CIVIL ENGINEERING DEPARTMENT CC304 – GEOTECHNICAL 1
NO. PARTICAL WORK
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TITLE OF PARTICAL WORK
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OBJECTIVE
ATTERBERG LIMITS LIMITS
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These tests are conducted to obtain the liquid and plastic limits of a soil sample for the purpose of identification and classification of the soil. The determination of these limits is also used to predict the sheer strength and settlement of soil.
THEORY
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The engineering behaviors of fine-grained soil depend on factors other than particle size distribution. It is influenced primarily by their mineral and structural composition and the amount of water they contain, which is referred to as water content (or moisture content). The liquid and plastic limits tests characterize the effects of water content on fine-grained soils and help to classify fine-grained soils and to assess their mineral composition and engineering properties. Water Content : Soils are made of solid particles with voids between. These voids are generally filled with air and water. The water content w of a soil is o
W w w x 100 (%) W s Where Ww is the weight of water removed from the soil by oven drying at 105 o to 110 o C and W s is the weight of the dried soil. A soil is considered to be dry when its mass does not change by oven drying, which may usually require about 12 to 24 h. w=
Liquid and Plastic Limits The mechanical properties of a clay are altered by changing the water content. A clay softens when water is added, and with sufficient water, forms a slurry that behaves as a viscous liquid; this is known as the liquid state. If the water content is gradually reduced by drying it slowly, the clay eventually begins to hold together and to offer some resistance to deformation; this is the plastic state. With further loss of water, the clay shrink and its stiffness increases until it becomes brittle; this is the semisolid state. As dring continues, the clay continues to shrink until it reaches a constant minimum volume. Beyond that point, further drying causes no further decrease in volume; this is the soild state. These four states are shown in figure 1. The change from one state to the next is not abrupt, but gradual.
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CIVIL ENGINEERING DEPARTMENT CC304 – GEOTECHNICAL 1
These smooth transitions are empirically defined by introducing the liquid limit LL , plastix limit PL , and shrinkage limit SL . The moisture content between PL and LL is the plasticity index PI : PI =LL – PL PI is a measure of the plasticity of a clay. The liquid and plastic limit tests provide a means of measuring and describing the plasticity range of clay soils. Liquid and plastic limits are also referred to as Atterberg limits , after the Swedish Scientist A. Atterberg, who first defined them for the classification of agricultural soils in 1911. Originally, the limits were determined by simple tests using an evaporating dish (Bauer, 1959) The procedures were defined more precisely for engineering purposes by Casagrande (1932)The mechanical device he designed for determining the liquid limit is still known as the Casagrande apparatus (Casagrande, 1958). Equipment
The equipment for determination of liquid and plastic limits includes:
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i. ii. iii. iv. v. vi.
Casagrande Apparatus Moisture Content can No. 36 Sieve (0.425mm) Porcelein dish Glass plate Distilled water
PROCEDURE :
This experiment is divided into two parts. 2
CIVIL ENGINEERING DEPARTMENT CC304 – GEOTECHNICAL 1
A. Determination of Liquid Limit 1. Prepare about 250g of soil passing through 0.425 sieve. 2. Check the drop of the casagrande bowl using the spacer gage on the grooving tool handle. This 10mm thick steel block should just pass between cup and base when the cup is at its maximum height. Tighten the locknut after adjustment and recheck the maximum height with the gage. 3. The soil used for Atterberg limits should not be dried prior to testing but may be air-dried if necessary. Mix the soil sample with distilled water on a glass plate by using a spatula until the mixture is uniform and behaves as a soft paste that can be shaped with a spatula. Separate approximately 30g soil mixture and place in a closed container for liquid limit determination. 4. Put some of the soil-water mixture into the casagrande bowl and level the surface using a spatula. Using a grooving tool, cut through the sample from back to front dividing it into two equal halves. The grooving tool must be kept normal to the cup and its chamfered edge in the direction of movement. The tip of the tool should scrape the bowl lightly. The completed groove must be clean and sharp. 5. Turn the crank handle at a steady rate of two revolutions per second so that the bowl is lifted and dropped. Continue turning until the two halves of the soil pat come in contact at the bottom of the groove along a distance of 13mm, as shown in Figure 2. During the test, the soil should slump and flow plastically in the bowl. Record the number of blows required to close the groove. 6. Take about 10g soil from the bowl to determine the moisture content. The extra soil is removed and placed into a porcelain dish. The casagrande bowl must be cleaned. 7. Mix the extra soil with more distilled water. Steps 3 to 6 are to be repeated at least 4 times to obtain a specimen between 50 to 10 blows. 8. Plot a graph, moisture content vs no. of blows on a semilog graph that produces a straight line and determine the liquid limit at 25 blows Reporting of results: The liquid limit shall be expressed to the nearest whole number. The percentage of material passing the 425µm BS test sieve shall be noted. The history of the sample shall also be noted, i.e. natural state, air dried, unknown. The method used to obtain the result shall also be stated, i.e. one point method using the Casagrande apparatus.
B. Determination of Plastic Limit 1. Divide the soil sample separated from procedure A to 4 samples and shape them into small balls (1-2 cm diameter). 2. Form the balls into threads by rolling them under your fingers against the glass plate surface. Use just enough pressure to roll the soil into a thread 3mm in diameter. If the thread 3
CIVIL ENGINEERING DEPARTMENT CC304 – GEOTECHNICAL 1
gets smaller than 3mm without crumbling, fold and knead the thread into a ball again and repeat the rolling process. Knead and roll the soil thread until it has dried to the point of crumbling and breaking into numerous pieces about 3 to 9mm in length when the thread diameter reaches approximately 3mm. 3. As soon as the soil thread crumbles, collect part of it and determine its water content. Repeat step 2 for the rest of the soil sample and check that two successive rums give approximately the same plastic limit. Reporting of result: The average of the moisture contents determined shall be taken as plastic limit (PL) of the soil and shall be expressed to the nearest whole number. If the two results differ by more than 0.5% moisture content, the test shall be repeated. The percentage of material passing the 425µm BS test sieve shall be noted. The history of the sample shall also be noted.
C. Determination of the plasticity index The plasticity index can be calculated from the equation: PI = LL - PL RESULT :
Use Form 2.D of the BS 1377 Plot the Moisture content vs number of blows on the semi log graph attached.
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CIVIL ENGINEERING DEPARTMENT CC304 – GEOTECHNICAL 1
JKA Laboratory area, Politechnic Kota Kinabalu
A
24 E
5
B
C
31
D
22
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CIVIL ENGINEERING DEPARTMENT CC304 – GEOTECHNICAL 1
CONCLUSION :
REFERENCE(S) 1.
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BS 1377-2:1990 – Page
9 – 14, Form 2.D at page 53.
