How to cut stair-steps in sloping bedrock for stone foundation

[jbd]

Hi, I'm preparing to start on a stone foundation for our house.  The house will be on slightly irregular/sloping limestone bedrock.  See photo:

http://i.imgur.com/aCUvPRu.jpg

I'm going to make an ~18" stemwall out of stone (limestone w/ mortar), so I feel like leveling the ground by cutting stair steps into the rock would be more sound than drilling and setting dowels/rebar into the bedrock, which people commonly do with monolithic concrete foundations on sloping bedrock.  Using dowels seems like it would create pressure points which I'd like to avoid since limestone is a softer rock.  Here's an illustration of the type of stair steps I'm talking about:

http://www.condor-rebar.com/news-articles/images/2011-03-sloped-vs-stepped-footings1.jpg

The only thing is, I'm not sure how to make the stair steps.  I find it very difficult to get that vertical face in the limestone with my stone mason hammer.  What do builders do in this situation?  Do I need to use a rock saw?  Is that something you can rent?

[flyingvan]

  I don't understand your thinking, really---limestone is a softer rock so you are going to cut into it?  You'd need a good soils guy to really answer this, but seems to me the drag coefficient between concrete and limestone is pretty high.  How steep an angle are we talking?  The picture doesn't look very steep at all.  Also by flattening it you're making a place for water to pool.  My builds both involved drilling and epoxying rebar into bedrock (but it was platonic schist, much harder than limestone)
   But that wasn't what you were asking.  I had to trim down some of the bedrock that was too high. You can rent a walk behind concrete cutter, if you can get one in there.  You have to make a series of wooden runners that are level to roll the cutter on, making the bottom of each kerf level.  My cuts were 1.25" apart--that's the biggest I could break with my stone chisel. 
   I don't have a picture of cutting the bedrock but here's the concrete cutter I rented---here I'm using it to cut the driveway.  For small areas, an old skilsaw with a diamond blade will work but you have to keep it wet, and cutting into the ground with an electric saw and water isn't exactly OSHA approved activity
   Also here's a compromise---could you pour your level steps out of concrete, pinned to the limestone as mentioned, then use that as a base for your limestone/mortar wall?

 

[flyingvan]

   Here, I found a picture of building right on the bedrock....THis is a 4:1 pitch

[jbd]

Thanks for the reply.

What I meant about limestone being a softer rock was that, while concrete may be able to handle the stress of shear force against rebar, I'm not so sure about limestone.

Good idea about pouring level steps out of concrete, but you have me wondering if it's even necessary.  I just measured the drop on each corner.  The NW corner is the highest, so using that as a reference point, the NE corner drops 7", SE corner drops 11", and the SW corner drops 10".  The footprint of the building is ~15'x19'.  So the biggest drop is from the NW to the SE corner, which I calculated to be a 3.8% slope.  Is that enough to be concerned about?  There are plenty of naturally irregular areas on the bedrock for the masonry to key into.

[Don_P]

I would not be concerned with sliding on that. You are describing what the codebook calls a rubblestone foundation, minimum wall thickness is 16". I've built a quickie plywood interior form and mix a wheelbarrow full of concrete and one of mortar. I set a course of stone in mortar and fill the backside to the form with rocks and concrete. Weaving some rebar into the wall is always a good idea.

[jbd]

Thanks Don, that gives me keyword to search for in the building code.  I looked this up in the California building code.  It says you need to take special measures when there is a >10 degree slope, so I guess I'm okay.

I'm planning on using the traditional "random rubble" rock wall building method, as described in "The Art of the Stonemason" by Ian Cramb.  I have heard of the slipform rock wall method, but hadn't considered doing it on one side only.  Interesting idea.