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Why Lightweight Aggregates May Be The Key To Improved Infrastructure

Lightweight aggregates made from expanded shale, clay or slate (ESCS) can improve soil conditions to help roadways and bridges

Every four years, the American Society of Civil Engineers (ASCE) issues a report card that grades America’s infrastructure. In the latest edition, the nation scored a C- on the overall assessment, indicating that while overall these systems are in fair condition, they exhibit deficiencies and increasing vulnerabilities. Roads, in particular, scored a D on the report, meaning they are, by and large, below standard and approaching the end of their service life. In an explanation of the grade, ASCE states that 43 percent of public roadways are in poor or mediocre condition.

As the nation looks for ways to improve this assessment for the 2025 report card, it is important for repairs and rehabilitation projects to consider long-term resiliency as an essential feature. In doing so, these improvements will not only raise the grade in the short-term but also create infrastructure that will remain fit for the future with minimal maintenance. One way to bolster the overall resilience of a roadway is to assess the stability and quality of soil beneath it.

Lightweight aggregates made from expanded shale, clay or slate (ESCS) can improve soil conditions to help roadways and bridges remain structurally viable for years to come. Likewise, ESCS can positively impact adjacent infrastructure systems that also need improvement such as energy and water (drinking, waste and storm), both of which were graded to be at risk for failure. As such, this material can be one of many “strategic investments” recommended to improve the
nation’s infrastructure.

Lightweight aggregates improve soil conditions

ESCS lightweight aggregates are produced by heating the raw materials in a rotary kiln to temperatures over 1000 degrees Celsius. At this temperature, air bubbles form and remain as the aggregate cools to create a network of unconnected voids. These voids reduce the weight of ESCS to approximately one-half of other common fills. They also contribute to a predictably high internal friction angle, which can reduce lateral loads by more than one-half. Further, because ESCS is free draining, it can withstand erosion caused by flooding or excessive rainfall. Combined, these qualities help soft soils become more stable so they can withstand live loads without settling.

The New Jersey Department of Transportation (NJDOT) understood these benefits when it started improvements to the I-295 to ROUTE 42 to I-76 direct connection in Camden County. The site needed a backfill that would minimize settlement and reduce loads in embankment areas and behind mechanically stabilized earth (MSE) walls. Further, the material used was needed to meet the New Jersey Department of Environmental Protection (NJDEP) requirements for flood hazard areas as well as freshwater and tidal wetlands. ESCS helped stabilize the soil. It is also free draining and chemically inert, so it helped this project improve soil conditions while minimizing the environmental impact of the project.

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Similarly, the North Carolina Department of Transportation (NCDOT) used ESCS to improve two bridge approaches by utilizing the soil stabilization capabilities of ESCS. The projects needed to improve the load bearing capabilities of the alluvial muck and loose, silty fine to coarse sand to ensure the project would stay structurally viable for future use. Because ESCS reduced the lateral forces acting on the embankments and the overall vertical loads on the soft soils beneath, it improved the bridge approaches’ ability to resist settling.

Increases MSE wall performance

Stabilizing poor soils is just one aspect of road improvement. ESCS can also improve the ability of MSE walls to maintain steep slopes. It does so by reducing the weight of backfill from approximately 100 pounds per cubic foot (pcf) to between 55 and 60 pcf, compacted, and by increasing the internal stability/shear strength of the soil to be between 40 and 46 degrees (compared to gravel’s 33 to 40 degrees).

These qualities helped reduce the lateral load forces exerted on MSE walls in a Connecticut section of I-95. One goal for this project was to increase the load accommodations from 40,000 vehicles per day from its original 1958 design to its current capacity—an excess of 140,000 vehicles per day. By utilizing over 100,000 cubic yards of ESCS, this rehabilitation project helped create a resilient section of a well-traveled portion of the interstate without having to build additional support systems for the MSE structures.

However, soil stabilization and improving the structural performance of MSE walls are not mutually exclusive. To alleviate congestion at an interchange between I-10, Bluebonnet Boulevard and Siegen Lane, the Louisiana Department of Transportation and Development (LaDOTD) sought to create a robust and intricate interchange. While an MSE retaining wall seemed to be a promising solution, the project’s geotechnical consultant discovered that due to the low bearing capacity of the area soils, the critical height for these walls would be around 22 feet.

Building above this height with normal weight backfill would result in unacceptable settlements—a major challenge given some of the proposed walls approached heights of nearly 40 feet. ESCS lightweight aggregate reduced the structure’s vertical pressure on the soil, allowing the walls to reach the proposed heights without substantial risk.

Reduced pressure on buried pipelines

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In addition to contributing to resilient roadways, ESCS can also benefit adjacent infrastructure systems including water, sewage and gas pipelines. When looking at these systems alone, ESCS can be a cost-effective alternative to expensive pile and cradle systems as well as sideline risks of conventional fills sloughing off and allowing pipelines to settle and crack. The key to the benefits is the material’s lightweight nature as it reduces pressure on the pipes and the materials under them.

As such, ESCS lightweight aggregate can be a solution for road rehabilitation projects that sit on top of underground pipelines. For example, the Indiana Department of Transportation (INDOT) constructed 3 new lanes along the southbound US Highway 31 in central Indiana. While the task was simple on the surface, the Panhandle Eastern Pipeline Company had buried gas lines approximately 33 feet below the project site. To help alleviate increased vertical load issues, INDOT replaced the in-place soil with ESCS lightweight aggregate.

Likewise, designers of a major interchange in Birmingham, Ala., used ESCS to reduce the weight exerted on a buried concrete box culvert. This drainage system is 20 feet underground and needed a significant amount of fill materials to bring it up to grade. Due to ESCS lightweight aggregates’ density and high angle of internal friction, the material significantly reduced the vertical pressure the nearly 2,500-foot drainage system would need to withstand, alleviating concerns that the project may crack the culvert.

Looking below to improve the surface

While most people’s experience of roads ends with the pavement that touches their vehicle’s tires, a key consideration for long-term resiliency is what’s under the street. The soil beneath the nation’s roads should resist settling due to increased traffic loads, higher water handling demands resulting from the increased frequency of severe weather events and other causes of settlement. Because ESCS lightweight aggregates weigh nearly half as much as conventional fills and have a high angle of internal friction, they can reduce vertical and lateral pressures to enhance the stability of soils and MSE walls. Further, because it is chemically inert and free draining, this material can withstand the demands of projects in areas prone to flooding and heavy rainfall.

As the referenced case studies demonstrate, the benefits of ESCS are not merely theoretical. Departments of Transportation across the country have been using this material for years because it provides long-term value to infrastructure projects. As the industry nears the next ASCE report card, ESCS could be an efficient means for improving the nation’s grade in road, water and energy infrastructure.

Source: csengineermag

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