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Ground Anchor Systems
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Differences Between Anchor Types

Rock & Soil
Anchors
Case
Histories
Corrosion
Protection
Bonded
Anchor
Differences Between Bonded and Mechanical Anchors Mechanical
Anchor

Mechanical Rock Anchors

Advantages

  1. Less drilling is necessary to develop the same shear cone as the bonded anchor system. Also, less grout is needed since there is less hole volume.
  2. The installer can pre-stress and grout the anchor in the same day.
  3. There is no cracking of the grout column, since the installer is pre-stressing the anchor before grouting.
  4. The oversized drill hole provides for excellent grout coverage.

Disadvantages

  1. The mechanical rock anchor should only be used in competent rock.
  2. The maximum working load for Williams largest mechanical anchor, utilizing a 2:1 safety factor from the ultimate tensile steel capacity, is 180 kips.

Grout Bonded Rock or Soil Anchors

Advantages

  1. Grout bonded anchors can be used in virtually all rock conditions and also in most soils.
  2. The maximum working load with a single Williams bar anchor or multi-strand tendon can exceed 1,000 kips.

Disadvantages

  1. The installer must wait for adequate compressive strength of the grout to be reached before pre-stressing the anchor.
  2. Deeper drilling is required to develop the design load in comparison to a mechanical anchor.
  3. In weak rock or soils, a test program or sample borings should be used to determine drill hole diameter and anchor lengths.

Resin Grout Bonded Rock Anchors

Advantages

  1. Pre-stressing can be accomplished within minutes of the installation.
  2. Resin bonded anchor bolts are one of the most economical temporary rock anchor systems available.
  3. Resin anchoring is successful in most rock types.

Disadvantages

  1. Resin anchors are difficult to protect against corrosion. They require tight drill holes for proper mixing of cartridges, resulting in only a thin resin cover. In addition, resin anchors cannot be centered in the drill hole, which allows the bolt to rest on the bottom or side of the hole. Resin is placed into the drill hole in a premeasured amount which does not account for resin loss into rock seams and cracks. Loss of resin creates unprotected gaps along the anchor, essentially reducing the safety factor of the system.
  2. Resin anchors with lengths over 25 feet are difficult to install because resin gel time often requires speedy installations. Couplings cannot be used with full column resin anchors because their outer diameter is too large relative to the drill hole diameter.
  3. Water presence can greatly reduce the holding capacity of the anchor or cause the anchors to be susceptible to creep.
  4. Temperature affects set and cure times of the resin.

Mechanical Soil Anchors (Manta Ray Anchors)

Advantages

  1. Problems associated with drilling anchor holes are eliminated because the anchor is driven into the soil.
  2. All anchors are tested during installation and provide immediate anchorage. Actual holding capacity is determined during pull testing.
  3. Time and expense associated with mixing and dispensing grout is eliminated.

Disadvantages

  1. The anchors are designed to hold no more than a 50 kip maximum working load. Holding capacity can be limited by the bearing strength of the soil.
  2. Corrosion protection is limited.
  3. Rocks or other obstructions in the installation path can prevent adequate embedment.
Williams Form Engineering Corp.
Rock & Soil
Anchors
Case
Histories
Corrosion
Protection
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