ELEMENTOS ORGANIZACIONALES

This indication of a tendency for current to flow becomes more important if something is known about the soil resistivity. High-resistivity soils may offer so much resistance

2,000

0

0 1000 2000 3000 4000

4,000 6,000 8,000 10,000 100,000

PIPELINE LENGTH-FEET SOIL RESISTIVITY OHM-CENTIMETERS

Figure 5.13 Soil resistivity survey.

to current flow that conditions are not as severe as the plotted potential data might lead one to believe. Conversely, severe potential peaks coupled with a relatively low resistivity environment may mean a truly serious condition. Frequent soil resistivity determinations are important when making a detailed survey on a pipe line. In addition to being a valuable aid when interpreting the severity of corrosive areas, a soil resistivity profile is extremely helpful in the later selection of sites for CP installations.Figure 5.13 is a plot of the soil resistivity measurements taken along the same section of pipeline used as a basis for the plots shown inFigures 5.4and5.12.

Data plotted in the figure represent average soil resistivity to approximate pipe depth.

In this example, a very wide range of soil resistivity is represented. Other cases would not necessarily have such a large difference between maximum and minimum resistivities along a similar length of line.

Along well-coated pipelines, measurements of soil resistivity can be of great assis-tance in the later selection of CP installations. This may be particularly true to identify areas where soils of suitable low resistivity for such installations.

Instruments used for measuring soil resistivity by the four-pin method are described in Chapter 6. Certain precautions to be observed in using the method are given below, together with suggestions for planning soil resistivity measurements.

In the example shown, resistivity of the soil was measured at∼100-ft intervals along the proposed alignment. Soil resistivity measurements were conducted by the Wenner four-pin method, utilizing a soil resistance meter (see Chapter 6). The Wenner method requires the use of four metal probes or electrodes, driven into the ground along a straight line, equidistant from each other, as shown inFigure 5.14. An alternating current from the soil resistance meter causes current to flow through the soil, between pins C1 and C2. The voltage or potential is then measured between pins P1 and P2. The meter then

a a a

P1 PIN

C1

PIN P1

PIN P2

PIN C2

C1

P2

C2

Soil Resistance Meter

Figure 5.14 Soil resistivity test set-up (Wenner four pin method).

registers a resistance reading. Resistivity of the soil is then computed from the instrument reading, according to the following formula:

ρ = 2π AR

whereρ = soil resistivity (ohm-centimeters) A= distance between probes (centimeters) R= soil resistance (ohms) {instrument reading}

π = 3.1416

The resistivity values obtained represent the average resistivity of the soil to a depth equal to the pin spacing. Resistance measurements are typically performed to a depth equal to that of the pipeline being evaluated. Typical probe spacings are in increments of 2.5 ft (76.2 cm).

If the line of soil pins used when making four-pin resistivity measurements is closely parallel to a bare underground pipeline or other metallic structure, the presence of the bare metal may cause the indicated soil resistivity values to be lower than it actually is. Because a portion of the test current will flow along the metallic structure rather than through the soil, measurements along a line closely parallel to pipelines should be avoided. When making soil resistivity measurements along a pipeline, it is good practice to place the line of the pins perpendicular to the pipeline with the nearest pin at least 15 ft from the pipe—further, if space permits.

Table 5.3 Format for Recording Soil Resistivity Measurements

Nominal Pin Resistivity in

Test Location Spacing, Feet Ohms Factor Ohm-cm

No. 1

Pipeline Station 2.5 4.40 500 2200

1000+ 00. Nearest 5 2.05 1,000 2050

pin 100 ft west of pipe. 7.5 1.26 1,500 1890

Line of tests perpendicular 10 0.96 2,000 1920

to pipe. Clay Moist 12.5 0.78 2,500 1950

15 0.62 3,000 1860

50 0.17 10,000 1700

No. 2

Station 1001+ 00. 2.5 1.30 500 650

Nearest pin 200 ft 5 0.60 1,000 600

east of pipe. Line of 7.5 0.45 1,500 675

tests parallel to pipe. 10 0.36 2,000 720

Edge of swamp. Wet 12.5 0.34 2,500 850

15 0.33 3,000 990

25 0.34 5,000 1700

Soil resistivity data taken by the four-pin method should be recorded in tabular form for convenience in calculating resistivities and evaluating results obtained. The tabular arrangement may be as shown inTable 5.3. Where many soil resistivity measurements are to be made, field time will be saved by using printed forms arranged for entering the necessary data.

With experience, much can be learned about the soil structure by inspecting series of readings to increasing depths. The recorded values from four-pin resistivity measure-ments can be misleading unless it is remembered that the soil resistivity encountered with each additional depth increment is averaged, in the test, with that of all the soil in the layers above. The indicated resistivity to a depth equal to any given pipe spacing is a weighted average of the soils from the surface to that depth. Trends can be illustrated best by inspecting the sets of soil resistivity readings inTable 5.4.

Soil resistivity is an electrical characteristic of the soil/groundwater which affects the ability of corrosion currents to flow through the electrolyte (soil/groundwater).

Resistivity is a function of soil moisture and the concentrations of ionic soluble salts and is considered to be most comprehensive indicator of a soil’s corrosivity. Typically, the lower the resistivity, the higher will be the corrosivity.

Table 5.5correlates resistivity values with degree of corrosivity. The interpretation of soil resistivity varies among corrosion engineers. However, this table is a generally accepted guide.

