Determination of Consistency of Soil -Liquid & Plastic Limit : Part III

INTRODUCTION


The Atterberg limits are a basic measure of the nature of fine-grained soil. Depending on
the water content of the soil, it may appear in four states: solid, semi-solid, plastic, and liquid.
In each state the consistency and behavior of soil are different and thus so are its engineering
properties. Thus, the boundary between each state can be defined based on a change in the
soil's behavior. The Atterberg limits can be used to distinguish between silt and clay, and it
can distinguish between different types of silts and clays. These limits were created by

Albert Atterberg, a Swedish chemist. They were later refined by Casagrande.


SCOPE


The liquid limit is significant to know the stress history and general properties of the soil met
with construction. From the results of the liquid limit, the compression index may be estimated.
The compression index value will help us in settlement analysis. If the natural moisture
content of the soil is closer to the liquid limit, the soil can be considered as soft if the moisture
content is lesser than the liquid limit the soil is brittle and stiffer. These tests are mainly used on
clayey or silty soils since these are the soils that expand and shrink due to moisture content.
Clays and silts react with the water and thus change sizes and have varying shear strengths.
Thus these tests are used widely in the preliminary stages of designing any structure to ensure
that the soil will have the correct amount of shear strength and not too much change in

volume as it expands and shrinks with different moisture contents.


THEORY


Plastic limit
The plastic limit (PL) is the water content where soil starts to exhibit plastic behavior. A
thread of soil is at its plastic limit when it is rolled to a diameter of 3 mm or begins to
crumble. To improve consistency, a 3 mm diameter rod is often used to gauge the thickness

of the thread when conducting the test.


Liquid limit
The liquid limit (LL) is defined as the arbitrary limit of water content at which the soil is just
about to pass from the plastic state into the liquid state. At this limit, the soil possesses a
small value of shear strength, losing its ability to flow like a liquid. In other words, the liquid
limit is the minimum moisture content at which the soil tends to flow like a liquid.

Plasticity index


The Plasticity index (PI) is the range of water content within which the soil exhibits plastic

properties; that is, it is the difference between liquid and plastic limits.


When the plastic limit cannot be determined, the material is said to be non-plastic (NP).
The plasticity index for sands is zero.


For proper evaluation of the plasticity properties of soil, it has been found desirable to use
both the liquid limit and the plasticity index values.


Atterberg limits are the limits of water content used to define soil behavior. The consistency
of soils according to Atterberg limits gives the following diagram.


APPARATUS REQUIRED


1. For Liquid limit determination
The apparatus required are the mechanical liquid limit device, grooving tool, porcelain
evaporating dish, flat glass plate, spatula, palette knives, balance, oven wash bottle with
distilled water and containers.


2. For Plastic limit determination
The apparatus consists of a porcelain evaporating dish, about 12 cm in diameter (or a flat
a glass plate, 10 mm thick and about 45 cm square), spatula, about 8 cm long and 2 cm wide
(or palette knives, with the blade about 20 cm long and 3 cm wide, for use with flat glass
plate for mixing
soil and water), a ground-glass plate, about 20×15 cm, for a surface for rolling, balance, oven,
containers, and a rod, 3 mm in diameter, and about 10 cm long.


Liquid limit device (Casagrande apparatus)
It consists of a brass dish, handle, and cam mounted on a hard rubber base. The dish
falls through a distance of 1 cm per rotation. A sample of soil 1 cm thick is placed in the dish
with a groove 11 mm wide at the top and 2 mm at the bottom. The number of jars required to
cause the 2-mm gap to close along 1/2 in (12.7 mm) is recorded.



Instructor: Dr. Radhakanta Koner, Department of Mining Engineering, IIT(ISM) Dhanbad

Students: MTech students of Mining Engineering, IIT(ISM) Dhanbad

Course Name: Geomechanics Practical (MNC505)

Session: 2020-21, Monsoon

Acknowledgment
Prabhat Kumar Mandal
Krishna Murari
Birju Gope
Vijay Karanga