Large Diameter Auger Boring in Illinois

On June 26, Midwest Mole Inc. began a railroad crossing project in the Town of Tolono, Ill., for Feutz Construction. The project consisted of 128 ft of 60-in. steel casing to serve as a storm water carrier as part of a new main trunk line storm water system.

The grade requirement of 0.15 percent made this project challenging. As this casing was to act as the actual carrier and there was no additional tolerance between the downstream and upstream structures, it meant the grade had to be extremely accurate.

With the grade restrictions and the minimal distance of 128 ft, it was decided a guided bore would be the most economical solution. This would ensure proper line and pitch to the new storm water carrier. Due to the casing’s large diameter, Midwest Mole had to use a new installation technique that it had yet to attempt. In order to get from the pilot tubes to the required size of a 60-in. diameter, Midwest Mole recognized that a “sequential” upsizing would be required. A typical guided bore consists of installing 4–in. diameter pilot tubes followed by a reaming head or swivel and 12- to 36-in. diameter steel casing.

For this project, Midwest Mole designed and fabricated a 24-in. by 60-in. “rocket ship” reaming head adapter. This would allow the use of a typical pilot tube installation, followed by a pilot tube to 24-in. diameter reaming head (common), then a short length of 24-in. casing, and then the 24- to 60-in. rocket ship. When reviewing the onsite soil conditions, it was found the onsite conditions were favorable to this type of installation. The soil borings provided showed the crossing traveling through a brown silty clay with trace sand and gravels on the upstream side with a brown sandy clay on the downstream side. These conditions led to the decision to proceed with this course of action.

The 24- to 60-in. reaming head was constructed using heavy wall steel casing and T1 steel for the cutting arms. The arms were constructed of T1 steel to withstand the jacking forces to be applied in this application. The reaming head and auger configuration allowed for a short piece of 24-in. auger to be installed in front of the 60-in. auger allowing the ingestion of earth ribbons thru the front section of the reamer, thus creating the 24-in. hole. The back end of the reamer was constructed in a similar manner. The 24-in. by .5-in. wall casing acted as the connection point for the cutting arms for the 60-in. section. The 60-in. section was constructed of 60-in. by .875-in. steel casing. This extension off the back end of the reamer allowed for the actual 60-in. casing as carrier to be affixed to the “rocket ship” for installation.

Ground Conditions

As installation began, the ground conditions were found to be even better than expected. The pilot tubes were installed without any problems. With the installation of the pilot on line and grade, the pilot tube to 24-in. reaming head was lowered into the main work pit to get all the required measurements for appropriate auger length to be installed inside the 24-in. and 60-in. casing. The auger was ran out into the 24-in. reaming section and the 60-in. reaming section as far as possible but still leaving enough room for the auger to “grow” as sections were added in the work pit. This eliminated the possibility of the auger being advanced into the back side of both reaming sections but kept the auger close enough to the earth ribbons being cut to be picked up as quickly as possible. With the proper spacing of the auger inside each section of casing spoil was removed efficiently and the actual push pressure remained low during the reamer advancement.

During casing advancement, a bentonite slurry mix was also introduced throughout the entire crossing to aid in the reduction of friction on the exterior of the steel casing carrier. On average, bentonite was pumped about 400 gals per 20-ft length of casing pipe. While the amount was found to be excessive, it was imperative to do whatever it took to limit the jacking force involved as to not exceed the push capacity of an American Augers 60-1200 auger boring machine. With this rate of bentonite introduction combined with the proper auger placement, the actual pushing pressure never exceeded 75 tons.

Initial production during installation was around 40 ft per day. Once the crew became accustomed to the setup and a routine was established, a production of 60 ft per day was observed. Careful, detailed planning in combination with excellent execution allowed this project to be completed ahead of schedule and under budget.

Brent Hofer is a project manager at Midwest Mole, based in Indianapolis.
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