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THE MICIDGAN COMBINATION DENSITY-MOISTURE GAGE FOR SOILS B. W. Pocock, L. W. Smith, W. H. Schwartje, and R. E. Hanna Research Laboratory Division Office of Testing and. Research Report No. 311 Research Projects 55 H-4, 56 H-5 Michigan State Highway Department John C. Mackie, Commissioner Lansing, March 1959 I / '

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Page 1: R-311 - The Michigan Combination Density-Moisture Gage for

THE MICIDGAN COMBINATION DENSITY-MOISTURE GAGE FOR SOILS

B. W. Pocock, L. W. Smith, W. H. Schwartje, and R. E. Hanna

Research Laboratory Division Office of Testing and. Research

Report No. 311 Research Projects 55 H-4, 56 H-5

Michigan State Highway Department John C. Mackie, Commissioner

Lansing, March 1959

I / '

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Introduction

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THE MICHIGAN COMBINATION DENSITY-MOISTURE GAGE FOR SOILS

The Michigan State Highway Department's combination density-mois­ture gage is a non-destructive device which depends for its operation on the interaction between nuclear radiation and matter. Instrumentation consists of two major components, the gage and a portable scaler (elec­tronic counter).

The gage is a stainless steel box about ten inches square, an inch and a half in height, with a handle across the top. Total weight is about 18 lb •. A sealed metal capsule about the size of a lead pencil eraser is embedded in a lead block inside the gage. This capsule contains a tiny amount of radium-beryllium, which furnishes the radiation for the interactions with the soil. The radiation is of two kinds, gamma rays and fast neutrons. Since radium has a half-life of 1620 years, the radiation is relatively con­stant.

The gamma rays on penetrating the soil undergo scattering as a re­sult of collisions with electrons contained in the soil atoms. The more soil atoms there are present, the more scattering takes place, and this scattering occurs in all directions. Simultaneously with the scattering, however, the gamma rays are absorbed in the soil, and the greater the density, the greater is this absorption. The scattered gamma rays which are not absorbed are detected by two gamma-ray counter tubes inparallel circuit in the gage. The number detected per minute varies inversely with the soil density.

The fast neutrons emitted from the capsule undergo elastic collisions with the nuclei of hydrogen atoms contained in the water molecules of soil moisture, and are likewise scattered in all directions. Each typical col­lision greatly reduces the velocity of the neutrons, so that after several collisions they are slowed to "thermal" velocities (about 7218 ft/sec). The number slowed by this process varies directly as the number of hydrogen atoms present, hence as the moisture content of the soil. The "thermal"

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or slow neutrons are detected by two slow neutron counter tubes in parallel circuit in the gage, which are insensitive to anything else. The number detected per minute varies directly with the moisture content.

Historical Development

The Michigan combination gage is the product of three years' re­search conducted in the Research Laboratory Division subsequent to investigations reported in the Michigan State Highway Department Office of Testing and Research Report No. 261, July, 1956, "An Analysis of Certain Mathematical Assumptions Underlying the Design and Operation of Gamma-Ray Surface Density Gages." A condensed version of this report appeared in the 1958 Proceedings of the American Society fo;: Testing Materials.

Preliminary work with individual pilot models in the laboratory and in the field indicated the soundness of the physical principles involved in measuring soil density and moisture by the nuclear method. Other workers reported favorable results and published designs for density gages and moisture gages which they considered satisfactory. At least one firm began making nuclear density gages and moisture gages avail­able commercially.

All of the nuclear gages reported or available commercially were of the individual type; that is, they either detected density or moisture but not both. The moisture gages usually used radium-beryllium as a source of neutrons, but the gamma rays from this source were totally neglected as undesirable. The density gages used either cesium 137 or cobalt 60 as a source of gamma rays.

The Michigan State Highway Department recognized the desirability of having a single nuclear gage which could be used for both density and moisture determinations, and research was undertaken to establish the feasibility of using the gamma radiation from the radium of the radium­beryllium source for accurate detection of soil density.

