The definition of the unconfined compressive strength (qu) is that it represents the amount of load that a cohesive soil’s cylindrical specimen can withstand per unit area before it fails in compression. In simpler terms, it is the maximum pressure that a cohesive soil sample can take before it collapses under its weight. The qu value is an essential parameter in geotechnical engineering as it helps in determining the strength and stability of soil. Knowing the qu value of a particular soil sample can help in designing and constructing safe and stable structures on that soil.
qu = P/A
Where P= axial load at failure, A= corrected area =
, where
is the initial area of the specimen,
The concept being discussed here is the axial strain and its relationship to the original length of a material. Axial strain refers to the change in length of a material in relation to its original length. This measurement is important in the study of various materials, including soil.
Another important concept related to soil is undrained shear strength, which is a measure of the soil’s ability to resist deformation under load without allowing water to escape. The undrained shear strength of soil can be determined through the use of unconfined compressive strength testing.
Interestingly, there is a mathematical relationship between the undrained shear strength and the unconfined compressive strength of soil. Specifically, the undrained shear strength of soil is equal to one half of its unconfined compressive strength. This relationship can be used to make predictions about the behavior of soil under various conditions and can be an important tool in the field of civil engineering.
Determine Unconfined Compressive Strength of Cohesive Soil
Equipment for the Test
The equipment list includes an unconfined compression apparatus of the proving ring type, which is equipped with a proving ring that has a capacity of 1 kN and an accuracy of 1N. Additionally, there is a dial gauge with an accuracy of 0.01 mm, a weighing balance, an oven, a stopwatch, a sampling tube, a split mould with a diameter of 38mm and a length of 76mm, a sample extractor, a knife, and vernier calipers.
The unconfined compression apparatus is a specialized tool used in geotechnical engineering to test the strength of soils. The proving ring type is a common design that uses a circular cell to hold the soil sample and a piston to apply a vertical load to it. The proving ring itself measures the force exerted on the piston, which is used to calculate the strength of the soil.
The dial gauge is used to measure the vertical deformation of the soil sample as it is compressed. This measurement is important for calculating the strain and stress within the sample.
The weighing balance is used to measure the mass of the soil sample, which is necessary for determining the density of the sample. The oven is used to dry the soil sample prior to testing to ensure consistent moisture content.
The sampling tube is used to collect undisturbed soil samples from the field. The split mould is used to prepare the soil sample for testing by shaping it into a cylinder. The sample extractor and knife are used to remove the sample from the split mould without damaging it. The vernier calipers are used to measure the dimensions of the soil sample, which are necessary for calculating its volume and density.
Lastly, the large mould is used for preparing larger soil samples or for casting concrete specimens. Overall, this equipment list provides everything needed for a comprehensive unconfined compression test on soil samples.
Procedure of Unconfined Compressive Strength Test
To prepare a soil specimen for compression testing, one should first place the soil sample in the large mould at the desired water content and density. If the sample is undisturbed, it should be pushed into the clay sample. After this, the soil sample must be saturated in the sampling tube using a suitable method. To ensure smooth extraction of the specimen, one should coat the split mould with a thin layer of grease and weigh it.
Next, the soil sample needs to be extruded out of the sampling tube into the split mould using a sample extractor and knife. The ends of the specimen should be trimmed, and the mould should be weighed again with the specimen. The split mould should be separated into two parts to remove the specimen. The length and diameter of the specimen should be measured using vernier calipers.
To begin the compression test, the specimen should be placed on the bottom plate of the compression machine and the upper plate adjusted to make contact with the specimen. The dial gauge and proving ring gauge should be adjusted to zero. The compression load should be applied to the specimen at a rate of ½ to 2% per minute, causing an axial strain. The dial gauge and proving ring gauge readings should be recorded every 30 seconds until a strain of 6% is reached. After that, the readings may be taken after every 60 seconds for a strain between 6% and 12%, and every 2 minutes beyond 12%.
The test should continue until failure surfaces have clearly developed or until an axial strain of 20% is reached. If possible, the angle between the failure surface and the horizontal should be measured. Finally, a sample should be taken from the failure zone of the specimen for water content determination.
Fig: Unconfined Compression Testing Machine (Spring Type)
Fig: Unconfined Compression Testing Machine (Proving Ring Type)
Data Sheet for Unconfined Compression Test
Initial length of the specimen
= Initial diameter of the specimen,
= Initial area of the specimen,
= Initial volume of the specimen,
= Mass of empty split mould = Mass of split mould + specimen = Mass of the specimen M = Bulk density,
= Water content = Dry density
= Specific gravity of solids, G= Void ratio,
Degree of saturation,
The curve represents the relationship between compressive stress, which is plotted on the ordinate, and axial strain, which is plotted on the abscissa. It visually displays how the compressive stress changes in response to different axial strains. The curve shows the behavior of a material under compressive load, indicating how it deforms or changes in shape as it is subjected to increasing amounts of stress. The compressive stress is the force per unit area applied to the material, while axial strain is the relative change in length of the material in the direction of the applied load. The curve can provide insights into the material’s mechanical properties, such as its elasticity, yield point, and failure point. It is commonly used in materials science, civil engineering, and other fields to analyze and understand the behavior of materials under compression, which is essential for designing and optimizing various structural systems and components.
Fig: Mohr’s Circle for Unconfined Compression Test
Results of the Test:From the plot, unconfined compressive strength,
= Shear strength,
=