The 2024 Washington State Building Code, which adopts IBC and references ASCE 7-22, governs every anchor embedded within Spokane County. The Inland Northwest presents a sharply layered subsurface—loessial silt over glaciolacustrine clay and dense basalt—that demands a careful distinction between active and passive anchorage. A tendon locked off at 70 percent of its ultimate tensile strength behaves differently in the Latah Formation than it does in Rathdrum Prairie gravel. In our experience across Spokane, the critical variable is rarely the steel; it is the grout-to-ground bond in saturated silt. For projects where the near-surface soil column is poorly characterized, a test pit investigation often reveals more about anchor feasibility than a dozen borings alone.
An active anchor controls movement; a passive anchor waits for it. In Spokane’s layered silts, that distinction decides whether the adjacent building cracks.
How we work
Local ground factors
Spokane sits at roughly 1,843 feet above sea level, where the winter freeze-thaw cycle reaches two to three feet into the ground. Anchored systems designed without attention to frost depth can see the upper bond length degrade within the first season. More consequential, however, is the seismic hazard: the 2001 Nisqually earthquake reminded the Pacific Northwest that deep intraslab events produce long-period motion amplified by Spokane’s basin fill. An anchor that performs adequately under static conditions may experience cyclic bond degradation under a scenario event. The current IBC assigns Spokane County to Seismic Design Category C, but site class effects can push the design spectrum upward. A seismic microzonation study helps refine the ground motion parameters before finalizing the free length and lock-off load.
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Applicable standards
ASCE 7-22 (Minimum Design Loads and Associated Criteria), IBC 2024 / Washington State Building Code, PTI DC35.1-21 (Recommendations for Prestressed Rock and Soil Anchors), ASTM A416 / A722 (Tendon Materials), AASHTO LRFD Bridge Construction Specifications
Associated technical services
Active Anchor Design and Load Testing
Full design package including bond zone determination, tendon selection, stressing sequence, and on-site verification via performance and proof tests per PTI recommendations.
Passive (Tieback) Anchor Analysis
Deformation-based design for soldier pile and lagging walls, anchored shotcrete, or deadman systems where movement tolerance governs.
Lift-Off Testing and Load Verification
Hydraulic jack lift-off tests on existing anchors to verify residual load, detect relaxation, and assess remaining service life.
Typical parameters
Quick answers
What is the difference between active and passive ground anchors?
An active anchor is stressed and locked off against the structure after installation, applying a precompressive force to the ground mass. A passive anchor is not stressed—it only develops resistance once the structure moves enough to stretch the tendon. Active systems control displacement from the start; passive systems tolerate some movement.
What is the typical cost range for anchor design and testing in Spokane?
For a complete design package including site investigation review, bond length calculations, and on-site load testing, the fee typically falls between US$1,110 and US$3,270 depending on the number of anchors and the testing protocol required.
How deep into basalt should an anchor bond length extend in the Spokane area?
In competent basalt of the Columbia River Basalt Group, bond lengths of 8 to 12 feet are common when using neat cement grout. However, the weathered upper surface and vesicular zones require careful probing; we specify water-pressure testing in the bond zone to confirm low permeability before grouting.
What corrosion protection is required for permanent anchors in Spokane?
PTI Class I protection is standard for permanent anchors: corrugated plastic sheathing over the full free length, epoxy coating or encapsulation in the bond zone, and a watertight trumpet assembly at the anchor head. Spokane’s seasonal groundwater fluctuation makes this essential.
How do you verify that an anchor has not lost load over time?
A lift-off test applies hydraulic pressure until the anchor head separates from the bearing plate, directly measuring the residual lock-off load. We compare the result to the original stressing record; a loss exceeding 5 percent generally triggers a re-stress or investigation into creep in the bond zone.