Before the Overlay: FWD Testing on Southeast Stoney Trail

Nov 4

4 min read

Know what your road needs before you pave

Cracks, rutting and visible patchwork can tell you a lot about a roadway’s condition, but they don’t show the full picture. Before planning an overlay, engineers need to understand how the asphalt, gravel base and underlying soil are working together to support traffic.

As Southeast Stoney Trail (SEST) approached its first planned overlay, ParklandGEO was asked to investigate its structural condition using Falling Weight Deflectometer (FWD) testing. Already providing construction Quality Control (QC) testing for the project, ParklandGEO used FWD data to help answer a practical question: Was the planned 50 mm overlay enough?

A Plan Written in 2012

Built between 2010 and 2013 through a public-private partnership (P3), SEST added 25 km of six-lane highway and nine interchanges to Calgary's ring road. Its pavement maintenance plan included a 50 mm asphalt overlay at about Year 15, and a mill and overlay at about Year 28.

That schedule reflected the traffic forecasts and pavement design assumptions available in 2012. By 2023, the operator had begun a five-year program overlay program. FWD testing, required by the P3 agreement, provided current field evidence to check whether the planned overlay remained appropriate.

Meet the Falling Weight Deflectometer

The machine behind the answer is a Falling Weight Deflectometer, or FWD. At each test point, the FWD lowers a loading plate onto the pavement and drops a weight onto it. On Southeast Stoney Trail, each drop delivered roughly 40 kilonewtons of force, roughly equivalent to the load on one side of a loaded single-axle truck.

Sensors spaced outward from the plate records how far the road surface deflects, or moves downward, under that load. These movements, measured in thousandths of millimeters, help engineers assess the pavement’s structural capacity and the support provided by the underlying soil. No digging. No cores. Each FWD test takes less than a minute. That makes it practical for a busy roadway like SEST, where Calgary’s 2024 traffic map shows approximately 59,000 vehicles per day south of Glenmore Trail.

ParklandGEO completed the testing at night using mobile traffic control and required no full road closures, allowing the roadway to remain open while protecting the testing crews and public drivers.

Why Not Just Look at the Road?

Visual condition surveys show surface distress and help schedule maintenance. But a road can look fine while its structure is weakening or look tired on top and still have plenty of strength left. Neither situation shows up in a visual survey.

Cores and boreholes show exactly what is below the surface, but only at the spot where they are taken, and they mean cutting into the road and closing a lane.

FWD testing fills the gap. It measures structural strength at many points along a corridor without damaging the pavement, so you can see where strength changes and where the treatment should change with it. ParklandGEO can pair FWD with coring, boreholes or Dynamic Cone Penetration (DCP) to confirm the existing pavement structure or can use existing as-built data for FWD calculations and other testing where needed.

An additional 10 mm of asphalt can cost roughly $10,000 per lane-kilometer for mix supply and placement alone. Actual pricing varies, but even a small increase in overlay thickness can add substantially to the paving budget.

What the Testing Found

ParklandGEO tested Stoney Trail from 52 Street to the southwest limits, including the Deerfoot Trail interchange and the east–west legs of the Cranston Boulevard interchange.

The results were compared with the 2019 testing to assess changes in pavement stiffness, effective structural capacity, and support from the underlying soil.

Traffic. Using 2024 Alberta Transportation traffic counts and vehicle classification data, ParklandGEO recalculated design traffic for each segment. It also added bus loading, based on the City of Calgary standard, which the original design had not accounted for. The updated projections came in lower than the original design had assumed.

Pavement. The effective structural capacity of the pavement was above what the original design expected.

Soil. Subgrade support was similar to, or slightly below, the original design values.

ParklandGEO calculated the overlay thickness each individual test location needed, not just a corridor average. These differences reinforced the value of testing individual locations. The FWD results allowed local conditions to be considered when assessing the planned overlay.

The result: The planned 50 mm overlay was enough, and the FWD testing gave the operator evidence to back up that decision. Some ramps required additional structural capacity, but those needs could be met within the planned overlay thickness.

"No Surprises" Is a Result Worth Paying For

Sometimes testing finds weak sections that need more work. Sometimes it shows a road can take a lighter treatment. On SEST, it confirmed the planned overlay.

The value is confidence before committing to construction. The cost of testing was small compared with the potential cost and disruption of an overlay that fell short and needed rehabilitation much sooner than expected.

What FWD Testing Can Do for Your Roads

Assess the existing pavement structure to support rehabilitation planning and design.
Verify design assumptions for the required overlay before the design is finalized.
Identify weaker sections so repairs go where they are needed, saving cost.
Check actual performance against original design assumptions.
Prioritize a road network rehabilitation using objective structural data instead of subjective visual surveys.
Support a capital plan or funding request with evidence.
Assess roads before heavier use to help determine whether strengthening is needed.
Investigate premature pavement distress.
Track structural performance over time.
Monitor pavement deterioration over time to track changes in structural capacity and help plan rehabilitation.

Why ParklandGEO

The FWD collects data. The value is in what happens next. FWD results are most useful when they lead to practical design decisions. ParklandGEO combines pavement analysis, geotechnical engineering and materials testing to assess existing support, update traffic loading and recommend overlay thicknesses and asphalt materials. ParklandGEO skill set is backed up by a robust team of Geotechnical and Materials Engineering Professionals with in-house CCIL certified laboratories across Western Canada.

ParklandGEO has provided FWD testing on Stoney Trail since 2023, helping the project team check the planned rehabilitation against current roadway conditions.

Have a Rehabilitation Program Coming Up?

Testing before the design is finalized gives you time to adjust overlay thickness, address weak sections, and budget for the work actually needed.

Share the roads you’re considering for upcoming construction seasons. ParklandGEO can help identify where FWD testing would be useful and develop a testing program to support your rehabilitation plans. Contact us today!