Microfracture beneath blunt disc cutters in rock

int J Rock Mech 'elm Set & Get.rnec~ 4b~tr

Pranted m Great Britain

~ot 25. No I. pp 35--38 1988

0148-906288 $300+000 Pergamon Press pk

Technical Note Microfracture Beneath Blunt Disc Cutters in Rock D F HOWARTH* E J BRIDGE*

exposed cutter tip wears rapidly leaving a relatively flat cutting edge While some studies on blunt disc perforrnance have been undertaken [1,23,24], it appears that there is httle or no published data concermng optimum S/P ratios for very blunt discs In this study a blunt disc cutter, based on a commercially available design (Fig. 1), was tested m simulated array Significant subsurface crack interaction and crater development were observed to occur up to a maximum S/P ratio of 40.

INTRODUCTION The correct selection o f the o p t t m u m separation distance between disc cutters m the cutting array is an ~mportant factor in T B M destgn T B M performance and o p e r a t m g costs are closely tied to this ,~alue A large number o f investigations (see Table 1) have been undertaken with both sharp and blunt disc cutters m order to determme o p t i m u m S/P (spacing/ penetratton) ratios For sharp cutters, o p t , m u m S/P ratios lie between 2 5 and 11 0. whereas, for blunt discs. the c o r r e s p o n d m g range ts 5 0-18 0 (see Table 1) It has been demonstrated [I] that the o p t t m u m S/P ratio mcreases with mcreasmg cutter t~p radtus, dtscs manufactured with a 3 m m ttp radms could be spaced 1 4 ttmes the distance used for sharp discs in order to attam o p t i m u m ( m m t m u m specific energy) S/P ratios C o m mercially avadable blunt discs tend to have cutter t~p geometries significantly blunter than those menttoned above In operation in hard and abraswe rocks the

EXPERIMENTS

* Department of Mmmg and Metallurgical Engineering. Umverstty of Queensland. St Lucia. Queensland 4067. Austraha

The experimental disc cutter Is shown m Fig 1 The cutter was m a n u f a c t u r e d f r o m heat-treated EN25 steel and had a Rockwell C hardness o f 55 T n a l experiments were undertaken using an extensively modified linear shaping m a c h m e Exhaustive tests showed that it was not possible to use thts type o f equipment to simulate field conditions where thrust forces could exceed 250 k N Subsequent expenments were conducted statically (no rolling action) m a 3600 k N capacity Avery testing m a c h m e (Ftg 2)

Table I Optsmum S P ratios tor sharp and blunt disc cutters'--a review of pubhshed hterature (references are m brackets) Type of disc cutter Optimum Sharp Blunt Wear tip radius [mm]b SP Int.luded angle (~) [degree~ "] ratio 60 70 80 90 100 10 15 20 25 30 [191 [19] 25 3O [14.191 35 [lSl [18.19I 4O [13] [13.181 [13] [131 [13.181 45 [18 191 5O Ill. 15.17 18] [21] [81 [I] [17] 55 [1] 60 ° [21] [121 6O [2.9.10 161 [2 9 10.15] [2.10] [2.9.10.181 [2, I01 [21 60 ° [t] 60° 65 7O [i]60[21 60 ° 75 [91 I00 ° IT] 8o ° 191 8O [20] [20I [14] 85 [41 80 ° 100 [3.4.S.7] 80 ° 161 110 [14] 120 [3.41 so ° 150 [3.22."1 80° 165 [41 80° 180 [41 80° "Optimum S P ratio i~ defined a~ occurring v,hen the specific energy Is a minimum and the rock yield is a max,mum [9] bAngle shown ts included edge angle CButton disc [22] 35

36

HO%-~,RTH a-d BRIDGE

s u b s u r f a c e c r a c k s'.~tem~ R o c k s u r f a c e s :z, >e ~ e r e p o h s h e d u ~ m g successl~el~ f i n e r g - a d o , c a r b t d e p a p e r G r i t sizes u s e d w e r e 12o 2"o, a n d 600 In o r d e r to h l g h h g h t crack_, a n d

) [ 2Omr'r~

TECHNIC._ \L \ O i -~

f r a c t u r e a n u l t r a ~ o l e t (u v ) p a m c u l a t e p e n t r a t m g d2re ~as used The d',e ~aa dtssol~ed m acetone and apphed t o t h e p o h s h e d r o c k s u r f a c e , e x c e s s d~e ~ a s r e m o v e d before the acetone completeb e~aporated Rock samples were examined using a standard laborator? m~croscope S a m p l e s v, ere p l a c e d u n d e r a u ~ h g h t m a d a r k e n e d r o o m f o r all ~tsual a n d p h o t o g r a p h l c o b s e r ~ a t m n s

[

F~g 1 E,tpenmental cutting tool--based on a commercmlly available TBM d~sc cutter de~_~m (Note that the destgn used m th~s stud~ onl', incorporates the commercml cutter ttp geometry, the commercml cutter ha~ a dtameter of 432 m m )

RESULTS

Rock specimens were diamond saw cut to approx~mate dtmensmns of 400 x 200 x 200 mm Only one stde of each block ~as used for the mdentatton experiments to avotd the posstbdtty of mtroducang overlappmg crack systems which would have made analys~s and mterpretatton of the results difficult The expenmental procedure mvolved two steps The first mvolved thrusting the disc cutter into the rock to the requtred load (200 or 250 kN) and recordmg the depth of penetratmn The requtred spacmg (at the selected S, P ratto) was calculated and the test spectmen posmoned accordingly The second step was to rodent the cutter to the same depth of penetratton m tts new posmon The load reqmred to effect the second step of rock penetratmn was, m most cases, different from that reqmred to produce the mmal penetratton A summary of the mechantcal and phystcal properttes

Test results are reported m Table 3

Selected photographs of subsurface cracks and craters are shown m Ftg

3a-f

DISCUSSION The photographs shown m Fig 3 clearly demonstrate stgntficant crack mteracuons and crater development up

o f t h e r o c k t y p e s t e s t e d ts g t v e n m T a b l e 2 The expertmental programme revolved two rock types (mmrosyentte and grantte), and one dtsc cutter tested at t h e f o l l o w i n g S P r a t t o s 10, 16, 20, 25, 30, 35, 40 a n d 45 O n e set o f e , ~ p e r t m e n t s w a s u n d e r t a k e n m m l c r o s y e m t e w h t l e t w o , t d e n t t c a l e x p e r i m e n t s , v, ere u n d e r t a k e n tn g r a m t e T h e a d d l t t o n a l t e s t s u n d e r t a k e n m grantte tested the reproduc~bthty of the results F o l l o w m g t h e t e s t s r o c k s p e c t m e n s w e r e c u t to r e v e a l

Fig 2 The e~penmental eqmpment--sho~.mg testmg machine platens disc cutter and rock spcmmen

Table 2 Rock propemes [25] Moog~rah mlcro~',enlte

&shgro~ c granlt~

I37 I -- 19 80 +_ I 9 2474 _+ 35 524+202 3932+161 49

234 _5_ + 13 152_+2 l 2618 4- 14 033±014 5310_+116 62

L nta,~ial compressl,,e strength--dry (MPa) Brazilian &sc tensde strength--dry (MPa) Dr', density (kg, m ~) Apparent porostt) (%) P-~,a',e velocxtv (m/see) S,.hmtdt rebound hammer ( - )

Table 3 Test results

SP ratms I0 16 20 25 30 35 40 45

Microsyemte Load IkN) lmt,al 2nd Pen indent rodent (mm) 200 200 20t) 200 200 200 ---

-240 196 198 262 220 ---

45

45 20 20 10 10 ---

Granite Load (kN) Inmal 2nd mdent rodent .

. 250 250 250 250 250 250

examined o f sl{lcon 320 400 zones of

.

.

.

. 172 225 309 295 265 282

Pen (ram) .

lnmal indent

Load (kN) 2nd rodent

Pen

(mm)

.

. I 05 1 05 1 25 1 25 105 t 05

. 250 250 250 250 250 250

222 ~ 273 28t 245 252

I)5

05 15 15 0O 00

HOWARTH and BRIDGE

TECHNICAL NOTE

37

Fig 3 Subsurface cracks and crater development (horizontal white line denotes the ongmal surface, the curved hnes indicate the posmons of the blunt disc cutter, the directmn of loading is normal to the plane of the ongmal surface, a mdhmetre scale is sho~n m either the Iov,er right- or left-hand corner of each photograph) (a) Mlcrosyemte--S,P = 16 P = 1 45 mm Note the crushed zone beneath each disc cutter and the interacting lateral cracks (marked vdth an arrov,) (b) Microsyentte-S:P = 20 P = I 2 mm Note the crater between the two mdentatmns (c) MIcrosyemte---S~P = 25 P = 1 2 mm Crater ~s shown by the v,hlte arro~ (d) Gramte---S P = 25, P = 1 05 mm Crushed zones beneath the cutters and a s~gmficant crater are evident (e) Gran,te---S P = 35, P = I 15 mm Note stgmficant crack interactions, extensional crack development and the crater (f) Gramte--S P = 40, P = I 05 mm Crater development and s~gmhcant extensmnal cracks are m evtdence

to a n S / P r a t i o o f 40 T a b l e 4 s u m m a r i z e s t h e p h o t o graphic records of crack mteracttons The criteria for a

