Measure—don’t assume soil thermal properties
The backfill and soil surrounding the duct bank control the rho value, along with the duct bank itself: its size, the number of conduits and the depth to which it’s buried. When designing trench systems, engineers often assume that the rho value in the soil is 90, as the NEC Guidebook, Division 26, specifies. However, thermal resistivity of soil is not a constant, as soil conditions vary from location to location. “I’ve seen rho values in native soils from 126 all the way up to 220,” says Jean-Philippe Thierry, US Ready-Mix Performance Director at LafargeHolcim. “And the one thing you cannot change about the design, at the point when everyone discovers that the assumption of 90 is wrong, is the soil.” Knowing that soil conditions vary, how did the guidebook decide to assume a rho value of 90 for soil? The reason is that there is no ASTM standard for thermal concrete, and conventional uses for concrete in construction have not required a lot of thought about its ability to conduct heat.
The key takeaway is that heat transfer in soil can be a complex process and 90 is not a magic number. A professional, safe installation of an underground system requires actual measurement and evaluation of thermal properties, which is relatively easy to perform in the field and in the laboratory.
Don’t rely on concrete alone to achieve performance
The performance and construction costs for underground transmission lines are influenced by five factors. Two of these factors are dictated by the project parameters: the electrical load—the amount of power demanded by the facility—and the cabling system. An efficient system maximizes the number of conduits and the size of the ducts while minimizing the depth of burial and cost of the backfill. Design, therefore, becomes a matter of carefully calibrated tradeoffs. How deep are engineers willing to dig the trench? The lower the duct bank is in the ground, the better, as topsoil moisture and ambient temperatures are cooler and less variable. However, deeper trenches increase excavation and spoils- handling costs during construction, as well as maintenance costs over the long term. It is tempting to make the thermal concrete and backfill improve the rho value, but concrete batch plants struggle to mix to a rho value specification. Frequently, batch plants that can meet the rho value specification in one mix have a hard time doing so consistently. When a batch plant can meet the specified rho value, the cabling system is selected, the depth and size of the bank is determined, and the number of conduits is set. However, the assumed rho of 90 for the native soils is not accurate.
Consult experts who marry science, soil, and concrete
Just like native soils, aggregates vary chemically by location. A mix recipe from another location may not perform the same way as the materials found in the location of your current project. To design an efficient and reliable system, it is important to work with experts who understand the base science of how soil, aggregates, and concrete work together to achieve a high level of performance. “Engage these experts at least 60 days before design begins so they can analyze the soil conditions, better predict the heat dissipation required for the buried cables and specify exactly what the performance for the conduit thermal concrete needs to be,” says Thierry. “This is the best way to ensure the system can provide predictable thermal performance and supply consistent electrical power.” Materials do matter—recent advances in thermal concrete LafargeHolcim, a global leader in building materials, has developed and pretested a proprietary concrete mix that offers a range of strengths and levels of thermal resistivity for optimizing the design of duct-bank systems for predictable thermal performance and potential project cost savings. Called Thermaflow™, this innovative thermal concrete product was developed based on extensive material know-how, an understanding of local soil and resistivity values, and rigorous testing. With the support of a world-class research network that understands the base science of the materials and the specific elements that impact thermal performance, LafargeHolcim’s family of companies have proactively designed and tested mixes using local materials.