Trench and rock VS backhoe, breaker, and auger

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rhodges

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Joined
Nov 13, 2005
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12
Well, I am running out of ideas. Argh. This project should have been pretty simple. I have a concrete slab and want to build a garage on it. Three sides are against a hill, which I will backfill with gravel and earth later. But before I start on the garage, I want to run perf pipe around those three sides for drainage so that water won't ever build up. The slab has floor drains too, so I want to take care of that too. This work area is on a hill, and I started trenching the drain from the slab down about 80 feet by the side of the driveway. I plan to put the perf pipe at the bottom of the trench and cover with gravel to make a kind of downhill leach field. Well, I can get about a foot or two down before my backhoe (Bobcat 910) hits rock and can't go any deeper. So I rented an 880 breaker for a few days to break up the rock in the trench so I can scoop it out. The breaker works nicely on my rock above grade, but for some reason it just does not work below grade in the trench. It really seems to be working, but when I get back to the backhoe, it is like I never used the breaker. So next I rented an auger, figuring that I could dig some holes next to each other so that the backhoe bucket can get in and pull up the rock. This rock is like some kind of cemented red sedimentary rock, and is very tough, but it does fracture and give if you can get underneath it somehow. So that was my idea anyway. This auger is a 15C with a 12 inch bit with carbide teeth, by the way. It takes about 20 minutes to dig a single hole. I counted about 58 RPM, which is about right for my 12GPM 825 on full throttle. The teeth just do not seem to grab and cut the rock, but instead I just get a very fine powder. I am thinking that if the auger could only bite into the rock and bust it up (rather than shaving it), I could do a lot better. I am wondering if the Bobcat rock bit would be a clear improvement over the heavy duty with carbide teeth. My rental place does not rent rock bits, so I can not just rent one and find out easily. Is there maybe some other good way to get my trench done through the rock? Thanks!
 

Tazza

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Dec 7, 2004
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Drilling through rock with a bobcat is really not the best, they just don't have the weight in the right place. All the weight is in the back end, you need it up front, thats why its just shaving through the rock. Where i live, we got some holes drilled with a large excivator, drilled through the shale wothout a problem, the last person that did it with a bobcat was there for ages drilling. Either keep going slowly, or pay someine in a large excivator to do the job.
Good luck with your project.
 

Tigerotor77W

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Apr 29, 2004
Messages
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Drilling through rock with a bobcat is really not the best, they just don't have the weight in the right place. All the weight is in the back end, you need it up front, thats why its just shaving through the rock. Where i live, we got some holes drilled with a large excivator, drilled through the shale wothout a problem, the last person that did it with a bobcat was there for ages drilling. Either keep going slowly, or pay someine in a large excivator to do the job.
Good luck with your project.
It's possible that a rented Cat XPS-unit (NOT! a Cat boost-flow unit, but one with the XPS high-pressure system) will fare much better, as it sounds like you could use more torque in drilling/augering.

I'm really not sure on this though -- it's just a throught.
 

rhodges

Member
Joined
Nov 13, 2005
Messages
12
It's possible that a rented Cat XPS-unit (NOT! a Cat boost-flow unit, but one with the XPS high-pressure system) will fare much better, as it sounds like you could use more torque in drilling/augering.

I'm really not sure on this though -- it's just a throught.
I don't think torqe was a problem. The auger did not bog down at all except when I accidentally changed the tilt angle and the auger caught on the sides of the holes. It seemed that the auger and the engine were loafing actually, while the auger teeth seemed to mostly slide over the rock. I have wondered whether the Bobcat rock bit would do well on this job, but I don't want to buy an auger and bit just to find out. I am pretty sure now that the rock is red sandstone, and I found that it reacts with acid, so I am thinking about pouring some hydrochloric acid into the trench to see if that softens up the rock enough for the backhoe to dig out.
 

Eric

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Jan 19, 2005
Messages
169
I don't think torqe was a problem. The auger did not bog down at all except when I accidentally changed the tilt angle and the auger caught on the sides of the holes. It seemed that the auger and the engine were loafing actually, while the auger teeth seemed to mostly slide over the rock. I have wondered whether the Bobcat rock bit would do well on this job, but I don't want to buy an auger and bit just to find out. I am pretty sure now that the rock is red sandstone, and I found that it reacts with acid, so I am thinking about pouring some hydrochloric acid into the trench to see if that softens up the rock enough for the backhoe to dig out.
Its all about psi. That bobcat cant put enough down force to chew through rock efficiently. Same applies towards digging in hard dirt with a tooth bucket on a Bobcat. Go from a 763 to a 963 and there is a huge difference in performance. This is why they make big rippers on the back of bulldozers. They dont use 10 rippers on the back,they use 1 to 3. This gives you more psi per tooth.
 

rhodges

Member
Joined
Nov 13, 2005
Messages
12
Its all about psi. That bobcat cant put enough down force to chew through rock efficiently. Same applies towards digging in hard dirt with a tooth bucket on a Bobcat. Go from a 763 to a 963 and there is a huge difference in performance. This is why they make big rippers on the back of bulldozers. They dont use 10 rippers on the back,they use 1 to 3. This gives you more psi per tooth.
Yep, it seems to be a problem of getting the pressure down on the teeth so they can bite the rock. I can get some pressure with the backhoe, but when I get the front weight of the Bobcat working for me, the stabilizers have lifted off the ground and the machine starts creeping forward. I even carved a shallow trench for the front tires to rest in, and that helps somewhat. Well, thanks for the ideas anyway.
 
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