UU Triaxial Failure Curve

To better demonstrate the reason why a CD triaxial curve is different than a UU triaxial curve, six Ziploc bags will be filled with the same number of peanuts to simulate the same initial void ratio in the soil. The first 3 bags will be utilized for a CD Triaxial test and the other 3 bags will be utilized for the UU Triaxial test. Each bag represents a different test specimen generated using the same soil and initial conditions. For the first three bags, each bag will have a different amount of water and be compressed differently to simulate different confining pressures during the consolidation phase of a CD Triaxial test. In other words, one bag will be as full with water as possible with less interaction/compression of the foam peanuts while another bag will have less water and be squeezed tightly to simulate increased effective stress during the consolidation phase. The instructor will then compress the foam peanuts inside the bag to simulate the second shear phase of the test. The goal is to demonstrate the differences in particle-to-particle interaction depending upon the initial confining pressure during the initial consolidation phase, and help the students understand how the failure curve is developed. Subsequently, no drainage will occur for the test specimens simulated using bags 4, 5, and 6. All bags will have the same number of foam peanuts and the same volume of water (same initial conditions). This replicates the unconsolidated phase of a UU triaxial test. The instructor will then squeeze these three bags to simulate the second shearing phase of the UU triaxial test. Through discussion, the reason for the horizontal failure curve of a UU triaxial test will become evident. This simple exercise enables the students to visualize and better comprehend soil behavior under various drainage and testing conditions.




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