Geosynthetics stabilize deteriorating mine roads

August 1st, 2014 / By: / Geogrids

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Introduction

The roads of a mine are the arteries through which all products (and by-products) pass. Throughout the day, metals and materials are recovered and large trucks make repetitive, usually slow, trips up and down mine haul roads. While mining companies understand how processes and products affect production, some overlook these roads when searching for ways to reduce costs and boost productivity.

Several mines in the western United States have found an affordable way to build better, longer-lasting haul roads and, in the process, reduce maintenance, maximize production rates, and extend vehicle service life.

Mine operators are increasingly turning to geosynthetic materials that can create a mechanically stabilized layer (MSL) within weak roadway subgrades. The MSL reduces in-service stresses by distributing wheel loads more uniformly. Strengthening the subgrade also helps minimize rutting, an advantage for mines that might be considering the new autonomous vehicles that are now entering the market.

Evaluating current road conditions

Haul roads are classified in a variety of ways, including temporary roads, semi-permanent roads, or permanent haul roads. When determining what type of road to build, there are factors to consider, such as the intended life of the road, the use of the road, the location of the road, and how much aggregate fill will be readily available from mining by-products.

Hard rock mines tend to have an abundance of sub-base and base materials available on site. But operators of soft rock mines must select good in-mine materials and then import or manufacture additional materials. In many cases, the materials for the surface layer of the haul road must be imported or manufactured.

While the cost of construction for these haul roads can vary greatly depending on the type of mine, and even within the same industry from mine to mine, geosynthetic materials ultimately turn out to be a good long-term solution that provides increased stability, safety, and efficiency at mine sites.

Large trucks wreak havoc on haul roads

The payloads of trucks that travel daily on a mine’s haul roads can range from 35 tons to 100 tons or more (A new class of autonomous vehicles or “ultra haulers” can carry 270 tons or more.) For comparison, a fully-grown, 100ft-long blue whale, the largest animal in existence, weighs about 170 tons. These huge loads can quickly degrade dirt and gravel haul roads that mine operators rely on.

This stress often results in rutting and surface erosion problems that can lead to safety issues and production delays—drivers have to reduce their speed and load size to ensure their own safety and to avoid vehicle damage. This slows down the overall mining process and decreases operational efficiency of the mine.

A solution for deteriorating roads

In the western U.S., one mine’s fleet of heavy trucks, along with other equipment, were rutting its haul roads so extensively that the “safe” travel speed had to be cut in half, to less than 10mph. The mine owners were assisted in the rebuilding of these roads to improve overall productivity by using two layers of geogrid combined with local aggregate material.

Installing a geogrid layer within the subgrade created a strong road structure that could handle heavy loads at 15–20mph. This increase in speed allowed for higher productivity on the mine site due to the increased ability to safely haul more material. The smoother roadway surface from the reinforced MSL of the road section also reduced the wear and tear on the equipment and truck tires. This translated into major savings for the mine owners. (One large mining truck tire can cost more than $100,000!)

After completion of the project, management at another mine in the same region heard about the stabilized haul road and wanted to learn more about the solution and the installation process.

In these cases, by reinforcing the MSL these companies reduced the cost of ongoing road and truck maintenance, increased the safety of the haul roads, and increased the long-term stability and performance of their haul roads.

Environmental challenges

Mechanical ground stabilization was also used to address an environmental issue at a third mine site in the western U.S. This mine had encountered variable soils while designing a holding pond for contaminated water and snowmelt. Part of the soil included hard travertine but there were also pockets of soft soils that needed to be addressed.

Consulting engineers on the project worked to design a platform for the pond to stiffen the softer areas before construction. Over these highly variable soils, project managers installed a bottom layer of large-aperture geogrid and topped by two layers of triangular-aperture geogrid. The project was a success and addressed a key issue—to ensure that contaminated water did not seep through the soft soil into a nearby stream.

Mine operators are increasingly addressing road stabilization and environmental issues by using geosynthetic solutions to increase mining productivity and safety.

Branden Reall is a Tensar International regional manager.Ron Bygness, Geosynthetics magazine senior editor, also contributed to this article.

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