HYDRO POWER CAVERNS | TECHNICAL


& Brown in 198010 19837


, and again by Hendron & Fernandez in , and were taken as starter values for the addition


of projects post-1980. The observation of adverse site conditions were taken


as risks for comparison with earlier editions of the hydro cavern tables introduction. As expected, three separate rock type groups show different contributions of risk groups, and the conclusions reached in the 2015 edition of the Yearbook are repeated here, i.e.: ● Sedimentary rocks are sensitive to adverse change in strong support measures, visible by the number of caverns with post-tensioned anchors and parameter variations for poor rock quality. This observation is similar in slightly metamorphic rocks.


● Bedded rocks are sensitive to structurally controlled cavern contour failure, clearly visible from this modelling item.


● Magmatic rocks are highly sensitive to the influence of lateral in situ stresses. This is also found to be similar in metamorphic rocks, which are sometimes found adjacent to them.


Adverse site conditions like high horizontal stress occur most frequently in 26% of magmatic rocks compared to 13% and 19% at the other two major rock formation groups. According to the results published in 1993 by Hibino et al8


higher on average for magmatic rocks than for other formations. Modelling of site-specific in situ stresses may be required at 25% of plants, regardless of rock formation. According to Hudson & Feng12


the typical ratio of


maximum/minimum in situ stress is between 1 and 3, with an average of 1.4 for 12 and 18 layered and magmatic rock sites, respectively. The typical ratio of intact rock compressive strength and maximum cavern contour stress is between 2 and 14, with an average of 5.2, for 12 and 18 layered and magmatic rock sites, respectively. The overall portion of caverns constructed in poor


quality rock is more than 10%, and a similar quantity is observed in China. The first 3-4 risk groups for each cavern may contain ~(2/3) and ~(6/7) of all risks, regardless of rock type. However, these results do not suggest that the two less frequent risks ~(1/7) should be disregarded. Hudson & Feng11,12


list 14 caverns with maximum wall


deformations by extreme values at seven plants, which is equivalent to 17% of the reported conventional hydro plants. Our estimate of 7.5% is therefore more than doubled and surely not biased by choice apart from the large size of turbines and caverns. From references it may be concluded that more


than 67% of the risks are geotechnical, while from site experience this is less than 40% if including all failures at construction sites. Typical cavern risk impacts are listed, and can


be divided into unacceptable, unwanted, acceptable (and insignificant) risks according to the sum of the


14 12 10 8 6 4 2 0


-200


300 800 1300 1800 2300 2800 3300 3800 4300 4800 Conventional


PSP


20 16 12 8 4 0


Poly. (Conventional) Poly. (PSP)


Observations


01 1


Calcar. Sediments 2


Conventional horizontal stress factors may be up to 25%


50 40 30 20 10


30


23 Pump storage


Effusive Intrusive 3


4 Hudson & Feng


5


y = 0.297x +19.4


y = 0.22x +12


4050 Trafo


Pillar 60 Linear (Trafo) 70 80 Linear (Pillar) 90


Top, figure 1:


Total capacity: evaluation of 64 conventional and 43 pumped storage plants in China Centre, figure 2:


Cavern host rock formations: evaluation of 49, 21, and 46 conventional, pumped storage, and plants referred by Hudson & Feng12 Above, figure 3:


Cavern group pillars related to power cavern size: evaluation of 40 conventional and 10 pumped storage plants


The seven most frequent geotechnical risks are:


Layered/intercalated rocks Structure as blocks/wedges Disturbed or clay-filled zones Valley effect, varied stresses Water inflow, permeability Very poor rock or rock burst Large deformation, squeeze


20% 20% 20% 15% 10% 7.5% 7.5%


Typical work performance risks are:


Delayed field investigations Delayed support installation Monitoring system ineffective Poor applied work sequence Poor support performance Poor shotcrete/grout quality Grout-take massively higher


~20% ~25% ~20% ~10% ~10% 7.5% 7.5%


September 2023


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