Parametric Study of Steel-Lined pressure Shafts in Anisotropic Rock
Steel-lined pressure tunnels and shafts are used to convey water from the reservoir to the turbines in high-head hydroelectric power plants. An axisymmetrical multilayer model is of-ten considered by engineers for the design of the steel liner. Stresses and deformations can then be computed with a closed-form analytical solution in isotropic rock. When the rock is anisotropic, the lowest elastic modulus of the rock measured in situ is often considered in the analytical solu-tion, which is regarded as conservative. In this work, the behaviour of steel liners in anisotropic rock was studied systematically by means of the Finite Element Method (FEM). The materials, namely steel, backfill concrete, near-field rock and far-field rock were modelled as linear and elas-tic, and a tied contact was assumed between the layers. The influence of geometrical and material parameters under a quasi-static internal water pressure was studied through an extensive para-metric study. It was observed that, compared to the corresponding results in isotropic rock, maxi-mum stresses in the steel-liner can be reduced up to 25% when anisotropy is considered. On the contrary, the maximum stresses in the far-field rock can be largely underestimated, namely up to 65%.
Design criteria applied for the Lower Pressure Tunnel of the North Fork Stanislaus River Hydroelectric Project in California
SummaryThe application of various criteria and certain new approaches to design is illustrated by the example of the Lower Collierville Pressure Tunnel of the North Fork Stanislaus River Hydropower Project in California. With a maximum internal water pressure of 72 bar, Lower Collierville Tunnel will be, when commissioned in 1989, the highest stressed pressure tunnel in the world not situated in granitic rocks.The geological conditions and the results of the geotechnical investigations are described briefly. For the steel-lined portion of the tunnel, the approach for determining the bearing capacity of the rock mass and the load sharing between steel and rock is discussed. The required length of steel liner was determined on the basis of rock mechanical (hydraulic jacking) and rock hydraulic (seepage losses and extension of saturated zone due to seepage) criteria. The use of a new theory allows the effects of mechanical-hydraulic interaction to be taken into account. Finally the methods of estimating the expected water losses and the sealing effect of the consolidation grouting are described.
New Strain Meters for Geophysics
IN the past two months, three ultra long period linear strain meters have been successfully operated in the United Kingdom. Their design is novel and of interest because of the growing attention being paid to long period seismology and the study of earth tides. Here we briefly describe the three, display the first records obtained from them and outline what we may expect to use them for in the future.