soil resistivity measurement and interpertation
soil resistivity measurement and interpertation
(OP)
Hi All,
I have a soil measurement as it is shown in the attached excel file. It is a Wenner method. I know measurements are not good. What is your suggestion to come of with a soil model to be a good representation of the soil model for the site.
Thanks,
I have a soil measurement as it is shown in the attached excel file. It is a Wenner method. I know measurements are not good. What is your suggestion to come of with a soil model to be a good representation of the soil model for the site.
Thanks,






RE: soil resistivity measurement and interpertation
RE: soil resistivity measurement and interpertation
RE: soil resistivity measurement and interpertation
RE: soil resistivity measurement and interpertation
RE: soil resistivity measurement and interpertation
The photo indicates this is some sort of rural area.
Are any water well records available for wells in the area?
Where I am our State Dept. of Natural Resources has put all well records, dating back decades, on line.
(They were always available the old-fashioned way, but it is much easier now.)
I have used these to gain insight into the soil characteristics in a couple of locations where we built substations.
These records should show the type(s) of soil encountered, whether or not there is bedrock, and the water table level.
If this is not available you could consider hiring a geo-technical consultant to bring his rig to the site and drill a few boreholes, as is normally done for foundation design.
Also, you may want to probe deeper using even wider electrode spacing, it appears that resistivity may be trending lower at deeper levels.
In that case some deep well type electrodes may be helpful to lower overall ground grid earth resistance.
RE: soil resistivity measurement and interpertation
For what it's worth, your apparent resistivity vs probe spacing measurements for most of your traverses are pretty consistent in terms of the curve shape. Your measurements look to me like there may be a very high resistivity layer pretty close to the surface.
Also keep in mind for shorter probe spacing on your Wenner measurements, you will likely have to correct the measured results based on the probe depth as the spheres of influence of your electrodes will overlap. Depth of your probes should be less than 0.05-0.10 x your probe spacing, which can be difficult to achieve in practice on short spacing measurements. This may not be the cause of your perceived issue, however is easy to overlook.
RE: soil resistivity measurement and interpertation
RE: soil resistivity measurement and interpertation
RE: soil resistivity measurement and interpertation
http://waset.org/publications/14223/soil-resistivi....
Soil Test comparison: The tests 3, 4 & 5 are consistently shown low resistivity on the top layer and very high resistance in the lower layer. Tests 1 & 2 are also consistent however, in conflict with the other tests performed in the vicinity.
Are the tests wrong? Not necessarily. In rocky or large layers of sandy soil structure, the lower layer could be of high resistance as shown in the data. It is not unusual in valleys of the West coast with dry desert region near rocky mountain to have such as high diversity of soil in small area.
Suggestions: follow some of the recommendation above with the support of a geotech engineer and verify with additional test to confirm the result of the first test. If this is confirmed, the grounding design and calculation could be still performed using the appropriate information even if there is a high soil resistivity.
For illustration, see the picture below. Good luck.
RE: soil resistivity measurement and interpertation
Clay:
Top Stiff to very stiff
Middle: Soft to medium stiff clayey silt
Bottom: Medium dense to dense Silty sand
RE: soil resistivity measurement and interpertation
Suggestions:
i) Use only the apparent (calculated) resistivity to avoid confusion.
ii) Apparent Resistivity (Rho) ~ 1.915*(probe Separation- A)*(Measured Resistance-R)
RE: soil resistivity measurement and interpertation
RE: soil resistivity measurement and interpertation
Consider the following data ranges:
1) Upper layer (above 3 ft) ................100 > ρ > 1.0 Ω.m (all measurements are consistent)
2) Mid layer between 3 to 6 ft ...........1000 > ρ > 100 Ω.m (large diversity measurement)
3) Lower layer between 6 ft to infinite.... 100 > ρ > 5 Ω.m for TR cases and
.................................................................. 1 > ρ > 0.01 Ω.m for the R and S cases
Suggested practical though:
The ground grids could be located 24 in below grade in the upper layer 100 and 1.0 Ω.m and ground rods could be used to bypass the mid layer and reach the lower layer at 10 ft. below grade (100 & 5 Ω.m or less). Therefore, the uncertainties on the mid layer measurement could have minimum impact in the overall ground grid design.
Noted that the low resistivity is associated with aggressive soil corrosivity that need to be considered in the size of the grid conductor and foundations rebar
Good luck