lab Reports# Determination of Atterberg's Limit (P.L &L.L) of soil sample

 

TITLE: DETERMINATION OF ATTERBERG'S LIMIT OF SOIL

OBJECTIVES:

• To determine the consistency limits of soil ie liquid limit & plastic limit.
• To determine the plasticity index of soil.

APPARATUS REQUIRED:

(A) Liquid limit

1. CassaGrande's Apparatus
2. 0.425 mm sieve
3. Grooving tool
4. Spatula
5. moisture cans
6. oven
7. scale
8. weight balance

( B) Plastic Limit

1. Glass plate
2. 0.425 mm sieve
3. oven
4. moisture can or crucibles
5. metal rod 3 mm
6. Balance
7. Spatula

MATERIALS REQUIRED:

1.Soil sample

THEORY:

Atterberg's Limit or Consistency Limit:

• The water content at which the phase change of soil occurs, are called the consistency limits.
• Consistency is the relative ease with which soil can be deformed. it is used mostly for clays.
• Attreburg's limits are basic measure of critical water content of a fine grained soil.
• Attreburg's limits or consistency limits represent the water content at which the soil changes from one state to another state of soil.
•  Depending upon the water content the soil mass has 4 stages
•    Below SL : Solid
•  In between SL and PL : Semi-soli
•  In between PL and LL : Plastic
•   In LL and above : Liquid

• Three limits of moisture at which fine soil changes its behavior is termed as consistency limit. The limits are defined by Atterberg, so also termed as Atterberg limit.

•            These limits are

•            Liquid Limit (LL)

•            Plastic Limit (PL)

•            Shrinkage Limit (SL)

(A) Liquid limit:

• The liquid limit is the water content at which soil changes from liquid state to  plastic state. It is the minimum water content at which the soil is still in liquid state but has a small shearing strength against flowing.

• The liquid limit is determined in the laboratory either by Casagrande's apparatus or  by cone penetration method.

• It can also be defined as, water content at which a groove of 2mm closes by 25 no. of blows in the casagrande's apparatus.

• From the curve, it is the water content corresponding to 25 blows.

(B) Plastic Limit:

• Plastic limit is the water content below which the soil stops behaving as a plastic material.

• At this water content, the soil loses its plasticity and passes to a semi-solid state.

• It is the minimum water content at which a soil just begins to crumble when   rolled into a thread of 3 mm in diameter.

Plasticity Index:

• Soil plasticity refers to the ability of soil to undergo deformation without cracking or breaking.
• It's a measure how easily soil can change shape under stress and then retain that shape when the stress is removed.
• The plasticity index (PI) is a measure of the plasticity of soil and is determined from the Atterburg's limits tests. These tests assess the fine-grained fraction of soil, particularly clay, and help characterize its behavior under different moisture content.
• PI=LL-PL
• Based on plasticity and soil types are as follows:

Plasticity Index (PI)	State of soil
PI = 0	Non-plastic soil
PI < 7	Low plastic soil
7 < PI < 17	Medium plastic
PI > 17	High plastic soil

  

Flow Curve and Flow Index:

• The flow curve can be obtained by plotting the water content with the corresponding number of blogs on semi-log graph paper.
• The liquid limit of the soil sample can be obtained from this figure.

• 

• Flow index is the slope of the curve obtained by flow curve as:
• Flow Index,

           If=(W1-W2)/log(N2/N1)

               Where, W1=water content corresponding to number of blows N1

                            W2=water content corresponding to number of blows N2

The lab procedure is explained below.

PROCEDURES:

(A)  Liquid Limit:

Ø   About  250  gm of the dried sample passing through 425 𝜇 IS sieve  was  taken in a dish and mixed with distilled water to form a uniform paste.

Ø   A portion of this paste  was  placed in the cup of the liquid limit device, and the surface was smoothened  and leveled with a spatula.

Ø   A groove was cut through the sample along the symmetrical axis of the cup, preferably in one stroke, using a standard grooving tool.

Ø   Then the handle was turned at a rate of 2 revolutions per second until the two parts of the soil sample      came into contact at the bottom of the groove along a distance of 12 mm.

Ø     T he number of blows was noted.

Ø   About 15 gm of soil near the closed groove  was  taken for water content determination.

Ø   The water content of the soil  was altered and the  process  was  repeated.

