Helical Tieback Systems
Helical tiebacks or anchors have established a consistent record of performance through extensive use in the tieback, electric-utility and petroleum industries. Construction applications for helical anchors in retaining-wall tiebacks continue to grow.
The helical anchor’s advantage is how it removes the performance uncertainties and costs associated with a grouted anchor when used in loose sandy soils and low shear-strength clay soils. When placed in the soil, the screw anchor acts as a bearing device. This is a fundamental difference compared to a grouted anchor formed in soil and reliant on friction between the soil and grout.
Collapse of a prepared hole can change a grouted anchor’s dimensions. There is little opportunity to assess the problem’s magnitude and exact location because it is in the hole, out of sight. Protecting grout from such an occurrence adds the extra costs of installing casing. A helical tieback averts these drawbacks by requiring neither an open hole nor a casing.
Helical Tieback Advantages
Several factors contribute to the lower installed costs of helical anchors:
- May be installed with conventional rotary drilling equipment used in grouted anchor tieback construction.
- Faster to install. No drilling or spoils removal.
- Typical installing time is 20 to 30 minutes.
- Testing is possible immediately following installation.
- Monitoring torsional resistance during installation gives an indication of soil strength that can relate to anchor load capacities and help prevent overstress.
- Screw anchors can be withdrawn and reused.
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