Atterberg limits

Atterberg limit tests: they are designed to give an idea or index, how the soil will act under certain conditions.

Bentonite clay has been added to increase the plasticity of the soil.

LL test

Liquid Limit is defined by the moisture content, in which a soil begins to act less as a viscous fluid and act more like a plastic putty.

By convention: this is defined as the water content at which a groove cut into a soil pat the casagrande device requires 25 blows to close distance of 13mm

PL test

plastic limit is defined as the moisture content at which a material begins to act less like a plastic putty and more like a brutal solid.

LL test procedure

• If the No. of blows falls between 15&20 :
  Scoop out some of the soil in place and one of the pre weight air cans and record the new weight for moisture content determination, place the can in the oven and reweight in 24 hours.

• If the No. of blows falls below 15 :

Mix the soil to dry it out, clean & dry the cup, repeat the test.

• If the No. of blows falls above 20:

Add moisture to the soil, mix it, clean and dry

The cup and retest.

• REPEAT the procedure to get test between :
  20 & 25

25 & 30

30 & 35

Plot the water content Vs. the blow counts and determine the liquid limit from the flow chart with moisture content plotted in the vertical axes and the blow count plotted in the Horizontal axes in log scale a linear gression can be established through the data and the liquid limit can be interpolated .

The plastic limit is the average of the moisture content when the soil starts to crumble when it is being rolled into a 3 mm fred .  

The method that has proved most suitable for engineering purpose was taken over from agronomy. It’s known as Atterberg’s method, and the water contents that correspond to the boundaries between the states of consistency are called the Atterberg Limits (Atterberg 1911)

The liquid limit Lw is the water content in per cent of the dry weight at which two sections of a pat of soil having the dimensions shown in Fig. 7.1 barely touch each other but do not flow together when subjected in a cup to the impact of sharp blows from below.

The personal equation has an importance influence on the test results.

The Plastic limit Pw or lower limit of the plastic state is the water content at which the soil begins to crumble when rolled out into thin threads.

The record of the results of the plastic-limit test should also contain a statement as to whether the threads, immediately before crumbling, were very tough like those of a gumbo, moderately tough like those of an average glacial clay, or weak and spongy like those of an organic or of a micaceous inorganic soil.

The Shrinkage limit Sw or lower limit of volume change is the water content below which further loss of water by evaporation does not result in reduction of volume. As soon as the soil passes below the shrinkage limit, it becomes slightly lighter in color.

The range of water content within which a soil posses plasticity is known as the Plastic range, and the numerical difference between the liquid limit and the plastic limit is the Plasticity Index I .

As a water content w of a cohesive soil approaches the lower limit P of the plastic range, the stiffness and degree of compaction of the soil increase.

The ratio,

                              Ii=-PwLw-Pw=-PwIw      

Is called the liquidity of the soil. If the water content of a natural soil stratum is greater than the liquid limit (Liquidity index greater than 1.0), remodeling transforms the soil into a thick viscous slurry. If the natural water content is less than the plastic limit (liquidity index negative), the soil cannot be remolded.

The unconfined compressive strength of undisturbed clays with a liquidity index near unity commonly ranges between 0.3 and 1.0 kg/cm2 .

If the liquidity index is near zero, the compressive strength generally lies between 1 and 5 kg/cm2

The plasticity of a soil is its ability to undergo deformation without cracking or fracturing.

plastic soil can be moulded into various shapes when it is wet. Plasticity is an important index property of fine-grained soils, especially clayey soils.

The presence of adsorbed water is necessary to impart plasticity characteristics to a soil.

The soil does not become plastic when it is mixed with non-polarizing liquid, such as kerosene or paraffin oil. These liquids do not have electromagnetic properties to react with clay minerals.

The soil becomes plastic only when it has clay minerals. If the soils contains only non-clay minerals, such as quartz, it would not become plastic whatever may be the fineness of soil. When such soils are ground to very fine size , these cannot be rolled into threads.

