In the city of bend, Oregan, a machine that is set to revolutionize precision boring has made it’s debut. The 300ft drive beneath a railroad has demonstrated the abilities of a remote controlled small boring unit (SBU) to stay on target. The SBU was driven through abrasive rock for some 100m and never wandered more than 17mm of course. If taken up by the industry, this machine, which cuts 36" (0.9m) bores in solid rock, would slash equipment costs by two thirds for projects where maintaining the line is critical, such as busy urban environments.

In the past, the way you avoided microtunnelling on a critical line is to bore a bigger hole and make up deviation inside the casing when you lay your pipe. However, this has a number of setbacks. First, and perhaps my crucially, if you only have space for a 36" (0.9m) pipe, you can’t bore a 48" (1.2m). Second, is price, a 48" (1.2m) casing is considerably more expensive than a 36" (0.9m) casing, the contractor is excavating less material and using less grout material in the annulus. But the machine has to be able to guarantee the grade.

Round the bend
In Bend, Oregon, USA, local contractor Stadeli Boring & Tunneling had a unique set of circumstances for a new gravity sewer interceptor. "We had a contract with general contractor Taylor NW to furnish and install 323ft (98m) of 36" (0.9mm) steel casing under railroad tracks. Line and grade were very crucial, and the tolerances were very close. We had to be right on," says Larry Stadeli, president and owner of Stadeli Boring & Tunneling.

"We met with Robbins in Ohio and told them what our needs were. They felt like their 36-inch (900 mm) prototype machine, would be a good fit. They listened to what we were wanting and needing to have done," said Stadeli.

At Robbins, Kenny Clever, SBU Sales Manager, and a group of engineers were honing the prototype machine that fit the bill. Known as the SBU-RC, for Remote Controlled Small Boring Unit, the machine was equipped with a smart guidance system by TACS.

Clever explains, "The TACS guidance system has been in the microtunnelling industry a long time, this is how they have steered microtunnelling machines for years, it is very, very effective. It was an easy conversion for us to put it in the SBU as the technology was already there, we didn’t have to do anything.

"You mount it up in the front of the machine, it feeds you back information giving projections on where you are headed if you continue on the same course. It doesn’t just tell you where you are, it is giving you forecasting information, so you are able to make heading corrections proactively, before you go off grade."

The machine itself works a lot like its larger cousin, the motorised SBU. Starting at 48" diameters the larger machine has able room for all the gadgets and luxuries an SBU needs, such as muck removal systems, and a spot for the drive. On the 36" machine, there is no such room. The shield is jam packed with the essentials for the bore, so any spare part, including the driver, had to go.

"The machine itself works on the same principle as the larger motorised, man operated SBUs. We have up front an electric driven motor, which drives through a gear reduction box and a bearing housing in the forward shield. So the cutterhead is turned by an electric motor, powered by a VFT [variable-frequency transformer] on the surface.

There is a jacking station in the pit to provide the thrust. There is no space in the shield for an invert auger to handle the muck removal. That all had to come out. We are instead using a vacuum system to pull the muck out.

The forward shield articulates in two degrees and in any direction. There are four small hydraulic cylinders up front, operated electronically. You steer it just like a 1.5m motorised SBU, the same principle. Everything is exactly the same, only now the package is so small we can’t fit a man in there to control it. So we have made it remote controlled with electric over hydraulics."

With the driver in a control cabin on the surface there is no visual reference as to what is going on below the surface. So the Robbins designers added in closed circuit TVs to monitor the machine.

"So we have two things going on," says Clever, "we have the TACs guidance system that we are watching from the control cabin, and we have the closed circuit TV to watch everything that is going on inside the machine. With that closed circuit TV we are watching the primary target, the laser coming down to hit that target; we are making sure that nothing is leaking, and that we are not getting an inflow of water into the machine. It is just good to see up in that machine to rest assured with what you see with the visual is showing everything is working as it should."

Maintaining suction
There is also a camera mounted up front to show that the pipe used for muck removal is not getting clogged.

Having a working vacuum muck removal system was crucial to the development of this machine. Clever says, "In the larger SBUs we use a small invert auger, right in the bottom of the casing. As the cutterhead brings the spoil through into the back of the cutterhead we pick it up with the invert auger and convey it back to the pit.

Now, because that area is so small and confined we can’t get an auger up in there effectively so now we are running a small tube up right behind the cutterhead, and we are running pvc connections all the way back to the launch pit, hooking it to a high suction vac truck. We are just sucking it right out of the cutterhead."

To carry chunks of volcanic basalt the size of golf balls the 300ft (91m) back to the launch pit requires the biggest vac truck available, with 6,000cu.ft (170cu.m) per minute of suction. Clever says, "We have designed, and in fact modified on the first bore, the cutterhead so that if the material passes through the cutterhead, it passes through the suction. So we have closed down that cutterhead not to allow bigger pieces to go by it because our first problem right out of the gate was that the muck was passing through in larger pieces than we could vacuum out. The machine has a mixed ground cutterhead on it so we’ve closed that up with grill bars. It is now typically coming back about the size of your thumb. But before adding the grill bars we were getting pieces about the size of a softball when in clumpy ground, now those pieces are held out of the cutterhead until they have been ground up.

