To determine the liquid limit of cohesive soil by Fall Cone or Penetrometer Method.

Object: To determine the liquid limit of cohesive soil by Fall Cone or Penetrometer Method.

Standard:

AASHTOT90-86

ASTMD423-82

BS1377:part 4

Apparatus:

• Penetrometer
  
• Cylindrical mould (5cm dia, 5cm height)
  
• Spatula
  
• Moisture containers and evaporating dish
  
• Electronic balance sensitive to 0.01gm
  
• Thermostatically controlled oven
  
• No. 40 sieve
  
• Distilled water
  

Theory:

• Liquid limit of soil is the minimum water content at which soil flows under its own weight. It may also be defined as the water content at the point of transition from plastic state to liquid state.
  
• Liquid limit is the measure of the shear strength of soil and is widely used for soil identification and soil classification.
  
• In this method, a 30 cone of stainless-steel having weight of 80gm is made to penetrate the soil paste. The liquid limit of soil sample is the water content corresponding to 20mm penetration in 5 seconds. The graph is plotted between the water content (w%) and penetration (mm).
  

Procedure:

1. Take about 200gm of cohesive soil sample and pass it through No.40 sieve, mix it thoroughly with distilled water in the evaporating dish so that uniform soil paste is formed.
   
2. Fill the soil paste in the cylindrical mould so that it is completely filled with no voids. Level off the soil with spatula.
   
3. Place the cylindrical mould under the cone of penetrometer. The cone should be in the center of mould and just touching the soil paste. Set the needle of dial of penetrometer to zero.
   
4. The cone is made to penetrate the soil paste for 5 seconds. Note the reading of penetration in millimeters from the dial of penetrometer.
   
5. Take some soil from the penetrated portion in the mould and determine its water content by oven drying method.
   
6. Repeat the procedure by increasing the water in the soil paste to get the penetration ranging from 10-40mm.
   
7. Plot the graph between penetration (mm) on the x-axis and water content (w%) on the y- axis. The graph is approximately the straight line.
   
8. From this graph, note the water content corresponding to 20mm penetration. This is the liquid limit of the soil sample.
   

Observations and calculations:

Results:

The liquid limit of given soil sample is.

Comments: