Experimental investigation of hard rock fragmentation using a conical pick on true triaxial test apparatus

Tunnelling and Underground Space Technology 79 (2018) 210–223

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Experimental investigation of hard rock fragmentation using a conical pick on true triaxial test apparatus

T

⁎

Shaofeng Wanga,b, Xibing Lia, Kun Dua, , Shanyong Wangb a b

School of Resources and Safety Engineering, Central South University, Changsha 410083, PR China ARC Centre of Excellence for Geotechnical Science and Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Hard rock fragmentation Conicalpick Peak cutting force Insertion depth Cuttability-improved approach

This study aims to determine the influences of confining stresses and cutting parameters on hard rock fragmentation using a conical pick. Using a true triaxial testing apparatus, single/double pick forces and static/ coupled static-dynamic pick forces were applied to granite rock specimens to break them confined by biaxial, uniaxial and no lateral stress conditions. The corresponding cuttabilities were estimated and compared by the peak pick force, insertion depth and disturbance duration at rock failure and the associated failure patterns. The results showed that excavation-induced unloading and cracking, which can change the biaxial confining stress conditions into the uniaxial and decrease uniaxial confining stress level, respectively, can significantly improve the hard rock cuttability. The experimental, theoretical and regressive results indicated that the hard rock cuttability initially decreases and then increases as the level of uniaxial confining stress increased. A moderate uniaxial confining stress instead improves the rock cuttability, but a high stress level may induce rock burst triggered by conical pick penetration. Therefore, only the hard rocks under stress-free and low uniaxial confining stress conditions can be easily fragmented with high safety and efficiency, as a complete splitting failure occurs. In addition, the hard rock cuttability can be also improved by the dynamic pick disturbance and the artificial defects such as excavation-induced fractures, pre-slits and boreholes in rock mass.

1. Introduction Mechanized excavation is a widely used approach in mining and tunneling engineering due to continuous and safe operation, high construction quality and low excavation damage (Bilgin et al., 2013; Wang et al., 2016a,b, 2017). Roadheaders and longwall shearers using rotary cutting of conical picks have been satisfactorily used to excavate soft to medium hardness ore-rock, such as coal, bauxite, and salt minerals, and continue to be promoted by the industrial development (Ergin and Acaroglu, 2007; Peng, 2008; Wang et al., 2016c). Nevertheless, these mining machines are unsuitable for breaking extremely hard ore-rock at present, in which the hard ore-rock under high geostress instead become a primary existing situation in deep underground. The reason for this is that the mining processes are prone to wear-out failure of cutter tool, low operation stability of machine and heavy dust production, resulting from low cuttability of hard rock having high strength, hardness, wear resistance and intactness and high geostress confinement (Ergin and Acaroglu, 2007; Yang et al., 2015; Dewangan and Chattopadhyaya, 2016a; Li et al., 2017a). Deep underground mining in hard ore-rock is a consequent trend with the consumption and depletion of shallow mineral resources. Field observations indicated

⁎

Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (K. Du).

https://doi.org/10.1016/j.tust.2018.05.006 Received 18 January 2018; Received in revised form 7 May 2018; Accepted 10 May 2018 0886-7798/ © 2018 Elsevier Ltd. All rights reserved.

that pre-existing rock fractures induced by excavation unloading disturbance of high geostress can improve rock cuttability (Kaiser, 2006; Yin et al., 2014b; Li et al., 2013, 2017b). Li et al. (2013) found that the pick consumption and dust production significantly reduced during excavation in an unloading-induced fractured zones under high geostress condition in hard phosphate ore-rock using roadheader, compared to a single-face excavation. Therefore, it is necessary to understand the cuttability of deep hard rock to promote the suitability of mechanized excavation approaches in upcoming deep underground mining. Rock cuttability is an integrated behavior that reflects the interactions between cutters and rock and is influenced by rock characteristics, cutter performances and stress conditions. Conical picks, which belong to point-attack picks with advantages of greater penetration depth, lower energy expenditure and longer life span, have been widely used for roadheader excavation and longwall shearer mining (Peng, 2008; Dewangan et al., 2015; Dewangan and Chattopadhyaya, 2016b; Nahak et al., 2017). The parameters of conical picks determine the fragmentation abilities of roadheader and shearer, which is significantly influenced by rock characteristics and stress conditions (Li et al., 2017a). Significant efforts have been undertaken to understand cutting

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failure patterns were measured and compared in the tests to estimate the influences of conical pick performances (loading velocity, load type and installation interval) and stress conditions (biaxial stress, uniaxial stress and stress-free conditions) on granite rock fragmentation.

behaviors of point-attack picks using experimental, theoretical and numerical investigations. Experimental studies were conducted to obtain specific cutting parameters of conical picks such as the cutter force, specific energy, fragment production, fractured surface roughness and fragment features influenced by rock properties and cutting parameters (Bilgin et al., 2006, 2013; Balci and Bilgin, 2007; Bao et al., 2011; Yang et al., 2015; Kang et al., 2016; Liu et al., 2017; Copur et al., 2017; Yasar and Yilmaz, 2017). Bilgin et al. (2006) undertook laboratory full-scale linear cutting tests to investigate the performances of conical picks on 22 different rock specimens varying from the soft to the hard under different cut depth and cutter spacing values. They found that the cutter force and specific energy values are positively correlated with rock properties especially expressed by uniaxial compressive strength and Brazilian tensile strength. Dewangan et al. (2015) and Dewangan and Chattopadhyaya (2016a,b) investigated the wear mechanisms of conical picks in coal cutting. Kang et al. (2016) made a new linear cutting machine and investigated the influences of specimen strength (low, moderate and medium), cutting depth and cutting spacing on the cutting force of conical pick. Copur et al. (2017) studied the influences of cutting parameters (single-, double- and triple-spiral cutting patterns, cutting depth and cutting speed) and rock properties (from soft to hard) on the cutting performances of conical picks (normal, cutting and side forces, specific energy, yield and fractured surface coarseness index) using a full-scale linear cutting apparatus. Several theoretical models were proposed to estimate the maximum and mean cutting forces of conical pick (Evans, 1984; Roxborough and Liu, 1995; Goktan, 1997; Goktan and Gunes, 2005; Bao et al., 2011). Obtained from these models, the cutting forces are correlated with rock properties, cutting depth, the geometric factors of conical pick and the friction coefficient between cutter and rock. In addition, a number of numerical simulations using the finite element and discrete element methods were undertaken to simulate rock cutting of conical picks to trace the complex process in more detail and reduce costs (Liu et al., 2014; Su and Akcin, 2011; Rojek et al., 2011). These studies provided valuable information to shed light on the rock cutting mechanism using conical pick. However, they did not consider the influence of geostress confinement, which can be ignored in shallow excavations but is a common, growing and serious factor in deep mines and tunnels (Li et al., 2017a). The influence of confining stress on cutting performances of other types of cutting tools such as wedge-shaped cutter and disc cutter has been investigated and found that a modest level of lateral stress could close the cracks and decrease the tensile stress around the cutting groove, and then increase the cutting difficulty (Pomeroy, 1958; Gehring, 1995; Bilgin et al., 2000; Innaurato et al., 2007, 2011). Nevertheless, other studies undertook field observations, laboratory tests and numerical simulations and found that confining stress has both beneficial and adverse influences on rock cutting using tunnel boring machine (TBM) cutters (Ma et al., 2011, 2016; Yin et al., 2014a,b; Liu et al., 2016a,b). Confining stress can facilitate rock fragmentation if it is high enough to induce rock pre-cracking near the excavation face; otherwise, it hinders rock cutting. The aforementioned studies focused on the cutting abilities of wedge-shaped pick and TBM disc cutter influenced by confining stress, but did not involve the conical pick. Unlike these cutting tools, a conical pick performs as a sliding indenter to crush and then fragment the rock, which is fundamentally three-dimensional and difficult to be simplified into a two-dimensional case (Bao et al., 2011). Moreover, the influence of confining stress on rock fragmentation does not occur as a single event, as determined by indentation tests of TBM cutters. In addition, the load types and the stress-redistribution factors induced by excavations and pre-existing defects have not been considered in previous studies. This study experimentally investigated hard rock fragmentation using a conical pick on an innovative true triaxial testing apparatus that can apply high biaxial or uniaxial confining stresses and variform axial pick forces to multi-size granite rock specimens. The cuttability indices such as peak force, insertion depth and disturbance duration and the

