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Unconfined Compression Test

The unconfined compression test is a type of triaxial test in which the confining pressure is taken as zero. The test is onlybe conducted in clayey soil specimens which can stand without confinement because of this case. The test is generally performed on intact (non-fissured), saturated clay specimens. Although the test can be conducted in a triaxial test apparatus it is more convenient to perform it in an unconfined compression testing machine. There are two types of machines as described below:

1. Machine with a spring
In this type of testing machine a loaded spring is used. It consists of two metal cones which are fixed on horizontal loading plates supported on the vertical posts. The upper loading plate is fixed in position, whereas the lower plate can slide on the vertical posts. The soil specimen is placed between the two metal cones

When the handle is turned, the plate attached with the spring is lifted upward. This plate is attached to lower plate. So the latter plate is also lifted. When the handle is turned slowly, at a speed of about half a turn per second, a compressive force acts on the specimen. Eventually, the specimen fails in shear. The compressive load is proportional to the extension of the spring.

The strain in the soil specimen is given by a chart fixed on the machine. As the lower plate moves upward, the pen attached to this plate swings sideways. The lateral movement of the pen (in arc) is proportional to the strain in the specimen.

The chart plate is attached to the yoke. As the yoke moves upward when the handle is rotated, the chart plate moves upward. The pivot of the arm of the pen also moves upward with the lower plate. The vertical movement of the pen relative to the chart is equal to the extension of the spring and hence the compressive force. Thus the chart gives a plot between the deformation and the compressive force. Springs of different stiffness can be used depending upon the expected compressive strength of the specimen

2. Machine with a Proving Ring
In this type of the unconfined compression testing machine, a proving ring is used to measure the compressive force. There are two plates, having cone seating for the specimen. The specimen is usually placed on the bottom plate to make contact with the upper plate of the instrument. Then the dial gauge and proving ring is set to zero to start the test. The compressive load is then gradually applied to the soil specimen by turning the handle. As the handle is turned, the upper plate move downward and causes compression. In some machines, the upper plate is fixed and the compressive load is applied by raising the lower plate. The handle is turned gradually so as to produce an axial strain of ½% to2% per minute. The shearing is continued till the specimen fails or till 20% of the axial strain occurs, whichever is earlier.

The applied compressive force can be determined from the proving ring reading and the axial strain can be found out from the dial gauge reading.

Presentation of results
In an unconfined compression test, generally the minor principal stress (σ3) is taken as zero. The major principal stress (σ1)is equal to the deviator stress and the found from the following equation


Where P=axial load and A=area of cross-section

The axial stress at which the soil specimen fails is termed as the unconfined compressive strength (qu) of the soil. A stress-strain curve can be drawn with the obtained results, which is between the axial stress and the axial strain at different stages before the failure of specimen.

While calculating the axial stress, the area of cross section of the specimen at that axial strain should be used. The corrected area can be obtained from the following equation

A=A0/ (1-ε)

The Mohr circle is generally drawn for stress conditions at failure of specimen. As minor principal stress is zero, the Mohr circle passes through the origin. The failure envelope is horizontal (Φu=0). The cohesion intercept is equal to the radius of the circle, i.e.

s = cu1/2=qu/2

Advantages and disadvantages of the test


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Kanwarjot Singh

Kanwarjot Singh is the founder of Civil Engineering Portal, a leading civil engineering website which has been awarded as the best online publication by CIDC. He did his BE civil from Thapar University, Patiala and has been working on this website with his team of Civil Engineers.

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