Featured Article: Pushing Pipe
When Akkerman, Inc., a southern Minnesota manufacturer of underground tunneling equipment, found
itself with too little time for the development of three new jacking frames, it turned to RFA for design help. Akkerman’s General
Manager said,
"I had good success with RFA on projects with my previous employer and I knew that RFA would do well."
Akkerman’s jacking frames are part of a system used to install gravity feed pipelines underground with great accuracy and minimal disruption of surface activities. The process used is one form of “trenchless installation,” a term that includes directional drilling and auger boring.
The jacking frame can be used to place piping under obstacles such as freeways, buildings, parks, etc. The first step is to dig a vertical, reinforced starting shaft, which can be just a few feet or as much as 85 feet deep. The jacking frame is then lowered into the shaft and anchored to the reinforced floor. The boring machine, which has a large flat-faced cutter with welded-on carbide tips, is then placed in the jacking frame. The cutter head turns, removing soil in front of the boring machine, while the jacking frame advances it forward. When the boring machine is jacked completely into the ground, the jacking frame is retracted and a section of pipe eight to twenty feet long is placed behind the boring machine. The cutter head is re-engaged and the jacking frame pushes both the boring head and the pipe into the ground. More sections of pipe are added until the pipe string reaches the target area on the other side of the obstacle. There the contractors have dug the “receiving shaft,” where the boring machine will be retrieved with a crane.
Soil conditions and the diameter of the pipe determine how much jacking force is required. Large diameter casings require a great deal of force just to overcome friction and the “overburden” or pressure from the ground above. The three jacking frames RFA helped Akkerman design were the MT 866, an 800 ton machine capable of pushing 66 inch diameter pipe through the ground, and the smaller MT 460 and MT 340 for 60 and 40 inch pipe, respectively.
RFA worked on the entire jacking frame, including base, extension arms, locking ring, track, brake and rollers, but the greatest challenge was designing the locking ring. Because the locking ring comes into direct contact with the pipe and bears the brunt of the force, it needs to be a certain thickness. But the thicker it is, the more pipe length is sacrificed – and the contractor wants to use the maximum length of pipe. It’s a trade-off. This led to another crucial part of the project, the analysis, done by manual calculation, of the locking ring, to insure that it would not collapse when force was applied.
Another challenge was the criteria that the locking ring would separate into upper and lower sections by disengaging two pins.
The top section of the ring could then be lifted off to allow quick insertion of the boring head. It then drops back down and the two pins are reinserted, locking it in place again.
All the design and detailing were done using CADKEY Rev. 7, as that was the software Akkerman was using at the time. “It’s very fast,” said RFA’s Project Engineer. “It is an excellent CAD program for a small design project like this with under 500 piece parts per machine.” Akkerman’s GM commented that “the greatest challenge in working with any outside engineering source is communication. It helped greatly that RFA was able to work on the same CAD system and was very familiar with that system.” RFA also knew what the client was looking for in terms of drawing quality, bill of materials and meeting the deadlines of a very accelerated program.
The marketplace is such that almost every jacking frame is unique and is custom-built for a particular job. Specifications are dictated by each individual contractor. There is a given size for the starting shaft and a given diameter for the pipe and Akkerman builds the jacking frame to fit. Akkerman’s conventional systems are manually operated, that is, an operator sits inside, five to six feet behind the face of the cutter, controlling the direction and RPM’s of the head. But the jacking frames RFA worked on were the modern microtunneling type with a computer-controlled, laser-guided boring head.
The six month long cooperative effort was a success for both RFA and Akkerman. Akkerman’s manager summed it up by saying, “I have had previous experience with other outside engineering firms, but RFA seems to have the best common sense engineering approach.”