There is always more than meets the eye when it comes to construction. In the pavement industry, the smooth, functional surface receives praise and, in the event of issues, cracks create criticism. No matter the situation, the subgrade is often the unsung hero or the culprit when addressing the current status of pavement.
While the concrete mix and structural design play essential roles in a long-lasting product, understanding the importance of good subgrade treatment is key to constructing durable surfaces.
Below, we will look at the role subgrade plays in pavement performance, the consequences of overlooking the condition of your subgrade, and ways to treat subgrade.
Subgrade and Pavement Performance
The primary function of the subgrade is to provide structural integrity by increasing the load-bearing capacity of the pavement. As a surface is subjected to traffic loads, the subgrade must absorb and distribute the weight to prevent damage. If the subgrade is weak or improperly treated, it will shift under stress, leading to cracks, potholes, and surface deformations. An appropriately treated subgrade creates a stable platform that accommodates the loads placed on the pavement over time.
Additionally, how the subgrade reacts to the expansion or contraction of elements like moisture or freezing will directly impact the pavement’s performance. In regions where wet weather conditions are common, water infiltration can cause the subgrade to swell or shrink, depending on the soil, causing degradation. Colder climates with finer soils become susceptible to frost heave. Properly treating the subgrade reduces the risk of such problems.
Consequences of Poor Subgrade Treatment
As you’ve probably guessed, failing to properly treat the subgrade can lead to several problems. Some consequences of poor subgrade treatment include:
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Premature Pavement Failure
Improperly treated subgrades lead to cracks, rutting, and potholes, requiring frequent maintenance and repair much earlier in the pavement’s life cycle.
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Safety Hazards
Consequently, as with number 1, the deteriorating surfaces pose risks to drivers and pedestrians alike. Poor subgrade can lead to surface unevenness and cracks, creating unsafe conditions and an eyesore. Pavement in volatile climates is more susceptible to issues if your subgrade is inferior.
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More Out of Pocket Costs
Roads built on weak or untreated subgrades require more frequent repairs, leading to higher maintenance costs. Over time, these additional costs can far exceed the initial investment in proper subgrade treatment and jeopardize relationships.
The Basics of Subgrade Treatment
There are several methods used to enhance the strength and stability of the subgrade, ensuring a well-performing pavement:
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Soil Compaction or Over Excavation
Soil compaction is one of the most common methods for improving subgrade strength. It involves compressing the soil to reduce air gaps and increase density, leading to better load-bearing capacity. If you are dealing with poor soil and compaction doesn’t work, while more expensive, you can try over-excavation, which brings in better load-bearing fill.
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Soil Stabilization
When the natural subgrade soil is too weak, soil stabilization techniques may be employed. These involve adding chemical agents with increased binding characteristics, such as lime, portland cement, or emulsified asphalt, to the soil to increase strength. Stabilized subgrades resist water infiltration better and have improved load-bearing capabilities.
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Add Geosynthetics or a New Base Course
Geosynthetics are polymer products often used to reinforce subgrades. These materials act as barriers between the subgrade and the pavement layers, distributing loads more evenly and preventing subgrade displacement. Adding a new granular base course is also a good option for areas with poor soil quality, just remember the new base course is held to the same compaction standards.
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Test, Remove, and Treat It
After trying one of the above options, employ the test, remove, and treat it method. Make sure that the treated subgrade can support your construction equipment. No ruts, no problem. Additionally, do a final sweep and remove any debris, rocks, or vegetation from the topsoil that could impact the subgrade. Finally, if vegetation is an issue, use an approved herbicide to help alleviate any unnecessary growth that will cause problems.
Don’t Ignore the Subgrade!
A good subgrade is the unsung hero and essential component of pavement design and construction. Properly treating the subgrade will increase the load-bearing capacity and durability of your pavement, ensuring a long-lasting product.