Rapid Measurement of Thermal Resistivity of Soil V. V. M A S O N

ASSOCIATE MEMBER AIEE M O S H E K U R T Z

ASSOCIATE MEMBER AIEE

THE EFFECT OF the thermal resistivity of the soil on the temperature of a buried cable is well known.

It is of more importance for the newer directly buried and pipe-type installations in which the temperature rise through the soil forms a larger percentage of the total temperature rise than is the case with the older concrete-duct types.

Laboratory methods of measuring the thermal resistivity of soils along a cable route are unsatisfactory because of the expense and the difficulty of obtaining an "undis­turbed" sample. The well-known method of field de­termination using the steady-state temperature rise of a buried heated body is accurate but expensive and too slow for a survey of a proposed cable route. An inexpensive cast-aluminum sphere developed for this purpose reduced the construction cost but the labor involved in taking read­ings was still prohibitive.

However, a rapid "transient" method of measuring the thermal resistivity of the soil now is available. This is based upon the fact that the rate of temperature rise of a heated body, as well as the ultimate temperature rise, will depend upon the thermal resistivity of the material in which it is buried. This method. has been developed from an idea put forth originally by B. Stalhane and S. P y k i n m i . 1

The results obtained with this apparatus were compared with the results obtained by the steady-state buried-sphere

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method and were found to agree closely enough for prac­tical purposes.

The apparatus used consists of a 1/4-inch steel tube approximately 8 feet long. The bottom 5 feet of this tube contain a heater and four thermocouples placed at 1-foot intervals. In use, this long tube or "needle" is thrust into the ground and allowed to reach temperature equilibrium with the soil. The heater is then connected to a battery and a constant power input is maintained while readings of the temperature of the thermocouples are taken at intervals of 30 seconds for 25 or 30 minutes. These temperature readings are plotted against time on semilog paper, see Figure 1. A straight line is drawn through the points from about 10 minutes to 30 minutes and the thermal resistivity is calculated from the slope of this line by multi­plying by the factor 4ττΖ,/2.303 p, where p is the power input of the needle in watts, and L is the length of the needle in centimeters. This gives the thermal resistivity of the soil in thermal ohm-centimeters (degrees centigrade-centimeters per watt).

In some cases the accuracy of the method may be im­proved by a graphical correction procedure. Although poor contact with the soil was feared as a possible source of error, no difficulties have been traced to this effect.

The size of the soil sample is of interest. It is not accurately definable but it appears that in the usual run of soils, the size of the sample seen by each of the thermo­

couples is of the order of 3 inches in diameter and 6 inches long. Thus, even with a common heater, each of the thermo­couples gives a value for the thermal re­sistivity at its particular depth. This allows the resistivity at different depths to be measured during a single 30-minute run. It is believed that the size of the sample may be increased by continuing the measurements for a longer time. This is limited, however, by the end effects of the needle becoming important.

The theoretical basis for this method is not as sound as is desirable and work is continuing on this matter. It is hoped further information can be made avail­able in the future.

REFERENCE 1. New Method for Determining the Coefficients of Thermal Conductivity, B. Stalhane, S. Pyk. Teknisk Tidskri/t (Stock­holm, Sweden), volume 61, number 28, 1931, pages 389-93.

Digest of paper 52-158, "Rapid Measurement of the Ther­mal Resistivity of Soil," recommended by the AIEE Com­mittee on Insulated Conductors and approved by the AIEE Technical Program Committee for presentation at the AIEE Summer General Meeting. Minneapolis, Minn., June 23-27, 1952. Scheduled for publication in AIEE Transactionst vol­ume 71, 1952. V. V. Mason and Moshe Kurtz are both with The Hydro-Electric Power Commission of Ontario, Toronto, Ontario, Canada.

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NOVEMBER 1952 Mason, Kurtz—Thermal Resistivity of Sou 985