While the machine is aimed at short drives in congested ground, Clever thinks the vacuum solution will continue to be effective over greater distances. "In Canada they have already done some long vacuum excavations for hydro projects," he says, "When you get further out you can Y pipe these machines together. Say I get out there 150m away and I start to lose suction and can’t excavate the face, I set another vac truck right next to the one I’m using, run two vac lines right down to the cutter head and Y pipe them into one, and now I have double the suction. You can get a lot of air moving with these trucks and you can piggy back them."

Maintaining line and grade
The Bend, Oregon project represents the target project for this machine according to Clever. "It was a gravity sewer, so they had already established the invert of the pipe on both sides of the railroad, and the exit pit was just barely big enough that they could land, because a manhole already had to go there."

This means there were restrictions on both the vertical and the horizontal tolerances.

"Most of the projects that this machine was designed for are 100m or less. The problem on these projects is line and grade. We are installing projects in areas where the ground is already so cluttered with other utilities that line and grade is critical.

You can’t deviate from your line or you will be into the sewer line that exists, or the water line that exists or gas main. This wasn’t intended to be out in the countryside boring under a road way, it is an innercity project where they dig down, it is solid rock and they need to avoid deviating from side to side or up and down. Or where they need to cross an interstate and enter an existing manhole," says Clever.

"In the past, in soft ground you would use a pilot tube system. It works very well in soft ground. You shoot that across, establish the line and grade and pull the product line across after it. In solid rock, that is just not possible. Guys have tried for years with directional drilling, with small diameter tricone bits, and still it is nearly impossible when you are looking at tolerances of 20-30mm. It is ridiculously difficult to hold a directional drill with those type of tolerances. Then the Axis system came out from Vermeer, this is very effective for getting a good pilot hole across and then you have to ream it open to the desired diameter, which is typically 30-36" (0.7-0.9m).

So what we have done is eliminated the need for a pilot hole because now we can steer it all the way across. This machine is not designed to have the ability to steer, it only has steering capabilities to maintain the line and grade we have already established. It is not designed to go around curves or go up and down. It is made to go straight and we want to keep it going straight. But the machine has a tendency to migrate towards a softer part of the rock formation, or the torque in the cutterhead might make it want to go to the right and climb upwards out of the hole.

"The ability to steer a cutterhead of that diameter really is a game changer. We can bore one pass and get across a 900mm diameter pipe, and maintain line and grade. We were never more than 17mm off line the whole way across. Half an inch was the most we were off in 300ft (91m). That is unheard of in solid rock.

Not only did the drive at Bend maintain a steady line, it also made good pace. "It is much quicker than a two-pass method. Once we were launched and got going we were achieving 30-40ft (9.1-12.2m) per day, a lot of the days were eight-hour shifts. One of the days we managed 50ft (15.2m). We went slow at the beginning intentionally just because we wanted to make sure we were hitting our line and grade. We were in some pretty abrasive ground and wanted to make sure we didn’t break anything. The when we got to a point where we felt comfortable we started picking up pace a little bit and setting two pieces of pipe a day. We were hoping for a 20ft (6.1m) per day production rate, when we were setting two pipes instead of one pipe we thought we didn’t need to do any better than that. The production rate was amazing. The pipe was installed two weeks ahead of deadline. This is solid rock, 10-15,000psi (69-103MPa) rock, this is not dirt work," says Clever.

Contractor Larry Stadeli adds, "We were able to cut off a couple of weeks of our schedule time. Taylor NW was very pleased about it. When you look down the pipe now after it’s finished, it looks like a rifle barrel. There is no sag, it’s all in one straight line."

Establishing demand
The past ten years have seen a change in demand for technology like this. "A decade ago there weren’t as many critical line of bore rock jobs. We have a 48 inch motorised SBU, so if someone had a project where line and grade were critical, the norm was to upgrade it to 48" (1.2m) so you can get a man in there and steer it across. For years that was the acceptable method," says Clever.

"But more and more over the past 10 years, that has not been acceptable. There has not been enough room to put a 48" (1.2m) pipe in there, because now you are talking about 6" (152mm) on both sides in clearance and there is not that much room for deviation. As these tunnel passes become smaller in target zone, we haven’t had the room to upsize the bore; we really need a smaller diameter."

In the past, if that were the case, microtunnelling was the option. But microtunnelling in solid rock is really not that productive. Advance rates are often poor. The contractor needs to have a slurry cleaning system on site, giving the project a huge footprint on the surface; it is not the ideal situation for a small bore dig.

The other reason microtunnelling is unpopular for these projects is cost. "The [SBU-RC] machine and the control cabin, less the vac truck and jacking station, is going to come in at about USD 650,000 to purchase. A microtunnelling system would be about three times that."

However, the future of this technology is going to be demand driven. The size of the market for these machines is unknown, it has just been launched, and the industry is only just learning that this solution exists.

"There has been a huge amount of interest from engineers and contractos alike. We really need to educate the engineers, and this will be a game changer as now they will be able to design project around equipment like this. They will know these tighter parameters can be achieved and can factor that into design," says Clever.