It was found that this gamma radiation could indeed be employed for accurate density determinations provided the gage were properly de­signed. Proper design included the introduction of gamma-ray couoter tubes separated from the radioactive source by a lead absorber of opti­mum thickness, as described in Report No. 261; a built-in electronic cathode follower circuit; placement of slow neutron couoter tubes with strict relation to their point of maximum sensitivity; and a built-in elec­tronic preamplifier circuit.

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Page 4: R-311 - The Michigan Combination Density-Moisture Gage for

Design Features

Illustrations, design features, and details of construction of the Michigan gage are shown in Figures 1 through 4. An operating manual was prepared for field density crews who might have to be trained to use the gage and was published as Michigan State Highway Department Office of Testing and Research Report No. 308, "Operating Manual for Soil Density and Moisture Measurements by Nuclear Methods," March, 1959.

In use, the gage is placed on the soil and is connected by a cable to a battery-operated portable scaler (electronic counter). A toggle switch on the scaler permits determination of the count rate from either the. gamma­ray tubes or the slow neutron tubes in the gage. The count rate from the gamma-ray tubes (toggle switch at "Density" on the scaler) is determined either with a stop watch or ~ith the timer built lnto the scaler, and is used to read the density of the soil on a density calibration curve directly in units of pounds per cubic foot, wet basis. The count rate from the slow neutron tubes (toggle switch at "Moisture" on the scaler), determined the same way, is used to read the moisture content of the soil on a mois­ture calibration curve directly in units of percent moisture by weight, dry basis.

Practical field experience on construction projects covering over 500 .. determinations of both density and moisture indicates that a one-minute

count is sufficient for determination of either density or moisture. This means that the operator may have both results in a little over two minutes, well within the limit of accuracy demanded for routine construction prac­tice.

Use of the Standard

The standard used with the gage is a wooden box filled with paraffin and capped with quarter-inch Masonite for durability. Dimensions are 7" high, 15" wide, and 20" long. Weight is 60. 5 lb. In use it is placed on an empty wood box with the Masonite side uppermost in order to main­tain a constant elevation above ground. The gage is positioned within the outline scribed on the Masonite and count rates are determined for both density and moisture. These rates are referred t() previously selected standard rates and all field count rates are adjusted by the difference for the next four hours in practice. The reason that paraffin is used as a moisture as well as a density standard lies in the fact that paraffin con­tains a copious supply of hydrogen atoms.

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Page 5: R-311 - The Michigan Combination Density-Moisture Gage for

Figure 1. Michigan Combination Density-Moisture Gage connected to companion portable scaler.

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Figure 2. Michigan Gage in use. Operators: Jack Lund and Larry Miller

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Figure 3. Method of running standard density and moisture count rates with the Michigan Gage. Operator: Jack Lund.

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MOISTURE PRE-AMP

"" 112W

,o,

""'~~fd

~~l:S'

;~;I ··~-hfC· """"""" OON8 = 2KV C"'----+ TM~SJ.TOR OOU..,-.,.TUO<O ---- MYT"EON

$ 2NII4

300~w~d

.. ~

300~,f,;"IO

~~ r-Jf-

~" "''' 0

'

"' lr2W

'"

Electrical Schematics for the Combination Gage.

DENSITY CATHODE

FOLLOWER

'l!e 1----- 4 ! j_ ' • 4 ~ ---d !- ' ' ' 1 / DAOLL 2 CL ffij?'"'\,AA ••~ rn• ,., _____ .........,.. ______ ..

-~-----------

c. IOL~ !'OR NO. 5 S><l". MET~L OC:Rtw

$0LOER<D

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SOLOERED

DETAIL

NOTE: ALL MATeRIAL ..... GAG~ STAINLESS STEEL VNL(S5 OTHERWISE SPECIFI(O.

