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P otential Liquefaction of a silt deposit due to rock blasting

P otential Liquefaction of a silt deposit due to rock blasting

P otential Liquefaction of a silt deposit due to rock blasting

I would like to get some information on the effect of rock blasting/excavation to potential liquefaction of soil deposits in the area.  The soil deposits are generally 10 to 15m thick,consisting of varved clay and with frequent thick layers of silt (up to 2-3 m thick).  The CPTU readings suggested that the tip resistance of this soil is almost uniform, about 1 to 1.5 MPa. Undrained shear strengths are about 50 to 100 kPa.

I went through a paper by Dr. Olson on the subject of liquefaction (also visited his website http://www.liquefaction.com), as well as additional research on the effects of vibrations during blasting, etc.  
Based on the above :
- There are relationships between liquefaction potential of a soil deposit and CPT or “N” values ,
or between “N” or CPT values and depth of deposit.  These relationships are always connected to
an M -7.5 earthquake.....  SSR vs normalized CPT or SPT
- Blasting causes vibrations and may effect nearby deposits which are susceptible to liquefaction.There are connections of damages to structures from certain blast (velocity versus damage), etc

My questions are :
- I have not found the relation of the two above (earthquake and blasting effect to liquefaction).
They seems to be treated as “different” world/field.  Should there be ?  
- How to relate the effect of vibration by blasting to SSR (Seismic Shear Stress Ratio) in the SSR vs normalized CPT relationship ?
- How to determine or estimate “a” (particle acceleration as a percentage of g) in the SSR equations for a blasting . I saw a blasting chart which came from one of our projects (it was from effect of pile driving), and showed 0.01 to 0.03g. What typical a is for blasting ?
SSR = 0.65 x amax/g x sigmavo/sigma’vo x rd (Seed and Idriss 1971)    
rd = 1 - (0.012z)   z= depth (Kayen et al,  1992)
- Does vibration /shock from blasting has same effects as in earthquake ?... May be not on the same scale, but I think there is a connection here.  (Most blasting info I know generally only discusses  velocity and “scaled” distance.  Blast Specs limits the velocity to avoid damages to existing structures).  

Any comments which can shade some “light” on this subject is appreciated.  Thanks


RE: P otential Liquefaction of a silt deposit due to rock blasting

Blasting is used specifically to densify materials, in principle, to effect liquefaction.  At present, to my knowledge, this is a rather "ad hoc" field.  Clearly research would be helpful, but as a first cut, the acceleration assumed in the SSR equations is NOT applicable.
The duration and frequency for blast vibration is very different, and has strong implications for liquefaction, which requires pore pressure changes that generally are accentuated by low-frequency vibration.  

Doug A

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