Development of LRFD Resistance Factors for Mechanically Stabilized Earth (MSE) Walls

Over 100 centrifuge tests were conducted to assess Load and Resistance Factor Design (LRFD) resistance factors for external stability of Mechanically Stabilized Earth (MSE) walls founded on granular soils. In the case of sliding stability, the tests suggest the LRFD Φ values vary from 0.74 to 0.94 and 0.63 to 0.68 using Rankine and Coulomb methods for lateral resultant force, respectively. These values covered wall heights of 8 to 14 ft with variable backfill having μφ=32° and CVφ= 11.7%. The study also revealed that the load factor for horizontal earth pressure agreed very well with current practice (AASHTO, 2012), i.e. 1.5. In the case of bearing stability, the tests support Φ= 0.47 and 0.45 (β= 3.09) for foundation soils with μφ= 26°-30° and 31°-33°, respectively, and Φ= 0.65 and 0.68 (β= 2.32); current practice uses Φ= 0.65. The tests also suggest the use of load inclination factors in estimating bearing capacity. The combination of axial and lateral force, i.e., inclined resultant reduces the depth and length of the bearing failure surface, which is reflected in a reduced capacity. From over 200 measurements, the load factor for vertical earth pressure was determined to be 1.87, which is larger than current practice, 1.35 (AASHTO, 2012), but agrees well with values reported by others for internal stability. In the case of bearing stability of MSE walls near slopes, it was found that current prediction methods (Bowles, Vesic, etc.) are too conservative. Moreover, based on observed failures of MSE walls near embankments, slope stability instead of bearing controls their design