The AASHTO 1993 Guide for Design of Pavement Structures remains the backbone of roadway engineering, but applying it in Salt Lake City demands more than a textbook approach. The valley's lacustrine sediments, deposited by ancient Lake Bonneville, create a subgrade that swells with moisture and shrinks in the arid summer heat. This cyclical movement destroys under-designed asphalt sections within a few freeze-thaw seasons. Our technical group starts every flexible pavement design with a forensic-level subgrade investigation, classifying fine-grained soils per ASTM D2487 and measuring the resilient modulus under conditions that mimic the local water table fluctuations. Once the expansive potential is mapped, we model the structural number required to protect the road base from both traffic loading and volumetric soil instability. The Wasatch Front's steep alluvial fans introduce additional complexity: aggregate base layers must be open-graded enough to handle spring runoff from the benches yet dense enough to prevent capillary rise from the high groundwater in the lower avenues. A CBR road test often supplements the resilient modulus data when clients need a direct correlation to the empirical design charts used by UDOT for collector streets.
A flexible pavement is only as reliable as the subgrade it floats on—in Salt Lake City, that means designing for a soil that remembers it was once a lakebed.
Service characteristics in Salt Lake City
Risks and considerations in Salt Lake City
Salt Lake City's grid was platted in 1847 with no knowledge of the lakebed clays lurking five feet beneath the surface. As the city expanded eastward onto the ancient Bonneville terraces and westward onto the saturated valley floor, pavement failures became a recurring budget drain for the municipality. The biggest threat to flexible pavement design here is differential heave: when a road section crosses from the dry, gravelly soils of the east bench into the plastic, high-liquid-limit clays of the central valley, the transition zone cracks within two winters. Frost depth of 30 inches per the IBC compounds this when moisture-laden base layers freeze and expand. Our risk assessment maps these soil transitions using historical borehole logs from the Utah Geological Survey, then specifies variable-depth asphalt sections and subdrainage where the soil plasticity index jumps above 20. Ignoring the Bonneville clay layer is not an option—post-construction forensic studies on Redwood Road and 2100 South have documented reflective cracking that traced directly back to inadequately dried subgrade during spring construction windows. We specify a minimum 72-hour evaporation period after grading before placing the base course when working in the Lake Bonneville sediment zone.
Our services
Flexible pavement engineering in the Salt Lake Valley goes beyond selecting asphalt thickness. Our scope covers the entire pavement structure from subgrade preparation to wearing course specification.
Subgrade Resilient Modulus Testing
Laboratory determination of Mr per AASHTO T 307 on undisturbed Shelby tube samples from the project site, providing the key input for MEPDG analysis instead of relying on default layer coefficients.
Pavement Structural Design and Life-Cycle Analysis
Full design of asphalt concrete thickness, base course gradation, and drainage layers for design ESALs over a 20-year performance period, including winter maintenance impact on surface course durability.
Forensic Pavement Investigation
Post-failure analysis using ground-penetrating radar, core sampling, and dynamic cone penetrometer testing to identify the distress mechanism—stripping, subgrade rutting, or thermal cracking—before designing the rehabilitation overlay.
Common questions
What resilient modulus value do you assume for Lake Bonneville clays?
We do not assume. We test. The resilient modulus for the Lake Bonneville sediment unit varies dramatically with moisture content, from 3,000 psi when dry to under 1,500 psi at optimum moisture. We run AASHTO T 307 cyclic triaxial tests on undisturbed samples from each soil layer identified in the boring logs, then input the actual Mr values into the MEPDG software. When clients need a fast preliminary number, we correlate from CBR tests with the standard Mr = 1500 x CBR equation, but only for feasibility studies, never for final design.
What does flexible pavement design cost for a commercial parking lot in Salt Lake City?
A complete flexible pavement design package for a commercial parking lot in Salt Lake City, including subgrade investigation, resilient modulus testing, and the structural thickness design report, generally falls between US$1,820 and US$4,990. The range depends on the number of borings needed to capture soil variability and whether we run full MEPDG analysis or AASHTO 1993 empirical design. Larger lots with multiple loading zones and truck traffic require more extensive testing and fall toward the upper end.
How does UDOT's pavement design differ from the AASHTO 1993 guide?
UDOT follows the AASHTO 1993 framework but applies regional calibration factors developed from decades of performance data on Utah highways. The biggest difference is the structural layer coefficient for dense-graded asphalt: UDOT uses a slightly more conservative value to account for thermal cracking from Salt Lake City's wide temperature swings. UDOT also mandates a minimum of 4 inches of asphalt concrete over 10 inches of aggregate base for collector streets, which exceeds the nominal AASHTO design for the same traffic level. We coordinate directly with UDOT's Materials and Pavement Section when designing for state-funded projects.
When can you pave in Salt Lake City given the winter restrictions?
The practical paving season in Salt Lake City runs from April through October, though UDOT specification 02745 allows hot-mix asphalt placement down to 35°F ambient temperature with an approved winter paving plan. The real constraint is subgrade moisture: the Lake Bonneville clays hold water well into May, and placing base course on saturated subgrade guarantees future rutting. We recommend scheduling subgrade preparation for June through September, then placing the asphalt in September or October when the air temperature is still warm but the pavement will cool quickly, reducing the risk of tender zone compaction issues. Night paving on I-15 is common in July and August to avoid daytime heat affecting the mix workability.
What geotextile or geogrid do you specify for soft subgrade?
For the soft, high-moisture clays common in western Salt Lake City near the airport and inland port, we specify a biaxial polypropylene geogrid placed at the subgrade-base interface. The geogrid mechanically interlocks with the aggregate base, spreading the wheel load over a wider area and reducing the required base thickness by up to 30 percent in some cases. For separation and filtration where silty subgrade is pumping up into the clean aggregate, we specify a non-woven geotextile with an AASHTO M288 Class 2 survivability rating. The choice between geogrid and geotextile depends entirely on the gradation of the subgrade soil and the groundwater elevation at the time of construction. We make this determination after the grain-size analysis and Atterberg limits testing are complete.