Project Name: Mine Closure in Mexico
Project Location: Near Chihuahua, Mexico
Mine Site Challenges:
Nestled in the Sierra Madre Occidental Mountain range, a remote mine site is surrounded by picturesque views of canyons, cliffs, and steep-walled gorges.
During the summer, the mine site experiences frequent, swift-moving storms that often cause washouts and severe erosion. To address major attrition concerns, project owners sought a long-term channel erosion protection solution that would also serve as a stormwater management system after the mine closure.
A channel armoring system had to be an extremely resilient, maintenance-free solution that could withstand high flows and, in some cases, even rockfalls. Post-closure monitoring would be limited because of the remote location and lack of personnel available to be on-site, so the solution had to be durable and reliable.
Prior to GEOWEB Channel System Installation
As one of the first mines to be decommissioned following the implementation of the International Cyanide Management Code (Cyanide Code), this mine had to meticulously follow the strict requirements of the code to ensure proper closure and safe management of cyanide.
“The Cyanide Code certification program was designed to improve the management of cyanide used in gold and silver mining operations. It was established to safeguard human health and to reduce environmental impacts from inadvertent releases or exposure to this chemical. When mines are decommissioned, the program allows for inspections to verify that cyanide and cyanidation activities no longer remain on site. These audits include evaluating the need to install measures necessary to control or manage surface or ground water, such as surface water diversions and pumping and treatment systems that would operate during the mine’s closure period.”
The original diversion channel design called for a grouted boulder wall or rip rap to stabilize the mine channels. After a hydraulic analysis was completed, the GEOWEB® Channel Protection System was determined to be the best solution.
In planning for severe storm events in this remote location, the canal would be used infrequently, but with very high expected flow rates. The canal also follows the contours of the mountainside, so there were numerous curves to account for in the design. Turbulent flow is most common in these curves, so the erosion protection must be sufficient to resist the impact and turbulent forces of the water. Therefore, a concrete-filled GEOWEB System with a bentonite blanket underneath was the right choice for this situation, and backed by accredited research.
GEOWEB® Geocells Concrete-Filled Performance Testing at Colorado State University (CSU):
Presto Geosystems has done extensive testing with the hydraulics lab at Colorado State University (CSU). This facility has a static drop of over 80 m (263´) for nearly limitless flows through their testing flumes.
A section of 75 mm (3-inch deep) GEOWEB Geocell with concrete infill was used for performance testing at CSU. Multiple tests were run with a fixed slope angle of 26.5 degrees, or 2H:1V. The concrete-filled GEOWEB System does not require any reinforcement, such as rebar or fibers, allowing for a fast and efficient install. The testing maxed out the flume at a velocity of 11 m/s (36 ft/s) with no degradation of the GEOWEB high density polyethylene (HDPE) structure or the concrete infill.
Furthermore, there was no visible loss of the soil that was placed beneath the concrete-filled GEOWEB sections, demonstrating the exceptional erosion protection offered by this system in channels with high velocity and shear stress. The GEOWEB system outperformed other geosynthetics materials, such as articulating concrete blocks, turf reinforcement mats, and erosion control blankets, and even maxed out the test flume at CSU during testing.
Mine Site Solution to Severe Erosion & Washouts
The GEOWEB Hard-Armored Concrete Channels
To ensure a structurally sound and cost-effective solution, the project engineers, consultants, and mine owners collaborated closely with Presto Geosystems’ design engineering team. Together, they worked to develop a value-engineered solution that met the project’s unique requirements and challenges.
The project leads completed a complimentary project evaluation that helped determine the appropriate GEOWEB cell size/depth and the accessories with the staking patterns.
The GEOWEB Geocell System included the following:
- GEOWEB® GW30V
- Mid-sized Cell
- 100 mm (4-inch) Depth
- Concrete Infill
- ATRA® Key—Geocell Connection Device
- ATRA® Tendon Clip—Load Transfer Device
- Tendon—TPP 55
** Calculations determined a 3-inch deep geocell with concrete infill was sufficient for the high flow channel. The mine client opted for a 4-inch deep geocell to permit excavator cleanout and as an additional factor of safety.
A unique aspect of this project is that there were multiple small intake channels that converged into one main channel throughout the mining area. Each of these input channels had different dimensions and anticipated flows. Rather than needing different layouts, forms, trusses, and anchors for each channel, the GEOWEB system was used to create a single cohesive system across the entire network of channels. Because the GEOWEB panels are the formwork and reinforcement for the concrete, it greatly reduced the amount of installation labor and cost of materials.
To ensure a consistent concrete depth and protect against erosion, the concrete was poured directly into the GEOWEB cells and leveled with the top of each cell, as shown in the photo above. This method effectively confines and reinforces the concrete, preventing it from shifting or eroding under high velocity and shear stress.
It is recommended to start filling the GEOWEB sections from the crest and work downward toward the bottom of the channel. This process ensures that the concrete remains intact and evenly distributed throughout the channel without any areas of weakness or vulnerability.
Presto Geosystems’ support, from design through installation and beyond, guaranteed site challenges were met successfully.
Shortly after the GEOWEB System was installed, heavy storms hit the region, resulting in flows of about 1 m depth (3.3´) in the channels. Some of the access roads (not protected with the GEOWEB System) were washed out, but the GEOWEB Channel Protection System held strong, and continues to provide resilient stormwater management, years after mine closure.