DETERMINATION OF ATTERBERG LIMITS

Objective

To determine the Atterberg Limits/Consistency Limits and classify the soil from the plasticity chart.

Need and scope

Consistency limits (LL and PL) are significant to understand the stress history and general properties of the soil met with construction. An estimate of Plasticity Index (LL-PL) is important to classify the soils particularly in highly expansive clays.

Apparatus Required

·        Balance 

·        Liquid limit device (Casagrande’s liquid limit device)  

·        Grooving tool 

·        Mixing dishes 

·        Spatula

·        Electrical Oven  

·        TxDOT recommended Plastic limit device (for this session).

a)       DETERMINATION OF LIQUID LIMIT

Procedure

1.  Determine the mass of each of the three moisture cans (W1).

2.  Make sure to calibrate the drop of the cup using the other edge of the grooving tool so that there is a consistency in height of drop.

3.  Put about 250 g of air dried soil passing # 40 into an evaporating dish and add a little water with a plastic squeeze bottle to barely form a paste like consistency.

4.  Place the soil in the Casagrande’s cup and using a spatula, smoothen the surface so that the maximum depth is about 8mm.

5.  Using the grooving tool, cut a grove at the centre line of the soil pat.

6.   Crank the device at a rate of 2 revolutions per second until there is a clear visible closure of 1/2” or 12.7 mm in the soil pat placed in the cup. Count the number of       

      blows (N) that caused the closure (make the paste so that N begins with a value higher than 35).

7.   If N ~ 20 to 40, collect the sample from the closed part of the pat using a spatula and determine the water content weighing the weight of the can + moist soil (W2). If the soil is too dry, N will be higher and reduces as water is being added.

8.    Additional soil shouldn’t be added to make the soil dry, expose the mix to a fan or dry it by continuously mixing it with the spatula.

9.   CLEAN THE CUP AFTER EACH TRIAL, obtain a minimum of three trials with values of N ~ 20 to 40.

10.  Determine the corresponding w% after 24 hrs and plot the N vs w%, called the “flow curve”.

Calculations

Water content, w% = 

Flow Index, F_I (slope of the flow curve) =  

b)      PLASTIC LIMIT TEST (TxDOT Procedure)

1.      Take approximately 20 g of dry soil and mix some amount of water from the plastic squeeze bottle.

2.      Determine the weight of empty moisture can, (W1).

3.      Prepare several small, ellipsoidal rolls of soil and place them in the plastic limit device. Place two fresh sheets of filter papers on either faces of the plates.

4.      Roll the upper half of the device which has a calibrated opening of 3.18 mm with the lower half plate.

5.      If the soil gets crumbled forming a thread of about the size of the opening between the plates, collect the crumbled sample, weigh it in the moisture can (W2) for       

       water content determination. Otherwise repeat the test with the same soil but drying it by squeezing between your palms.

6.      Determine the weight of the dry soil + moisture can, (W3).

7.      The water content obtained is the plastic limit.

Calculations

Plastic limit, PL = 

Plasticity Index, PI = LL-PL.

Photos

LIQUID LIMIT

Obtain 250 g of soil passing # 40 - step 3

Obtain 250 g of soil passing # 40 - step 3

Cassagrande's cup and TxDOT plastic lomit device

Calibrate the fall - step 2

Add water - step 3

Step 3

Fill the cup with soil - Step 4

Cut the Grove using the groving tool - step 5

Cut the Grove using the groving tool - step 5

Visible Closure of 12.7 mm - step 6

Collection of sample across the closed part - step 7

Sample for w% determination - step 7

Sample for w% determination - step 7

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Obtain 250 g of soil passing # 40 - step 3

 

PLASTIC LIMIT

TxDOT recommended plastic limit device - step 1

Ellipsoidal soil mass - step 3

Slide the halves of the device - step 4

Repeat the test after drying the soil - step 4

Obtain crumbles of app. 3 mm dia - stpe 5

Sample for w% determination - step 6

Sample for w% determination - step 6

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TxDOT recommended plastic limit device - step 1