Twin pipelines are often constructed in one trench for effectiveness of water management. However, in the Japanese current design for pipelines, interaction of twin buried pipes is not considered at all and they are treated same as single pipes. In this study, in order to clarify the influence of the sheet pile extraction on twin buried pipes, model experiments were conducted. As a result, it has been revealed that the deflection of the twin buried pipes is less than that of a single pipe and the reduction of the deflection depends on the distances between the twin pipes.
Twin pipelines are often constructed in one trench for the efficient distribution of agricultural water as shown in Fig. 1. In the current Japanese design guidelines for irrigation pipelines (MAFF, 2009), the distance between twin pipes has been described. However, this distance was decided only due to the workability and economy for the construction of pipes regardless of the soil-pipe interaction. In other words, the influences of the distance on the mechanical behavior of pipes have not been considered yet. Additionally, sheet piles have often been used in Japan to install pipelines under poor ground conditions such as the soft ground and high ground-water levels or because of limited work sites. The installed sheet piles are generally extracted after the completion of backfilling. During sheet pile extraction, large deformation and change of stress condition are caused in the surrounding soil. Hence, it has been reported that excessive deformation and cracks occur in flexible and rigid pipes respectively.
In many past researches, the influence of the sheet pile extraction on single pipes has been studied. For instance, Mohri et al. (1987) conducted field tests for single buried FRPM (Fiberglass Reinforced Plastic Mortar) pipelines with a large-diameter to examine the changes of the deformations of pipes and the earth pressure distribution due to the sheet pile extraction. Yoshimura et al. (1997) conducted centrifuge tests for single buried flexible pipes to investigate the mechanical behavior of the pipes with the extraction. As the study on twin buried pipes, Kawabata et al. (1991; 1992) conducted a series of loading tests and FEM analyses for twin buried flexible pipes, and investigated the mechanical behavior of the pipes. Mohri et al. (1999) conducted field tests for twin buried pipes with large-diameter, and evaluate the safety of the twin pipes. Moreover, Young et al. (1984) proposed the calculation methods for the vertical pressure applied to the twin or more rigid pipes in single ditch, but these methods didn't include the case of the flexible pipe and the coefficient of subgrade reaction.