Classification and Specification of Bedding and Backfill for Buried Pipelines

Current pipeline design practice within water, wastewater, and drainage pipe industries is largely based on the American Water Works Association (AWWA) design manuals and standards, American Society for Testing and Materials (ASTM) standards and American Association of State Highway and Transportation Officials (AASHTO) codes and design practices, respectively. AWWA, ASTM and AASHTO practices recognize the importance of proper specification and installation of bedding and backfill for achieving the intended structural performance of buried rigid and more importantly flexible pipelines. However, the guidance provided by various AWWA design manuals, specific for pipe materials, ASTM standards, and AASHTO codes use different terminology and requirements to classify and specify bedding and backfill materials based on the pipe material. This variability of bedding and backfill classification and specification requirements creates unnecessary confusion amongst pipeline designers and introduces a potential for unintended use of bedding and backfill materials. More importantly, the use of inconsistent and non-standard soil classification terminologies makes the interdisciplinary communications difficult, potentially resulting in inappropriate and inconsistent selection of engineering properties for the pipeline design. With an understanding of the potential risks from these different specifications based on pipe material, the development of a uniform classification of bedding and backfill materials has been proposed in the past. Due care must be exercised in selecting such a uniform classification system for bedding and backfill. This paper presents the opinion that such a uniform classification and specification system should be based upon fundamental soil mechanics principles, the Unified Soil Classification System (USCS), and engineering properties of soils. The system should also consider the past and present practices of pipeline design. In an attempt to develop such a system, this paper presents the following: 1) A summary of the current practices of classification and specification of bedding and backfill for water/wastewater and storm drainage pipelines. 2) A comparison of current classification and specification practices of bedding and backfill for different pipe materials noting common and differentiating parameters. 3) A proposed uniform framework for classification and specification based on USCS and engineering properties of the bedding and backfill materials.