Unconfined Compression Test
Geotechdata.info - Updated 19.04.2014

Unconfined Compression Test (UCT) is a simple laboratory testing method to assess the mechanical properties of rocks and fine-grained soils. It provides a measures of the undrained strength and the stress-strain characteristics of the rock or soil. The unconfined compression test is often included in the laboratory testing program of geotechnical investigations, specially when dealing with rocks.

The primary purpose of the Unconfined Compression Test is to quickly determine a measure of the unconfined compressive strength of rocks or fine-grained soils that possess sufficient cohesion to permit testing in the unconfined state. This measure is then used to calculate the unconsolidated undrained shear strength of the clay under unconfined conditions. In general, The UCT can be conducted on rock samples or on undisturbed, reconstituted or compacted cohesive soil sample.

In the unconfined compression test, the sample si placed in the loading machine between the lower and upper plates. Before starting the loading, the upper plate is adjucted to be in contact with the sample and the deformation is set as zero. The test then starts by appling a constant axial strain of about 0.5 to 2% per minute. The load and deformation values are recorded as needed for obtaining a reasonably complete load-deformation curve. The loading is continued until the load values decrease or remain constant with increasing strain, or until reaching 20% (sometimes 15%) axial strain. At this state, the samples is considred to be at failure. The sample is then removed for measurement of the water content.

As for the results, the axial stress is usually plotted versus the axial strain. The maximum axial stress, or the axial stress at 20% (sometimes 15%) axial strain if it occurs earleir, is reproted as the unconfined compressive strength σc. The undrained shear strength then reads

S_u= σ_c / 2

S_u : ndrained shear strength

σ_c : unconfined compressive strength

• BS 1377-7:1990
• ASTM D2166 - 06 Standard Test Method for Unconfined Compressive Strength of Cohesive Soil Link

  

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1. Principles of Unconfined Compression Test, Chap 7.2 of Experimental Soil Mechanics, Bardet, J. P., Experimental Soil Mechanics, 1997, Prentice-Hall, Upper Saddle River, New Jersey.
2. Detailed description of Unconfined Compression Test, Chap 7.3 of Experimental Soil Mechanics, Bardet, J. P., Experimental Soil Mechanics, 1997, Prentice-Hall, Upper Saddle River, New Jersey.
3. Detailed description of Unconfined Compression Test with photos and an example data, Engineering Properties of Soils Based on Laboratory Testing, Unconfined Compression (UC) Test, Prof. Krishna Reddy, UIC
   
4. Detailed description of Unconfined Compression Test with photos and example, University of Texas at Arlington, Geotechnical Engineering Laboratory, Unconfined Compressive Strength Test, Lecture Notes # 9

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