Mechanized tunnelling in hydrothermally altered grounds: The effect of hydrothermal fluids on the rock behaviour in the central Iran

Abstract

The Kerman tunnel is located in the southeast of Urmia-Dokhtar Magmatic Arc (UDMA) in central Iran that is known as the Kerman Cenozoic Magmatic Arc (KCMA). This arc is one of the most important altered areas in Iran, which has caused significant physical and mechanical variations in characteristics of rocks. The relationship between the type of hydrothermal alteration and rock properties is considerably variable depending on several factors including the parent rock, PT-conditions, chemical composition, and fluid type, as well as the period of fluid-rock interaction. Studies showed that high-temperature fluids increase consolidation and strength by decreasing porosity and permeability of rocks in the area while the mechanism of alteration by low-temperature subsurface fluids is more complicated resulting in various properties. The most important effects are the decrease in density, strength, and elastic modulus. In Kerman tunnel, these variations in the rock properties led to unexpected behavior of rocks and hazards such as (1) water inrush in some parts of the tunnel where the RQD value is high, and Lugeon value is low, (2) wearing of disc cutters in low or non-abrasive lithologies, (3) intense and mainly unexpected variations in penetration rate, which show considerable influence of the alterations in the area. These are some unknown mechanisms during the tunnel boring in the area. In this study, it is tried to detect these mechanisms and determine the risks of mechanized tunnelling in hydrothermally altered grounds.