Japanese Earthquake Definition of Bedrock

[NUCENG]

On the Japanese Earthquake: Nuclear Plants thread in PF Nuclear Engineering forum we have been having a lot of trouble figuring out descriptions of mudstone and plant foundations built on solid bedrock. Can anyone help me on this.

I followed some research clues and found the following:

A geological definition of bedrock:

http://www.expertglossary.com/water/definition/bedrock

Bedrock

The solid rock at the surface or underlying other surface materials. Rock of relatively great thickness and extent in its native location. A general term for any solid rock, not exhibiting soil-like properties, that underlies soil or other unconsolidated surficial materials. As distinguished from boulders. The consolidated body of natural solid mineral matter which underlies the overburden soils. The solid rock that underlies all soil, sand, clay, gravel, and other loose materials on the Earth's surface. Any sedimentary, igneous, or metamorphic material represented as a unit in geology; being a sound and solid mass, layer, or ledge of mineral matter; and with shear wave velocities greater than 2500 feet per second.

Then from the following link:

http://erosion.com.co/index.php?option=com_docman&task=doc_download&gid=287&Itemid=237

I found a graph (attached) of shear wave velocity vs Compressive wave velocity for different types of rocks. There are three samples for intact mudstone and all have shear wave velocities greater than 2500 fps or about 760 m/s. If I am reading this right mudstone MAY meet the definition of bedrock.

http://www.iitk.ac.in/nicee/wcee/article/9_vol3_733.pdf
One of the previous posts discussed seismic response at unit 6 in a study of the Seismic Soil Structure Interaction

http://www.iitk.ac.in/nicee/wcee/article/9_vol3_733.pdf

Table 2 shows a graph of shear wave velocity vs depth that tops out and then remains constant just over 500 m/s from a depth of 12 m to 143 m. There is a discussion that the velocity may be reduced by fractures, folding and other discontinuities. I wonder with the history of seismic activity in Japan whether there is a lot of intact bedrock anywhere in the country. Are people who are saying that mudstone isn't bedrock emphasizing the word mud over the word stone?

Can a large seismically qualified structure be built on mudstone?

 

[NUCENG]

Oops forgot the figure.

 

[Studiot]

I have some difficulty with your definition of bedrock.

Here is another

Barren formation (seat, earth, clay 'farewell rock') underlying the exploitable part of a mining deposit.

Bedrock, it seems, is a general term which has more than one interpretation.

 

[NUCENG]

I have some difficulty with your definition of bedrock.

Here is another

Barren formation (seat, earth, clay 'farewell rock') underlying the exploitable part of a mining deposit.

Bedrock, it seems, is a general term which has more than one interpretation.

Thank you for the reply. In trying to find a definition I found a number of qualitative descriptions as you indicate. The one I quoted was the only one I found with a quantifiable characteristic - shear wave velocity.

If it is really subject to multiple interpretations, does mudstone meet one of them?

 

[Dotini]

Mudstone is an argillaceous rock, similar to shale in non-plasticity, cohesion, and lower water content, but lacks the bedding plane fissility.
Source: Dictionary of Geology, Brooks and Whitten, Penguin, 1972.

Like shale, it may disintegrate when immersed in water. <--My interpretation.

Respectfully submitted,
Steve

 

[Studiot]

Well mudstone is not normally found at the surface because it weathers readily. I am not familiar with japanese geology, but I have never seen it massively bedded. In my experience it forms variable, often thin, layers, often well broken and often mixed with softer strata.

Mudstone strata overlap the foundation scenarios when the engineer considers soil mechanics or rock mechanics in design.

All that said, mudstone is quite capable of offering a competent foundation to significant structural loads. BS8004 suggests sound mudstone to offer around 2000 kN / sq m bearing capacity.

But any strata, especially softer sedimentary ones, should be investigated for geological structure when considering sensitive structures in case underlying features.

I have built a reservoir on mudstone, but not a nuclear reactor.

go well

 

[~kujala~]

I found a graph (attached) of shear wave velocity vs Compressive wave velocity for different types of rocks. There are three samples for intact mudstone and all have shear wave velocities greater than 2500 fps or about 760 m/s.

