By Dr. Fariborz M. Tehrani

The ground supporting a structure plays an essential role in what is possible for that project. The stability of soil can determine a structure’s height, weight, and even how sustainable or resilient its construction can be. This vitality can be especially true for projects requiring ground improvement, retaining systems, and other ground and foundation stabilization forms. These projects may incorporate more raw materials in their planning and may also need heavier-duty foundation systems to provide an adequate base. Both needs can potentially enlarge a project’s carbon footprint or make sustainability benchmarks difficult to achieve.

However, for structures built on soft soils, engineers and building professionals can reduce a build’s ecological impact by considering what materials are used as geotechnical backfill. For example, lightweight aggregates made from expanded shale, clay, or slate (ESCS) significantly reduce structural demands and possess a high internal friction angle, further reducing lateral loads. This reduction, in turn, eases material needs for foundation support. Due to their low density, ESCS lightweight aggregates can also reduce transportation costs—financially and ecologically. In fact, from mining to placement, ESCS presents a sustainable option for projects that require soil remediation or ground improvement with substantial needs for moving the earth. 

Figure 1. ESCS reduces transportation footprints to one-sixth of those for conventional backfills.
Photo: ESCSI

Responsible mining and low-impact manufacturing

Sustainable construction encompasses more than the use phase of a building product—it stretches into a product’s future with end-of-life considerations and into its past to the extraction of the very raw materials it is made from. This consideration can include responsible mining practices.

Responsible mining involves determining if a mine is an appropriate use of land and if its development is environmentally responsible, beneficial to workers and affected communities and other societal and ecological considerations. For ESCS, the mining process can benefit local environments and communities by slowing the rate at which materials are taken from one area and transported to another. 

For example, naturally occurring geotechnical fill materials such as gravel or limestone require a one-to-one ratio between extraction and use—every cubic yard or meter of specified material requires an equal amount to be extracted from the earth. This ratio can easily exceed the unit value when considering waste and losses associated with unprocessed mining materials. Hence, this approach can quickly strip an area of resources and profoundly impact local ecosystems. However, because shale, clay, and slate are expanded two to three times their original volume, they can reduce the speed at which a mine extracts materials without reducing the total amount of geotechnical fill provided to job sites. Further, the engineered process technology minimizes waste and recycles losses in production to maximize productivity and reduce mining footprints.

While ESCS requires rotary kiln firing to expand the raw materials, the energy and carbon emissions represented in this process are offset by the reduction of raw material needed as well as the other sustainable benefits across the material’s life and how it reduces material needs in adjacent and complementary building systems.

Reduce fuel needs and emissions caused by transportation

In addition to needing to extract fewer materials from mines, engineers who utilize ESCS lightweight aggregates as a geotechnical backfill can also expect to see significant reductions in transportation costs compared to other materials. This saving is due to the low density of the aggregate. Whereas traditional quarried materials have a typical dry loose bulk density of 95 – 135 pounds per cubic foot (1500 – 2200 kilogram per cubic meter), ESCS aggregates land between 35 – 55 pounds per cubic foot (560 – 880 kilogram per cubic meter).

Figure 2. The overall fuel, energy, emissions and freshwater savings of ESCS use are significant. Photo: ESCSI

Not only does this minimize the number of trucks, railcars, or ships needed to transport the material (thereby streamlining construction schedules), but it also reduces the amount of fuel needed to get the aggregates from manufacturer to site by 50 to 67 percent. That is, while the heavy weight of normalweight aggregates prevents reaching even a struck capacity of many hauling bodies, the low density of ESCS allows heaped capacity and, therefore, facilitates better fleet management with less cost of wear and tear on hauling carriers. On the one hand, this can positively impact a project’s bottom line since lower loads result in higher fuel efficiency, and less fuel generally reduces overall construction costs. Moreover, it drives sustainable practices as reduced fuel consumption translates to fewer nonrenewable resources consumed and less greenhouse gasses emitted. 

WTC Memorial Gardens. ESCS has a high internal friction angle for more stability. Photo: ESCSI

Supports long-term soil stability without contaminating local ecosystems

While ESCS is substantially less dense than other quarried materials, it is not the lightest geotechnical fill available. However, unlike other, lighter geotechnical materials, its ability to reduce the environmental impact of transportation is bolstered by its chemical inertness, easy compactibility, and ability to reduce material needs of overlapping systems. 

When a project site requires ground improvement, it is crucial that the geotechnical materials used do not leach harmful or potentially harmful chemicals into soils and waterways or contain them within their bodies. ESCS, as a geotechnical fill, is chemically inert, non-degradable, non-corrosive, and free draining, all of which ensure the material can handle strain from areas below the water table and locations prone to flooding from heavy rainfall. 

Likewise, ESCS can be easily compacted to save fuel costs as well as reduce fuel and energy needed, greenhouse gasses emitted, and freshwater used in compaction efforts. Lightweight ESCS aggregates can use lighter compacting machines with higher fuel efficiency while achieving 95 percent compaction rates. 

Works with foundation systems to improve overall building design

Finally, ESCS lightweight aggregates have a predictably low density and high internal angle of friction, which can exceed 35 degrees, to significantly reduce lateral earth pressure and dead loads by up to 50 percent. This characteristic is why ESCS is often used to increase the stability of soils in previously unusable or difficult-to-build-on project sites. 

That said, the reduced dead loads and lateral forces also help facilitate more sustainable construction practices when a project is assessed as a complete whole. Due to its ability to stabilize soils, ESCS lightweight aggregate can reduce the material demands of retaining walls, piles and foundations. For example, when designing a new terminal at Port Canaveral, engineers used lightweight aggregate to reduce the diameter and gauge of pipe piles. 

Low density aggregates reduce energy needs for compaction. Photo: ESCSI

While several factors drove this decision, it resulted in fewer materials consumed to build the terminal and less construction waste overall. Therefore, ESCS also addressed the high energy input and emission of other construction materials like concrete and steel. Additionally, the lighter gauge foundation systems also contributed to reduced fuel costs to transport materials to the site. Similar results can be seen in highway stabilization projects, projects on riverfronts that need soil remediation and more. ESCS lightweight aggregates alleviate the strength requirements of neighboring systems to reduce material use and waste—a benefit that increases as foundation size and complexity increase.

Sustainable building aggregates impact every part of a project

From extraction to placement, ESCS lightweight aggregates can help reduce the environmental impact of construction. Taking fewer materials from the earth, reducing fuel consumption for transportation and mitigating material use and waste across the entire structure, geotechnical fills made from ESCS help mitigate the environmental cost of a project. Therefore, the material can be a viable option for engineers when ground retainment or improvement is necessary for a project and sustainable construction is a goal. 

While fill material may not be the most talked-about part of sustainable construction, it nevertheless can contribute to more ecologically conscious building practices. Its role will become necessary as developers look to marginal sites with soft or suspect soils for new construction. However, the shift toward sustainable development highlights the application of ESCS in all projects concerned with public health, safety and well-being.

Dr. Fariborz M. Tehrani, Ph.D., PE, ENV SP, PMP, SAP DSW, F.ASCE is a Full Professor in CSU and the Director of ESCSI with expertise in sustainable and resilient SEMM. Fariborz is a voting member of ACI, ASTM, and TRB, EMI’s Liaison in ASCE STC, and EMI ORC Vice Chair. He is the recipient of ASCE Region 9 Outstanding Faculty Advisor, CHESC Best Practice, and Fresno and San Francisco ASCE Research Awards. Fariborz received BSc from SUT, MSc from AUT, and MS, Deg. of Eng., and PhD from UCLA.