Background
In 1993 the City of Chicago Department of Water began to receive an increase in the number of complaints from customers regarding the taste and odor of the City's water. Chemical analysis of the raw water drawn from Lake Michigan revealed that the taste and odor problem was caused by two organic compounds in the vicinity of the existing raw water intakes. The presence of these compounds are thought to have occurred as a result of the exponential increase in the population of zebra mussels in the inshore zone of Lake Michigan.

Since the treatment process required to control the taste and odor problems is expensive, the Department of Water identified four potential offshore sites for new water intakes. A new tunnel will be constructed to the intake site. The current concept for the tunnel is a 10-meter diameter Tunnel Boring Machine (TBM) bore, approximately 30 meters below competent carbonate bedrock. The alignment currently under investigation extends up to 15.2 kilometers beyond the Chicago shoreline. Ancillary nearshore facilities may include additional filled land at the shoreline, new head house intake and pumping facilities, and a large diameter vertical intake and construction access shafts.

Fugro's Role
Fugro performed specialized geotechnical services for the project including:

• Drilling 23 boreholes from a self-elevating stationary platform capable of drilling in water depths of between 3 and 30 meters
  
  
• Solving the unique challenges associated with the site conditions and requirements.
  
  
• Positioning of the rig using Fugro's OmniSTAR Differential Global Positioning System network.
  
  
• Drilling eight deepwater boreholes to depths of up to 125 meters below the lakebed.
  
  
• Drilling three inclined boreholes at an angle of 25 degrees to the vertical in order to examine the near-vertical structure of the bedrock.
  
  
• Rock coring with the Geobor-S system in conjunction with standard HQ wireline coring equipment through the glacial till (and softer infill) and into the dolomite. Core recovery averaged 95%.
  
  
• Extensive soil sampling using a variety of methods including piston samples in the softer lake sediments and standard penetration test (SPT) samples in the harder clays and dense sands.
  
  
• Drilling fifteen boreholes to appraise the subsurface conditions in the proposed cofferdam and headhouse extension areas.
  
  
• Recording parameters such as mud pressure, torque, depth, penetration rate, and rpm during drilling using the Jean Lutz system.
  
  
• Permeability testing of the bedrock materials to assess the rate of water ingress during operation of the tunnel boring machine.
  
  
• Back-pressure (draw down) testing to assess the rate at which the pressure head recovered to hydrostatic level, or possibly higher (i.e. artesian levels).