Karstic problems in the construction of Milwaukee's Deep Tunnels

A critical component of Milwaukee's $ 2.8 billion Water Pollution Abatement Program is a 31.2-km inline storage system comprising three Deep Tunnel sections that were bored between 1984 and 1993 at a depth of 80-100 m within Silurian-aged Niagara dolostone. Construction of these Deep Tunnels proved more difficult and expensive than estimated because the karstic nature of the dolostone, particularly its hydrology, had not been fully appreciated. Rock collapse, subsidence and groundwater intrusion necessitated remedial grouting and lining of about 45% of the tunnels, costing some $ 50 million above estimates and delaying completion by 9 months. Tunnel performance since completion continues to be controversial.