Engineering Properties based on RQD

[4n6]

I am designing an short (6' or less) MSE type wall in a cut situation. The retained material is a poor quality limestone. The RQD (Rock quality designation) ranges from 8-17.

A texas cone penetrometer advanced 0" at 100 blows in most areas.

Can any one recommend a phi angle that should be assigned to this material?

I thank you in advance for your help.

 

[jdmm]

I don't know what a Texas cone penetrometer is (TCP?) but I expect that it is at least twice the size and weight of a normal penetrometer!

Is the limestone undisturbed or is it broken up backfill material? Are you using the limestone as reinforced backfill or is the limestone to be retained and you will use a different material in the MSE wall?

If the limestone is intact and to be retained with penetration refusal it must have a high strength based on C' and phi or phi plus a dilatancy. If this is the case then the earthpressure would be very low (like a very high phi) in which case you would design the wall on the basis of the reinforced backfill used to construct the MSE wall.

 

[4n6]

jdmm,
Thanks for the info. To clarify -the retained limestone in question is undisturbed and in place.

We will be using a crushed rock backfill for the reinforced zone. I am designing with a phi = 34 for the crushed rock zone.

Would I be okay, based on the parameters described in the first post to assign a higher phi (36 to to the retained, undisturbed limestone?

Thanks

 

[BigH]

What type of MSE wall? Are they geogrids or RE Wall strips?

6 ft high wall is very low - if you are in limestone - whether it is highly weathered or not - I don't see a problem - your mass of the MSE (plus the backfill in the strip/geogrid zone) will be enough to hold back the wall. I doubt that I would have even "tried" an analysis unless you have a high sloping backfill behind.

Phi of the backfill zone material of 34 is quite low. We are using medium to fine sand alternating with sandy silt fly ash both having 36deg minimum direct shear strength on a job I am doing now - it is up to 9m high; metal strips. {by the way - we have a soft clay foundation with Su about 20 kPa, if that, so we will be building in stages and expect about 800mm or so of total settlent - have little choice.}

One caution, is that if your fill material is quite angular, etc., I would be somewhat careful - I would, as a gut feeling, be leery of damaging the strips/geogrids if you put a roller on to compact.

Most important item, in my view, is to ensure you have a proper base onto which to put the initial layers of your MSE wall.

 

[doresh]

In order to avoid damage to the reinforcement units you can use backdumping work in order to avoid as much as can straight damage. Also, a thin protection layer of sand can be lay on top of any reinforcement level in order to minimize the damage.