The first set of data in Table 5.4, Set A, represents a uniform soil conditions. The average of the readings shown (∼960 ohm-cm) represents the effective resistivity

Table 5.4 Typical Soil Resistivity Readings Using 4 Pin Method

Soil Resistivity (ohm-cm) Pin Spacing

(Feet) Set A Set B Set C Set D

2.5 960 1100 3300 760

5 965 1000 2200 810

7.5 950 1250 1150 1,900

10 955 1500 980 3,800

12.5 960 1610 840 6,900

15 955 1710 780 12,500

that may be used for design purposes for impressed current ground beds or galvanic anodes.

Data Set B represents low-resistivity soils in the first few feet. There may be a layer of somewhat less than 1000 ohm-cm around the 5-ft depth level. Below 5-ft, however, higher-resistivity soils are encountered. Because of the averaging effect mentioned ear-lier, the actual resistivity at 7.5-ft deep would be higher than the indicated 1250 ohm-cm and might be in the order of 2500 ohm-cm or more. Even if anodes are placed in the lower-resistivity soils, there will be resistance to the flow of current downward into the mass of the earth. If designs are based on the resistivity of the soil in which the anodes are placed, the resistance of the completed installation will be higher than expected. The anodes will perform best if placed in the lower resistance soil. The effective resistivity used for design purposes should reflect the higher resistivity of the underlying areas. In this instance, where increase is gradual, using horizontal anodes in the low-resistivity area and a figure of effective resistivity of∼2500 ohm-cm should result in a conservative design.

Data Set C represents an excellent location for anode location even though the surface soils have relatively high resistivity. It would appear from this set of data that anodes

Table 5.5 Soil Resistivity vs. Degree of Corrosivity

Soil resistivity

(ohm-cm) Degree of corrosivity

0–500 Very corrosive

500–1,000 Corrosive

1,000–2,000 Moderately corrosive 2,000–10,000 Mildly corrosive Above 10,000 Negligible Reference: NACE Corrosion Basics.

located>5-ft deep, would be in low-resistivity soil of ∼800 ohm-cm, such a figure being conservative for design purposes. A lowering resistivity trend with depth, as illustrated by this set of data, can be relied upon to give excellent ground bed performance.

Data Set D is the least favorable of these sample sets of data. Low-resistivity soil is present at the surface but the upward trend of resistivity with depth is immediate and rapid. At the 7.5-ft depth, for example, the resistivity could be tens of thousands of ohm-centimeters. One such situation could occur where a shallow swampy area overlies solid rock. Current discharged from anodes installed at such a location will be forced to flow for relatively long distances close to the surface before electrically remote earth is reached. As a result, potential gradients forming the area of influence around an impressed current ground bed can extend much farther than those surrounding a similarly sized ground bed operating at the same voltage in more favorable locations such as those represented by data Sets A and C.

One mathematical procedure, known as the Barnes method, is based on calculating the resistivity of the soil in each incremental layer of soil. This is done by using the data from the four-pin soil resistivity test but extending the calculations by determining the conductivity of each incremental layer and converting this conductivity to resistivity.

Applying the procedure to the Set B soil resistivities (Table 5.6) provides a demonstration of the method.

The Barnes method of analysis is not infallible because its accuracy requires soil layers to be of uniform thickness and parallel to the surface. In cases, where this is true, each added layer of earth must increase the total conductivity from the surface to the bottom of the added layer, no matter what the resistivity of the added layer may be. If, as

Table 5.6 Calculation of Soil Resistivity by Layers

4-PIN DATA, SET B¹ BARNES PROCEDURE

Layer Resistivity

Spacing R1 Resistivity Mhos² Mhos³ R2Ohms and Layer

Feet Ohms Factor Ohm-Cm 1/R1 11/R1 1/1 1/R1 Factor⁴ Ohm-cm Depth, Feet

2.5 2.2 500 1100 0.455 — — — 1100 0–2.5

5.0 1.0 1000 1000 1.0 .545 1.84 500 920 2.5–5.0

7.5 0.833 1500 1250 1.2 .20 5.0 500 2500 5.0–7.5

10.0 0.75 2000 1500 1.33 .13 7.7 500 3850 7.5–10

12.5 0.645 2500 1610 1.55 .22 4.55 500 2275 10–12.5

15.0 0.57 3000 1710 1.75 .20 5.0 500 2500 12.5–15

1FromTable 5.4

2This is the conductivity in mhos (reciprocal of resistance) to the indicated depth.

3This is the increase in conductivity caused by the added layer of earth.

4The factor used here is for nominal 2.5 foot layer increments. If other layer thickness are used, the factor must be changed accordingly (191.5× layer thickness in feet).

in the sample calculations above, the conductivity (the column headed 1/R1) continues to increase with depth, conditions appear to approach the ideal closely enough to make the method usable. Decreases in the conductivity at any point in a series are an indication that soil layers are too distorted to permit use of the method for analysis of data at that depth. For example, if the Barnes method procedure is applied to soil data Set D, results cannot be calculated by this method below the 5-ft level. One inference from this type of data is that the low resistivities observed near the surface indicate the presence of a limited pocket of favorable soil in an area of soil having predominantly very high resistivity.

In some areas, experience will show that soil resistivities may change markedly within short distances. A sufficient number of four-pin tests should be made in a ground bed construction area, for example, to be sure that the best soil conditions have been located.

For ground beds of considerable length (as may be the case with impressed current beds), four-pin tests should be taken at intervals along the route of the proposed line of ground bed anodes. If driven rod tests or borings are made to assist in arriving at an effective soil resistivity for design purposes, such tests should be made in enough locations to ascertain the variation in effective soil resistivity along the proposed line of anodes.