1 I I I 111

:--~- ""~ : r i: ~ ' I: J

PART A

.~,·~-::~·-~-,~- rfflt 11 lr1 ~~"""

LJ:I. I f-----·~--1.1 ~

PART D

,,II 1: ,-f J,~~ -------~-- I ---------- T

I ' • ' --·---------- 11

otl~ -l~ 16 ~I~---~

"' ORILL 13 PICOT "OLES ft:<l NO.O SHT. METAL SCRCW

f- j CASE DETAIL PART G

Detail Drawings of the Combination Gage.

-!-:. ·•;;-7If 4 NOTE ....... 2

~~ 4SI'OTWELO

I -- I~· ...II3-c " I.!..!-· PART B -1!~- J.

c-.::::::~=:j:F _J = 4" ....! -lti-

~. fl It t:t=;;-~

' CJJD

PART E

~J l+2:L:lLtR. HOLE

FOR 5·32 MAC><. SCREW

PART H

OOTE: MME 2, I RIG"T ~ 0 LEFT

PART C

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8•32 MM:"- scRtw - ~""'"'\.

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PART F

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PART

Figure 4. Engineering Drawings of the Combination Gage.

PART<h

' (f-~::;~R~~~~~:c't:'g'uc

AM PHENOL (~o":!E.'-'~~~.s

=an-'-II- -PARTe

EiL ~" COPPER CV.O P>IENOUC l-t12X:!'-•112 IOICHES

~~~~·~Ill 'A

:~X~-~ t~~;~:z~~=~tn~ ~~'" ... Plan View of the

Combination Moisture and Density Gage.

Exploded View Assembly drawing of the Combination Gage.

Page 9: R-311 - The Michigan Combination Density-Moisture Gage for

It has been found that two standard determinations are usually enough for a normal working day, one in the early mon1ing and one in the early afternoon. Variations in the density standard count rates are due to changes in the local background radiation caused by different amounts of atmospheric radon and thoron; variations in the moisture standard count rate reflect changes in absolute atmospheric humidity. Although no temperature dependency of any part of the gage or scaler has been de­tiwted within the zero to 90 degrees F. range experienced so far, any such dependency would certainly be minute and should be compensated for by this method of using the standard.

Calibration Curves

Figures 5 and 6 show the calibration curves for density and mois­ture. Count rates for density are adjusted according to the count rate on the standard, referring to a standard count rate of 13, 000 counts per minute. Count rates for moisture are similarly referred to a standard count rate of 2,900 counts per minute, and are adjusted accordingly.

The first 100 density determinations obtained with the gage on a colliltruction project for which determinations were also made with the balloon apparatus are plotted in Figure 7, which shows nuclear results plotted agaillilt conventional results. The correlation coefficient, r, for this graph is 0. 941.

The first 100 moisture determinations obtained with the gage on the same project, for which determinations were also made by driving the water off over a hot plate, are plotted in Figure 8, showing nuclear re­sults against conventional results. The correlation coefficient, r, is 0.979.

Soils on which these determinations were made included loamy sand backfill, both with and without the presence of large stones; clay and sand mixtures; sand; sand and gravel mixtures; sand, gravel, clay, and silt mixtures; stony material; and clay.

Comp_arison with Moisture on a Volume Basis

Others who have reported results of moisture determinations by the neutron method have invariably established their calibration curves by plotting count rates against known weights of water per cubic foot of soil. Conversion from pounds per cubic foot to percent by weight is then made

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Page 10: R-311 - The Michigan Combination Density-Moisture Gage for

900

16,000

800 15,000

::; :::;

' 14,000 ' 700

0 0

13,000 600

0 0

"' 12,000

"' 1- 1- 500

0 11,000

0

"' "' 400 10,000

0: 0:

0: 0: 9,000 300

0 0

u 0

e,ooo 200

7,000

100 40 60 80 100 120 140 160 180 0 5 10 15 20 25 30

DENSITY C WET BASIS) LBS/FT3 PERCENT MOISTURE- DRY BASIS

Figure 5. Calibration Curve for Density Figure 6. Calibration Curve for Moisture M.s. H. D. Combination Nuclear Density-Moisture Gage. M. s. H. D. Combination Nuclear Density-Moisture Gage.

Page 11: R-311 - The Michigan Combination Density-Moisture Gage for

20

170 L I 19

" § 18

"' 160 f-- I