Thanks for the source!
They have huge readings for mudstone, more than 2 500 m/s!
But the main thing is that the results are for the rock that is "intact".
In real life rock is hardly intact - it has gone through a great deal of erosion and weathering.
I think what they have might done is this: They have taken a small sample of "intact" rock, then studied it in a laboratory environment.
The result hardly tells anything about real life situation.
This is my guess how they were able to record such high readings.

Table 2 shows a graph of shear wave velocity vs depth that tops out and then remains constant just over 500 m/s from a depth of 12 m to 143 m.

Thanks again!
So here we have a real life measurement, I guess. I captured the graph to this link:

http://varasto.kerrostalo.huone.net/lietekivi_4.png

If you compare this result to the soil types in San Fransisco area (again real life situation) you can see that this velocity goes exactly where it's supposed to go:

http://varasto.kerrostalo.huone.net/lietekivi_2.png

750 m/s > Vs > 350 m/s

So this would indicate that S-wave velocities for mudstone in Fukushima and San Fransisco do not differ that much.

Are people who are saying that mudstone isn't bedrock emphasizing the word mud over the word stone?

I think it's more a problem with language.

In Finland we have a word "peruskallio" which means a deeper and harder layer of bedrock (level A/B in the San Fransisco area soil type graph). "Kalliopohja" on the other hand means the same as "bedrock" in its general meaning in English.

But the translator machines for English->Finnish may give a translation for "bedrock" as "peruskallio" which is not what it is supposed to mean.

And I think even native English speaking persons might sometimes assume that "bedrock" only means this deeper/harder layer of bedrock.

Now I think mudstone is a part of bedrock. My explanation here:
https://www.physicsforums.com/showthread.php?p=3308680&highlight=bedrock#post3308680

Of course, if it would be defined by shear wave velocity it would be much more difficult. In San Fransisco area soil type graphs only soils A/B would go above > 750 m/s.

Can a large seismically qualified structure be built on mudstone?

I have asked the same question.

A raft foundation / pile raft foundation can be even used in much weaker soils.
They are a using pile raft foundations in Dubai Towers / Dubai Burj Califa skyscrapers.

First we should know the exact foundation type of Fukushima plants to evaluate it further.

 

[~kujala~]

http://web.ics.purdue.edu/~ecalais/haiti/macrozonation/formation/6_Soil%20Amplification%20and%20Topo%20Effects.pdf

In this paper they also seem to suggest that 760 m/s is some kind of border between bedrock and ground-surface:

Model full Vs profile of soil from bedrock
(Vs~760 m/s) to the ground surface

(page 16)

 

[~kujala~]

I found a source where they are trying to make a clearer definition of bedrock. I don't know if they have succeeded?
http://iisee.kenken.go.jp/net/yokoi/bedrock/index.htm

"Engineering bedrock" is defined as an interface having shear wave velocities 300 m/s - 700 m/s:

In the viewpoint of Earthquake Engineering, it has been proposed, based on the followings, to use the shallower interface of which underlying stratum has from 300 to 700 m/sec of the shear wave velocity. This interface is called "Engineering Bedrock".

Based on these velocities the interface would be roughly the same as class C soil in San Fransisco area (350 m/s - 750 m/s):
http://varasto.kerrostalo.huone.net/lietekivi_2.png

On the other hand "Seismic bedrock" is defined as a deep interface of having shear wave velocity of about 3 000 m/s:

As the shear wave velocity of upper Earth crust is as homogeneous as from 3000 to 3500 m/sec, the upper interface of the upper Earth crust having 3000 m/sec of the shear wave velocity is called "Seismic Bedrock".

This would be class A in San Fransisco area or perhaps only a subset of class A.

Note also:

In the textbook of Earthquake Engineering, "Bedrock" and sometimes "Seismic Bedrock" is used in the context where "Engineering Bedrock" should be used. In contrast, the textbook of Seismology uses "Bedrock" where it should be called "Seismic Bedrock". It is important to distinguish the definition of the terms where they are used, by the context or the glossary.

It's complicated. The geologists again may have their own definition of "bedrock". And the linguists may have their own version.

My conclusion is that it is impossible to define "bedrock" as having some kind of universal meaning. Or if you should have a "general" definition it should be the definition made by the linguists as they are in charge of how words are used in everyday life.

