 2:9:1 :1990
Test procedure
- Select and prepare specimen
- Oven dry, cool and weigh
- Select sieves
- Pass through sieves
- Weigh each size fraction
- Calculate the cumulative percentage passing each size
- Plot grading graph
Minimum quantities for particle size test
The specimen to be used is obtained from the original sample by riffling. The minimum quantity depends on the maximum size of the particle. Completely dry specimen and weigh to the accuracy of 0.1% of its total mass. Place it into the top sieve with the lead fitted
The complete nest of sieves are placed in the shaker and secured down. set the timber to the required time. A minimum of 15 to 20 minutes should be standard practice to obtain consistent results.
Sieves must not be overloaded.
- Empty each sieve in turn into a pan on the electronic balance. Remembering to tare the balance after each load. Any particles lodged in the aperture of the sieve should be carefully removed with a sieve brush.
- The masses retained are recorded against the sieve aperture size on a particle distribution work sheet.
The moisture content test
Test procedure
- Weigh a clean dry container
- Weigh the container with the wet soil sample
- Dry in the oven or microwave
- Weigh the container and dry soil sample
- Calculate the results
-
The test specimen must be a proper representation of the soil sample, taken from the middle of the mass of soil. Dry in the oven at the temperature of 1100c or if the microwave is used, on full power.
How much material
Homogenous clays or silts. 30g weigh to 0.01g
Medium grained soils. 300g weigh to 0.1g
Course – grained soils. 3kg weigh to 1g
Moisture content (w) = Mass of the moisture
Mass of the dry sample
Conclusion:
The conclusion to my lab report is that the liquid limit and the plastic index gave an indication that the plasticity is low thus means that the soil is good for building/ construction. If the soil was of high plasticity, then it would not be suitable for use in many engineering projects.
For the liquid limit by BS standard it is suggested that there be 4 sets of reading but due to lack of time within the lab we were only able to get 2 sets of readings this means that the results would not as accurate as inconsistencies are harder to find because the two sets can not be differentiated as to which one is correct having a third reading would be able to spot and odd reading or one that is out of place. Having only two sets of results means that there are only two points that we are able to plot on the graph which make the results inconsistent.
Ideally in the plastic limit test 3 results would have been preferable to get a more accurate average but time could allow us only to produce 2 sets of results.
Reference
Peter L Berry and David Reid, 1988, An introduction to soil mechanics, McGraw- hill book company, maidenhead Berkshire, England.
