Rigid Pavement Design in Billings, Montana

Billings sits at 3,123 feet above sea level, where the Yellowstone River has carved through sandstone for millennia. When you are placing a concrete pavement slab on the Rimrock or the alluvial terraces of South Billings Boulevard, the subgrade is rarely uniform. In our expertise, the biggest variable is not the traffic count but the moisture sensitivity of the native clay seams that interbed with the sandstone. A rigid pavement design that survives 20-plus years here must address frost depth at 42 inches, sulfate attack potential from the local groundwater, and the 30-degree diurnal temperature swings that open joints wider than the AASHTO default tables predict. We combine the CBR road subgrade assessment with slab curling analysis to keep joint faulting below 0.10 inches over the design life.

A concrete pavement on Billings clay without a capillary break will pump fines within the first three freeze-thaw cycles—the slab loses support before it ever cracks.

Our service areas

Our approach and scope

The geology under Billings is dominated by the Fort Union Formation: interlayered sandstone, siltstone, and claystone that weathers into a medium-plasticity fat clay across much of the Heights and West End. We pull Shelby tube samples from the top 3 feet and almost always find PI values between 20 and 40, which puts the subgrade squarely in the A-7-6 classification. That soil will heave if we do not control the moisture regime beneath the slab. Our rigid pavement designs incorporate a 6-inch capillary break layer of crushed aggregate meeting Montana DOT gradation, and we specify dowel bar baskets at every contraction joint to transfer 40 percent of the wheel load across the joint—backed by Westergaard edge-load calculations rather than rule-of-thumb spacing. For industrial yards near the refinery corridor, we add a lean concrete subbase to push the modulus of subgrade reaction above 200 pci, which drops the required slab thickness by nearly an inch while keeping the tensile stress ratio under 0.50.

Rigid Pavement Design in Billings, Montana — Technical reference — Billings

Local geotechnical context

The IBC 2021 references ASCE 7-22 for seismic loads, and Billings sits in Seismic Design Category B with a 0.10g short-period spectral acceleration. That is moderate enough that most rigid pavement designs do not require base isolation, but it is high enough that a jointed plain concrete pavement can rack at skewed intersections if the subgrade is saturated during a 475-year return period event. The bigger risk on the Rimrock is differential heave: a slab poured half on cut sandstone and half on fill will see a 3/8-inch step within two winters. We call for geogrid-reinforced fill compacted to 95 percent of modified Proctor density and a 12-inch transition wedge at the cut-fill line, because replacing a faulted panel on a commercial driveway costs more than the original subgrade preparation ever would.

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Relevant standards

AASHTO Guide for Design of Pavement Structures (1993) and MEPDG, ACI 330R-08 Guide for the Design of Concrete Parking Lots, ASTM D2487 (Unified Soil Classification), ASTM D1586 (Standard Penetration Test), IBC 2021 / ASCE 7-22 (seismic provisions), Montana DOT Standard Specifications Section 400

Technical data

Parameter	Typical value
Design method	AASHTO 1993 / PCA / MEPDG
Design traffic (ESALs)	500,000 to 25 million
Slab thickness range	6.0 to 11.5 inches
Subgrade k-value target	≥ 100 pci (treated), ≥ 200 pci (industrial)
Joint spacing	12 to 15 ft (unreinforced), per ACI 330
Frost penetration depth	42 inches (Billings building code)
Load transfer efficiency	≥ 75% at contraction joints

Q&A

What is the typical cost for rigid pavement design in Billings?

For a commercial or municipal project in the Billings area, the design package—including subgrade investigation, k-value determination, thickness design, and jointing plan—usually falls between US$1,850 and US$5,800. The range depends on the number of borings, the size of the paved area, and whether we need to run FWD testing on an existing pavement for an overlay design.

How deep should the subgrade investigation go for a concrete pavement?

We sample to at least 4 feet below the proposed subgrade elevation, which puts the borehole bottom below the 42-inch frost line in Billings. If the site has fill thicker than 2 feet, we go deeper to check for buried organic layers or old foundation debris that would create differential support.

Do you use AASHTO or PCA for rigid pavement design?

We use both, depending on the project. AASHTO 1993 is the default for public streets because Montana DOT and the City of Billings accept it. For industrial yards with heavy concentrated loads—forklifts, crane outriggers—we run a PCA fatigue analysis because it handles axle configurations that are outside the AASHTO equivalent single-axle load model.

How do you handle freeze-thaw in Billings concrete pavements?

We specify air-entrained concrete at 6±1 percent air content and a maximum water-cement ratio of 0.45 for exposure class F2 per ACI 318. Under the slab, we place a non-frost-susceptible granular base, and we daylight the base layer to a positive drainage outlet so water does not sit at the subgrade interface during the spring thaw.

Location and service area

We serve projects in Billings and surrounding areas.

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