Blasting Bedrock Next to Existing Structure 2

[LPPE]

I have an existing medium-rise building with a one level basement. This building is founded on bedrock just below the basement slab.

The owner wants to put a new structure right next to the existing, with a two level basement.

Now, this will require the removal of bedrock, wether by blasting or chipping. I think that this rock removal will fracture the bedrock under the existing building, possibly causing undermining the foundation. I want to have the owner pull his new structure away from existing X amount of feet.

Does anyone know of any problems associated with removing bedrock 13' down from face of existing structure founded on rock? And what is a safe distance to move the new rock cut away from existing structure?

 

[ishvaaag]

I think the risk exists mainly for cases in which the rock is itself prone to crack unreliably. Still I feel on should intent what the owner wants if feasible. If sound rock you can proceed hard till 25 or 30 ft of the wall, then with small or medium automatic hammers. What you see will guide you. Just in case, check the ability of the esxisting foundation to take the loads at edges in a "pulling beam" scheme; this will ensure a partial lack of support won't turn critical. Explosives? If feasibe, better NO.

Upon excavation, the released of restraint adjacent rock sometimes expands -what by the way affects a little the adjacent building- and it may be then interesting to delay construction of the wall or allow 100 mm or more of compressible material in-between of our new basement walls and rock.

Although not extense in the subject, you can give a look to

Rock Engineering Applications
Franklin and Dusseault
Mc Graw Hill

 

[JAE]

pylko, my first response in your situation would be to get recommendations from a geotechnical engineer who can analyze the rock and provide you with some guidance on cutting adjacent to and below your existing footings. It probably depends a lot on the cometency of the rock (weathered vs. non-weathered) as well as the magnitude of the load the existing footings are supplying.

 

[aurelio]

hi, I and sure that with blasting NO, chipping is the way, but also depend of the rock quality. Is bether if you could made a geological study to know more about the rock, if the rock is granite or other hard rock, follow with your idea, if not, you are buying a sue.

 

[LPPE]

The rock is a gneiss, slightly weathered at the surface.
Someone told me of a procedure like blasting - The rock is drilled (like blasting), but instead of using explosives, the drill holes are pressure injected with water, and when the pressure is high enough the rock fractures. Anyone hear of this? Doesn't seem that possible, because what about fissures and such in the rock?

 

[DaveVikingPE]

The International Society of Explosives Engineers (ISEE) -

http://www.isee.org

- should have some good resources/people. I recently took a course from them (Blasting Safety) and it was excellent. Had a lot of examples of blasting under structures and such.

 

[redhead]

There are hydraulic rock splitters, and they are very effective. We did a building opposite the State House in Trenton, NJ using this technique because blasting was not allowed. The rock was a very hard gneiss with virtually no natural fracture planes. The area of influence of the splitters is controlled by the hole pattern.

 

[visitor]

While doing blasting use some "delays" which controls the
blasting and reduces the amount of shock waves produced.

 

[Rowland33]

A good powderman should be able to peel an orrange beside a building. Here in Ottawa Canada downtown is on limestone rock and buidings are put up side by side with foundations four levels deep.
I agree with Visitor, use delays and above all do a crack survey of all surrounding buildings, the claims for cracks will come in thick and fast at the sound of the first charge.
A Geotech shoould be consulted to ensure that joints and faults dont provide any unpleasant surprises and monitor all blasting with seismic apparatus just for records.

 

[Viscous]

A good blasting plan can remove the rock without damaging the rock that will remain under the existing building.

You may want to try the International Society of Explosives Engineers - I think they are at

http://www.isee.org

- to find a blasting specialist in your area.

 

[Peabody]

The method I would use is called Controlled Blasting or Contour Blasting.It requires that you Line Drill along your building line.Line drilling consists of a row of holes, 2 or 3 inch diameter, spaced 2 to 4 diameters apart.These holes will not be loaded.The burden (distance from "line drilling" to the next row of holes) should be 1/2 to 3/4 the burden used for your production shots.the spacing between the holes in this row would be 3/4 the spacing for your production holes.No stemming should be placed around the explosives."Stick" explosives, no poured explosives,are used in the holes.The holes are to be plugged with wadded paper or a commercial plug.Aboove the plug the hole is stemmed.

Production blasts should advance to the third line from the line drill holes.

If the rock dips toward the face the rock may have to be pinned or rock boltsd to prevent slippage in the rock that supports the adjacent building.

References:

ISEE,"Blasters Handbook", 17th Edition,International Society of Explosive Engineers, 1998,

http://www.isee.org.See

Chapter 31, pages 479 -506,Controled Blasting.

Berta,"Explosives: An Engineering Tool",Italesplosivi-Milano Italia 1990, Chapter 3, Pages 77-108

Konya & Walter,"Surface Blast Design",Prentice-Hall,1990,Chapter 10,Controlled Blasting, pages 217-238

Persson,Holmberg & Lee,"Rock Blasting and Explosive Engineering",CRC,1994,Chapter 9,Contour Blasting,pages 265-285.