The development of the Ust-Jaiwa potash mine involves the construction of two vertical shafts that are sunk by the application of the freeze shaft method. While the frost wall of the Ust-Jaiwa shafts only needs to fulfill water cut-off functions over most of the shaft depth, there are freeze shafts sections to be sunk through low-strength rock mass material where structural capacities of the frost wall are required. In this paper, a combined approach of rock testing, in-situ rock characterization and both thermal and rock mechanical modelling is reported, implemented in order to determine a reliable strategy of shaft sinking through low-strength rock mass that is stabilized by a combination of frozen ground and temporary shaft lining. The advantages of the application of numerical calculation methods for back-analysis and subsequent predictive modelling are demonstrated. The components of the investigation are outlined and discussed, and its results are conclusively summarized.

1 Introduction

The freeze shaft method has been used in the mining industry since the late 1800s. During the period of shaft construction, this method provides temporary ground support and water control through artificial ground freezing.

The development of the Ust-Jaiwa potash mine, located in the Perm Region of the Russian Federation, commenced early 2012. It involves the construction of two vertical shafts with a final diameter of 8.0 m and a depth of approximately 500 m each. Both shafts are constructed through the application of the freeze shaft method with a freeze depth of 250 m. A freeze plant with a freeze capacity of 3.0 MW was installed in order to cope with local ground freezing challenges.

A comprehensive investigation of shaft wall stability during construction was required due to required sinking through low-strength salt-marl strata with a melting temperature of -21°C. Ground characterization, frost body monitoring and laboratory investigations were conducted, aiming to optimize the sequences of the conventional drill and blast sinking operation, in which a cast-in-situ concrete liner, installed by the jumpform method serves as temporary excavation support. In particular, the determination of the maximum unsupported height of shaft wall not resulting in shaft wall failure was of interest, as this parameter significantly impacts on sinking performance due to the employed construction method.

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