Full Description

Horizontal directional drilling (HDD) is a relatively new technology fortrenchless pipe installation. It has proved to be an effective, cost-savingmethod for installing polyethylene (PE) pipes for municipal water and sewerapplications. During installation, pipes are pulled into a pre-drilled boreholewhere they are encapsulated by a mixture of drilling mud and earth cuttings(mud-cuttings slurry). The construction technique as well as the installed earthloads on the pipe differs from pipes installed in open-cut trenches. Thus,commonly used design equations for buried pipes are frequently inappropriate fordirectionally drilled pipes. This paper discusses both installation andpost-installation design considerations for directionally drilled PE pipes.During installation the pipe must safely withstand the pull-in force (or"pullback force" as pipes are pulled back from one side of the crossing to theother). The pullback force is the force required to overcome the frictionalresistance between the pipe and the borehole wall and the pipe and the groundsurface. In addition, the drilling mud creates an external fluid head pressure onthe pipe. The pipe must be sufficiently stiff to resist this pressure. After thepipe is installed, the loads are essentially radially directed pressures due tosoil weight, live-load and groundwater. The soil load will depend on thesoil-characteristics of the borehole. In some soil formations, the borehole isstable and does not deform thus acting much like a tunnel. In other soils, theborehole may deform and transfer soil pressure to the pipe. Where the soil has ameasurable internal-friction angle, the pressure is usually less than thegeostatic stress (prism load). This paper also discusses the calculation ofinstallation loads, post-installation loads, and pipe reactions for directionallydrilled pipes. Pipe reactions include longitudinal tensile strain, ring buckling,and ring deflection. Safety factors and allowable deflections are suggested forPE pipes. Includes 10 references, tables, figures.

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Edition: Vol. - No. Published: 06/01/2001 Number of Pages: 17File Size: 1 file , 280 KB