Modelling of soil diffusibility on leakage characteristics of burried pipes

Abstract

In water distribution systems, water is transported via underground pipes normally made of cast iron or, more recently, PVC or polyethylene. Breaks in pipes are caused by a number of factors - movement of surrounding soil (e.g. freeze/thaw events), pressure surges or as a result of poor installation(joint leaks etc.). For assessing leakage levels in a network, implementing pressure control or locating leaks it is important to understand the nature of leakage and how it varies with pressure. It is normally assumed that the leakage characteristic for a leak (the relationship between leakage flow and water pressure) follows a square root power law as predicted by physical principles. However, recent experiments show that the leakage characteristic can exhibit a higher power law depending on pipe material, defect type or the surrounding soil. The effects of pipe material and defect type on the leakage characteristic is the subject of a lot of current research and publications are routinely appearing. This paper purely addresses the effect that soil has on the leakage characteristic and will supplement the work describing the other factors. The true behaviour is obviously a combination of all these things and will ultimately be determined by pulling them all together. The research in this paper considers the idealised case of leakage from a circular orifice modelled using the Darcy-Weisbach equation and a two-dimensional model of soil, represented by the standard diffusion equation. Combining these two equations into one general steady state model allows the effects of soil diffusibility on the pipe leakage characteristic to be determined. This is shown in terms of the flow discharge over varying pressure for several different soil diffusibilities. The validity of the model has been confirmed with practical experiments performed on a test facility. A large box, with holes drilled into the side to allow drainage, was filled with different types of compacted soil and a leaking pipe placed in the centre. The water pressure in the pipe was increased from 10m-50m and the corresponding steady state flow measured (once soil was saturated). The main result of the work is that for high soil diffusibility (a sandy soil, for example) then the leakage characteristic is similar to that for discharge to air - i.e. a square root law. As the diffusibility of the soil decreases, then the exponent increases and the overall discharge level drops. This pattern is most pronounced for very heavy soil such as clay where the exponent approaches one and the flow is very low. Copyright ASCE 2006.