Overcoming Challenging Conditions with Innovative Culvert Design

By Hans Arnett


The Alaska Department of Transportation and Public Facilities (DOT&PF) is replacing the structurally deficient Little Goldstream Creek Bridge on a section line easement road near Nenana. The bridge was classified as scour critical by DOT&PF, which means the bridge foundation is susceptible to undermining and instability due to scour. A significant amount of material had already eroded beneath both bridge abutments, and one abutment had experienced differential settlement of more than one foot. DOT&PF determined a culvert would be the most appropriate replacement solution, and developed a design for a 94’-long structural plate pipe arch (SPPA) culvert with a span of 19’ 11” and a rise of 12’ 10”. The culvert was designed using stream simulation principles to provide fish passage.

Challenging Geotechnical Conditions
Geotechnical investigations showed the crossing site is underlain by frozen, fine-grained, thaw-unstable soils. DOT&PF geotechnical engineering staff was concerned the installation of the SPPA would alter the thermal regime of the stream. The specific concerns were that the thermal alteration could induce freeze-down to the underlying permafrost layer, and cause frost heaving of the culvert and formation of aufeis (ice deposits produced by overflow). The DOT&PF SPPA design included excavation of the frozen material to a depth of 7.5’ below the bottom of the stream bed to address these geotechnical concerns. The resulting excavation would be extremely large, and have a bottom width of 40’ and a length of 114’. The design called for the excavation to be lined with geotextile fabric and 4” thick polystyrene insulation board prior to bedding the pipe and backfilling the excavation.

Finding the Best Solution
DOT&PF realized dewatering the site and controlling sediment pollution in the thaw-unstable soils would make construction of the proposed SPPA design extremely challenging. They contacted R&M and requested we provide a dewatering plan and Erosion and Sediment Control Plan. However, after a discussion regarding design options with the Department’s Project Manager, Lauren Little, PE, our role was re-scoped to include an alternatives analysis. The analysis included three alternatives – a modified version of DOT&PF’s original design with vertical headwalls and wingwalls to reduce the size of the required excavation and two open-bottom arch culvert alternatives, both with vertical headwalls and wingwalls, but with different foundation types.

R&M performed a site visit and background research; developed a Site Visit Memorandum; and provided an Alternatives Technical Memorandum that provided 35% designs and cost estimates for the three alternatives, and presented advantages and disadvantages of each. DOT&PF selected an open-bottom arch culvert with a sheet pile wall foundation and corrugated aluminum headwalls and wingwalls as their Preferred Alternative. This alternative allows construction of a new crossing structure without the need for dewatering, a large excavation in the stream bed or the placement of insulation.

Advancing the Preferred Alternative
DOT&PF requested that we move forward with the design of the Preferred Alternative. They opted to keep the overall design of the project in-house and have R&M provide plans, specifications and estimate (PS&E) for the hydraulic engineering portion of the project. This included the arch culvert and its sheet pile wall foundation, corrugated aluminum headwalls and wingwalls, overflow pipes and riprap erosion protection. We performed hydrologic and hydraulic (H&H) analyses while developing the hydraulic design, including flood frequency analyses and development of a one-dimensional flood water surface profile model of existing and proposed conditions using the HEC-RAS analysis program. The H&H analysis work was documented in an H&H Report.

Scour of the stream bed is a major design concern for open-bottom culverts, which experience hydraulic forces similar to those occurring at bridge abutments. The foundation walls were designed to resist scour through a combination of sheet pile embedment depth and an anchoring system that ties the top of the walls back to concrete deadmen buried in the road embankment. We designed riprap erosion protection to resist contraction scour within the barrel of the culvert and local scour concentrated at the corners of the sheet pile foundation at the inlet. We also included riprap in the design to protect against erosion at the base of the headwalls and wingwalls.

Innovative Culvert Design Solution
The presence of fine-grained permafrost soils at the project site presented major geotechnical, constructability and environmental challenges for the design and construction of a replacement crossing structure. These challenges would have been difficult to overcome using a conventional culvert crossing design. The unusual and unconventional design R&M developed effectively addressed the most critical design and constructability issue – thawing of exposed, fine-grained permafrost soils – through the use of an open-bottom structure with a pile foundation. This allowed the crossing to be constructed without excavation and exposure of frozen soils, and with minimal disturbance of the existing stream channel. As an added benefit, the cost to construct R&M’s innovative design solution was very similar to the cost of DOT&PF’s original, more conventional culvert crossing design.

“I like the tricky projects best, especially ones where there isn’t an obvious solution at first for a very unique set of design challenges. The Little Goldstream project definitely falls into the tricky project category with its Alaska-specific geotechnical and hydrologic design issues. These are the types of projects that R&M really excels at.”

Hans is R&M’s Group Manager of Water Resources. He has 27 years of H&H experience in Alaska, with a broad technical background in hydrology, hydraulics, fluvial geomorphology and stream restoration. Hans’s work experience includes hydrologic and hydraulic investigations; stream relocation design; stream channel restoration; stream valley reclamation; flood studies; water surface profile modeling; bridge, culvert and low water crossing hydraulic analyses; culvert fish passage analyses; feasibility studies; river erosion control and training structure analysis and design; bridge and buried pipeline scour analyses; storm water studies; stream discharge measurements; and stream gaging.