A Mini-Mole crawler winds its way down snow-covered stairs for a backyard job in March, drilling 50-foot boreholes for a stability investigation. Source: Ryan Jackson
Whenever I read an issue ofNational Drillermagazine and see an article about limited-access drilling, I get a little excited. But then I usually feel a bit disappointed when I realize that it’s about limited access for foundation drilling. While it certainly is important to have buildings with a properly designed and built foundation, that’s not the only type of limited access drilling. In my case, the discipline is geotechnical and environmental drilling.
There are countless drilling projects going on in all types of limited-access areas. You probably don’t see too many of them because they are hidden: inside buildings, in backyards and alleyways. I can’t begin to count the number of projects I’ve completed in residential backyards, driveways, garages, basements of strip malls, and inside homes, convenience stores, malls, restaurants, boiler rooms, parking garages and elevator shafts. The list goes on. Each one has its own unique problems and solutions. Just when I think I’ve seen it all, I come upon a new challenge to deal with.
This is the first of a series covering this type of limited-access drilling, and we start with the basics: assessing job sites, power, torque and pullback.
Site Visits
A client who hires you and your rigs frequently will probably have a good idea about the capabilities of your equipment and personnel, and whether or not you can get one of your machines into a hole location. Some circumstances, however, require you to visit a site beforehand. First, to make sure you can even do the job and, second, to make adequate preparations. Is it a simple case of taking a few hand tools to disassemble a fence? If necessary, can you obtain permission from a neighboring property owner to cross their land? Do you need to perform a miracle to access your hole locations?
Working inside is just one of many situations where a visit may be necessary. The biggest issue I’ve had with working on inside jobs, besides an occasional lack of private utility locates, is right above my head. A client may need standard penetration tests for a geotechnical investigation in specific locations, but if there isn’t enough overhead clearance then it might be impossible. Some locations I’ve been asked to work in have barely enough headroom for the average person to stand, let alone lift and drop a hammer 30 inches, whether that’s a 70-pound or 140-pound hammer.
Working in a ditch beside a road may necessitate the removal of a guard rail to get the rig through. If that’s not an option then you may need a boom truck to lift the drill. This makes things even more complicated. If you’re on a busy road, someone will need to arrange for traffic control, road occupancy permits might be needed and blockage of lanes may only be permissible during certain hours. Adding complexity adds cost. Crane rental and traffic control aren’t cheap, and then you have to build in extra time to set up lane closures, ready the machine and move the drill past any obstructions. Multiply that by the number of holes needed. All of this is time you will charge the client for during which you won’t be drilling, and it can be a budget breaker. But an efficient plan and execution can minimize the time it takes to get everything in place.
One job I worked on a few years back comes to mind. It was at a conservation area where the client wanted us to make the minimal disturbance necessary to get to the hole locations. A larger track-mounted rig could have made short work of getting through by running over brush and smaller trees, with a chainsaw taking care of the rest. But this was not permitted because of the sensitive nature of the area. The borehole locations were far back in the bush, so a crane was out of the question.
I went for a visit and was amazed at what I saw, but not in a good way. With a big grin on my face, I told the client we could get the rig in to the borehole locations without any disturbance at all ... using a helicopter! Unfortunately, this was well beyond the budget. I was quite confident that we could do the job, but I knew it was going to be a lot of work
On the day the job started, I have to say I was a bit nervous. Sometimes, it’s really difficult to tell exactly how hard it will be to get the rig to a spot until you get it there. Even with my company’s small track-mounted rig, snaking our way through the woods, around stumps, in between trees and over logs, there were still many spots where the only option was to chop, saw and hack our way through. Seven hours after we entered the woods we made it to the farthest hole location, only a few hundred yards from our entry point. The entire day was taken up with moving the drill, finding a trail and laying out mats and wooden blocks to get us unstuck and to cross over soft, muddy spots. Boy, were my assistant and I sore after that!
Fortunately, our client was very understanding about the whole ordeal and was able to budget for the extra time necessary for us to move to the hole locations. There was no way I could have honestly told him that we would have been able to get in the woods and back out without some disturbance to ground and plant life. Had we been allowed to cut down anything we felt necessary to make our path, instead of having to snake our way through the brush when we were able to, we would have made it to the hole in half the time. But, then again, were that the case, the job probably would have been done by a full-size track rig. When all was said and done, we cleaned up our tracks as best we could and the conservation authority planted native shrubs to conceal our path.
