McCook Reservoir is a part of the gigantic Chicago Underflow Plan, which started in the seventies. A major part of the work includes the overburden cut-off wall, where a grout curtain is installed to minimize the flow of water in and out of the reservoir.
Wassaras water-powered DTH hammer was chosen as it is the most cost effective drilling method for the deep holes required. The quality control indicated that Wassara had an average hole deviation of just over 1%.
Part of a gigantic plan
The McCook Reservoir Project is located in Illinois, USA and is a 38 million m3 (10 billion gallon) reservoir that will capture combined sewer overflows that cause flooding and watercourse contamination in Chicagoland and benefits the greater Chicago area. It is a part of the gigantic Chicago Underflow Plan, designed to protect the city of Chicago from flooding. Underground tunnels connect the reservoirs to the sewer system of the city.
A test project was conducted to compare the water-powered DTH hammer and rotary drilling methods. The primary objective of the McCook Reservoir grouting test was to evaluate the effectiveness of grouting in this geologic environment while determining the most appropriate drilling method to complete the perimeter grouting of the proposed reservoir. This included grouting of the upper bedrock and integration with the overburden cut-off wall that was constructed previously.
The test program consisted of two parallel rows of grout holes, each row containing 128 holes drilled to a depth of 135 meters. One line was drilled with Wassara, the other using rotary drilling. The two rows were 300 meters (1 000ft.) apart. The test results showed that the Cubex rig with Wassara drilling system was 100% more efficient than the rotary drilling method.
A schematic view of the overburden cut-off wall
The report concluded that:
- “The water driven, down-hole hammer drilling is the most cost effective drilling method for the deep holes required.”
The complete test results are available in the report:
The allowed borehole deviation was set to 4% at full length. The quality control indicated that Wassara had an average hole deviation of just over 1%.