Modifications to Inuvik, N.W.T. Airstrip Ground Temperature Installations

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Technical Note (National Research Council of Canada. Division of Building Research), 1962-01-01

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Johnston, G. H.

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DIVISION OF BUILDING RESEARCH

TECHN][CAlL

NOT FOR PUBLICATION

360

FOR INTERNAL USE

PREPARED BY G. H. Johnston CHECKED BY CBC APPROVED BY _NBH

PREPARED FOR

Record Purposes

January 1962

SUBJECT

MODIFICATIONS TO INUVIK, N.W.T. AIRSTRIP

GROUND TEMPERATURE INSTALLATIONS

In March ＱＹＵＷｾｦｯｵｲ thermocouple cables were installed at the Inuvik, N.W.T. airstrip to measure ground temperatures. Two cables were placed under the centre line of the runway, one at the centre of the parking apron and a fourth in an undisturbed area adjacent to the airport (Fig. 1). These installations were made to determine the effect of the crushed rock fill (of which the airstrip is constructed) on permafrost conditions in the underlying foundation material and the rate of seasonal thaw and freeze of the fill.

The work was undertaken as a co-operative venture of the Department of Transport and the Division of BUilding

Research of the National Research Council. The Division supplied all the equipment and supervised the installation of the

instruments. In turn the Department of Transport undertook to make the observations of ground temperatures that have

been taken on a weekly basis since June 1957. Full information on these installations, including details of the ｩｮｾｴｲｵｭ･ｮｴｳ

and soil and permafrost conditions at the site, have already been given to the Department of Transport. Further details

of the completed installations are given on Drawing No. AKLAVIK 26, Department of Transport, Air Services Branch, Construction Division, Edmonton District dated May 1958.

The purpose of this note is to describe certain modifications which were made to the three thermocouple installations located on the airstrip, in September 1961 by members of the Division of Building Research. These changes

2

-were essentially made to simplify the taking of readings and to provide a more convenient installation.

Description of Original Installations

The airstrip thermocouple cables were installed in test pits dug to a depth of 15 ft below the original ground'

surface prior to the construction of the airstrip. The top

portion of each of the cables, which contained the sensing points for measuring temperatures in the fill, was positioned as the crushed rock was placed during the construction period. Each cable terminated in a special, weatherproof switch box enclosed by a heavy steel box, about 2 ft square, to protect

it from damage. When the airs·trip was completed, the boxes

were located just under (from 6 - 18 in.) the surface of the fill and directly above the thermocouple cable in the ground. The cable located in the undisturbed area terminated in a similar switch box placed on the ground surface.

Each time readings were taken it was necessary to remove the crushed rock covering the switch box and'when the

readings were completed, to ｢｡｣ｫｾｦｩｬｬ and compact the disturbed

fill. During the Winter, when the snow cover on the airstrip

was rolled and compacted,

6 - 9

in. of very dense snow or ice

had to be removed as well as the frozen backfill covering the

switch box. Not only was this a tedious and time consuming

chore, but the removal and subsequent replacement of this material each week left a soft spot on the runway and apron at each location, that could prove hazardous to aircraft using the airport.

Ground ｴ･ｭｰ･ｲｾｴｵｲ･ measurements were obtained

initially with a portable potentiometer. The sensitivity

of this instrument is adversely affected when used with installations of long lead length and consequently of large

resistance. The airstrip installation leads were near the

maximum permissible length so that the switch boxes had to be placed in the fill and could not be moved to a more

con-venient position. To counteract this, the Division modified

a laboratory-type electronic temperature indicator for field

use. The sensitivity of this instrument is not affected by

the lead lengths normally used for ground. temperature

installations. Powered by a gas engine driven generator, it

has been used to measure all ground temperatures at the airstrip since the fall of 1960.

With this instrument now in use, the decision to move the switch boxes to a more convenient location could be

made. In September 1961, they were moved from under the

surface of the fill, at the centre of the runway and the parking apron, to the sides'of the airstrip where they were

placed above the ground surface. The old location of the switch boxes, under the centre of the fill, would prove troublesome if the decision to pave the airstrip were to be made in the future.

Modifications to the Ground Temperature Installations

The modifications made to the three airstrip

instal-lations included: the removal of the rotary selector switches

from the old switch boxes, the splicing of one ｾｮ､ of lengths

of special thermocouple lead wire to the ground temperature measuring thermocouple wires in the old switch' box (which was left in place in the fill), and the connection of the other end of the special lead cables to another type of rotary

ｳ･ｬ･｣ｴｾｲ switch housed in specially made weatherproof switch

boxes at the edge of the fill. The special lead cables were

placed inside black polyethylene pipe and buried in a trench

about 12 in. deep under the surface of the fill. The cable

was placed at the same depth as the runway lighting cables but could not be buried any deeper because of large-size rock

encountered at that depth. The trench was excavated from the

location of tne original switch box at the centre of the

airstrip to the new location of the switch box at the edge of the fill, backfilled and compacted by a grader •.

