CA1089211A - Thermal stabilization of soil - Google Patents

Abstract

ABSTRACT
A stabilizing agent and backfill composition for providing a thermally stable environment for buried electrical equipment comprising from about 90 to about 99 percent by weight soil and from about 1 to 10 percent by weight of the stabilizing agent, which is con-prised of a mixture of clay and a dispersing agent therefor, wherein the dispersing agent comprises at least about 0.25 percent by weight of the stabilizing agent.

Description

This invention relates ~o a process for providing a thermally stable environment for buried electrical transmission and distri~ution equipment, , e.g. cables and transformers.
In recent years, because of ecological as well as operational rea-sons, electrical equipment such as high voltage ~ransmission and distribution ~- power lines, transformers, etc. have been placed underground. One of the ; most severe limitations on the capabilities of such installa~ions is thedissipation of heat generated by the flow of electrical power therethrough.
If the thermal resistivity of the environment surrounding the buried equip-ment is unsatisfactorily high, the heat generated during functioning of the equipment can cause an increase in the temperature of the equipment which is ~, .
beyond the tolerable limits thereof, and on extended operation at such tem-peratures, failure or destruction of the equipment may acaur.
' For this reason, underground facilities must be typic~lly designed `; aacording to their expected thermal environment. Furthermore, since most of the thermal impedance from the heat source, i.e. the electrical conductor, to the air, resides in the intervening earth, the earth becomes an overwhelming factor in calculating equipment size. In such calculations, li~itations re-sulting from non-uniformity of the surrounding earth environment must be taken into account. For example, earth found along .: .
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the route of an underground cable typically varies widely in heat conducting properties~ and as such would require cable sizing compatible with 90~il areas having the highest thermal resistivity. For th~s reason, native soil is seldom returned to a transmission cable trench.
To alleviate this situation, presenk commercial practice dictates the use of a prepared backfill makerial having known resistivity characteristics to replace the - nakive soil. This backfill material has kypically been a ;
well graded soil exhibiting resistivity within a satis-factory range for an assumed khermal history, i.e. duration of expected subsurface temperature and moisture availability.
In many ca~es, however, this backfill musk be transported to the construction site, greatly increasing the cost o~
the pro~ect. Furthermore, in many inskances, the replaced native soil must be transported from the construction site.
As such, the installation costs for the utility are greatly increased. ~`
One solution to the problem of heat dissipation for buried electrical transformers is disclosed in U.S.
Patent No. 3,212,563, wherein ~acketed transformers are taught, with cooling water being piped to and rrom the transformer Jacket to dissipate the heat generated during normal operation. While the cooling water does indeed function to dissipate the heat generated, such a design would greatly increase transformer costs and maintenance problems on such a system would tend to render the system commercial:Ly infeasible.
: Another solution to this problem, as disclosed in U.S. Patent No. 3,719,511, utilizes a weak mix concrete .

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to encase the electrical equipment therein. There is some difficulty in reentry once the concrete mix is utilized, however, and also some cracking of the concrete system may occur, thereby detracting from its capabilities as an effective backfill material.
Still another backfill material, as defined by United States Patent ~o. 3,082,111, utilizes a composition having particularly defined percentages of sized sand particles, assertedly to optimize packed density and corres-pondingly the thermal resistivity. Such a material would still not reduce the costs involved with transporting backfill to the con~truction site, etc.
It has now been found that ~oil can be simply treated with a stabilizing agent comprising a clay, preferably a kaolinitic or montmorillo-nite clay, with a dispersing agent therefor, to produce a backfill composition having improved thermal resistivity properties. Depending on the charaater~
istics o the nativ~ soil excavated during trenching operations, in many ln~tances the Bame 50il can be treAted and returned a~ bac};~ill l:o the trench.
In accordance with the invention there is provided a process ~or providing a thermally stable environment for underground electrical equipment comprising the steps of:
~a) placing said equipment in an open trench;
~b) backfilling said trench with soil in which from about 1 to about 10 percent by weight of a stabilizing agent is thoroughly mixed in the presence o water, said stabilizing agent comprising a mixture o~ clay and a disper6ing agent ~or said clay, wherein said disperslng agent comprise~ at least about 0.25% by weightosaid stabilizing agent.
The clay/dispersant mixture unexpectedly reduces the thermal resist-ivity of the backfill material below that 1~89Zl~

