New closed form solutions accounting for non-homogeneous, elastic plastic or visco-elastic soil properties have been established for a single pile subjected to vertical loading, and for vertically loaded pile groups. The solutions are based on load transfer approach, with the pile-soil interaction being characterised by springs distributed down the pile shaft and at the pile base. As such, the accuracy of the solutions lie in the load transfer factors linking the gradient of the load transfer curves with elastic modulus values. This paper presents an extensive investigation into the load transfer approach, by comparing the new closed-form solutions for a pile embedded in an elastic medium with rigorous numerical analysis. By this means: 1. The rationality of the load transfer approach has been extensively explored; 2. Load transfer factors have been calibrated against the numerical solutions to account for a number of factors, such as the ratio of the depth to the underlying rigid layer and the pile length, the degree of soil non-homogeneity, Poisson's ratio for the soil, and the pile slenderness ratio; 3. Expressions for estimating the load transfer factors have been provided. With the new load transfer factors, the closed form solutions may be applied with high accuracy over a very wide range of pile geometries and soil conditions. (C) 1998 Elsevier Science Ltd. All rights reserved.