(2) b e c a u s e t h e t e s t s w e r e s t a t i c t h e r e is t h e p o s s t b t h t y that dtsc penetratmn would have been greater m the

p o s i t t v e r e s u l t m T a b l e 4 w e r e (1) s t g m f i c a n t c r a c k interactions (overlapping and/or

coalescmg)

and

(2)

e v i d e n c e o f c r a t e r f o r m a t i o n (I e r e m o v a l o f all o r p a r t o f the matertal between the two i n d e n t a t i o n s ) W h d e the test r e s u l t s a p p e a r t o ha~,e s l g m f i c a n t t m p h c a t t o n s f o r T B M d e s t g n , a t t e n t m n ts d r a w n to a s e r t e s o f mmgatmg

factors

(1) t e s t s w e r e s t a u c - - t h e d~sc d t d n o t roll a c r o s s t h e r o c k s u r f a c e as m n o r m a l T B M o p e r a t m n Fracture p a t t e r n s d e x e l o p e d m t h e t w o m o d e s o f o p e r a u o n m~ght be d i f f e r e n t ,

Table 4 Summary of stgmhcant crack mteract~ons and crater de,,elopments observed m photographic records S/P ratios

Mlcro~emte

10

16 20 25 30 35 40 45

~,

,~ --

-

Trial I

Gramte Trial 2

--

--

--

--

×

,' x x

x

38

HO'0, 4,RTH and BRIDGE

rolhng c o n d m o n , (due to the reduced area o f dtsc contact) Increased penetrauon may have some effect on fracture patterns However, Roxborough and Phflhps [9] showed that peak values of thrust force were essentmlly

the same for static and rolhng condmons (depth of penetrauon remammg constant) Th~s was explained b), the observation that m practice the broken rock behind the dtsc remains in place, and that the buned sector of the dtsc remains m beanng contact wtth the surfaces of the cut groove dunng rotatmn On th~s basts, mcreased penetration m the rolhng c o n d m o n would be neghg~ble, (3) determmattons o f optimum S/P ratms were not possible since the expenments were static (rolling force could not be calculated), (4) expenments were conducted under stmulated array condmons, thts ~s not constdered to be a problem smce a large number of T B M are destgned to have adjacent cutters staggered some d~stance behmd the leading cutter, (5) the test surfaces were smooth rather than rough as would be the case m the field, (6) the test dtsc was approxtmately ~, o f the dmmeter o f the commercml disc, thts was not constdered to be a problem smce tt has been shown that spectfic energy ~s mdependent o f d~sc dtameter [9] Despite the d~scusston above ~t ts suggested that the tests do not depart too far from reahty and that the results are s~gntficant Further work ts necessary to determine whether or not optsmum spacing to penetratton rattos for blunt disc cutters m the field, do exceed those recorded m the pubhshed hterature The photographtc observations shown m Ftg 3 tnd~cate that with some minor mechamcal assistance, a much larger crater volume could be excavated between the d~sc

cutter grooves It ts suggested that the interacting crack systems between the grooves could be exploited by the apphcat~on of drag ptck or water.let Such a hybrid rock cutting system would have the potentml to be far more efficient than any of the component systems when acting alone

TECHNICAL NOTE

2 Bflgm N ln~est~ganons m to t-e mechanical cutting charactenst~c~ 3 4

5 6 7

8 9 lO 11

12 13 14 15 16

17 18 19

20 Acknowledgement--The assistance of the Head and techmcal staff of the Department of Mining and Metallurgical Engmeenng, University of Queensland ts acknowledged

21

22 Recetred 22 Aprd 1987, ret:sed 23 June 1987 23

REFERENCES 1 PhdhpsH R , B d g m N and PriceD L Themfluenceoftyrettp geometry on the destgn of d,sc cutter arrays 3rd Australasian Tunnelling Conf, pp 48--52 Australasmn Insmute of Mmmg and Metallurgy, Melbourne (1978)

