Earth anchor

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Single helix earth anchors
Guyed mast anchor

An earth anchor is a device designed to support structures, most commonly used in geotechnical and construction applications. Also known as a ground anchor, percussion driven earth anchor or mechanical anchor, it may be impact driven into the ground or run in spirally, depending on its design and intended force-resistance characteristics.

Earth anchors are used in both temporary or permanent applications, including supporting retaining walls, guyed masts, and circus tents.

History[edit]

The first practical earth anchor was invented in 1912 by Albert Bishop Chance in Centralia, Missouri, in response to an ice storm that knocked down his company’s telephone poles.[1] The town of Centralia holds an annual Anchor Day Festival.[2]

Applications[edit]

An earth anchor for guy wires on a guyed mast near Thabazimbi, South Africa

Earth anchors are typically used in civil engineering and construction projects, and have a variety of applications, including:

Types[edit]

Deadman[edit]

A deadman is one of the simplest ground anchors in terms of equipment needed, and is suitable for light loads or temporary installations. It is a horizontal beam, such as a log or steel girder, placed crosswise to the load and buried in a hole in the ground. It can be constructed with as little as a tree branch, adequate rope and adequate digging.[7]

A deadman may also be placed on the surface, held in place by a number of picket stakes. This allows a greater load to be taken than a single row of pickets.

Performance[edit]

Once installed and load-locked, an earth anchor exerts effort to the soil above it, with the soil in turn providing resistance.[8] Upward soil compression created by the anchor is typically exerted in a frustum shaped cone,[9] reflecting:

  • The shear angle of the soil
  • The depth at which the anchor has been installed
  • The load applied to the anchor
  • The size of the anchor and size and angle of its lateral surfaces

When angled these lateral surfaces generate greater cone-shaped soil resistance than a simple cylinder created by purely perpendicular design.[10]

Installation[edit]

Site analysis determining soil load resistance is often required before earth anchor installation.[11] Included are depth that the anchor is to be driven, and soil strength, moisture content, and corrosivity.[12] When appropriate, test installations are done to determine optimal anchor design or conformance with project specifications.

Installation methods differ depending on soil composition and moisture.[11] Earth anchors are commonly driven into the ground using a drive rod and impact hammer. Pilot holes are required in denser soils. After an impact driven anchor has been installed, the drive rod is removed and the anchor load-locked, typically by rotating it ninety degrees. For lighter anchors a hand tool is often sufficient.

References[edit]

  1. ^ "Ground Anchors – The History".
  2. ^ "Annual Anchor Festival".
  3. ^ [1][dead link]Archived 2012-08-21 at the Wayback Machine Core Applications, retrieved 2012-07-09
  4. ^ Geotechnical Engineering Circular No. 4 retrieved 2012-07-09
  5. ^ [2] Archived 2012-08-21 at the Wayback Machine Platipus Anchors, retrieved 2012-07-09
  6. ^ Patent 2765764 Summary
  7. ^ "6: Rigging" (PDF). Steelworker, vol 2. US Navy. p. 6-26. NAVEDTRA No: 14251 – via GlobalSecurity.org.
  8. ^ "Helical Anchors - A complete guide".
  9. ^ "How A Mechanical Anchor Works". Archived from the original on 2012-08-21. Retrieved 2012-07-16.
  10. ^ "Canadian Intellectual Property Office: Patent 2175673 Summary".
  11. ^ a b "An Earth Anchor System: Installation and Design Guide" (PDF).
  12. ^ "Ground Anchor Practice in New Zealand - A Review of Applications, Design and Execution" (PDF). Archived from the original (PDF) on 2012-07-11. Retrieved 2012-07-16.