case 2: Tunnel bore stable, tunnel heading not stable
10.11 Rock mechanics for underground hydroelectric power
10.11.5 Modern measurement techniques in underground
Future success in design and construction of underground works will depend, to a large extent, on improved measurement techniques really adapted to the sequence of operations, excavations, lining and grouting.
The classical jacking tests - on which earlier tunnel designers relied - do not yield very convincing results. The stress pattern (compression stresses) under the jack does not compare with the stress pattern about a pressure tunnel.
Several authors favour the 'hydraulic pressure chamber tests' which put the rock under tensile stress in the circumferential direction and yield more convincing values for the E modulus of the rock about the excavation. But this method is costly.
Seismic waves have been used by Kujundzic (Kujundzic, Javanovic &
Radosavljevic, 1970) to check the varying E-value of the rock about a circular tunnel. The result has confirmed the classical Talobre diagram. It is likely that correlations between the dynamic E modulus, measured by the waves, and the static is-value, measured in hydraulic pressure chambers, can be established, as indicated by some remarks of Kujundzic. It is essential to know this lvalue for deformations of the tunnel and convergences of the rock to be correctly interpreted.
The fc-value can be measured in situ either with the 'stress tensor gauge' of the Laboratorio Nacional de Engenharia Civil (Lisbon) (Rocha & Silverio, 1969) or with the simple 'doorstopper' of E. R. Leeman (1969) (South Africa).
The stress tensor gauge consists essentially of a plastic cylinder in which electrical strain gauges are embedded. A 7-5-cm-diameter borehole is drilled to the required depth, a coaxial hole with diameter 3-7 cm and a length of about 90 cm is drilled at the bottom of the previous hole, and the stress tensor gauge is cemented to the walls of this hole. With the doorstopper method, a BX borehole is drilled to the required depth and its end is flattened and polished with diamond tools. A strain gauge rosette is glued to the flat end of the borehole and strain readings recorded. Then the borehole is extended with the BX diamond coring crown, thereby overcoring the rosette support and stress-relieving the core. The BX core, with rosette attached, is removed and final strain readings taken. At an early design stage, some in situ readings are taken giving preliminary information on the probable k- values. Later measurements from galleries and adits can provide some useful checks on the ^-values at selected points of the underground system.
The present attitude of designers is to favour systematic and continuous measurements of rock strains and tunnel deformations. The measured strains should be compared with the local measured E moduli (seismic waves method) and then with the computed values for a series of possible alternatives (assuming the jE-value, the jointing and friction factors, etc.).
From recently published case histories, it can be seen that expert designers favour long cable extensometers penetrating deep into the rock and anchored beyond the estimated RL radius. Such extensometers have been used not only for checking tunnel and cavern roofs, but also in increasing numbers for checking the stability of cavern walls. The measurement of tunnel conver- gence, commonly used in mines, should be introduced in civil engineering for checking vertical and horizontal displacements of opposite points of tunnel circumferences. The rock jointing and the A>value may cause interest- ing distortion of the primitive circular tunnel shape.
Ten cable extensometers were used for the Lago Delio cavern (Italy) (Mantovani, 1970 and Fanelli & Riccioni, 1973) and 28 for the difficult Veytaux power cavern (Switzerland) (Rescher, 1968). As previously men- tioned, Kujundzic used strain meters in the concrete lining of the Rama tunnel and checked the dynamic E modulus with seismic waves. Extensive measure- ments have also been carried out in Japanese underground works.
324 Underground excavations
Measurements made inside or about the St Gotthard Road tunnel have been mentioned in section 10.10.6. More information on modern measuring techniques will be given in section 16.3 dealing with Waldeck II underground power-station. Other methods are described in a paper on Saussaz, a station excavated in creeping rock (Bozetto, 1974). Pressure from the vertical rock walls is supported by two horizontal concrete beams, heavily steel reinforced.
Rock deformations were measured with an extensive system of extensometers, and checked by direct optical measurements (fig. 10.65g).
There is one vital parameter on which we really lack information: the increase in volume AF of loosened 'visco-plastic' rock. Few authors have commented about it and more information is urgently required on this point.
Reports submitted to recent International Congresses show an interesting trend in rock mechanics, which gives a more coherent insight into the behav- iour of rock masses. Modern problems have been discussed for their relevance to civil engineering. It is obvious that additional fundamental research, as opposed to the technical approach, on the behaviour of jointed, fissured, fractured rock masses under strain and stress (see Bieniawski, 1969) is required. New, vitally important, fields will have to be investigated in the laboratories, in conjunction with the tests and measurements in situ, if real progress is to be achieved.
11 Rock mechanics and dam foundations