Having solid ground under your feet is important no matter the context, but in road construction it’s critical. Without a strong base underneath to support them, roadways are in danger of buckling, sinking, or crumbling under heavy use and environmental conditions. Before a single foot of asphalt or concrete is laid, road construction crews have to pay close attention to how the soil under it will behave and take the appropriate steps to ensure it delivers the right performance.
The Basics of Roadway Soil Mechanics
OSHA defines four main types of soil classifications:
- Type A is stable, cohesive soil that has a naturally high degree of compressive strength.
- Type B describes mainly cohesive soils that don’t offer the same stability as Type A.
- Type C soils are the least stable, and include sand, gravel, or mud.
- Stable Rock is completely solid mineral, offering the highest degree of stability for road construction.
One of the key metrics for determining the stability of soil is measuring its plasticity. This is a measurement of how much the soil can be deformed before it breaks. Soils with high degrees of plasticity are better at absorbing and retaining water, which makes them poor foundations for road construction projects.
Plasticity is a core component of a soil’s Atterberg limits, which determine the consistency of soil in terms of how plastic and absorbent it is. Engineers often test jobsites for Atterberg limits to understand what they can expect from the soil and whether it will require stabilization to ensure the overall success of the project. One of the most common forms of testing is the California Bearing Ratio test, which measures the amount of pressure needed to penetrate the surface of the soil.
The Role of Compaction and Dry Density
Loose soils where the particles are separated by tiny air cavities can create major issues for roadway construction. Building directly on top of loose soil means the road eventually will force these particles of soil closer together, which contributes to sinking and shifting that will result in potholes, cracks, and other issues that will need to be addressed through more construction.
This is why compaction is a critical element of most road construction projects. Typically, this involves the use of a compactor or roller to compress the soil so any pockets of air are forced out and the soils are pushed as close together as possible. The success of these efforts is measured by dry density, which is the measurement of a soil’s mass per unit of total volume. It’s a key indicator in determining the strength of the soil and how well it will support the resulting roadway. The Proctor test is the primary method used for finding the proper level of moisture for optimal dry density of a given soil.
Preparation of the Subgrade for Added Strength
Compaction is just one element of subgrade preparation, which is the process through which crews ensure the ground underneath the road will be suitable for long-term support. After evaluating the soil, engineers will determine whether topsoil or any other unsuitable elements should be removed. From there, the surface is graded to meet the desired elevations and compacted to improve the density if needed. In certain instances, the use of geotextiles may be called for to hold soils in place. This is done to prevent erosion and preserve the strength and stability of the ground.
Soil Stabilization Techniques
For many projects, compaction and geotextiles aren’t enough to provide long-term stability for roadways. This why soil stabilization may be needed beyond these mechanical methods. The addition of chemical agents such as quicklime and cement helps remove moisture from the soil and give it more strength. In recent years, newer additives have been developed such as enzymes that convert soil into stronger forms or polymers that bond with the soil particles to hold them together with more success.
The Importance of Moisture Control
Ensuring proper drainage of stormwater in road construction is crucial for many reasons. For example, soils that become waterlogged during heavy rainfall can become less stable and lead to roads deteriorating or shifting around under force. Any moisture present in soil also may be susceptible to freeze-thaw cycles in certain climates. This swelling and shrinking of the soils can exert unexpected and unpredictable pressure on the roadway, causing it to buckle and crack.
For these reasons, it’s critical for engineers and builders to consider the moisture control needs of the site before starting construction. Ensuring proper drainage may call for installing surface methods such as ditches or subsurface drains and permeable bases to allow water to drain away from the road. In addition to Proctor tests, engineers often use the nuclear density gauge method to determine moisture content. This process uses radioactivity to track the speed at which particles emitted from the device are bounced back, which helps engineers find the moisture content and density of the soil.
Work with Rock Solid for Optimal Soil Conditions
The soil underneath a roadway can determine the project’s success or failure before any actual work is completed. Having the right expertise in soil stabilization working alongside engineers and contractors can go a long way toward ensuring how long the resulting roadway will last. Rock Solid Stabilization & Reclamation, Inc., is a leading provider of soil stabilization services aimed at making sure roads are built to be as strong as they need to be. To learn more, reach out and connect with a member of our team today.