Electrical and Civil Engineering Challenges for Underground Cable Installations in Urban Settings

Heat-producing power cables are enemies of one another due to mutual heating. Above-ground electrical infrastructure (overhead lines) experience natural and forced convection in air and is unaffected by proximity to other equipment, while power cables are exposed to thermal conduction through soils and backfills that is far less efficient. Buried cables suffer from mutual heating that is exacerbated by some of the design challenges civil engineers must address:

physically placing the cable conduits and splice vaults in the ground along congested city streets. Close proximity and burial depth are at odds with better cable ratings, yet these are sometimes the only options for civil engineers to find alignments. Placing cable circuits close together provide mutually detrimental reductions in ratings on the cables, and mutual heating effects become exaggerated as burial depth increases. However, dealing with limited space within the public right-of-way can require compressing the cable trench with smaller phase spacing, putting a new cable trench close by to other existing utilities, or putting new circuits deeper, but all of which lower the circuit ratings with other factors held constant. Utility companies seldom want or allow that new circuits reduce capacity on existing circuits where parallel runs or crossings occur, so there are design challenges. Placement of splice vaults especially where dedicated manholes for each circuit are required is an additional challenge.

This paper describes strategies for civil and electrical engineering disciplines to collaborate to successfully design underground cable circuits considering the various design criteria. Various construction alternatives may be considered including splitting duct banks, open-cut trenching, pipe-jacking, microtunneling, directional drilling, and box tunnels, all of which have associated impacts on cable ratings, extent of permits needed, property rights, and conflicts with stakeholders including traffic management. Information is provided on the fundamental aspects of each construction method, and associated impacts on a normalized rating scenario. In some situations, selecting certain construction alternatives have a more significant impact on cable size selection or the ability to meet a specific rating requirement. The reader will benefit from understanding these alternatives to be better able to make design choices.