Integrated Steep Channel Design Protects Canadian Highway Bridge

摘要

A large eroding gully immediately adjacent to the Old Nipawin bridge approach posed highway stability concerns due to erosion. The drainage slope composed of silt with fine sand, stretched 60m down a 1.2:1 slope where it emptied into the North Saskatchewan River. The gully receives water from over 3000 m~2 of surrounding area, all which is essentially impervious to water and exhibits high runoff velocities due to the upslope grade as well as year round spring fed water. Freshly eroded soils depositing into the river was the main concern. Given the slope and access options, the site had limited equipment access. Rock filled gabions were the original plan for this erosion problem, but given site conditions, cost, and health & safety concerns, another solution was required. The alternative system of turf reinforcement mats (TRM) and natural vegetation was the answer based on cost and ease of installation.Given flow values and other data from Saskatchewan Highways, a porous ditch check system was installed to decrease the initial velocities on the TRM until vegetation was established. A fall rye with perennial and annual mix of grasses was used for the vegetation cover. Installation started in July 2005, and work on the steep slope proved a challenge. The TRM was installed under the existing gabion baskets 3 rolls wide. 8" staples along with 18" stakes were put in at 7/m~2 to secure the blanket to the soil. Ditch checks were then installed at 1 m intervals along the channel bottom and partially up the slope to dissipate flow energy of the water. The fall rye emerged very quickly along the bottom. The side slopes were spotty due to the inability to hold seed on a 1:1 slope. The area was reseeded with the mixture in the Fall in hopes of getting more vegetation on the side slopes. Considerable rain events on the structure to date have led to few problems.Overall the project went well. The cost savings on this project were substantial, at least 1/4 the price of the original quotes. The time required for installation was also shortened by 75%. Since it has been installed, it has seen over 225mm of rain and working as expected. Given the performance to date, Saskatchewan Highways will definitely look at using TRMs instead of gabions for cost savings, ease of installation, and functionality.