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Using RMR System to Determine Mass Strength of Jet Grout or Concrete

Using RMR System to Determine Mass Strength of Jet Grout or Concrete

Using RMR System to Determine Mass Strength of Jet Grout or Concrete

I am wondering if anyone has used the RMR system or Q system to rate or determine the mass strength of anything other than rocks such as unique concretes like soilcrete.

I am trying to apply the RMR or Q system to insitu Jet Grout material. I am having issues as the RMR system is mainly governed by discontinuities within rocks whereas my insitu jet grout material does not have any major discontinuities. Instead, it is full of soil inclusions and voids.

If there are any other ideas on ways to rate and evaluate the mass strength of a large mass (width = 20 m length=6 m depth=25 m) of soilcrete (approx 2 Mpa) underground, I would be very interested to hear them.

Additional Information: The jet grout is being used as a headwall for a tunnel boring machine and its primary function is groundwater cut-off with a secondary function as part of the support of excavation for the exit shaft of the tunnel boring machine.


RE: Using RMR System to Determine Mass Strength of Jet Grout or Concrete

I take it that you have a 20 m wide by 25m tall section of this grout and it is 6 m thick in the path of the TBM. The TBM will cut through this as it daylights into the receiving pit? Will the ground water be sealed by the outer walls of the TBM? Is the pipe right behind the TBM? I would not use the grout as a means of supporting the excavation except within 20-30 cm from the TBM wall.

Richard A. Cornelius, P.E.

RE: Using RMR System to Determine Mass Strength of Jet Grout or Concrete

Dicksewerrat, I chose those dimensions as hypothetical values with a hypothetical function of the jet grout. I agree it may not function as a SOE as described but the design details are not in the scope of my work. My main concern is that the jet grout must be low permeability to act as a water cut-off and also meet the required mass strength for other purposes.

My main problem is to come up with an ideal way to assess the overall mass strength and integrity of the jet grout. If a test pit is created with test jet grout columns and sequentially excavated to allow mapping of the grout, what rating system would be most applicable to this material. RMR and Q seem difficult to adapt to a soilcrete since they are mostly geared towards rock.

Thank you.

RE: Using RMR System to Determine Mass Strength of Jet Grout or Concrete

It would be incorrect to use RMR or Q on concrete blend products. The "mass" strength has no applicability here. Rock mass strength is an intermediate strength between that of discontinuities and intact rock. Using the Hoek-Brown method, this strength is estimated using GSI (or RMR) as one of the inputs.

The only way to get the strength of your soilcrete is to prepare samples and have them tested (unconfined compression).

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