24 25

of some medmm and htgh strer, gth rocks Ph D thesis Umsersl,~ of \e'aca~tle upon T~ne (19--, Sno~don R ¢ R~!e~ M D and Temporal J A study of disc cutting m selected Brmsh r~k~ In, J Rotk ~[e~h ~Itn Set & Geomech 4bstr 19. 107-121 tY982) Snov, don R A R~,le.',M D Temporal J and Crabb G 1 The effect of h~drauhc stiffness on tunnel bonng machine performance lnt J Rock ~Iech ~hn S t t & Gtomech 4bstr 20,203-214 (1983) Temporal J and Snov, don R -~ The eff~t ol h)drauhc stiffness on tunnel bonng mat.hme performance Tunnel~ Tunnelhng 149, 11-13 (March, 1982) Sno~,don R A , Temporal J and Hignett H J A linear rock cutting ng Transport and Road Research Laboratory Supplementary Rept 588 TRRL, Cro~thorne Berks (1981) Snowdon R A and R)le~ M D Single and multiple pass disc cutting m shap gran,te Tunnel~ Tunnelhn_g 15, 15-19 (November, 1983) Rad P F and Olson R C Tunnelling mat.hme research USBM RI 7881 (1974) Roxborough F F and Phdhps H R Rot& exca',atton by disc cutter lnt J Rock ~hxh ~[ln Set & Geom~ch 4bstr 12, 361-366 (1975) Hew~tt K S Aspects of the design and apphcat~on of cutting systems ,n rock excavauon Ph D thesis, Umverstty of Newcastle upon T.~ne (1975) Roxborough F F and Phdhps H R The change m performance of disc cutters v, hen stalled 4th 4ustralastan Tunnelhng Conf, pp 361-368 Aust IMM Melbourne (1981) Phdhps H R The mcchamcal cutting characteristics and properties ot selected rock formatmns Rept to TRRL Umverslty of Nev,castle upon Tyne (1975) Roxborough F F Ro~.k exta~auon by ma~.hme--a comparative study of picks and dlscs 2nd 4ustralastan Tunnehng Con]', pp 133 141 Aust IMM Melbourne (1976) Crabb G I and Htgnett H J A laboratory and pilot scale study on the cutting of chalk containing flints Tunnels Tunnelhng 12, 2%33 IJanuary 1980) O'R:elI~ M P and H~gnett H J Rock cutting tools--their arrangement on full lace tunnel machines Chart ~[ech Engr~ 2,4, 47-51 (1977) Rad P F and McGarry F J Thermally assisted cutting ofgramte In Prot 12th U S S~mp on Rock ~,techantcs, pp 721-757 American lnsmute of Mining, Metallurgical and Petroleum Engineers New York (1970) Hlgnett H J and O'Rlelly M P The arrangement of rock cutting tools on full-face tunnell boring machines, TRRL Laboratory Rept 376 TRRL, Crowthrone, Berks (1978) Roxborough F F and R~spm -X The mechamcal cutting charactensucs of the Iov,er chalk Tunnels Tunnelhng 5, 45-67 (January, 1973) O'R~elI3, M P , Roxborough F F and H~gnett H H Programme of laboratory, pdot and full-scale experiments m tunnel bonng Pro~ Tunnelhng 76p 287.-299 lnsntutton of Mmmg and Metallurg), London (197~) Roxborough F F Resear~.h m m~ham~.al rock excavation progress and prospet.t~ Proc R E T C, 1985, Vol l, pp 225-244 AIMME New York (1985) Fenn O Protheroe B E and Joughm N C Enhancement of roller cutting b) means of ~ater jets In Prot R E T C 1985Vol I, pp 341-356 AIMME Ne'.,, ~ork (1985) Hamilton Vv H and Dolhnger G L Opummng tunnel bonng machines and cutter design for greater boreabdity Proc R E T C , 1979Voi 1 pp 280-296 AIMME New York (1979) Ozdem~r L M,ller R and %ang F D Mechamcal tunnel bonng machine pred~cuon and ma~.hme de~tgn Evcatatton Engineering & Earth ~[echamcs lnstttut~ (NSF APR73o07776-,~03) Colorado School of Mines Den,,er ~1973~ Rad P F Bluntne~ and wear ot rolhng disk cutters lnt J Rock Mech ~lm S~t & Geome{h 4h~tr 12, 93-99 (1975) Ho',,,arth D F and Rowland~ J C Quanmatwe assessment of rock texture and correlanon v,~th dnllabdtty and strength properues R,,c~ ~[eth R,w/, Er~'r.r 20 57-85 (1987)