SHORING IN ROCK

[voi]

Hi.

I am designing a shoring system up to 15 m deep. The soil investigation indicates the strata is soft rock. I am wondering in my analysis should I assume it to be sand based on Peck's pressure envelopes or ?. I am unable to get any information on how to analyse shoring on rocks. All information is on granular or cohesive soils.
thanks

 

[FixedEarth]

You can assume soft rock as being similar to dense granular soils. There are many analysis methods. You can use Blum's free cantilever method, which equates active thrust to passive resistance to get the required embedment. All the equations you need are in the 2nd edition of "Foundation Engineering" by A.R. Jumikis.

 

[PEinc]

Before assuming the rock acts as a soil, you may want to check if the rock has any sloped bedding, joints, or soil seams that could cause a slide. Slide pressure may be greater than soil pressure. You may need rock bolting, possibly with line drilling, shotcrete, or chain link mesh, rather than sheeting. Also, trying to sheet a rock cut can be more expensive that rock bolting, etc.

http://www.PeirceEngineering.com

 

[voi]

I appreciate the contribution. Basically the site is somewhere in Africa and the strata is Volcanic tuff. When it comes in contact with water it just disintegrates. You can actually crush it with your palm of your hand when it is fully saturated. When drilling you obtain complete intact samples and the UCS is in excess of 3000 kPa. In its dry state it is quite solid and intact. There are no bedding or seams visible in the rock cores.

voi

 

[GeoPaveTraffic]

Based on your description, it would seem that shot crete and maybe some rock bolts could be used to stabilize the rock as the excavation progresses. I would not try to drive sheets into that material unless local practice indicates that sheets have been driven in the past.

Mike Lambert

 

[PEinc]

Rather than sheet piling, drilled-in soldier beams could be used but there still could be more than normal earth pressure. If the "rock" deteriorates easity when wet, rock bolts with shotcrete would probably be most economical. Chain link mesh would not prevent deterioration.

http://www.PeirceEngineering.com

 

[voi]

Let me understand you PEinc. Are you advising that I should consider drilled in soldier beams supplemented with rock bolts and shotcrete? Or only rock bolts with shotcrete. I was thinking of using soldier piles with wales and lagging. could you be more specific on what you mean by "more than normal pressure"
Voi

 

[cvg]

the problem with sheet piling is that you are likely to run into a mixture of Tuff along with other coarse pyroclastic rocks. that could make driving (or drilling) piles problematic. You might be able to confirm this with the drilling logs. I believe I would just stick with rock bolts. Provide perimeter drainage to avoid saturating the subgrade. Shotcrete facing would help to keep the face from raveling.

 

[PEinc]

voi,
If you have rock, consider rock bolting and shotcrete.
If you are set on using sheeting, consider drilled-in soldier beams with lagging and tieback anchors.
Drilling in rock can be expensive, but it can be done.
Sheet piling usually cannot be driven through rock or very dense decomposed rock.
Carefully choose your lateral pressures. A rock slide can produce greater lateral pressures than a "normal" excavation in soil.

http://www.PeirceEngineering.com