Atterberg Limits and Plasticity Questions and Answers

Answer: C

It is the Shrinkage limit.

As a saturated clay dries, it shrinks in volume until it reaches the shrinkage limit; below that water content the soil particles are in contact and further drying removes water from the pores without reducing the volume. The shrinkage limit is the lowest of the Atterberg limits.

The liquid limit marks the change from liquid to plastic state, the plastic limit from plastic to semi-solid, and the plasticity index is the difference between them, not a moisture content at which shrinkage stops.

Answer: C

A soil with a plasticity index of zero is Non Plastic.

The plasticity index is the liquid limit minus the plastic limit, the range of water content over which the soil behaves plastically. If it is zero, there is no such range, as with sands and non-plastic silts, so the soil is called non-plastic.

In the usual classification, a plasticity index below 7 is low plasticity, 7 to 17 medium, and above 17 high. All three describe soils that do have some plastic range, unlike a soil with an index of zero.

Answer: C

At the liquid limit the shearing strength is small but definite.

The liquid limit is the water content at the boundary between the plastic and liquid states. The soil still has a small, measurable shear strength there, of the order of 2 kN/m², and the standard cup test is in effect a way of finding the water content at which the soil has that strength.

The soil is fully saturated at the liquid limit, so A is wrong. Zero strength would describe a true liquid, beyond the limit. Easy moulding describes the plastic state between the plastic and liquid limits rather than the limit itself.

Answer: D

Clay has the highest plasticity index.

The plasticity index is the range of water content over which a soil behaves plastically, the liquid limit minus the plastic limit. Clay minerals are fine, plate-shaped and electrically charged, so they hold water films and stay mouldable over a wide range of moisture; clays typically have plasticity indices well above 17.

Silt has low plasticity. Sand and gravel are coarse and non-plastic, with a plasticity index of practically zero.