2. Experiment method 2.1. Test apparatus This study was performed on an innovative testing apparatus using the TRW-300 electro-hydraulic servo system for true triaxial coupled static and dynamic loading tests. The apparatus consisted of X, Y and Zdirection loading units, dynamic disturbance unit, external digital controller, hydraulic pump controller, control software for true triaxial static loading and dynamic disturbance, camera, and video surveillance, as shown in Fig. 1. The static loading capacities of the apparatus were 0–2000, 0–2000, and 0–3000 kN in X-, Y- and Z-directions, respectively. The dynamic disturbance allowed 0–500 kN amplitudes and 0–70 Hz frequencies (Du et al., 2016). 2.2. Experiment conditions Rock fragmentation is a complicated process with the interaction between rock and cutter influenced by rock characteristics, cutter performances and geostress conditions (Li et al., 2017a). Therefore, this study mainly investigated the influences of rock characteristics, loading parameters of conical pick and confining stress conditions on the hard rock fragmentation on granite specimens. 2.2.1. Rock specimens The rock material used in tests was extracted from homogeneous matrix blocks of granite. A series of cylindrical granite specimens with sizes of Φ50 × 100 mm and Φ50 × 25 mm were prepared to determine the conventional physical and mechanical parameters of the rock material, including its density, Young’s moduli, Poisson’s ratios, tensile strengths, cohesions, friction angles, uniaxial compressive strength (UCS), and conventional triaxial compressive strengths under 30 MPa (TCS-30) and 60 MPa (TCS-60) confining stresses. There were five specimens for each conventional test, and the average values of tested results were listed in Table 1. Then, the piecewise linear and non-linear peak strength curves were regressed from the experimental peak strength values under the different confining stresses based on MohrCoulomb and Hoek-Brown strength criteria expressed as linear Eq. (1) and non-linear Eqs. (2) and (3), respectively (Hoek et al., 2002; Brady and Brown, 2006; Zhang and Zhao, 2014a; Wang et al., 2018). The measured strength values and regressive strength curves were plotted in Fig. 2, and the corresponding regressive coefficients were shown in Table 1. The strength grades of the granite used for rock fragmentation tests were very strong according to these strength parameters, and the granite belongs to a stiff and brittle material. As shown in Fig. 2, the failure envelope curves regressed by piecewise linear criterion using Eq. (1) and modified non-linear criterion using Eq. (2) can reflect the full strength characteristics of granite from tensile to compressive stress conditions. However, the failure envelope curve regressed by original non-linear criterion using Eq. (3) can only reflect the strength characteristics of tested rock under compression tests and is unsuitable for determining tensile failure.

σ1 =

1 + sinφ 2C cosφ σ3 + 1−sinφ 1−sinφ

(1)

a

σ σ1 = σ3 + σc ⎡m 3 + s⎤ ⎢ ⎥ ⎣ σc ⎦

(2) 0.5

σ σ1 = σ3 + σc ⎡m 3 + 1⎤ ⎢ ⎥ ⎣ σc ⎦ 211

(3)

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Z-direction loading Camera

External digital controller

Dynamic disturbance

Hydraulic pump controller

Y-direction loading

X-direction loading

Video surveillance

Cutting force Input sine wave

Output sine wave Control software for dynamic disturbance

Control software for true triaxial static loading

Fig. 1. TRW-300 electro-hydraulic servo system for true triaxial coupled static and dynamic loading test.

of pick and artificially induced defects on rock fragmentation using a conical pick. Cuboid rock specimens with sizes of 200 × 200 × 200 mm, 300 × 150 × 80 mm and 400 × 100 × 100 mm were used to determine the influence of applied distance between the two conical picks on rock fragmentation. 2.2.2. Size and material of conical pick A typical conical pick was used for tests, which consisted of pick body and a conical tip embedded in pick body. The pick body and embedded tip are made of cast iron and cemented carbide, respectively. The shape, size and material of conical pick are shown in Fig. 3. 2.2.3. Loading parameters of conical pick Rock fragmentation involves a direct interaction between conical picks mounted on roadheader or shearer and rock mass at their interfaces. A concentrated force supplied by a conical pick will be applied to the exposed ore-rock surface to fragment rock mass and strip rock blocks. There is an economy and suitable cutting power supplied from mining machine for a specific rock mass. Only when the cutting force and speed of mining machine satisfy the cutting power requirement, the saving mechanized mining of hard ore-rock can be achieved. Meanwhile, the high mining efficiency is another pursuing goal. The mining efficiency of picks is generally determined by the mechanical energy applied on the rock mass per unit time that is influenced by loading parameters of conical pick: loading velocity, load type and installation interval. In tests, six displacement-controlled loading velocities of 0.3, 1, 3, 15, 30, 60 mm/min supplied from Z-direction loading

Fig. 2. Regressive peak strength curves using Hoek-Brown and Mohr-Coulomb strength criteria.