Rock flour which contains very fine particles of non-clay particles does not become plastic.

Consistency Limits :

The consistency of a fine grained soil is the physical state in which it exists. It is used to denote the degree of firmness of a soil.

Consistency of a soil is indicated by terms such as soft, firm or hard.

In 1911, a swedish agriculture engineer Atterberg mentioned that a fine-grained soil can exist in four states, namely, liquid, plastic, semi-solid or solid state. The water contents at which the soil changes drme one state to the other are known as consistency limits or Atterberg’s limits.

The water content alone is not an adequate index property of soil. At the same water content, one soil may be relatively soft, whereas another soil may be hard. However, the soil with the same consistency limits behave somewhat in similar manner. Thus consistency limits are very important index properties of fine-grained soil.

A soil containing high water content is in a liquid state. It offers no shearing resistance and can flow like liquids. It has no resistance to shear deformation and, therefore, has shear strength equal to zero. As the water content is reduced, the soil becomes stiffer and starts developing resistance to shear deformation. At some water content, the soil becomes plastic . the water content at which the soil changes from the liquid state to the plastic state is known as liquid limit(LL, 1). In other words, the liquid limit is the water content at which the soil ceases to be liquid.

The soil in the plastic state can be moulded into various shapes. As the water content is reduced, the plasticity of the soil decreases. Ultimately, the soil passes from the plastic state to the semi-solid state when it stops behaving as a plastic. It crackes when it moulded. The water content at which the soil becomes semi-solid is known as the plastic limit (PL.1). In other words, the plastic limit is the water content at which the soil just fails to behave plastically.

The difference between the liquid limit and the plastic limit is known as plasticity index(PI,I1). The soil remains plastic when the water content is between the liquid limit and the plastic limit.

The plasticity index is an important index property of fine-grained soils.

When the water content is reduced below the plastic limit, the soil attains a semi-solid state. The soil cracks when moulded. In the semi-solid state, the volume of the sil decrease with a decrease in the water content till a stage is reached when further reduction of the water content does not cause any reduction in the volume of the soil. The soil is said to have reached a solid state. (In solids, no appreciable change in volume is observed with a change in water content). The water content  as which the soil changes from the semi-solid state to the solid state is known as the shrinkage limit.

Below the shrinkage limit, the soil does not remain saturated . Air enters the voids of the soil. However, because of capillary tension developed, the volume of the soil does not change. Thus, the shrinkage limit may also be defined as the lowest water content at which the soil is fully saturated .

Fig. 4.1 shows sudden changes in the states of the the soil at different consistency limits. Actually, the transition between different states is gradual. The consistency limits are determined rather arbitrarily, as explained in the following sections.

[NOTE: in liquid state, the soil is like soup ; in plastic state, like soft butter ; in semi-solid state, like cheese ; and in solid state like hard candy ]

AT THE LIQUID LIMIT, the liquid is practically like a liquid, but possesses a small shearing strength. The shearing strength is the smallest value that can be measured in the laboratory.

THE LIQUID LIMIT of a soil depends upon the clay minerals present. The stronger the surface charge and the thinner  the practise, the greater will be the amount of adsorbed water, and therefore, the higher will be the liquid limit.

THE LIQUID LIMIT is determined in the laboratory either by Casagrande’s apparatus or by Cone penetration method.

THE CASAGRANDE TOOL is recommended  for normal fine-grained soils, whereas the ASTM tool is recommended for sandy fine grained soil, in which the Casagrande tool tends to tear the soil in the groove.

Reference 

• Atterberg Limit Tests (LL and PL) -Uploaded by ronaluna - Youtube .

• Soil Mechanics - Principle and Practise
• Introductory soil mechanics  
• Soil Mechanics - R. F. Craig
• Soil Mechanics in Engineering Practice - Karl Terzaghi, Ralph B. Peck