Seven difficult cases encountered during the excavation of large-scale caverns are explained:
the conditions of these cases,
the adopted countermeasures, and
the causes of difficulties.
The major cause of problems was due to the weak geological condition that was hard to predict. Therefore, information-aided construction is indispensable to constructing large-scale caverns, By analyzing the measurement results during the excavation of the caverns the followings were revealed:
the most important point in designing is to pay attention not to stress but to the deformation of the rock mass;
the amounts of deformation of rock mass structures vary depending on the kinds of the surrounding rock masses even if the rock stress and the deformability of the rock masses are the same; and
in the case of jointed rock mass a rock mass structure becomes larger, the rock mass may sometimes behave as an anisotropic body.
Over 20 large-scale caverns for pumped storage power stations have been excavated in Japan. Dimensions of those caverns are approximately 50 m high, 25 m wide, and 150 m deep with the section areas of over 1000 m2. This report discusses the following: basic cavern designs derived from various observations and measurements during cavern excavations; characteristics of rock mass behavior; and causes of difficulties in seven different cases encountered during the excavation of the large-scale caverns.
Because of space limitation, it is essential to assess (1) Conditions of these cases, (2) the adopted countermeasures, and (3) the causes of difficulties in these cases. The scale and rock mass conditions of each cavern are shown in Table 1.
Destruction of PS-bolts and local increase of deformation in the cavern wall (Site 1KSY 1968, Table 1) (Hayashi & Hibino 1969)
(I) A number of pre-stressed (PS) bolts (Φ27mm, strength of 608 kN, length of 15 m) began to be torn off at their top heads and the broken heads flew like a bullet when the excavation proceeded down to an elevation of 42 m of the main part (A cavern consists of two parts of ‘an arched part’ and ‘a main part’ as shown in Figure 5). The concrete lining surface began to crack as well. When the excavation proceeded down to an elevation of 40.5 m (77% volume of the main part), horizontal displacement increased up to 35 mm on the draft tube side of the cavern wall at No.1 generator and the deformation did not cease after the blasting. (2) After having examined countermeasure reinforcements using the ‘excavation analysis method (Hayashi & Hibino 1968)’, steel struts of 6 sets in 2 tiers (a set consisted of 4 H-steels) were installed to suppress the deformation of the walls to continue excavating the cavern (Figure I). (3) There was a soft slate on part of the draft tube side of the cavern.