Construction & Contractors/Rock Blasting in Sensitive Area
Hi Robert, I am bidding a bridge job which requires some rock excavation. The rock is under about 16' of sand overburden and i need to excavate a volume of rock 75' x 38' x 15'thick. The rock is classified as Granitic Gneiss: Gray, M-F Grained, Moderately weathered, closely fractured, friable. There is a 24" city water line about 15' away from the edge of the excavation volume but above the rock. We can shut down the water main but dont want to damage it. You think blasting the rock volume without damaging the water main is a possibility? More importantly can we convince the NYDOT that we can do it?
I take it the water main, being above the rock, lies in the overburden. You say there are 16 ft of sand overburden and the main is 15 ft from the edge of the excavation, so my first question is what is going to be done about overburden ground loss into the excavation, because that would reduce lateral confinement on the water main.
That is relevant to the rock excavation question because the protection of the temporary excavation slope will also be critical to supporting the water main under any ground movements (vibrations) induced by the rock excavation.
You didn't say what kind of main it is or what condition it is in. In general, buried pipelines are not very susceptible to blast-induced ground vibrations because the vibration occurs as body waves at the pipeline rather than surface waves. As long as the waves are not low-frequency and very high amplitude, a combination which is very unlikely when shooting close to a pipeline, the induced strains in the pipeline tend to be very small. However, in old pipelines, especially DIP pipes that may be under pressure and subject to corrosion losses, even small strains may not be tolerable, but that circumstance is much more rare than people think. And in that case your issue would again revert to the ground strains induced by overburden removal.
There are references that document pipeline blasting parameters pretty well that can support your assessment of blasting feasibility, and that can be used to support your case to the public officials.
For the reason cited above, the main concern for damage of buried pipelines when shooting close in is block movement (mass displacements as opposed to small-strain vibrations). Block movements result from surface cratering (too thick a stemming height, usually the result of the blaster being over-cautious of fly rock) and/or too much burden (blast holes having to do too much work individually so they develop too much back pressure when releasing the rock to their front, causing block movements outside the intended excavation limits). This is of particular concern when the pipeline is above the bottoms of the blast holes.
The solution is to reduce the blast hole diameter and spacing, and ideally the lift height too. The energy induced into the rock mass is the same, but each individual hole does less work and the stemming heights can be reduced because of the smaller hole diameter.
As an example, I once shot hard, thick-bedded limestone within 6 ft of a historic masonry structure that was not only tied into the same rock mass on the back side, but was already cracked (we had strain gauges to assess whether we widened any existing cracks). This was for a road cut 20 ft deep and 70 ft wide. We started far away from the structure and progressively narrowed each shot as the margin of the cut was reached. At the end we were on 2 rows of a 4x5 pattern of 2.5-inch-diameter holes, each carefully loaded with stick powder so we knew exactly what the loading was. One crack was widened by about 0.015 inches which was shrugged off as insignificant.
Your situation somewhat different, but as a technical problem, much easier because of the fracturing and because the depth of the excavation lends itself to two lifts. So yes, you absolutely can blast the rock you have, but it will be more costly than ordinary shooting.
It should be noted that the means of stabilizing the slope in the overburden must also be protected from the blasting, and the conservative approach necessary for that will also protect the pipeline.
My final observation is that if the rock is as you say, you might not want to incur the expense of blasting if that is all the rock excavation you actually have. There are other aspects of blasting that arise whether you are shooting one hole or 1000 (permits, submittals and approvals of blasting plans, monitoring of adjoining areas, pre-and post-blasting surveys of adjoining structures) that escalate the cost and have to be spread out over whatever volume of rock you shoot, so if those are applicable the cost really goes up for small jobs. You are probably talking about $25+/cy, not including mobilization and those incidentals I just mentioned.
You didn't say what the seismic velocity is (hopefully someone ran some refraction surveys) but a weathered, friable, closely-spaced gneiss, while easy to shoot, should also be amenable to excavation with mechanical equipment.
If you find it cost-effective to blast, but the public officials balk at shooting right to the side of the excavation, you could offer to shoot the middle and leave the last 6 ft or so for mechanical excavation as a buffer.