Hegan, B.D. 1981 Engineering geological appraisal of the upgrading proposals for Mt Messenger tunnel, SH3. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 356 14 p.
Abstract: Rock condition of the Urenui Siltstone in the Mt Messenger Tunnel, SH3, is favourable for the proposed tunnel widening. Reinforcement of the northern portal will be necessary and support of the tunnel roof will be required if the proposed profile is adopted. Minor instability at the northern approach cut batter is anticipated, as in this area slightly open joints striking parallel to the proposed cut dip steeply out of the batter. Investigations to ensure that the foundation of the northern approach retaining wall is not sited on an old tip head are recommended and the excavation should be logged when completed.