Pit & Quarry, June 2017

managing civil engineer Rainwater from residences and business districts all goes into the same combined sewer system when big storms hit Its too much water for that pipe to carry which causes it to back up into homes and basements In the late 1960s studies were conducted to evaluate the existing storm water system and determine what could be done to improve and update the infrastructure Part of the ideas coming from the studies included plans to capture and hold storm water runoff until it could be treated and placed back into the waterway in a controlled manner More than 50 alternative projects were reviewed for development of the TARP Fitzpatrick says The MWRD identified the most economical and beneficial alternatives and adopted the overall TARP plan It has been the largest public works project in the Chicagoland area In all the MWRDs TARP plan involves more than 100 miles of deep tunnels that have been constructed about 300 ft below ground The tunnels generally follow local waterways capturing combined sewage runoff at 400 locations Tunnels transport the water to reservoirs like Thornton Reservoir which is one of three major TARP reservoirs Once the mix of rainwater and sewage reaches the Thornton Reservoir its stored there until the storm is over and treatment plants have capacity to clean the water Fitzpatrick says Overall the Thornton Reservoir can hold about 79 billion gallons of floodwater HANSON MATERIAL SERVICE MWRDs relationship with Hanson Material Service the company mining the Thornton Quarry began in 1998 The two entities reached an agreement that over a period of 15 years 1998 2013 Hanson Material Service would complete limestone mining activities in the northern part of the quarry and turn it over to MWRD for use in the TARP system The last blast in the quarrys north half intended to loosen the final 36000 tons of limestone that has been mined from the quarry since the 1830s was completed in September 2013 Once mining activities were completed at the quarry we had a lot of work to do to prepare it for use in the TARP system Fitzpatrick says The first step in prepping the 320 ft deep quarry was to make it water tight so no seepage occurred While most rock quarries are naturally water tight rock fracturing actions can lead to seepage A cement grout curtain placed The first step in prepping the 320 ft deep quarry was to make it water tight so no seepage occurred around the quarrys perimeter ensured that none of the water would escape Grout was injected into fractures in the rock to reduce permeability Fitzpatrick says The quarry perimeter is more than a mile long Along that perimeter about every 5 to 10 ft holes were drilled down 500 ft from the surface The holes were injected with grout under pressure to seal any rock fractures Just beneath the quarry floor is a layer of shale which has very low permeability so we didnt need to seal the quarry floor A great deal of engineering work was necessary to address each aspect of the quarry preparation Among the plans was sealing two tunnels in the quarry which allowed trucks to move rock from the fracture site to the quarry plant The tunnels were plugged with reinforced concrete to avoid flooding the still active section of the quarry A 1300 ft long 30 ft diameter tunnel was constructed to connect the quarry to the existing TARP system Stainless steel 18 ft wide wheel gates weighing 54 tons were lowered more than 300 ft down into the quarry where they will be raised and lowered to regulate the flow of storm and wastewater into the quarry and hold it there The gates are 29 ft high and 4 ft wide Fitzpatrick says They not only regulate water flow but can also be used to isolate the quarry for any necessary maintenance The final phase of prepping the quarry pitandquarry com June 2017 PIT QUARRY 33