The bridge site is located in an area with several geotechnical challenges including sporadic pockets of permafrost, and soft/weak clay layers causing long-term settlement and stability concerns for fill embankments. Furthermore, the design was required to meet the stringent seismic performance requirements mandated for a “lifeline” bridge as defined in the CSA S6-14. Despite the modest levels of seismic activity, the poor geotechnical conditions were found to cause large amplification of the earthquake motions, thereby increasing the seismic demand and causing relatively large ground displacements that could impact the bridge foundations. Moreover, to achieve the required design speed, large volumes of cuts and fills were required. Some of the cuts were in competent bedrock and permafrost rich soils, causing several construction challenges. A large volume of cut material could not be utilized for embankment construction due to high fines content while granular material sources for fills were scarce and located some distance from the project site.

The geotechnical investigation followed a phased approach, given the high permafrost potential of the site. Accordingly, the geotechnical investigation included a geophysical investigation using electro resistivity scanning to locate permafrost pockets followed by an intrusive geotechnical drilling investigation to confirm the soil and groundwater conditions and derive necessary input parameters for geotechnical analysis. As fill embankments up to 9 m high were constructed on soft weak clay layers adjacent to permafrost pockets, a detailed instrumentation program was implemented to track the long-term settlement concerns during construction and embankment instability concerns that may require staged construction. A toe berm was constructed to improve the stability by utilizing the excess cut material in turn reducing the hauling quantities.

We were able to overcome a number of geotechnical challenges, most notably the sporadic pockets of permafrost, in the design and construction. From an environmental standpoint, we were able to develop a spawning and rearing side channel as well as reconnect a land-locked pond to the mainstem of Crooked Creek and install two large woody debris arrays, which are expected to increase and diversify fish habitat resources in the vicinity of the new bridge.