Tailings pond rehabilitation
LOCATION: Northern Alberta (Oilsands)
TIMEFRAME: Winter 2010
PRODUCT: High Strength Geotextiles
Material Supplier: Tencate Mirafi
Fabrication and Installation: Layfield
The scale and scope of capping oilsands tailings ponds has limited reclamation work in the past. The presence of persistent clay in oilsands tailings has made consolidation of tailings a difficult task and many ponds remain too soft to cap many years after they are full. The tailings pond in this example was a Mature Fine Tailings (MFT) pond. This project profile outlines the work done in the winter of 2010 to start the capping of this tailings pond.
Capping soft areas such as sludge ponds is a common practice with high strength geotextiles. The geotextile is sewn into one large piece at the edge of the pond and then pulled into place in one deployment step. In this tailings pond example the size of the pond (2 by 3 kilometers) precluded the use of a single piece geotextile panel. The design required a network of roads to be built on the pond when it was frozen in the winter. These roads would then provide a working surface to complete the capping of the pond in the future. This project was the initial construction of the roads during the winter of 2010.
The basic design was to install a network of roads on the frozen surface of the tailings pond. The roads were 100m wide and about 2 to 3 kilometers long depending on the span of the pond. Altogether there were 13 kilometers of roads planned for an amount of geotextile of 1,300,000m2.
One of the major issues in this project was the planning of this large quantity of material. The scale of this order required that the manufacturer produce the material at full capacity and that Layfield fabricate the panels of fabric at a capacity to match production.
The installation challenge was a daunting task. Rarely has a high strength geotextile project of this size been installed and never on the surface of a frozen pond. Our installation planners had to overcome issues of deployment on ice, sewing in the cold, and testing of the material on site. We developed a portable sewing shelter that allowed us to keep our sewing activities warm to maintain seam quality. We also designed and built a portable wide width tensile testing machine that could be deployed to site to avoid testing delays. We accomplished all of this while staying within strict safety guidelines that required that all staff wear floatation devices and that all equipment had to float in case of ice failure.
With the project awarded on December 23rd we started shop sewing on December 29th and had our first panels ready on January 4th. The sewing shelter and portable tensile tester were designed and built in less than 2 weeks and were ready for construction start on January 11th.
It took a few days of production to sort out how to deploy materials efficiently on the ice. Sewing in the heated sewing shelter worked very well with no problems encountered even at -26C.
Each fabric panel was delivered to site 100m long (road width) and 22 m wide (5 fabric rolls wide). These panels were sewn across the road width to add 22 m to the length of each road. We had no difficulty in placing the panels on the ice and the sewing was consistent and reliable.
Quality testing in the shop and in the field showed that we were able to maintain a project specification strength of 82.5 kN/m across the seams. In the shop we fabricated 631 panels most of which were 5 rolls wide. Of the 2,474 seams sewn only one panel fell short of the shop tensile criteria. This panel was set aside and was used in a non-critical area. All seams sewn on site were tested and met specification.
Although the initial plan indicated that the sewing of the geotextile would be bottleneck in site production it was clear a few weeks into the project that the delivery and placement of fill materials would slow the project considerably. The sewing crews had sufficient capacity to place and sew the geotextile but the earthworks could not keep up.
By late February the weather began to warm up and by early March it was clear that the project was not going to be able to continue. With standing water reaching ½ m deep in places it became impossible to continue to place and sew geotextile and the job wrapped up in mid March. We look forward to getting out on this pond and completing this job in the years to come.