by Michael P O’Reilly, University of Nottingham, and Abir Al-Tabbaa, Ove Arup & Partners. This paper was first published in GE’s June 1990 edition.
Piles are often constructed in soil which subsequently heaves as a result of vertical stress reduction, for example by basement excavation or rising groundwater level. A simple procedure is described for calculating the magnitude f the tension forces induced into such piles and hence the required capacity of tension reinforcement. While the general principles of heave induced tension are widely understood by practising engineers, the authors are not aware of any previous publication which has proposed a one-dimensional soil-pile interaction solution as outlined in this note. This method is able to account for soil stiffness (including non-linear stiffness if appropriate), pile stiffness, pile spacing, the friction properties of the soil-pile interface and time-related effects (eg the rate of swelling). The routine described uses simple assumptions and may easil programmed. However, it is not so complicated that it cannot be carried out by hand. This note concentrates specifically on the case of straight-shafted bored piles constructed in soil subjected to heave pressures before the imposition of externally applied loads from buildings or other sources. Building loads usuall y induce compressive stresses into the pile and hence the critical case for tension will generally occur before the pile experiences such loading. A short note on the extension of the method outlined here to the analysis of cases involving externally applied loads appears at the end of this technical note.