By Chip Fuller
A Canadian firm, Continental Gold Ltd., is a precious metals mining company and it currently operates the largest gold mine in Colombia, the Buriticá project. Mining products provide approximately 20% of Colombia’s exports and 2.5% of its GDP. The Buriticá project covers more than 13,500 hectacres (33,345 acres) and is located 75km (47 miles) from Medellin in a mountainous area of the country. This construction project included the design/build of an equipment staging platform 6km (3.7 miles) away from the main paved road, the Pan-American Highway.
One of the most critical areas was known as the Platanal–Yaragua Ramp and is the subject of this case study.
Expanded mine operations required the construction of two tunnels through a mountain that would provide access for needed drilling and construction equipment, which necessitated the construction of a 3m-wide unpaved access road, plus a staging platform for equipment.
The site’s difficult topography created challenges related to designing and building common retaining walls and steep slopes that form the staging platform, which is designed to support the high surcharge loads from heavy mining vehicles.
Syntex Colombia, a distributor of geosynthetic materials in Colombia, worked with engineers at Continental Gold and Strata Systems to design a mechanically stabilized earth (MSE) system, using uniaxial layers of polyester geogrid combined with the native soil compacted in thin lifts to 95% standard Proctor density. The resulting reinforced soil structure is comprised of geogrid reinforcement of sufficient strength, spacing, and length to address internal and external stability of the gravity mass.
The design was based on the methodologies published in the FHWA–NHI technical papers 10-024 and 10-025, “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes” and the AASHTO guide, “Standard Specification for Highway Bridges.” The design addresses internal and external stability of the reinforced soil mass and general slope stability using limit equilibrium methods.
The facial stability was achieved by wrapping geogrid around synthetic bags that were filled manually on the jobsite with local backfill. A permanent turf reinforcement mat was placed over the face and vegetated.
Overall, this technique allows the construction of steep slopes with allowable factors of safety.
The MSE wall was constructed by a crew that included one engineer/inspector, three machine operators, and six laborers. The workers had no previous experience building MSE walls, but were advised by Syntex on the jobsite.
The construction equipment included a small combination loader with backhoe, a skid-steer loader, and a small vibrocompactor.
Construction of the structure took only five days, approximately 25% of the time other methods may have taken.
The resulting benefits of this MSE solution included:
- All of the retaining wall construction could be performed using local labor forces in this remote area.
- No specialized equipment was required.
- All materials were lightweight and were easily transportable to the site.
- The design incorporated the use of on-site soils.
- A 30% cost reduction vs. any other solution was realized.
This project illustrates the effectiveness of building large retaining walls or over-steepened slopes by reinforcing the native soils with geogrids on a remote jobsite. Well-established engineering standards and design software are available, and great economic benefits can be realized.