Buried pipeline performance under surface loading using finite eelement analyses and innovative soil box testing methodologies.

Buried pipelines are used extensively in the utility industry to deliver natural gas to customers. Reasonable estimates indicated that there are approximately 26,000,000 miles of pipeline, of various diameters throughout the United States (PHMSA, 2018). For protection, and to relieve urban congestion, these pipelines are nearly always buried under ground. After the pipes are installed, however, there is little opportunity for inspection, or for supplemental protection, when needed. Thus, to reduce the likelihood of rupturing a natural gas pipeline, design guidelines that ensure consistent, minimum protection of these utilities, have been developed and implemented. Pipeline design and installation regulations for the natural gas industry are encoded in 49 C.F.R. § 192.327. Within the regulations, minimum burial depths are mandated as an alternative to bury depth requirements, utilities may provide protection to shallow buried pipelines. The regulations do not specifically state alternative protection mechanisms, but most often, additional protection is provided by 1) pouring a concrete slab across the trench wherein the pipeline is buried; 2) place a steel plate across the trench wherein the pipeline is buried. Unfortunately, these protection methods have not been studied. The degree to which these systems protect the shallow buried pipelines is unknown. Additionally, the required degree to which any system must protect shallow buried pipelines is not clearly defined. This study was conducted to better understand the performance of common utility pipeline protection systems. The protection systems were analytically studied using finite elements (FEM), and subsequently modeled in laboratory conditions. Through this combined approach, the accuracy of the finite element method developed by this investigation could be verified against laboratory measurements. This combined approach ensured the accuracy of the modelling. Once the minimum protection state was defined, mathematical (finite element) models of pipelines at shallow burial depths, but with protection systems, were developed. In addition to reviewing the standard buried pipe protection methods, a novel protection system using a steel dome placed over the pipeline was also considered. This study also evaluated the application of digital image correlation to the accurate measurement of a continuous strain field within the cross section of soil beneath a surface load. Specifically, laboratory soil box testing was used with digital image correlation to measure deformations of buried pipes. This digital image correlation software allowed strain measurements of soil and the buried pipes without the impact that other strain measurement methods experience. The soil box tests, and image correlation technology showed that the finite element models were able to predict the behavior of buried pipe systems. Once these models were expended to full scale, concrete slab and ½ pipe arches can be used to protect gas pipelines buried at shallow depths.