TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment

TRRL LABORATORY REPORT 714

THE PARTICLE-SIZE DISTRIBUTIONS OF DEBRIS PRODUCED DURING TUNNELLING TRIALS

by

A F Toombs

R A Snowdon

and

M P O'Reilly

Any views expressed in this Report are not necessarily those of the Department of the Environment

Tunnels Division Structures Department

Transport and Road Research Laboratory Crowthorne, Berkshire

1976 ISSN 0305-1293

Abstract

1.

2.

3.

4°

5.

6.

7.

Introduction

Experimental procedure

Discussion of results

3.1 Square chisel picks

3.2 Disc cutters

3.3 General considerations

Conclusions

Acknowledgements

References

Appendix

CONTENTS

Page

1

1

1

2

2

3

3

4

4

5

6

© CROWN COPYRIGHT 1976 Extracts f rom the text may be reproduced, except for

commercial purposes, provided the source is acknowledged

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

THE PARTICLE-SIZE DISTRIBUTIONS OF DEBRIS PRODUCED DURING TUNNELLING TRIALS

ABSTRACT

This report presents data on the particle-size distributions of debris and the amounts of energy expended while cutting chalk with drag picks and disc cutters during the tunnelling trials at Chinnor, Oxfordshire. Samples of debris were sieved and generalised relations were found between the coarseness index and the specific energy values. However there was considerable scatter in the results much of it attributable to variations in rock strength. Because of this only primary trends could consistently be distinguished and it was concluded that a debris-size distribution approach is unlikely to provide a useful method of assessing tunnel boring machine performance.

1. INTRODUCTION

In tunnelling there are considerable advantages in being able to select and use t ypesand arrangements o f

rock-cutting tools which expend as little energy as possible in excavating the tunnel. Laboratory studies have

shown that the particle-size distributions of debris and the amount of energy used in cutting are related 1'2'3

Thus, in a given rock formation, cutting systems using the smaller amounts of energy would be expected to

produce coarser debris than systems using the greater amounts of energy 4 '5 '6 '7 . The particle-size of debris

produced in tunnel excavation is also required for the design and selection of face-mucking and conveyor

belts and may also be important if they are to be used as fdl material in earthworks.

This report describes the particle-size distribution measurements made during the tunnelling trials in

Lower Chalk at Chinnor, Oxfordshire 8. The cutting head of the 5m diameter tunnel boring machine used

was designed so that either picks or discs could be fitted at a number of spacings; the cone angle on the head

could be varied from 0 ° to 9 °. The forces on cutting tools, the power consumption, head speed and rate o f

machine advance were recorded during the trials 9 on a computer-based data logging and processing system 10.

2. EXPERIMENTAL PROCEDURE

Cutting tests were carried out as detailed in Fig 1. The cutting tools under investigation were square chisel

picks 25, 50 or 75mm wide with rake angles of 15 ° or 30 ° (Plate 1) and disc cutters of 280mm diameter with

a 90 ° cutting edge angle (Plate 2).

The cutting tools, either picks or discs, were mounted on the variable coned head in a single spiral

array.

The debris samples were collected immediately behind the head of the machine on the primary

conveyor. This procedure was adopted to avoid the possibility of further breaking-down of the larger debris

sizes at the conveyor transfer points. The machine was operated with the intended tool array and at a pre-

determined rate of advance. When the required cutting data had been obtained, the conveyor was stopped,

and the head" rotated one revolution in order to obtain a sufficient quantity of debris from which to sample.

A representative sample of approximately 20 to 30 kg was gathered in a polythene bag, and labelled for

identification purposes.

In most cases, the whole sample was used to determine the particle-size distributions. The British

Standard method of dry sieve analysis 11 was used throughout, using 75, 37.5, 20, 10, 5 and 1.2mm British

Standard sieves of 450ram diameter. The sample was air-dried before sieving, and the sieving procedure

adapted to take into account the friable nature of the chalk. The confined space of the tunnel made it

difficult to obtain truly representative samples and while every care was taken in handling the material it is

almost inevitable that some degradation of the larger particles, particularly of the weaker chalk, occurred in

the machine head prior to their ejection onto the primary conveyor.

Values of the energy used to excavate a unit quantity of chalk, (specific energy) were obtained from the

data collection and processing system 10 after allowing for the power absorbed to rotate the head and in the

muck disposal system.

