Determining critical particle velocity for a nearby berth.

[goingCoastal]

Hello Tippers,

I have question on determing critical partical velocity for a concrete wharf, or the natural frequency of the structure. Blasting operations will be near the structure and need to provide a upper limit which a blaster cannot exceed. Is there a proven method which I can quickly calculate or understand. To better aid your response, the following parameters are:

Structure to Remain:
The Wharf is supported on 24" Square Concrete Bearing Piles. All piles were driven to refusal to a depth of 45 feet. The structure has no batter piles. The uniform Live Load for the structure was design to be 800 psf.

Structure to Remove:
The derelict structure is 80 feet waterward from the face berth, and is submerged to a depth of 35 feet and is 15 feet deep.

Geology:
The area is comprised of Clay,sand, sandstone, Limestone. Both structures are resting on limestone.

Any hints, suggestions, books, programs will be very useful and handy.

V/r

 

[dgillette]

Hello gC.

I pulled out some references that I collected for a paper I wrote for a grad soil dynamics class in 1982, which was a while back.

For residential construction, the recommendation seems to be to keep it below 2-3 in/s. 4 or 5 in/s is the typical threshold reported for plaster damage. The sort of structures you are dealing with are probably much less vulnerable than houses with lath and plaster. I haven't found anything applicable to reinforced concrete structures in my quick search, although I think I have some old Bureau of Mines reports in one of several large boxes that have been lurking under my table since I last moved office 4 or 5 years ago. Try hunting for their publications on line.

Best regards,
DRG

 

[MarineConstruction]

How close to the structure? Typical limits here in Canada for blasting close to structures or in urban areas is 12.7mm/s (0.5 in/s). As dgillette stated, the peak particle velocity required to cause damage is significantly greater than this but provides a cushion for the blaster, especially during the first few blasts. Using this criteria we have blasted very close and underneath existing infrastructure (within 1m of a 1800mm live full flow sanitary sewer and within 5m of a live 500mm natural gas pipeline) using this criteria and have never had any damage.

You can use this limit of peak particle velocity to work back and determine what your charge load is going to be. By doing this, you will be able to see what type of measures may need to be employed to limit vibrations (MS timing, decking of holes, different products, different hole spacing and diameter, etc...). To truly know how the site and structure will respond, you need to perform small trial blasts with an linear array of seismographs from which you can develop a site specific attenuation equation.