Construction & Contractors/Pouring cement on rock

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Question
Rock
Rock  

Slope
Slope  
I have a steep hillside with some loose rock on top of solid rock.  I want to build a path down the hill but it's too steep for excavator. Can I do some kind of retaining wall strong enough to bring in fill dirt for path? Do I need to drill into rock for building retaining wall? What's least expensive route?
Additionally, I want to pour some concrete steps at bottom of hill. Can I pour concrete onto solid rock?
Thank you!

Answer
Yes you can pour concrete onto solid rock, the only exception being that some rock types, chiefly those that contain a lot of hydrated silica like chert, may adversely react with the concrete.  That is rare though.  The main threat for pouring concrete onto rock is attaining sufficient adhesion that the constant exposure to the elements may cause shifting of the concrete.  This is why it is important to clean the rock thoroughly before pouring, and the best way to do that is by pressure washing the surface with clean fresh water, although you can also use a blowpipe.  The advantage of pressure washing is that it wets the rock and prevents dehydration of the concrete at the roco interface, which improves bonding to the rock.  The disadvantage is that if you use too much water you keep washing debris onto your own work.  If the rock slope is gradual, say not steeper than 25 degrees or so, or if it is composed of flat and very steep sections that, when you pour continuous concrete, act to create a very rough rock surface beneath the concrete, you can get away without adding shear resistance in the form of dowels.  If it's a lot steeper, like 30 degrees and steeper or if the rock surface is slick, or if you suspect drainage may run along or over the concrete, you should consider doweling the concrete to the rock.  I would use rows of 2 short sections of #4 or #5 rebar drilled and grouted 1 ft minimum into the rock with a row every 4-6 ft (closer if the rock is really steep) and extending up about half the thickness of the concrete.  You need at least a couple of inches of concrete cover over any steel in a case like this.  

Now, as to the trail.  If it's a foot trail and not too wide, the cheapest method I can think of would be to build up steps using the loose material, and retain the front of each step  with natural rock masonry, treated timber (anchored in place with rebar dowels), or block masonry. Depending on the drainage you may have to protect the outboard edges of the steps from erosion by wrapping the step-face retainage around the side a little, and you need to be sure the drainage is prevented from washing down or over the steps.  One way to do that would be to form a rock-lined channel against the slope (if the material is highly erodible, place high-survivability nonwoven geotextile under the rock) and place collection basins every 20-50 ft or so with PVC pipe crossing the trail under the steps so that it can discharge on the downslope side.   

If you want to make a continuously-sloped and more elaborate dirt trail then you need to make a retaining wall on the outer side.  That will require a wall foundation by cutting out the side of the trail area and pouring a lean concrete footing for the wall to sit on.  If the wall is only a few ft high and you can construct the base as a series of horizontal reaches and then step down from one reach to the next as the trail descends, then you don't need to do a lot of anchoring of the foundation.  If the wall is nore than a few ft high then overturning and base sliding must be considered and more than likely you will have to anchor the wall, use a reinforced earth type wall, or pour a cast-in-place concrete wall.  You might want to consider, in that case, segmental block walls that make use of geogrid in layers to reinforce the earth.  In these kinds of walls the earth is already reinforced so the blocks don't have to retain much and can just be stacked.  There are examples all over the Internet on those kinds of walls.  Some of them have interlocking features or counterforts that extend into the fill, but most rely on reinforcing the earth with geogrid.  Some examples are Allan Block, Keystone Walls, and so on.

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Robert Cummings, P.E.

Expertise

I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.

Experience

30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.

Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.

Publications
Mining Engineering, AEG Bulletin.

Education/Credentials
BS and MS Geological Engineering

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