Saturday, September 19, 2020

Unconfined Compressive Test

Determination of unconfined compressive test of a soil sample

 Unconfined Compressive Test

The bearing capacity of soil should find out, but always should not possible to conduct the test in same field. Sometimes  the Unconfined compressive test is conducted to laboratory by taking the undisturbed soil sample from the field. Other hand, the Unconfined compressive test is conducted to choose the best material for the embankment. The process are enlisted below to easily perform the unconfined compression test on undisturbed and remolded soil sample and the detailed  investigate experimentally the strength of a given soil sample.

Unconfined compressive test title image


The main concept of Unconfined compressive test is, a cylindrical form of a soil sample  is tested to failure in simple compression, without lateral support, at a constant rate of strain. The Unconfined compressive strength is defined as The compressive load per unit area required to fail the specimen. 


Primarily We should find out the diameter and length of the specimen.


  • Loading frame of capacity of 2 t, with constant rate of movement. What is the least count of the dial 2.gauge attached to the proving ring.
  • Proving ring of 0.01 kg sensitivity for soft soils; 0.05 kg for stiff soils.

  • Frictionless end plates of 75 mm diameter (Perspex plate with silicon grease coating).

  • Soil trimmer.

  • Evaporating dish (Aluminum container).

  • Soil sample of 75 mm length.

  • Dial gauge (0.01 mm accuracy).

  • Balance of capacity 200 g and sensitivity to weigh 0.01 g.

  • 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.

  •  Sample extractor and split sampler.

  • Dial gauge (sensitivity 0.01mm).

  • Vernier calipers.


A. Undisturbed specimen -

  1. Record the specific sample number, bore hole number and the depth excavated at which the sample was taken for further reference.
  2. The sampling tube should be cleaned and remove the protective cover (paraffin wax).
  3. The sampling tube extractor are placed to the sampling tube and push the plunger till a small length of sample moves out.
  4. Carefully 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. There should be taken care to cut out this sample carefully and hold it properly 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 -

  • For the determination of Unconfined compressive test , calculate the weight of the dry soil (Ws) for the desired water content and the dry density, required for preparing a specimen of 3.8 cm diameter and 7.5 cm long.
  • Now the required quantity of water (Ww) is added to this soil, and mix it properly,
                          Ww =( WSxW)/100 gm

  • The wet soil is placed in a tight thick polythene bag in a humidity chamber.
  • After 24 hours the soil take out from the humidity chamber and the soil should be placed in a constant volume mould, having an internal height of 7.5 cm and internal diameter of 3.8 cm.
  • The lubricated moulded should be placed with plungers in position in the load frame.
  • The compressive load is applied till the specimen is compacted to a height of 7.5 cm.
  • Then the specimen should be ejected from the constant volume mould.
  • The correct height, weight and diameter of the specimen should be recorded in a proforma for calculation purpose.


  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 h) 5 mm compression.
  8. Continue loading till failure is complete.
  9. Draw the sketch of the failure pattern in the specimen.


Code followed:    IS : 2720 (part 10) : 1991

Type UD/R : soil description

Specific gravity (GS)=  2.71                                                Bulk density =

Water content = W                                                       Degree of saturation =    %

Diameter (Do) of the sample=  3.8cm                     Area of cross-section =    cm2

Initial length (Lo) of the sample = 76 mm  


Unconfined compressive test table image

Unconfined compression strength of the soil = qu =

Shear strength of the soil = qu/2 =

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

Information Civil

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