Introduction Experimental Setup Data Analysis Results Theory

Unconfined Compression (UC) Test

Need and Scope:

It is not always possible to conduct the bearing capacity test in the field. Sometimes 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 (UC) test on undisturbed or remoulded soil sample.

Concept:

An Unconfined compression test is also known as uniaxial compression tests, is special case of a triaxial test, where confining pressure is zero. UC test does not require the sophisticated triaxial setup and is simpler and quicker test to perform as compared to triaxial test. 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.

Unconfined Compression (UC) Test

Experimental Setup:

1. Loading frame of capacity of 2 t, with constant rate of movement.

2. Proving ring of 0.01 kg sensitivity for soft soils; 0.05 kg for stiff soils.

3. Frictionless end plates of 76 mm diameter (Perspex plate with silicon grease coating).

4. Dial gauge (0.01 mm accuracy)

5. Soil sample of 38 mm diameter & 76 mm length

Specimen Preparation:

__a)Undisturbed Specimen(UDS): __

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 (38 mm dia) extractor and push the plunger till a small length of sample moves out.

4. Trim the projected sample using a wire saw, and push the plunger until a 76 mm long sample comes out.

5. Cutout this sample carefully and hold it on the split sampler so that it does not fall.

6. Take about 10 to 15 g of soil from the tube for water content determination.

7. Note the container number and take the net weight of the sample and the container.

8. Measure the diameter at top, middle, and bottom of the sample. Find the average and record the same.

9. Measure the length and weight of the sample and record.

__ b) Remoulded Specimen(R): __

1. For the desired water content and the dry density, calculate the weight of the dry soil Ws required for preparing a specimen of 38 mm diameter and 76 mm long.

2. Add required quantity of water W_{w} to this soil.

W_{w} = W_{s} x W/100 gm

3. Mix the soil thoroughly with water.

4. Place the wet soil in a tight thick polythene bag in a humidity chamber.

5. After 24 hours take the soil from the humidity chamber and place the soil in a constant volume mould, having an internal height of 76 mm and

internal diameter of 38 mm.

6. Place the lubricated mould with plungers in position in the load frame.

7. Apply the compressive load till the specimen is compacted to a height of 76 mm.

8. Eject the specimen from the constant volume mould.

9. Record the correct height, weight and diameter of the specimen.

Experimental procedure (IS 2720 Part 10):

1. Take two frictionless bearing plates of 38 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. In UC test, the commonly used loading rate is 1.25 mm/min. For harder specimens 1.5 mm/min or 2.25 mm/min can also be used.

9. Continue loading till failure is complete, and then draw the sketch of the failure pattern in the specimen.

Unconfined Compression (UC) Test

Observation Sheet:

Sample no:_________________________Depth/Location:______________________

Weight of Sample:_____________________ In-situ density: _____________________

Diameter:_________________________ Area:__________________________

Initial Water Content:___________________ Deformation rate:__________________

Least count of dial gauge:_________________ Proving ring constant: _________________

Calculations:

1. Axial stress = (Proving ring reading x Proving ring constant) / A_{corr}

2. A_{corr}= A_{0}/(1-ε); A_{0} is initial cross-sectional area of the soil specimen, ε is the axial strain at that point of loading.

3. Maximum axial stress is obtained, which is also considered to be the failure point of the specimen.

4. Repeat the test 3 times. Find the average value of maximum axial stress obtained in all three UC tests.

5. Unconfined compression strength of the soil, q_{u} = average value of maximum axial stress of three tests

6. Shear strength of the soil (cohesion, c) = q_{u}/2

7. Sensitivity = (q_{u} for undisturbed sample)/ (q_{u} for remoulded sample).

Unconfined Compression (UC) Test

Graphs:

1. Axial stress versus Axial strain relationship

Example:

The UC test has been performed at loading rate of 1.25mm/min on specimen of 38 mm dia & 76 mm ht.

__Results:__

Unconfined Compressive Strength (q_{u}) = 267 kPa

Cohesion (c) = 133 kPa

Internal friction angle (Φ) = 0 deg

General Remarks

• Minimum three samples should be tested; correlation can be made between unconfined strength and field SPT value N.

• Up to 6% strain the readings may be taken at every 1/2 min (30 sec).

• UC test is recommended for cohesive soils, or which can stand without lateral support.

Unconfined Compression (UC) Test

Theory:

In UC test, a cylindrical soil specimen of standard size (normally 38 mm dia & 76 mm ht) is loaded axially monotonically under compression till failure at a constant strain rate / deformation rate in a loading frame. Since soil specimen is not kept laterally confined, it is termed as Unconfined Compression test. The axial/vertical compressive stress is the major principal stress and other two principal stresses are considered to be zero.

Since soil specimens are kept unconfined, these tests cannot be performed on soils which cannot stand without lateral support. These tests are performed only on cohesive soils in undisturbed or remoulded conditions. A loading frame with proving ring and a dial gauge to measure axial load and axial deformation, respectively. Maximum axial stress (compressive stress) causing failure is obtained from stress-strain plot and is defined as unconfined compressive strength of soil. When failure point on the curve is not defined, the stress corresponding to 20% axial strain is used to represent the strength values. During the test, No drainage of water from the sample takes place; and to maintain same water content of the sample, the test is run at a fast strain rate. Failure plane is not predetermined and takes place along the weakest plane.

Mohr circle of stresses at failure in unconfined compression test and loading conditions are given below: