road research laboratory rrl report lr 272b.s. 1377:1967 contains two methods for determining...

ROAD RESEARCH LABORATORY

Ministry of Transport

RRL REPORT LR 272

AN EXAMINATION OF METHODS OF DETERMINING THE SPECIFIC GRAVITY OF SOILS

by

J. V. Krawczyk

Road Research Laboratory Crowthorne, Berkshire

1969

CONTENTS

Abstract

1. Introduction

PART 1

2. Development of an improved procedure for the present B.S methods of determining specific gravity

2.1 The British Standard methods

2.2 Development of improved test procedure

3. Examination of alternative procedure

3.1 Soils used

3.2 Experimental procedure

3.3 Investigation of factors affecting the accuracy of the test

3.3.1 Effect of shaking time

3.3.2 Effect of initial degree of pulverisation of the soil

3.4 Discussion of results

4. Conclusions

10.

11.

12.

Appendix 1.

Appendix 2.

PART 2

5. An examination of the Loebell pycnometer for the determination of the specific gravity and moisture content of soils

6. PrincilSle of the Loebell air pycnomet~*r

7. Requirements in the accuracy of the volume measurements

8. Determination of specific gravity



9. Tests with a modified form of the Loebell air pycnometer

9.1 Modifications made to the apparatus



Conclusions

Acknowledgements

References

Determination of the specific gravity of soils

Principle of the Loebell air pycnometer

Q CROWN COPYRIGHT 1969

Extracts from the test may be reproduced provided the source is acknowledged

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Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on l S t April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

AN EXAMINATION OF METHOD5 OF DETERMINING THE SPECIFIC GRAVITY OF 5OIL5

ABSTRACT

This Report describes the results of an investigation made to find methods for the determination of the specifi c gravity of soils that are less subject to error than those given in B.S. 1377:1967.

Part 1 of the Report shows that the present B.S. method for coarse-grained soils can be made more reliable and used for all types of soils if the a i r is removed from the soil by a shaking technique in place of removal under vacuum.

In Part 2 of the Report the use of a Loebell air pyenometer is described and it is shown that this apparatus can be adapted for the rapid determination of the specific gravity of soils. The time taken is less than 15 minutes and the accuracy is sufficient for most purposes.

I. INTRODUCTION

The determination of the specific gravity of soils is in principle one of the easier tests to perform

as it only requires a knowledge of the mass and volume of the soil particles. Accurate weighing of

the soil should not present a problem. However, the determination of the volume is made by a method

involving the displacement of water (or other suitable liquid) by the soil particles and this can give

rise to errors because of the difficulty of completely removing the air from the soil.

In practice, therefore, the specific-gravity results are subject to quite large errors as can be

seen in Fig. 1 which gives the results of specific-gravity determinations made on three soils by 36

different laboratories. An examination of the causes of the large variations given in Fig. 1 suggested

that failure to remove all the air was the major cause of error. ,Investigation has revealed that there

is little appreciation by the laboratories of the primary importance of this factor.

In view of the apparent difficulties with the existing British Standard method the Laboratory has

undertaken an examination of alternative methods of measuring the specific gravity of soil particles.

This Report describes the results of an investigation of two alternative procedures. Part 1 deals

with the development of an improved procedure of the existing methods described in British Standard

1377:1967 and Part II describes an investigation of an air pycnometer method.

PART I

2. DEVELOPHENT OF AN IHPROVED PROCEDURE FOR THE PRESENT B.S. HETHODS OF DETERHINING SPECIFIC GRAVITY

2.1 The British Standard methods

B.S. 1377:1967 contains two methods for determining specific gravity - Test 6 (A) for fine- grained soils and Test 6 (B) for medium and coarse-grained soils.

The method for fine-grained soils utilises a 50 ml density bottle and the determination is made

on a sample of oven-dry soil weighing approximately 1%. The volume of the soil is obtained by

measuring the volume of water or other suitable liquid displaced by the soil particles. It is essential that all the air be removed from the soil and this is achieved by placing the density bottle and con-

tents under vacuum for a period of time. Unfortunately there is no simple means of knowing when all

the air has been removed and the bottle therefore needs to be left under vacuum for several hours.

Failure to leave the bottle under vacuum for long enough, coupled with the use of inefficient vacuum

pumps, is commonplace in soil-testing laboratories and could introduce errors of the magnitude necessary to produce the scatter of results shown in Fig. 1.

The method for medium and coarse-grained soils utilises a 21b Kilner jar as the density bottle (the method is often referred to as 'the pycnometer method' but all density bottles are pycnometers).

