These guidelines apply to the use of HPCA in tunnelling (including shaft sinking). To date, such work has been mainly associated with TBM interventions for maintenance and repair. However it is foreseeable that conventional tunnelling operations may have to be undertaken in HPCA. The guidelines are intended for use by regulatory authorities, clients, designers, contractors, insurers and others involved in HPCA work. Whilst this guidance is intended to apply to HPCA work as defined above, the guidance is equally applicable to intermediate pressure exposures above 3.5 bar.