The Old Happy Valley Tunnel was constructed around 1895. It is about 8000 feet long; fully concrete lined, and 6.5 feet in internal diameter. It has been operating as a free flow tunnel ever since. It is now proposed to operate it as a pressure tunnel so that it can be operated readily in conjunction with the New Happy Valley Outlet Tunnel. Modifications at present being carried out in the Old Tunnel include: • Installation of 180 feet of steel liner, grouted into place, immediately upstream of the outlet portal • Installation of a manifold at the outlet portal At the request of Mr H. Kimber, Assistant Engineer for Water Supply, Engineering and Water Supply Department, an examination has been made of the tunnel to investigate the possibility of leakage from it during operation as a pressure tunnel. This investigation has consisted of: • An inspection of the tunnel lining • Surface mapping of the outlet portal area • An examination of old construction record plans During these investigations it became apparent that leakage would be likely to occur from the downstream portion of the tunnel and a programme of backfill grouting behind the concrete lining was carried out.