The strength of the chalk was measured at intervals as the tunnel progressed using an impact

penetrometer 12. The mean penetration strengths along the tunnel line are shown in Fig 1. It will be noted

that the experiments with picks and discs were carried out in chalk having a mean strength of about 2.5MN/m 2

and 2.0MN/m 2 respectively. Details of rock discontinuties were also recorded.

In analysing the results it is convenient to express the particle size distribution of each sample of debris

as a single number and the coarseness index introduced originally by Barker (1964) has been used for this

purpose 1. The coarseness index is calculated by summing the cumulative percentage by weight retained on

each sieve and the percentage passing the smallest sieve i.e. 1.2mm in this instance. With the seven fractions

used for the calculation a minimum value of 100 would indicate that all the sample passed the 1.2ram sieve

whilst a maximum value of 700 indicates that all the material was retained on the coarsest sieve i.e. 75mm in

this instance. Examples of the calculation of coarseness index are given in the Appendix.

The 'bulking factor '13 was determined by weighing a known volume of the sample and comparing the

loose density calculated from this with the in-situ density of the chalk taken to be 1.90Mg/m 3. The bulking

factor ranged between 1.4 and 1.6 for the samples tested and compares well with the value of 1.5 commonly

used in earthmoving and quarrying to estimate the volume of loose material for haulage after excavation.

3. DISCUSSION OF RESULTS

3.1 Square chisel picks

Fig 2 shows all the data obtained on coarseness index related to the corresponding values of specific

energy obtained in the trials using square chisel picks. The results show that each cutting head configuration

produced reasonably defined zones of results and show a definite trend for the coarseness index to increase

i.e. the particle-size distribution to become coarser, as specific energy decreased. However the correlation

2

coefficient, r value, - 0.61, of the regression line is rather low 14. To explain some of the scatter those results

where the mean chalk strength determined by the measurements with the impact penetrometer was 2 and

3MN/m 2 were plotted (Fig 3). The regression lines obtained show r values of - 0.94 and - 0.89 respectively

indicating good correlation between coarseness index and specific energy when allowance is made for rock strength.

Figs 4 to 6 show plots of coarseness index against depth of cut for pick widths of 25, 50 and 75mm

and head cone angles of 0 ° and 5 °. There is some slight tendency in all the plots for the coarseness index to

increase slightly as the depth of cut increases but the scatter of the results is considerable in chalk with a

range in average impact penetration strength values at the face of 1.5 to 3.3MN/m 2. This poor correlation is

borne out by the low r values ranging from 0.25 to 0.71 for the data shown in Figs 4 to 6. The summary of

regression lines on Fig 7 does again show, however, that coarseness index does separate out the different

cutting configurations although there is much overlapping of the individual results.

The relations obtained between coarseness index and tool width for ratios of tool spacing to depth of

cut (s/d) of 1.5, 2.0,420 and 6.0 are shown in Fig 8 (extrapolated points have been calculated from the

regression lines shown on Figs 4 to 6). These show that the 25mm wide picks produced an average debris

having a lower coarseness index than either the 50 or 75mm wide picks for any of these four s/d ratios.

Typical particle-size distribution curves, determined for the debris produced by two cutting head

configurations considered to have high and low cutting efficiencies respectively, are shown in Fig 9, and

photographs of these samples are shown in Plates 3 and 4. They show that approximately 25 percent of the

debris produced by the efficient cutting system consisted of particle-size greater than 75mm, whereas the

less efficient system produced a debris, all of which passed the 75mm sieve.

3.2 Disc cutters

All the data obtained on coarseness index and the corresponding values of specific energy obtained in

the trials using 280mm diameter disc cutters is shown in Fig 10. The relation between coarseness index and

specific energy is ill defined (r = - 0.37 and - 0.11) with the closer spacing, (smaller pitch) between discs

using the least energy but producing coarseness index values almost identical in range with the larger pi tch.

Fig 11 shows plots of coarseness index against depth of cut for the two pitches between disc cutters.

The correlation is poor (r = - 0.27 and 0.36) and is unexpectedly negative in one of the cases.