A sample of soil of approximately 500g is placed in the bottle and covered with water; air is removed

either by placing the jar under vacuum or by stirring and shaking. In this test, also, the difficulties of removing air are not fully appreciated and results for specific gravity that are too low are frequently reported.

2.2 Development of improved test procedures

The major difficulty with both tests is the provision of a satisfactory vacuum and measuring the

length of time required to remove the air completely. The first requirement in the development of an

alternative procedure, therefore, was to find another method of removing the air. One possibility that

is made use of in some test procedures is to boil the contents of the density bottle but this is time-

consuming as the bottle and contents have to be cooled back to room temperature. Another alternative and relatively simple procedure is to shake the soil-water suspension to remove the air, as is sug-

gested in Test 6 (B) of B.S. 1377. The method as given in the B.S. involves shaking by hand and it

would clearly be preferable to mechanise this and to standardise the length of time require for shaking.

Tests were therefore carried out to find if a shaking procedure could be standardised. It is probable that many of the shaking machines available commercially would be suitable but a Wagner shaking machine (Plate 1) which will shake ten 1-1itre capacity gas-jars was available in the laboratory and this was the apparatus examined. In the event gas-jars proved to be very suitable as

pycnometers and were preferable to Kilner jars largely because of the ease with which gas-jars could

be topped-up with water and the end closed with a glass plate (Plate 2). Air trapped under the plate can be very easily seen, whereas it is difficult to know if air is trapped in the top of the Kilner-jar pycnometers which have a metal cone and ring for closing the jar.

2

3. EXAMINATION OF ALTERNATIVE PROCEDURE

3.1 5oils used

The properties of the six soils used in this part of the investigation are given in Fig. 2. No

special preparation was given to the soils at this stage except that clay aggregates were broken down

into particles having a diameter of less than 12mm.

The bulk samples of Weald clay, London clay, sandy clay and fine sand were divided by suc-

cessive riffling so that a number of representative sub-samples of each soil weighing about 200g were

available for determining the specific gravity bythe revised procedure and 4 sub-samples of each soil weighing about 12g were available for determining the specific gravity by B.S procedure.

In the case of the coarse sand and gravel-sand-clay the soils were riffled into sub-samples

weighing about 400g and the specific gravity of these samples determined by both the revised and

B.S. procedures.

All the samples were oven-dried at 105-110°C before test and in carrying out the test by B.S.

procedure particular attention was given to making sure that all the air had been removed.


The experimental details for carrying out specific-gravity determination by the alternative pro-

cedure are given in Appendix 1. All the tests were carried out at constant temperature. Preliminary

tests showed that de-mineralised water and tapwater gave the same results and tapwater was there-

fore used for most of the work described.

3.3 Investigation of factors affecting the accuracy of the test

After the preliminary tests had shown the feasibility of the alternative test procedure a more

detailed investigation was made to find the effects of shaklng-time and the initial degree of pulyeri

sation of the soil.

3.3.1 Effect of shaking-time To find the effect on the specifiC-gravity results of varying the shaking-time, eight 400g and 200g samples of ejach soil (depending on soil type) were obtained in the

manner described in Section 3.1. The specific gravities of duplicate pairs of samples were then determined by the procedure given in Appendix 1 using shaking-periods in the mechanical shaker of

15 minutes, 30 minutes, 60 minutes and 120 minutes. The results obtained are given in Tables 1 and

.

3.3.2 Effect of initial degree of pulverisation of the soil Although the samples of Weald clay, London Clay, sandyclay and fine sand had no individual particles that were retained on the No.7 (2.4ram) B.S. sieve, the samples as they existed in their natural condition after oven-drying contained

aggregations of particles up to 12ram in diameter. These aggregations had to be broken down to pass a No.7 (2.4ram) B.S. sieve before the specific gravity could be determined by Test 6 (A) of B.S.1377: 1967. A series of experiments was therefore made to find if it was also necessary to break the aggregations down before determining the specific gravity by the newly developed alternative procedure.

(a) (b) (c)

T A B L E I

Comparison of spe.cific gravity results obtained by:-

B.S.1377:1967 method As (a) but using end-over-end shaking As (b) but on material less than 12 mm in particle size

Soil

Weald clay

London clay

Sandy clay

Fine sand

(a) Standard

Lab. method

B.S. 1377

S.G.

2.776 2.785 2.777 2.779

Mean 2.78

2.792 2.785 2.791 2.780

Mean 2.79

2.716 2.718 2.713 2.710

Mean 2.71

End-over-end shaking

Shaking time

(b) Sample of soil passing No. 7

(2.4 mm) test sieve

(rains) S.G. ! S.G. Mean S.G.

