To determine the plastic limit of cohesive soil sample by Glass Plate Method.

Object: To determine the plastic limit of cohesive soil sample by Glass Plate Method.

Standard:

AASHTOT91-86

ASTMD424-82

BS1377:part4

Apparatus:

• Glass plate
  
• Moisture containers
  
• Spatula
  
• Evaporating dish
  
• Electronic balance sensitive to 0.01gm
  
• Thermostatically controlled oven
  
• No. 40 sieve
  
• Distilled water
  

Theory:

• Plastic limit of the cohesive soil is the water content at the point of transition from semi- solid state to plastic state. It may also be defined as the water content at which the soil, when rolled into threads of 3mm diameter, starts crumbling.
  
• The plastic limit test is performed by repeated rolling of an ellipsoidal soil mass with palm of hand on the glass plate.
  
• Plastic limit is also the measure of shear strength of the soil. The shear strength of a soil at the plastic limit is a measure of the toughness of the clay.
  
• Plasticity index (PI) is the numerical difference of liquid limit and plastic limit. Plasticity index (PI) = liquid limit – plastic limit
  

Procedure:

1. Take about 200g of cohesive soil sample and pass it through No:40 sieve, mix it thoroughly with distilled water in the evaporation dish so that uniform soil paste is formed.
   
2. Take some portion of soil paste and prepare ellipsoidal shaped soil mass by squeezing the soil with fingers.
   
3. Roll the ellipsoidal soil mass on the glass plate using palm of the hand to make the soil threads. Rolling should be done at the rate of 80 strokes per minute.
   
4. When the thread being rolled reaches 3mm diameter then break up into several pieces and again form ellipsoidal mass.
   
5. Repeat the above step until thread starts crumbling when it reaches the diameter of
   

3mm.

1. Collect the small crumbled pieces into the moisture container and determine its water content by oven drying method. This water content is the plastic limit of the soil sample.
   
2. Repeat the procedure with the same soil sample or ellipsoidal soil mass and calculate the average value of water content or plastic limit of the soil sample.
   

Observations and calculations:

Results:

The plastic limit of the given soil sample is.

Comments: