The correct formulation of the constant quadrupole element is derived first. An indirect boundary element algorithm is then presented which uses this element to redistribute the out-of-balance stresses. Three numerical examples of slopes involving heterogeneous soils and underground water are shown as illustrations.
Jiang, YS., & Zimmermann, T. (1992). Indirect Boundary Element Algorithm for Slope Stability Analysis. Engineering Analysis with Boundary Elements, 9(3), 209-217. https://doi.org/10.1016/0955-7997(92)90095-O (Original work published 1992)