The subgrade beneath Northeast Boise near Table Rock behaves nothing like the deposits south of the airport along the Boise River. In the foothills you find stiff silts mixed with weathered basalt fragments, while the valley floor carries layers of fine sandy alluvium that shift moisture content with each irrigation cycle. These differences directly control flexible pavement performance. A section designed for the Harris Ranch bench won't transfer to the Vista neighborhood without adjustment. In our experience, pavement failures here rarely start in the asphalt itself. They begin in the base course or the subgrade. We use site-specific CBR values, resilient modulus estimates, and local frost penetration data — typically 24 to 30 inches per Ada County Highway District requirements — to build up the structural number layer by layer. When the subgrade shows plasticity above 20, which happens often in the clay lenses west of downtown, we look at stabilization or a thicker aggregate base. For projects where deeper strength profiling is needed, CBR testing for road subgrades gives us the soaked and unsoaked values that feed directly into the AASHTO 93 equation.
In Boise, pavement life is decided underground. Get the subgrade modulus and drainage path right, and the asphalt almost takes care of itself.
Our approach and scope
Local ground factors
The IBC references AASHTO pavement design standards, but the local enforcement through the Ada County Highway District adds a layer of specificity that catches out-of-town engineers. ACHD Standard Specifications require the pavement design report to demonstrate how the structural section handles the projected traffic load index and subgrade quality, with particular attention to frost protection. In Boise, the risk isn't just the cold — it's the combination of freeze-thaw cycling on silty soils and the irrigation-driven moisture fluctuations that keep the subgrade in a near-saturated state through much of the spring. That sustained moisture cuts the effective resilient modulus by 30 to 50 percent compared to summer conditions. For flexible pavements, the failure mode we see most often is fatigue cracking that starts at the bottom of the asphalt layer and propagates upward, accelerated by a base course that has lost drainage capacity because fines migrated into the aggregate from an unprotected subgrade. A geotextile separator between subgrade and base adds modest cost and eliminates that migration path. For commercial projects along the Chinden Boulevard corridor where the water table sits high in winter, that single design choice often doubles the functional life of the pavement.
Relevant standards
The design process references the AASHTO Guide for Design of Pavement Structures (1993), the ACHD Standard Specifications for Highway Construction (latest edition), the Idaho Transportation Department Standard Specifications for Highway Construction, ASTM D1883 (CBR test), and ASTM D6951 (Dynamic Cone Penetrometer).
Other technical services
Pavement Subgrade and Materials Investigation
Field CBR testing, DCP profiling, soil sampling, and laboratory classification to determine the subgrade resilient modulus and the structural number required for your traffic loading. We deliver a pavement design memorandum that meets ACHD submittal requirements.
Full Pavement Structural Design Package
AASHTO 93 structural design with layer thicknesses, drainage analysis, frost protection verification, and base course specification. Includes ESAL projections, terminal serviceability calculations, and recommendations for geotextile or stabilization where subgrade conditions warrant.
Typical parameters
Common questions
What subgrade CBR value should we assume for flexible pavement design in Boise?
There is no single safe assumption. The silty sands along the Boise River corridor can yield soaked CBR values of 6 to 10, while the clayey silts on the Bench often test at 3 or below. We recommend site-specific testing because assuming a generic CBR of 5 can under-design by 40 percent in weak zones or over-design in the better-draining gravels of the foothill alluvial fans.
Does ACHD require a pavement design report for commercial site development?
Yes. For any development that will dedicate right-of-way to ACHD or connect to the public street system, the District requires a sealed pavement design report that documents subgrade conditions, traffic projections, the structural design methodology, and layer thicknesses. The report must be stamped by a licensed civil engineer.