The effective length D determines the pile length, which effectively transfers the load from the pile into the soil. If the entire pile is placed into the resistant soil, in which the load is transferred by skin friction, then the effective length corresponds to the pile length below terrain - see Fig. A. In case of layered medium, highly compressible layers (in which the load is not transferred into the soil by skin friction) and the layers above are not counted into the effective length D - see Fig. B. Introduction of effective length into the analysis and its magnitude are set in the frame "Vertical capacity".
Fig. A Effective length in resistant soils
Fig. B Effective length in layered medium