This paper reports the result of closure measurements in roadways driven in salt at a depth of 11OOm. The results obtained from a variety of mining geometries are compared and the implications for the design of underground nuclear waste repositories are discussed.

Introduction

Ever since Project Salt Vault (Bradshaw, 1971) isolation of nuclear waste in mined openings in rocksalt has been rigorously investigated. To eliminate the possibility of radio-nuclide escape to the biosphere, a primary requirement of the design of an underground repository must be the long term stability of the openings. The stability of excavations in rocksalt is mainly a function of the mining layout, the percentage extraction, depth below surface and the time dependent behavior of the material. Additional factors to be considered in the case of nuclear waste storage are the influence of heat and radioactivity on the behavioral properties of the material. Generally speaking evaporate mining practice is based on either regular room and pillar layouts with fully load bearing pillars or the use of relatively narrow, high extraction panels separated by wide barrier panels. Internal panel pillars are small and designed to yield (Serata, 1976).

This paper focuses on a study of the closure behaviour of a variety of mining layouts and configurations driven in salt at a potash mine in North Yorkshire, including:

  • Isolated twin roads.

  • Roads in low extraction panels (25–35%) with 40m pillars

  • Roads in high extraction panels (60–70%) with 4 - 6m wide pillars

  • Roads with 60m pillars driven in the shaft pillar.

All roadways are rectangular, 6.7m wide by 3.5m high, driven by means of mechanical miners, except the shaft pillar roads which were drilled and blasted to roughly the same shape and dimensions. The depth below surface is 11OOm given an approximate overburden stress of 25MPa. Roof-floor closure measurements have been taken over periods ranging from days to years.

PRESENTATION AND DISCUSSION OF RESULTS

Closure measurements from the four basic layouts described above have been collected and are presented in tabular form. The variation of closure rate with time has been found (Hebblewhite, 1977) to follow the curve given by an equation of the general form: C= AtB where c is the closure rate in mm/day, t is the time from date of excavation in days, and A and B, constants. Experience based on visual inspection over several years has shown that for roadways to be stable they must have closure rates which fall within the envelope given by the equation: C = 34.81 t−o·785

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