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In the standard triaxial tests, the values for cohesion and internal friction angle are determined through a series of tests on a soil sample run at several confining pressures. Traditionally, the failure envelope is drawn after the results of the triaxial tests are performed by graphical means using Mohr's circle.
This process can be done mathematically. The calculations are relatively simple using a programmable calculator. Below these calculations can be done online.
Enter two sets of values for the initial confining pressure (s3 or p3) and the failure stress (s1 or p1) and submit them. The resulting cohesion and internal friction angle (assuming a straight line failure envelope) are returned. Only two sets of data can be entered at one time because, barring perfection, a straight line is tangent to only two circles. For more than two sets, the calculations must be run for each combination of data sets. Significant variations in values of cohesion and internal friction angle indicate either problems in the results or a curved failure envelope.
If you encounter an error, your data is probably faulty, or you have entered it improperly.