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The Perils of Paving on a Soft Subbase/Subgrade: Understanding the Risks

When it comes to constructing parking lots, subdivision roads, walkways, and athletic surfaces, the importance of a solid foundation cannot be overstated. The subbase or subgrade, which serves as the supporting structure beneath the pavement, plays a critical role in ensuring the longevity and stability of any paved surface. Unfortunately, the temptation to cut corners or rush through the preparation phase can lead to disastrous consequences. Paving on a soft subbase or subgrade is a risky proposition that can lead to a host of problems. Below, we discuss the dangers associated with such practices.

Poor drainage on soft subgrade

1. Inability to Maintain Grade:

One of the primary issues with constructing asphalt pavement on a soft subbase or subgrade is the inability to maintain the desired grade. A soft foundation is prone to settling and shifting, which can result in an uneven pavement surface. This not only poses safety risks but also leads to drainage problems, as water may pool in low spots causing premature failure of the paved surface. Maintaining a consistent grade becomes an ongoing battle, requiring frequent and costly repairs.

2. Inconsistent Compaction of the Asphalt Pavement Layer:

Proper compaction of the subbase and/or subgrade is crucial for providing a sound base on which to build the asphalt pavement structure.  Attempting to construct asphalt pavement on a soft foundation often results in inconsistent compaction.

Compacting the asphalt pavement over a soft subbase can be challenging, as it tends to deform under pressure.  Hot mix asphalt pavement requires sufficient compaction to maximize the bond between the hot liquid asphalt and the aggregate.  Without a solid base to provide an equal and opposite force, the asphalt pavement structure cannot attain its optimum density.  This inability to achieve density will lead to a reduction in the structural integrity of the pavement. As a result, sections of the asphalt pavement may be inadequately compacted, leading to differential strength and assured cracking of the paved surface over time.

Deteriorated pavement due to soft subgrade and subbase

3. Trapped Moisture Leads to Frozen Subbase:

In regions with cold climates, trapped moisture within a soft subbase can lead to freezing during winter months. When water in the subbase freezes, it expands, causing heaving and disruption of the pavement above. The repeated freeze-thaw cycles can cause significant damage to both the subbase and the pavement, resulting in a bumpy and hazardous surface that is costly to repair.

4. Inability to Withstand Design Loads:

Pavements are designed to withstand specific loads, whether from vehicles, pedestrians, or other sources. Paving on a soft subbase or subgrade compromises the load-bearing capacity of the pavement system. The weak foundation can lead to excessive stress and deformation, causing premature pavement failure, including cracking, rutting, and potholing.

Poor aesthetic on soft subbase and subgrade

5. Poor Aesthetics:

Beyond the functional concerns, paving on a soft subbase or subgrade often results in poor aesthetics. The uneven surface caused by settling and shifting not only detracts from the visual appeal of the pavement but also poses tripping hazards for pedestrians. Additionally, the presence of cracks and deformations can make the paved area appear unkempt and poorly maintained.

In Conclusion:

The dangers of paving on a soft subbase or subgrade cannot be underestimated. While it may be tempting to expedite the construction process, the long-term consequences can be financially and logistically burdensome. Proper site preparation, including appropriate subbase and/or subgrade preparation and compaction, is an investment in the durability and safety of the paved surface. By prioritizing a solid foundation, you can avoid the pitfalls associated with a soft subbase and ensure the longevity and performance of your paved areas. Remember, a strong foundation is the key to a successful pavement project.  Click here for ways to repair and provide a solid subbase and/or subgrade.

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