Rock mass rating | EPFL Graph Search

The rock mass rating (RMR) is a geomechanical classification system for rocks, developed by Z. T. Bieniawski between 1972 and 1973.
Since then it has undergone multiple modifications out of which, RMR89 is commonly used. Recently RMR14 has been proposed to improve the RMR performance by incorporating new experiences from tunnel practices. Continuous functions and a software "QuickRMR" for RMR89 and RMR14 have also been proposed by Kundu. RMR combines the most significant geologic parameters of influence and represents them with one overall comprehensive index of rock mass quality, which is used for the design and construction of excavations in rock, such as tunnels, mines, slopes, and foundations. Definition The following six parameters are used to classify a rock mass using the RMR system #Uniaxial compressive strength of rock material #Rock quality designation (RQD) #Spacing of discontinuities #Condition of discontinuities. #Groundwater condi

Influence of geometrical imperfections and flaws at welds of steel liners on fatigue behavior of pressure tunnels and shafts in anisotropic rock

Alexandre Jean Pachoud

High-head steel lined pressure tunnels and shafts may be considered as critical infrastructures especially when rock overburden is small. In the case of lining failure, catastrophic damages can occur when a large amount of water can reach the steep valley slopes and induce dangerous mud and debris flow. In the last decades, high-strength steel has been used more frequently for such high-head pressure tunnels and shafts mainly for economic and construction reasons. Under the more and more rough operation conditions of storage hydropower plants, high-strength steels, which are generally thinner than lower grade steels, are more prone to fail by fatigue. Design guidelines for the application of high-strength steel for steel liners are still missing. With his research project, Dr Alexandre Pachoud made an important and novel contribution for the design of pressure tunnels and shafts considering the influence of anisotropic behavior of rock as well as the geometrical imperfections and flaws at welds on the fatigue resistance of the steel liners. For the first time, Dr Pachoud studied systematically the influence of rock anisotropy on the deformation of the lining system comprising steel liner, backfill concrete and near as well as far field rock mass. He performed an extensive parametric study with the finite element method (FEM) over a wide range of geometrical and material parameters. Normalized stresses and displacements were analyzed in the steel liner and the far-field rock mass and correction factors to be included in the analytical solution for isotropic rock conditions could be derived. For transversely anisotropic rock mass the analytical solution allows a simple and fast estimation of the maximum stresses in the steel liner by a correction applied to the isotropic analytical solution with a high accuracy. Based on extensive FEM simulations Dr Pachoud derived in a further step parametric correction factors, which allow estimating stress concentrations and structural stresses in steel liners considering geometrical imperfections. Dr Pachoud obtained also Stress Intensity Factors (SIF) for axial cracks in the weld material of the longitudinal joints by means of computational Linear Elastic Fracture Mechanics (LEFM) and could propose new parametric equations for the weld shape correction. For fatigue assessment, a probabilistic model for steel liner crack growth and fracture was developed by using the above mentioned new parametric equations for considering geometrical imperfections. Finally, Dr Pachoud illustrated the implementation of all the developed parametric equations in a probabilistic model for crack growth in the steel liner under dynamic loading with a case study for a high-head power plant. The probabilistic model allows determining the acceptable undetected initial crack sizes in the steel liner depending on the choice of the steel grade, which is crucial for engineering practice using high-strength steels for pressure tunnels and shafts.

EPFL-LCH2017

Mechanical-hydraulic interaction in the lining cracking process of pressure tunnels

Anton Schleiss

Pressure tunnels are in operation subjected to internal water pressure. When the hoop tensile stress acting at the lining intrados exceeds the tensile strength of concrete, longitudinal cracks occur in the concrete lining. As a consequence of crack openings, the internal water pressure will act at the lining extrados and cause high local water losses. If left untreated, these losses will induce the washing out of joint fillings and increase the risk of hydro-jacking of the surrounding rock mass. When pressure tunnels are situated close to valley slopes, excessive water losses can endanger the stability of the rock mass and provoke landslide. Whether or not the internal water pressure is fully effective at the lining extrados, it depends predominantly on the number of cracks and the width of crack openings. The width of cracks can be estimated based on the total circumferential deformation of the rock mass, which is governed not only by mechanical boundary pressures, but also by seepage pressures. In turn, seepage pressures generate water losses from the tunnel, which are depending not only on the permeability of the rock mass, the grouted zone, and the concrete lining, but also, if any, on the width of the crack openings. The estimation of pressures transmitted to the rock mass requires therefore solutions using iterative methods dealing with this coupling behaviour. This paper presents a method to estimate the distribution of seepage pressures and water losses around concretelined pressure tunnels pre-stressed by grouting, which considers the lining cracking process due to a high internal water pressure. The mechanical and hydraulic behaviour of cracked concrete-lined pressure tunnels is presented so as a first step for more elaborate numerical studies.

2013

Influence of geometrical imperfections and flaws at welds of steel liners on fatigue behavior of pressure tunnels and shafts in anisotropic rock

Alexandre Jean Pachoud

EPFL-LCH2017