Soil model with out intended resistivity values 1

[saadicarnot]

Normally as most earthing design practitioners take the resistivity values before excavating for the earthing mash. But in our case the site is already excavated for the said purpose and the resistivity data is available further bellow that level. So can any one recommend a technique which we don’t have to back fill the site to calculate the resistivity values and excavate it again to lay the mesh? Some of my pals are talking of extrapolation but still it won’t represent the actual thing as the error is quite high.

 

[jghrist]

If you know or can estimate the resistivity of the backfill material, you can use a soil model with an extra layer. For instance, say you measure the resistivity of the soil after excavation and calculate a two-layer soil of 100 ohm·m from the excavated surface to 10 meters and 200 ohm·m below 10 meters. If the final site will have 5 meters of fill with a resistivity of 50 ohm·m, then your final soil model will be a three-layer soil with 50 ohm·m from the 0-5 meters depth, 100 ohm·m from 5 to 15 meters depth, and 200 ohm·m below 15 meters.

If you will fill the site with the excavated soil, then assume that the fill will be the same resistivity as the measured upper layer.

 

[saadicarnot]

The site is excavated till a level of 10 meter, so don’t you think that a two layer soil model be used instead of three layer one. Thanks.

 

[ELEP]

As soil is excavated and the same soil will be backfilled then there is no question of change in soil resistivity. But, possiblity can't be denied that backfilled soil may have higher resistivity initially, the reason is backfilled soil is not compacted so it forms cavities which may contain air, also during excavation moisture in the soil may be evaporate that can also increase the resistivity. But once this soil is compacted by vehicle movements & weather effects like rain it may regain origional resistivity.

Hence, it can be benificial to use resistivity values taken before excavation.

 

[saadicarnot]

Yep I know that and that is the main problem that even after backfill the values will not be very accurate and because of this the system will be over designed. Normally it is not a big problem to over design but you see cost cuts are desired in recession. Thanks.

 

[jghrist]

The site is excavated till a level of 10 meter, so don't you think that a two layer soil model be used instead of three layer one. Thanks.

If you measure after excavation and find a 2-layer soil model is best, then if you add 10 meters of a different resistivity, you will be adding a third layer.

If the excavated soil is replaced and the backfill is compacted, then I'd say you could keep the 2-layer model but add 10 meters to the top layer. If the soil is replaced without compacting, then you might want to increase the resistivity and use a 3-layer model. Why do you excavate 10 meters and replace the soil? If you do not compact, won't you worry about settlement in the future?