Trenchless Methods Continue to Advance in India

In the 100-plus-degree heat of Bombay, India, men and machines were put to the test to install pipe casing under the Central Railway tracks. Packed with hundreds of commuters twice daily, the trains traveled to andfrom their central city destination, undeterred by the construction activity beneath them.

Michigan Engineers Limited (MEL), engineers and contractors for 25 years in the Bombay area, was chosen to install the 1m diameter casing to host a 600mm cast iron forced main carrier pipe for a sewage project for the Government of India Rashtriya Chemical Fertilizers Corporation. MEL has plenty of experience driving casing underground. Much of its work in India has involved heavy engineering construction - constructing bridges, pumping stations, marine plants, industrial structures and pile foundations and sheet pile driving. Driven by its growing reputation for meticulous planning and efficiency, MEL chose to pipe ram this current project employing a 400mm-diameter HammerHead Mole pneumatic tool.

View of the ramming site under the Bombay rail track.

The job was originally planned as a pipe-jacking project, similar to the 900mm pipe-jacking jobs that MEL had completed in the past year, jacking in concrete and steel pipes with the conventional method. But the heavy, wet clay ground conditions and the high water table that eventually necessitated continual dewatering of the launch pit led MEL to consider pipe ramming with pneumatic tools, a new technology for the Bombay area.

"Staying on schedule in all phases of a project is crucial to us and to our customers," stated Dr. M.N. Patel, Managing Director of MEL. "We are constantly looking for ways to improve our operations. This new technology of pipe ramming in casing aids in planning because of the relative simplicity of the procedure when compared to older established methods. There is less equipment, less set-up time and, as it turned out, less actual installation time."

This ramming project called for the 1m-diameter casing to be rammed 47m under the railbed of the twin rail tracks. In addition to the train tracks, there were two drainage ditches, the bases of which had to be bored under. The grade of the entire 47m ram was 2.5 percent, which meant that there would be a 1m drop over the entire run of the casing. That grade was designed so that the top of the casing would be far enough under the bottom of the second canal on the far end of the ram.

To prepare for the actual ram, the 10m-long launch pit was graded with a one-inch gravel bed of stone. Dewatering pumps were set up to move the ground water out of the launch area so the launch bed base would remain graded and not deteriorate. The launch pit was dug deep enough so that the casing would be a minimum of 2.5m below the tracks throughout the ram.

The ram was to be done with eight sections of 6m-long steel casing with 14mm wall thickness. To ensure the quality of the casing and to make sure that casing would be delivered on schedule to the job site, MEL rolled and welded its own pipe in its fabrication workshop.

Soil shoes were welded onto the lead casing on the outside and inside. These 14mm-thick fabricated "shoes" were necessary in the thick, wet clay conditions. The outside diameter of the soil shoe would be wider than the outside diameter of the pipe acting to lessen the anticipated friction between the casing and the earth. The same effect works inside the pipe. As the spoil fills up the pipe, the inside shoe lessens the friction between the spoil and the inside diameter of the casing.

To further reduce the friction on the outside and the inside of the casing, a lubrication line was added. The lubrication also improved production and facilitated the cleanout of the spoil inside the casing once the ramming was finished. The lubrication line was a steel tube welded onto the top of each of the casing sections as they were brought into the pit. Flowing through the lube line was a mixture of polymer and water to slicken the inside and outside of the lead edge. It was estimated that more than 1,000 gallons of water was used over the 47m ram.

The last major part of the careful preparation by Michigan Engineers Limited was the I-beam steel rail onto which the casing would ride as it was hammered by the Vermeer Mole into the ground.

Not only did this steel rail allow the casing to ride on a carefully staked and graded rail, but one other refinement was added. MEL fabricated a slide so that when the HammerHead Mole was placed into the adapter ring and collets at the rear end of each pipe section, the tool would be kept perfectly aligned with the center of the collets. Any downward pressure added from the weight 2,700-pound machine would rest on the slide and not affect the vertical alignment of the lead casing, with the trench face and the target grade.

Once the launch pit and casing were prepared, and the 1m collar and 600mm collets were placed in the back of the first section, the HammerHead Mole was lowered into the launch pit, and taper-locked into the collets. "The productivity was outstanding," described Dr. M.N. Patel. 'The first section went in at a rate of 1m every three minutes, so the first 6 m were installed within a half-hour.'

Before each new section was strap-welded, approximately 1.5m of spoil were quickly shoveled out of the back end of the preceding section. Removal of some of the wet, packed spoil helped speed ram productivity.

Once the ram had passed under the first canal, beneath the 16.5m that comprised the width of the railbed, and was 10m from the end of the ram length, an exposure pothole was dug to check the grade of the casing. In the upcoming 8m, they would have to pass under the concrete-lined drainage canal and the engineers wanted to make sure that the casing would pass safely under the concrete structure and not damage it. The inspection revealed that the grade of the ram was within 100mm of target. With the grade confirmed, the ram continued and passed a comfortable 0.4m below the bottom of the concrete canal.

When the ram reached its destination, the spoil was removed through air pressure pushing the spoil back into the launch pit. The entire project took a mere week including all excavation, pit preparation and spoil removal. "We started on a Monday and were done by Friday," described Dr. M.N. Patel. "We had planned on one week for ramming alone, but the HammerHead Mole was so efficient that the actual ramming, including the welding time, took four days. Although this was our first pneumatic pipe-ramming project, we feel comfortable adding it to our list of services we can offer. Pipe-ramming worked is a fast and clean method when compared to what we would have had to do. This project would have taken us three weeks with our pipe-jacking method. I anticipate using the HammerHead tool to ram in almost all of the pipe sizes we encounter."