SHAFTS, CAVERNS - HYDROPOWER | TECHNICAL


are so large as to be cavernous in themselves, such as rock trap and settling chambers upstream in headraces, or downstream of the power plants where some tailraces can large. But they are not the focus here.


CALL FOR CAVERNS There are many more underground hydropower projects across the world than pumped storage projects. More of each have been built over recent decades and the trend is continuing. The increase in activity comes as renewable power projects are boosted in the energy mix of mature economies, more water resources need developed, and further benefits of having these projects include their fast- response generation capability to quickly meet variable load demands on electricity grids but also support stability of the networks. But the major undertakings of the last few decades have


been in rapidly developing economies in Asia. The abundant water resources of long and wide rivers, and many mountains, plus accelerating demands for power generation capacity have been combining to see many medium-sized and large hydropower and pumped storage projects built and more being planned. Often the caverns associated with projects in these regions are enormous and the largest in their class, as discussed below. But while we are looking at the largest hydro and


pumped storage caverns it must be noted that there are many more across the world and, while smaller, they have each called for significant tunnelling undertakings, often in remote and challenging regions. Around the world many such smaller and medium-sized underground caverns are in operation and more of these are also being built.


VOLUME Considering excavated volumes, China dominates the ranking of largest hydropower power plants for the powerhouse caverns, the biggest being Xiluodu, Baihetan, Three Gorges, Longtan, Nuozhadu, Xiangjiaba, Jingping 2, Wudongde, Laxiwa, Pengshui, and proposed for Longpan. The last five are near 500,000m³ while the others are even larger in their volumes. Pumped storage cavern rankings have more of an international spread but by volume they are – while big – much smaller than those of hydropower powerhouse caverns. The eight largest range in volume from 170,000m³ to 260,000m³ with the smallest (Linth-Limmern) and largest (Lago Bianco) of the range both located in Switzerland. Two others are from China (Qingyuan, Tongbai) and one is in South Africa (Lima/Tubatse). Japan shows a notable presence with three of the largest


pumped storage caverns by volume in the top rankings (Kannagawa, Shin Takase, Kazunogawa), within a tight range of 220,000m³ to 250,000m³. The underground projects also, typically, have


second cavern in their powerhouse complexes – a Transformer Cavern. The second, smaller cavern holds the huge electrical transformers, having them located separate but near the main Powerhouse Cavern, which houses the hydro-mechanical and turbine-generator equipment, and through the bottom of which the water enters and leaves for discharge.


September 2026 | 27


Largest hydro caverns according to volume (m3


Xiluodu, China (x2) Baihetan, China (x2)


Three Gorges, China (x2) Longtan, China Nuozhadu, China Xiangjiaba, China Jinping 2, China


Wudongde, China (x2) Laxiwa, China Pengshui, China


Longpan, China (planned)


):


770,000 700,000 660,000 660,000 580,000 530,000 500,000 480,000 470,000 460,000 490,000


Largest PSP caverns according to volume (m3


Lago Bianco, Switzerland Kazunogawa, Japan Shin Takase, Japan Kannagawa, Japan Tongbai, China


Lima/Tubatse, South Africa Qingyuan, China


Linth-Limmern, Switzerland ):


260,000 250,000 220,000 220,000 213,000 194,000 175,000 170,000


While they are the smaller of the two, the caverns


holding the transformers are large by volume in their own right, and the largest nine in the listings range in size from 116,000m³ to 336,000m³. As such, their excavated volumes are more in the scale of those of the largest pumped storage powerhouse caverns. China, again, dominates the listing with six of its caverns


taking the top places in the Transformer Cavern rankings, being at least 138,000m³ with two of the three largest at 200,000m³ and 220,000m³, and the largest – by far – being the Laxiwa project (336,000m³). Outside of China, others in the listing, and are smaller, include Iran’s Karun 3 and Tajikistan’s Rogun.


AREA, HEIGHT, LENGTH Conventional hydro caverns generally have larger caverns than pumped storage projects and so, too, then are their linear and area dimensions greater.


Alto Maipo powerhouse cavern under


construction, Chile. Image credit: Strabag.


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