3.3 General considerations

A number of laboratory studies of rock-cutting have investigated relations between the particle-size

distributions of debris and the efficiency of the rock excavation process. Barker (1964) 1 and Roxborough

and Rispin (1973) 2 showed that reasonable agreement exists between the energy expended in breaking out

rock and the coarseness index of the debris produced in the laboratory and the indications were that the

method provided a means of determining the optimal tool depth - spacing arrangements. More recently

Rad and Olson (1974) 3 have demonstrated relations between disc spacing and particle-size distributions in the

laboratory. However the variability of individual results in their investigations was quite large and variations of

10-20 percent in the percentage of debris retained on an individual sieve was commonplace. There is some

indication too that the correlation between debris-size distribution and the efficiency of rock excavation was

stronger when cutting was carried out using picks rather than discs.

3

The breaking-out of rock particles from the solid with metal tools is a complex process and energy is

expended in a number of operations 6'15. There are considerable similarities in the comminution process

for industrial purposes 16. There it has been suggested from energy balance considerations that practically

all the energy is expended in shearing and frictional processes; and once the depth of cut begins to exploit

the inherent inhomogeneities of the material and particle size exceeds a few microns, specific energy decreases

as particle size increases. In rocks there will also be discontinuities on a macro-scale by comparison with the

above but often of the same order of size as the cutting tools. Fig 1 shows that in the sections being considered

here the mean discontinuity spacing varied from 0.12m to 0.08m while the maximum width of tool used was

0.075m (75ram); this factor alone might well explain the smaller than expected improvement obtained with

the 5 ° coned head using the 50 and 75mm wide chisel picks.

Because of this complexity only empirical relations have been found to exist between debris-size

distributions and the energy expended in producing them and no method has yet been devised whereby

acceptable or optimal particle-size distributions of debris could be predetermined for a given tunnelling

situation 6. Given this the method can only be used in a comparative manner to indicate whether one

particular cutter arrangement is better or worse than another. The present investigation has shown that

considerable scatter occurs in the debris-size distribution produced during actual tunnelling and that

considerable additional.investigations of rock strength and discontinuity spacing is required before meaningful

interpretation o f t h e data is possible (see Figs 2 and 3): even when this is done only primary trends can be

distinguished and it is not possible for example to determine the optimal depth of cut for a given cutter

arrangement.

4. CONCLUSIONS

The investigation showed that the relations found in the laboratory between the particle-size distribution of

debris and the amount of energy use in cutting could also be found in the field. The range of particle-size

distribution curves and consequently the scatter of coarseness index values in substantially the same operating

conditions was large enough to preclude the use of the method as a simple and practical means of pre-tuning

the cutter arrangement and operation on a tunnelling machine in the working situation. Changes in the size

of cutting-tool or the cone angle of the head of the tunnelling machine were needed before the coarseness

index values were able, in some instances (e.g. 25mm wide chisel picks on 0 ° and 5 ° coned heads) to clearly

distinguish between the efficiency of the cutter arrangements. In the weaker rocks at least there would seem

to be little prospect of this line of at tack providing a useful means of improving tunnel boring machine

performance. In harder rocks the position for picks might differ but the evidence from laboratory tests on

disc cutters is not encouraging.

5. ACKNOWLEDGEMENTS

The work described in this report was carried out in the Tunnels Division of the Structures Department of

the TRRL.

The authors wish to thank all those who assisted in the trials in particular the Consulting Engineers,

Mott , Hay and Anderson, the main contractors Sir Robert McAlpine and Sons Ltd and the quarry owners

t he Rugby Portland Cement Company Ltd.

The research team consisted of G Alderman, R A Snowdon, J Temporal, D M Tilley and A F Toombs.

4

1.

2 .

.

6. REFERENCES

BARKER, J A. A laboratory investigation or rock cutting using large picks. Int J of Rock Mech. Mining Sciences, Vol 1, Pergamon Press, 1964.

ROXBOROUGH, F F and A RISPiN. The mechanical cutting characteristics of the Lower Chalk. Tunnels and Tunnelling, January/February 1973, pp 45-67.

RAD, P F and R C OLSON. Tunnelling machine research. Size distribution of rock fragments produced by rolling disc cutters. US Bureau of Mines Report of Investigations, 7882 (Washington) 1974.

4. ALLINGTON, A V. Rock machining. The Mining Engineer, August 1969, pp 654-660.

5.

.

7.

.

.

10.

11.

12.

GAYE, F. Efficient excavation. Tunnels and Tunnelling, January/February 1972.

HUGHES, H M. Some aspects of rock machining. Int. J of Rock Sciences, Vol 9, Pergamon Press, 1972.

RAD, P F. Correlation of laboratory cutting data with tunnel boring machine performance. US Bureau of Mines Report of Investigation, 7883 (Washington) 1974.

