Civotec Blog

Pavement Isn’t Just Skin-Deep

To understand why trenches are problematic, it helps to revisit the way flexible asphalt pavements are designed. Pavement systems are essentially layered structures meant to distribute traffic loads across increasingly softer layers—typically an asphalt surface, a granular base, and the native soil or subgrade.

When a trench is dug and backfilled, this layered system is disrupted. The backfill material, if not properly engineered, may not match the stiffness or strength of the surrounding soil. This creates a mechanical mismatch beneath the pavement, often leading to strain concentrations, differential settlement, and localized cracking at the surface.

Cracking, especially when driven by tensile strain, is a major mode of asphalt failure. It allows water infiltration, accelerates aging, and diminishes the pavement’s service life. Understanding how backfill stiffness, cohesion, and internal friction affect these strain zones is key to building longer-lasting restorations.

Numerical Modeling Offers New Insight

While many studies have focused on field measurements or empirical observations, this research performed by Civotec Consultant leveraged finite difference and finite element tools to simulate how trench backfill materials influence surface behavior under traffic loading.

The model involved a simplified two-dimensional pavement section over a utility trench. A standard truck tire load was applied above the trench centerline. By varying the stiffness, cohesion, and internal friction angle of the trench backfill while holding other conditions constant, the study was able to isolate the role of each parameter in controlling stress and strain.

Key output variables included:

• Surface vertical displacement (to assess settlement)

• Horizontal strain at the top and bottom of the asphalt layer (linked to fatigue and top-down cracking)

• Shear stress within the asphalt (associated with internal material failure)

• Vertical stress distribution (to evaluate load transfer efficiency)

What Happens When Backfill Is Too Soft?

Among all variables tested, stiffness—measured as the ratio of the backfill’s modulus to that of native soil—had the most immediate effect. As the stiffness ratio dropped (i.e., the backfill was significantly softer than the surrounding soil), both surface deformation and asphalt tensile strain increased sharply.

In one test case, when the stiffness ratio dropped to 0.1, vertical deformation reached nearly 3 mm above the trench, and tensile strains exceeded commonly cited cracking thresholds. This suggests that soft backfill not only promotes settlement but also accelerates crack initiation—particularly bottom-up fatigue cracking from the asphalt layer’s base.

When the backfill stiffness matched the surrounding soil (stiffness ratio = 1), the system responded more uniformly. Strain concentrations flattened, and stress was more evenly distributed across the subgrade. While perfect matching isn’t always practical, this result emphasizes the value of engineered backfills with adequate support properties.

Shear Strength Matters Too

Beyond stiffness, the study looked at two shear strength parameters: cohesion and internal friction angle. Both were found to significantly affect pavement strain and deformation, though their influence behaved differently.

• Increasing cohesion from 0 to 20 kPa reduced surface strain and vertical settlement—but gains plateaued after 15 kPa, suggesting a limit to cohesion’s benefits.

• Increasing internal friction angle, on the other hand, produced consistent improvements across the full range studied (10° to 40°). Strain and displacement declined steadily, with no clear threshold effect.

In both cases, stronger materials led to more efficient stress transfer and reduced deformation. However, friction appeared to offer more linear, scalable performance gains, especially when trench width or depth becomes significant.

Why Surface Strain and Shear Track Together

One of the more interesting observations was the strong correlation between tensile strain and shear stress within the asphalt. In all three parametric studies, reductions in strain were mirrored by reductions in shear.

This suggests that when backfill properties improve, the pavement system as a whole becomes more structurally integrated. Instead of concentrating stress above the trench, it distributes load more broadly, reducing the mechanical drivers of both cracking and shear failure. In practical terms, choosing the right backfill mix can help prevent multiple forms of pavement damage—not just one.

Implications for Utility Design and Maintenance

What this study confirms is that trench restoration should be treated as more than just a compaction job. The mechanical properties of the backfill—particularly stiffness and internal friction—play a central role in determining how the pavement will perform years down the line.

For city engineers, contractors, and utility agencies, this has several implications:

• Specify minimum stiffness and strength requirements for backfill materials, especially in high-traffic corridors.

• Avoid using trench spoils or low-strength soils without modification.

• Prioritize cohesion and frictional resistance in backfill mix designs.

• Where space or cost limits trench width, use higher-quality materials to minimize strain and settlement.

• Consider lifecycle costs when evaluating trenching permits and restoration methods.

These insights can feed into better restoration specifications, more effective fee structures for utility cuts, and long-term reductions in pavement maintenance costs.