Ø   The water content was adjusted in such a way that the number of blows to close the groove may fall within the range of 5 to 40 blows and carry out at least four tests.

Ø   A plot was made between the water content as ordinate and the number of blows on the log scale as abscissa.

Ø   The plot  was  approximately a straight line which is referred  to  as  a  flow curve.

Ø   From this plot, the liquid limit  was  obtained as water content corresponding to 25 blows.

( B ) Plastic Limit: 

•  About 30 gm of an air-dried sample passing through 425 𝜇 IS sieve was taken in a dish and it was mixed thoroughly with distilled water till it became plastic and can       be easily molded with fingers.
•  About 10 gm of the plastic soil mass  was  taken in one hand and a ball  was  formed.

• The ball was  rolled with fingers on a glass plate to form a soil thread of uniform diameter. The rate of rolling is kept  at  about 80 to 90 strokes per minute.

•   If the diameter of the thread becomes smaller than 3 mm, without crack formation, it shows that the water content is more than the plastic limit.
•  The soil  was  kneaded further.

•  The soil was re-rolled  and the procedure  was  repeated until the thread crumbles.

• The water content at which the soil can be rolled into a thread of approximately 3 mm in dia. without crumbling is known as a plastic limit.
• The test was repeated, taking a fresh sample each time. The plastic limit was taken as the average of three values.
• The pieces of the crumbled thread were collected for moisture determination.

OBSERVATIONS AND CALCULATIONS:

Details of the sample..........

Natural moisture content=0% (dry sample was taken)

Room temperature = 23 degrees cel.

 (A) Liquid Limit:

Observation Table:

Determination Number	1	2	3
Container number	S-8	S1-16	S3-18
Weight of container, gm (W1)	22.5	9.8	22.2
Weight of container + wet soil, gm (W2)	46.1	26.5	47.7
Weight of container + dry soil, gm (W3)	41.7	23.2	43
Weight of water, gm Ww=W2-W3	4.4	3.3	4.7
Weight of solid, gm Ws=W3-W1	19.2	13.4	20.8
Water content, w=Ww/Ws*100%	22.916	24.526	22.596
no. of blows	22	16	26

• The relationship between water content (on y-axis) & number of blows (on x-axis) was plotted on semi-log graph.

• The curve obtained is called flow curve.

• The moisture content corresponding to 25 no. of blows was read from the represents liquid limit.

• It is usually expressed to the nearest whole number.

RESULT:

• Thus, from the above lab experiment the liquid limit (LL) of given soil sample was found to be LL=22.6% from graph corresponding to 25 blows.
• from flow curve ,let

               W1=26%  W2=22%   N1=10  N2=30 

• Flow index (FI)=(W1-W2)/log(N2/N1)-slope of the flow curve =(26-22)/log(30/100=8.387

(B) Plastic limit :

Observation table: 

Determination Number	1	2	3
Container number	S4-18	9S2-13	S5
Weight of container, gm (W1)	9.7	10.4	6.8
Weight of container + wet soil, gm (W2)	13.3	11.2	7.5
Weight of container + dry soil, gm (W3)	12.6	11.1	7.4
Weight of water, gm Ww=W2-W3	0.7	0.1	0.1
Weight of solid, gm Ws=W3-W1	2.9	0.7	0.6
Water content, w=Ww/Ws*100%	24.13	14.28	16.66

Result:

• Average Plastic limit=(w1+w2+w3)/3=(24.13+14.28+16.66)/3=18.356.%

• Plasticity Index(P.I)=L.L-P.L=22.6-18.356=4.243%

• Toughness index (TI)=Ip/If=4.243/8.387=0.506

CONCLUSION:

since , PI=4.243%, 

F.I=8.387

T.I=0.506

The classification of the soil is given below based on these properties:

-slightly plastic
-contains silt
-partly cohesive

PRECAUTIONS:

1. Cleanliness of all equipment should be ensured.
2. Soil samples should be handled carefully to prevent contamination.
3. Soil samples should be adequate to remove moisture.
4. Should use distilled water to avoid impurities.
5. Measurement of masses should be precise to minimize errors.
6. The no of Blows should read accurately with the joint of soil paste in the cup.
7. The crumble diameter should be near 3mm while collecting samples for water content.
8. All the equipment should be handled carefully.