TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment

TRRL REPORT LR 584

SHEAR-BOX TESTS ON GRADED AGGREGATES

by

D.C. Pike

Materials Division Highways Department

Department of the Environment Crowthorne Berkshire

1973

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 St 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.

Abstract

I.

2.

.

.

5.

C O N T E N T S

5.2

5.3

Introduction

Preliminary work

2.1 Choice of method for shear strength test

2.2 Preliminary shear-box tests

2.3 Modified equipment

2.3.1 Shear-box machine

2.3.2 Compaction rig

2.4 Selection of experimental materials

2.5 Compaction tests

Shear-box tests

3.1 Preparation and compaction of specimens

3.2 Schedule of tests

Wet-sieve analysis

Discussion

5.1 Effects of machine variables

5.1.1 Normal stress

5.1.2 Displacement rate

Effects of materials variables

5.2.1 Compaction

5.2.2 Grading

5.2.3 Moisture content

5.2.4 Aggregate type

Application to engineering problems

5.3.1 Granular sub-bases

5.3.2 Bound materials

Page

1

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7

7

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7

13

13

13

14

14

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16

16

18

18

21

21

.

7.

8.

Conclusions

Acknowledgement

References

Page

22

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Q CROWN COPYRIGHT 1973 Extracts from the text may be reproduced

provided the source is acknowledged

LR 584

SHEAR-BOX TESTS ON GRADED AGGREGATES

ABSTRACT

As part of an investigation into the mechanical properties of aggregates to develop a rational classification system, shear-box tests have been carried out on typical, graded, road aggregates at normal stresses up to 1 MN/m 2. Non-linear relations were found between peak shear stress and normal stress, with shear strengths of various aggregate types converging at higher levels of normal stress; however an arbitrary index based on logarithmic plots plots of the results has been developed to allow the comparison of shear strength of aggregates over a range of normal stress. The effects of aggregate type, degree of compaction, grading and moisture content upon shear strength have been quantified by correlating proportional changes in dry density produced by these variables, with proportional changes in the tangent of the angle of shearing resistance (tan q~ ). The next stage of the work will be to apply these findings to the solution o f practical engineering problems: a series of pilot-scale trials to investigate the stability of unbound sub-bases has been started.

1. INTRODUCTION

A great variety of road aggregates is used in Gt. Britain; currently about 80 million tonnes each year are drawn from up to 1500 sources whose products differ widely in engineering properties and are used in several classes of road-making materials. To take account of these differences in nature and in end-use, specifications for road materials must classify aggregates. At present this is often achieved by the use of classifications based on broad trade groups. This system generally provides suitable road-making aggregates, but sometimes excludes the use of cheap and satisfactory materials which cannot be fitted easily into an arbitrary classification scheme.

If a rational scheme were substituted, based on meaningful, simple and reliable tests, savings could be expected. These savings would arise partly from reduced haulage charges if the use o f locally available materials could be increased, and it is also probable that a rational classification system would allow better use to be made of currently acceptable aggregates.

Because of the many important properties of aggregates as road-making materials, and because o f the different levels of significance attached to these properties for various end-uses, it is obvious that a whole range of tests is needed. Tests for some properties are already available, but no satisfactory method has hitherto been developed to classify aggregates in terms of their shear strengths which can be important in many aspects of the structural performance of roads. This Report describes the first stage in the development of a suitable test for shear strength of graded aggregates, namely a series of tests with a large shear-box.

2. P R E L I M I N A R Y WORK

2.1 Choice of method for shear strength test

The complexity of the stresses and strains to which road materials are subjected is well established 2' a It is extremely difficult to model these factors accurately in the laboratory. Simple mechanical tests for aggregates, usually carried.out only on single-sized fractions, have been incorporated in BS812 4 t o permit assessments to be made of relative resistance to crushing and of relative compactability etc., but these tests are by their nature, incapable of predicting the stability of the compacted, graded materials that form the bulk of the aggregates used in modern roads.

A compromise between close simulation and simplicity is offered by shear strength tests which have been developed for soils. These tests include triaxial and plane shear tests, both of which have been used widely to investigate the mechanical properties of granular materials. At the outset of this investigation it was decided to use a shear-box test because at that time, it seemed that it was simple in operation and analysis, and especially because of the known difficulties of compacting specimens containing coarse, angular particles to high levels of dry density within the rubber membranes used in the triaxial test.

2.2 Preliminary shear-box tests

As a first step, a standard 300mm square shear-box already installed at the Laboratory was used for a limited series of tests to determine whether the method could be used to distinguish between aggregates known to confer different levels of mechanical stability on both bound and unbound road-making materials. Five well-graded aggregates were compacted to refusal with a hand-held vibrating hammer of the type used in standard compact ion tests s, and were then sheared at levels of normal stress up to about 160 kN/m 2 applied by a lever-and-weight system.

The results o f these tests are plotted in Fig 1 and show that, as expected, the angular and rough-textured aggregates (crushed limestone 9 and crushed slag 17) gave higher levels of peak shear stress at all levels of normal stress than the sands and gravesl (rounded flint 1, partly crushed quartzite 5, and rounded but rough-textured gritstone 10). It seemed possible, therefore, that graded aggregates could be classified in terms of their relations between peak shear stress and normal stress.

The differences found in peak shear stress, at the highest level of normal stress (160 kN/m z) that could be achieved on the standard machine, appeared to be significant, despite some variability about the relations drawn in Fig 1. It seemed logical that these differences could be accentuated by testing at higher levels of normal stress, and it was reasoned that tests should be made at the levels of stress up to those to be found in practical conditions of road construction and trafficking. It was therefore decided to construct modified apparatus to carry out shear-box tests at normal stresses up to 1 MN/m z.

2.3 Modified equipment

The layout of the modified shear-box machine and its ancillary equipment is illustrated in Plate 1.

2.3.1 Shear-box machine

The modified shear-box machine has a maximum shear-force capacity of 250 kN and operates at normal forces of up to 100 kN applied to specimens about 300 mm square in plan, with a continuously variable range o f displacement rates between about 8 x 10 -9 and 8 x 10"s m/s. It is equipped with devices to facilitate the preparation, testing and disposal of specimens including:-

2

(a) A shear-box mould which can be moved into the adjacent compaction rig and back again along steel runners, and which is fitted with a hinged base- plate to permit the removal of spent specimens.

(b) A hydraulic normal loading system.

(c) An electronic shear-force measuring and readout system.

2.3.2 Compaction rig

Because of the need to control the dry densities of test specimens accurately and at high levels, a compaction rig was constructed to match the modified shear-box. It employs a heavy-duty, vibrating hammer having a power consumption of 1.5 kW and operating at a frequency of about 14 Hz. The hammer is attached to a double-acting, hydraulic ram which supplies a steady force of about 4.5kN to the hammer during compaction. The whole system is fixed within a rigid frame which is mounted on damping springs and located in a cabinet to reduce noise levels. Compaction is effected through a foot which occupies the whole surface area of the mould. By instrumenting the shank of this foot it has been found that the blows of the hammer develop dynamic forces of up to about 20 kN.

2.4 Selection of experimental materials

As previously described 5 ranges of standard aggregate types (17 sources) and of standard gradings (14 gradings) have been selected to represent the main types use in Great Britain. Summ~iries of petrological types and of particle shape and texture characteristics are given in Table 1, and Gradings I, II, III, X and XI are plotted graphically in Fig 2. Because aggregates from any one source can vary in properties, the selected materials should not be regarded as typical of actual production from their sources. Further, in nocircum- stances should any result given in this Report be considered as typical of materials occurring in a particular region.

2.5 Compaction tests

Because the degree of compaction of graded aggregates can markedly affect their shear strength it was necessary to set standards of dry density for the specimens to be tested in the modified shear-box. To provide estimates of the levels of dry density likely to be achieved in the field when granular materials are compacted with heavy plant, a series of BS vibrating-hammer compaction tests was carried out on the experi- mental aggregates. As has been previously reported s, it was not possible to establish immediately from the results of these tests the standard parameters of "maximum dry density" and "op t imum moisture content" because of the convex-downward shape of the compaction curves; additionally, differences in specific gravity made difficult the comparison of compactability of the standard aggregates.

To overcome these problems the results were transposed to a volumetric basis, which employs the following main terms and symbols:-

(a) Proportion of volume occupied by solids

Unit:- per cent by volume

Symbol:- V s

Given by:- the ratio of dry density to oven-dried specific gravity, multiplied by 100.

(b) Maximum proportion of volume occupied by solids

Unit:- per cent by volume

3

(c)

Unit :-

Symbol:-

Given by :- aggregate at MPVS, by the relation:-

OMC = I00 (lO0 - MPVS) + W Abs

MPVS x SpG

Symbol:- MPVS

Given by:- V s obtained from BS vibrating hammer test on dry aggregate.

Optimum moisture content

per cent by weight

OMC

the content of water required to saturate exactly a sample of compacted, graded

............... :.(1)

where SpG = oven-dried specific gravity, W Abs = water absorption value per cent

TABLE i

Sources, petrological types and descriptions of the shape and surface texture of the coarse particles of the 17 experimental aggregates

Ref Name

n o .

1 Bridport 2 Chertsey 3 Longfield

4 Branston 5 Rugeley 6 Hartshill

7 Cerney 8 Carnforth 9 Holcombe

10 Stanley Ferry 11 Scorton 12 Haughmond

13 Harehill 14 Llwyn 15 Croft 16 Clee Hill

17

General type*

G G G

G G R

G G R

G G R

G G R R

Petrological type

Flint Flint Flint

Quartzite Quartzite Quartzite

Limestone Limestone Limestone

Gritstone Gritstone Gritstone

Igneous Igneous Igneous Igneous

Description of coarse particles

Shape

Very rounded Irregular Angular

Rounded Mixed Angular

Rounded,flaky Mixed Angular

Rounded Angular Angular

Rounded Mixed Angular Angular

Texture

Very smooth Smooth Very smooth

Smooth Mixed Rough

Smooth Mixed Rough

Rough Very rough Very rough

Mixed Mixed Very rough Rough

Corby S Slag Angular Very rough

* G = sand and gravel, R = crushed rock

4

It is emphasised that estimates of OMC obtained in this way are applicable only to laboratory conditions using fairly well-graded aggregates; in full-scale conditions adjustments may have to be made; for example, unbound sub-bases and road bases may be too unstable at saturation, whereas concretes require higher moisture contents to achieve practical levels of workability.

It has been shown s that values of MPVS can be used as an index of compactability, higher values being obtained both by improving grading (i.e. by increasing coefficient of uniformity) and by decreasing angularity and surface roughness of particles. In the BS tests it was difficult to maintain a very close control over the grading of specimens, even though they had been carefully prepared because of degradation during compact ion which produced finer gradings in many cases. Also, fine aggregate was forced out of some specimens by the strong vibrating action of the hammer. A modified mould has since been produced to prevent losses of fines, and results of modified tests largely confirm the results found with the BS tests.

A detailed analysis of results of both BS and modified compaction tests (some 180 individual determinations) shows that the typical relation between logarithm of uniformity (log CU) and MPVS is more complex than is indicated in Fig 15 of LR 447 s, and can be more closely modelled by the form shown in Fig 3 of this Report. The steeper slope at lower values of CU is probably partly associated with the increasing influence of edge- effects 6 with increase in uniformity (i.e. decrease in CU).

The slopes of the parts of the typical relation shown in Fig. 3 can be used to correct values of MPVS for small errors or changes in grading to provide a common basis for comparing the effects of particle shape and texture upon compactability. This is done by using the simple equation:-

A MPVSxl = MPVSx2 - m ( l o g x 2 - 1 o g x 1) . . . . . . . . . . . . . . . . . . . . ( 2 )

A where MPVS = corrected value of MPVS

MPVS = measured value of MPVS

m = appropriate value of slope (i.e. 20, 7 or 0 )

x 1 = nominalvalue of CU

x 2 = measured value of CU

/N Values of MPVS for the experimental aggregates which were used in the shear-box tests are given in Table 2. These values are based on means of sets of five modified tests and are estimated to have a repeatability of about 1 per cent. It will be seen that the values in Table 2 are similar to those given in LR 447 s in the order given to the several aggregate types. The anomalous result achieved with the limestones was confirmed; in all other petrological groups an increase in angularity produced a decrease in MPVS.

5

TABLE 2

Results o f compac t ion tests*

Reference nos

Agg Grading

1 I 2 I 3 1 4 I 5 1 6 I 7 1 8 i 9 i

IC I II | 12 I 13 1 14 I 15 I 16 I 17 I

4 I1 4 II1 4 X 4 X1

15 II 15 III 15 X 15 XI

Measured MPVS-

per cent

90.1 87.0 85.8 92.3 88.4 87.1 88.3 89.6 89.6 88.5 85.6 84.3 88.4 87.9 84.5 84.9 86.5

80.5 92.8 72 .6 87.3

75 .0 89.2 63.6 84.7

Log CU

Nominal

1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893 1.6893

0 .9129 1.8696 0.3945 1.2471

0 .9129 1.8696 0 .3945 1.2471

: g *

Measured

1.8338 1.7343 1.8041 1.8062 1.8181 1.8062 1.7338 1.7664 1.7577 1.8196 1.7599 1.7305 1.7142 1.7391 1.7158 1.7757 1.7456

1.1073 1.8890 0.5577 1.2778

1.0786 1.8888 0.5632 1.2575

Corrected MPVS - per cent

89.1 86.7 85.0 91.5 87.5 86.3 88.0 89.1 89.1 87.6 85.1 84.0 88.2 87.6 84.3 84.3 86.1

76.6 • 92.7 69.3 86.7

71.7 89.1 60.2 84.5

NOTES: * Averages f rom sets o f 5 mod i f i ed tests ** Measured on samples af ter c o m p a c t i o n *** F o r basis o f co r rec t ion see Sect ion 2.5

6

3. SHEAR-BOX TESTS

3.1 Preparation and compaction of specimens

The specimens for the shear-box tests were prepared from bulk supplies of the standard aggregates by wet-sieving, drying and then recombining the appropriate size fractions incorporating ground silica flour as a non-plastic filler where necessary. The materials were made up in 9 or I 0 kg batches, three batches being

• used for each specimen which were compacted in three separate layers. Care was taken to avoid segregation, and to roughen the interfaces between layers to maximise homogeneity. After compaction the height of each specimen was measured, and after shear-testing the mass of each specimen was checked so that accurate values of dry density (Vs) could be obtained.

3.2 Schedule of tests

Several programmes of tests, not fully reported herein, were necessary before a satisfactory method was developed to produce reliable results. Based on these early trials a broad series of tests was then carried out according to the schedule set out in Table 3, which indicates the variables examined.

4. WET-SIEVE A N A L Y S I S

To check on the gradings of the materials used, wet-sieve analyses were carried out on all combinations of aggregate type and grading both before and after compaction and shear testing. The spent shear-box specimens were too large to wet-sieve, so they were riffled twice to give sets of four sub-samples. Average grading results are given in Table 4.

TABLE 3

Schedule of shear-box tests

Reference nos.

Test Agg. Grading

1 4 I ~ , , 9

99 ~

~9 9~

,9 9,

6 " 9'

7 " " 9, ,,

Initial moisture content

-%

Dry 9 9

9 9

9 9

} 9

Compaction time per layer- s

60 ~9

9~

9~

} ,

~ 9

~9

Nominal applied normal stress kN/m 2

1 0 0 0 30

100 300

1 0 0 0

30

Displacement rate- m/s

8 x 1 0 "s

~9

7 8x10" 8 X 1 0 -6

8x10 "7 8x 10 -6

To investigate the effects of

Normal stress

Displacement rate

7

T A B L E 3 ( C o n t i n u e d )

R e f e r e n c e n o s .

Tes t

9 10

11

12

13

1 4

15

16

Agg.

15 ~9

17 ~9

9 9

9 9

Grading

17 1 "

! 8 2 "

19 3 "

2 0 4 "

21 5 "

2 2 6 "

23 7 "

2 4 8 "

2 5 9 "

2 6 10 " 2 7 11 9'

2 8 12 "

2 9 ' t 3 "

3 0 14 "

31 15 9,

3 2 16 "

33 17 "

3 4 4 II

3 5 . . . . 3 6 " III

3 7 . . . .

3 8 " X

3 9 . . . . 4 0 " XI

41 . . . . 4 2 15 II

4 3 . . . . 4 4 " 1II

4 5 . . . . 4 6 " X

4 7 . . . . 4 8 9, XI

4 9 "

Ini t ia l

m o i s t u r e

c o n t e n t -%

Dry

C o m p a c t i o n

t i m e per

l a y e r - s

60

N o m i n a l

appl ied

normal

stress k N / m 2

30

D i s p l a c e m e n t

rate - m / s

8 x l O -s

T o invest igate

the e f f e c t s o f

9 ,

9 9

9 9

9 9

9 ,

9 9

~9

~9

9 9

5 ,

9 ~

9 9

9 9

$ ,

9 '

9 ,

9 ,

9 ,

99

9 9

9 ,

9 ,

9 9

9 9

9 9

9 9

9 9

9 9

9 9

100

3 0 0

1000

30

100

3 0 0

1000

2 0 0 9 9

, 9

~9

~9

, 9

9 9

9~

9~

9 9

9 '

9 9

~9

9 9

9~

9~

9 9

, $

,J9

N o r m a l stress,

aggregate type

" Aggregate type 9~

~ 9

9 9

~ 9

9 9

, 9

9 9

9 9

~ 9

9 9

9 ,

9 9

9 9

"J9

9 9

~9

9 9

$ 9

9 9

, 9

9"J

9 9

9 9

9 9

~ 9

9 9

, 9

9~

, $

, 9

9 9

9 9

9~

9 9

, 9

99

9 ,

3 0

1000

3 0

1 0 0 0

30

1 0 0 0

3 0

1000

30

1000

3 0

1000

30

1000

3 3 0

1 0 0 0

8 x 1 0 "s 9~

9 9

9~

9~

9 9

~ 9

9 9

9 9

9 ,

Grading , aggregate t y p e

TABLE 3 (Continued)

Reference nos .

Test Agg. Grading

50 4 I

51 . . . .

52 . . . .

53 . . . . 54 . . . .

55 . . . . 56 15 "

57 . . . . 58 . . . . 59 . . . . 60 . . . .

61 . . . . .

62 4 " 63 . . . . 64 . . . . 65 . . . . 66 . . . . 67 . . . . 68 15 "

69 . . . . ~/0 . . . . 71 . . . . 72 . . . . 73 . . . .

Initial

moisture

content -%

PMC

OMC

OMC+2

PMC

OMC

OMC+2

Dry

Compaction time per

layer - s

60

4

15

240

4

15

240

Nominal applied

normal stress kN/m 2

30

1000 30

1000

30

1000 30

1000 30

1000

30 1000

30 1000

30 1000

30 1000

30

1000 30

1000 30

1000 L

Displacement

rate - m/s

8 x 1 0 -5

To investigate the effects of

Moisture conten t ,

Aggregate type

Compact ion , aggregate type

Note: 1 Actual intial moisture contents used in Tests 50 to 61 were as follows:-

Note: 2

PMC

OMC

OMC+2

Agg 4 Agg 15

1.9

4.4

6.4

(all given in terms of per cent by dry weight)

2.3

6.4

8.4

Actual values of normal stress achieved were usually higher than nominal values (See Sect ion 5.1.1)

9

.,< [-.,

~ 3

0

i

~ 3

0

0 , . o

0 c~

c ~

I !

O 0

O 0

0

x 0

L"',. oO 0", ~ ~ '.,0 O0 ~ ~ ~

1 0

0 p~

~.~ o

Q~ 0

o;

o;

0

°~

0

0

0

0

c5

c5

c5

c5

oo ~f

o<

~ ~ 0 ~ ~ ~ ~ ~ ~ ~ 0 0 O ~ ~ ~ . .

0 0 c ~

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O 0 O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

c ~ ..=

0

0

o c6

X 0

. o

11

0

..<

0

c5

~_ c~

° ~

. 0 u'~

0

0

Ox

00

° ~

o

.<

X

0 0 0

O 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

o

0

0

. . o . ~

~ 8

.o

0 Z

12

5. DISCUSSION

5.1 Effects of machine variables

5.1.1 Normal stress

The results from the preliminary programme shown in Fig 1. have been plotted in accordance with the standard procedure given in basic soil mechanics literature to give linear relations over a limited range between peak shear stress and normal stress. I~hese relations have the general form:-

A OS = c + O n t a n ¢ . . . . . . . . . . . . . . . . . . . . . . . . ( 3 )

A where os = peak shear stress

a n = normal stress

and c = "apparent cohesion" '

¢ = "angle of shearing resistance"

c and t~ are shear-strength parameters which can be used in calculations relating to stability of soils and similar materials. The basic literature suggests that, for the materials examined in this programme, values of c should be very low, while ¢ should be not greater than about 50 degrees. Although the maximum values of c and ~b shown in Fig. 1 are both higher than expected, more or less linear relations can be drawn, and the results for the 'strongest" aggregate (1~7) appear to be diverging quite markedly from those for the "weakest" aggregate (1) at normal stresses up to about 160 kN/M 2.

An extension of these relations, by testing at higher levels of normal stress, was expected to yield and even greater difference between Aggregates 1 and 17, thus facilitating the construction of an index by which materials of intermediate shear strength could be classified, l-'lowever, as shown in Fig. 4, tests with the modified shear-box at normal stresses at about 1 MN/m 2 showed only small differencesbetween peak shear stresses measured on different aggregates, and all these shear stresses were lower than those to be predicted by extrapolation from Fig.1.

The fundamental reasons for these non-linear results have been discussed by Bishop 7, and in detail by Billam 8 and others. The behaviour of granular materials subjected to shear stresses is held to be governed by a combination of dilatancy and degradation and the balance between the influences of these two factors is itself affected by the level of normal stress and by aggregate properties, such as angularity and resistance to crushing. At low normal stresses dilatancy will generally predominate in sheared specimens of dense, graded road aggregates, whereas at high normal stresses crushing will become more pronounced. In short, the volumetric strain behaviour of materials of the type examined in this programme is complex and a full elucidation is beyond the scope of the original simple aims of this programme. A simple method of energy analysis is, however, being developed for future studies.

Experimental work conducted on sands at high confining pressures in triaxial cells by Vesic and Clough 9 also showed non-linerarity of strength envelopes, and they proposed a logarithmic law. Before considering a logarithmic solution, however, attention must be paid to the precise determination of values of normal stress. At first sight the normal stress is given simply by the applied force divided by the original cross-sectional area of the sample, but because this area diminishes with increase in horizontal displacement a correction must be made for the diminution of area at failure. Further, when a specimen dilates vertically during the shearing process it carries with its top half, the upper shear-box and other ancillary apparatus; hence the weights of

1 3

these parts must be added to the applied force, in this case supplied by an hydraulic ram. Because the modified shear-box was constructed to operate at higher stresses the weight of the robust upper box plus its ancillaries was significant (adding about 15 kN/m 2 to the nominal normal stress) especially at low levels of normal stress. Lastly, it was found that, despite the use of pressure-control devices, it was impossible to control applied normal forces accurately, again especially at low levels, and hence it was necessary to monitor these forces for each test. The values of normal stress shown in this Report have been calculated taking into account all these factors.

A selection of results is plotted in Fig.4 to show the relations found between peak shear stress and normal stress up to 1MN/m 2 ; it will be seen that, owing to the curved shape of the relations drawn, values of c much lower than those shown in Fig.1 are indicated and that values of ~b at normal stresses higher than about 200 or 300 kN/m 2 are much nearer the levels originally expected (i.e. less than about 500). These data are replotted on a logarithmic basis (log o n versus tan ~b*) in Fig. 5, and more clearly illustrates the convergence of shear strengths of different aggregate types.

Because of the non-linearity of the shear strength properties of the materials examined, the investigation of the effects of the other variables had to be carried out at both high and low levels of normal stress. In general, the results of tests at low normal stresses were more variable than the results at o n = 1MN/m 2, and a significant proportion of this variability can be associated with the difficulties in controlling normal stress that have been described.

5.1.2 Displacement rate

(Tests 1, 5, 6 and 2, 7, 8). It was found that change of displacement rate in the range 8 x 10 "7 to 8 x 10 -5 m/s made only slight differences in values of tan q~ measured at high normal stresses, and hence the fast rate was used for the bulk of the tests described. Large variations in tan q~ obtained at low normal stresses could not be correlated sensibly with changes in displacement rate and were attributed to poor repeatability (see Section 5.1.1 .)

5.2 Effects of materials variables

5.2.1 Compaction

(Tests 1, 63, 65, 67 and 2, 62, 64, 66 and 9, 68, 70, 72 and 12, 69, 71, 73).

From early trials with the compaction rig it was found that dry shear-box specimens compacted for 60s in each of three layers achieved levels o f dry density roughly equivalent to values of MPVS from BS vibrating- hammer compaction tests, and hence this compaction time was used for the bulk of the tests described herein, with the results summarised in Fig. 6. I t will be seen that equivalence is particularly good at higher levels of dry density, but the more open-textured specimens (i.e. to Gradings II and X) gave higher va1aes of Vs than those to be predicted from values of MPVS. These differences can be attributed to the greater susceptibility of the more open-textured materials to degradation and to differences in edge-effects. 6. Taken as a whole, the data shown indicate that the shear-box specimens were tested at levels of density up to and above those to be found in real conditions.

The effect o f degree of compact ion on shear strength was investigated by varying compaction time on specimens made with Aggregates 4 and 15 to Grading I, with the results shown in Fig. 7. Increases in shear strength with increase in dry density are seen for all four groups of remits, but a transformation to a common basis is needed to seek a general relation. This can be done, as shown in Table 5, by expressing the data in relative terms. This process yields data which are replottted in Fig. 8, giving a highly significant correlation** between parameters of dry density and shear strength. The slope of the regression line drawn in Fig. 8. implies

* Note: Hereafter in this Report it is assumed that c = 0 and therefore that tan ~b is given by the ratio of peak shear stress to normal stress at peak.

** Where the terms " significant" and "highly significant" appear in this Report they mean statistical significance at the 5 and 1 per cent levels respectively.

14

that an increase in dry density o f 1 per cent, produced by an increase in compactive effort, yields an increase in shear strength o f about 5per cent*. There is evidence that the slope of the relation is influenced by normal stress, and that a higher value (i.e. 7 to 8) could be expected at o n = 50 kN/m 2 : a slope of 3 to 4 is obtained by analysing the results at o n = 1MN/m 2.

1.

TABLE 5

Estimates of relative changes in dry density and shear strength

1. Changes resulting from changes in compaction time.

A From Table 2 we find MPVS49 values (i.e. to Grading I) for Aggregates 4 and 15 to be 91.5 and 84.3 per cent respectively, and by interpolation in Fig.7. we find values of tan ¢ at MPVS as follows :- 49

Agg. ref. no.

4 15

O n 50 kN/m z**

2.94 2.94

(I n 1000 kN/m 2 .*

1.82 1.59

.

Test no.

2 62 64 66

1 63 65 67

Using these interpolated values as standards* (i.e. 100 per cent relative density, 100 per cent relative strength) we can express the actual values obtained in proportional terms, by the following process:-

Agg ref.

Or * *

k~/m 2

50

1000

d e

Vs %

91.3 87.8 89.8 92.4

90.4 86.3 89.3 91.7

tan ¢

2.48 1.89 2.41 3.61

1.63 1.28 1.49 1.89

/N MPVS49

91.5

tan ¢

at A MPVS49

h=lOO(l+(d-f)) f

2.94

1.82

,7

Relative density - %

99.8 96.0 98.2

101.0

98.8 94.3 97.6

100.2

j=100(l+(e-g)) g

Relative strength - %

84.4 64.3 82.0

122.8

89.6 70.3 81.9

103.8

Note: This statement must be interpreted strictly upon the basis used for the calculations shown in Table 5.

** Approximate values

15

TABLE 5 (continued)

Test no.

9 68 70 72

i2 69 7i 73

Agg ref.

15

k°~i/m 2

50

i000

1

V s %

87.4 78.3 82.2 87.1

86.4 77.4 8!.5 85.8

e f g

tan q~ A

MPVS49 tan q~

M~'~$49

h=lOO(l+(d-f)) j=l O0(l+(e-g)) f g

Relative density - %

Relative strength- %

3.36 1.92 2.48 4.10

1.68 1.21 i .44 1.66

84.3 2.94

1.59

!

100.7 92.9 97.5

103.3

102.5 91.9 96.7

101.3

114.3 65.3 84.4

139.5

105.7 76.1 90.6

104.4

Note: The chosen standard values are not critical to the slope derived in Fig.8.

5.2.2 G r a d i n g

(Tests 1, 35, 39, 41 and 2, 34, 36, 38, 40 and 9, 42, 44, 46, 48 and 12, 43, 45, 47, 49). Because, as shown in Fig. 3, an "improvement" in grading, as represented by an increase in coefficient of uniformity, can produce substantial increases in dry density (cf Fig.3), it was expected that marked differences in shear strength would be found between "dense" (e.g. I and III) and "open" (e.g. II and X) gradings, and possibly that the changes in shear strength per unit change in dry density caused by changes in grading would be of the same order as those caused by changes in compactive effort. The results of tests made on Aggregates 4 and 15 to Gradings I, II, III, X and XI are shown in Fig 9 in terms of absolute values of Vs and tan ~b, and again in relative terms in Fig 10, based on the correction procedure given in Table 6. A highly significant correlation is found by this method between parameters of shear strength and dry density, but the slope of the relation shown in Fig. 10 is well below that in Fig. 8, and implies that for an increase o f i per cent in dry density produced by improvement in grading an increase o f strength o f only 1 to 2 per cent will be achieved. Nevertheless, substantial differences of strength (up to 30 per cent) were recorded between specimens made to extremes of grading in common use for road-making materials.

5.2.3 Moisture content

(Tests 1, 51, 53, 55 and 2, 50, 52, 54 and 9, 56, 58, 60 and 12, 57,59, 61). That moisture content can influence the dry density of granular materials is very well known, and hence it is reasonable to expect a related influence on shear strength. As previously reported 5 and summarised in Section 2.5, vibrating-hammer compaction tests carried out on the aggregates used in this investigation gave a convex-downward "compaction curves" and hence a modified method of establishing optimum moisture contents (OMC) had to be developed.. Additionally the points at which maximum bulking occurred were identified as "pessimum moisture contents" (PMC).

1 6

TABLE 6

Estimates of relative changes in dry density

and shear strength.

2. Changes resulting from changes in grading

1. Using the standard values derived in Table 5 we can now express the results shown in Fig. 9 in relative terms.

c d e f

Test Agg O n Vs tan t~ no. ref. kN/m 2 %

2 4 50 91.3 2.48 91.5 34 . . . . 83.2 2.10 " 36 . . . . 92.3 2.81 7,

38 . . . . 84.3 2.05 " 40 . . . . 87.0 2.11 "

1 1000 90.4 1.63 " 35 " 83.6 1.39 " 37 " 92.3 1.65 " 39 " 75.0 1.34 " 41 " 86.2 1.43 "

9 15 50 87.4 3.36 84.3 42 . . . . 77.8 2.78 " 44 . . . . 89.5 3.28 "

46 . . . . 64.9 2.59 " 48 . . . . 82.7 2.20 "

12 " 1000 86.4 1.68 " 43 . . . . 77.5 1.34 " 45 . . . . 90.0 1.92 " 47 . . . . 68.7 1.16 " 49 . . . . 84.4 1.66 "

/N MPVS 49 tan ~b

at/N

MPVS49

2.94

1.82

2.94

1.59

h=lOO(l+(d-f)) f

Relative

density - %

99.8 90.9

100.9 92.1 95.1

98.8

91.4

100.9 182.0 194.2

100.7

92.3 106.2 77.0

98.1

102.5

91.9 106.8

81.5 100.1

j=lOO(l+(e-g)) g

Relative

s t r eng th - %

84.4 71.4

95.6 69.7 71.3

89.6 76.4

90.7 73.6 78.6

114.3

94.6 111.6

88.1 74.8

105.7

84.3 120.8

73.0 104.4

To investigate the effects of moisture content specimens were made up from Aggregates 4 and 15 to Grading I at PMC, OMC and OMC plus 2 per cent, and tested in the normal way except that, addi t ional ly , residual moisture contents were obtained by weighing the whole specimens before and after dry ing at 105°C for 24 hours. The results of these tests are summarised in Fig. 11, which shows the characteristic shape of the compaction curves, reflected below in the relations between parameters of moistures c o n t e n t and shear strength.

17

To test these data, obtained on two aggregates at two levels of normal stress, for some general relation, the procedure employed in Tables 5 and 6 is repeated in Table 7 to give farther relative values of dry density and shear strength. These relative data are replotted in Fig. 12, and yield another highly significant correlation. It can be seen from Fig. 12 that the slope of the regression line is similar to that in Fig. 8, and hence it appears that, over the range of conditions examined, the effects of changes in moisture content upon shear strength are paralleled by the effects of compaction upon shear strength, because a change in dry density o f ] per cent produced by either variable results in a change in shear strength o f about 5 per cent.

Before leaving the topic of moisture content, attention is drawn to the slight down-turns cf the curves at the highest moisture contents drawn in the bot tom part of Fig. 12. The initially oversaturated specimens (at OMC + 2) lost considerable quantities of water during compaction, and hence pore-water pressures must have been developed during the compaction period. Because of the non-plastic nature and limited content of the fines added to the experimental aggregates, these pore-water pressures could be expected to dissipate rapidly, but the downturns in the strength curves serve as a warning that, at moisture contents at or above OMC, the relation between moisture content and shear strength may not be of the linear type discussed above, especially if higher proportions of fines were used or if these fines were plastic.

5.2.4 Aggregate type

e l l e c LS (Tests 17 and 33) After the broad pattern of the ~'~ * of machine and materials variables upon measured shear strength had been established, consideration was given to the choice of a set of standard test conditions under which to investigate the full range of 17 standard aggregate types. To avoid the problems of convergence andpoo r repeatability a nominal normal stress of 200 kN/m 2 was selected for this part of the programme. The specimens were made up dry, to Grading I, and were compacted for 60s in each layer. The results of these tests are plotted in Fig. 13, on the same basis as was used for the results already discussed, i.e. in terms of parameters of dry density versus shear strength; once more to seek some general law, these data are rendered in relative form in Table 8 and replotted in Fig. 14.

Taken as a whole, the data in Fig. 14 do not provide a significant correlation but if the results from the limestones (7, 8 and 9) are excluded, a significant correlation is achieved. The slope of the regression line drawn implies that an increase o f I per cent in dry density resulting from a change in aggregate type, produces a decrease in shear strength o f about 3 per cent. The negative slope of the relation could be expected by reference to Fig. 13 of LR 447Swhich shows that, with the exception of the limestone group, increases in particle angularity and roughness (which can be associated intuitively with increases in shear strength) produced decreases in compactabili ty. Hence the effect of aggregate type, via dry density, on shear strength is opposite to those shown for compaction, grading and moisture content.

5.3 Application to engineering problems

To make fuller use of locally available aggregates, engineers require an index of mechanical stability with values set for particular purposes (perhaps on lines similar to the PSV test 4, which is used in conjuction with a set of standards specified I for skid-resistant roads). The original concept of this programme was that the shear strength of aggregates could be assessed simply in terms of q~, but the non-linearity of the relations found between ~s and o n prevent this. A distinction can be made between aggregates of different type

by measuring tan ~b under standard conditions, allowing for the effects of changes in dry density caused by compaction, grading and moisture content which have been quantified approximately herein. It is possible to construct an index of the type shown in Fig. 15 by plotting a 'fan' of lines radially from a point o f convergence derived f rom the relations in Fig. 5, at constant increments of tan ~b chosen for convenience to span the range of results obtained. Superimposed on this index are a broad group of results which were obtained from tests on dry aggregates, to Grading I, compacted for 60s per layer. It can be seen that Aggregates 4, 15 and 17 are given roughly constant ratings in the index (i.e. 30 to 40, 50 to 60 and 70 to 80 respectively) over the whole range of normal stresses and that the results for the other aggregates are distributed in a sensible order. It is apparent f rom the index that some sands and gravels are much more stable than others, and indeed some are shown as being "stronger" than the least stable crushed rocks. The next step is to check that this method of classification can be related to practical, full-scale conditions, and to determine what

18

TABLE 7

Estimates o f relative changes in d ry densi ty and shear s t rength

3. Changes resulting from changes in mois ture con ten t

Again, using the standard values derived in Table 5, we can now express the results shown in Fig. 11 in relative terms.

b c d e

Test Agg o n Vs tan ~b

no. ref . kN/m 2 %

2 50 52 54

I 51 53 55

9 56 58 60

4 50 91.3 2.48 . . . . 88.4 2.45 . . . . 91.5 2.73 . . . . 92.5 3.02

" 1000 ! 90.4 1.63 . . . . 88.4 1.39 . . . . 91.6 1.68 . . . . 91.6 1.56

15 50 87.4 3.36 . . . . 83.9 2.85 . . . . 86.1 3.22 . . . . 86.4 2.94

12 " 1000 86.4 1.68 57 . . . . 83.4 1.37 59 . . . . 86.7 1.56 61 . . . . 86.7 1.69

A MPVS49

91.5

84.3

tan q~ a t / N

MPVS49

2.94

1.82

2.94

1.50

h=l O0(t+(d-.___O) f

Relat ive dens i ty - %

99.8 96.6

100.0 101.I

98.8 96.6

100.1 100.1

100.7 99.5

102.1 102.5

102.5 98.9

102.9 102.9

j=l O0(l+(¢-g)) g

Relative s t rength - %

84.4 83.3 92.9

102.7

89.6 69.0 92.3 85.6

114.3 96.9

109.5 100.0

105.7 86.2 98.1

106.3

1 9

4.

TABLE 8

Est imates of relative changes in dry densi ty

and shear strength.

Changes resulting from changes in aggregate

type.

1. Because Tests 17 to 33 were carr ied out at o n -----200 kN/m z a change from the standard values used in Tables 6 to 8 is necessary, and means of tan 4~ and Vs from Tests 17 to 33 are used instead.

a b

Test Agg no. ref

17 1 18 2 1 9 3 20 4 21 5 22 6

23 7 24 8 25 9

26 10 27 11 28 12 29 13 30 14 31 15 32 16 33 17

Ws %

92.2 89.9 86.5 90.7 88.8 87.9

89.7 90.5 88.7

88.9 87.2 86.4 88.6 89.3 87.4 84.9 86.3

tan ~b

2.01 2.12 2.23 2.02 2.35 2.32

2.43 2.49 2.89

2.23 2.46 2.40 2.48 2.33 2.54 2.39 2.75

Averages 6 = 88.5 ~1= 2.38

e=l O0(l+(c-~) ) C

Relative densi ty - %

104.2 101.6

97.7 102.5 100.3

99.3

101.4 102.3 100.2

100.5 98.5 97.6

100.1 100.9

98.8 95.9 97.5

f=l O0(l+(d-~l) ) -d--

Relative strength - %

84.5 89.1 93.7 84.8 98.7 97.5

102.1 104.6 121.4

93.7 103.4 100.8 104.2

97.9 106.7 100.4 115.5

2 0

minimum index values are required for particular purposes. Also, because a classification scheme of this type must be suitable for general application a simpler method of estimating index values should be sought.

5.3.1 Granular sub-bases

Sub-bases pose special problems for engineers. Unlike the materials comprising the upper pavement layers their quality, within broad limits, is relatively unimportant to the structural lives of roads, but this quality is most important during the construction phase. Sub-bases must provide stable, weatherproof platforms to carry construction traffic and subsequently to accept the laying o f the upper pavement courses.

If a good-quality sub-base is laid too thinly on a weak sub-grade deformation in the sub-grade will occur after only a few passes of heavy construction vehicles, leading eventually to disruption of the sub-base. Even on stronger sub-grades, sub-bases can fail under construction traffic if they themselves exhibit inadequate shear strength. In the case of Type 2 sub bases I which are permitted to contain a proportion of plastic fines, stability may be very high in dry weather, but prolonged rainfall can speedily reduce the shear strength of these materials because the plastic fines interfere with the mechanical interlock of the larger particles. Difficulties experienced with some Type 2 materials in winter conditions have led to a preference for Type 1 aggregates for the upper part of sub-bases laid on major road contracts.

Type 1 sub-bases can also fail if they exhibit low levels of shear strength. Even if they contain non- plastic fines, they can exhibit poor mechanical interlock if insufficient attention is paid to compaction, grading and particle shape. Experience has shown that rounded, smooth gravels cannot support the weight of heavy lorries and especially of wheeled paving machines. Because of this, all sands and gravels are currently excluded from Type 1 sub-bases.

Experience has shown, and is confirmed by the tests reported herein, that the term "sand and gravels" includes a wide range of materials, some of which exhibit much higher levels o f shear strength than others. Consideration of Fig. 15 leads to the tentative conclusion that an aggregate having a value on the shear strength index of at least 40 would be required to provide a sub-base having sufficient stability to support wheeled pavers in realistic conditions. This conclusion is based partly on field experience with materials roughly comparable with Aggregates 3 and 10, but must be validated under carefully controlled conditions. A series of pilot-scale trials designed for this purpose is under way at the Laboratory. These trials will include as variables aggregate type, compaction, grading and moisture content, to determine whether the index shown in Fig. 15 must be modified to take account of practical factors.

5.3.2 Bound materials

It is likely that some of the results of this investigation on unbound materials will also shed useful light on the properties of bituminous and cement-bound road-making mixtures because they contain, on average, 90 to 95 per cent by weight of graded aggregates. It has long been recognised that some types of bituminous mixtures made with some sands and gravels may be unstable and, more recently, it has been claimed that pavement quality concretes made with sands and gravels exhibit lower tensile strengths than those made with crushed rocks. On the other hand, provided that ~mixtures are properly designed, satisfactory and economic materials can be made with most sands and gravels for most purposes. Particular difficulties have been found with road-making materials containing flint gravels because o f the extreme smoothness of this hard mineral. The relative positions of Aggregates 1, 2 and 3 in the shear strength index shown in Fig. 15 therefore parallel field experience, and this suggests that the index could be used to judge an important aspect of the probable performance of aggregates in bound materials.

Lastly, the procedure developed for identifying optimum moisture contents of the experimental aggregates may be of use in identifying optimum binder contents for dense bituminous mixtures to replace the current recipe specifications, which have been shown to have limitations in certain respects. Further work is in hand to establish this point.

21

6. CONCLUSIONS

Using a modified shear-box it has been shown that:-

1. The relations between peak shear stress and normal stress measured on compacted, graded, non-plastic aggregates are non-linear over extended ranges of normal stress, but these relations can be expressed simply in logarithmic terms.

. Considerable differences in shear strength, between aggregates of various types were measured at low (50 kN/m 2) levels of normal stress, but these differences decreased with increase in normal stress. An arbitrary index can be constructed to ascribe values to these aggregates to order them in a scale of mechanical stability over a range of normal stress.

3. The degree o f compaction, grading and moisture content of aggregates all influence their shear strength; the scale of influence can be assessed via the relations between changes in density caused by these variables and a parameter o f shear strength (tan q~). An increase in dry density of 1 per cent caused by an improvement in grading leads to an average increase of shear strength of I to 2 per cent but the same change in dry density caused by increased compactive effort or optimising of moisture content leads to an average increase of about 5 per cent in shear strength.

. An increase in particle angularity or roughness generally leads to a decrease in dry density at a given level o f compactive effort but also to an increase in shear strength at low and intermediate levels of normal stress. A 3 per cent increase in strength was found for a i per cent decrease in dry density attributable to an increase in angularity. Limestones as a group, however, exhibit higher levels of compactabili ty (and hence of shear strength) than would be expected from simple measurements of the angularity of their coarser particles.

. A necessary next stage of the work will be to relate the findings of this laboratory-scale investigation to the solution o f practical engineering problems; a series of pilot-scale trials to investigate the stability of unbound sub-bases has been started.

7. ACKNOWLEDGEMENT

This Report was prepared in the Materials Division (G F Salt, Division Leader) of the Highways Department. The work described forms part of the programme o f the sand and Gravel Association's Co-operative Research Team. The research team included S.M. Acott, N. Mills, R.D.C. Potter, A. Singh and M.V. Wells. The shear-box machine was designed and constructed under the supervision of D.A. Smyth, formerly of the Experimental Equipment Section.

8. REFERENCES

1.

.

.

DEPARTMENT OF THE ENVIRONMENT. Specification for road and bridgeworks, London, 1969

(Her Majesty's Stationery Office).

LISTER, N.W. The transient and long-term performance of pavements in relation to temperature. Proceedings of the 3rd International Conference on the Structural Design of Asphalt Pavements. September 1972. University of Michigan. Vol. 1, p. 96, Table 1.

THROWER, E.N., N.W. LISTER and J.F. POTTER. Experimental and theoretical studies of pavement behaviour under vehicular loading in relation to elastic theory. Proceedings of the

22

.

.

.

.

8.

.

3rd International Conference on the Structural Design of Asphalt Pavements, September 1972. University o f Michigan, Vol. 1, pp 521-35.

BRITISH STANDARDS INSTITUTION. Methods of sampling and testing of mineral aggregates, sands and filler. British Standard BS 812: 1967. London, 1967 (British Standards Institution).

PIKE, D.C. Compactability of graded aggregates 1. Standard laboratory tests. Department of. the Environment TRRL Report LR 447 Crowthorne, 1972 (Transport and Road Research Laboratory).

LEES, G. Influence of boundary effect on the packing and porosity of granular materials. QJEngng Geol 1969, 2 pp. 129-47.

BISHOP, A.W. The strength o f soils as engineering materials Geotechnique Vol. 16, 2, pp 91-130.

BILLAM, J. Some aspects of the behaviour of granular materials at high pressures. Proceedings of the Roscoe Memorial Symposium, 29th - 31st March 1971, Cambridge University, pp 69-80. Henley-on-Thames, 1972 (G.T. Foulis and Co.).

VESIC, A.S. and G.W. CLOUGH. Behaviour of granular materials under high stresses. Journal of the SoilMechanics and Foundations Division, ASCE, Vol 92, SME3, Proc. Paper 5954, May 1968, pp 661-688.

23

A

O =

E z v

t ~

500

450

400

350

300

250

200

150

100

50

O

O Aggregate 1 • Aggregate 5 A Aggregate 10 • Aggregate 9 [] Aggregate 17

Sources are given in Table 1

100

Normal stress (kN/m 2)

200 300

Fig.1. R E S U L T S F R O M P R E L I M I N A R Y S H E A R - B O X T E S T S

®'E _~ N

0-05 0.10 0.20

90 A

E

80 t ~

70 o~

~' 6o

50 O

O

~- 40 £ Q .

"~ 30

~ 20

lO

o

Primary group

Secondary group

100 0"50

Xl

; I 0 ~ 1 " "nm .075 5 0

33. No. 200 1100

, /

30

52

1.0 2.0

/

/

J /

J

5-0

I

4

10 mm

33. No. 72 36 18 10 ~in }in { in I I I I I I I

20

7/I 71 rl /

38

1½

1 in I

40

F i g . 2 . S T A N D A R D G R A D I N G S I, I I , I I I , X A N D X l , S H O W I N G W I D E R A N G E

COVERED BY THESE D I S T R I B U T I O N S

A

E)-

o

O

¢ ) . o

E

E

I "

0 0

O •

oJ

Regression lines ~ -

O

0 O

O

O

O

O

O

@ O

Notes

1 .0 = BS tests, • = modif ied tests

2. Best f i t t ing s lopes: -

(a) CU<20, m-~ 20

(b) 20<CU<100 , m-~7

3. Insuff ic ient data to deduce slope at CU>100.

4. Relations shown are based on the results o f 180 tests on Aggregates 4, 11, 15 and 1 reduced to a common basis.

5. Relations shown are sign- i f icant at the 1 per cent level.

6. Results at low levels of CU are probably very dependent on e~ condit ions.

O

O

20 50

Coef f ic ient o f un i fo rm i t y (log scale)

100 200

Fig.3. E S T I M A T E S OF SLOPE OF RELA] ION B E T W E E N LOG CU A N D M P V S

1600

E

u ~

1400

1200

1000

800

600

I

400

200

0 ,t

0

1800

//

O Aggregate 4 [] Aggregate 15 Z~ Aggregate 17

400

Normal stress (kN/m 2)

800 12o0

Fig.4. INFLUENCE OF NORMAL STRESS UPON PEAK SHEAR STRESS

4"0

O

II

E

o~

e -

l l

;10 z

+. ,

3-0

2.0 i

1.0

20 30

A

5'0

A ¸

[] o~

O Aggregate 4 [] Aggregate 15 A Aggregate 17

100 300

Normalstress, log scale(kN/m 2)

1000 3000

Fig.5. DATA FROM Fig4. REPLOTTED ON A LOGARITHMIC BASIS

100

95

A

t -

8 90

Q . v

Q.

N 85

Q.

E 8 80

E

'9 X 0

-Q 7 5 J

Regression line slope = 0.77 intercept = 20-8 Corre lat ion coef f ic ient

70

65,

= 0.94

60

60

Fig. 6.

70 80

MP~S - f rom compaction tests (per cent)

90

RELATION BETWEEN COMPACTABILITY OF AGGREGATES USED IN

SHEAR-BOX TESTS AS PREDICTED BY VALUES OF MPVS AND

ACTUAL VALUES OF Vs OBTAINED AT 60s COMPACTION PER LAYER.

RESULTS FROM TESTS 1 TO 49.

f-

4.5

4-0

3"5

3-0

2.5

2.0

1.5

1.0

0.5

I I

Approx normal stress O 50 kN/m 2

Aggregate 4 • 1000 kN/m 2

Aggregate 151~ 50kN/m2 1000 kN/m 2

76

Fig.7.

80 84 88 92

Vs (per cent)

THE INFLUENCE OF COMPACTION ON SHEAR STRENGTH- ABSOLUTE VALUES OF TAN 0 AND Vs.

