STUDY ON LONG-TERM BEHAVIOUR OF UNDERGROUND POWERHOUSE CAVITIES IN SOFT ROCKS

Abstract

Monitoring the long -term behaviour of underground rock o penings may be recognized today as an important and essential aid in the design and construction of excavations. Systematic insitu monitoring of the performance of both the rockmass and the support has been found to be one of the most promising developments in underground construction in recent years. In India, several river valley projects have been proposed for construction, particularly in the Himalayan region. These have severe geological, structural and construction problems. Current experience of excavation in the lesser Himalaya has been discouraging mainly due to support problems in the soft rocks under complex hydro-geo1ogica1 conditions and tectonic influences. Further, these problems could be attributed to the non-homogeneous and anisotropic nature of rockmass and their time-dependent behaviour. Consequently, construction of large underground openings in this region has become a challenging task. The Chhibro underground powerhouse complex has set a major precedent*by being the first venture of its type in the lesser Himalaya. The complex is located in poor quality dolomitic limestones which include shears, faults and other discontinuities. A major shear zone (approximately 25 m thick) is within 10 a of the lowest draft tube in the powerhouse area. The excavation is aligned normal to the strike of the rock formation. The long -term behaviour through field observations with strain meters, rock bolt load cells etc. installed in the steel rib supports and in between the cable anchors of the powerhouse (vi) cavity have been studied. The influence of earthquake on the wall support pressure appears to be significant in neighbourhood of the underlying thick shear zone. A new hypothesis on empirical support pressure is proposed for support pressure assessment in seismic region such as in lesser Himalaya. The analysis of instrumentation data for 8-10 year period shows that the roof support pressure increases significantly after charging the water conductor system and during heavy rainfall period. Further, it has been observed that the stresses in the steel-rib supports vary with the quantity of water drawn for power generation. This is due to the effect of adverse hydrogeological condition prevailing around powerhouse complex, which causes seepage problem on the roof of the cavern. Consequently, Barton's theory for ultimate support pressures is found to be inadequate as the same does not take into account the effect of seepage after commissioning of the project. The seepage problem is observed after charging of the water conductor system due to which time-dependent deformations appear to be affected significantly as observed in the roof of the powerhouse cavity. Therefore, Barton's support pressure theory in case of powerhouse cavity should be applied taking into account the effect of seepage after commissioning of the project. This would lead to reduction in the value of joint water reduction factor (Jw) according to seepage pressure at a later date. The analysis of data also suggests that the time - dependent behaviour is not significant, where rockmass is dry, i.e., on the roof of surge tank and the walls cf powerhouse cavity. The time -dependent deformations have been noticed only where there is seepage problem and soluble joint til lings, i.e., (•Qui ot the (vii) powerhouse cavity and also near thick shear zone. The roof of the powerhouse cavity is observed to be completely wet due to seepage from surge tank. So the stresses on the steel rib due to time-dependent deformations (extrapolated for 100 years) is estimated to be approximately 3.4 times the stresses due to longterm support pressure and 5.9 times the stresses due to short-term support pressure from Barton's theory. Near shear zone, it is about 4.2 times the long-term support pressure from above theory. Hence, the stresses in steel ribs near thick shear zone due to time-dependent deformations are found to be approximately 1.27 times the stresses observed at a far distant place which is not under the influence of shear zone. It. therefore, appears that the ratio between the long-term and short-term support pressures is not a constant equal to 1.7 as commonly assumed but may vary from nearly 1 to 5.9 depending upon seepage condition and soluble nature of joint fillings. The observed stresses on the dry root arch of surge tank have indicated a periodic type of rock movement. There is no increase in support pressure with time in this case. The analysis of instrumentation data shows that the earthquake vibrations do not appear to affect significantly the support pressure on the steel ribs. The effect could be instantaneous but the same has not been recorded. However, the wall support pressure in the powerhouse cavity have been observed to increase temporarily during the rainy season and during earthquakes. The percentage increase in static anchor loads due to single small earthquake l<5 M) is less than 2%. The anchor loads on the wall of powerhouse cavity are also found to increase with time due to effect of underlying thick (viii) shear zone and recurring earthquakes which in turns appear to cause accumulation of strains in the rockmass. This problem may not be observed if shear zone is far away, i.e., 1.5 times the span of the opening. The effect of earthquake vibrations has not been observed at all in the pressure shaft liner since the shaft is inherently more stable than high caverns. The assessment of support pressures have been carried out from different existing theories and the results have been compared with the observed value. Underground wedge analysis by Hoek and Bray (I960) may give a realistic range of support pressures when the seepage water pressures after commissioning of the project are taken into account according to joint water reduction factor (Jw) in Q system. Self-draining support system in the roof of the power cavern is recommended as the same has been found to drain out the seepage water, where adverse hydro-geo1ogica1 conditions prevail. The analysis of field instrumentation data has provided useful insight into the long-term behaviour of the underground powerhouse cavities in soft rockmass condition such as in the lesser Himalaya.