CORRELATION COEFFICIENT I I ~ 17 ~ I CORRELATION COEFFICIENT

r = 0.941 z r = 0.979 Q ~ 16

" "' 0 > ~ 150 ~ 15

"' u ::;

~ 14 z 0 tJ 0 a: J 140 :t 13 <(

~- 12r / • CD

,.: • • • ... > • a 130 :!i II

• .... CD !i:' .J 10

" > i:i !:::: 120 ;:: 9 ., • ~ z • :t

• • "' ... y.,· " • z • • •

"' • • ... • 110 f-- /. z "' 8

"' 61 a: I!L ~ :::>

100 f-- / I Iii 5 0

::;

4

3 100 110 120 130 140 150 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20

DENSITY, LB/CU FT. NUCLEAR METHOD MOISTURE CONTENT BY WEIGHT 1 DRY BASIS, PERCENT. NUCLEAR METHOD

Figure 7. Conventional vs Nuclear Density. Figure 8. Conventional vs Nuclear Moisture,

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by dividing the result by the unit weight of the soil. This method, of course, has two disadvantages: the extra computation plus the necessity of knowing the density. ·

The Michigan calibration curve was established by plotting count rates against known percentages of moisture by weight, dry basis. Re­sults are thus obtained directly in units desired, and it is not necessary to know the density in order to determine the moisture content. The only rational explanation of this seeming paradox is that the volume of the sphere of influence of the neutron or moisture portion of the gage must vary inversely with the density.

A comparison of the accuracy of both methods was made by multi­plying the moisture results obtained by conventional means for the 100 moisture points, by density results obtained with the balloon equipment for the same samples. This converted the conventional moisture results into units of pounds per cubic foot. These results were then plotted agai.nst the count rates obtained with the nuclear gage, and the resulting calibration curve is shown in Figure 9. This curve is a straight line, as reported by other workers. Moisture contents on the volume basis were read from the curve for each of the 100 points, and these were divided by the conventionally-determined densities, ttus converting back to engineering units. These results were plotted against conventional re­sults as shown in Figure 10. The correlation coefficient, r, for this comparison is o. 945. Our conclusion based on the evidence to date is that the Michigan calibration curve for moisture is at least as accurate as calibration curves established on a volume basis, with the advantages that results are obtained in immediately useful units and that it is not necessary to know the density in order to determine the moisture content.

Health. an~J.Y

With reference to health and safety aspects, it is felt that the design of the Michigan gage effectively precludes any hazard to personnel oper­ating the gage according to instructions. Practical common-sense safety rules are included in the operator training program.

As a legal precaution, arrangements should be made, and are made in Michigan, for all gage operators to have semiannual health examinations including blood counts and urtnalyses, and to wear film badges sensitive to ionizing radiation including neutrons when working with the gage. Several commercial firms such as Tracerlab of Boston, Massachusetts,

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Page 13: R-311 - The Michigan Combination Density-Moisture Gage for

900

BOO

[ VOLUME BASIS I ... I 0

1- 700

0 :::> 0

z 0

::; 600 J 0 0

0

0: o/oo 00

... 0

._sao 0

"' t- 0 of. 0 1-

(f .. dL 0 0

0 0 0

0 Oo)o 0 0

300

200

IOOL-__ L_ __ L_~L_~--~--~--~--~---L __ _L __ _L __ J

0 4 8 12 16 20 24 MOISTURE CONTENT, LB/ CU FT

Figure 9. Calibration Curve for Moisture. Volmne Basis

20

19 0 0

~ 181

~ 17 0 >= z 16r ... ~ 8 15

,.: ~ 14 u 0:

~ 13

<!1.' -~ 12 ., ~ II 0

... -~ 10

... ;: 9 > ., !z 8

... 1-z 7 8 ... 6 0:

~ ~ ::;

4

3~ 4

I

0

CORRELATION COEFFICIENT

0 0

r = 0.945

0

0

/o 0

0

5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20

MOISTURE CONTENT BY WEIGHT, DRY BASIS, PERCENT. NUCLEAR METHOD (R~ULTS CONVERTED FROM CAI..IBRATION CURVE ESTABLISHED ON VOLUME BASIS)

Figure 10. Conventional vs Nuclear Moisture. Nuclear results converted from calibration curve

established on volume basis,

