Unconfined Compressive Strength test for rocks

Determining the Unconfined Compressive Strength of Rock Materials

Unconfined Compressive Strength test for rocks 

Objective

The aim of this test is to measure the unconfined compressive strength (UCS) of rock materials. This test helps classify rocks based on their strength.

Standard used 

IS 9143:1979

Apparatus Used

1. Loading Machine

Capacity: 5000 kN and shall be capable of applying load at a rate conforming to the requirements.

Least Count/Accuracy: 2.5 N

2. Vernier Caliper

Capacity: 30 cm

Least Count/Accuracy: 0.1 mm

Procedure

1. Specimen Preparation as per IS 9179:1979

UCS sample of sand stone

• Test the rock specimen at a moisture content as close to field conditions as possible.
• The length-to-diameter (L/D) ratio of the specimen should preferably be between 2 and 3. If the L/D ratio is less than 2, apply a correction to the results.
• The diameter of the specimen should be at least 10 times the diameter of the largest grain size. A preferred diameter is 45 mm, but it should not be less than 35 mm.
• Ensure the cylindrical surface of the specimen is smooth and free from abrupt irregularities.

2. Measuring Specimen Dimensions:

• Measure the diameter of the specimen to the nearest 0.1 mm.
• Measure the diameter at three points: upper height, mid-height, and lower height, ensuring the measurements are taken at right angles to each other.

3. Testing:

UCS testing machine 

• Clean the surfaces of the two bearing discs and the specimen.
• Place the specimen on the lower disc of the loading machine.
• Align the specimen so that its axis is centered with the loading machine's thrust center.
• Apply the load continuously at a constant stress rate between 0.5 MPa/s and 1.0 MPa/s.
• Record the maximum load at failure with an accuracy of 1%.

4. Number of Specimens:

• At least 5 specimens are required to obtain a representative average value.

Calculation

• To determine the UCS, use the formula:
• UCS= Maximum Load / Average Cross-Sectional Area
• Where: Maximum Load is the highest load the specimen can support before failing.
• The average Cross-Sectional Area is calculated from the diameter measurements.

Reporting

• Report the UCS value for each specimen in the sample.
• Provide the average UCS value for the sample.
• Express the UCS values to three significant figures.

Reference: IS 9143:1979, IS 9179:1979 and ISRM.