HIGNETT, H J and J B BODEN. Chinnor tunnelling trials - background and progress. Tunnels and Tunnelling, Vol 6, No 6, pp 65-70.

SNOWDON, R A. Machine performance data recorded during the Chinnor tunnelling trials. Department of Environment, TRRL Supplementary Report 196UC. Crowthorne, 1975. (Transport and Road Research Laboratory)~

NEW, B M and J TEMPORAL. A tunnelling machine data acquisition and processing system. Int.

Symposium, Tunnelling.London, 1976. (Institution of Mining and Metallurgy).

BRITISH STANDARDS INSTITUTION. British Standard No 1377 " 1975. Methods of testing soils for civil engineering purposes. London, 1975.

HUDSON, J A and S D DREW. An impact penetrometer for assessing the cuttability of soft rock.

Department of the Environment, TRRL Report LR 685, Crowthorne 1975. (Transport and Road Research Laboratory).

13. HAYIR, J et al. A simple method of determining the coefficient of loosening of rocks. Int J of Rock Mech. Mining Science, Vol 8, pp 97-103, Pergamon Press, 1971.

14. MORONEY, M J. Facts from Figures. Pelican Book A236, Penguin Books, England.

15. OSBURN, M J. Wear of rock-cutting tools. Powder Metallurgy, 1969, Vol 12, No 24, pp 471-502.

16. WALKDER, D R and M C SHAW. A physical exploration of the empirical laws of comminution. Mining Engineer, March 1954, pp 313-320.

5

7. APPENDIX

7.1 Calculation of coarseness index

The values of coarseness index determined during the investigation were calculated for each of the debris

samples as shown in the following examples:

Sieve aperture

Total weight of sample 24797 g

Weight retained in

sieve

Cumulative weight retained

mm g g

75.0 37.5 20.0 10.0

5.0 1.2

Passing 1.2

3860 10411

4658 2685 1750

807 556

3860 14341 18999 21684 23434 24241 24797

Cumulative percentage

retained

15.6 57.7 76.6 87.4 94.5 97.8

100.0

Coarseness index = Sum of cumulative percentage retained 529.6

Total weight of sample 22276 g

Sieve aperture

m_rn

75.0 37.5 20.0 10.0

5.0 1.2

Passing 1.2

Weight retained in

sieve

0 3215 4557 4693 4968 3159 1684

Cumulative weight retained

0 3215 7772

12465 17433 20592 22276

Cumulative percentage

retained

0 14.4 34.9 56.0 78.3 92.5

100.0

Coarseness index = Sum of cumulative percentage retained 376.1

6

~ ) ~ ) o o o o o 0 I ~ 1.0 1.0 k.e) ~ 0

._m

~ " o o r- C,) (~,) 0

o m

--~ o

~ ~ ~ EE cO rr m EE

~" E E E E E E E E 5 ~ o ° I E E E E E E E E I

0 I,D 1,0 tZ) 1.0 0 0 0 Z m ~ r - r - - ~ m m m Z :

~ : I---

r r E E ~ EE

o,~ ~ ~ o ~ o ~ o ~ ~ " "

o ~ o~. '-o (") m • .~. r~ : 5 :

0 U.I 0 0 0

I.- "1- I.-

Z 0 I.

i i i o0

O

A

o

d e

A

o d v O

A O o d

d e

o .E d •

¢ -

8 E

o

d O

I I o o ~ e4

A co o

c- o

0

o

J ¢.o o c',,,i

I I I I I I

- - 0

o0

co

co

I o

¢.,o 00

co

A

E

o..

E .o

Q;

r "

z ~ c.)

0

(.0 ¢.0

CO

GO

1.0

Z 0 I -

I--

I.LI

Z

I.LI " I- I - n,- 0 I.i..

I.LI

Z 0 I -

n "

1.1= Z 0

I ,LI " r

Z !-'- I - -

i i o f,/';

I - IJ.I a

° ~ M,.

