Reinforced Concrete Pipe Design under Extreme Loading Conditions Using ASCE Standard 15-98 and Beyond

Saskatoon is a city in central Saskatchewan, Canada, on the South Saskatchewan River. It has a population of approximately 265,000 people. The City's Circle Drive project is a $245 million design build project involving the construction of a bridge over the North Saskatchewan River, several overpasses, and a large land drainage system (LDS) complete with outfall to the Saskatchewan River. The LDS trunk sewers in this project range considerably in size and applied loading with extreme loading conditions that included a 3050 mm diameter reinforced concrete pipe (RCP) with covers approaching 16 m and a 2700 mm diameter RCP pipe with covers of 21 m. Based on contractor preference and staging logistics the pipes were designed and installed utilizing open cut methods. The paper provides an overview of the LDS trunk sewer design and construction processes used and provides insight into both the advantages and challenges of direct design when used with very high applied load scenarios. Open cut installations under high covers produce very large external loads on rigid pipes which induce considerable shear and radial tension forces within the pipe wall. The RCP trunk sewers subject to high external loads on this project were designed utilizing the ASCE Standard Practice for Direct Design of Buried Precast Concrete Pipe Using Standard Installations (15-98). Direct design pipe was chosen for this project as it directly considers shear and radial tension within the pipe wall where traditional indirect design methods do not. As the pipes were exposed to open cut embankment conditions as well as additional loads due to large offset embankments; ASCE Type 1 and 2 Standard Installation Direct Design (SIDD) installations were utilized to develop enough soil structure to reduce the bending moment forces and shear in the invert area of the pipe to within acceptable levels. The 2700 mm pipeline required additional reinforcing beyond this as even SIDD Type 1 installations required reinforcing schemes that were not feasible within a standard pipe wall section. The designs were also undertaken during a time when the AASHTO standard for the Standard Installations was under revision and the ASCE Standard of Practice was being revisited to limit the use of self-compacting materials in the middle third area of the pipe bedding. This required the design process to re-visit and incorporate many of the original design concepts for SIDD as published by Frank Heger in the late 1980's. The use of conventional RCP for this project pushed the practical limitations of both the manufacturing and installation process. The paper provides an overview of both as well as initial results of inspections that were carried out after representative loading had developed on the pipe to confirm that the pipe response to applied loads was consistent with the design intent.