Getting Geotechnical: All Roads Lead to Soil Improvement

The soil beneath a pavement often is integral to long-lasting infrastructure. It supports the structural and load requirements of roads, bridges and abutments while contributing added resilience to flooding and erosion by allowing free water drainage. When soil amendments are recommended, engineers have a few paths toward improvements.

For example, soil compaction and stability offer different benefits to infrastructure projects and are governed by distinguished standards. This column looks at both methods and how they may provide long-term structural viability of infrastructure development.

Paths Toward Soil Improvement

Depending on a project’s location and constraints, engineers may have a few choices regarding how to amend soil to satisfy the foundational requirements of roads and bridges. While multiple soil improvements can be used simultaneously, it’s crucial to determine their exact contributions to ensure cost efficiency.

Although not the most-desired engineering property, density is an essential parameter for soil performance, because soil compaction reduces the air among soil particles to improve its strength and resistance to settlement. However, compacted soil often is more prone to flooding and holding standing water, which may prohibit its use in rain-prone locations. In addition, denser soils may burden the construction of mechanically stabilized earth (MSE) walls. As such, soil density isn’t necessarily the determining factor for long-term pavement integrity.

In contrast, soil stability may provide a better-fit performance-based measure for long-term pavement integrity. Soil stabilization through stiffness improvements can help minimize lateral forces exerted on retaining walls and approaches—especially when lightweight aggregates such as expanded shale, clay and slate (ESCS) are used. It also can support more-efficient structures by reducing the size and strength required of foundations and MSE walls. These outcomes help reduce settlement and erosion to support the pavement’s structural integrity.

How Soil Stabilization Benefits Infrastructure

Soils amended with ESCS can provide more-stable sloping due to the material’s predictably high internal friction angle. These lightweight aggregates impart an internal friction angle of up to 46 degrees, significantly higher than achievable with average compacted soils. Further, this measurement is mainly unaffected by heavy and repeated rainfall. As such, it reduces the chances of infrastructure failure in most climates.

Stabilizing soils can offer benefits similar to compaction while preserving free drainage. As such, this soil-improvement method reduces flooding risks and the amount of standing water in an area. This supports the usability of a road and mitigates the potential risk of damage caused by repeated floods, including soil erosion.

In addition, soil-stability improvements can minimize lateral forces acting on retaining walls. MSE backfills using lightweight aggregates can reduce lateral forces by half to support heavier surcharge loads without bolstering retaining-wall structures. Because these aggregates have a lower density, they need low compaction efforts.

Testing Standards for Soil Stability

Just as there are many paths toward soil improvement, there are several standards for testing the efficacy of soil amendments—all with their parameters and uses. When amending soils, it’s essential to conduct tests that provide a large amount of data without causing undue risk to the locations or those performing the tests.

With this in mind, and according to the Louisiana Transportation Research Center 17-2B research study, it’s recommended to use a dynamic cone penetrometer instead of nuclear moisture-density gauges. The penetrometer reduces safety concerns, training requirements and federal regulations or licensing without compromising the quality assurances provided by soil tests.