Plastic Pipes Conference Association # 2023 Lake Buena Vista
Amster Howard
This paper explores the use of a composite soil stiffness as a valuable design tool to consider when a pipeline traverses through poor soil conditions. Thermoplastic pipe are flexible pipe and require good soil support at the sides of the pipe to minimize deflection. However, sometimes the trench walls are weak soils and decrease the effectiveness of the compacted embedment beside the pipe. Some of the options are to encase the pipe, widen the compacted embedment to support the pipe, or to (preferred) to use the concept of composite E prime. Based on the work of Leonhardt that was incorporated into the European ATV method, the current procedure in the United States is to use tables of values and determine the type of soil and percent compaction for the embedment material. This procedure has now been adopted in several AWWA and ASTM standards and manuals. Several consulting firms use it routinely.
The stiffness of the trench wall soil and the stiffness of the embedment soil are combined into a composite value that can be used in various equations to estimate what the deflection may be. In addition to stiffness, the width of the trench at springline (horizontal diameter of pipe) also affects the composite value.
When pressurized, the pipe will tend to re-round but sometimes the pipe may sit for months, or even years, before it goes into operation. The pipe may be continually deflecting during that time. The more the pipe deflects, the harder it might be re-round the pipe. Over deflection may threaten the integrity of the pipe. While not a common problem, the pipe behavior during sitting empty for some time should be checked.
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