The Hoek and Brown yield criterion was adopted for the prediction of convergence in a number of tunnels in the Himalaya region and compared with measurements in squeezing ground for a hydro-electric power development.
Le critère de rupture sous contrainte maximale de Hoek et Brown a ete verifie dans un tunnel hydroelectrique de l'Himalaya et une relation simplifiee a ete selectionnee pour estimer la convergence des terrains fortement constraints.
Das Hoek und Brown Kriterium warde fuer die Vorhersage von Untertagekonvergenzen benutzt und mit Gelandemessungen in verschiedenen Himalaja-Tunneln verglichen.
In so far as assumptions of forces in tunnel design are conserned, the remark of Terzaghi in 1942 that there was little doubt in his mind that such forces which acted on the tunnel are very much smaller than those assumed by the designers holds good even today. Inadequacy of knowledge regarding the real intensity of stress distribution makes one to lean on customary method of design. This opens up large scope for under standing of the same and every contribution however small, in this field is likely to lead towards rationalization and economy in design.
The state of the art regarding design practice of tunnels virtually being at stand-still most of the designs methods in use today are based on empirical formulations. It is heartening that emphasis is presently being given to the observational approach adopting ‘build as you go’ technique. Improvements in the design methods is the need of the day. This evidently calls for an increased awareness of the mechanism and modes of behaviour of the system composed of tunnel and surrounding medium. Such awareness can best be obtained by a combination of theoretical considerations and analysis of data of the observed behaviour of tunnels in the field.
The Himalayas form the north-western. boundary of the Indo-Australian plate. This is a continent to continent collision boundary, the deliniation of which runs along the axis of the Himalayas. The region remained under water for the greater part of its history. This continental collision has resulted in some of the highest mountain ranges and deepest valleys surpassing th9se On any of the other continental plate boundaries.
The Himalayan region has a potential in abundance for the development of hydroelectric power by virtue of the available water and enormous potential heads.
Construction of tunnels in such a terrain has been extremely difficult. The ground squeezes enormously and continues to do so for long periods. It is seen that the convergence confinement method of rock-support interaction is more applicable in these conditions than the empirical formulations.
The technique of employing two characteristic, curves - one for the ground and the other for the support - has been described in detail by Lombardi(1970), Daeman and Fairhurst(1970), Ladanyi(1974)and Lombardi (1977). It is considered as one of the most promising methods for understanding the mechanics of tunnel deformation and development of rock loads. The evaluation of the characteristics calls for assumptions that do simplify the theoretical solution. obtained for the complex practical problem but the analytical model can be made sufficiently comprehensive to cover a wide range of the practical situations.
The ground convergence characteristic is the relation between the inward radial displacement at the tunnel periphery and the radial pressure or support pressure applied at the periphery. It is assumed that the sequence of face advance and excavation can be represented by a gradual continuous increase of the tunnel convergence and the ground characteristic is then obtained by calculating the support pressure that would be required to maintain equilibrium at each point of this convergence curve. The support characteristic is the load deflection curve of the support. The inter-section between the two characteristic curves refers to the equilibrium point between the ground movement and the support reaction and thus the load mobilised by the support to maintain the stability of the tunnel at the corresponding convergence(fig.1). A complete evaluation of the two characteristic curves requires knowledge of the virgin stresses and the strength and deformation characteristics of the rock mass and the support.