I performed the independent structural QA review of pipe loading calculations for an existing 1680 mm (66") Class IV reinforced concrete storm sewer pipe in Abbotsford, BC. The pipe sits beneath the proposed south abutment of the Peardonville Road underpass, part of the Highway 1 widening project. With 7 m of embankment fill and ~80 kPa of surcharge loading planned above the pipe, the question was whether the 37 m section of Class IV pipe could remain in service during construction or whether it would fail.

My role

I was the Structural Engineer responsible for the independent QA review of the pipe loading calculations, assigned based on my pipe design experience and analytical background.

What I did

Scope definition and technical questioning:

  • Reviewed the geotechnical data (Thurber Engineering settlement and stress profiles) and the original calculations before starting the formal review.
  • Raised 7 critical questions about the analysis assumptions: pipe diameter discrepancies (68" as-built vs. 1650 mm in the geotech report vs. 1680 mm per CSA A257-1974), whether differential settlement was being considered as a separate failure mode, whether the negative projecting embankment assumption was verified against actual positive projecting conditions, and whether crane loading should be included.
  • Defined the QA scope to focus on the Class IV concrete pipe section only, excluding Class III sections (no significant load increase) and the tunnel liner section (protected by soil arching).

Calculation verification - three independent methods:

  • Method 1 - Indirect design (trench configuration): Verified the Marston equation coefficients (Cd1 = 2.077, Cd2 = 1.836), backfill loads (Wd1 = 238 kN/m, Wd2 = 299 kN/m), and required bedding factor (Bf = 1.857 vs. available 1.9). Found that even without the surcharge, the pipe has only a 2.3% margin. Calculated the combined loading with surcharge: Bf = 3.25, which far exceeds the available bedding factor. Identified that this combined calculation was missing from the original document (Finding F-06).
  • Method 2 - Indirect design (negative projecting embankment): Confirmed that the actual calculation was absent from the document - only the Cn coefficient table was included (Finding F-07). Based on the QA review discussion, the method barely passes at ultimate D-load, which means it fails at the 0.25 mm crack serviceability limit. I also noted that the actual installation is positive projecting (more stringent), making this result unconservative.
  • Method 3 - Direct design (Heger earth pressure distribution): Verified the prism load (295.7 kN/m), earth load with surcharge (577.85 kN/m), Heger pressure coefficients, and flexural steel area calculations. The required inside cage reinforcement is 11.72 cm2 against 10.8 cm2 available per CSA A257-1974 Table 4A - an 8.5% overstress confirming flexural failure. Identified that the springline thrust N2 equals exactly We/2, implying zero lateral earth pressure (conservative but worth documenting).

Concrete strength discrepancy:

  • Identified that the calculations used f’c = 27.6 MPa, but CSA A257-1974 Table 4A specifies 34.5 MPa for the 1680 mm pipe size (Finding F-04). Recalculated with the correct strength - required As drops to 11.43 cm2, which still exceeds the 10.8 cm2 available. The failure conclusion holds regardless.

Critical gap identification:

  • Flagged differential settlement as an unaddressed failure mode. Rigid concrete pipes experience structural stress concentration at joints during profile settlement, and this was not included in the transverse loading analysis. Joint locations would be needed to assess this.
  • Identified that the surcharge source (p_fill = 80 kPa) was back-calculated but not explicitly traced to the Thurber Figure A2-SS, creating a traceability gap for audit purposes.

Deliverables

  • Produced a formal QA review report with 10 findings (2 high severity, 3 medium, 5 low), covering calculation errors, missing documentation, and scope limitations.
  • Confirmed the failure determination: the existing Class IV pipe fails all three loading checks under construction loading conditions.
  • Provided 7 actionable recommendations: correct the concrete strength, add surcharge to Method 1, include the Method 2 calculation, fix the crack width label, add source references for critical inputs, document the diameter resolution, and note the differential settlement limitation.
  • Delivered the review within the compressed timeline to support the March 30, 2026 IFT submission deadline for the overall Highway 1 widening project.