Problem Solving with Polystyrene

MTO is well known for its ability to design innovative, practical solutions to complex engineering problems. The ministry showcased this ability once again by successfully completing a challenging culvert replacement at McKellar Creek, located on Highway 17 east of Terrace Bay, using large blocks of expanded-polystyrene (EPS) foam confined to a unique matrix system (for those unfamiliar with EPS, think of foam coffee cups!). This system produced an efficient replacement with significant cost savings.

The culvert replacement was tendered in spring 2004, as the creek's existing timber culvert structure was experiencing failure due to age and wood decay. The highway geometry at the site was constrained by soil conditions in rugged terrain, which impeded the safe flow of traffic. Northwestern Region (NWR) was faced with the challenge of replacing the deficient culvert and improving both the grade and alignment of the roadway while operating under several design constraints. The site featured poor soil condition and a weak load-bearing foundation. As a result, no weight could be added to the soil substrate under the roadway, the highway footprint could not be altered, and the toe of slope (the points at which the natural terrain and the highway fill intersect) could not be increased. Environmental factors heavily impacted the project, as construction needed to take place without disrupting the creek's highly sensitive fish habitat. Furthermore, all surplus rock produced during construction had to be hauled off-site due to limited work zone space and environmental concerns.

In order to compensate for these design constraints, NWR decided to use deep EPS fills confined in an innovative, stage-constructed matrix. The new road structure is a matrix consisting of an EPS core, steel cellular wall facing and elastizell foam Crete filler. These components were tied together with steel stressing rods, and all steel components were doubling zinc-galvanized to produce added longevity. The structure, which is both lightweight and rigid, offers many advantages.

EPS is a light, low-density foam fill material that allowed for effective construction in poor soil conditions. The highway at the site featured a steep slope, a potential roadside hazard. EPS foam (up to 6 m thick) was used to raise the grade of the highway by approximately 3 m, which effectively removed the slope and straightened the curvature of the highway over a length of 250 m. The result was a significant improvement to the vertical and horizontal geometry of the roadway, producing a suitable clear zone between the highway and roadside hazards. EPS has been used previously as a fill material in Ontario, but never in this massive confined matrix.

The installation of the new culvert system was completed in two halves. Each stage involved three procedures: first, a temporary Bailey Bridge was erected over the existing culvert; then the top and walls of existing timber were removed from beneath the temporary bridge; the new culvert and H pile foundations were then installed along with EPS approach fills. Staged construction allowed for continual traffic flow through the site without the use of expensive detours.

Environmental concerns were addressed through consultation with the Ministry of Natural Resources. As the new culvert featured an open footing structure, all construction took place outside of the waterway. This allowed the replacement to be completed without causing disturbance to the creek's fish habitat.

The culvert replacement at McKellar Creek was completed in the spring of 2005. The project was a resounding success that effectively saved the ministry millions of dollars, as potential alternatives included the costly construction of an entirely new route through rugged terrain. The matrix design implemented at McKellar Creek remains an advantageous option available to the ministry when faced with other sites featuring similar constraints.