Machine Compressibility
For consolidation during a test, data are collected that relate to the total deformation which occurs throughout the system (e.g., frame, load cell, porous stones, filter paper, loading cap, etc.). For sandy and stiff soils, an appreciable portion of the total deformation may be caused by the deformation of the load frame itself. Consequently, corrections must be applied so that the report presents the deformation of only the specimen. ASTM D2435 states that vertical deformations for the apparatus flexibility must be accounted for when the correction exceeds 5% of the measured deformation and in all tests where filter paper disks are used. Refer to ASTM D2435 for details.
The calibration of machine compressibility, also referred to as frame deformation, produces the needed corrections for the report. When a report is generated, the deformation measurements will be automatically adjusted to account for the machine compressibility before they are written to data files. Machine compressibility calibration should be performed for the equipment when it is first placed in service, and when apparatus parts are changed (including porous stones, filter paper, etc.).
The Machine Compressibility Calibration is run using an incompressible calibration disk instead of the soil specimen. The calibration disk should be made of copper or hard steel with the same height as the test specimen and a diameter 1 mm (0.04 in) less than the test specimen. Figure 1 shows a typical machine compressibility calibration disk.
Figure 1. Machine Compressibility Calibration Disk
Prepare the test cell by placing a piece of moist filter paper on the bottom stone, inserting the calibration disk, placing another piece of moist filter paper on top of the disk and adding the top cap. Put the cell on the LoadTrac-II platen and make any necessary alignment adjustments.
Bring down the Calibrate menu and click the Machine option to open the Machine Calibration window shown in Figure 2.
Figure 2. Machine Calibration Data Window
In the first column, enter the stress values that are to be applied for each step during the machine compressibility test. The values should be those which are in the first column of the Consolidation Table (on the Consolidation Table page of the Properties Sheet). After entering the values in increasing order, continue the entries using the same values but in decreasing order.
Click the Step button. The platen will rise until the first stress value you entered has been reached. When the platen stops moving, click the Step button again. Values will appear in the second and third columns, and the platen will again move up. When it stops, again click the Step button so that values will be put in right-hand two columns. Continue pressing Step until you have gone through a cycle of increasing values and decreasing values. Generally, a single cycle will be sufficient, but you may have to cycle the loading sequence several times to get rid of any plastic deformation in the frame. By doing so, you will account only for the elastic deformation.
If, at the end of a step you click the Read button instead of the Step button, values will be recorded in the second and third columns but the next step will not begin. This allows you to look at the graph of the results (by clicking the Graph button) while the calibration is in progress. To start the next step, click the Step button.
When you have completed the calibration, we recommend you view the graph of the values (by clicking the Graph button) and examine it for reasonableness. Figure 3 shows a typical machine calibration graph. You may edit the values in the two End of Step columns on the Machine Calibration window, making any changes you consider necessary.
Figure 3. Machine Calibration Graph
When you are satisfied with the compressibility calibration, lower the LoadTrac-II platen using the keypad on the front of the unit, then close the Machine Calibration window. Finally, save the results to your setup file for use in future tests by pulling down the File menu and clicking Save.
As stated earlier, the machine compressibility table includes deformation from the frame, the load cell, the stones, the filter paper, etc. If any of these items change, you should run a new compressibility calibration so that the CRC program will subtract the latest machine deformation data from the test measurement values and obtain the actual soil deformation.
Should you have any issues with the above steps or further questions, please open a ticket by clicking "New Support Ticket".
GeoComp Support Team