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supply satisfactory film badge service. A typical film badge is shown in Figure 11. All records should be maintained for the life of the individual.

Regu1atory Agencies

The United States Atomic Energy Commission possesses no regula­tory power over naturally-occuring radioactive materials. Since radium falls in this category, the radium-beryllium source is generally con­sidered to be outside the scope of AEC jurisdiction. All radioactive materials, however, come under the authority of most state health de­partments, and Michigan is no exception. The Michigan Department of Health requires copies of normal records including purchase date, owner­ship, intended use, etc., in addition to the results of periodic tests for possible leakage of the isotope from the sealed capsule. Relevant infor­mation and test results are furnished by the Research Laboratory Division.

In view of the complicated nature and many possible ramifications of radioactivity, it is felt very strongly that supervisory personnel should possess knowledge of this subject sufficient for AEC approval. Such knowledge can be obtained by taking the four-weeks radioisotope techniques course conducted by the Oak Ridge Institute of Nuclear Studies. The value of this course cannot be too highly stressed. Arrangements can be made through Dr. Ralph T. Overman, Chairman, ORINS, P. 0. Box 117, · Oak Ridge, Tennessee.

Individual nuclear density gages are available commercially for $1285. The companion moisture gages are priced at $1750. The Michi­gan combination gage costs approximately $600 for all parts . including the standard. It is estimated that construction costs are in the neighbor­hood of $400 for labor, on a unit basis. The portable scaler used with all these instruments, also available commercially, sells for $1345.

A conventional field density control unit in Michigan consists of a vehicle, two men, and all the usual equipment for conducting balloon method density tests and hot-plate moisture tests in addition to sand cone and Proctor determinations. It usually averages out that conventional routine density and moisture control tests require about 30 minutes. Preliminary experience based on over 500 nuclear determinations shows that over ten density and moisture determinations can be obtained during the time it would take to obtain one determination of density and moisture by conventional methods.

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Figure 11. Typical film badge carried by operators using the Michigan Gage.

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Although this potential has . occasionally been realized, it is the ex­ception rather than the rule, due to time lost in moving from place to pl'ace on the grade. It is felt that much of this lost time can be eliminated by equipping the two-man crew with a panel truck, thus making it un­necessary to disconnect the scaler from the gage when changing locations, and at the same time furnishing plenty of room for such accessory equip­ment as that used for determinations of percent of maximum compression.

One complete nuclear device including the scaler costs about half as much as ten conventional sets of density-moisture apparatus, yet the re­sults are roughly comparable. Ten conventional setups require 10 men and 10 vehicles. At present one nuclear device requires two men, one of whom determines percentages of maximum compaction and records data, and one vehicle.

Qperator Training

No outstanding educational background or unusual talents, other than normal intelligence and a willingness to learn, are required in order to operate the Michigan gage, although a three-day training program is re­commended for new operators. During this period the trainee studies the operating manual and uses the gage under close supervision. He is given some background in elementary nuclear concepts and is made to feel that he is "getting in on the ground floor" in a new area of engineering technology. The response has invariably been excellent.

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