“When working with rockfall protection, you don’t do the same thing twice,” says Tim Pfeiffer, senior geotechnical engineer for Foundation Engineering in Portland, OR. And because rockfalls can be triggered by a variety of manmade and natural causes, chances are that no one solution will apply to an entire job.
In addition to careful slope preparation (removing loose rocks and vegetation), successful rockfall protection involves a range of techniques used singly or in combination. For example, draping slopes with wire mesh (galvanized or PVC coated to blend with the terrain), reinforcing the mesh with aircraft cable or cable netting, installing rock bolts, revegetating slopes, and constructing catch basins, dirt berms, retaining walls, and catch fences. The rule of thumb holds that rockfall protection systems that dissipate rather than resist energy are the most cost-effective, efficient, and low maintenance.
Designs for rockfall protection systems must consider rock and soil types, the angle of the slope and conditions on top, and the toe of the affected area. Installation problems, which can be complicated by existing vegetation, access, aesthetics, and environmental issues or regulations, typically fall to the constructor responsible for installing rockfall-protection measures.
The hexagonal wire mesh that is fundamental to slope protection, 0.118-in. galvanized wire or 0.146 in. when PVC coated, with openings measuring 3.25 x 4.5 in., is different from standard gabion wire in a number of aspects. Rockfall mesh is double twisted (one twist more than its chainlink cousin), which prevents stretching and, when properly installed, allows rock under the mesh to move farther down a slope. The double twist also ensures that a cut or a splice won’t cause an entire section of mesh to unravel and compromise the integrity of the remaining installation.
Rockfall mesh is manufactured in rolls that are 3-15 ft. wide and up to 300 ft. long, whereas gabions are compartmentalized and manufactured as finished products according to customer specifications. Once draped on the slope, the mesh rolls are woven together with the same gauge wire and held in place by anchors – sometimes a cross-slope cable system, sometimes with tensioned steel bolts installed at the top and down the slope if the mesh is designed to adhere to the contour of the slope.
For anchoring mesh in fissured hard rock that is not subject to deterioration, the rockfall mesh supplier recommends simple and/or double expansion bolts as anchors, 0.4-0.8 in. in diameter and at 3.3- to 6.6-ft. centers. For compact rock subject to deterioration, the recommendation is a series of 0.5- to 0.9-in.-diameter hooked steel reinforcing bars installed with mortar or resin. In compact soils, 3.3- to 4.9-ft. steel reinforcing bars, 65-100 ft. in diameter, can be driven into the ground and fitted with a hook to which the mesh is attached using a steel rope. In situations where the mesh is applied close to the slope with the goal of preventing rock fragments from falling, anchors the same size as the steel bars typically used for rock bolts, 1-1.25 in. in diameter, are installed every 162-323 ft.2 of covered surface.
Rockfall mesh can be also reinforced by a network of crisscrossed galvanized steel aircraft cables or overlaid with cable netting 0.24 or 0.31 in. in diameter, with openings of 0.5-1 in., the most common being 12 x 12 in. Installation of mesh or netting might require cranes or helicopters, especially on steep slopes or where access is limited. Rockfall mesh and cable netting can be anchored at the bottom or, when the plan is to leave room for debris to flow beneath the mesh, left open so the debris can be caught in a catch fence or a retaining wall at the toe of the slope.
Anping Zhuoda hardware wire mesh is a professional manufacturer of gabion nets, Reno cushions, and landscape gabions. Find out more at https://www.zhuomaoyp.com.
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