Trusted Foundation Repair from Residential to Heavy Commercial
What are tieback anchors?
Tieback anchors are either solid bar or multi-strand tendons that consist of a bond length, free stressing length and anchor tail.
How are tieback anchors installed?
The anchors are installed in a drill hole, which is accomplished using open hole or cased hole drilling techniques. The anchor is grouted, allowed to cure a minimum of 3 days and tensioned, allowing the load capacity derived deep in the ground to be transferred to some structure at the ground surface.
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The bond length of the anchor is founded in competent material behind a failure plane, whether it is a defined slip surface or an active wedge of material that tends to slide due to soil properties. The bond length, drill hole diameter and soil properties determine the geotechnical capacity the anchor will develop. The structural capacity is based on the tendon diameter, tendon strength and number of tendons in an anchor.
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The free-stressing length is the portion of the anchor that has a grout bond breaker around it and allows the steel tendon to elongate during tensioning and lock-off, thus permitting load transfer to an above ground structure. The bond breaker will consist of a greased and sheathed length of tendon, typically PVC and grease for solid bar tendons and grease and extruded HDPE for strand tendons. Minimum unbonded lengths are 10’-0” for ASTM A722 150 KSI solid bar tendons and 15’-0” for ASTM A416 multi-strand 7-wire 270 KSI tendons.
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The anchor tail is required above grade to allow stressing equipment to be installed to place tension on the anchor. The anchor tail contains the anchorage that effectively transfers the load derived deep in the ground to the surface structure and consists of steel bearing plates and hex nuts for solid bar tendons and wedge plates, wedges and steel bearing plates for multi-strand anchors. A hydraulic center-hole jack and stand-off chair is placed over the anchor tail, around the anchorage so crews have access to the hex nut or wedges. The anchor is either proof or performance tested and then returned to a specified lock-off load where the hex nut is fastened securely to the bearing plate, or wedges seated in the wedge plate to hold the tension on the face of the structure after the load is released from the center-hole jack.
What benefits can tieback anchors provide on my project?
Straight Line Construction and Ram Jack companies have installed tieback anchors on many projects in the Rocky Mountain Region.
Tieback anchors have many uses and can be temporary or permanent. They can be used for temporary or permanent excavation support, support of existing structures, seismic upgrades of existing structures, permanent foundation walls, tie-downs for dams, towers and buoyant structures and landslide mitigation.
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Tieback anchors and anchor systems are designed in accordance with FHWA-IF-99-015 Geotechnical Circular No. 4 “Ground Anchors and Anchored Systems”, as well as the Post Tensioning Institutes “Recommendations for Prestressed Rock and Soil Anchors”, 5th edition, 2014.
What are the advantages or tieback anchors?
Advantages of tieback anchor systems include:
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Can develop very large structural capacities
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Geotechnical capacities can be increased by increasing drill hole diameters and bond lengths or utilizing pressure grouting and post-grouting techniques
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Can be very long (anchors over 50’-0” in overall length typically become multi-strand anchors due to economy and practicality of installation)
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Suitable for all ground conditions
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Limited access installations can occur with multi-strand anchors
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Each anchor is tested during tensioning verifying the anchor capacity and the depth in the anchor at which the load transfer is occurring
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Utilization of high tensile strength, low relaxation steel tendons ensures tension on the structure is maintained
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There are many options for corrosion protection, promoting very long design life
