Every project should have a Plan B. As projects become more complex it becomes even more important to expect the worst. This is especially true if the costs and risk to the traveling public are going to be high should the project take a nasty turn sideways.
This particular project done by Marathon Drilling was a 1,829-mm diameter pipe ram that was part of a larger scope of work. The rammed culvert was designed as a large bypass culvert to take the water flow from the adjacent existing box culvert so it could be rehabilitated. Open-cut was out of the question because of the risk of ongoing settlement and due to the high traffic volumes 24/7. Also, there was no reasonable bypass road available in the area so a trenchless solution had to be designed.
The pipe casing wall thickness was a heavy 22 mm and was to be rammed in 12-m lengths. The lead pipe had a 125-mm by 14.3-mm thick steel band (casing shoe) with a cutting wedge. This band is welded to the front edge of the installation pipe to provide reinforcement against boulders and other obstructions as the pipe advances through the ground. The outside of the casing is lubricated by Baroid Bore-Gel that pumped in through 12-mm steel pipes welded to the outside of the casing. This is fairly standard operating procedure for pipe ramming.
There were several factors that made this project interesting and presented elements to make it risky for any kind of tunneling operation. The pipe had to be rammed under a major traffic corridor, Hwy 401. This East-West highway makes up part of the TransCanada highway that connects the East and West coasts of Canada.
Further, the site had high water swamps on both the North and South sides of the new culvert installation site that required us to install substantial waterproof piling and dewatering measures. However, the most challenging aspect of the ram was that the path of the casing had less than 1 m of cover between the top of the steel casing and the road surface itself. This lack of “adequate” cover between thousands of cars and trucks traveling at 110 and 140 km per hour (68 to 86 mph) always makes one stop and think twice about a tunneling project.
Fortunately, Marathon Drilling had good geotechnical information to understand the risks involved in advancing the pipe so close to surface. From all indications, the crew could expect to encounter engineered granular bed under the asphalt that would, at times, be just above the top of the casing as it advanced. Also indicated were regions that would contain rocks, soft till and possibly wood material. Tricky enough with deep cover but at this shallow depth, we proposed a pipe ramming solution with many levels of contingency. Public safety was the chief concern. The crew set up 24/7 real time settlement monitoring along the pipe ram path and had traffic control crash trucks, beacons and emergency backfill material and road equipment available on site to be able to respond within minutes — an organized Plan B is what takes some of the risk out of the equation. As the pipe rammed forward, the crew closed off on the lanes as the pipe advanced to ensure that the lane above the face was never carrying dynamic live loads from traffic.
Once the water was managed and the pipe ram started, the crew made it across the first section of paved highway without any indications of subsurface problems. It was at the shoulder of the median that Marathon Drilling crew encountered the first signs of trouble. The pipe stopped advancing under the hammers thrust and the shoulder began to shift. The crew stopped the ramming operation and set up traffic control to investigate the median. The shoulder had definitely buckled and been pushed out by the subsurface obstruction. Crewmembers pulled the TT Taurus hammer off the push rings and started to clean out the pipe to get to the face of the casing.
As the spoils were removed, the crew found the casing contained no real surprises. Loose granular cobbles and mixed till were expected; however, when the crew got closer to the front, it reached the obstructions. Anyone involved in the trenchless industry won’t be surprised but for the crews, it is always a nuisance.
It doesn’t take too many of these large boulders jammed up inside the pipe to cause pressure to build up in the casing and stop its progression to a crawl. The crew had planned to try and keep an earth plug in the face of the casing to ensure good face control but these large boulders forced us to modify the plan. The boulders were removed with chains and bolts and were pulled out with the crew’s excavator. Some of the stones were so large that they had to be drilled and split with jackhammers. It is a lot of hard work on the part of the crew but true to form they didn’t complain.
The crew activated its short duration traffic control plan and stabilized the ground at the shoulder of the highway using granular material and tamper. The pipe rammer was set up again and the operation continued for 12 m without much indication of trouble. And that’s when the settlement tests showed some signs of settlement in the fast lane of the highway. The crew immediately moved its traffic control pieces into place and slowed the traffic down to safe speeds around the traffic cones and work area. They excavated down with its shovel and found a void that had be created when a large diameter stone was struck in such a way that it was pulled down and into the casing but left a large cavity in its place. Since there was such minimal cover above this stone, the space it occupied grew significantly as the softer material around it vibrated down into the new void created by its movement into the casing. The angle and elevation the crew struck the stone at must have certainly contributed to the size of the void. Fortunately, the crew was well prepared and knew the road repair procedures; the void was repaired with a roller using concrete fill, compacted granular and asphalt topping. Still, the operation took several hours and did cause a few hours of traffic backup and general public consternation. It happens.
With the road stabilized, the ramming was re-started. The crew welded another section of pipe to continue its push under the eastbound lanes. At this point, given the geology just encountered, the crew began to expect the worst. Those expectations were confirmed several hours later when the pipe advancement stopped under the eastbound fast lane. Again, since the lane was closed during this operation, there was no danger to the traveling public but it always a bit of a scramble to investigate and ensure that the road is reinstated quickly if you can. When the crew cleaned out the casing, it found another massive nest of large hard rocks jammed inside the casing; they were removed with a lot of manual back-breaking hard work with drills and jackhammer.
Once the pipe was cleaned out and the road backfilled and re-instated, the pipe ramming continued relatively smoothly until the exit pit was finally reached.
One thing that is certain is that in any kind of project there is always going to be a challenge. As trenchless practitioners, we all understand that there is always soil condition uncertainty and that problems we encounter are both time consuming, and can be expensive. The key is to understand the risk indicators and make plans to mitigate them quickly as they occur. Even in the face of risk and operational challenges, these higher risk projects are incredibly rewarding because they enable plans to be tested and improved without compromising public safety or quality of workmanship for the project owner. The successful completion of a project is always something to celebrate but there are times when we forget the efforts our crew and team members take to help make sure things move forward. It is these challenging projects that put the spotlight on the importance of proper planning and good teamwork.