Tieback Anchors

Since the 1950-s, the Chance Company has been manufacturing multi-helix screw anchors. These anchors have established a consistent record of performance through extensive use in the tieback, electric-utility and petroleum industries.

Construction applications for screw anchors in retaining-wall tiebacks continue to grow. The screw 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 soil.

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. This leaves little opportunity to assess the problem's magnitude and exact location. To protect grout from such an occurrence adds the extra costs of installing casing. A screw anchor averts these drawbacks by requiring neither an open hole nor a casing.

Installation torque limitations may preclude the application of screw anchors in some extremely-dense soils. Torque-monitoring during screw-anchor installation provides a means of quality control. It can disclose valuable information where subsurface conditions vary significantly from those anticipated.

Screw Anchor Advantages

Several factors contribute to the lower installed costs of screw 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. Immediate testing following installation since grouting is eliminated.
• Monitoring torsional resistance during installation gives an indication of encountered soil parameters that can relate to anchor load capacities and help prevent overstress.
• Screw anchors can be withdrawn and reused.