In this paper, the 't-z method' is employed to describe the nonlinear behaviour of a single pile and is used to obtain simplified predictions of pile group behaviour by considering the interaction between two-piles in conjunction with the Interaction Factor Method (IFM). The principal inconvenience of the t-z method arises from the determination of the resisting curve's shape; an improvement upon this aspect is the main aim of this study. Partial slip is considered using a new analytical approach which is an adaptation of a model based on bond degradation. Pile installation effects and interface strength reduction are uncoupled and considered explicitly in this study. Lateral profiles of mean effective stress after pile installation and subsequent consolidation which were representative of predictions determined in a previous study using a modified version of the cavity expansion method (CEM) are adopted; these predictions are subsequently used to relate installation effects to changes in soil strength and stiffness. In addition, the 'reinforcing' effects of a second, 'receiver', pile on the free-field soil settlement is considered using a nonlinear iterative approach where the relative pile-soil settlement along the pile shaft is related to the soil spring stiffness. Through comparisons with previously published field test data and numerical simulations, the results indicate that the proposed approach provides a sufficiently accurate representation of pile behaviour while conserving considerable computing requirements. (C) 2016 Elsevier Ltd. All rights reserved.