where σ1 and σ3 are axial peak strength and confining stress, respectively; φ is friction angle; C is cohesion; m, s and a are empirical coefficients in Hoek-Brown strength criterion model. Cubic rock specimens that were 100 × 100 × 100 mm and 150 × 150 × 150 mm in sizes were manufactured and used to investigate the influences of confining stress, loading speed and load type Table 1 Physical and mechanical parameters of the granite rock material. Density (g·cm−3)

2.604

Young’s modulus (GPa)

69.56

Poisson’s ratio

0.21

Tensile strength (MPa)

Cohesion (MPa)

Friction angle (°)

7.56

15.45

62.50

23.90

48.52

UCS (MPa)

TCS-30 (MPa)

TCS-60 (MPa)

Coefficients m

212

126.24

337.43

544.85

s

a

13.84 0.8493 0.6629 Full envelope curve regressed by Eq. (2) 26.20 – – Envelope curve under compression (σ3≥0) regressed by Eq. (3)

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transportation. During mining operation, ore-rock in the exposed entry walls is fragmented and stripped from the mineral deposits into free faces. The exposed ore-rock mass is under biaxial confining stress condition at single-face driving entryway. In surrounding rock mass of peninsula or island pillar, the exposed ore-rock mass is confined by uniaxial stress. In underground mining practice, some artificially induced defects, such as excavation-induced fractures, pre-slits and preboreholes in the mining or driving faces would be produced to improve the cuttability of hard rock mass (Li et al., 2017a,b). Stress redistribution often occurs in excavation-disturbed surrounding rock mass of deep mining stope, during which the abundant fractures are produced with stress concentration and stress release (Li et al., 2014, 2016; Cao et al., 2016; Xie et al., 2017). Those excavation-induced fractures can decrease the integrity of deep hard rock. In addition, pre-slits or/and pre-boreholes will be performed in the rock wall at mining face to create a low-stress condition with the confining stress relief. The uniaxial stress significantly decreases and can even completely be removed, resulting from stress release and relief in excavation fractured zone and around pre-slits or pre-boreholes in entry wall. To reproduce the stress situations mentioned above, rock fragmentation tests were performed on the cubic granite specimens under different confining stress conditions in laboratory. Two pairs of end-surfaces of cubic granite specimen were subjected to biaxial stresses supplied by two jacks in two mutually perpendicular loading directions to create a biaxial confining stress condition. A pair of end-surfaces of cubic specimen was subjected to uniaxial stress supplied by a jack to produce a uniaxial confining stress condition. The rock specimen that only subjected a concentrated force supplied by conical pick is under the no confining stress condition. In static rock fragmentation tests, the biaxial, uniaxial and no confining stress conditions were supplied by Xand Y-directions loading jacks, Y-direction loading jack and no jack, respectively, as shown in Fig. 5a–c. During coupled static and dynamic rock fragmentation tests, these three types of confining stress conditions were created by X- and Z-directions loading jacks, Z-direction loading jack and no jack, respectively, as shown in Fig. 5d–f. The confining stresses were applied to the granite specimens using a force-controlled loading approach with a loading velocity of 0.5 kN/s in all tests.

31.33 mm

45.95 mm

65.4 mm

37.88 mm

12.58 mm

67.71 mm

75.84 mm

(a)

(b)

Fig. 3. Size and material of conical pick used in tests, which include (a) the shape and size and (b) the physical image and material of conical pick.

(b)

(a)

Z-direction loading Double picks Z-direction loading Rock specimen

Holder

Holder Conical pick Rigid block X-direction loading

Rock specimen

(c) Y-direction loading

Z-direction loading Disturbing bar

2.3. Evaluation indices of rock cuttability Conical pick Y-direction loading

An ideal cutting situation of mining machine should be that the rock fragments are easily spalled from rock mass using small applied force and insertion depth of conical pick. The peak force and insertion depth of conical pick at rock failure determine the failure energy requirement and influence the wear life of conical pick and the rock dust emissions. Meanwhile, the peak pick force at rock failure is a primary parameter to determine the feasibility of continuous mining machine with many rotary conical picks in hard ore-rock excavation. Therefore, the peak force Fc and insertion depth Dc of conical pick at rock failure were the primary measured indices to evaluate the hard rock cuttability and to investigate the influences of designed experiment conditions on rock fragmentation in laboratory static rock fragmentation tests. As shown in Fig. 6, these two evaluation indices can be extracted from the correlation curves between applied force and insertion depth of conical pick during pick penetration into rock specimen, which were measured and recorded in rock fragmentation tests using force and displacement sensors installed in true triaxial test apparatus. For a coupled static and dynamic rock fragmentation test, a dynamic disturbing load with constant amplitude and frequency was superposed on a pre-loading static force applied to conical pick to fragment rock specimen. Disturbing duration, which can reflect the amount of mechanical energy applied to rock by conical pick, was a primary parameter to evaluate the rock cuttability in coupled static and dynamic rock fragmentation, which can be achieved via force-time curve of conical pick shown in Fig. 7.

Fig. 4. Illustration of loading approaches of pick force and confining stresses at rock fragmentation, including (a) static rock fragmentation using a conical pick, (b) static rock fragmentation using double conical picks and (c) coupled static and dynamic rock fragmentation using a conical pick.

jack were designed to investigate the influence of loading velocity of conical pick on the rock fragmentation (Fig. 4a). Two load types of pick force, which included static force (Fig. 4a and b) and coupled pre-static and dynamic disturbance (Fig. 4c), were utilized to break rock specimens. The static pick force, pre-static pick force and dynamic disturbance were applied on the free end-surfaces of cubic rock specimens and supplied from Z-direction loading jack, Y-direction loading jack and Y-direction disturbing bar, respectively. In addition, a holder shown in Fig. 4b was designed to mount two conical picks and to allow them free movement in a chute to change their installation intervals from 70 to 200 mm. In tests, four interval values of 80, 100, 120, 160 mm were performed to find a suitable installation interval between picks. 2.2.4. Confining stress conditions Entryways are typically prepared to expose the underground mineral deposits to be mined and provide laneways for ventilation and 213

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(a)

(b)

FZ

Y

Fig. 5. Illustrations of loading approaches for creating confining stress conditions including the (a) biaxial, (b) uniaxial and (c) no confining stress conditions in static rock fragmentation tests and the (d) biaxial, (e) uniaxial and (f) no confining stress conditions in coupled static and dynamic rock fragmentation tests, where σX , σY and σZ are confining stresses, and FY and FZ are coupled static-dynamic pick force and static pick fore, respectively.

(c)

FZ

FZ

Y X

(d)

(f)

(e) Z

Z

FY

FY

FY

X

3. Results and discussion 60

3.1. Effect of loading velocity of conical pick

Third fracture

40 30

Second fracture 20

First fracture

10 0 0.0

0.5

1.0

2.0

1.5

Varying loading velocity can change the strength and fracture behaviours of rock materials (Li, 2014; Zhang and Zhao, 2014b). In rock fragmentation tests, the conical picks with the different loading velocities of 0.3, 1, 3, 15, 30 and 60 mm/min were performed to fragment the 100 × 100 × 100 mm cubic granite specimens under no confining stress condition. The change curves of peak force and insertion depth of conical pick at rock failure as loading velocity of pick increases and the corresponding failure images of rock specimens were shown in Fig. 8. Increasing loading velocity of conical pick can change the peak force and insertion depth of pick at rock failure, which increased first and then decreased, although the failure patterns were all in split. A dynamic enhancement occurred as loading velocity increased from 0.3 to 30 mm/min. The pick force could be stably applied to rock surface, and the chiselled pit produced by conical pick presented regular shape at low loading velocity. Therefore, uniform local stress supplied from pick force, which was uniformly applied to the chiselled pit, generated that the rock specimen could be split using a small pick force and corresponding shallow insertion depth. As loading velocity increased, the chiselled pit became irregular and relatively large because the small

2.5

3.0

Insertion depth of conical pick D/mm Fig. 6. Correlation curve between applied force and insertion depth of conical pick during pick penetration into rock.

4

100.0

Coupled static and dynamic pick force/kN

160

Pick force at rock failure Fc /kN

Dynamic disturbance Pre-loading static force

140 120 100 80 60 40 20 0 20

30

15

3

87.5

Force at failure Insertion depth

75.0

62.5

60 0.3

3.5

3

2.5

1

50.0

2

37.5

1.5

Duration 21

22

23

24

25

26

27

28

29

30

31

32

1

25.0

33

0

Time/s

10

20

30

40

50

Insertion depth of pick at rock failure Dc /mm

50

Failure

Peak pick force at rock failure Fc

Applied force on conical pick F/kN

Insertion depth of pick at rock failure Dc

60

Loading velocity of conical pick/mm·min-1

Fig. 7. Typical actual force-time curves of rock fragmentation using a conical pick with ideal coupled static and dynamic loads of 80 ± 60 kN.

Fig. 8. Change curves of applied force and insertion depth of conical pick at rock failure as loading velocity of pick increases. 214

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corresponding insertion depth of conical pick at rock failure were plotted in Fig. 9. It was ascertained that the peak forces and corresponding insertion depths of conical pick at rock failure first increased, reached their maxima at uniaxial confining stress of 40 MPa, and then decreased with increasing uniaxial confining stress. At the low uniaxial confining stress level less than 10 MPa, the relatively small peak force and corresponding insert depth could fragment hard rock. In this case, the cubic rock specimens occurred complete splitting failure and were split into two pieces of fragments with equivalent sizes separated toward free ends. The reason for that result was that the end-face friction, friction-induced lateral confinement and uniaxial confining stress-induced confinement were low at the low level of uniaxial confining stress. As the uniaxial confining stress increased to the middle level from 20 to 60 MPa, the variations of peak force and insertion depth at rock failure were not obvious and had relatively large values. The rock specimens mostly like to be partially split, and the stripped fragments were separated off from free ends, resulting from high end-face friction and lateral confinements. Under high uniaxial confining stress, typically exceeding 80% of the UCS of normal granite specimen, the high elastic energy was stored in the rock. The stored energy would convert into kinetic energy to produce rock burst with a rapid ejection of rock fragments, triggered by penetration-induced damage in rock during rock fragmentation using a conical pick. In rock burst, the initial rapid ejection of slabbing fragments in the confining stress direction was triggered by pick penetration and powered by elastic energy storage in rock accumulated by high confining stress, as shown in the high-speed images in Table 3. The failure phenomena mentioned above indicated that the confining stress conditions determined the failure patterns of cubic granite specimens during rock fragmentation using a conical pick. Near the chiselled pit produced by penetration of conical pick, the rock was compressed by conical tip on the head of pick, which produced a flat shear failure surface with low roughness and lots of crushed material. However, the rock far away from chiselled pit was relatively subjected to a point load from conical pick, which induced a rough tensile failure surface with high roughness and little crushed material. The tested results showed that the hard rock can be easily and safely cut under the low uniaxial confining stress, especially under stress-free condition. Middle uniaxial confining stress would confine the rock fragmentation. High uniaxial confining stress would increase the risk of rock burst. In mining practice, entries orthogonal to single-face laneway should be prepared to increase the number of free faces of ore-rock to be mined. The biaxial confining stress condition can be transformed to uniaxial confining stress condition in surrounding rock mass. Meanwhile, the stress concentration and subsequent stress release will occur with the stress redistribution, in which the uniaxial confining stress eventually decreases. Furthermore, sometimes the pre-slits and/ or pre-boreholes should be prepared in exposed ore-rock to relieve stress so that the confining stress approaches zero. Through the measures mentioned above, the hard rock cuttability can be improved significantly.

