UNCONFINED COMPRESSION TEST

OBJECTIVE
 determine shear parameters of cohesive soil

Need and Scope of the Experiment

It is not always possible to conduct the bearing capacity test in the field. Some times it is cheaper to take the undisturbed soil sample and test its strength in the laboratory. Also to choose the best material for the embankment, one has to conduct strength tests on the samples selected. Under these conditions it is easy to perform the unconfined compression test on undisturbed and remoulded soil sample. Now we will investigate experimentally the strength of a given soil sample.

  Planning and Organization

We have to find out the diameter and length of the specimen.

  Equipment

  1. Loading frame of capacity of 2 t, with constant rate of movement. What is the least count of the dial gauge attached to the proving ring!
  2. Proving ring of 0.01 kg sensitivity for soft soils; 0.05 kg for stiff soils.
  3. Soil trimmer.
  4. Frictionless end plates of 75 mm diameter (Perspex plate with silicon grease coating).
  5. Evaporating dish (Aluminum container).
  6. Soil sample of 75 mm length.
  7. Dial gauge (0.01 mm accuracy).
  8. Balance of capacity 200 g and sensitivity to weigh 0.01 g.
  9. Oven, thermostatically controlled with interior of non-corroding material to maintain the temperature at the desired level. What is the range of the temperature used for drying the soil !
  10. Sample extractor and split sampler.
  11. Dial gauge (sensitivity 0.01mm).
  12. Vernier calipers

Experimental Procedure (Specimen)  

  1. In this test, a cylinder of soil without lateral support is tested to failure in simple compression, at a constant rate of strain. The compressive load per unit area required to fail the specimen as called Unconfined compressive strength of the soil.

  Preparation of specimen for testing

A.     Undisturbed specimen

    1. Note down the sample number, bore hole number and the depth at which the sample was taken.
    2. Remove the protective cover (paraffin wax) from the sampling tube.
    3. Place the sampling tube extractor and push the plunger till a small length of sample moves out.
    4. Trim the projected sample using a wire saw.
    5. Again push the plunger of the extractor till a 75 mm long sample comes out.
    6. Cutout this sample carefully and hold it on the split sampler so that it does not fall.
    7. Take about 10 to 15 g of soil from the tube for water content determination.
    8. Note the container number and take the net weight of the sample and the container.
    9. Measure the diameter at the top, middle, and the bottom of the sample and find the average and record the same.
    10. Measure the length of the sample and record.
    11. Find the weight of the sample and record.

  B.     Moulded sample

  1. For the desired water content and the dry density, calculate the weight of the dry soil Ws required for preparing a specimen of 3.8 cm diameter and 7.5 cm long.

                   

  1. Add required quantity of water Ww to this soil.

                          Ww = WS ´ W/100 gm

  1. Mix the soil thoroughly with water.
  2. Place the wet soil in a tight thick polythene bag in a humidity chamber and place the soil in a constant volume mould, having an internal height of 7.5 cm and internal diameter of 3.8 cm.
  3. After 24 hours take the soil from the humidity chamber and place the soil in a constant volume mould, having an internal height of 7.5 cm and internal diameter of 3.8 cm.
  4. Place the lubricated moulded with plungers in position in the load frame.
  5. Apply the compressive load till the specimen is compacted to a height of 7.5 cm.
  6. Eject the specimen from the constant volume mould.
  7. Record the correct height, weight and diameter of the specimen.

  Test procedure

  1. Take two frictionless bearing plates of 75 mm diameter.
  2. Place the specimen on the base plate of the load frame (sandwiched between the end plates).
  3. Place a hardened steel ball on the bearing plate.
  4. Adjust the center line of the specimen such that the proving ring and the steel ball are in the same line.
  5. Fix a dial gauge to measure the vertical compression of the specimen.
  6. Adjust the gear position on the load frame to give suitable vertical displacement.
  7. Start applying the load and record the readings of the proving ring dial and compression dial for every 5 mm compression.
  8. Continue loading till failure is complete.
  9. Draw the sketch of the failure pattern in the specimen.

Project :                                                 Tested by :     

Location :                                               Boring No. : 

Depth :

  Sample details

Type UD/R : soil description

Specific gravity (GS) 2.71                                                Bulk density

Water content                                                        Degree of saturation    .%

Diameter (Do) of the sample     cm                     Area of cross-section =    cm2

  Initial length (Lo) of the sample = 76 mm  

Elapsed     time   (minutes)  

       1

 Compression dial           reading (L)   (mm)    

           2

 Strain             L ´ 100/Lo       (%)     (e)

                3                  

 Area A           Ao /(1-e)    (cm)2  

     4

Proving  ring  reading (Divns.)      

5  

 

Axial  load  (kg)

  6  

 

Compressive stress

(kg/cm2)    

7  

 
     

 

 

        

Interpretation and Reporting

Unconfined compression strength of the soil = qu =

Shear strength of the soil = qu/2 =

Sensitivity = (qu for undisturbed sample)/ (qu for remoulded sample).

  General Remarks

Minimum three samples should be tested, correlation can be made between unconfined strength and field SPT value N. Upto 6% strain the readings may be taken at every ½ min (30 sec).