150

A ¢-

8 Q.

> <O-

"6 ¢-

O

O t~ o ¢L

140

130

120

110

100

90

80

70

60

50

/

Regression line slope = 5.3 intercept = - 4 2 9 correlat ion coeff ic ient = 0.84

/ /

/ )

I o

O

,/. i /

O = Results where O-n -~ 50 kN /m 2 • = Results where O-n ~- 1 MN/m 2

90 95 100

Vs as propor t ion o f MPVS49(per cent)

105

Fig.8. DATA FROM Fig.7 COMBINED ON A PROPORTIONAL BASIS TO QUANTIFY INFLUENCE OF COMPACTION ON SHEAR STRENGTH (cf TABLE 5)

p-

4"0

3"5

3"0

2"5

2"0

1"5

1"0

0"5

[3

i

J

7

t

[ ] J [ ]

O

O °Oo7

m

Approx. normal stress

I O 50 kN/m 2 Aggregate 4 • 1000 kN/m 2

15 t [] 50 kN/m 2 Aggregate • 1000 kN/m 2

62 70 78 86 94

Vs (per cent)

Fig. 9. EFFECT OF GRADING ON SHEAR STRENGTH- ABSOLUTE VALUES OF TAN 0 AND Vs

115

110

105

100 A +~

C

e.I

i f ) > 95

e -

c o

o

o

90

85

80

75

70

[ ]

Regression line

Slope = 1.3 Intercept = - 3 3 . 5 Correlat ion coeff icient [ ] = 0.65 I

/ i /

O

O

/ O

[]

O

1-1

/

Approx . normal stress

f O 50 kN/m 2 Aggregate 4 1 • 1000 kN /m 2

Aggregate 15 i [ ] 50 kN/m 2 t l l 1000 kN /m 2

75

Fig. 10.

85 95 ^

Vs as proport ion of MPVS49~per cent)

105

DATA FROM Fig.9 COMBINED ON A PROPORTIONAL BASIS TO QUANTIFY INFLUENCE OF GRADING ON SHEAR STRENGTH (cf TABLE 6)

96

A

O-

92

88

84

80

-••'--• t l Aggregate 4~OS0kN'~ 2r°ss r

IG 1000 kN/m 2 )[3 50 kN/m 2

Aggregate 15 ~== 1000 kN/m 2

-4 0 4 8 12

Proportion of volume occupied by free residual water (per cent)

16

F-

4-0

3"0

2"0

1"0

J J

-4

Fig. 11.

0 4 8 12

Proportion of volume occupied by free residual water (per cent)

THE EFFECT OF CHANGE IN MOISTURE CONTENT ON DRY DENSITY AND SHEAR STRENGTH-

ABSOLUTE VALUES OF TAN ~) AND Vs

16

115

u

O.

>

r -

"6 o

O. o Q.

F-

110

105

100

95

90

85

80

75

70

90

Regression line

Slope = 4.7 L Intercept = -372 /~ Correlation coefficient /

= °7 i \ o

!

I D I •

I . I e I

0 0

Approx. normal stress

Aggregate 4 ~O 50 kN/m 2 • 1000 kN/m 2

~E] 50 k N/m 2 Aggregate 15 t [] 1000 kN/m 2

"1 , l

95 100

as proportion of MPVS49(per cent) Vs

105

Fig. 12. DATA FROM Fig.11 COMBINED ON A PROPORTIONAL BASIS TO QUANTIFY INFLUENCE OF MOISTURE CONTENT ON SHEAR STRENGTH

(cf TABLE 7 )

%

o o c~

3"00

2"80

2"60

41

2.40

I..-

2"20

2"00

8 4

Slag

/

I I

Igneous Rocks

\ L 1:3

\ \

Limestones

z~

Gritstones I Q

Flints

Quartzites

I L L I L 86 88 90 92

Ms (per cent)

Fig. 13. RESULTS OF STANDARDISED SHEAR-BOX TESTS ON THE 17 STANDARD AGGREGATES TO GRADING 1

125

A

Q.

~6

r -

E

t -

.9

0 C). o Q.

- I

a)

120

115

110

105

100

95

90

85

80 90

Fig. 14.

Regression line (excluding limestones) f ~ Slope = --2.7 " Intercept = 368 Correlat ion coeff ic ient = 0-71

• Limestone 0 Others

0 o

o

95 1 O0

Measured Vs as pr()~ort ion o f mean Vs (per cent)

105

DATA FROM Fig. 13 COMBINED ON A PROPORTIONAL BASIS TO

QUANTIFY INFLUENCE OF AGGREGATE TYPE ON SHEAR STRENGTH

(cf TABLE 8)

A 0

LJ

E

o

0

II

~1 ° Z

0

4"00

3"50

3"00

2"50

2"00

1 "50

' ~ 1. Origin of index has co- i ~ ~ I I ordinates of 1500, 1"50

Radials are spaced at intervals in tan ~ of 0.25 at O-n = 100 kN/m = \ ,m0ers, c,rc, give index values

3. Unringed numbers are I aggregate reference

I I I numbers

17 4. See text, for test conditions

' \ J I

@ J I I

20 30 100 300

0"5, log scale (kN/m 2)

1000 3000

F ig .15 A N A R B I T R A R Y I N D E X OF S H E A R S T R E N G T H S

t¢6 g

7 ~

El

_ IIIIIIII I " ~ IIIIIIII ]

~ ~ Iit11111 i ( II111,111 t ~ IIIIitlt I IIIII1#1 I -~

111111111 ~. IIIIIIII I IIIIIIII I

7

. _

0 0

> i

E ~ - o c~

> ~ x a

. . Q

r- ¢ - " - -

( " 9 C r--

F-- "~ ~ < ~ ' r -

i . . ° - -

< 8

O 7

(1548) Dd635221 3,500 8/73 HP Ltd., So ' ton G1915 PRINTED IN ENGLAND

A B S T R A C T

Shear-box tests on graded aggregates: D.C. PIKE, Department of the Environment, TRRL Report LR 584: Crowthorne, 1973 (Transport and Road Research Laboratory). As part of an investigation into the mechanical properties of aggregates to develop a rational classification system, shear-box tests have been carried out on typical, graded, road aggregates at normal stresses up to 1 MN/m 2. Non-linear relations were found between peak shear stress and normal stress, with shear strengths of various aggregate types converging at higher levels of normal stress; however, an arbitrary index based on logarithmic plots of the results has been developed to allow the comparison of shear strength of aggregates over a range of normal stress.zThe effects of aggregate type, degree of compaction, grading and moisture content upon shear strength have been quantified by correlating proportional changes in dry density produced by these variables, with proportional changes in the tangent of the angle of shearing resistance (tan ~b). The next stage of the work will be to apply these findings to the solution of practical engineering problems: a series of pilot-scale trials to investigate the stability of unbound sub-bases has been started.

A B S T R A C T

Shear-box tests on graded aggregates: D.C. PIKE, Department of the Environment, TRRL Report LR 584: Crowthorne, 1973 (Transport and Road Research Laboratory). As part of an investigation into the mechanical properties of aggregates to develop a rational classification system, shear-box tests have been carried out on typical, graded, road aggregates at normal stresses up to 1 MN/m 2. Non-linear relations were found between peak shear stress and normal stress, with shear strengths of various aggregate types converging at higher levels of normal stress; however, an arbitrary index based on logarithmic plots of the results has been developed to allow the comparison of shear strength of aggregates over a range of normal stress. The effects of aggregate type, degree of compaction, grading and moisture content upon shear strength have been quantified by correlating proportional changes in dry density produced by these variables, with proportional changes in the tangent of the angle of shearing resistance (tan ~b). The next stage of the work will be to apply these findings to the solution of practical engineering problems: a series of pilot-scale trials to investigate the stability of unbound sub-bases has been started.