The moral of the story here is that, had I not gone to see the site beforehand, we would have been ill prepared for the job and the client might have been very displeased, given the amount of time and trouble necessary to access the borehole locations.
Power Struggle
Drilling with a machine suited for limited access usually means limited power. Harder soils, gravel, cobble and rubble can present huge challenges for smaller equipment. Less torque, pullback and lower weight all play a role. Think of it as trying to drive a railroad spike with a finishing hammer. The problem is you just don’t have the room to swing a 10-pound sledgehammer to drive that spike.
Harder soils naturally require a rig with more weight. Any drill rig worth its hourly rate has more than enough power to easily lift itself off the ground when you feed down too hard or too quickly. I can’t count how many times I’ve been on a site and had the drill lifted off the ground and gotten nowhere in stiff materials. It’s not always practical, or effective, to anchor the drill to the ground. A more aggressive bit can certainly help, but unfortunately is not the end all, be all for penetration problems.
Case in point: I had a recent job where we were drilling at the bottom of a parking garage in really hard till material. Using a more aggressive bit provided absolutely no advantage. In fact, the bit had been hard faced recently, and hadn’t been used until this job. By the time we finished the 25-foot hole, the hard facing had been completely worn off one wing of the bit. The whole thing had been hard faced at the same time and only one part of it was bare-the “wing” that stuck out slightly farther than the others and was more in contact with the side of the hole. Our field tech had to take pictures of it to show to the project manager, who was in disbelief.
Torque
A smaller machine having less torque poses two problems. In gravel and cobble or in coarse, wet sand while using hollows, the lack of torque can turn what could have been an easy, half-day job into a long frustrating day with the potential to accomplish very little. Drilling in wet sand, or other caving conditions with hollows might not slow down a large truck-mounted rig much, but with a smaller machine going from a four-inch solid stem to a 6- or 8-inch outside diameter hollow is a big step (in the wrong direction) that can seriously slow penetration rates.
The other problem that I’ve seen frequently, especially in cobble, rubble and gravel, is the rig moving around. Many has been the time, when drilling, that I’ve started to grind on a rock and the auger gets stalled out-only to have the whole drill swing around on me. Or had the rig lift up and move to the side, causing the augers to go crooked and making it necessary to stop, realign and continue, only to have it happen again. Even with slow rotation and gentle feeding, this can still be a problem. This then goes back to anchoring the drill to the ground. You hit a rock below the surface and the augers go crooked. On a bigger rig with a base that can slide in four directions you can compensate, but with a head that has only one drilling position and swings out of the way to pull augers or to sample, it’s just not always practical to anchor it. This can be especially frustrating for clients working with budget constraints.
Pullback
Not having enough pullback is something that even the guys running bigger rigs can relate to. Cleaning out the hole is necessary, not just to be able to get augers out for sampling, but also to continue going deeper. Again, with a smaller rig and limited power, it can be a challenge in the right (wrong) conditions. In harder, dry till materials for example, I find that even feeding up and down numerous times won’t necessarily clean out the cuttings enough to make it easy to pull augers out. All that powder can really hold augers in there, and bind them up so the head can’t even rotate them. I’ve had a few instances where adding water worked like magic. From spending several minutes trying to get augers to even turn one revolution to suddenly being able to turn and lift the augers with just moving the control levers halfway. Had the machine had more pullback, I may have been able to do without the water and just using brute force to pull the augers up. Needless to say, the hole went quite a bit smoother after that, and we were able to get to the required depth.
Another common problem with smaller machines and drilling inside older buildings is fill consisting of bricks, rocks, chunks of wood, concrete and just about everything else you can think of. In the old days, it seemed that if a contractor had a hole to fill before building something he asked all his friends if they had any junk they needed to get rid of and told them to throw it in. Clay bricks aren’t that much of a problem, but cinderblock, chunks of concrete, rocks, wood and metal can cause the same problems as drilling through gravel and cobbles. And under old buildings it seems that encountering that type of stuff is the rule, not the exception. This is the kind of stuff that can bend your auger flights, break bits off and knock out pins, leaving auger strings stuck down the hole.
Power, torque and pullback are just some of the challenges of limited-access drilling. In the next installment, we’ll discuss geotechnical and environmental applications, as well as job pricing.
