Spokane’s pavement sections face a dual challenge that many generic designs overlook: the region’s basalt-derived silts combined with an average of 50 freeze-thaw cycles each winter. The Spokane River valley, perched at roughly 1,900 feet elevation, sees frost depths that can reach 24 inches, heaving moisture-sensitive subgrades right under the asphalt layer. A flexible pavement design that works in Seattle often fails here because it does not account for the low CBR values typical of weathered basalt residuum. Our lab runs soaked CBR per ASTM D1883 and resilient modulus estimates tied to WSDOT’s pavement design manual, producing cross-sections that hold up under I-90 corridor traffic and Spokane County arterial loads. Before finalizing the asphalt thickness, many projects benefit from correlating subgrade stiffness with a plate load test to verify in-situ modulus and reduce over-design in marginal cut areas.
A CBR of 3 on a South Hill basalt silt demands nearly double the base thickness compared to a CBR of 8, which is why site-specific testing pays for itself in material savings.
How we work
Key design inputs we control:
- Hot-mix asphalt (HMA) dynamic modulus at local binder grades (PG 58-28 or PG 64-22 depending on traffic tier).
- Crushed surfacing base course gradation meeting WSDOT 9-03.9(3).
- Subgrade resilient modulus back-calculated from laboratory R-value or CBR correlations validated for Eastern Washington soils.
- Drainage coefficients tied to Spokane’s 16-inch average annual precipitation and spring snowmelt timing.
- Terminal serviceability index (pt) set at 2.5 for arterials and 2.0 for collectors, per AASHTO recommendations.
Local ground factors
A common mistake in Spokane County is placing pavement directly on moisture-conditioned basalt silt without a separation geotextile or adequate base aggregate. During the spring thaw, the subgrade loses bearing capacity and the base course becomes contaminated with fines within two to three seasons. The result is alligator cracking and rutting that show up long before the design ESAL life is reached. Another risk is ignoring the frost penetration depth when selecting the total pavement section. If the combined asphalt and base thickness is less than 60 percent of the design frost depth, ice lenses can form in the subgrade, causing differential heave that tears the mat apart by March. Our flexible pavement design reports always include a frost protection check and a constructability review of the subgrade preparation sequence.
Applicable standards
ASTM D1883 (CBR), ASTM D2487 (USCS classification), WSDOT M 25-30 (Pavement Design Manual), AASHTO 93 (Flexible Pavement Design), ASTM D4123 (Indirect Tensile, HMA modulus)
Associated technical services
Structural Section Design
Full AASHTO 93 pavement design with traffic projections, subgrade resilient modulus, and seasonal adjustment factors. Deliverable includes AC thickness, base course depth, and subbase recommendations calibrated to Spokane materials.
Forensic Pavement Evaluation
Field coring, dynamic cone penetrometer (DCP) testing, and laboratory modulus checks to diagnose premature failures in existing flexible pavements. We identify whether the root cause is subgrade, base contamination, or mix design issues.
Typical parameters
Quick answers
How much does a flexible pavement design package cost for a Spokane subdivision road?
For a typical residential collector in Spokane County, the structural design package ranges from US$1,680 to US$5,560 depending on the number of borings, laboratory CBR samples, and whether a seasonal modulus study is required. A small cul-de-sac with two borings and basic CBR testing falls near the lower end, while a mile-long arterial with multiple soil zones and resilient modulus correlation work approaches the upper end.
What subgrade preparation do you recommend for basalt silts on the South Hill?
We typically recommend undercutting the top 12 inches of moisture-sensitive silt and replacing it with a geotextile separator plus 8 to 12 inches of crushed surfacing base course compacted to 95 percent of modified Proctor. Lime stabilization is an alternative when the plasticity index exceeds 10 and the project schedule allows curing time before paving.
Do you use the 1993 AASHTO guide or the mechanistic-empirical (MEPDG) method?
We primarily use the AASHTO 93 method because WSDOT still references it for most local agency projects, but we can run MEPDG Level 2 or 3 analyses when the traffic data supports it. For I-90 or Division Street corridors with high truck percentages, we often supplement the AASHTO design with an MEPDG check to validate the bottom-up fatigue and top-down cracking predictions.
What is the typical turnaround time for a pavement design report?
A standard flexible pavement design for a Spokane project takes 10 to 15 business days from the completion of field sampling. Expedited reports can be delivered in 5 to 7 business days when the subgrade data is already available and the traffic analysis is straightforward.