 

[Studiot]

As I mentioned in post#6
Wave velocity by itself is a very unreliable indicator of competency.

You really do need to also study the structural geology when considering sensitive structures.

I note your reference states that there is not much of such information 'below many cities in Japan.'

Except of the plains around the Tokyo metropolitan area, Osaka City and Nagoya City, there is not any information about such deep structure as "Seismic Bedrock".

 

[darksociety]

Thanks for the source!
They have huge readings for mudstone, more than 2 500 m/s!
But the main thing is that the results are for the rock that is "intact".
In real life rock is hardly intact - it has gone through a great deal of erosion and weathering.
I think what they have might done is this: They have taken a small sample of "intact" rock, then studied it in a laboratory environment.
The result hardly tells anything about real life situation.
This is my guess how they were able to record such high readings.

More likely that figure (2500 m/s) comes from a seismic line, thus reflecting a real life situation.

Terminology of 'bedrock' simply refers to any indurated lithology (if sedimentary) that does not exhibit significant weathering. In some areas the bedrock may be exposed at the surface (and likely metamorphic or igneous), and in others may be buried up 10 of metres of soil or till.

 

[~kujala~]

A couple of notes from an earlier document which contained geological information about Daiichi. I took a few screenshots which might be copyrighted material so I am perhaps not keeping it long in my webpage.
https://www.physicsforums.com/showpost.php?p=3345188&postcount=9259

In this picture I took away the ground they shaved off so what is basically left are the layers below OP+10 m (the current ground level). I used the translations posted earlier. I am not sure if I understood correctly which layer is weathered mudstone and which is non-weathered mudstone:

http://varasto.kerrostalo.huone.net/lietekivi_5.png

Anybody knows what do the numbers 24.86 kg/cm3 - 37.55 kg/cm3 stand for? To me they seem rather "big". Any charts for making comparisons?

Anyways, if one compares this information with the one below which is from an earthquake study, the information is almost same. The first 10 meters is weaker soil and below that the shear-wave velocity is 520 m/s (= mudstone):

http://varasto.kerrostalo.huone.net/lietekivi_4.png

Terminology of 'bedrock' simply refers to any indurated lithology (if sedimentary) that does not exhibit significant weathering. In some areas the bedrock may be exposed at the surface (and likely metamorphic or igneous), and in others may be buried up 10 of metres of soil or till.

In Daiichi this probably means that below first 10 meters of sand/weathered mudstone is the layer which could be called 'bedrock' (= mudstone + sandstone).

In another part of the above picture there were also some velocities presented (500 m/s - 1,700 m/s):
http://varasto.kerrostalo.huone.net/lietekivi_6.png

For mudstone it is 1,600 m/s - 1,700 m/s. Any ideas what velocities these are and how they have been measured? The actual measured shear-wave velocity was about 520 m/s from 12 m below ground to 143 m below ground for two different earthquakes.

Finally, there was a picture of the original groundwater level. The strange thing here is that the distance from EL + 9.273 to EL + 14.5 is quite long compared to the distance from EL +14.5 to EL +24 as though there was some kind of distortion in the drawing:

http://varasto.kerrostalo.huone.net/lietekivi_7.png

Nonetheless, what the picture suggests is that the current level of groundwater could be near the current ground level. So the Daiichi could be floating on water even without tsunami.

 

[Astronuc]

Anybody knows what do the numbers 24.86 kg/cm³ - 37.55 kg/cm³ stand for? To me they seem rather "big". Any charts for making comparisons?

Possibly a pressure gradient, as in (kg/cm²)/cm.

 

[~kujala~]

The discussion in this thread could perhaps be directed to this thread as it contains general discussion about Fukushima Daiichi geological issues:

https://www.physicsforums.com/showthread.php?t=507369

 

[Ophiolite]

Anybody knows what do the numbers 24.86 kg/cm3 - 37.55 kg/cm3 stand for? To me they seem rather "big". Any charts for making comparisons?

The units are obviously those of density, but the values are four orders of magnitude greater than they should be in this context. 0.0486 kg/cm^3 would be a typical value for these sort of sediments. Also the symbol used in the diagram, for which these are the values, I cannot recognise. If it is meant to be rho, the normal greek letter for density, it is a poorly written one.