found uslng the components of the m~xture spearately with soil. -He~t transfer through soils ls, of course, a complex phenomenon, because o-f the-complexity of the soil itself. As used herein, soil is composed of solid matter, such as sand, clay or silt, and finally air or water.
The thermal resistivity, which is of primary interest herein, is typically dependent upon-soil compos~tion, ~ ~
density, moixture content, particle size, size distribu- -tion, etc.
The primary physical characteristic of a thermall~
stabillzing medium for backfilling of buried electrical equlpment, such as cables, is a resistivity unit known as the "thermal ohm", which is defined as the number of de~rees Centi~rade of temperature drop throu~h a cube having sides measuring one centimeter, through which heat is flowing at the rate of one watt, i.e. one ~oule per second. It is -designated either by the Greek letter ~'p", or "rho".
As solids have the lowest thermal resistivity, a high solids content is, of course, desirable. For example, solld quartz, the principal constituent of silicic sand, has a thermal resistivity of approximately 11C-cm/watt. Water, on the other hand, has a resistivlty of about 1~5C-cm/watt, and air about 4000C-cm/wattO
From these Pigures, lt ls obvious that for minimum resistivlty, the soil should contain the max~mum amount of solid particles and a minimum amount of air. It is, o~
course, impossibie to pack the solid particles without having lnterstlces therelnO In fact the ob~ect of afore-mentioned U.S Patent No. 3,082,111 is to provlde a . .
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39Zll composltion-haYing-particularly deflned-par~lcle sizes so as to attempt to maximize the packing density of the back-fill material and thereby effectlvely reduce resistivity.
From the standpoint of physical composition, ideal backfill materials for thermal stability should have a low thermal resistivity which ls stable over a wide range of climatic conditions, has good moisture retention char-acteristics, is easily re-wetted, and is easily handled.
Such characteristics are contributed to by a high soLids content composed of a material having inherently low thermal resistivity.
At the present time, one of the most suitable of such materials is a quartz sand composed of a wide range o~ particle sizes graduated in such a manner as to provlde a dense mixture. However, the moisture retent~vity of sands is poor. Also, the high density of such a mixture depends on the particles of each size being thoroughly mixed.
Particles of a relatively dry sand will, however, segregate very readily when handled. Presently, compensation for such shortcomings is made by including in the compositlon a small percentage of clay, i.e., about 5 to about 10 percent by welght, whlch is suf~icient to hope~ully provide a thin coating on the indlvidual sand partlcles. Thls provides to the mlxture the moisture retentivity of clay, increases the contact area between ad~acent sand particles, and provldes sufficient adhesion between particles to reduce segregationO
- Clay is also one of the components useful in our thermally stable composition, It has been determined that - up to about 1~ percent by weight of the clay ln soil will provide satisfactory reduction in resistiv~ty when ut~lized ~089Zl~L
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in conjunction with the dispersing agent herelnafter dis~
cussed. Exemplary and preferred clays include kaolinitic clay, such as the commercially available Dixie~Clay~ (from the R.I. ~randerbilt Company), and montmorillonite clay, with ;~
the kaolinitic clay being most preferred.
It has been unexpectedly found that a synergistic effect on the thermal stabilizatlon of soil-can be attained by ~ncluding with the clay a material which is a dispersing agent therefor. The inclusion of such an agent together with the clay and soil has!been found to cause a reduction in thermal resistlvity below that found when either component is us~d singularly with the soil.
Apparently, the dispersing agent functlons so as to dlsperse the clay particles so that they are inhibi~ed from settling in an aqueous medium, apparently by plac~ng a charge on the clay particles. The electrostatic repulsion between these charged particles effectuates their separation and inhibits their settling, allowing for more efficlent mixing with the soil. Furthermore, it is believed that a secondary effect of the inclusion of a dispersing agent in the composition may be the lowering of the surface tension, which would tend to promote more effective particle wetting.
Examples of preferred dispersing agents found tv function effectively herein include anionic materials, such as Darvan~No. 1 and Darvan~No. 2, commercially available from the R.T. ~Janderbilt Corp. Darvan No. 1 is comprised of sodium salts of polymerized alkylnaphthalene sulfon~c acids and Darvan~No. 2 is comprised of sodium salts of polymerized substituted benzold alkylsulfonic acids. An exemplary dlspersing agent which is cationic in nature ls Atlas G-3570, ~em~' .
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commercially available from the Atlas Chemical Co., and an exemplary non-ionic dispersing agent is Atlas~G-14410 The anionic dispersants are preferred.
It has been determined that at least about 0.25 percent by weight of the clay/dispersant mixture must be the dispersing agent to effectively reduce the thermal reslstiv-ity of the compositlon. While no upper concentration limit for the dispersant has been found, at dispersant concentra-tions exceeding about 2.0 percent by weight only minor beneficial decrease in thermal resistivity of the soil has been noted.
The use of such a simple treatment composition to effectuate thermal stabilizat~on of soil allows the use of many native solls removed from the trenching site to be reused by ~ixing ~ame with the clay/dispersant combination at the construction site, thereby effectively ellminating the necessity for the transportation of stabilizing backfill materials and the removal of the native soil from the site.
The manner of addition of the clay/dispersant stabilizing agent is unimportant as long as thorough mlxing is undertaken. In fact, the clay and dispersant can be added separately to the soil and still effectuate the stabilizing thereof. Water is, of course, utilized to aid in the dispersing of the stabilizing agent. The water can be present in the soil or added with the stabllizing agentO
Our invention will now be more specifically defined by the aid of the following non-limiting-examples, wherein all parts are by weight unless otherwise specified.

