ACI 423.7-14 Guide: On-Site Design and Construction Rules for Prestressed Ground Anchors

For engineers and contractors working on deep excavations, earth retention systems, and slope stabilization, the stability of the entire project often hinges on the performance of tensioned ground anchors. ACI 423.7-14 provides the critical, field-focused framework for the design, construction, testing, and long-term monitoring of these vital structural elements. This guide translates its requirements into actionable steps for on-site implementation, moving from theory to verified, in-ground performance.

What is ACI 423.7-14 and When Do You Use It?

ACI 423.7-14, “Specification for Unbonded Single-Strand Tendons and Commentary,” is the American Concrete Institute’s dedicated standard for post-tensioning applications outside of building frames—most notably, for permanent and temporary ground anchors. On-site, you encounter this standard when:
* A shoring design for a deep basement calls for tie-back anchors.
* A landslide mitigation project specifies anchored retaining walls.
* A project specification mandates compliance with ACI 423.7 for all prestressed ground elements.

It fills the operational gap between geotechnical design assumptions and the physical reality of installing and stressing a high-capacity anchor. While a geotechnical report provides soil parameters and required capacities, ACI 423.7-14 gives the field team the “how-to” for achieving that capacity reliably and verifying it before the structure assumes the load.

Core On-Site Problems This Standard Solves

Without a unified specification like ACI 423.7-14, projects face significant risks:
* Inconsistent Installation: Variable grouting, stressing, and locking procedures lead to unpredictable anchor capacity and service life.
* Corrosion Failure: Inadequate corrosion protection for the unbonded strand, especially in the critical transition zone, can cause sudden, catastrophic anchor failure.
* Unverified Performance: Lack of standardized proof and performance testing protocols means anchors may be installed below design capacity without detection.
* Documentation Gaps: Missing or non-standard records of installation and testing complicate liability assignment and long-term maintenance planning.

This standard is critical for infrastructure, commercial, and heavy civil projects across North America and is frequently referenced in regions adopting U.S.-based design practices.

Key Technical & Safety Requirements for Field Application

ACI 423.7-14’s power lies in its specific, sequential requirements for anchor construction. Key differentiators from general post-tensioning or anchor guidelines include its holistic treatment of the unbonded tendon system.

1. Material and Component Control: The standard mandates strict traceability. On-site, you must verify mill certificates for the prestressing strand and have documentation for the proprietary anchor head (wedge plate, trumpet, etc.) and encapsulation system (sheathing, grease). The sheathing must be inspected for damage before insertion, as any breach compromises the essential unbonded, corrosion-protected status of the strand in the free-stressing length.

2. Step-by-Step Installation and Grouting Protocol:
* Drilling & Assembly: The borehole must be cleaned per geotechnical requirements before inserting the tendon assembly. The tendon must be assembled with centralizers to ensure grout cover around the bonded (ground) length.
* Grouting the Bond Length: This is a critical phase. The standard emphasizes a continuous, tremie-like grout placement from the bottom of the hole upward to avoid voids or segregation. Grout mix proportions and fluidity tests are required before injection.
* Protecting the Free Length: The unbonded free-stressing length must be reliably isolated from the grout, typically using a corrugated plastic or steel sheath, and filled with corrosion-inhibiting grease.

3. Unique On-Site Verification: The Stressing and Testing Regime
This is the core of on-site compliance. ACI 423.7-14 outlines a mandatory multi-stage testing sequence that must be witnessed and recorded:
* Proof Test: Every production anchor is proof-tested to a load typically 20-30% above its design lock-off load. This verifies the anchor’s immediate capacity and load-displacement behavior.
* Performance Test: Conducted on selected “test anchors” (often the first 2-3 anchors of each type), these are taken to higher loads, sometimes to ultimate failure, to validate the geotechnical design assumptions for the site.
* Lift-Off Test: After locking off and seating losses have stabilized (often 24-72 hours later), a lift-off check is performed to confirm the locked-in load matches the design requirements. This catches any significant load loss due to creep or wedge seating.

On-Site Safety Control Point: The stressing operation itself is a high-risk activity. The standard implicitly requires the use of calibrated, remote-readout jacking equipment, proper anchor head safety cages, and exclusion zones behind the jack during stressing to protect personnel from potential strand or component failure.

Regulatory Context and On-Site Compliance Workflow

ACI 423.7-14 is often invoked by the project’s geotechnical engineer of record (EOR) and is a common requirement in project specifications to satisfy building code (e.g., IBC) and OSHA requirements for earth retention systems. Compliance documentation is not optional; it is the primary evidence for regulatory inspections and permit sign-offs.

The On-Site Compliance Packet for Each Anchor Must Include:
1. Anchor Log Sheet: Unique ID, location, installation date/time, drilling records.
2. Material Certifications: For strand, grout, sheathing, and grease.
3. Grouting Records: Mix design, batch tickets, injection pressures and volumes.
4. Stressing Log: The most critical document. It must record jack calibration date, incremental load steps, corresponding elongations, hold periods, final lock-off load, and lift-off test results. The log must be signed by the stressing technician and the inspecting engineer.

Regional Implementation Note: While other standards like BS EN 1537 (Europe) cover ground anchors, ACI 423.7-14 is distinctly focused on the unbonded, single-strand tendon system prevalent in U.S. practice. Its testing and lift-off verification protocols are more explicitly detailed for field execution compared to some broader international codes.

Target Professionals and Risks of Non-Compliance

Who Uses This On-Site: Geotechnical Field Engineers, Earth Retention Contractors, Special Inspectors (often hired per IBC Chapter 17), Construction Managers, and Project Foremen.

When It’s Used: During the pre-construction submittal and shop drawing review phase, daily during anchor installation and grouting, and at the critical moment of every stressing operation.

On-Site Risks of Ignoring ACI 423.7-14:
* Catastrophic Structural Failure: An under-capacity or corroded anchor can lead to shoring wall collapse, resulting in project devastation, injury, or loss of life.
* Massive Rework and Delay: Failure of a proof test may require redesign or ground improvement, halting excavation and crippling the schedule.
* Regulatory Stop-Work Order: An inspector rejecting inadequate documentation or procedures can shut down all earthwork.
* Long-Term Liability: Failure of a “permanent” anchor years after project completion can lead to major litigation, with the absence of compliant installation records being a severe liability.

Real-World On-Site Scenario

A field engineer on a 40-foot deep excavation for a new hospital is reviewing the stressing log for anchor #15. The log shows the anchor met its proof test load, but the recorded elongation was 15% less than calculated. Per ACI 423.7-14 guidance on evaluating load-extension curves, the engineer suspects “drag” in the unbonded length. They order a re-stress after re-greasing the trumpet, and the elongation then matches the calculation. This simple, standard-prescribed check prevented accepting an anchor that had hidden friction, which could have led to premature lock-off load loss and potential wall movement.

Common On-Site Misconceptions

1. “Grout is Grout.” Neglecting the fluidity and expansion requirements for bond length grout can result in incomplete encapsulation of the tendon, reducing bond capacity and compromising corrosion protection.
2. “A Passed Proof Test is Enough.” Skipping the subsequent lift-off test misses the critical verification of actual locked-in load. An anchor can pass a proof test but still lose significant load during wedge seating, leaving the wall under-supported.

By treating ACI 423.7-14 as your daily field manual for ground anchors—from material receipt to final test signature—you transform a hidden, geotechnical element into a verified, reliable, and compliant structural component.