INTRODUCTION
Consolidation is a process which involves a decrease in water content of saturated soil without replacement of water by air (Terzaghi 265). During the process, two changes occur: (1) reduction in soil volume and (2) increase in soil strength. The results of this test are used in the study of fine-grained, cohesive soils such as clay and clay-silt. Generally, an undisturbed sample is fitted into a consolidation device.
This consolidation device consist of a heavy brass metal ring to hold the soil sample being tested and porous stone discs to cover the top and bottom surface of the sample. The discs enable pore water to escape from the soil as compression is taking place. The attached micrometer dial gage is used to determine the displacement. For each load, the amount of compression and time required for compression are recorded over a two-week period.
The consolidation test is used in order to determine the compression curve of a specific soil sample. From this graph, four parameters are quantified: (1) the coefficient of consolidation, (2) the compression index, (3) the swelling index and (4) the pre-consolidation pressure of the soil being tested. The load applied to the soil specimen is increased and decreased gradually in specified intervals consisting of loading and unloading phases. In the duration of the experiment, the settlement of the soil specimen is measured with respect to time. The procedure in this experiment is aligned with ASTM D 2435- Standard Test Method for One-Dimensional Consolidation Properties of Soils.
PROCEDURE
First, the soil sample is prepared. Second, moisture content of the sample is analyzed. In this step, the sample is placed in the oven for 24 hours. The difference between the initial and final weights divided by the final weight is the moisture content of the soil sample. The sample is then placed in the device. To start the consolidation test, a load of 20 kg is added. The load applied is then doubled as the test goes on until the load reaches a maximum of 160 kg. The load is then reduced until it goes back to 20 kg. The loading data of 20 kg, 40 kg, 80 kg and 160 kg helps in estimating compression index while the loading data of 160 kg, 80 kg, 40 kg and 20 kg is used in estimating swelling index.
Recorded data include dial gage readings and time. The dial gage readings is related to the volume reduction happening during consolidation. As time increases, this volume reduction approaches a constant value. From the experiment, a semilog plot of gage reading versus logarithm of time is used to calculate the coefficient of consolidation and void ratio for each pressure. The coefficient of consolidation describes the rate at which soil consolidation occurs. The higher the coefficient, the faster the consolidation happens. Another soil parameter is void ratio. It is the ratio of compressed volume to the original volume. It is noted that both coefficient of consolidation and void ratio is dependent on pressure. A plot of void ratio versus pressure was obtained. From this plot, the compression index, the swelling index and the pre-consolidation pressure are estimated graphically. The compression index indicates the compressibility (volume reduction) of normally-consolidated soil. The swelling index (or recompression index) indicates the compressibility of over-consolidated soil. Lastly, the preconsolidation pressure refers to the maximum pressure over which overconsolidation happens. The parameters obtained describe the characteristic behavior of soil (particularly clay) under loading. This is useful in establishing data of structural foundations in clay soil.
References
Terzaghi, Karl. Theoretical Soil Mechanics. New York, John Wiley & Sons Inc.: 1945. Print.
Reddy, Krishna. Experiment 11: Consolidation Test. University of Illinois at Chicago. Web. November 11, 2014. http://www.uic.edu/classes/cemm/cemmlab/Experiment%2011-Consolidation.pdf.
