ATTERBERG LIMITS

ATTERBERG LIMITS

The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil, such as its shrinkage limit, plastic limit, and liquid limit. As a dry, clayey soil takes on increasing amounts of water, it undergoes dramatic and distinct changes in behavior and consistency. 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 a soil is different and consequently 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.

PURPOSE
This lab test is performed to determine the plastic and liquid limits of a fine-grained soil. The liquid limits (LL) is arbitrarily defined as the water content in percent, at which a part of soil in a standard cup and cut by a groove of standard dimensions will flow together at the base of the groove for a distance of 13mm when subjected to 25 shocks from the cup being dropped 10mm in a standard liquid limit apparatus operated at a rate of two shock per second. The plastic limit (PL) is the water content, in percent at which a soil can no longer be deformed by rolling into 3.2mm diameter threads without crumbling.

STANDARD REFERENCE
ASTM D4318- standard test method for liquid limit, plastic limit and plasticity index of soils.

SIGNIFICANCE
The Swedish soil scientist Albert Atterberg originally defined seven ‘Limits of consistency’ to classify fine-grained soils, but in current engineering practice only two of the limits, the liquid and plastic limits are commonly used. (A third limit, called the shrinkage limit is used occasionally). The Atterberg limits are based on the moisture content of the soil.

The plastic limit is the moisture content that defines where the soil changes from a semi-solid to a plastic (flexible) state. The liquid limit is the moisture content that defines where the soil changes from a plastic to a viscous fluid state. The shrinkage limit is the moisture content that defines where the soil volume will not reduce further if the moisture content is reduced.

The original liquid limit test of Atterberg involved mixing a part of clay in a round-bottom porcelain bowl of 10-12cm diameter. A groove was cut through the pat of clay with a spatula and bowl then struck many times against the palm of one hand.

Casagrande subsequently standardized the apparatus and the procedures to make the measurement more repeatable. Soil is placed into the metal cup portion of the device and a groove is made down its centre with a standardized tool of 13.5mm (0.53in) width. The cup is repeatedly dropped 10mm onto a hard rubber base at a rate of 120 blows per minute during which the groove closes up gradually as a result of the impact. The number of blows for the groove to close is recorded. The moisture content at which it takes 25 drops of the cup to cause the groove to close over a distance of 13.5mm (0.53in) is defined as the Liquid Limit. The test is normally run at several moisture content which requires 25blows to close the groove is interpolated from the test results. The liquid limit test is defined by ASTM standard test method D4318. The test method also allow running the test at one moisture content where 20 to 30 blows are required to close the groove. Then a correction factor is applied to obtain the liquid from the moisture content.

The materials needed to do a liquid limit test are as follows;
• Casagrande cup (liquid limit device)
• Grooving tool
• Soil pat before test
• Soil pat after test

IMPORTANCE OF LIQUID LIMIT TEST
The importance of the liquid limit test is to classify soils. Different soils have varying liquid limits. Also, one must use the plastic limit to determine the plasticity index.

DERIVED LIMITS
The values of these limits are used in a number of ways. There is also a close relationship between the limits and properties of a soil such as compressibility, permeability and strength. This is thought to be very useful because as limit determination is relatively simple, it is more difficult to determine these other properties. Thus the Atterberg limits are not only used to identify the soils classification, but it allows for the use of empirical correlations for some other engineering properties.

PLASTICITY INDEX
Plasticity index is a measure of the plasticity of the soil. The plasticity index is the size of the range of water content where the soil exhibits plastic properties. The plasticity index is the difference between the liquid limit and the plastic limit (PI = LL – PL). Soils with high plasticity index tend to be clay, those with a lower plasticity index tend to be silt and those with a plasticity index of 0 (non-plastic) tend to have little or no silt or clay.

 

PLASTICITY INDEX AND THEIR MEANINGS
• 0 – 3 = Non-plastic
• 3 – 15 = Slightly plastic
• 15- 30 = Medium plastic
• > 30 = highly plastic.

LIQUIDITY INDEX
The liquidity index (L.I) is used for scaling the natural water content of a soil sample to the limits. It can be calculated as a ratio of difference between natural water content to plastic limit: L.I = (W – P.L)/ (L.L – P.L). Where W is the natural water content.

 

 

APPARATUS

1. Sand cone apparatus which consist of a one gallon plastic bottle with a metal cone attached to it.
2. Balance sensitive to 1 gram.

• One gallon plastic can with cap.

 

1. Tools for excavating a hole in the ground like chisel, hammer etc.
2. Metal tray with a hole in the centre
3. Plastic air tight bag for carrying wet excavated soil from field to the lab.

• Scale (weight balance).

 

PR0CEDURES

The following procedures were carried out;

1. Go to the field were the soil’s unit weight is to be measured, place the metal tray and fasten the four (4) screws. (See Fig A. below).
2. Dig up to 10 to 15 cm deep.

• As you are digging the hole, put the retrieved soil into the plastic bag in other that the soil does not loose moisture. All of the soil including the soil at the bottom of the hole is poured into the bag as well.(See Fig. B below)

1. Having the valve closed, turn the gallon + cone upside down and place the cone in the centre hole of tray and open the valve so that sand flows down to the hole.(See Fig. C below)
2. After the flow of sand stops, close the valve and pick the assembly up, the sand in the cone will be poured into the tray. This sand will be left there in the field.

NOTE; The plastic bag should be kept close while transferring it to the lab to avoid moisture loss and consequently weight of the soil.

1. Measure the weight of soil (laterite).

• Measure the weight of sand in cone.
• Measure the weight of sand after pouring.

1. Measure the weight of sand before pouring.
2. Pour the soil (laterite) into the speedy moisture tester (SMT) and add SMT reagent to find the moisture content.

 

 

CALCULATIONS

The following parameters were obtained from an experiment carried out, a case study of Gilmor Engineering (Nigeria) Limited.

Location Layer Soil type Sand bottle No.   Wt. of wet soil from whole W gm Wt. of sand in cone w7 Wt. of sand before pouring w5gm Wt. of sand after pouring. W6 gm Wt. of sand in hole + cone. W5 – W6 gm Wt. of sand in hole. W8 = w5 – W6 ­­- W7gm Wet density Dw = WXS/W8 g/ cm3 Mean Moisture content % Dry density Dsg/cm3 Ds = 100 Dw 100 +m Laboratory compaction details O.M.C Compaction M.D.D % Compaction

Culvert 2nd Layer Bank filling Laterite CL 10   2711 420 3000 874 2126 1706 2.05 11.0 1.85     14.0 1.85 100%