15 2.769 2.780 2.77 30 2.770 2.773 2.77 60 2.776 2.773 2.77

120 2.765 2.780 2.77

(c) Sample of soil

less than 12 mm in particle size

Mean S.G. S.G. S.G.

2.782 2.769 2.78 2.771 2 .771 2.77 2.783 2.792 2.79 2.785 2.781 2.78

15 2.784 2.796 2.79 2.733 2 .790 2.76 30 2.802 2.782 2.79 2.795 2.800 2.80 60 2.792 2.792 2.79 2.794 2.792 2.79

120 2.793 2.790 2.79 2.779 2 .791 2.79

15 2.719 2.714 2.72 2.717 2.706 2.71 30 2.728 2.726 2.73 2.708 2.722 2.72 60 2.720 2.714 2.72 2.712 2.717 2.71

120 2.722 2.722 2.72 2.699 2.720 2.71

2.675 15 2.661 2.664 2.66 2.676 2.659 2.67 2.668 30 2.667 2.672 2.67 2.669 2.652 2.66 2.670 60 2.651 2.661 2.66 2.665 2.652 2.66 2.667 120 2.659 2.659 2.66 2.651 2.656 2.65

Mean 2.67

Samples of each of the four soils were obtained in two conditions. The samples in one set

were in the natural oven-dried condition with aggregations up to 12mm diameter and the samples in the other set had been crushed to pass a No. 7 (2.4mm) sieve.

The results of the specific-gravity determinations made on both sets of samples are given in Table 1.

3.4

4

Discussion of results

The results in Table 1 show that for the Weald clay, London clay, sandy clay and fine sand a

shaking time of 30 minutes was sufficient to obtain values in close agreement with those obtained

by B.S. procedure. There were no differences between the results obtained with the samples crushed

to pass a No. 7 B.S. sieve and those obtained with samples used in an aggregated condition.

The results in Table 2 for the gravel and coarse sand show that even after 2 hours of shaking

the specific gravity values tended to be lower than those obtained by B.S. procedure. It was found

that this was due to the difficulty of removing air from the larger stones. When the samples were

allowed to soak for 5 hours before shaking the results were again in very close agreement with those

given by the B.S. procedure.

(a) (b) (c)

TABLE 2

Comparison of specific gravity results obtained by:-

B.S.1377:1967 method As (a) but using end-over-end shaking As (b) but on material immersed in water before testing

Soil

Gravel - sand - clay -

Coarse sand

(a) Standard

Lab. method

B.S. 1377

S.G.

2.665 2.664 2.658 2.666

Mean 2.66

2.691 2.688 2.693

Mean 2.69

Shaking time

(rains)

15 30 60

120

15 30 60

120

End-over-end shaking

(b) Sample of soil passing No. 7

(2.4 mm) test sieve

Mean S.G. S.G. S.G.

2.613 2.611 2.61 2.623 2.624 2.62 2.620 2.622 2.62 2.634 2.643 2.64

2.663 2.667 2.67 2.665 2.660 2.66 2.672 2.672 2.67

2.678 2.675 2.68

(c) Sample left immersed

for 5 hours before shaking

S.G,

2.627 2.660 2.659

2.687

Mean S.G. S.G.

2.657 2.64 2.657 2.66 2.653 2.66

n

2.684 2.69

4. CONCLUSIONS

The results of this investigation show that the proposed alternative procedure for determining speci-

fic gravity is able to give results that are as accurate as the B.S methods. It has a number of advan-

tages over the present B.S. methods, the most important being that:-

1. The same procedure is applied to all soils.

2. No special preparation of the soi l is required. 3. The mass of sample that can be used for fine-grained soils is increased from 10g to 200g.

This reduces the effect of sampling and weighing errors. 4. The method of removing the air from the soil is more reliable and takes only 30 minutes.

5

In view of these advantages it is suggested that consideration be given to incorporating the new method into the British Standard for testing soils.

PART 2

5. AN EXAMINATION OF THE LOEBELL PYCNOMETER FOR THE DETERHINATION OF THE SPECIFIC GRAVITY AND HOISTURE CONTENT OF SOILS

The Loebell air pycnometer 2 (illustrated in Plate 3) permits the volumes of solidsto be determined

within a few minutes. Work carried out at the Laboratoire Central des Ponts et Chaussees in Paris 3

has shown that the apparatus can be used to determine the volume of samples of soil. If the soil is dry the specific gravity can then be calculated and if the soil is wet the moisture content can be calculated provided that the specific gravity of the soil particles is known.