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EXA~PLE_l The determination of thermal resistivity is undertaken by utilization of a thermal needle, which ln effect utilizes the relationship existing between the thermal resistivity of a substance and the temperature rise of a line source of heat within that substance. As is pointed out in A.I.E.E. Trans. (Power Apparatus and Systems), ~Jol 79, pp. 836-843 (1960), the thermal needle contains a heater element and thermocouple, whereupon one can observe the temperature - time characteristics resulting from a given heat input. In accordance with the discussions in the above-referenced article, a 4" long laboratory thermal needle was prepared.
Utilizing thi~ equipment, the technique known as the transient method was utilized to determine thermal resistivity. The transient method is in general based on ~ `
the theory that the rate of temperature rise of a body is dependent upon the thermal properties of the substance in which it is placed. The basis of the method has been described in A.I.E.E. Trans. (Power Apparatus and Systems), Volume 71, Part 3, at pages 570-577 (1952). Utillzing a data acquisition system, the millivolt output of the thermo-couple of the thermal needle were recorded automatically at preselected time intervals. In general, a 10 second time interval was utilized and the experiments were run for no longer than 8 minutes. The current and voltage to the needle were monitored every 2 minutes, and an average of these readings was utilized to calculate the power input thereto. Analysis of the data was accomplished by utlllzing a least squares fit computer program to mathematlcally 8~

