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Determine Rock Excavation Difficulty

Determine Rock Excavation Difficulty

Determine Rock Excavation Difficulty

Have a project where we will need to excavate a large amount of rock to build some tanks. We have a preliminary geotech report that gives us an RQD (varies from 73% up to 100%), and an unconfined compressive strength (279 psi). This is claystone.

From those values any way to determine effort required for excavation? Can this stuff be ripped, require a hoe ram, or is blasting a must?

We are in the very early stages of design and haven't contracted for geotechnical for design yet. The current report was done for a different consultant and we just have a copy.

RE: Determine Rock Excavation Difficulty

Your RQD number implies massive competant rock with minimal fractures but your USC value seems very low, Are you sure this isnt a typo?? Kinda important if your looking for feedback

RE: Determine Rock Excavation Difficulty

Is there any info on rock bedding orientation? Strike and dip info would help in evaluating ahead of time.

RE: Determine Rock Excavation Difficulty

my (limited) experience with claystone is it doesnt have joints, it is not crystalline and so it is considered sound "rock". Claystone is weak and behaves more like a very stiff clay soil than rock. It is highly plastic and muck can become very sticky when wet. it can be interbedded with sandstone which might be a problem

RE: Determine Rock Excavation Difficulty

Caterpillar has a handbook of ripping that provides rippability for different caterpillar equipment, however it's based primarily on shear wave velocity. Komatsu also produces similar information. I believe it is conservative in that the shear wave velocity based systems don't take into account increased ease of rippability from joints / joint spacing / dip (i.e. overall rock mass)

There are other methods (eg. Kirsten's Excavatability Index) based on UCS, RQD, and joint spacing / dips. If they still have the core the geotech should be able to provide enough information for this method.

Finally, you could also phone up local contractors and see if anyone has experience excavating / ripping in similar material.

RE: Determine Rock Excavation Difficulty

Quote (your USC value seems very low)

40Ksf... seems to be OK depends on the rock.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?


RE: Determine Rock Excavation Difficulty

We encounter rock with RQD of 70% plus with UCS of 2MPa routinely.

The Pettifer and Fookes 1994 method for assessing ripability is a good place to start. (Note the red line was added by me for a project.)

You will need to know you Is50 point load index and fracture spacing. Review your logs, fracture spacing should be shown. If not you will have to get the core boxes out and measure the distance between natural factors.

And a word of caution, rippability can often end up in litigation. Contractors price jobs based on the ease of excavation and if you get it wrong they will claim against the main Client and say that the material is not what was priced and I will come back to you!

RE: Determine Rock Excavation Difficulty

was the RQD calculated properly? I ask because any piece of core that can be broken by hand is not to be included in RQD assessment, irrespective of length.

If the rock breaks by hand the RQD is zero.

Beyond that use seismic velocity.


ípapß gordo ainÆt no madre flaca!

RE: Determine Rock Excavation Difficulty

Thanks all for the responses. I'll address everyone below. Before I got onto the project another engineer told the PM on this project the RQD indicated the rock was like quartz so they are freaking out and throwing a huge $$ figure at excavation. I don't think it'll be as hard as they do.

miningman: No, the 279 psi value is correct.

oldestguy: There is no strike or dip info in the preliminary report. Their description for the boring nearest my structure is "Sedimentary bedrock - claystone, gray, moist, slightly fractured, fresh to slightly weathered, weak field strength". They did some test pits/trenches and for those some descriptions say "tightly foliated/bedding planes".

cvg: That's my experience as well. I've worked and played around it a bit and I know the claystone cliffs in the area can be scratched pretty easily.

geotechguy1: Shearwave velocities are 2600 ft/s which barely puts it into "rock" per ASCE7. I'll see if I can find those charts from Catepillar and a good idea about talking to contractors. I know a few in that area.

EireChch: Thanks for that chart. I don't have the Is50 but when we get into the project I'll get it. I don't intend on giving any rippability recommendations. That'll be up to the contractor.

fattdad: I can't comment if they did the RQD right or not. Their pictures show fairly long completely intact cores for the most part.

RE: Determine Rock Excavation Difficulty

Here are two of the Caterpillar charts:

As cvg mentioned, claystone is much different for rippability than crystalline rock. In our specs on a big job in a massive claystone (Clay Shale, RQD below ~4' is 90+, UCS 420 psi) we define "rock" as a crystalline structure. Our experience is the claystone we have is easily excavated with a conventional excavator.

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