The apparatus is portable and requires no electricity supply and therefore appeared to be worthy of investigation to see if it could be used for the rapid determination of moisture content and specific gravity. This part of the Report describes the results of these investigations.

6. PRINCIPLE OF THE LOEBELL AIR PYCNOHETER

The principle of the Loebell air pycnometer is described in Appendix 2. To determine the value of

a soil sample the level of mercury is lowered to a point below the junction of the two tubes (as in

Plate 3). The soil is placed in the pressure-pot and clamped in position; at this stage the pressure

in the apparatus is equal to atmospheric pressure. The level of mercury is then raised until the

pressure in both the left and right hand tubes is again equal as indicated by the mercury level being

the same in both tubes. The volume of the soil sample (V) is then read off on the scale which is calculated in cubic centimetres.

If the soil sample is dry the specific gravity of the soil (Gs) can then be determined from the equation:-

G s = W

V where W = the mass of the dry soil sample in grams.

The relation between specific gravity and volume for 125g and 250g samples of dry soil is given in Fig. 3.

If the soil is wet and the specific gravity of the soil particles is known, the moisture content (m) can then be calculated from the equation:-

m = 100 VGs - W s per cent Gs (W s - V)

where W s = the mass of the wet soil sample in grams.

The relation between moisture content and volume for 100g and 200g samples of wet soil (G s -- 2.70) is given in Fig. 4. 6

7. REQUIREMENTS IN THE ACCURACY OF THE VOLUME MEASUREMENTS

The form of the equation linking the height of the mercury column (He) to the volume of the sample

(V) (See Appendix 2) is such that the scale on the apparatus which reads the volume of the sample

directly in cubic centimetres is not linear and is more accurate at the higher volumes (Fig. 5). It is

therefore desirable to use as large a volume of soil as possible. The maximum volume that the appa-

ratus will measure is 120cm 3 but the maximum amount of dry soil that the 'pressure-pot, supplied with

the apparatus, will hold is about 250g. This corresponds to a volume of about 90cms for most soils .

As will be seen in the next section it did not prove possible to measure the volumes of oven-dried

soils with any accuracy and known values of water had to be added before the volumes could be

accurately determined. In practice therefore the maximum weight of dry soil that could be effect ively

used was about 125g corresponding to volumes of soil in the region of 50 cm3.

The scale on the pycnometer is calibrated in intervals of 1 cm 3 and can only be read to an • average accuracy of + 0.2 cm3. This means that for 'a sample of soil weighing 125g, because of

difficulties in reading the scale, the specific-gravity results are subject an uncertainty of _+ 0.01

(Fig. 3) and moisture-content results to an uncertainty of + 0.5 per cent (Fig. 4). This degree of

error is acceptable for most purposes but tests with the apparatus filled with known volumes of water

and mercury showed that the reproducibility of the results was no better than +_ 1 cm 3 which would

give a variation of + 0.06 in the specific gravity of a sample weighing 125g and an error of + 2.4 per

cent of moisture on a wet sample of soil weighing 100g.

I t w a s therefore concluded, and subsequent test confirmed, that the apparatus is not suitable

for accurate determinations of moisture content. As far as the determination of specific gravity was

concerned the apparatus, as purchased, did not seem to be capable of giving suff ic ient ly accurate

results but it was decided to confirm this by carrying out some actual determinations of specif ic

gravity. These investigations are described in the remainder of this Report.

8. DETERMINATION OF SPECIFIC GRAVITY


Six soils (listed in Table 4) of accurately known specif ic gravity were used for this stage of the

investigation. A known weight of each soil was placed in the pressure-pot of the pycnometer and its

volume and hence specific gravity determined.

Whilst the results for the two sandy soils and the sample of p.f.a, were of the right order it was

immediately apparent that the volumes recorded for the three clayey soils were far too low thus giving

absurdly high specific-gravity results (Table 4).

It was considered that the results for the soils might be improved by adding a known volume of

water to the dry soil and determining the volume of soil + water. This was done by placing a known

weight of dry soil in the pressure-p6t as before, then adding 50 ml Of water from a pipette. The

volume of the soil sample is thus obtained from the value of (volume of soil + water) - 50.

The determinations on each soil were made in duplicate, each individual determination taking

less than 5 minutes. The results are given in Table 3.

