Soil resistivity opinion

[cuky2000]

Dear colleagues,

Please help us with your assessment in regard to the following statement

"The upper layer resistivity varies significantly with the season where low temperature and low moisture increase the soil resistivity. The lower layers within a few feet below grade are les vulnerable to seasonal changes."

Please see thread238-111645 for additional background regarding EE discussion in this matter.

Thanks for your help.

 

[fndn]

Resistivity is a method used in soil engineering to measure the RESISTANCE of soil to the flow of electric current. It has units of Ohm-m. Resistivity surveys are used to locate depth to water table, depth to bedrock or changes in soil layers.

The resistivity of a soil is mainly a function of its water content and dissolved ions. So Resistivity is INVERSELY proportional to soils having high water content like saturated silts AND high dissolved ions like fat clays.

Then it would appear that dry soils, like sand and even berdock/gravel would have high resistivity. So for example below is a table of likely values:

Saturated Clays 50 ohm-m
Saturated Silt 100 ohm-m
Saturated Sand 400 ohm-m
Dry Sand 1200 0hm-m
Gravel 5000 ohm-m
Bedrock 6000 ohm-m

So in the statement "low moisture increases resistivity" is clear from above table. As for "the lower layers within a few feet below grade---" statement, it refers to the deeper the soil layer is, the less frost and seasonal moisture variations would have any impacts on the engineering soil properties.






A Member of

http://www.civilvillage.com

 

[cuky2000]

Thanks fndn,

I concur with you. However, other colleague had challenge this statement since the enclose graph of “seasonal ground rod resistance variation with 10' rod varies from about 10 ohms to 30 ohms (300%), but the 3' rod only varies from about 20 ohms to 40 ohms (200%)”.

Any though or advise in this matter.

Source:

http://www.transcat.com/TechReference/Newsletters/Why_Soil_Resistivity.html

 

[fndn]

Cuky2000-Thanks for your comments. What are you tring to do with Resistivity test results? Maybe other nondestructive methods may be applicable as well.

A Member of

http://www.civilvillage.com

 

[BigH]

cuky - how did you do that?

 

[cuky2000]

There are several interesting features in this forum that could be used to enhance your post. Use the [/color blue] tag to show images from any web server.
Be sure to use the entire url to the image you want to use including the [COLOR=blue][img]"[URL unfurl="true"]http://"[/URL] [/color blue] _{(no line breaks, no quotes)}
For additional details, click Process TGML that can be found the bottom of the response window.

NOTE:
The server address URL of any image shown in a web page may be obtained as follow:
- Place the cursor in the image
- Click the right mouse bottom.
- In the pop-up menu, go to “property”
- Highlight the image URL address.
- Hit simultaneously [CTL] + [C] keys to copy this address
- Type

“image address copied above”[/i]][/color blue]
- Before submit the post this can be tested using the Preview bottom .
- At this point, either submit or cancel the post deleting what is in the window.

As alternate procedure, place the image in your own web page. If you do not have web page, you may obtain space from one of the free sites such as

http://www.250free.com/manager/user.cgi

Place the image and follow the above procedure.

Here is a sample posted in a web page:

http://cuky2000.250free.com/Soil_Res.jpg

 

[cuky2000]

This is in response to the question posted by Fndn ...What are you trying to do with resistivity test results?

I would like to clarify that in our discipline, Elect Power Eng, we use the ground resistivity as essential parameter to calculate and design a safety grounding systems for electrical projects such as high voltage substation, power plants and others facilities.

Logically speaking, appears that upper soil layer is more exposed to interact with changes in environment conditions such as rainfall (moisture), temperature, etc. However, the enclose graph appear not supporting this logic because its shows more seasonal variation at 10 ft than at 3 ft below grade (see post above please).

We welcome any additional comments from any of the geotechnical members of this forum.

Thanks in advance.

 

[nsong]

Hi guys,

I am working on a project that requires reviewing some soil resistivity data. The number I have are aroung 800,000 ohms-cm, they were measured in damp to moist silty and clay sands, are these numbers possible? I read some literature suggesting that the maximum values are about 500,000 ohms-cm. Where could I found some sound reference?

Thanks for your help!