Time-dependent interaction of soil and flexible pipe

Abstract

In recent years, there has been an increase in the use of flexible plastic pipes as buried conduits even though much of the soil-pipe behavior has not been fully understood. Present design approaches either involve the use of theory modified with various factors to match field and laboratory data or are simply empirical formulations which describe observed data. Most procedures predict the pipe deflection and do not address the pipe strain which is a more suitable measure of the structural integrity of the pipe. Also, when considering the time-dependent behavior of the soil-pipe system, little theory had been introduced. It is the fact that buried pipes creep, that this study was initiated to develop a design procedure which can systematically show the effects of various design parameters on the deflection of buried pipes, to accurately predict the structural integrity of the soil-pipe system and to be able to describe its behavior over a period of time. The new method that is presented here involves a viscoelastic solution which was transformed from an elastic solution obtained through a factorial analysis, using the "correspondence principle". The elastic solution was developed from more than 700 data points generated using a nonlinear finite element program called Culvert ANalysis and DEsign [CANDE]. The parameters that were used in the factorial analysis include material and strength properties of the soils in the three zones in a trench installation condition, namely, the embedment soil, the backfill and the in situ soil; the pipe stiffness, the trench width, the depth of cover and the level of the groundwater. The hyperbolic stress-strain relationship was assumed for the soils. Time-dependent properties of high density polyethylene which is the pipe material used, were obtained through laboratory testing. The viscoelastic solution obtained is comprised of various components which will show the influence of the various parameters such as the pipe stiffness, the degree of soil arching and the presence of groundwater. It also predicts the thrust and the bending moment in the pipe wall, the changes in the strain-level and the pipe deflection for different time and under different loading conditions. As a result of the availability of this procedure, the relaxation properties of the soil around a pipe can be determined from pipe deflections measured over a known period of time. The new procedure is shown to be accurate in matching CANDE results as well as in predicting deflections and strains of pipes buried over a period of time and thus fulfills its purpose.