The Deformation And Failure Mechanism Of HDPE Pipes In Horizontal Directional Drilling Back To The Traile And The Numerical Simulation

As one of the most rapid developed, highly technical, well-equipped and widely adopted non-excavating construction method, Horizontal Directional Drilling is the dominant technique in the non-excavating construction. High-density polyethylene(HDPE) pipe has been widely and most commonly adopted material in the Horizontal Directional Drilling because of its special property. However, one major problem can occur in the process of laying HDPE pipe by Horizontal Directional Drilling technique: The section dimension deformation of the HDPE pipe occurring in the process of back-dragging can only be noticed, which will lead to the failure of the pipe laying project.The reasons that count for the failure of the pipe laying are the short history and the insufficiency of the basic relevant research. The former researches have not specially touch the problem mentioned above. Nowadays, the deformation of the HDPE can only be controlled according to the empirical knowledge due to the lack of the relevant study. Nevertheless, in the practice, because of the special property of the strata, the complexity of environment, the difference of the pipe material, the variety of the borehole trajectory and the diversity of construction technology, problems can arise by relying on the Empirical observations, thus leading to the failure of the pipe laying. As a result, the mechanism of the deformation of the HDPE pipe in the Horizontal Directional Drilling back-dragging is both a basic theoretical and engineering technical problems. It is of great scientific and practical significance in reducing the construction risk, initiating a new field and enhancing both the economic and security.On the basis of reviewing and absorbing the research result, this paper fully analyzes the mechanical model of laying HDPE pipe in the Horizontal Directional Drilling from the mechanics and geometry perspectives, and at the same time analyzes the deformation mechanics of the HDPE pipe. It also explores the factors that lead to the pipe deformation like the geologic factor, the engineering practice factor, the pipe material factor and so on. Thus, establish the exterior pressure calculating model of the pipe and avoid the defect of only calculating the axial tension.The numerical stimulation is carried out by adopting the large finite elements software and then compared with the theoretical result, thus testing their conformity. After analysis, the conclusion can be drawn that the elastic modulus, standard size ratio, diameter deformation ratio and the axial tension are the important factors influencing the pipe deformation.This paper also analyzes the tensile stress and the security coefficient of anti-insecurity when the pipe is full of the inshore liquid by referring to the engineering practice. It establishes the relationship between the maximum stress and the deformation in the condition of complex loads by combining the finite element and theoretical calculating results, so that put forward the method that can effectively prevent the pipe deformation.The following conclusions can be drawn after the exploration of the mechanics leading to the pipe deformation by combining the theoretical model and finite element as well as the specific engineering case.1. The most important factors that influence the pipe deformation are the geological, the engineering construction and material ones. The pipe especially tends to deform in the sandy gravel strata.2. It establishes the external pressure calculating model and makes the theoretical analysis, thus compensating the defect of only calculating the axial stress.3. It makes the theoretical analysis of the pipeline damage mechanism, and the judgment by the third strength theory. It shows that the pipe deformation will occur when the net external pressure exceeds the limitless one.4. The results obtained from the large-scale finite element software simulate confirm with the theoretical ones. The analysis shows: the limitless external pressure increases with the increase of the elastic modulus and the decrease of the diameter-thickness ratio, the diameter deformation ration and the axial stress of the pipe. The minimum reduction diameter coefficient deformation rate can reach 0.28, while that of the axial stress can reach 0.65. 5. It has been proved by both theory and practice that it not only reduces the maximum back-dragging mechanic but also greatly improves the anti-instability security and effectively prevents the deformation of the pipe when the pipe is full of the liquid.