10.4 Minimum overburden above a pressure tunnel
10.4.3 Types of rocks and galleries; drainage
Concrete dams are built on competent or acceptable rock whenever possible.
On indifferent rock, rock-fill dams would be the preferred alternative.
Tunnels may have to cross different types of rock, some of which are of poorer quality. Difficult problems can seldom be avoided along the whole trace of a long power tunnel.
A tunnel excavated in competent rock, whether lined or unlined, offers few hazards. A simple theory, which will be developed in this chapter, solves the problem of the minimum depth of overburden for a tunnel excavated below a horizontal plain or near an inclined slope. Some of the very large tunnels in the Province of Quebec are of this type. They are concrete-lined, mainly in order to reduce friction losses.
In fissured, well-drained rock, conditions are similar. The stress calculation takes into account the reductions in strength caused by rock fissures, which are assumed to be in a radial direction - the most unfavourable direction. A concrete lining is required which can be considered to be watertight pro- vided its permeability is low compared with that of the rock masses. Con- siderable progress has been made in obtaining watertight linings with the Austrian techniques of shotcrete: a very flexible thin concrete layer, often reinforced with wire mesh, with or without normal concrete lining for additional strength. Soft rock with a low modulus of elasticity, Er9 may cause the concrete lining to be fissured when the tunnel is filled with water under pressure. Use of shotcrete behind the lining, reinforced or not, may be the cure. Rock squeezing inwardly when under excavation has been described for the Malgovert tunnel (section 2.2). These are dangerous rock condi- tions requiring special analysis.
Where there is no natural drainage of the rock masses, then full account has to be taken of water pressure in the rock fissures or dangerous uplift conditions may occur. Alternatively, the rock masses may be artificially drained. This was the case in the Gondo pressure shaft in Switzerland, and the Kemano pressure shaft in Canada. The Gondo, with a steel-lined pressure shaft (fig. 10.13) crosses a geological fault, maximum surge level reaching
1289 maximum surge \ levels in metres
i(U0_a d it AgfrV $Pj%L' /^external water pressure grouting pressure,
6 kg/cm2
^external water pressure
*27 kg/cm
Fig. 10.13 Gondo pressure shaft: diagram of external water pressure; drainage through adit.
1289 m. An adit at level 1040 m was used for draining the fault. Several bore- holes were drilled in the direction of the fault to assist the drainage of the rock and to relieve substantially the cleft water pressure on the outside of the steel lining. The gross head on the Kemano steel-lined pressure shaft is
242 Underground excavations
790 m (fig. 10.14). The steel lining is backed by 'Prepakt' concrete, reputed to be impervious. In order to reduce the outside water pressure on the very high shaft, an intermediary adit was designed to drain the rock masses.
/powerhouse
main pressure tunneb
tank
8 0 0 m
shaft
adit 400 600 '800 1000 m
Fig. 10.14 The Kemano pressure shaft. The adit is used for decreasing the pressure of the interstitial water.
Drainage of rock near vertical unlined shafts leading to an underground power-house is also recommended in order to avoid build up of hydrostatic pressures on the power-house wall (fig. 10.15). The design of drainage
unlined or concrete lined
shaft
* Steel
lining
Fig. 10.15 Suggested drainage of the rock near a pressure shaft.
systems requires great care, as proved by the following case history. The most spectacular failure recorded in recent times in the Alps, was that of the steel-lined pressure shaft of Gerlos (Austria). The rock consists of schists, mostly of doubtful quality, and the overburden is meagre. A drainage gallery was running in the rock parallel to the 1-70-m-diameter conduit.
Ruptures occurred at four points on the lower part of the conduit, in October 1945 and September 1948, where the static pressure was about 413-5 m (highest point of rupture) and 628-0 m (lowest point). Although the steel lining was designed to withstand the full hydrostatic pressure without reach- ing yield point it was, in all four cases, cracked along lines parallel to the
drainage gallery. A considerable quantity of rock was washed away leaving large excavations in the rock masses (fig. 10.16). The generating sets were flooded to the point of immersion. It is believed that this rupture was caused
inclined pressure shaft
drainage gallery
V
Fig. 10.16 Gerlos: damaged pressure shaft. Rupture occurred along the drainage gallery.
by uneven subsidence of the concrete packing along the drainage gallery, which allowed bending stresses to develop in the steel lining.
Power tunnels or discharge galleries passing through the rock abutments of high concrete dams create special problems when they are under full hydrostatic pressure, in spite of shallow rock cover. Such problems have to be analysed with special care in relation to the general stability of the dam abutment. Grouting and drainage systems must be adapted to local condi- tions (see section 13.2).
In recent years experts have considered the possibility of storing gas at very high pressures in underground excavations, old mines or galleries.
Gas-tightness can be achieved with plastic materials. Extremely high pres- sures could be transferred to rock which may be overstressed and crushed to some depth round the cavity.