TECHNICAL | SHAFTS, CAVERNS - HYDROPOWER
FOCUS ON LARGE POWER CAVERNS
A look at the major caverns excavated across the world for major underground hydropower complexes.
TVA’s Racoon Mountain pumped storage plant in Tennessee, US. Image credit: Stantec.
Large, voluminous rock caverns are excavated for major hydrocarbon storage and hydropower power projects, primarily, while some are constructed for specific transport rail/metro stations, utilities with water/sewage treatment works, scientific laboratories doing high-energy particle physics research or materials storage, and there is also civil defence subsurface infrastructure, which occasionally doubles up with leisure space. The specific caverns of many are almost filled with
structural levels and have only limited free open space; the huge and open voluminous caverns are for those designed to hold the likes of hydrocarbons, or those that partly- filled but still boast significant open space, such as with the powerhouse and transformer caverns of underground hydropower complexes. Hydro tunnel infrastructure does occasionally also include fully open caverns too, for specific purposes, such as surge chamber to throttle and dampen the hydraulic shocks and flows. Military and salt mine storage caverns are not
considered here. For this consideration of caverns, we turn our attention
to the many excavated for hydropower complexes over the decades. In particular, the spotlight is on the largest caverns, huge in length and width and height. We can provide this focus, in overview, thanks to a detailed and extensive study
26 | September 2026
made available by courtesy of T&TI’s sister title International Water Power & Dam Construction. Cavern sizes can be considered by various metrics, such
as volume, area of cross-section, and single dimensions of length, width and height (L × W × H). Within underground hydropower developments, the range of projects cover both conventional and pumped storage projects – the former letting flow go, the latter working with to & fro of flows to gain benefit each way. Data can also consider caverns in relation to the
maximum hydraulic heads associated with their locations and with which to drive the water flows under gravity; or, the installed generation capacity; or, the design flows that are conveyed from the headrace feeder and then high-pressure penstocks through the power units of the powerhouse cavern, and then onward into the tailrace, and discharge. While interesting, in many ways in how civil engineering and long water tunnels serve the energy sector, those are not the focus of this brief overview which will stick with large caverns. Constructing such massive voids for powerhouse
complexes in rock calls for significant quantities of additional excavations for adits and other much longer tunnels as part of the projects, and neither are these included in this overview. However, some hydraulic tunnels
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