rock fragments would be quickly squeezed and ejected, and then the small point pick load was transferred into a relatively large arc-face load to be used to split rock specimen. Subsequently, the rock fracture before rock failure did not happen as loading velocity increased from 30 to 60 mm/min. The rock blocks near chiselled pit were too late to be squeezed and ejected, and only a peak force for rock failure and a small chiselled pit would occur. A small pick force was instead required to split rock specimen at high loading velocity. Therefore, the actual loading velocity of conical pick in mechanized mining practice in hard rock mass should be set to greater than 60 mm/min to obtain the high mining efficiency. However, for achieving the stable and safe experiment process and detailed test data and for saving consumed time, the loading velocity was set to 1 mm/min in the subsequent laboratory tests. 3.2. Effect of confining stress condition The surrounding ore-rock of underground excavation structure is subjected to different stresses, which affect the cutting capacity of the conical pick. Soft rock, such as coal and bauxite, can be easily fragmented by the rotating picks mounted on roadheader even under a biaxial confining stress condition in the single-face driving operation (Peng, 2008; Wang et al., 2016c). For deep hard rock, high stress and excavation-induced stress redistribution are the unignorable conditions that influence the rock cuttability. Therefore, a variety of rock fragmentation tests were performed to understand the hard rock cuttablities under the different confining stress conditions. The experimental results of rock fragmentations on the 150 × 150 × 150 mm and 100 × 100 × 100 mm cubic granite specimens were listed in Tables 2 and 3, respectively. As shown in Table 2, a concentrated force given by a conical pick was utilized to dig cubic granite specimen with size of 150 × 150 × 150 mm under biaxial, uniaxial and no confining stress conditions. The measured peak forces and insertion depths showed that the hard rock cuttability increases in order from rock fragmentations under biaxial confining stress condition to uniaxial confining stress condition and then to no confining stress condition. A uniform conical chiselled pit occurred and gradually increased in width and depth as the pick gradually penetrated into rock specimen, and the corresponding pick force increased under lithostatic confining stress condition. Confined by lithostatic biaxial confining stress, the flake-shaped fragments can be stripped only from the free end-face of rock even with a large pick force, and many shear cracks were produced when rock was squeezed and compressed by conical pick. Under biaxial confining stresses with a deviation, the rock specimen was partially split and separated along the direction of smaller confining stress. Large pick force and insertion depth were required to fragment hard rock under biaxial confining stress. However, conical pick mounted on roadheader/shearer was difficult to penetrate rock because the mining machine could not produce a large enough force, resulting in the prolonged friction between pick and rock surface and then causing serve pick wear and heavy dust. These results illustrate that it is infeasible to utilize roadheader or shearer to economically excavate hard rock in single-face driving process. The peak force and insertion depth of conical pick at rock failure reduced to 547.42 kN and 17.03 mm, respectively, under uniaxial confining stress of 30 MPa. The rock specimen was partially split and separated from lateral free end-face of rock specimen. Especially under no confining stress condition, only 103.39 kN pick force and corresponding insertion depth of 3.39 mm can trigger the complete split of rock, producing two pieces of rock fragments with equal sizes. These showed that removing and decreasing confining stresses can improve the hard rock cuttability. Then, the 100 × 100 × 100 mm cubic granite specimens were used to understand the rock fragmentation behavior under higher uniaxial confining stress and its wider variation range. The tested results were shown in Table 3, and the change curves of peak force and

3.3. Effect of load type from conical pick The ore-rock to be mined is subjected to a quasi-static concentrated force provided by a conical pick with low velocity rotary cutting during mechanized mining using roadheader or shearer. Recently, a novel rock fragmentation approach, which employed rotary cutting accompanied by dynamic disturbing, was proposed to fragment hard rock (Li et al., 2017a), based on a confirmed phenomenon that coupled static and dynamic loading is conducive to rock fracture (Li et al., 2008, 2009). Thus, a static pick force with 1 mm/min penetration velocity and a coupled pre-loading static force (1 mm/min displacement-controlled loading velocity) and dynamic disturbing load (sine wave with 5 Hz frequency and different amplitude) were applied to conical pick to fragment 100 × 100 × 100 mm cubic granite specimens. The tested 215

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Table 2 Experimental results of static rock fragmentation using a conical pick on 150 × 150 × 150 mm cubic granite specimen under different confining stress conditions. Size

150 × 150 × 150 mm

Confining stress/ MPa

Peak force at failure Fc/kN

Insertion depth at failure Dc/ mm

Failure pattern

Failure image of rock specimen

σX

σY

13

13

679.52

21.5

Surface stripping

4

22

648.77

20.49

Partial splitting

0

30

547.42

17.03

Partial splitting

0

0

103.39

3.79

Complete splitting

could improve the application of mechanized mining in hard rock. Long disturbance duration of the conical pick until rock failure with only surface stripping indicated that it was difficult to cut the granite effectively, although the coupled static and dynamic pick forces were applied to the rock under biaxial confining stress condition. Under no confining stress condition, the complete rock splitting occurred in a short time when the sum of the pre-loading static force and disturbing amplitude approximated the pick force at failure in static rock fragmentation. Meanwhile, the disturbance duration until rock failure

results were shown in Table 4. Failure images of rock specimens in Table 4 showed that the load types on conical pick used in tests did not change the failure patterns. The failure pattern in the coupled static and dynamic rock fragmentation tests were the same as the results from the static rock fragmentation tests. Nevertheless, transforming single static loading of conical pick into a coupled pre-loading and dynamic disturbing would increase rock fragmentation efficiency, which meant that the fragmentation approach employing the coupled rotary cutting and dynamic disturbing 216

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Table 3 Experimental results of static rock fragmentation using a conical pick on 100 × 100 × 100 mm cubic granite specimen under different uniaxial confining stresses.

σY

0 0 0

130a 120 100

Dc/mm

Failure pattern

37.94 114.27 140.14

2.71 3.74 5.39

Rock burst → shear Rock burst → shear Rock burst → shear

Failure image of rock specimen

Pick

1.043s

1.045s

1.046s

1.047s

ıy

ıy

1.054s

1.265s

ķ

1.461s

0

60

162.33

12.19

Partial splitting

ķRock burst (Slabbing ejection)

σX

Fc/kN

ĸShear failure

100 × 100 × 100 mm

Confining stress/MPa

ĸ

Fz I-I I

I

Chiselled pit ız

Split fragment ıy 0

40

206.28

12.27

Fz

Partial splitting

I-I I

I

Chiselled pit

Split fragment

ız

ıy 0

20

203.44

11.25

Partial splitting

Fz I

I

Chiselled pit

Split fragment

ıy 0

10

51.64

5.34

I-I

ız

Complete splitting

Fz I-I I

I Chiselled pit Split fragment

ıy 0

5

63.77

3.43

ız

Complete splitting

Fz I-I I

I

Fracture

Size

Chiselled pit ıy 0

0

49.99

2.66

Split fragment

ız

Complete splitting

(continued on next page) 217

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Table 3 (continued) Size

Confining stress/MPa

Fc/kN

Dc/mm

Failure pattern

Failure image of rock specimen

σY

σX

Fz I

Fracture

I

I-I

Chiselled pit Split fragment

ız

a The UCS (135.72 MPa) of cubic granite specimen used for rock fragmentation is slightly larger than the UCS (126.24 MPa) of normal cylindrical specimen because of the differences in size, shape and end-face friction effect.

Pick force at rock failure Fc /kN

data1

Force at failure

250

data1

Insertion depth 10

200

150

5

100

50

0

0 0

20

40

60

80

100

120

140

which included peak forces and insertion depths at rock failure and associated failure images. The 200 × 200 × 200 mm cubic granite specimen could be completedly split into two fractured rock blocks along the connected fracture passing through the chiselled pits produced by the double conical picks having 80 mm installation interval. The wing fractures also occurred through a chiselled pit to produce some spalling fragments. However, the double conical picks using 120 mm installation interval could not completedly split this kind of rock specimen, only producing some wedge-shaped fragments peeled off from corners of rock specimen along wing fractures. Cuboid granite specimen that was 300 × 150 × 80 mm in size could be fragmented into four pieces of fractured blocks as the wing and connected fractures occurred by the double conical picks with 100 mm installation interval. For the 400 × 100 × 100 mm cuboid granite specimens, the perforative wing fractures produced by one of the conical picks and the connected fractures between the double conical picks could not occur at the same time during rock fragmentations using the double conical picks with 120 mm and 160 mm installation intervals, resulting in the difficulty of controlling the size of the rock fragments. Therefore, the suitable installation interval between conical picks should be set less than 100 mm and maybe better in the range from 80 to 100 mm in mechanized mining practice in hard ore-rock, according to the tested results mentioned above.

