Lateral pressure from rock ka and c using Wallap

[bratty]

In using Wallap, the input required is material friction angle and cohesion. When the soldiers are in rock, It is appropriate to use rock's ka and c in rock to determine the lateral pressure? Where can I find the ka and c values? Some GI reports do not give any indication. Some will recommend to use ka = 0.1 to 0.15 and do they include any cohesion values?

 

[PEinc]

Rock mechanics is not the same as soil mechanics. Lateral pressures in rock can be greatly affected by the amount of fracturing and the inclination of bedding in the rock. Will there be sliding wedges of rock? Extremely rarely (almost never?) is bedding information given in geotech reports for projects. I don't believe there is a simple answer to your question.

http://www.PeirceEngineering.com

 

[EireChch]

I agree with PEinc, rock mechanics is a different beast altogether. Your rock could be of sufficient strength where there is no lateral earth pressure at all. You could cut it vertical and it would stand forever.

Or it could be a poor quality rock with low RQD and joints that are dipping towards wall, this could exert a lot of lateral earth pressure. all depends on your rock. If you tell us more we might be able to help.

But my main recommendation would be to request that the geotech, who prepared the report, provide the ka etc.

 

[bratty]

Thanks for the comments.

I read some geotech Investigation report. Some stated to take Ka = 0.25 in weathered rock and 0.15 in higher strength rock. but Geotech Investigation report would recommend friction and cohesion values. So L am totally confused as which to take. the lateral pressure will be different between these 2 approaches.

 

[EireChch]

Ideally the friction angle in the report would give you the ka in the report. Unless the author is trying to be conservative and give a higher ka than the friction angle would give. I would be worried if the ka from the friction angle was higher than the stand alone ka value in the report.

Cohesion is not included in the calculation of ka. Ka is only a function of phi.

Cohesion is included as part of your active pressure (Pa) calculation.

Pa = H x gamma x ka - 2 x c x sqrt(ka)

 

[bratty]

EireChch

Thanks for your reply

As many correlations has been carried in Geotechnical Engineering, is there are books or established publication on various kind of rock's ka and cohesion values? Where can I find them?