The paper presents the analysis of instrumentation data over a ten year period for the first large cavern constructed and instrumented in soft rock of lower Himalayan region which is seismo-tectonically active. The influence of earthquake on the support pressure has been studied and found significant in the neighbourhood of underlying thick shear zone.


Cette thèse faire I'analyse dell instrumentatoin pendent une periode de dix ans de la première caverne construit et instrumente' dans les rochers mousses de la region de la basse Himalya qui est acttf à cause de tremblement de terre. L influence de tremblement de terre sur la pression significatif dans les environs de rochers dejà inidique à ‘etc’ etudier.


Der Beitrag analysie of die MeBdaten fuer eine Daur von 10 yahren in der essten groBer Kaverne, die in der unteren Himalya-Region in weichem Gesten in scismisch-tektonisch aktivem Gebiet gebolirt und instrumentie of werde. Der EinfluB der Eradbeben anf den auflagedruck in der darmter liegenden dicken Scherzone wurde untesucht und fuer signifikant befunelen.


The underground structures in the rockmass present major design and construction problems in Hydroelectric projects. The design of the powerhouse cavities is highly empirical and cannot be based solely on geological and petrographic description of rocks and on the past experiences particularly in rocks as prevailing in the Himalayas. The theoritical analysis of rock mechanics problems associated with the design of large underground excavations can be relied reasonably when it is based on field measurements. Keeping the above objectives in view, instrumentation data for studying the long -term behaviour of the underground powerhouse cavern have been analysed. The field data has been collected for 8 - 10 year period from Yamuna Hydroelectric Project stage-II Part-I for Chhibro underground powerhouse complex, which was constructed about 25 years ago and has set a major precedent by being the first venture of its type in the soft rocks of the lesser Himalaya.


Lesser Himalayas owing to their unique tectonic history presents a complex hetrogenous mass. The rockmass formations, in general have many folds, faults and thrusts which have been formed due to tectonic deformations. The slopes of Himalayan valleys are steep and unstable. The weak and soft rockmass are susceptible to weathering and erosion. The young rock formations comprises mainly of sandstones with bands of steep dipping shear zones of clay and siltstones. Such rocks are highly weathered and jointed. The major structural features in this area are the two main boundary faults running from Punjab to Assam along the foothills of the Himalaya. The faults are known locally as the Nahan and the Krol thrusts. The project area lies in the Garhwal Lesser Himalaya adjacent to the main boundary fault. The region is transversed by a number of secondary faults and thrusts and is known for intense tectonic activity. From the existence of major thrust and a great number of secondary planes of movements, it is evident that the region under reference has been subjected to seismic activity in geologically recent times. The age of these movements is not known with any certainity but it is considered that the Krol thrust might have resulted from the seismic activity spread over a long period of time from pre-Pliocene to post-Pliocene. In recent times, the only major earthquake reported anywhere near the area were the 1905 Kangra and 1991 Garhwal earthquakes. Other indications of the recent tectonic activity in the region are huge boulders of quartizites found embedded in the breciated, pulverised and gouged material along the Nahan Thrust (Jethwa, 1981).

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