Insertion depth of pick at rock failure Dc /mm

15

300

Uniaxial confining stress/MPa Fig. 9. Change curves of applied force and insertion depth of conical pick at rock failure with increasing uniaxial confining stress.

decreased as pre-loading static force increased, even though the disturbing amplitude decreased to maintain a constant sum of static and dynamic pick force applied to rock. Under a high uniaxial confining stress of 100 MPa, a rock burst phenomenon with rapid ejection of slabbing chips and ultimate failure of shearing cones could be triggered by a coupled static preload of 80 kN and disturbing amplitude of 60 kN that only took 7.44 s. That result indicated that the disturbing load produced by cutter was prone to triggering rock burst under high uniaxial stress condition.

3.5. Effect of artificially induced defects in rock mass Field observations indicated that pre-existing excavation-induced rock damage ahead of the cutting machine as a result of unloading disturbance of high geostress can improve rock cuttability (Blindheim and Bruland, 1998; Kaiser, 2006; Yin et al., 2014b; Li et al., 2013, 2017b). Abundant cracks/fractures exist in excavation damage zone around surrounding ore-rock mass of cave/entry/stope/face for mining preparation, which can decrease rock strength to improve its cuttability. Meanwhile, the ore-rock in excavation damage zone is under the low confining stress condition due to stress release in stress redistribution process, and has the high cuttability confirmed by the above experimental results showing that hard rock was the easiest to be split safely and efficiently under low uniaxial confining stress condition. In addition, pre-slits and pre-boreholes, which are prepared in island or peninsula pillar beforehand, can result in stress relief in surrounding ore-rock in whole or in part to create a low confining stress or even stress-free condition to improve the cuttability of hard ore-rock. Thus, mechanized mining in hard ore-rock using conical picks should be performed in excavation damage zones to the maximum extent possible, in which the fractured rocks can be easily cut by mining machine. Pre-slits and pre-boreholes that are chiseled in rock pillars can be selected to further improve the hard rock cuttability.

3.4. Effect of installation interval between picks There are many conical picks mounted on longwall shearer or driving roadheader, which are set in holders arranged in spiral lines. The installation interval between picks is an important parameter influencing the fragmentation efficiency, which has had a reasonable value in mining machines using in coal mining from numerous laboratory tests and field practices (Peng, 2008). However, it is vague for hard ore-rock fragmentation to determine the suitable installation interval between picks. Thus, the granite specimens with large sizes were used to perform rock fragmentation tests in laboratory using double conical picks to determine a rational range of installation intervals. Double conical picks with adjustable intervals in a designed holder were synchronously pushed forward by displacement-controlled loading with 1 mm/min velocity in Z-direction to fragment granite specimens under no confining stress condition. The tested results corresponding to the different installation intervals were shown in Table 5, 218

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Table 4 Experimental results of coupled static and dynamic rock fragmentation using a conical pick. Static pre-loading/ kN

a

30 (60% )

Confining stress/MPa σX

σX

0

0

Confining stress/ MPa

Disturbing frequency/ Hz

Duration/s

Failure pattern

20

5

7.39

Complete splitting

Failure image of rock specimen

Split fragment

20 (40%b)

0

0

30

5

16.14

Complete splitting

Split fragment 10 (20%c)

0

0

40

5

51.5

Complete splitting

Split fragment d

80 (57% )

0

100

60

5

7.44

Rock burst

Shear cone Chips from rock burst

20

40

110

5

93.13

Surface stripping

Stripped fragment

360 (72%e)

a ,b,c The 60%, 40% and 20% represent that the pre-loading static forces applied to conical pick approximate to 60, 40 and 20% of peak fore (49.99 kN) at rock failure under no confining stress condition. d The 57% represents that the pre-loading static force applied to conical pick approximates to 57% of peak force (140.14 kN) at rock failure under uniaxial confining stress of 100 MPa. e The 72% represents that the pre-loading static force applied to conical pick approximates to 72% of peak force (500.48 kN) at rock failure under biaxial confining stresses of (σX, σZ) = (20, 40) MPa.

4. Estimation model of peak cutting force

σt

Exposed ore-rock in underground mining stope is generally subjected to uniaxial confining stress, resulting from stress redistribution after excavations of entries and mining face. Experimental tests mentioned above determined that the low uniaxial confining stress resulting from stress release and stress relief could provide advantages for mechanized mining of hard ore-rock. Therefore, rock fragmentation using conical pick under uniaxial confining stress is a feasible and promising mechanized mining approach in hard ore-rock. For the cubic rock specimen using in fragmentation tests, the illustrations of geometric model and force analysis were shown in Fig. 10. Assuming that tensile failure occurred on fracture plane, the peak force applied to pick at rock failure satisfies Eq. (4).

∬S sin αds + 2fσy SB =

namelyσt SA + 2fσy SB =

Fct π tan θ

Fct π tan θ

(4)

(5)

Thus, the peak force of pick at rock failure can be expressed as

Fct

= π tanθ (σt SA + 2fσy SB )

(6)

where Fct is the peak force applied to pick at rock failure based on tensile strength, σt is the tensile strength of rock, σy is the uniaxial confining stress applied on rock, SA is the projected area of fracture plane S on the yz plane, SB is the contact area between rock fragment and rigid loading block in the y direction, θ is the half conical angle of the conical pick, α is the tangential inclination of fracture line in the x 219

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Table 5 Experimental results of static rock fragmentation using double conical picks with different installation intervals under no confining stress condition. Size (Length × width × height mm)

Installation interval between picks/mm

Peak force at failure Fc/kN

Insertion depth at failure Dc/ mm

200 × 200 × 200

80

239.99

4.07

Failure image of rock specimen

Chiselled pit Connected fracture Fragments Wing fracture Chiselled pit

200 × 200 × 200

120

170.07

Fractured block

3.70

Chiselled pit Chiselled pit

Fragments

Fractured block

Wing fracture

300 × 150 × 80

100

171.29

3.75

Fractured block

Chiselled pit Connected fracture

Wing fracture

Chiselled pit Wing fracture

400 × 100 × 100

120

142.99

3.07

Chiselled pit

Chiselled pit Fractured block

Wing fracture

400 × 100 × 100

160

109.94

Wing fracture

2.84

Chiselled pit

Fractured block Connected fracture

Chiselled pit

Wing fracture

Wing fracture Fragments

direction, and f is the friction coefficient between rock surface and loading block. Assuming that shear failure occurred on fracture plane, the peak force applied to pick at rock failure satisfies Eqs. (7)–(8).

