Assessing TBM performance in heterogeneous rock masses

A major challenge that TBM performance is requested to deal with for a successful and effective progress is tunnelling through lithologically and geomechanically heterogeneous rock masses. Such heterogeneous environments are common and recent tunnel examples in the UK include the Hinckley Point C offshore cooling tunnels being driven through interbedded carbonaceous mudstone/shales and argillaceous limestone and the Anglo American’s Woodsmith Mine Mineral Transport System tunnel in Redcar Mudstone with beds of ironstone. This inherent geological heterogeneity leads to difficult tunnelling conditions that initially stem from predicting a sound and representative ground model that can be used to preliminary assess the TBM performance. In this work, an exhaustive review of existing TBM Penetration Rate (PR) methods identified that no models address the issue of parameter selection for heterogeneous rock masses comprising layers with different rock strengths. Consequently, new approaches are required for estimating rock mass behaviour and machine performance in such environments. In the presented work the Blue Lias Formation (BLI), which is characterised by its layered rock mass, comprising very strong limestone, interbedded with weak mudstone and shales, is investigated. BLI formation is considered herein being a representative example of lithological heterogeneity. Based on the fieldwork carried out in three localities in the Bristol Channel Basin (S. Wales and Somerset), geological models are produced based on which a geotechnical model is developed, and four ground types are determined. Implications of the current findings for TBM performance are assessed, including faulting, groundwater inflow and excavation stability with a particular focus on both PR and advance rate. A modified approach using the existing empirical models is proposed, developed and presented in this paper that can be used as a guide to determine TBM performance in heterogeneous rock masses reducing the risk of cost and time overruns.

Methodology for Calculating the Stability of the Polymer Operating String in Permafrost

The goal of this paper is to develop a methodology for calculating the stability of an annular cross-section string casing made of polymer material used for the development of mineral deposits by underground leaching in permafrost formations. The aim was to determine the geometric parameters of the casing and to ensure its operational reliability.The relevance of the research is associated with the peculiarities of external influences on the string, for example, a geotechnological well operating in permafrost formation under conditions of additional exposure to ice pressure during freezing of water in the borehole annulus. This effect is usually accompanied by deformation of the casing due to ice pressure, which can lead to string collapse, abnormal operation and the risk of contamination of the geological environment.The proposed calculation method for a polymer casing is based on simulation of objects using the finite element method. We used the spatial finite elements to model the interaction of key elements of the geotechnological natural-technogenic complex: a polymer casing, ice in the annulus space and homogeneous or heterogeneous rock masses adjacent to the well.The results of the study are presented in the form of tables and patterns of displacements, which reflect stresses and strains in the elements of the calculation scheme. The analysis of the obtained results confirms the possibility of using polymer casings of different technological purposes in various conditions of permafrost formation, including extreme ones. The results of the redistribution of pressure created by ice during the freezing of water in the borehole annulus to the rock mass and the string are evaluated. Interdependent deformations of the rock mass and operating string during freezing of water in the borehole annulus are determined. The necessity of considering the properties of the rock mass in determining the pressure on the string is established. The conditions for the collapse of the string with a different combination of its parameters are revealed.