(~W/NIAI ) q).6UgJ:l .S u 0 ! : l . g J ~ a u e d

700

X Q~ "ID

(M c"

O

6 0 0

5 0 0

4 0 0 "

300

0

r = - -0 .61

,

0 000Z~ &°A Z

O0 Z~ O~ • 0 A •AA~

"A 0 • • \

0 50ram picks -- 0 ° cone Z~ 50ram picks -- 5 ° cone • 25mm picks -- 0 ° cone • 25ram picks -- 5 ° cone

Z~ A

Z~

Z~

0 A •

1.0 2 .0 3.0

Spec i f i c energy (M J i m 3 )

Fig. 2 R E L A T I O N B E T W E E N C O A R S E N E S S I N D E X A N D S P E C I F I C E N E R G Y

W H E N U S I N G S Q U A R E C H I S E L P ICKS

X

" O

t -

C.3

700

600

500

4 0 0

300

Mean cha lk s t reng th 2 . 0 M N / m 2

r = - 0 . 9 4 • o "1

O 50mm picks -- 0 ° cone A 50mm picks -- 5 ° cone • 25mm picks -- 0 ° cone • 25mm picks -- 5 ° cone

Mean cha l k s t r eng th 3 . 0 M N / r n 2 ~ ~ r = - - 0 . 8 9

0 1 . 0 2 . 0

Spec i f i c energy ( M J / m 3 )

3 . 0

Fig. 3 T H E E F F E C T O F C H A L K S T R E N G T H O N T H E R E L A T I O N S B E T W E E N COARSENESS I N D E X A N D S P E C I F I C E N E R G Y

700

600

500

¢ -

400 0

300

700

r = 0.25

"b z " "

Cone angle= 5 °

f = D

I I

10

L

15 20 25

Depth of cut (mm)

I I I 5 4 2 1.5

Space/depth ratio

30

I 0 . 8

X

c -

¢ -

600

500

400

300

r = 0.40 Cone angle =0 ° - -

0 0 ( ) •

• • A • w

5 10 15 20 25

Depth of cut (mm)

I I I I

3 2 1.5 1

Space/depth ratio

30

I

0.8

Fig. 4 R E L A T I O N S BETWEEN COARSENESS INDEX A N D DEPTH OF CUT WHEN USING 25mm PICKS A T 25mm SPACING A N D

CONE A N G L E S OF 0 A N D 5 DEGREES

X

"(3 c-

¢-

~o

O c.)

X

"O ¢-

. ~

Q~ c

700

600

500

.400

300

700

600

500

400

300

r = 0.51

L - - - - - - - - I

Cone angle = 5 °

i r e - "

10

5 4

15 20 25

Depth of cut (mm)

30

I I I

3 2 1.5 Space/depth ratio

35

r = 0.59

q

W v

Cone angle = 0 °

10 15 20 25 30 35 40

Depth of cut (mm)

I I I I I

4 3 2 1.5 1.2

Space/depth ratio

45 50

I

1.1

Fig. 5 RELATIONS BETWEEN COARSENESS INDEX AND DEPTH OF CUT WHEN USING 50mm PICKS AT 50mm SPACING AND

CONE ANGLES OF 0 AND 5 DEGREES

X

t- o~

t-

O

700

600

500

400

300

r = 0.71

10 15

Depth of cut (mm)

I i I I I I I 10 9 8 7 6 5 4

Space/depth rat io

20

Fig. 6 R E L A T I O N S BETWEEN COARSENESS I N D E X A N D DEPTH OF CUT WHEN USING 7 5 m m PICKS AT 7 5 m m SPACING A N D

CONE A N G L E OF 5 DEGREES

X

-O c "

. _

t -

c.~

600

5=50

500

450

400

/

_ % . i J 10 15

J _11 J

f

i J

J

I o ( ~ - 25ram picks 0 cone angle~_ (~) 25 . . . . 5 ° " Fig. 4 . .

( ~ ) - 50 " 0 ° t (~)-- 50 . . . . 5 ° ,, . ~ Fig. 5

(~ ) - 75 5 ° " Fig. 6

20 25 30 35 40 45 5 0

Depth of cut (mm)

Fig. 7 S U M M A R Y OF R E L A T I O N S O B T A I N E D BETWEEN COARSENESS I N D E X A N D DEPTH OF CUT WHEN USING SQUARE CHISEL PICKS

700

6O0

" X

" O ¢ -

® 5 0 0 t -

O

4 0 0

300 10

Extrapolated

J '

I

! Cone angle = 5 °

7 /

q ' ~ ' ~ _ _ E x t r a p o l a t e d P+ ~ points i

poln[s , .

