True cohesion of recently deposited phosphogypsum

Standard penetration tests were conducted and undisturbed samples of phosphogypsum were obtained from within two temporary storage piles at the PCS Phosphate Mining Facility located in Aurora, North Carolina. One of the storage piles was under construction at the time of sampling and the other pile had been completed three years prior to the time of sampling. Laboratory strength tests consisting of unconfined compression tests, soaked unconfined compression tests, and consolidated undrained triaxial tests with pore pressure measurements were conducted on the undisturbed samples. The test results indicate that the recently deposited gypsum and gypsum within the upper 50 feet within the older pile has no true cohesion. Below 50 feet within the older pile, the gypsum has a true cohesion which ranges from approximately 2,600 psf to 21,000 psf. The effective angle of internal friction equals approximately 44°. Because of the presence of true cohesion, steep excavation slopes can be used during teardown operations within lower portion of older piles.