Question

LL=liquid limit , PL=plastic limit

2- What are the possible sources of errors in performing the LL and PL tests?

Answer 1

The greatest source of error in both liquid and plastic limit tests is lack of care in performing ,operator error, because these tests are very precise and requires very exact following of procedures, any mistake and throw off the data substantially. As an example, the soil must be rolled out to the correct diameter and in plastic test the exact rate of blows should be performed correctly. 2. Plasticity index = Liquid limit – Plastic limit LL and PL obtained from the experiment, 57.74 and 19.91 respectively, Plasticity Index = 57.65 - 19.91 = 37.74. 3. The classification of this soil in the Unified Soil Classification System with a LL of 57.65would have to be CH (Clay of high plasticity) because PI is plot above A line in plasticity chart .

4. If the height of fall in the LL test to be too large, the measured LL would be less than the true LL. because larger fall would cause the gap to be closed with fewer numbers of blows as more force is introduced to close the gap per cycle. 5.The sample that is not dried at all will have the higher liquid limit because drying the sample will destroying the organic matter and brings about changes to the liquid limit. Soils with little organic matter in them seems to suffer less of a reduction. 6-Plastic limit (PL) would be too low. The water content required for the clay to be rolled into a thinner thread is more than the water content required for a thicker thread. Therefore, using the moisture content of a ¼ inch thread would be much smaller than a 1/8 inch thread. 7-An example that could be given would be the excavation the foundation for a large structure.A hole will have to be dug into the ground and soil samples from multiple sites will have to be analyzed to determine strengths and properties. One may argue that the results of the tests will have little or no bearing on the behavior of undisturbed samples of soil. This may be true, a sun disturbed soils will have different physical properties than disturbed soils. However, the engineering properties of the material itself will be very similar. Therefore, the tests is reliable for the purposes of Geotechnical Engineering.