7

TABLE 3

Results of specific-gravity determinations on 6 materials carried out in a Loebell pycnometer

Material

Sulehay Sand

Organic Sand

Pulverised Fuel Ash (Uskmouth)

Brickearth

Gault Clay

Weald Clay

Specific-gravity by B.S.

procedure

2.67

2.66

2.05

2.74

2.76

2.78

Specific-gravity results obtained with

a dry sample

a. b. Meal

2.5~ 2.58 2.58

2.49 2.49 2.49

2.23 2.06 2.15

3.46 3.40 3.43

4.1 - 4.1

3 . 9 - 3 . 9

Specific-gravity results obtained by, adding 50 m l water to dry sam des

a. b. Mean

2.61

2.57

1.92

2.66

2.74

2.75

2.61 2.61

2.57 2.57

1.88 1.90

2.71 2.69

2.70 2.72

2.79 2.77


The results of the specific-gravity determinations made on the dry soils (Table 3) show that

with the possible exception of the p.f.a, sample the values obtained were seriously in error, this was

particularly so in the case of the three clayey soils. The magnitude of the error was unexpected

since previous French work3 with the Loebell pycnometer had shown that it could be used for the determination of the specific gravity of dry soils.

However, Jamison4 had found when using another type of air pycnometer that false values were

obtained with dry soils and he showed that this was due to absorption of air on the surface of the

clays and other colloidal materials present in the soil. A similar phenomenon presumably occurred

in the present work as the low volumes recorded :for the clayey soils are consistent with absorption

of air on the clay. Jamison's work also showed that absorption decreases rapidly as the moisture

content increases. This would explain why the specific-gravity results of the soils to which 50 ml of water had been added were much closer to the actual values.

An examination of the results obtained after the addition of water to the soils show that the

errors were much as expected from the arguments advanced in Section 7. The errors were too large

to make the pycnometer in its standard form acceptable as an alternative method for determining the

specific gravity but the results were sufficiently close to the actual values to justify further investigations to find if the accuracy could be improved.

:8

9. TESTS WITH A MODIFIED FORM OF THE LOEBELL PYCNOMETER,

9.1 Modifications made to the apparatus

For the reasons already given, the maximum weight of soil that could be used in the pycnometer

was about 125g. Fig. 3 shows that, if a sample of 250g of dry soil could be taken instead, the error

arising from a difference of 1 cm 3 in reading the pycnometer could be halved (i.e. + 0.06 to + 0.03).

The apparatus was therefore modified by replacing the pressure-pot with a much larger vessel having

the same diameter but with the volume increased from 150 cm 3 to 600 cm3. This pot when filled with

475 ml water gave readings on the scale of about 20 cm3. When 250g dry soil were added to the pot

the volume reading was then about 110 era3 and the volume of the soil obtained from the difference of

the two readings.


The soils listed in Table 5 were used in this part of the investigation and the volume of 250g

samples of each soil was determined in duplicate. This was done by first placing approx. 475 ml

water in the pressure-pot and noting the reading on the pycnometer and then adding exactly 250g of

the soil to the water in the pot and noting the volume of soil + water. The pycnometer readings were

taken in triplicate and the volume of soil calculated from the difference of the mean of the two

readings. The reproducibility of the readings for the volume of each individual sample was always

within + 0.5 cm3. Each determination took about 10 minutes. The results are given in Table 4.

TABLE 4

Results of specific-gravity determinations on 11 materials carried out in a modified Loebell pycnometer

Material

Specific- gravity by B.S.

procedure

( i ) (2)

Sulehay Sand Leighton Buzzard Sand Hertingfordbury Sand Clayey Gravel Pulverised Fuel Ash

(High Marnham) Unburnt Colliery Shale Colliery Tailings Briekearth Weald Clay Gauh Clay London Clay

2.67 2.65 2.69 2.69

2.15 2.15 2.28 2.74 2.78 2.76 2.79

Specific-gravity results obtained with the modified Loebell

pycnometer

a. b. Mean

(3) (4) (5)

2.70 2.69 2.70 2.63 2.64 2.64 2.68 2.71 2.70 2.68 2.61 2.65

2.15 2.13 2.14 2.15 - 2.15 2.37 2.40 2.39 2.77 2.76 2.77 2.77 2.79 2.78 2.76 - 2.76 2.86 2.82 2.84

Difference (5) - (2)

(6)

+ 0.03 - 0.01 + 0.01 - 0.04

- 0.01 0.00

+ 0.11 + 0.03

0.00 0.00

+ 0.05


The results (Table 4) show that the modifications to the Loebell pycu'ometer generally permitted

the specific gravity to be determined within the predicted limits of + 0.03 (Fig. 3). The only excep-

tion to this were the results for the sample of colliery tailings (0.1 high) and London clay (0.05 high).

