Large underground cavities are proposed to be excavated in the Nathpa Jhakri Project located in the Himalayas. The paper deals with the measurement and interpretation of results of hydraulic fracturing/overcoring tests conducted at various locations of the project.
Situe dans les himalayas, Ie projet Nathpa Jhakri propose à excaver de vasees cavites Souterraines. La note est lur la mesure et l'interpretation des resurtats de la fracture hydraulique et d'autres tests effectives dans pursieurs locarux du projet.
Grosse untergrund Hohlungen sind vorgeschalgen Zur Ausgrabung in der Nathpa Jhakri Projekt, der sich in Himalaya befindet. Das Papier bestatigt sich mit den Massnahmen und Interpretation der Ergebnissen von hydrauliche Brechun tests, die an verschiedene Projektorten ausgefuehrt sind.
The measurement of the state of in-situ rock stress has been recognised for some time as providing essential data for the rational design of underground excavations, based on the principles of rock mechanics. Attempts at estimating the stress state from depth of cover have generally provided unsatisfactory in the Himalayan region. The results of direct stress measurements have revealed a sub-horizontal stress magnitude substantially in excess of the corresponding vertical stress magnitude in this area. The impact of the high horizontal stress is obvious in many of the underground excavations in this region. Direct stress measurements have been few in this area but wherever undertaken have revealed the influence of geological structures (folding, faults, dykes, etc.) on the stress field. Changes to the orientation of the stress field and the relative magnitudes of the principal stress components associated with such situations have been shown to have significant impact on excavation conditions. To date, there is no reliable way of quantitatively estimating such perturbations to the stress field, other than by direct measurements. In the planning and construction of hydro-electric power projects, the design of the pressurised portion of the headrace tunnel requires a balance of geotechnical risks and economic considerations. The lining requirements can have a very significant impact on the placement and design of the machine hall, transformer hall and other excavations and greatly affect the cost of the pressure tunnel itself. This is particularly important in situations of extreme topogaphical relief which are common specially in Himalayan regions of India (Gowd et aI 1983, 1986, 1987 and Srirama Rao et aI 1990). The general aim in the design of pressure tunnels is to minimise the lining requirements without risk of water leakage. Pressure tunnels which are unlined or have concrete linings, over the major portion of their length, have been widely used in hydro-electric projects, for many years when suitable conditions are encountered with no significant permeable rock structures or erosional paths intersecting the tunnel and sufficiently high rock stress conditions to overcome hydraulic pressures. The rock stress conditions at any depth will be a function of overburden and tectonic forces, as modified by rock structural features, topography and groundwater. Numerical methods are often used these days for stress analysis at a given site. However, these modelling methods require a quantitative stress data as input, and direct measurement of stress conditions can be considered essential for their implementation. For assessment of risk of water leakage, rock stress measurements and hydraulic jacking tests should be carried out. This paper presents example of recent stress measurements carried out by Central Soil and Materials Research Station (CSMRS) for the design of powerhouse cavity, desilting chamber and pressure tunnel. Results are discussed in terms of stress variability associated with structural and topographical features.
Stress measurements from the surface and from the underground have been carried out by CSMRS at Nathpa Jhakri Project at the following locations.