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Can There Be Cheap Yet RELIABLE Scientific Option For Conventional

Engineering Investigation Of Foundation ?

Electrical Resistivity Method (ERM) is used mainly for knowing sub surface rock characteristics

so as to locate favorable place to drill for underground water under which suitable rock

formations for ground water storage are present.

The data acquired can also be used for civil engineering foundation study. We can know sub

surface geology with more accuracy & this can be used as an alternative to conventional

method of foundation study/analysis either, or as a complimentary scientific approach that in

all help to reduce time & cost of investigation of foundation.

Table showing Resistivity data analysis for knowing the rocks at subsurface.

Actual field

Resistivity Values in

Ohm-mtr.

Depth in

Meters atwhich

Strata

encounter

ed from

Surface

CalculatedThickness

of

rock/strat

a

Data

Interpretations

/Results

Remark

5.258576 20-2= 2

meters.

Loose soil, clay Remove

8.164 32-3=

1meters.

Compact soil,

clay

Remove

17.16533 55-3=

2meters.

Highlyweathered Basalt

Partly Stable atpresent

environment

756.9912 77-5= 2

meters.

Hard, compact

Basalt.

Excellent For

Foundation of

Structure.

108.4713 1010-7=

3meters.

Hard, Fractured

Basalt

Provides good

support to compact

Basalt

47.59455 1313-10= 3

meters.

Highly Fractured

Basalt

Provides support to

compact Basalt

170.1252 1616-13= 3

meters.

Hard, Fractured

Basalt

Provides good

support to compact

Basalt

26.376 2020-16= 4

meters.

Soft, inter

trappean , ash

bed, red bole.

Stable at present

environment

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Field study by engineering method samples the foundation rocks/strata. The core log analysis

by engineering method gives a broad spectrum idea about the real nature of foundation

rock/strata. This is direct method. Resistivity data acquired on the surface using scientific

instruments is indirect method, but, the interpretation of resistivity data has ability to cover

maximum bands of spectrum which is generated by direct method.

Its the matter of interpretation & scope given to geology for the aspect of reliability. Apparent

resistivity values can be correlated with the known sample interpretated using engineering

methods & related engineering parameters are suggested.

The table indicates interpretations in the terms in Remarks column, the PRACTICAL view about

the foundation. These practical terms, that we reach to, at the end of any analysis, Isnt it ?

May it be VIA engineering methods !

The following explanation may help you further realize the time & money saving strength ofElectrical Resistivity Methods-

Why Resistivity Method Option ?

Present area (Maharashtra, India) is a basaltic terrain. With slight variations, the subsurface

geological setup of rocks presently considered to be same at all bore well locations as seen in

the diagram.

As the bore hole locations 1, 2 & 3, as in diagram, are placed at near distances (clustered); it is

hypothetically considered to be having same sub surface setup of rocks at all locations.

Hypothesis has to be confirmed!

To confirm this possibility of similar subsurface setup, resistivity readings will be taken at two

locations i.e. at bore hole no. 1 & 4. If it is showing same readings, the sub surface setup at

Bore Hole2 & 3 has to be same as that of Bore Hole1 &4. This is the approach to the problem.

This approach will be confirmed further by taking actual drill core work at Bore Hole1 & 3. If it

is deriving the same geotechnical results, this will confirm the positive co-relation between

resistivity data & drill core data. (Samples needs to be checked in a reliable, reputed lab & not

by the agency who is going to take the geotechnical investigations of the same work/area)

About the bearing strength:

Bearing strength calculation of any kind of formation is beyond the limit of the resistivity

method. Hence I have said earlier that, bore hole data is complimentary to the resistivity

method & not the 100% substitute.

What we can do is investigate the locations geotechnically which I otherwise am suggesting

you to rely only on resistivity and see that it was true. Then we can apply the resistivity

approach to the other clustered locations of towers to save the cost against geotechnical

investigations.

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Resistivity readings are based on electrical properties of subsurface setup(strata). Data analysis

of which tells us about the type, depth& thickness of rocks/strata. It does not generate direct

data as in geotechnical laboratory investigations.

It is like chemically analyzing the white crystals and saying it to be sugar, what I am going to do

is taste it, realize it to be sweet & then say it to be sugar ! Geophysical observations are in closeproximity to the actual borehole data obtained.

What I wish to say is, you will be taking bore hole at all four locations which will cost a lot. My

role here is to show you scientifically that; without taking bore holes at all locations, same

results can be inferred using resistivity method. Yet geotechnical investigation is not waved off

at ll.

Joint/Fault

Present rates of drill core acquiring, analyzing & report generation; all together costs more than

Rs. 28,000/- per bore well sample. Locations covered by this method will be at the most two

per day. Few service providers charges machinery idle charges & shifting charges additionally.

Great !

On the other hand, Electric Resistivity method costs Rs. 10,000/- per location & five locations

can be covered in a day ! Yet reliable being scientific . . . & the project owner/manager knows

Time is Money. . . .

No money for machinery being idle &/or shifted is charged additionally.

Simply Great !

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Imagine the cost cutting of the project if the point locations are 200 in number !

Its nearly Rs. 50,00,000/- Count the Zeros Guys !

I have seen mega projects that demands 400 point locations or more analyzed.

One more important point; the ERM gives more information about the subsurface setup as it

reflects the deeper data than the conventional boring way ! Absolutely satisfies the demand of

heavy structure, for probable foundation settlement issues.

Drill core just cant reliably preserve weak zones like fragile layers, joints, fractures, faults

present in rocks, ERM, by a wise geologist does. Imagine just for a moment, if core logs at Bore

Location 1 & 2 comes out INTACT (lets say), without any indication of weak zones, BUT what if

a real problem lies deeper, between or around these two point locations in the form of joint,

fault, shear zone . . .? Simply missed at 1 & 2. . . ? But not by a vigilant geologist ! Who else ?

But . . .

The God saves this project !

Surface erosion, Drainage System, Rate of weathering, Earthquake & Tectonics,

Hydrogeology are the absolutely bypassed issues . . . .

Talk something scientific guys !