Ground Anchors or Rock Anchors are Deep Foundation elements designed to transmit a tensile load to the soil or rock into which they are embedded. They are often used in applications with uplift in bridge structures, cantilevered foundations, buoyant uplift in structures below groundwater tables, and horizontally to provide resistance to lateral earth pressures in retention structures.
Post-tensioned ground anchor design and installation is covered by specifications from the Post Tensioning Institute (PTI), Deep Foundation Institute (DFI), and the International Association of Foundation Drilling Contractors.
Post-tensioned ground anchors consist of a steel tendon subdivided into three sections:
Anchorage - a combination of the anchor head, bearing plate, and trumpet that connects the anchor to the resisting structure;
Bonded Length - the area permanently grouted in place to the rock or soil around it;
Unbonded (Free) Length - the portion of the anchor that is "free stressing" between the anchorage and the bonded length. Free Length is created by encasing the unbonded length of the tendon in (typically plastic) sheathing.
Ground Anchors are characterized by the fact that they are Post-tensioned (pre-stressed) after being grouted in place.
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Ground Anchors may be installed to counter vertical uplift in a structure created by hydrostatic or overturning forces. Examples include Rock Anchors to tie down a cell tower to counteract overturning due to wind loading or Ground Anchors to resist uplift created by a high underground water table.
Used for temporary Support of Excavation (SOE) or permanent anchored walls, Ground Anchors can be employed to stabilize slopes or excavations by providing lateral resistance to the horizontal pressures applied to the vertical elements of the wall.
Ground Anchors are post-tensioned after the grout has set. A hydraulic ram is used to pre-stress the free length and then the anchorage is installed.