Case Study: Soil Resistivity Test by Wenner 4 Points Method and Polar Curve

Case Study: Soil Resistivity Test by Wenner 4 Points Method and Polar Curve 

 

        Soil resistivity measurements is the pre determination of the resistance of earth. Commonly a found probe method (widely known as Wenner method) is used for measuring soil resistivity.

        The steps of soil resistivity measurement by wenner method are as follows:

        Step 01: Drive four equally spaced test electrodes to a depth of not greater 5% of their spacing. It is important to ensure that their resistance areas do not overlap (Figure 1)

                                                                        Figure 1

        Step 02: Pass current between the two outer electrode

        Step 03: Measure the earth potential between the two inner electrodes

      Step 04:The resistance R should be taken as the ratio of the voltage between the inner electrodes and the current between the outer electrodes. Finally the average resistivity ρ in ohm metres (Ωm) taken by the below equation:

                                                ρ = 2 π a R

Where:

• a is the spacing between electrodes, in metres (m);
• R is the resistance measured between the middle electrodes, in ohms (Ω).

 

CASE STUDY: (LOCATION A)

First Step: Spacing Calculation

The Wenner 4-point soil resistivity test uses 4 probes spaced at equal distances across the surface of the earth in a straight line. 

The distance that the probes are spaced determines the testing depth into the soil.

Multiple tests are typically conducted at a variety of probe spacing.

Data is then be entered into our specialized computer software and specific soil resistivity readings at various depths can be calculated.

Four equally spaced test spikes should be driven to a depth of up to 6 inch, the depth not exceeding 5% of their separation. It is important to ensure that their resistance areas do not overlap. Current should be passed between the two outer electrodes and the resistance R may be found as the ratio of the voltage between the inside electrodes to the current conducted through the outside electrodes. It should be noted that environmental conditions such as temperature have an impact on earth resistivity with a corresponding decrease in resistivity as temperature rises.

Following table includes the data of spacing among the electrodes and depth of electrodes during test-

Second Step: Electrode Arrangement as per Spacing Data (Wenner Array)

As we have the data of spacing which are 2, 4, 6, 7, 8, 10, 12,14m following arrangement of electrodes have been made before taking instrument reading.

3^rd Step: Taking Test Data & Results

 

Direction:  North-South	Test No	Spacing	Reading (Ω)	Soil Resistivity (Ω-m) ρ = 2 π a R	Average
	1	2	3.39	42.60	23.20
	2	4	1.27	31.92
	3	6	0.43	16.21
	4	7	0.37	16.27
	5	8	0.29	14.58
	6	10	0.28	17.59
	7	12	-
	8	14	-
Direction:  East-West	Test No	Spacing	Reading (Ω)	Soil Resistivity Ω-m	Average
	1	2	4.0	50.27	48.85
	2	4	3.0	75.40
	3	6	0.2	7.16
	4	7	1.3	57.18
	5	8	0.7	39.71
	6	10	0.8	50.27
	7	12	0.4	31.67
	8	14	0.9	79.17

 

4^th Step:  Calculating the area from plotted curve and determining Soil resistivity from polar graph

Resistivities for at least four equally spaced directions from the centre of the site are measured. These resistivities are plotted on a graph sheet in the appropriate directions choosing a scale. A closed curve is plotted on the graph sheets jointing all the resistivity points plotted to get the polar resistivity curve. The area inside the polar resistivity curve is measured and equivalent circle of the same area is found out. The radius of this equivalent circle is the average resistivity of the site under consideration. The average resistivity thus obtained may be used for the design of the earthling grid and other computations.

 

Max of resistivity Ω-m	Three Calculated Values from Polar Curve	Area (m2)	Soil Resistivity (Ω-m)  from polar graph
50.27	Uniform Value	1984.44	25.14
75.40	Highest value	4465.11	37.71
16.21	Lowest Value	206.37	8.11
57.18
39.71
50.27
31.67
79.17

From the test results the apparent resistivity values varied from 7.16-79.17 Ohm-meters and average is 36.02 Ohm-meters. From polar curve calculation the highest value is 37.71 Ohm-meters, lowest value is 8.11 Ohm-meters and uniform value is 25.14 Ohm-meters.

 

Written By:

Jewel Sikder Joy (B.Sc. in EEE, M.Sc. Engg.)  

Remon Das (B.Sc. in EEE, MBA)