you need to provide more information including:

type of rock, hardness
depth / width of rock cut
weathering, fracturing, jointing of the rock
is blasting allowed?
are you in close proximity to other utilities? structures?

Also you need to advise on:

Access (could you use a ripper or milling machine on a backhoe)?
Groundwater?
Underwater trench?
Noise and dust limits?

For rough, preliminary estimating, blasting ~ $70/CY in PA, USA.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

First of all, I don't know how to respond to a particular member. CVG asked me for more speicfic information and I responded in this thread.

Original Thread:
I'm in search of rock excavation (Bedrock)cost for trench.
My project is a 24-inch gravity sewer approximately 2600-feet long. Any elaborations will be very appreciated. I have to excavate soil prior to the rock.

CVG's response:

you need to provide more information including:

type of rock, hardness
depth / width of rock cut
weathering, fracturing, jointing of the rock
is blasting allowed?
are you in close proximity to other utilities? structures?

My answer to CVG:

Rock description: a mixture of highly weathered shale and hard limestone (compressive strength of limestone goes up to 750 ksf)
Depth: Ranges from 4 feet to 20 feet beneath soil
Trench Width: 48"
Blasting is limited. In areas of suspected trenching into the hard stuff, blasting is allowed. Yes I am in close proximity to another structures, which is being replace by the new and bypass operation will be in place.

a rock trencher could be used, although I'm not sure you can go the full 20 feet deep.  It will go through the weathered shale quite well, but may have difficulty with the limestone.  This will slow down production and require frequent replacement of cutting teeth.  For such a small job, the cost to mobilize this equipment may be excessive.

A hoe ram can be used to break up the rock, again the production rate depends on the amount and hardness of the limestone.  

$70/cy sounds reasonable

You may want to consider an excavator mounted cutter head or cutter bucket.  These are catching on in Europe and are just starting to be used in the US.  These cutters are nice because a much narrower trench width can be excavated which reduces backfill volume.  They also have lower noise and vibrations than impact hammers (and drill and blast) so operation in sensitive or small places is less troublesome.  The cutter-bucket allows excavation and material removal without switching attachements.  Plus the unit can act as a crusher on site so the cut material can be reused as pipebedding.  One place where these tools can be found is through www.alpinecutters.com.  They are located in Pennsylvania.  This is not intended as a plug but these tools aren't widely known or used.  

If your excavator has the requires reach, cutting that rock of that strength would be no problem.  

What is your excavator make and model? That info would be required for a production estimate.

Trench blasting:

8' cut = $22/linear foot.
20' cut = $40/linear foot.

Frank Lucca M.I.Exp.E.
www.terradinamica.com

All I can say is, this looks like trouble, and you'd better have lots of money to handle it.
I have worked as an estimator and project manager for a pipeline construction company for a lot of years, and we ran into a similar situation on a project in the middle of a city street about 15 years ago.  There were commercial buildings on each side of that street, which did not help.  The hard rock is only the beginning of your problem -- the shale is possibly the most treacherous thing to excavate, as it may be difficult to cut, but then, once you get about 10-15 feet down, the shale may fracture and huge blocks break off and fall into the ditch, without any warning.  This is especially dangerous because you have to excavate that hard rock down below--and that cannot be readily done without creating a whole bunch of vibration that will make those top layers want to cave into the ditch. Adjacent (and not so adjacent) structures can be affected.  I'd have to say that this situation was the most dangerous place to dig that I have ever seen.
Our solution was expensive, but may be necessary in your case:  pre-drill soldier piles (H-piles) about 5-10 feet deeper than your trench goes into that hard rock, then drop in lagging (either trench plates, or wood) to brace the walls of the excavation.  You probably will need an excavator-mounted hoe-ram to break the rock.  
Good luck!
PS I make my living as an estimator, and I am not green enough to offer an off-the-top-of-my-head price on this one.

I think one thing to really keep in mind is to start your calculations for removal quantities by accomodating an overcut of rock starting at the bottom of the trench.  I know it sounds simple, but..please consider that a work width or installation width of 36" wide at 20' deep requires some type of sloping or shoring to be employed.  This means either a shelf must be cut for a box or the trench is sloped per OSHA standards.  (Type A soil is difficult to argue with a compliance officer unless your geologist will step out on a limb) To cut a ledge for a box certified for 20' deep in what is likely Type B soil), means the box would likely be an 8" wall x 2.  This adds 1.33' to your previous 36".  Thus a trencher with a trench cut straight up would need to have a 4.33 to 4.5 wide chain.  Lots of HP (750-1000hp) and hundreds of teeth @ $10-$15/each!

If you are using a hoe ram, you must employ a "shelf" technique to be able to successfully get down to the required subgrade.  Consider a hammer on a 100,000 excavator is approximately 24" wide (from center of tip of bit to outside edge of hammer housing).  In order to get 20' down stepping in 1' at a time, the trench would need to be 40' at the top, stairstepping down and in.

My point is this - rock is expensive to remove no matter which way it is removed.  It is extremely risky for all the parties involved especially contractors.  To accomodate the overexcavation required at this depth or the cost of a massive trencher, do yourself a favor and figure a removal budget about 2-4x's what it theoretically costs to remove a CY!  Good Luck!