provide a plot of temperature rise of the samples versus time.
The particular soil utilized for much of the testing hereinafter discussed is termed Ottawa sand, which meets the designations contained in AST~ C-109. `This sand is a natural silica sand from Ottawa, Illinois, and ls typically of uniform size and of rounded shape. In accordance with aforementioned discussion, both of these characteristics provide the sand with high thermal resistivity, i.e. on the order of 270C-cm/watt.
Soil samples were typically compacted prior to testing. The compaction procedure utilized is detailed in ASTM D698-70. Furthermore, samples were thoroughly drled prior to testlng to insure unlformity in the testing and to measure on the basis Or maximum thermal impedance.
To ascertain the effectiveness of our invention on Ottawa sand, a sample of Ottawa sand containing 5 weight percent water therein was compacted pursuant AST~
D698-70. After drying the sample to remove water, the thermal resistivity thereof was measured by use of the thermal needle, whereupon the resistivity was ascertalned to be 275C-cm/watt. A second sample of Ottawa sand thoroughly mixed in a Hobart mixer with 2.94 parts by weight of Dixie Clay. The Dixie Clay was provided in an aqueous suspension by mixing 5 parts of water with 2.94 parts of clay and adding same to the dried Ottawa sand.
The thermal resistivity of the sample was ascertained to be 137C-cm/watt. A similar sample was prepared wherein the Dixie Clay was eliminated from the formulation~ and instead o . o6 parts by-weight of Darvan #1 in 5 parts of ^~ ~08~

water was thoroughly mixed wlth the soil in a Hobart mixer. The thermal resistivity of the sample was determined to be 270C-cm/watt. Finally, to a ~ourth sample of dried Ottawa sand, 5 parts by weight of water, 2.94 parts by weight of Dixie Clay, and 0.06 part by weight of Darvan ~1 were added. The thermal resistivity of the sample was determined to be 94.3C-cm/watt.
The effect of the components in the thermal stabilization of soil was also tested utilizing a soil denominated as Class 5 soil, which is a relatively well-graded soil used typically in roadbed installations. The compacted Class 5 soil itself, dried to remove water~ with-out addition of the clay or dispersant thereto, had a thermal resistivity of 68.2C-cm/watt, indicating the greater erfectlveness in terms of resistivity when utilizing a well-graded soil as opposed to one having substantially uniform particle sizes therein. i.e. the Ottawa sand.
When the soil was tested by adding thereto 2.94 parts by weight of Dixie Clay, the thermal resistivity thereof was determined to be 70.7C-cm/watt, indicating that very little effect was noted by the simple addition of the clay to the well-graded Class 5 soil. When 0.06 part of Darvan ~1 was added to the soil and thoroughly mlxed therewith in a Hobart mixer, the thermal resistivity thereof was determined to be 69.7C-cm-watt, again indicating very little effect on the soil by addlng a dispersant thereto. However, when the class 5 soil was well mixed with 2.94 parts of Dlxie Clay and o.o6 parts of Darvan #1, the thermal resistivity thereof decreased to 57.0C-cm/watt, clearly indicating . .

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EXA~PLES 2-13 To ascertain the effectivenes.s of the clay/
dispersant combination on other soils, samples of soils were obtained having the analysis indicated in Table 1 below.
The soils tested typify in general the various soil types found in the United States as classified by the U.S. Department of Agriculture.

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The results of treatment of each soil with the clay/dispersant mixture provided resistivity data as illustrated in Table 2.

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~9~11 As can be ascertained from Table 2, the treatment of the various soils with the clay/dispersant mixture provided a reduced thermal resistivity in most instances.
Water content of all control and stabilized samples was selected to maximize compaction of the samples and correspondingly optimize resistivity thereof. In the dispersant - only samples, the water content of the control was utilized.

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for providing a thermally stable environment for under-ground electrical equipment comprising the steps of:
(a) placing said equipment in an open trench;
(b) backfilling said trench with soil in which from about 1 to about 10 percent by weight of a stabilizing agent is thoroughly mixed in the presence of water, said stabilizing agent comprising a mixture of clay and a dispersing agent for said clay, wherein said dispersing agent comprises at least about 0.25% by weight of said stabilizing agent.

2. The process of claim 1 wherein said clay is selected from the group consisting of kaolinite and montmorillonite clays.

3. The process of claim 1 wherein said dispersing agent is an anionic dispersing agent.

4. The process of claim 1 or 2 wherein the dispersing agent is a sodium salt of a polymerized alkylnaphthalene sulfonic acid.

5. The process of claim 1 or 2 wherein the dispersing agent is a sodium salt of a polymerized substituted benzoid alkylsulfonio acid.