Oedometer & Mathematics Essay
Oedometer & Mathematics
The aim of this experiment was to explain how to use two type of oedometer to calculate compression index (Cc) and re-compression index (Cr), measuring the pre-consilidation (s’vc), and to predict the coefficient of consolidation (Cv). The soil used in this experiment is Kaolinite clay, which has Cv = 8.69 sqm/year at 300 kPa (hydraulic result) and Cv = 2.98 sqm/year at 288 kPa (manual result). This result then would be compare with some theoretical calculation by using the isochorne plot.
Oedometer test is test to present and measure the consolidation of a soil. Consolidation is a process when an external load is given at the top of a saturated soil which will cause the water to be squeezed from the soil and finally will lead to the settlement process. This long process is not as simple as it looks, however it may take several years for the soil until the final settlement is reached. In this case oedometer will be used to stimulate the real consolidation situation process of the site. This report will explain how oedometer works, how to process oedometer result for further calculation, and explain the difference between two types of oedometer that are operated in the experiment. Experiment Description
Two type of oedometer were used in this experiment. They were manual oedometer and hydraulic oedometer. The sample that was used was Kalinite clay. A 34.2 mm diameter of ring was used to put the sample during the experiment. Each experiment was done separately because all the method and the technique were different.
Two sets of data were collected at the same time. The first data were obtained by applying pressure to compact the clay. This was done by added or
removed a load every 25 minutes and measure the gauge depth from the dial gauge. The compaction and the recovery of the soil will take time to perfectly occur. However, it is assumed that in 25 minutes, 95% will occur. At the end of the experiment, a set of data would be used to plot the void ratio versus vertical stress graph. This plot then used to calculate the Cv and Cc value which were obtained by calculating the slope of each curve. Beside, pre- consolidation stress could also be obtained by applying the Cassagrande’s Method.
The second set of data were collected from at the first 25 minutes of the previous experiment. It worked by applying a fix amount of pressure (288kPa) and measure the depth of the dial gauge at 10s, 20s, 30s, 1min, 1.5min, 2min, 3min, 4min, 6min, 9min, 12min, 16min, and 25min. This set of data the would be used to create a plot of settlement vs square root of time. Then, this graph would be used to calculate the coefficient of consolidation (Cv) at 90% of primary consolidation.
Initially, hydraulic oedometer was set with 250 kPa of applied pressure, 202 kPa of pore pressure, and 200 kPa of back pressure. The sample thickness was 20 mm, with 75.4 mm ring diameter and volume of 45149 mm3. The hydraulic oedometer consist of two outputs, the volume and the back pressure. The volume was obtained from the digital display while the back pressure was read from the multimeter.
These two types of information were then colleted for every selected interval of settlement time. After the experiment was finished, the applied pressure was increased to 300 kPa. Then, the collected data created a plot of settlement vs square root of time which lately was used to measure the coefficient of consolidation at 90% of primary consolidation. Besides, this plot was used to performhe variation of pore pressure vs time, which then determined the theoretical variation of the pore pressure by using the isochrones plot.