Shear-Induced Failure and Static Liquefaction in Reinforced Soil and Triaxial Tests

In the Federal Highway Administration (FHWA) laboratory, a phenomenon was identified that reduces soil bearing capacity. This phenomenon arises in common situations, for example, reinforced soil, triaxial tests, mat foundations on sand over bedrock, and other stratified media. In reinforced soil, this reduction is based on reinforcement spacing S and soil particle diameter D. This study provided a generic derivation showing that Coulomb's failure criterion is triggered by shear stress associated with not only S and D, but also the other dimensions of a soil structure. Although identified and validated in experiments, the S:D instability was verified by finite-element analysis when the number of elements equals the number of soil particles. Static liquefaction was identified with instability and collapse of force chains. Although analysis was performed in the idealized context of unfaced soil structures, FHWA test data demonstrated violent effects on faced structures when the S:D stability limit was exceeded. For narrow structures, this study also evaluated the S:B instability, where B is the breadth of the reinforcement, triaxial test specimen, or foundation mat.