| A structural system consisting of micropiles fixed by a cap beam can be used to provide an engineered solution for stabilization of landslides. For this technology to gain wider acceptance by practicing engineers, a reliable design procedure is required. The most widely-cited method is described in the micropile design manual published by the Federal Highway Administration (Sabatini, et al., 2005). The method is based on the assumption that landslide forces are transferred to the micropile wall by two mechanisms: (1) side shear between the sliding soil mass and the micropiles above the slide plane and (2) as a concentrated force acting at the slide surface and distributed to the micropiles, each of which is modeled as a single free-headed pile. This model fails to account for (a) load transferred between the micropiles through the cap beam, i.e., the micropile wall is a structural system, not a collection of individual, free-standing elements and (b) load transferred laterally from the moving mass of soil, as opposed to a concentrated force at the slide surface. The limiting condition using this method is bending resistance of the individual micropiles. However, case studies of instrumented micropile walls in Alabama, Ohio, Canada, and Wyoming demonstrate that micropiles develop resistance to sliding primarily through mobilization of axial load, and that bending moments are much smaller than predicted by the FHWA method. In this thesis, a new design procedure is proposed in which the micropile wall is modeled and analyzed as a structural frame. Using this approach, micropiles resist distributed soil loads through axial tensile and compressive forces rather than in bending. The computed axial loads control the design of the structure and provide a more realistic representation of structural performance. Based upon determined maximum axial loads, micropiles can then be designed structurally, and geotechnically for proper embedment based upon pile side friction. The proposed method is described and example calculations are presented to illustrate its application to slope stabilization. |
