Twin 12.5 m diameter, 470 m long road tunnels were constructed in the Newhaven Chalk Formation of Southwick Hill, NW of Brighton, UK. The tunnel was supported using shotcrete and untensioned bolts, the support being adjusted on site using the Q classification system and deformation monitoring. Other than some grade B4 chalk at one portal the ground conditions were similar to the optimistic prediction and significantly better than the conditions used for design, which were greatly influenced by poor quality core. Six metres of heading advance per 24 h day was normal in the 7 m by 12.5 m heading, peaking at 8 m in 24 h. This paper discusses the attempts to deal with conflicts in the site investigation data and the assessment of the ground reference conditions, and compares these with the actual ground conditions encountered. A summary of the main geological features affecting tunnel stability is presented.

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