Evaluation of Deep-Seated Slope Stability of Embankments over Deep Mixed Foundations

When embankments are constructed over soft foundations, deep-seated slope stability often becomes one of the controlling factors in design. Deep mixing methods have been commonly used as an alternative to solve the deep-seated slope stability problem. Bishop's modified method is a commonly adopted approach for analyzing the slope stability of embankments on deep mixed foundations. Bishop's modified method assumed slopes fail along a circular slip surface and the soils along this slip surface provide shear resistance. However, experimental studies have showed that deep mixed columns under a combination of vertical and horizontal forces could fail due to shearing or bending. The possible failure modes depend on the combination of the forces, the strengths of soft soils and deep mixed columns, dimensions and arrangements of deep mixed columns. A numerical method was used in this study to evaluate the factors of safety varying with the strengths of deep mixed columns and their arrangements with three rows of columns having two different thickness. The numerical analysis indicated that the critical slip surface of the deep-seated slope failure was not circular when the deep mixed columns were used. The factors of safety obtained using the numerical method were compared with those using Bishop's modified method and Spencer's three-part wedge method. The comparisons indicated that Bishop's modified method yielded significantly higher factors of safety than the numerical method, especially when the deep mixed columns had higher strengths. The Spencer's three-part wedge method yielded lower factors of safety than the numerical method.