The reason for the differences in these two instances is not clear; the agreement between the dupli-

cate determinations were good enough to rule out the possibility of leakages in the apparatus. In the

case of the colliery railings the Loebell pycnometer result may be more accurate; this material was

porous in nature and the specific gravity determined by B.S. procedure could well be low.

10. CONCLUSIONS

It is concluded that with the modifications to the pressure-pot described in Section 9 the Loebell air

pycnometer gives results for the specific gravity of soils that are sufficiently accurate for the control

of earthworks. The big advantage of the method is its rapidity - results are obtained in less than 15 minutes - and simplicity. Against this must be set the cost of the apparatus (£150).

The Loebell pycnometer cannot be recommended for the rapid determination of moisture content. It can be predicted (Fig. 4) that, even with the larger pressure-pot, its accuracy will be less than

_+ 1 per cent of moisture. It is unlikely that this would be acceptable when a moisture-content deter-

mination that is more accurate can be obtained just as quickly by the use of a Speedy Moisture Meter5 which is less than half the cost of a Loebell pycnometer.

I I . ACKNOWLEDGEMENTS

The work described in this Report was carried out in the Earthworks and Foundations Section of the

Construction Division of the Road Research Laboratory under the general direction of W. A. Lewis. The Research Team consisted of P. T. Sherwood, J. V. Krawczyk and R. G. Pocock.

12. REFERENCES

1. BRITISH STANDARDS INSTITUTION Methods of testing soils for civil engineering purposes British Standard 1377:1967, London 1967.

2. LOEBELL R. Luft pyknometer Verlag ~/asser und Boden Hamburg 1955.

3. SCHON C. Les picnometres a air Bull. Lias. Labs. Routieres Ponts et Chaussees 1963, 4, 22-1.

4. JAMISON V. C. The significance of air absorption by soil colloids in picnometric measurements (Proc. Soil. Soc. Amer.)1953, 17, 17-19.

5. CRONEY D and J. C. JACOBS. The rapid measurement of soil moisture content in the field Rds. and Rd. Constr. 1951, 29 (343).

10

APPENDIX I

Determination of the specific gravity of soils (With improved method of extracting air)

I. Scope • This method covers the determination of the specific gravity of soil particles. It is not suitable

for soils containing more than 10 per cent of stones larger than 40 mm diameter and such stones

should be broken down to less than this size.

.

.

Apparatus

(1) A gas-jar 1 litre in capacity fitted with a rubber bung (see Plate 2)

(2) A ground-glass plate for closing the gas-jar (see Plate 2) (3) A mechanical shaking apparatus capable of rotating the gas-jar at about 50 rev/min

(see Note 2) e.g. of the type shown in Plate 1.

(4) A balance readable and accurate to 0.2g. (5) A thermometer to cover the range 0-50°C readable and accurate to 1°C.

Procedure

(1) The bulk sample shall be dried (oven-drying is permissible if the soil is not to be used for

any other tests) and any aggregations of soil particles broken down into particles less than

12 mm in diameter.

A representative sub-sample weighing 200g in the case of fine-grained soils and 400g

in the case of medium and coarse-grained soil shall be obtained by riffling from the bulk sample. This sub-sample shall be oven-dried and stored in an air-tight container until

required.

(2) The gas-jar and ground-glass plate shall be dried and weighed to the nearest 0.2g (W2).

(3) Approx. 200g of fine-grained soil or 400g of medium - or coarse-grained soil shall be

introduced into the gas-jar direct from the container in which it has been cooled. The

gas-jar, ground-glass plate and contents shall be weighed to the nearest 0.2g (W2).

(4) Approximately 500 ml of water at a temperature within _+ 2°C of the average room tempera-

ture during the test (see Note 3) shall be added to the soil. The rubber stopper shall then

be inserted into the gas-jar and in the case of medium - and coarse-grained soils the gas- jar and contents shall be set aside for at least 4 hours. At the end of this period, or

immediately after the addition of water in the case of fine-grained soils, the gas-jar shall

be shaken by hand until the particles are in suspension and then placed in the shaking

apparatus and shaken for a period of 20-30 minutes.

