Auger Boring in the Sand
May 14, 2014Regardless of how many projects a company completes during a year, there are always one or two that stand out as worthy of sharing — Either as a warning (not to try it again) or as a beacon of hope that one can always find a solution, albeit a painful one.
Take a complex and wet geology, an environmentally sensitive area, a creek, two rail crossings, long and multiple diameter sewer installations and group them together, and you have a recipe for an interesting and challenging scopes of work.
The full scope of this New Brunswick project included 1,700 ft of boring to install 24-in. diameter gravity collector sewers and a sanitary forcemain installation in the City of Saint John, New Brunswick, Canada. Typical of these types of projects, there were multiple stages and sites. One particular bore stands out as challenging for an auger boring project. It involved a jack-and-bore of 455 ft of 36-in. steel casing pipe to accommodate a 24-in. sewer installation.
The project geotechnical reports showed a mixed geology of silty clay, sands, peat, organic silts, gravel, reddy brown clay, shale and various fill materials but nothing untoward in terms of major challenges. However, it is underground and anything is possible, as things turned out.
The project started with a series of challenges with the pit excavations; wet and soupy silts and clays and a high water table made traditional trench boxes impossible to use. This unforeseen issue forced Marathon Drilling crews into a substantially different pit design. Anyone who has tried to dig a sand castle moat at the seaside can attest to how hard it is to keep water and sand stable. One of the factors that came to light was that there were only two boreholes logged for this bore and the conditions crews encountered were dramatically different from the borehole logs. Given the new situation, Marathon Drilling designed and installed a more suitable entrance and exit pit consisting of liner plate, supports, high-volume dewatering pumps and thick drill pad of stone, rebar and concrete.
Getting the Equipment Right
Matching the right equipment to the bore requirements is critical. While there are times when you can push the limits of your equipment on a bore, any substantial length you have pay attention technical details and ensure that you have enough horsepower to deal with changing ground conditions and work stoppages. You don’t want to have to stop augering and have problems starting again if your machine is under powered.
In light of the length of this bore, Marathon Drilling used its American Augers 72-1200 Boring Machine. This piece of equipment is powerful and robust; equipped with a 6-speed Caterpillar C-7 ACERT Tier III 250 hp (186 kW) diesel engine, it has 1.2 million lbs of thrust and 200,000 ft-lbs of output torque. It is certainly a go-to machine for any jack-and-bore project requiring a bit of backup brute force.
At the start of the bore crews encountered many loose rocks — which can be problematic when your machines are set up for sands and silts. Water control and pit flooding danger are always a top concern. With proper protection and safety measure in place, the rocks encountered were more of an aggravation than any real risk. It does slow you down. On multiple occasions crews had to pull the augers to remove large rocks by hand. Even with the power to drill through them, there is always a risk of damaging your steering head or waterline.
Crews welded a second set of waterline levels and bentonite lines on the steel casing as they advanced to mitigate the risk of losing one due to rock damage. With grade being critical and the ground conditions being so variable, crews paid close attention to the waterline to steer the casing accordingly. Rocks in and around the casing initially forced it to ride up at times and crews had to adjust rotation speed, thrust pressures and wing cutters to accommodate the rough and smooth conditions encountered.
Once crews moved out of the rock and cobble zone, they encountered very wet soupy ground conditions. While it a nice change from rocks and boulders, it presents a contractor with a bit of a race against time. Welding large diameter casing in a small pit in wet conditions — while your casing is sitting is soupy silts and clay — can cause it to want to lose sink and head downward. The pit could only accommodate 20-ft length of casing, where ideally 40-ft lengths (or longer ) would have been preferable. The welding time during which the boring machine was idle did pose a steering and grade control problem for the drill operators.
It was at this time that Marathon Drilling started to run 24-hour operations to ensure that the pipe was not sitting longer than it had to. The contractor tackled the issue by adding extra welders to reduce the welding time and focused on running the machine at different speeds and pressures. Crews worked on the principle of More Pressure to Start, Slower Rotation and No Stopping where possible. This brought the head back up.
Dealing with Challenges
At about 180 ft into the bore, crews encountered some added anomalies — boulders, wood and bricks. Certainly a bit odd, but manageable using the 72 to 1,200’s variable rotation speed and thrust controls; listening to the machine, watching the spoils and having a clean picture of what is happening at the drill head is critical to overcoming obstacles in the ground. Crews start each new casing push at 2,000 to 2,200 psi and finish with approximately the push with 1,200 psi on the gauge. The head remained stable to around 300 ft into the bore. It was then that crews started to experience ground resistance and sand blockage that raised the pressure to 2,800 to 3,000 psi at the beginning of each new casing push.
As crews advanced, they continued to encounter ever increasing running sand and water flows and it became more difficult to keep the pit from flooding. At 380 ft of the 455 ft, crews had several large pumps pumping and they could see a small ground settlement at the casing head. Fortunately, it was at the edge of a parking lot and could be rehabilitated after the bore was completed. Though small, it was still was an indication that more water and material was coming in and around the casing than we would have liked.
Toward the last 40 ft of the bore, the pressure jumped and spiked to 3,800 psi and crews had to keep the casing moving to keep it from locking up in the sand. Though out this project, water in the pit was always a problem for welding and was a general nuisance, but it became serious at this point. Crews would arrive in the morning to be greeted with 4 ft of water in the pit, resulting in a quick dewatering program rethink and they sealed the pit as best they could and started on the last 20 ft of the bore. Even with the power of the 72 to 1,200, sand binding proved to be a challenge and crews had to reverse rotation several times to free up the augers. It is not wise to force the augers to rotate if they are stuck; many have learned the hard way and broken many augers, hexes and keys trying to use pure machine power to get the augers moving. Not only does this pose a danger to the operator and the machine, but it inevitably leads to a substantial loss of production when you have to repair your equipment and retrieve your augers.
As we advance toward the exit pit, crews pulled the augers back 20 ft from the front of the casing to create a sand plug in order to control the inflow of material into the casing. To keep the augers from binding and minimize spoil retrieval we kept the machine in first gear and a close eye on everything. Crews pushed the last 20 ft and once they were in the exit pit, added a string of augers to the boring machine and pushed out the sand plug.
The bore was completed on line and grade and the sewer was installed and grouted into place in a fairly textbook manner. Everyone loves a happy ending.
Andrew McPhedran is vice president of business development at Marathon Drilling Co. Ltd., based in Greely, Ontario, Canada.