Stresses in cast-iron pipes due to seasonal shrink-swell of clay soils

The causes of pipe damage that lead to leakages and bursts in water distribution networks are numerous, and the significance of each possible mechanism is not yet fully understood. This paper reports field monitoring and numerical modelling carried out to evaluate the potential for vegetation-induced desiccation to impose significant bending and therefore additional stress on buried water pipes. Over a period of two years, vertical and horizontal deflections were measured along two flexible pipes buried in London Clay in the vicinity of an oak tree. Meteorological measurements made at the site allowed soil moisture deficit to be computed, and these values were compared with regional values supplied by the UK national meteorological office. Finite difference continuum analyses were carried out in order to permit the interpretation of measured deflections in terms of the equivalent stress increases that would be imposed on a rigid cast iron water pipe. The calculated maximum tensile stress increase was found to be significant in terms of the residual strength of a corroded cast iron pipe.