The stopper shall then be carefully removed and any soil adhering to the stopper or

the top of the gas-jar carefully washed into the jar; any froth that has formed shall be

Ii

dispersed with a fine spray of water. Water shall then be added to the gas-jar to within

2 mm of the top. The soil shall be allowed to settle for a few minutes and the gas-jar then

filled to the brim with more water. The ground-glass plate shall then be placed on the top

of the jar taking care not to trap any air under the plate. The gas-jar and plate shall then

be carefully dried in the outside and the whole weighed to the nearest 0.2g (W3).

(5) The gas-jar shall be emptied, thoroughly washed out, and completely filled to the brim with

water. The glass plate shall be placed in position taking care not to trap any air under the

plate. The glass jar and plate shall then be carefully dried on the outside and the whole weighed to the nearest 0.2g (W4).

(6) (1), (2), (3) and (4) shall be repeated on a second sample of the same soil so that two values for specific gravity are obtained.

4. Calculations

The specific gravity of the soil particles shall be calculated from the formula:-

G s = W 2 - W 1

(W4 - Wl) - (W3 - W2)

where W 1 = Mass of gas-jar and ground-glass plate

W 2 = Mass of gas-jar, plate and soil

W 3 = Mass of gas-jar, plate, soil and water

W 4 = Mass of gas-jar, plate and water

5. Report ing of resu l ts

The specific gravity of the soil particles shall be reported to the nearest 0.01. If the two results differ by more than 0.03 the tests shall be repeated.

NOTES ON TEST

Note 1. A gas-jar has been found to make a very effective pycnometer but any container of similar

capacity can be used provided that it can be shaken in a mechanical shaking apparatus and

provided that the mouth can be sealed in such a way that its volume is constant.

Note 2. An end-over-end shaker has been specified but shaking machines giving a vibrating shaking

motion would also be suitable. The choice of shaking machine depends on the type of pycnometer used.

Note 3. If there is a large difference between the air temperature and water temperature sufficient

water should be drawn for the required number of tests and allowed to stand in the room in

which the tests are being done until the temperature is within the given range.

12

Pressure Pi

Principle of the Loebell air pycnometer

The two figures given below give a diagrammetric representation of the Loebell air pycnometer at

the beginning and end of the test:-

Pressure Pi I Pressure Pf Pressure pfl

o . . . . .

/ { v~.vq: v" [ length=L)

Vu ( tcngth= Ho)

1 - -Mercury

S z

(A) Beginning of test (B) End of tes t

APPENDIX 2

v~ (length=He)

At the beginning of the test:-

Volume of air in the left tube = V k + Vq + V u

Initial pressure = Pi

13

When a sample of volume V is in place, volume of air in the left tube = V k - V + Vq + V u

At the end of the test:-

Volume of air in the left tube = V k _ V

Final air pressure = pf

Provided the temperature remains constant and assuming that Boyle 's law is obeyed:-

P i ( V k - V + V q + V u ) = p f (v k _ V ) . . . . . . . . . . . . . . (1)

Similarly fo r r igh t tubep .1 (V~+V1) = p~ V~ . . . . . . . . . . . . . . (2) l q

The apparatus is used in such a manner that Pi = p1 and Pf = p1 i f

• " P i ( V k - V + V q + V n) = P f ( V k _ V )

and Pi (V~ + V 1) = Pf (V~) q

V l ( v k - v + v q + V u ) = k

• . . vq+vu

. ' . V I ( V q + V u) k

D

B t

V 1 = q

V 1 k

( V ~ + V 1) (V k - v ) q

V k V 1 - V V 1 q q

V 1 (V k - V) q

Vq + V u

V k - V . . . . . . . . . . . . . . (3)

Substituting lengths and cross-sect ional areas for volumes:-

Vq = (H e _ no)S 1

V 1 = H e S 2 q

V 1 = L S 2 0

V k = V o - V q = V o - ( H e _ H o ) S 1

V • = V 1 - v 1 = L S 2 - H e S 2 = S 2 ( L - H e ) o q

14

which will give in replacement in relation (3)

(He - H o) S 1 + V u H e m

V k - V L - H e

Hence

H e = L I V u - S 1 Ho ]

Vo + Vu - LSi - v

(4)

Let A = L (V u - S 1 H o) and B = V o + V u - LS 1

Equation (4) can then be written as H e = A ,'which re la tes the height of the column of B-V

mercury (H e) to the volume (V) in terms of twoeons tan t s A and B which are a function of the

dimensions of the apparatus.

15

4 -

3 -

2

1

I

2.54 2-58 2.62 2"66

Speci f ic g r a v i t y

Specif ic grav i ty soit 'B' = 2.72

i I I I I I I I

2.70 2.74 2-78 2"80

I i l

I / I

t . .

