Prestressed concrete-lined pressure tunnels : towards improved safety and economical design; Dissertation, UNESCO-IHE Institute for Water Education, Delft.

At the global scale, nearly two billion people are still lacking reliable electricity supply. Hydropower can be a source of sustainable energy, provided that environmental considerations are taken into account and economic aspects of hydropower design are addressed. Pressure tunnels are relatively expensive constructions, particularly when steel linings are used. Concrete linings can be economically attractive; however, their applicability is limited by the low tensile strength of concrete. Techniques to improve the bearing capacity of concrete tunnel linings have become one of the interesting topics in hydropower research. One of the techniques available is through prestressing the cast-in-place concrete lining by grouting the circumferential gap between the concrete lining and the rock mass with cement-based grout at high pressure. As a consequence, compressive stresses are induced in the lining. This is meant to offset tensile stresses and avoid tendency for longitudinal cracks to occur in the lining due to radial expansion during tunnel operation. Moreover, as the grout fills discontinuities in the rock mass and hardens, the permeability of the rock mass is reduced. This is favourable in view of reducing seepage. In order to maintain the prestressing effects in the concrete lining, the rock mass has to be firm enough to take the grouting pressure. The grouting pressure, taking into account a certain safety factor, should remain below the smallest principal stress in the rock mass. Since the prestress in the concrete lining is produced by the support from the rock mass, this technique is also called the passive prestressing technique. A classical approach to determine the bearing capacity of such tunnels does exist; but, it is based on the theory of elasticity assuming impervious concrete. Due to the fact that the rock mass in nature is non-elastic and concrete is a slightly pervious material, doubts were fostered by experiences with tunnel failures resulting in loss of energy production, extensive repairs, and even accidents. Record shows that some of the tunnel failures are associated with hydraulic jacking or fracturing. While the former is the opening of existing cracks in the rock mass, the latter is the event that produces fractures in a sound rock. The overall objective of this research is to investigate the mechanical and hydraulic behaviour of pressure tunnels. By means of a two-dimensional finite element model, the load sharing between the rock mass and the concrete lining is explored. This research deals with the effects of seepage on the bearing capacity of pre-stressed concrete-lined pressure tunnels. A new concept to assess the maximum internal water pressure is introduced. The second innovative aspect in this research is to explore the effects of the in-situ stress ratio in the rock mass on the concrete lining performance. The rock mass supporting the tunnel is distinguished based on whether it behaves as an elastic isotropic, elasto-plastic isotropic or elastic transversely isotropic material. In the final part, this research focuses on the cracking of concrete tunnel linings. A step-by-step calculation procedure is proposed so as to quickly quantify seepage and seepage pressure associated with longitudinal cracks, which is useful for taking measures regarding tunnel stability. If the assumption of elastic isotropic rock mass is acceptable, this research suggests that the load-line diagram method should only be used if it can be guaranteed that no seepage flows into the rock mass. Otherwise, seepage cannot be neglected when determining the bearing capacity of prestressed concrete-lined pressure tunnels. In cases of pressure tunnels embedded in an elasto-plastic isotropic rock mass, the Hoek-Brown failure criterion is applicable for investigating the behaviour of the rock mass. When pressure tunnels are constructed in an inherently anisotropic rock mass, the rock mass can be idealized as an elastic transversely isotopic material. Regarding the behaviour of the concrete lining, the combined Rankine-Von Mises yield criteria can be used. While the former controls the response of the concrete lining in tension, the latter in compression. When dealing with a three-dimensional problem of tunnel excavation and eventually the load transferred to the support, the limitation of two-dimensional models can be solved by means of the convergence-confinement method. However, this is not the case when the in-situ stresses in the rock mass are non-uniform. In such cases, the simultaneous tunnel excavation and support installation is acceptable provided that the radial deformations at the shotcrete-concrete lining interface are reset to zero to avoid the lining being influenced by the previous deformations during prestressing. It is evident that the load sharing between the rock mass and the lining determines the bearing capacity of prestressed concrete-lined pressure tunnels. Particularly in the lining, longitudinal cracks can occur along the weakest surface that is submitted to the smallest total stress in the rock mass. When pressure tunnels embedded in elasto-plastic isotropic rock mass, longitudinal cracks may occur at the sidewalls if the in-situ vertical stress is greater than the horizontal. If the in-situ horizontal stress is greater than the vertical, cracks will occur at the roof and invert. When pressure tunnels are embedded in transversely isotropic rocks and the in-situ stresses are uniform, the locations of longitudinal cracks in the lining are influenced by the orientation of stratification planes. If the stratification planes are horizontal and the in-situ vertical stress is greater than the horizontal, cracks can occur at the sidewalls; whereas if the stratification planes are vertical and the in-situ horizontal stress is greater than the vertical, cracks can occur at the roof and invert. When the stratification planes are inclined and the in-situ stresses are non-uniform, longitudinal cracks will take place at the arcs of the lining, and their locations are influenced by the combined effects of the in-situ stress ratio and the orientation of stratification planes in the rock mass. Since crack openings in the lining are difficult to control with the passive prestressing technique, it is essential to maintain the lining in a compressive state of stress during tunnel operation. The attractive design criteria for prestressed concrete-lined pressure tunnels are therefore: avoiding longitudinal cracks in the lining, limiting seepage into the rock mass, and ensuring the bearing capacity of the rock mass supporting the tunnel. All in all, this research demonstrates the applicability of a two-dimensional finite element model to investigate the mechanical and hydraulic behaviour of pressure tunnels. Remaining challenges are identified for further improvement of pressure tunnel modelling tools and techniques in the future