The purpose of this research study is to perform in situ and laboratory direct shear tests to evaluate the shear strength parameters of rock piles deposited during surface mining operation at the Chevron Mining Inc. (formerly Molycorp Inc.) located in Taos County, New Mexico and evaluate the effect of weathering on these shear strength parameters. Weathering of rock piles is an important process that can change the shear strength (cohesion and friction angle) of these materials. A modified in situ direct shear test apparatus was designed and used to evaluate in situ cohesion and friction angle of rock pile materials. Two test apparatuses were constructed; a 30 cm square metal shear box and a 60 cm square metal shear box. The main difference between the in situ shear box and its conventional laboratory equivalent is that the in situ shear box consists of a single box that confines an excavated block of rock pile material. The lower half of the block consists of the rock pile material underneath the shear plane that is a semi-infinite domain. This book gives an in-depth describtion of the device, test performed and results analyses.