4

3 -

2 -

1

0 2"58

I t 2"62 2.66

Specif ic grav i ty soi['G'= 2.76

2.71 2 -74 Speci f ic grav i ty

I I I I I I ! 2"78 2-82 2"84

_

4

_

2 -

1 -

0 2-51

, I I 2.62

Specih'c grav i ty soit~W'= 2.78

-iF I i

2"66 2.70 2.7~ 2.78

Specih'c g rav i ty

I I 2"82 2"84

Fig. 1. VARIATION OF" SPECIFIC - GRAVITY RESULTS

N_

"O

o "O

~ v ~ v

e=. O O O~ CO

~u~ssod ~6o~u~J~d

0

0 0

L )

E

C

0

E

J ~

¢.-

0

ffl ~ I z

m

0 ~

~ w L,. o 0

. . J

e -

¢',,,I 0

0 0

m

• -~ U.J Q=" O. t19

O

o--

u.J Q~

Z

z

N

, - J

¢v)

E u

...a

" O

O~

U 3 b ~

O

E

96

95

For 125g dry soil Icm 3-0 .06 SG For 250g dry soit lcm 3-0-03 SG

93

92

91

- - -L . . . . . . . . ~ . . . . . . . . . . . . .

\ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . :L- . . . . . . . . . . . . . . . . . . . . .

0 -- . . . . . . . ~ - ~ ,

2"60 2'65 2.70 Specific gravity

2 '75 Z,5

2'80

Fio,3. RELATION BETWEEN VOLUME AND SPECIFIC GRAVITY FOR 125g AND 250g OF ORY SOIL

48

4? E U

O u'l

:o

46 ~

E

A

E u

. . J

O I/!

O Q t 'N

e~

For 100g wet soil. lcm ~ = 2.4 percent moisture 100 For 200g wet soit Icm 3" 1.2 percent m 0 i s ~ ~ "

98

96

9/,

92 15 20 25 30

Moisture content (percent)

Fig.&. RELATION BETWEEN VOLUME AND MOISTURE CONTENT FOR lOOg AND 200g WET SOIL (SPECIFIC GRAVITY OF SOIL = 2.70)

Sl

A

50 " ' E

0 u~

49

8

E

z,6

47

300

250

20O E E

t - '

ol c 150

u

oo

100

50 /

/

J 0 20 40

/ /

/

60 80 100 120

Votum¢ (cm 3)

140

Fig.5. RELATION BETWEEN LENGTH OF SCALE AND VOLUME RECORDED ON THE LOEBELL AIR PVCNOMETER

Neg. No. H449/68

Plate 1

End-over-end shaking machine used to remove air from soil specimens

~i ¸ i ¸~¸¸¸¸ • i ~

Neg. No. B3379/68

Plate 2

Gas-jar with g!ass plate and rubber stopper for the determination of the specific gravity of soils

'~Y..

I<

Neg. No. H432/68

Plate 3

The Loebell Air Pycnometer

ABSTRACT

An examination of methods of determining the specific gravity of soils: J. V. KRAWCZYK: Ministry of Transport, RRL Report LR 272: Crowthorne, 1969 (Road Research Laboratory). This Report describes the results of an investigation made to find methods for the determination of the specific gravity of soils that are less subject to error than those given in B.S.1377:1967.

Part 1 of the Report shows that the present B.S. method for coarse-grained soils can be made more reliable and used for all types of soils if the ~,lr is removed from the lsoil by a shaking technique in place of removal under vacuum.

In Part 2 of the Report the use of a Loebell air pycnometer is described and it is shown that this apparatus can be adapted .for the rapid determination of the specific gravity of soils. The time taken is less than 15 minutes and the accuracy is sufficient for most purposes.

ABSTRACT

An examination of methods of determining the specific gravity of soils: J. V. KRAWCZYK: Ministry of Transport, RRL Report LR 272: Crowthorne, 1969 (Road Research Laboratory). This Report describes the results of an investigation made to find methods for ,the determination of the specific gravity of soils that are less subject to error than those given in B.S.1377:1967.

Part 1 of the Report shows that the present B.S. method for coarse-grained soils can be made more reliable and used for all types of soils if the air is removed from the soil by a shaking technique in place of removal under vacuum.

In Part 2 of the Report the use of a Loebell air pycnometer is described and it is shown that this apparatus can be adapted for the rapid determination of the specific gravity of soils. The time taken is less than 15 minutes and the accuracy is sufficient for most purposes.

road research laboratory rrl report lr 272b.s. 1377:1967 contains two methods for determining...

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