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Home › Roadway › Flexible pavement design

Flexible Pavement Design Services in Eugene Oregon | AASHTO-Based Analysis

Geotechnical engineering with regional judgment.

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Applying the AASHTO 1993 Guide for Design of Pavement Structures in Eugene, Oregon, requires a granular understanding of the local subgrade—specifically the alluvial silts deposited by the Willamette River. The city sits at an elevation of roughly 430 feet, and seasonal moisture fluctuations here are brutal on under-designed asphalt. We run resilient modulus tests on undisturbed Shelby tube samples to feed the structural number equation, but the real work happens when you reconcile those lab values with the drainage characteristics of the site. For projects near the wetlands of West Eugene, the design often shifts toward a thicker aggregate base layer to prevent pumping and fatigue cracking. Before finalizing the layer coefficients, many engineers pair this with a grain-size analysis to confirm fines content, which directly impacts the drainage modifier in the AASHTO equation.

Resilient modulus is not a single number. In Eugene’s seasonal saturation cycles, it swings by 40% between August and February, and your pavement section needs to survive the low end of that curve.

Our service areas

Slopes & Walls

Slope stability analysis ›Active/passive anchor design ›Retaining wall design ›
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Underground Excavations

Geotechnical analysis for soft soil tunnels ›Geotechnical design of deep excavations ›Geotechnical excavation monitoring ›
VIEW ALL UNDERGROUND EXCAVATIONS ›

Roadway

Flexible pavement design ›Rigid pavement design ›CBR study for road design ›
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Scope of work

The core of our methodology revolves around the triaxial cell and the repeated load testing frame—equipment that lives in our soil mechanics lab and gets calibrated every 200 cycles against a proving ring. We compact specimens at the target density from a Proctor test and then subject them to cyclic haversine loads that mimic the pulse of a dual-tire assembly. The output is a resilient modulus curve plotted against confining pressure, which gives you the effective roadbed soil modulus (MR). In Eugene’s silty clay subgrades, you typically see MR values clustering between 4,000 and 8,000 psi during the wet season, dropping sharply if the heavy clay content exceeds 30 percent. This is why we push for stabilized subbase layers when the plasticity index creeps above 15—the extra cement or lime treatment adds about 12 bucks per square yard but doubles the expected ESALs before the first overlay.

We also incorporate the Oregon Department of Transportation (ODOT) supplemental specifications, which often override generic AASHTO defaults with regionally calibrated layer coefficients. The asphalt concrete layer coefficient here is typically taken as 0.42 for dense-graded HMA, while the crushed aggregate base sits at 0.14. That might seem standard, but the sensitivity analysis we perform accounts for the specific gradation band ODOT enforces for base rock sourced from the McKenzie River quarries.
Flexible Pavement Design Services in Eugene Oregon | AASHTO-Based Analysis
Technical reference — Eugene Oregon

Area-specific notes

The Willamette Valley’s fluvial-lacustrine deposits put Eugene squarely on top of alternating layers of silt, clay, and poorly graded sand—the kind of profile that traps water right beneath the pavement section. During the winter months, the groundwater table in parts of the Whitaker neighborhood rises to within three feet of the surface. If the design skips a proper subsurface drainage analysis, you end up with a saturated subgrade that loses bearing capacity and triggers alligator cracking within five years. The risk is compounded by the fact that many older subdivisions in Eugene were built on compacted fill that doesn’t meet current ODOT density specs. When we see a site like that, the pavement design has to treat the fill as a high-moisture-susceptibility subgrade, often requiring a geotextile separator and an additional four inches of open-graded drainage aggregate.

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Standards used

AASHTO Guide for Design of Pavement Structures (1993), ASTM D7369-20 (Resilient Modulus by Cyclic Triaxial), ODOT Pavement Design Guide (current supplement to AASHTO)

Technical parameters

ParameterTypical value
Design MethodAASHTO 1993 / AASHTOWare ME
Traffic InputESALs (18-kip equivalent single-axle loads)
Reliability Level (Urban Arterial)R = 85–95% per ODOT
Serviceability Loss (ΔPSI)1.7–2.2 for flexible pavements
Layer Coefficient (HMA)a1 = 0.42 (ODOT dense-graded)
Layer Coefficient (Base)a2 = 0.14 (crushed aggregate)
Drainage Coefficient (mi)0.8–1.0 depending on saturation
Subgrade MR Typical Range4,000–8,000 psi (wet season, Eugene silts)

Common questions

What’s the typical structural number for a residential street in Eugene?

For a local residential street with an ADT under 400 and about 50,000 ESALs over 20 years, the structural number typically falls between 2.8 and 3.2. That usually translates to 3 inches of HMA over 8 inches of aggregate base on a prepared subgrade with an MR of at least 5,000 psi. The actual number shifts depending on whether the street is in the flatlands or up in the South Hills, where the residual clay soils have lower bearing capacity.

How do you handle the wet winters in the Willamette Valley for pavement design?

We sample for resilient modulus in the wet season or condition specimens to a moisture content that mirrors the saturation levels observed from February through April. The drainage coefficient in the AASHTO equation is set to 0.8 or lower if the base course sits within the capillary zone. We also specify edge drains when the roadway cross-section traps water, which is common in the flat terrain west of Highway 99.

What’s the cost range for a flexible pavement design package in Eugene?

A complete package—including field sampling, resilient modulus testing, and the structural design report—runs between US$1,600 and US$5,300. The spread depends on the number of borings, whether we’re testing for both HMA and base course properties, and the complexity of the traffic forecast. A small commercial parking lot would be at the lower end; a collector road with staged construction analysis lands at the upper end.

Do you use the AASHTO 93 method or the newer mechanistic-empirical approach?

We’re set up for both. Most projects in Eugene still go through the AASHTO 93 framework because ODOT’s local calibration is mature and the review cycle is fast. When we move to AASHTOWare Pavement ME, we use the same resilient modulus data but add climate modeling pulled from the Eugene airport weather station, which gives us hourly temperature and moisture gradients through the pavement layers over a 20-year horizon.

What’s the minimum subgrade strength you’d recommend before requiring stabilization?

We draw the line at a soaked CBR of 3 or an MR below 4,500 psi. Below that, the required asphalt thickness becomes uneconomical, and we shift to a lime-treated subgrade. In the Eugene area, the fine-grained Missoula Flood deposits often test at CBR 2 after saturation, so stabilization is more the rule than the exception in those pockets.

Location and service area

We serve projects across Eugene Oregon and its metropolitan area.

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