Pillar, repair

The strengthening work necessary at Tunnel Quarry consisted of the repair of existing pillars and the provision of further permanent works to ensure that the overhead cover was adequately supported. It was considered at the time that the roof was so sound and the overhead cover so deep that there was no danger of it breaking up imder bombing attack, but there was some doubt that it was sufficiently supported as a mass. Colonel Minnis noted to the War Office Tunnel Quarry Committee that ... 

Broomfield (2000) carried out regular surveys using LPR (see Section 4.12) on some reinforced concrete support pillars which had calcium chloride cast in as a set accelerator. These had been repaired with vapour phase corrosion inhibitors applied. Measurements from 1995 to 1999 showed the increase in the corrosion rate in treated and untreated control areas, with the corrosion rate peaking at around 0.5 to 1.0 A/cm (5-10 mm per year) with cracking seen approximately one year later. This demonstrated that the inhibitor applied with the chloride content of the structures was too high to stop corrosion. It also showed that a factor of two difference in corrosion rate between treated and untreated area made no significant difference to the rate and extent of cracking. 

The finest particles (<600 xm) are hard to handle, cake easily, and have the least commercial value. They often are used for maintenance inside the mine, as in road building and repair, and to build temporary barriers. The latter often are erected between pillars for control of ventilation. 

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