2fσy SB + τ

∬S cos αds =

Fcs +σ π tan θ

∬S sin αds

2fσy SB + τ

∬S sin αds + σ ∬S cos αds =

Fcs 2

τ = σ tanφ + c

(8) (9)

Eqs. (7) and (8) can be also written as (7)

2fσy SB + τSC = 220

Fcs + σSA π tanθ

(10)

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Fig. 10. Schematic diagram of rock fragmentation using a conical pick, which includes (a) geometric model and force analysis of rock fragment peeled off from specimen, (b) force analysis of conical pick in chiselled pit, (c) stress on fracture plane based on tensile failure, and (d) stresses on fracture plane based on shear failure.

Conical pick

Fc y

SC

Fc x S

SA

Dc

SB

Tip angle

y

z

(d) y

2fσy SB + τSA + σSC =

Fcs 2

t

where σc is the uniaxial compressive strength of the rock, d is the cutting depth of conical pick. The above equation did not reflect the cutting mechanics satisfactorily because the boundary and interaction effects were ignored, and the V-shaped chip and uniform stress distribution did not represent the actual situation. Goktan (1997) considered the effect of friction between rock and pick and proposed Eq. (15) to estimate the maximum cutting force of conical pick.

(11)

Thus, the peak force of pick at rock failure can be solved and expressed as

Fcs =

2π tanθ [2fσy SB (SA + SC + tanφSA−tanφSC ) + c (SA 2 + SC 2)] 2SC + 2tanφSA + π tanθSA−π tanθtanφSC

(12)

Fcs

Fc =

is the peak force applied to pick at rock failure based on shear where strength, SC is the projected area of fracture plane S on the xy plane, σ and τ are the normal and tangential stresses on fracture plane, respectively, c and φ are the cohesion and friction angle of rock material, respectively. Assuming that the fracture plane is a part of spherical surface with radius of R and center of (R, 0, 0) , the parameters of SA , SB and SC will be determined by the radius R of spherical surface and the size a of cubic rock specimen. The radius of fracture plane is influenced by the uniaxial confining stress. In addition, the rock failure on the fracture plane is a coupled tensile and shear failure in the actual fragmentation test. Thus, the peak force applied to conical pick at rock failure can be simplifiedly written as

Fc =

St t S Fc + c Fcs S S

16πσt2 d 2 σc cos2 θ

(15)

where γ is the friction angle between rock and pick. The effects of rock scale, end-face friction and confining stress were ignored in Eqs. (14) and (15). Small-scale experiments were performed in true triaxial hard rock fragmentation tests to physically simulate the high confining stress condition. Thus, the rock scale, end-face friction between rock surface and loading block, and confining stress will significantly influence the measured peak cutting force of conical pick to break rock specimen in this study. Considering the scale effect and stress effects causing end-face friction and axial confinement, the semiempirical equation expressed as Eq. (16) was proposed to estimate the peak cutting force based on Goktan’s formula.

Fc =

(13)

12πσt d 2sin2 (θ + γ ) ks 12πσt d 2sin2 (θ + γ ) ks f (σy ) = · cos(θ + γ ) cos(θ + γ )

(mσy2 e−nσ

where St and Sc are the areas of tensile and shear fractured zones on the fracture plane S . The peak force of conical pick required for rock fragmentation under uniaxial confining stress is influenced by the tip angle of conical pick, the geometrical and geomechanical parameters of rock specimen, the uniaxial confining stress level and the friction coefficient between rock surface and loading block. The rock fragmentation using a conical pick is fundamentally threedimensional, and is difficult to be simplified into a two-dimensional case, especially under confining condition. As a result, only a few theoretical models have been proposed in the literature to estimate the cutting force of conical pick, although their force estimations often draw support from semi-empirical formulas based on the full-scale laboratory experiments (Evans, 1984; Roxborough and Liu, 1995; Goktan, 1997; Goktan and Gunes, 2005). Evans (1984) proposed a formulation shown in Eq. (14) by taking the equilibrium of the force on half of the V-shaped chip using a limit analysis to estimate the maximum penetration force of the conical pick to break the rock material.

Fc =

12πσt d 2sin2 (θ + γ ) cos(θ + γ )

y

)

+1

(16)

where ks is the influence coefficient of rock scale, f (σy ) is the influence function of uniaxial confining stress resulting in end-friction and axial confinement, and m and n are the influence coefficients in the influence function of f (σy ) . The calculated results of peak cutting force were plotted in Fig. 11, which included the theoretical results calculated by Eq. (13) and the regressive results according to Eq. (16). The proposed theoretical model shown in Eq. (13) was satisfactory to estimate the peak cutting force of conical pick at rock failure under low and middle levels of uniaxial confining stress. However, this model could not be used to calculate the peak cutting force under high uniaxial confining stress when the rock burst occurred due to pick penetration. The regressive model expressed as Eq. (16) based on Goktan’s formula and experimental values could reflect the entire variation of peak cutting force from stress-free condition to high uniaxial confining stress condition. The proposed model can provide theoretical support for cuttability estimation of hard orerock under uniaxial confining stress condition due to excavation-induced stress redistribution in underground peninsula- or island-type stope.

(14) 221

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fractured zone with low confining stress and rich fractures, relieving confining stress by artificially induced defects, and adding dynamic disturbing load to cutter can significantly improve the suitability of non-explosive mechanized continuous mining in deep hard rock. Acknowledgements The work described in this paper was supported by the State Key Research Development Program of China (No. 2016YFC0600706) and the National Natural Science Foundation of China (Nos. 41630642, 51774326 and 51504287), for which the authors are very thankful. The first author thanks the Chinese Scholarship Council for financial support toward his joint Ph.D. at the University of Newcastle, Australia. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.tust.2018.05.006. Fig. 11. Theoretical and regressive results of peak cutting forces to break granite rock.

