Expert: Robert Cummings, P.E. - 1/4/2008

Question
There is a property that I wish to build on that is at the end of a culdesac in an established subdivision.  The site slopes towards the street and is almost entirely bedrock, visible at the surface.  Is there any economical means of trenching bedrock to allow connection with city services beneath street level?  Blasting is impractical due to the proximity of existing homes within 50' of the sewer hookup point.

Answer
Actually, you can blast that close.  You just use small holes on close spacings, little charges, and cover the blast area with blasting mats.

There is another option -- you drill a lot of closely-spaced (1.5-2 ft centers, 3x3 pattern) holes at least 3 inches diameter (the larger the better) about 1 ft deeper than the sewer trench needs to be, and have a hoe-ram chop the rock out.  Use a hoe-ram of at least 5,000 ft-lb capacity.  The cost for the hoe-ram may be $300/hr, and it needs to be accompanied by an excavator to remove the loose rock before it gets muck-bound.  Another $150/hr.  Drilling costs are probably $250/hr.  For my money if you are going to drill that many holes you might as well set small charges and blast.

If you do blast be sure to hire a competent blaster and have pre-blast surveys done of the neighboring houses, so you won't get blamed for pre-existing damage.  

If acess is an issue or the rock is very hard, and you cannot blast, you may be relegated to using a rock trencher.  These look like a Ditch Witch on steroids -- carbide teeth on a belt, big noisy machine, but I have seen them cut granite 10 ft deep.  They are only available to general contractors with diverse capabilities.  Not cheap.  Check the Vermeer website for examples.

Blasting would be cheaper and I think feasible for the depths typical of most sewer work (8 ft or less).