case 2: Tunnel bore stable, tunnel heading not stable
10.11 Rock mechanics for underground hydroelectric power
10.11.2 Classification of rock types for underground hydro-
Basically any decision on the design of an underground power station depends on the strain-stress pattern to be expected around the large cavities in relation to the intrinsic curve of the rock mass, in situ (see fig. 4.10).
(a) In case the most dangerous Mohr circles do not touch the intrinsic curve, the design could, theoretically, dispense with systematic rock support.
(Cables could be used locally for reinforcing some less competent rock zones.) This is the case for several underground stations excavated in the Scandina- vian granite, where there is no support for the rock vault nor for the rock walls. It is also possibly the case for Cabora Bassa (Jaeger, 1975/76) where rock is excellent. But at Cabora Bassa residual rock stresses stored in the rock mass are extremely high, far higher than the stresses to be expected from the rock overburden. The model tests showed high stress concentrations along the cavern walls, mainly where galleries cut the hall at a right angle, and where stresses are very near the ultimate rock strength.
(b) In many cases the designer realizes that rock is overstressed. That this is the case may be due to general weakness of the rock. In this case the approach by Fenner-Talobre, or better the method developed by Kastner will relate the required rock support to the depth of the damaged rock around the cavern and the physical rock characteristics.
Other possible cases concern local weak rock seams and the possible rupture by shear failure along these seams.
(c) Creeping rock conditions: slow creep of rock masses has been observed when slow rock deformation extends over many months. Special designs have to be worked out to contain such deformations. The concrete founda- tions of the heavy rotating runners and generators must be erected with the utmost precision and be protected against any slow rock deformation. The Saussaz underground power station is an example of a large excavation in creeping rock masses (Bozetto, 1974 and Jaeger, 1975/76).
in m 655.4 -
640.0 632.7.
628.5
19,50
discharge canal
;enerator
valve chamber (a)
249.6
247.1 240.3 ,
discharge canal
warm-air outlet
access and ventilation
tunnel to expansion
chamber
sluice gates
Elevation in m
323.5 320.2 318.4 314.9 313.6
discharge canal.
draught tube (d)
(c)
m ằ•
15 10 5 0 5 10 15
Fig. 10.65 Types of underground power-station: (a) Innertkirchen, Switzerland (gross static head 672 m); (b) Santa Massenza, Italy (gross static head 590/460 m); (c) Santa Giustina, Italy (gross static head 182/95 m); (d) Isere-Arc, France (gross static head
152 m).
25.65 m
vertical section horizontal section
(0
730.34
extensometer L2
(reinforced concrete- frame)
*"* <L>
ằSe^ opposite
Fig. 10.65 (e) cross-section of the machine hall at Saussaz power station showing the steel supporting struts (after Bozetto, 1974 and Jaeger, 1976); (/) reinforcement to help prevent deformation at Kisenyama underground power station, showing steel reinforcing frame (after Yoshida & Yashimura, 1970 and Jaeger, 1976); (g) the method of measuring rock displacement using Monta-Mess measuring rods at the Saussaz power station (Bozetto, 1974; Jaeger, 1976).
H.W.L.
el. 436 ft
38 ft 0 in dia.
concrete lined manifold •
surge tank
^ max. surge level el. 476 ft
38 ft 0 in dia. concrete lined conduit
penstock steel liners 497ft *
\ \
(b)
n powerhouse j penstock steel liner
l. 49 ft
Fig. 10.67 Plan and profiles of Bersimis no. II development, (a) profile of supply tunnel and penstock;
(b) plan of manifold, penstocks and power-house; (c) profile of penstock (after McQueen et al., 1958).
manifold and surge chamber
_6Oft__O_in _^
o 10 20 30scale in feet
Fig. 10.68 The Chute des Passes power-house and draft tube manifold (after McQueen et al., 1958).
Hayashi has investigated the case of progressive rock deformations penetrating inside the rock mass around a cavern. The deformations are considered as being a function of the time (Hayashi, 1970).
In all these cases (a) to (c) engineering geology plays a major part in the choice of the site and in many detailed decisions. There is no sound engineering without sound geological advice. The geologist should warn about rock masses being part of an unstable area. Faults have to be carefully investi- gated for possible unstability. The old rules, often advocated in the past, concerning the position of the cavern long axis relative to the most dangerous family of joints, stratifications and faults, still hold.
(d) Despite thorough investigation errors may still occur. The cavern for the underground power-house of Verbano, of the Maggia system (Switzer- land) had the correct orientation. During excavations a rock slide occurred at one end and the cavern had to be shifted along its axis and relocated in a better rock mass.
A large geological fault crossed the rock downstream of the surge tank of the Santa Giustina power-station (Italy). The pressure tunnel and surge tank are located in fissured hard dolomite. It would have been possible to locate the power-house in the dolomite and cross the fault with the tail-race tunnel. In the final design the power-house is located downstream of the fault in plastic marl. Because of this decision the pressure shaft crosses the fault. Instead of a conventional steel-lined shaft, the designers decided to build a self-supporting pressure pipeline, capable of withstanding the full hydrodynamic pressures, located in a steeply inclined gallery and wide enough for inspecting the pipeline (fig. 10.65c).
Elevation Two 225-ton cranes with in ft. 25-ton auxiliary hooks
exhaust fan transformer vault
800 ft of rock cover
284
247
218
m210
196 184
suspended ceiling
ventilation tunnel
permanent access^
fc tunnel between turbines 8 and 9
high voltage cable
•tunnel opposite turbine 5
construction .access tunnel at
turbine 4
control cable gallery
tailrace tunnel / tailwater for '
Q = 5000 cfs bus gallery
m^^^Tiu^d
j ^ . valve chamber access tunnel s between turbine units 8 and 9
5 ft dia. penstockpipe 141 ft 9 in
turbine
_j scale in feet 0 10 20 30 40 50
Fig. 10.69 Underground power-station at Kemano, Canada (gross static head 790 m).
valve .ằ- chamber
6 in-dia.
penstock pipe 51 in double seal
sphere valve
!l5l !22l J2li J20| !19i j l 8 i j 17 j j l 6 j
draft tube manifold plan
section B-B section A-A
Fig. 10.70 Bersimis no. 1 power-house, excavation sequence.
Because of the pressure which may develop in the plastic marl, the power- house excavation is oval-shaped, heavily lined, with a thick horizontal beam which reinforces the structure at generator floor level (fig. 10.65c). Strong horizontal steel beams were also adopted for stabilization of the vertical walls of the Saussaz machine hall.