References Balci, C., Bilgin, N., 2007. Correlative study of linear small and full-scale rock cutting tests to select mechanized excavation machines. Int. J. Rock Mech. Min. Sci. 44 (3), 468–476. Bao, R.H., Zhang, L.C., Yao, Q.Y., Lunn, J., 2011. Estimating the peak indentation force of the edge chipping of rocks using single point-attack pick. Rock Mech. Rock Eng. 44 (3), 339–347. Bilgin, N., Copur, H., Balci, C., 2013. Mechanical Excavation in Mining and Civil Industries. CRC Press, Taylor & Francis Group, Boca Raton. Bilgin, N., Demircin, M.A., Copur, H., Balci, C., Tuncdemir, H., Akcin, N., 2006. Dominant rock properties affecting the performance of conical picks and the comparison of some experimental and theoretical results. Int. J. Rock Mech. Min. Sci. 43 (1), 139–156. Bilgin, N., Tuncdemir, H., Balci, C., Copur, H., Eskikaya, S., 2000. A model to predict the performance of tunneling machines under stressed conditions. In: Proceedings of the World Tunnel Congress, Johannesburg, May 13–18, pp 47–53. Blindheim, O.T., Bruland, A., 1998. Boreability testing, Norwegian TBM tunnelling, 30 years of experience with TBMs in Norwegian tunneling. Norwegian Soil Rock Eng. Assoc. 11, 21–28. Brady, B.H., Brown, E.T., 2006. Rock Mechanics: for Underground Mining. Springer, Dordrecht. Cao, W., Li, X., Tao, M., Zhou, Z., 2016. Vibrations induced by high initial stress release during underground excavations. Tunn. Undergr. Space Technol. 53, 78–95. Copur, H., Bilgin, N., Balci, C., Tumac, D., Avunduk, E., 2017. Effects of different cutting patterns and experimental conditions on the performance of a conical drag tool. Rock Mech. Rock Eng. 50 (6), 1585–1609. Dewangan, S., Chattopadhyaya, S., 2016a. Characterization of wear mechanisms in distorted conical picks after coal cutting. Rock Mech. Rock Eng. 49 (1), 225–242. Dewangan, S., Chattopadhyaya, S., 2016b. Performance analysis of two different conical picks used in linear cutting operation of coal. Arab. J. Sci. Eng. 41 (1), 249–265. Dewangan, S., Chattopadhyaya, S., Hloch, S., 2015. Wear assessment of conical pick used in coal cutting operation. Rock Mech. Rock Eng. 48 (5), 2129–2139. Du, K., Tao, M., Li, X., Zhou, J., 2016. Experimental study of slabbing and rockburst induced by true-triaxial unloading and local dynamic disturbance. Rock Mech. Rock Eng. 49 (9), 3437–3453. Ergin, H., Acaroglu, O., 2007. The effect of machine design parameters on the stability of a roadheader. Tunn. Undergr. Space Technol. 22 (1), 80–89. Evans, I., 1984. A theory of the cutting force for point-attack picks. Int. J. Min. Eng. 2 (1), 63–71. Gehring, K., 1995. Design criteria for TBMs with respect to real rock pressure. In: Proceedings of the Tunnel Boring Machines: Trends in Design and Construction of Mechanized Tunneling, Hagemberg, pp 43–53. Goktan, R.M., 1997. A suggested improvement on Evans cutting theory for conical bits. In: Gurgenci, H., Hood, M. (Eds.), Proceedings of the Fourth International Symposium on Mine Mechanization and Automation, Brisbane, pp. 57–61. Goktan, R.M., Gunes, N., 2005. A semi-empirical approach to cutting force prediction for point-attack picks. J. South Afr. Inst. Min. Metall. 105 (4), 257–263. Hoek, E., Carranza_Torres, C., Corkum, B., 2002. Hoek–Brown failure criterion—2002 edition. In: Proceedings of the Fifth North American Rock Mechanics Symposium, Toronto, Canada, vol. 1, p. 267–273. Innaurato, N., Oggeri, C., Oreste, P., Vinai, R., 2007. Experimental and numerical studies on rock breaking with TBM tools under high stress confinement. Rock Mech. Rock Eng. 40 (5), 429–451. Innaurato, N., Oggeri, C., Oreste, P., Vinai, R., 2011. Laboratory tests to study the influence of rock stress confinement on the performances of TBM discs in tunnels. Int. J. Miner. Metall. Mater. 18 (3), 253–259. Kaiser, P.K., 2006. Rock mechanics considerations for construction of deep tunnels in brittle rock. In: Rock Mechanics in Underground Construction, ISRM International Symposium, 4th Asian Rock Mechanics Symposium, Singapore, pp. 47–58.

5. Conclusions Cuttability of hard rock is effected by many factors, such as rock properties, cutting parameters, and stress conditions. Using TRW-300 true triaxial electro-hydraulic servo system, the influences of conical pick performances such as loading velocity, load type and installation interval and stress conditions including uniaxial stress, biaxial stress and stress-free conditions on rock fragmentation of stiff and brittle granite were investigated. Peak force and insertion depth of conical pick at rock failure can reflect rock cuttability. The large peak force and insertion depth mean the low cuttability of rock. For the stiff and brittle granite specimens used in rock fragmentation tests, the following conclusions can be drawn: The cuttability initially decreases and then increases with increasing loading velocity. The effective loading velocity of pick should be set to larger than 60 mm/min, and the installation interval between picks should be arranged less than 100 mm and better from 80 to 100 mm during mining practices. The high preload applied to pick can achieve the high cuttability, and a certain dynamic disturbance added to original cutting force of pick can improve the cutting efficiency during coupled static and dynamic rock fragmentation. The theoretical and regressive models were proposed to estimate the peak cutting forces of conical pick to break rock under the different uniaxial confining stress conditions, considering the effects of rock scale, end-face friction and confining stress. Experimental results indicated that the cuttability decreases in order from the hard rock under stress-free or low confining stress condition in excavation damaged zone or stress-relief zone to uniaxial confining stress condition in peninsula- or island-type pillar, then to biaxial confining stress condition in single-face excavation. Under uniaxial confining stress condition, the experimental, theoretical and regressive results showed that the rock cuttability initially decreases and then increases with increasing uniaxial confining stress, and the uniaxial confining stress greater than 40 MPa instead improves the rock cuttability. However, the high uniaxial confining stress larger than 80% of uniaxial compressive strength may induce rock burst triggered by conical pick penetration. Therefore, only the hard rocks under stress-free and low uniaxial confining stress conditions can be easily cut with high safety and efficiency. In addition, the excavation-induced fractures, the pre-slits and boreholes in rock mass and the coupled static pre-loading and dynamic disturbing can improve hard rock cuttability. The concluding remarks suggest that some measures, e.g. increasing the amount of free surfaces by induced excavations to create uniaxial confining stress condition, triggering stress release by unloading and support methods to create 222

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