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SUMMARYThe use of drilled shafts to stabilize an unstable slope has been a widely accepted practice. There are two basic design and analysis issues involved: one is to determine the global factor of safety of the drilled shafts stabilized slope and the other one is to determine the design earth thrust on the drilled shafts for structural design of the shafts. In this paper, a limiting equilibrium method of slices based solution for calculating global factor of safety (FS) of a slope with the presence of a row of drilled shafts is developed. The arching mechanisms due to the presence of the drilled shafts on slope were taken into account by a load transfer factor. The method for calculating the net force applied to the drilled shaft from the soil mass was also developed. The interrelationships among the drilled shaft location on the slope, the load transfer factor, and the global FS of the slope/shaft system were derived utilizing the developed numerical closed-form solution. An illustrative example is presented to elucidate the use of the solution in optimizing the location of the drilled shafts on slope to achieve the desired global factor of safety of the slope/shaft system.

SUMMARYTwo-dimensional slope stability analysis for a slope with a row of drilled shafts needs a mechanism to take into account the three-dimensional effect of the soil arching due to the spaced drilled shafts on slope. To gain a better understanding of the arching mechanisms in a slope with evenly spaced drilled shafts socketed into a stable stratum (or a rock layer), the three-dimensional finite element modelling technique was used for a comprehensive parametric study, where the nonlinear and plastic nature of the soil and the elastic behavior of the drilled shafts as well as the interface frictions were modelled. Various factors were varied in the parametric study to include (1) the rigidity of the drilled shafts as influenced by its diameter, modulus of elasticity, and total length; (2) shafts spacing and location on the slope; (3) the material properties of rock and the socket length of shaft; and (4) the soil movement and strength parameters. Evidences of soil arching and reduction in the stresses and displacements through the load transfer mechanisms due to the presence of the drilled shafts were elucidated through the finite element method (FEM) simulation results. Design charts based on regression analysis of FEM simulation results were created to obtain a numerical value of the load transfer factor for the arching mechanism provided by the drilled shafts on the slope. Observations of the arching behavior learned from the FEM simulations provide an insight into the behavior of drilled shafts stabilized slope.

The use of drilled shafts to stabilize an unstable slope has gained popularity in highway applications, mainly because it is a structural fix that does not require additional right-of-way. An analysis method for determining the factor of safety of a drilled shaft or slope system and for determining the earth thrust on the drilled shafts for structural design is introduced. The concept of the analysis is cast in the limiting equilibrium approach via the method of slices, while incorporating the drilled shaft–induced arching effects as the soil mass moved downslope and around the drilled shafts. The mathematical equations based on the limiting equilibrium calculation, together with the load transfer factor for accounting for the drilled shaft–induced arching effects, are presented. The three-dimensional finite element model parametric study using ABAQUS program was used to derive the semiempirical equations for quantifying the arching effect. A UASLOPE computer program was written to incorporate these algorithms for applications to real cases. A case study of a fully instrumented and monitored slope stabilization project, ATH-124, in Ohio, is presented. The analysis of the slope at the project site using finite element modeling and the computer code UASLOPE is presented, together with field-monitored data. On the basis of field monitoring data and the comparison between the finite element analysis results with the computer code UASLOPE results, the suggested analysis and design approach appears to be reasonable.