~ ~ ~ m Cone~ angle = 0 °r

O S/D ratio- 1.5 • S/D ratio -- 2.0 Z~ S/D ratio-- 4.0 • S/D ratio -- 6.0

20 30 40 50 60 70 Tool width (mm)

Fig. 8 CALCULATED RELATIONS BETWEEN COARSENESS INDEX AND WIDTH OF CUTTING TOOL (SQUARE CHISEL PICK) FOR

HEAD CONE ANGLES OF 0 AND 5 DEGREES

80

100

/ / 9°

0.1

Coarseness index = 430

Square chisel picks

Coarseness index = 550

/ /

1 10

Particle size (mm)

8O

70

60

50

40

30

20

10

0 100

i f / Coarseness index = 400 /

CO arxe=ne /

280mm d i a m . disc cutters

100

90

80

70

60

50

40

30

20

10

0

r -

E =o

a _

0 3 ¢--

t ~

E L..

O .

0.1 1 10 100

Particle size (ram)

Fig. 9 TYP ICAL PARTICLE-SIZE DISTRIBUTIONS PRODUCED BY USING SQUARE CHISEL PICKS AND 280mm DIAMETER DISC CUTTERS

700

X

¢..

¢.-

600

500

400

300

r = --0.37

0

0

0

0 •

0 = 75mm pitch • =lOOmm pitch

r = --0.1 1

..1 I"

0 0 t

0 •

0

° i

8

0 1.0 2.0 3.0

Specific energy (MJ/m 3 )

Fig. 10 R E L A T I O N S B E T W E E N C O A R S E N E S S I N D E X A N D S P E C I F I C E N E R G Y W H E N U S I N G D ISCS

600

X

" 0

.E 500

E

?~ 400 ~J

300

700

r = --0.27

.=

Pitch =75mm

10

I I I i 1 0 9 8 7

15 20 25 30 35 40

Depth of penetration (mm)

i I I I I 5 4 3 2.5 2.0

Pitch/depth ratio

X

@ C

¢-.

(.~

6 0 0

500

400

3 0 0

r = 0.36 Pitch= lOOmm

D r •

5

I

15

10 15 20 25 30 35 40

Depth of penetration (ram)

I I I I I I I I 1 0 9 8 7 6 5 4 3

Pitch/depth ratio

Fig. 11 RELATIONS BETWEEN COARSENESS INDEX AND DEPTH OF PENETRATION WHEN USING 280ram DIA. DISCS AND

CONE ANGLE OF 3 DEGREES

L~ r ~

C~ r ~ 0 0

0 t -

Z

Q.. i.m

el c~

~ r CtJ

r ~

0

o c cr~

Z

I

h

E

w ~ c~

t -

.--]

° - - c~

PLATE 3 Neg no B611/75

A typical debris sample having a coarseness index of 550

PLATE 4 Neg no B612/75

A typical debris sample having a coarseness index of 430

(711) Dd209447 2M 5/76 HPLtd So ' ton G1915 PRINTED IN ENGLAND

ABSTRACT

The particle-size distributions of debris produced during tunnelling trials: A F TOOMBS, R A SNOWDON and M P O'REILLY: Department of the Environment, TRRL Laboratory Report 714: Crowthorne, 1976 (Transport and Road Research Laboratory). This report presents data on the particle-size distributions of debris and the amounts of energy expended while cutting chalk with drag picks and disc cutters during the tunnelling trials at Chinnor, Oxfordshire. Samples of debris were sieved and generalised relations were found between the coarseness index and the specific energy values. However there was considerable scatter in the results much of it attributable to variations in rock strength. Because of this only primary trends could consistently be distinguished and it was concluded that a debris-size distribution approach is unlikely to provide a useful method of assessing tunnel boring machine performance.

ISSN 0305-1293

ABSTRACT

The particle-size distributions of debris produced during tunnelling trials: A F TOOMBS, R A SNOWDON and M P O'REILLY: Department of the Environment, TRRL Laboratory Report 714: Crowthorne, 1976 (Transport and Road Research Laboratory). This report presents data on the particle-size distributions of debris and the amounts of energy expended while cutting chalk with drag picks and disc cutters during the tunnelling trials at Chinnor, Oxfordshire. Samples of debris were sieved and generalised relations were found between the coarseness index and the specific energy values. However there was considerable scatter in the results much of it attributable to variations in rock strength. Because of this only primary trends could consistently be distinguished and it was concluded that a debris-size distribution approach is unlikely to provide a useful method of assessing tunnel boring machine performance.

ISSN 0305-1293