At the three airstrip locations, ground temperatures were measured by three thermocouple points placed at each of the follOWing depths - 2.5, 5, 10 and 15 ft below the original ground sUrface, at the ground surface and also at ｩｾｴ･ｲｶ｡ｬｳ

of 2, 4, 6 and 8 ft above the ground surface in the rock fill. In addition, one thermocouple point measured the air

tempera-ture in the switch box. A total of 28 thermocouple measuring

points were therefore connected to a 28-point LEWIS switch. Two 12-point LEEDS and NORTHRUP rotary selector switches were used in the new switch boxes for each installation

modified. Thus, four measuring points, the switch box ｡ｩｲｾ

temperature thermocouple and the three measuring points placed in the fill 8 ft above the original ground surface, have been eliminated at each of the newly modified airstrip installations.

Lengths of "THERlVIO-CABLE" type PPeK-12-20-DD,

containing 12 copper-constantan wire pairs were used as lead cables from the thermocouple installations to the new switch

boxes. The individual conductors in this cable, are covered

with polyvinyl chloride and the 9undle of conductors is

wrapped with aluminum-backed Mylar tape. The cable is

finished with an over-all jacket of polyethylene. The wires

are of premium grade copper and constantan. Two lengths of

"Thermo-Cablen were used at each installation and were placed

4

-pipe (C.G.S.B. spec. 41-GP-5A) for protection and buried side by sid'e in the trenoh. A total length of 1000 ft of "Thermo-Cable" was used for the three installations.

At each installation the lead cables entered the old switch box (where the splice was made) thrOUgh a hole cut in the side of the protective steel box. The splice was made by tWisting the matching copper and constantan wires together and wrapping each splice individually with electrician's tape. The first 12 pairs of wires were spliced to "Thermo-Cable" lead cable No. 1 in order from the lowest thermocouple point in the ground. The second group of 12 pairs was then spliced to cable No.2. When all splices were made, a 9-in. length of I-in. dia. polyethylene pipe, that had been slipped over one end of the 3/4-in. protective plastic pipe before the connections were made, was then pulled over the junction and filled With sealing compound. The switch box lid was then· tightly clamped in place and the heavy steel cover replaced on the protective box. Fine crushed rock backfill was then carefully placed and tamped around and over the installations and brought to grade.

The two lead cables enter the new switch boxes, which consist of a 9-in. length of 6-in. dia. ｳｾ･･ｬ pipe supported on three legs, through special connectors on the bottom plate which is welded to the main body of the box. The copper wires from each cable are soldered to the appro-priate lugs on the two switches in the box. All constantan wires from both oables are connected to a common oonstantan wire by a Marr conneotor. The common copper leads from the

switohes and the common constantan lead terminate at a

special oonnector in the box to whioh the lead wire from the reading instrument is attaohed when readings are taken. When the lid is placed on the switch box and tightened in place by the thumb screws provided, the switches and connections are enolosed in a weatherproof box.

The only modification made to the ground tempera-ture installation in the undisturbed area near the airport was the placing (in the old type switch box which was not removed) of a new type of connector to which the lead cable from the measuring instrument is connected to take readings. The 28-point LEWIS type switch is still used, and 28 tempera-ture measurements are obtained at this installation. All soldered constantan connections at each of the four installa-tions have been eliminated by the use of Marr connectors.

Details of the location of the ground temperature installations, the lead cables and the new switch boxes are given in Fig. 1. The form (Northern N-l) used to record the ground temperature observations has been revised as a result of the modifications made; a copy of this is attached to this

5

-note. It will be noted that thermocouple point four at

installation No.3, indicates the temperature in the junction box (old switch box) under the airstrip. This ground tempera-ture point in the ground was found to be dead; the copper-constantan pair in the "Thermo-Cable" lead cable, at the

junction box, splice, was then made into a temperature measuring point.

Aclmowledgment

The co-operation and assistance provided by the Department of Transport Inuvik airport maintenance staff

(Mr. R. J. Aubrey, Airport Manager) during the course of this work is gratefully aclmowledged.

,

• -INUVIK N.W. T. AIRSTRIP GROUND TEMPERATURE OBSERVER _ DATE _

NORTHERN RESEARCH GROUP DIVISION OF BUILDING RESEARCH NATIONAL RESEARCH COUNCIL

FORM N-I (REVISED OCT. 1961)

#1

#2

#3

#4

0 UNDISTURBED 0 WEST END EAST END

SWITCH DEPTH AREA DEPTH PARKING APRON AIRSTRIP AIRSTRIP POINT (FT) _{TEMP (OF)} (FT) _{TEMP (OF) TEMP (OF) TEMP (OF)}

I 15 15 2 15 15 3 15 15 4 10 10 0 5 10 10 6 10 10 7 5 5 8 5 5 9 5 5 10 4 2·5 II 4 2·5 12 4 2·5 13 3 0 14 3 0 15 3 0 16 2 -2 17 2 -2 18 2 -2 19 I -4 20 I -4 21 I -4 22 0·5 -6 23 0·5 -6 24 0·5 -6 .. 25 0 _REMARKS 2& (j 27 0 28 AIR SNOW

I

DEPTH

o DEPTH BELOW ORIGINAL GROUND SURFACE

NEGATlVE SIGN INDICATES AEIOVE ORIGINAL GROUND SURFACE (ie. IN ROCK FILL) • TEMPERATURE IN JUNCT I-ON BOX - ONE FOOT BELOW

ROA.O TO TOW/lJSITE. L1Gl-l1 . ,

e

CABL£,. "1 LlG\.IT "'7

PLAN OF AIRSTRIP

INUVI K N.W.T.

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DETAILS OF GROUND TEMPERATURE

INSTALLATION LOCATIONS